WO2006093964A1

WO2006093964A1 - Diversity antenna control for implantable medical device telemetry

Info

Publication number: WO2006093964A1
Application number: PCT/US2006/007092
Authority: WO
Inventors: Hui Li; Joseph E. Bange; Sylvia Quiles; Prashant Rawat; Vineel Vallapureddy; Earle Roberts; Jeffrey A. Von Arx
Original assignee: Cardiac Pacemakers, Inc.
Priority date: 2005-02-28
Filing date: 2006-02-28
Publication date: 2006-09-08
Also published as: EP1863565A1; JP2008532426A

Abstract

A far-field radio frequency (RF) telemetry system for communicating with an implantable medical device includes a diversity antenna system. In one embodiment, an antenna control circuit selects an antenna of the diversity antenna system for transmitting the outgoing data frames and/or receiving the response data frames based the quality of signal reception associated with response data frames sent by the implantable medical device. In another embodiment, an antenna control circuit selects one or more antennas of the diversity antenna system for reducing potential data transmission errors associated with nulls encountered by the telemetry system due to environmental reflections of RF electromagnetic waves.

Description

DIVERSITY ANTENNA CONTROL FOR IMPLANTABLE MEDICAL DEVICE TELEMETRY

CLAIM OF PRIORITY

Benefit of priority is hereby claimed to U.S. Patent Application Serial Number 11/068,476, filed on February 28, 2005 and U.S. Patent Application Serial Number 11/068,478, filed on February 28, 2005, which applications are herein incorporated by reference.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is related to co-pending, commonly assigned, U.S. Patent Application Serial No. 11/068,497, entitled "DIVERSITY ANTENNA SYSTEM FOR COMMUNICATION WITH AN IMPLANTABLE MEDICAL DEVICE," filed on February 28, 2005, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

This document relates generally to telemetry for implantable medical systems and particularly to an external telemetry system having diversity antennas for communicating with an implantable medical device.

BACKGROUND

Medical devices are implanted in human bodies for monitoring physiological conditions, diagnosing diseases, treating diseases, or restoring functions of organs or tissues. Examples of such implantable medical devices include cardiac rhythm management (CRM) devices, neural stimulators, neuromuscular stimulators, drug delivery devices, and biological therapy devices. When an implantable medical device is intended for long-term use in a patient, its size and power consumption are limited by implantability and longevity requirements. Consequently, many implantable medical devices depend on external systems to perform certain functions. Communication between an implantable method device and an external system is performed via telemetry. Examples of specific telemetry functions include programming the implantable medical device to perform certain monitoring or therapeutic tasks, extracting an operational status of the implantable medical device, transmitting real-time physiological data acquired by the implantable medical device, and extracting physiological data acquired by and stored in the implantable medical device. One type of telemetry between the implantable medical device and the external system is based on inductive coupling between two closely-placed coils using the mutual inductance between these coils. One of the coils is part of the implantable medical device, and the other coil is part of the external system. This type of telemetry is referred to as inductive telemetry or near-field telemetry because the coils must be closely situated for obtaining magnetically coupled communication.

Far-field radio-frequency (RF) telemetry provides another means for communications between the implantable medical device and the external system. The far-field RF telemetry is performed using an RF transceiver in the implantable medical device and an RF transceiver in the external system. The far-field RF telemetry frees the patient from any body surface attachment that limits mobility and is more suitable for use when the patient is at home, without the attendance by the physician or other professional caregiver.

The far-field RF telemetry between the implantable medical device and the external system often operates in an environment where RF electromagnetic waves are reflected from various kinds of surfaces. Destructive interference between the incident and reflective waves results in nulls, where the incident wave and reflected wave cancel out. The far-filed RF telemetry link is substantially interrupted when an antenna encounters a null. While such a null is moving and usually transient, the interruption to the telemetry link may last long enough to cause a data transmission error.

Therefore, there is a need for ensuring the quality of far-field RF telemetry between an external system and an implanted device when nulls are present.

SUMMARY

A far-field RF telemetry system for communicating with an implantable medical device includes a diversity antenna system. In one embodiment, multi- frame messages each including multiple outgoing data frames are transmitted to the implantable medical device. In response, the implantable medical device transmits response data frames each following one or more of the outgoing data frames, according to a predetermined communication protocol. An antenna control circuit selects an antenna of the diversity antenna system for transmitting the outgoing data frames and/or receiving the response data frames based the quality of signal reception associated with the response data frames. In another embodiment, an antenna control circuit selects one or more antennas of the diversity antenna system for reducing potential data transmission errors associated with nulls. hi one embodiment, a system for communicating with an implantable medical device includes a diversity antenna system, a transceiver, an antenna interface circuit, and an antenna control circuit. The diversity antenna system includes a plurality of antennas for transmitting an outgoing signal to the implantable medical device and receiving an incoming signal from the implantable medical device. The transceiver transmits outgoing data frames by modulating the outgoing signal and receives incoming data frames by demodulating the incoming signal. The antenna interface circuit includes a switch circuit that connects an antenna of the diversity antenna system to the transceiver according to an antenna selection signal. The antenna control circuit produces the antenna selection signal and includes an incoming signal monitoring timer, an incoming signal monitoring circuit, and an antenna selector. The incoming signal monitoring timer generates incoming signal monitoring signals based on a predetermined communication protocol. The incoming signal monitoring circuit detects at least one measure of quality of the incoming signal in response to each of the incoming signal monitoring signals and produces an indication of quality of the incoming signal. The antenna selector adjusts the antenna selection signal based on the indication of quality of the incoming signal.

In one embodiment, a method for operating a telemetry system communicating with an implantable medical device is provided. A diversity antenna system is used for transmitting an outgoing signal to the implantable medical device and receiving an incoming signal from the implantable medical device. The diversity antenna includes a plurality of antennas. Outgoing data frames are transmitted by modulating the outgoing signal. Incoming data frames are received by demodulating the incoming signal. An antenna of the diversity antenna system is selected to be an active antenna according to an antenna selection signal. Incoming signal monitoring signals are generated based on a predetermined communication protocol. An indication of quality of the incoming signal is produced by detecting at least one measure of quality of the incoming signal in response to each of the incoming signal monitoring signals. The antenna selection signal is adjusted based on the indication of quality of the incoming signal.

In one embodiment, an external system communicating with an implantable medical device includes a diversity antenna system, a transceiver, an antenna interface circuit, and an antenna control circuit. The diversity antenna system includes a plurality of antennas for transmitting an outgoing signal to the implantable medical device and receiving an incoming signal from the implantable medical device. The transceiver transmits outgoing data frames by modulating the outgoing signal and receives incoming data frames by demodulating the incoming signal. The antenna interface circuit includes a switch circuit that connects an antenna of the diversity antenna system to the transceiver according to an antenna selection signal. The antenna control circuit produces the antenna selection signal and includes a fading detector, an antenna selector, and an antenna switching timing circuit. The fading detector detects a transmission failure deemed to be associated with a null. The antenna selector adjusts the antenna selection signal for connecting a different antenna of the diversity antenna system to the transceiver in response to a detection of the transmission failure. The antenna switching timing circuit holds the antenna selection signal while a frame of the outgoing data frames is being transmitted or a frame of the incoming data frames is being received.

In one embodiment, a method is provided for operating a telemetry system communicating with an implantable medical device. Using a diversity antenna system including a plurality of antennas, an outgoing signal modulated by outgoing data frames is transmitted to the implantable medical device, and an incoming signal modulated by incoming data frames is received from the implantable medical device. An active antenna is selected from the diversity antenna system according to an antenna selection signal. A transmission failure deemed to be associated with a null is detected. In response to a detection of the transmission failure, the antenna selection signal is adjusted for selecting a different active antenna of the diversity antenna system. The antenna selection signal is held while a frame of the outgoing data frames is being transmitted or a frame of the incoming data frames is being received. When no data frame is being transmitted or received, the different active antenna is selected from the diversity antenna system according to the adjusted antenna selection signal. In one embodiment, a telemetry system for communicating with an implantable medical device includes a diversity antenna system, a transceiver, an antenna interface circuit, and an antenna control circuit. The diversity antenna system includes a plurality of antennas for transmitting an outgoing signal to the implantable medical device and receiving an incoming signal from the implantable medical device. The transceiver transmits outgoing data frames by modulating the outgoing signal and receives incoming data frames by demodulating the incoming signal. The antenna interface circuit includes a switch circuit that connects an antenna of the diversity antenna system to the transceiver according to an antenna selection signal. The antenna control circuit includes a selection sequence generator that produces the antenna selection signal for selecting an antenna of the diversity antenna system to be connected to the transceiver in response to an antenna switching timing signal. In one embodiment, a method is provided for operating a telemetry system communicating with an implantable medical device. Using a diversity antenna system including a plurality of antennas, an outgoing signal modulated by outgoing data frames is transmitted to the implantable medical device, and an incoming signal modulated by incoming data frames is received from the implantable medical device. An active antenna is selected from the diversity antenna system according to an antenna selection signal. The antenna selection signal is produced for selecting a new active antenna of the diversity antenna system on a predetermined periodic basis.

In one embodiment, an external system communicating with an implantable medical device includes a diversity antenna system, an external system controller, a transceiver, and an antenna control circuit. The diversity antenna system includes a plurality of antennas for transmitting an outgoing signal to the implantable medical device and receiving an incoming signal from the implantable medical device. The transceiver transmits outgoing data frames by modulating the outgoing signal and receives incoming data frames by demodulating the incoming signal. The transceiver includes a plurality of receiving modules and a switch circuit. The receiving modules each have an input coupled to an antenna of the diversity antenna system and an output. The switch circuit connects the output of one of the receiving modules to the external system controller according to a receiving path selection signal. The antenna control circuit produces the receiving path selection signal and includes a signal quality assessment circuit and a receiving path selector. The signal quality assessment circuit produces an indication of quality for the incoming signal processed by each of the receiving modules. The receiving path selector adjusts the receiving path selection signal based on the indications of quality for the incoming signal produced for the plurality of receiving modules.

In one embodiment, a method is provided for operating a telemetry system communicating with an implantable medical device. Using a diversity antenna system including a plurality of antennas, an outgoing signal modulated by outgoing data frames is transmitted to the implantable medical device, and an incoming signal modulated by incoming data frames is received from the implantable medical device. The incoming signal is processed through a plurality of processing paths each coupled to an antenna of the diversity antenna system. An indication of quality is produced for the incoming signal processed by each of the processing paths. One of the processing paths is selected based on the detected indications of quality associated with the processing paths.

This Summary is an overview of some of the teachings of the present application and not intended to be an exclusive or exhaustive treatment of the present subject matter. Further details about the present subject matter are found in the detailed description and appended claims. Other aspects of the invention will be apparent to persons skilled in the art upon reading and understanding the following detailed description and viewing the drawings that form a part thereof. The scope of the present invention is defined by the appended claims and their legal equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, which are not necessarily drawn to scale, like numerals describe similar components throughout the several views. The drawings illustrate generally, by way of example, various embodiments discussed in the present document.

FIG. 1 is an illustration of an embodiment of a CRM system including an implantable medical device and an external system and portions of an environment in which the CRM system is used.

FIG. 2 is a block diagram illustrating a specific embodiment of the external system.

FIG. 3 is a block diagram illustrating an embodiment of a telemetry system of the external system. FIG. 4 is a block diagram illustrating an embodiment of an antenna control circuit of the telemetry system.

FIG. 5 is a block diagram illustrating an embodiment of a response failure detector of the antenna control circuit of FIG 4.

FIG. 6 is a block diagram illustrating another embodiment of the antenna control circuit.

FIG. 7 is a block diagram illustrating another embodiment of the antenna control circuit.

FIG. 8 is a block diagram illustrating another embodiment of the antenna control circuit. FIG. 9 is a block diagram illustrating another embodiment of the antenna control circuit.

FIG. 10 is a flow chart illustrating an embodiment of a method for operating a telemetry system communicating with an implantable medical device. FIG. 11 is a flow chart illustrating an embodiment of a method for adjusting an antenna selection signal.

FIG. 12 is a flow chart illustrating an embodiment of another method for adjusting the antenna selection signal.

FIG. 13 is a flow chart illustrating an embodiment of another method for adjusting the antenna selection signal.

FIG. 14 is a flow chart illustrating an embodiment of another method for adjusting the antenna selection signal.

FIG. 15 is a flow chart illustrating an embodiment of another method for adjusting the antenna selection signal. FIG. 16 is a block diagram illustrating an embodiment of a telemetry system of the external system.

FIG. 17 is a block diagram illustrating a specific embodiment of the telemetry system of FIG. 16. FIG. 18 is a block diagram illustrating another specific embodiment of the telemetry system of FIG. 16.

FIG. 19 is a block diagram illustrating another specific embodiment of the telemetry system of FIG. 16.

FIG. 20 is a block diagram illustrating another specific embodiment of the telemetry system of FIG. 16.

FIG. 21 is a block diagram illustrating another specific embodiment of the telemetry system of FIG. 16.

FIG. 22 is a block diagram illustrating another embodiment of the telemetry system of the external system. FIG. 23 is a block diagram illustrating another embodiment of the telemetry system of the external system.

FIG. 24 is a flow chart illustrating a method for operating a telemetry system communicating with an implantable medical device.

FIG. 25 is a flow chart illustrating another method for operating a telemetry system communicating with the implantable medical device.

FIG. 26 is a flow chart illustrating another method for operating a telemetry system communicating with the implantable medical device.

DETAILED DESCRIPTION In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that the embodiments may be combined, or that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description provides examples, and the scope of the present invention is defined by the appended claims and their legal equivalents. It should be noted that references to "an", "one", or "various" embodiments in this disclosure are not necessarily to the same embodiment, and such references contemplate more than one embodiment.

This document discusses an RF telemetry system for bi-directional communication between an implantable medical device and an external system. The external system includes an external telemetry system that uses a diversity antenna system and an antenna control circuit that selects one or more active antenna of the diversity antenna system to ensure quality of communication. An active antenna is an antenna that is currently used to transmit and/or receive signals. In one embodiment, the antenna control circuit selects one or more active antenna of the diversity antenna system for reducing or minimizing data transmission errors associated with nulls. A null is a point where the destructive interference causes a substantial loss of the RF telemetry including interruption of data communication. The antenna control circuit selects the one or more active antenna based on the quality of signal reception of the diversity antenna system. The signals transmitted to the implantable medical device include multi- frame messages each including a plurality of data frames. The implantable medical device transmits response data frames after one or more of these transmitted data frames. The antenna control circuit assesses the quality of signal reception when the response data frames are expected to be received by the external telemetry system according to a communication protocol. In another embodiment, a different active antenna is selected when a transmission failure such as a data transmission error or a sudden signal strength drop is detected. Such a transmission failure is deemed to be associated with a null resulting from destructive interference between the incident and reflected electromagnetic waves. In another embodiment, a new antenna is selected to be the active antenna on a regular basis, such as on a periodic basis. This reduces the probability for a currently active antenna to encounter a null. In another embodiment, the telemetry system includes multiple processing paths each associated with an antenna of the diversity antenna system. A different processing path is selected when the transmission failure is detected.

In various embodiments, the circuits described in this document are implemented by hardware, software, firmware, or any combination thereof. In various embodiments, the circuits or portions thereof described in this document are each an application-specific circuit constructed to perform one or more particular functions, a general-purpose circuit programmed to perform such function(s), or a combination thereof.

FIG. 1 is an illustration of an embodiment of portions of a CRM system 100 and portions of an environment in which system 100 is used. System 100 includes an implantable medical device 110 and an external system 120. In the illustrated embodiment, after being implanted into a patient's body 101, implantable medical device 110 is coupled to the patient's heart 102 through a lead system 105. Examples of implantable medical device 110 include pacemakers, cardioverter/defibrillators, cardiac resynchronization therapy (CRT) devices, cardiac remodeling control therapy (RCT) devices, neural stimulators, drug delivery systems, biological therapy devices, and patient monitoring devices. External system 120 allows a physician or other caregiver to interact with implantable medical device 110 through an RF telemetry link 115, which provides for bi-directional data communication between implantable medical device 110 and external system 120.

RF telemetry link 115 provides for data transmission from implantable medical device 110 to external system 120. This includes, for example, transmitting real-time physiological data acquired by implantable medical device 110, extracting physiological data acquired by and stored in implantable medical device 110, extracting therapy history data stored in implantable medical device 110, and extracting data indicating an operational status of implantable medical device 110 (e.g., battery status and lead impedance). RF telemetry link 115 also provides for data transmission from external system 120 to implantable medical device 110. This includes, for example, programming implantable medical device 110 to acquire physiological data, programming implantable medical device 110 to perform at least one self-diagnostic test (such as for a device operational status), and programming implantable medical device 110 to deliver at least one therapy. RF telemetry link 115 is a far-field telemetry link. A far-field, also referred to as the Fraunhofer zone, refers to the zone in which a component of an electromagnetic field produced by the transmitting electromagnetic radiation source decays substantially proportionally to 1/r, where r is the distance between an observation point and the radiation source. Accordingly, far-field refers to the zone outside the boundary of r = λ/27r, where λ is the wavelength of the transmitted electromagnetic energy. Li one embodiment, a communication range of RF telemetry link 115 (a distance over which data is capable of being wirelessly communicated) is at least ten feet but can be as long as allowed by the communication technology utilized. Unlike an inductive telemetry link using a coil placed near implantable medical device 110, attached to the patient, and electrically connected to external system 120 with a cable, using RF telemetry link 115 frees the patient from any physical restraints caused by the coil and the cable and allows external system 120 to be placed entirely away from the sterile filed during an operation such as the implantation of implantable medical device 110.

RF telemetry link 115 is supported by an implant telemetry system of implantable medical device 110 and an external telemetry system 122 of external system 120. External telemetry system 122 includes a diversity antenna system 126, an antenna interface circuit 128, a transceiver 130, and an antenna control circuit 132. Diversity antenna system 126 includes a plurality of antennas to transmit an outgoing signal to implantable medical device 110 and to receive an incoming signal from implantable medical device 110. Antenna interface circuit 128 includes tuning circuitry for diversity antenna system 126 and routes the outgoing and incoming signals between diversity antenna system 126 and transceiver 130. Transceiver 130 transmits outgoing data frames by modulating the outgoing signal and receives incoming data frames by demodulating the incoming signal. The outgoing data frames and the incoming data frames are each a frame being a logic unit of data including a header, a payload, and a trailer. The header includes a "comma," which includes a unique set of bits for signaling a receipt of a frame. A lack of comma, or failure to receive the comma, indicates a failure to receive a frame. The payload includes the data block being transmitted. The trailer includes a cyclic redundancy check (CRC) character having a value generated by a transmitter. A receiver receives that CRC character and also recalculates the CRC character based on the received data block and compares the result to the received CRC character in the trailer. The data is deemed to be correctly transmitted if the recalculated CRC character matches the received CRC character. A CRC error refers to a mismatch between the recalculated CRC character and the received CRC character. Depending on the specific communication formats, the header and the trailer each include additional information for flagging, control of data recovery, and/or synchronization of the receiving device. In various embodiments, the outgoing data frames include multi-frame messages each including a plurality of data frames. The incoming data frames include response frames each being an incoming data frame produced by implantable medical device 110 in response to one or more outgoing data frames. Antenna control circuit 132 controls the operation of antenna interface circuit for an approximately optimal performance, or at least an acceptable performance, of diversity antenna system 126. In various embodiments, antenna control circuit 132 selects an active antenna of diversity antenna system 126 based on the quality of the incoming signal. Such quality is measured by, for example, strength of the incoming signal and/or integrity of the incoming data frames. Antenna control circuit 132 controls the timing for assessing the quality of the incoming signal based on a predetermined communication protocol according to which bi-directional communication is performed via RF telemetry link 115. In one embodiment, antenna control circuit 132 selects a different active antenna of diversity antenna system 126 in response to the detection of a transmission failure in the incoming signal. Such a transmission failure may be associated with a null or an antenna that is damaged or otherwise with a degraded performance. External telemetry system 122 is connected to an external system controller 124, which allows external system 120 to receive information acquired by implantable medical device 110 and to control the operation of implantable medical device 110. External system controller 124 receives the incoming data frames from transceiver 130 and sends the outgoing data frames to transceiver 130. A user interface 125 allows the physician or other caregiver to view the received information and to enter commands and parameters to control the operation of CRM system 100.

In one embodiment, external system 120 includes a programmer. In another embodiment, as illustrated in FIG. 2, external system 120 includes a patient management system.

FIG. 2 is a block diagram illustrating an embodiment of external system 220, which is a specific embodiment of external system 120. As illustrated in FIG. 2, external system 220 is a patient management system including an external device 234, a telecommunication network 236, and one or more remote devices 238. External device 234 is placed within the vicinity of implantable medical device 110 and includes external telemetry system 122 to communicate with implantable medical device 110 via RF telemetry link 115. Remote device(s) 238 are in one or more remote locations and communicates with external device 234 through network 236, thus allowing a physician or other caregiver to monitor and treat a patient from a distant location and/or allowing access to various treatment resources from the one or more remote locations.

FIG. 3 is a block diagram illustrating an embodiment of an external telemetry system 322, which is a specific embodiment of external telemetry system 122. External telemetry system 322 includes a diversity antenna system 326, an antenna interface circuit 328, a transceiver 330, and an antenna control circuit 332.

Diversity antenna system 326 is a specific embodiment of diversity antenna system 126 and includes two or more antennas 340A-N. Each of antennas 340A-N allows for transmitting an outgoing signal to implantable medical device 110 and/or receiving an incoming signal from implantable medical device 110. In one embodiment, diversity antenna system 126 includes two antennas. In a specific embodiment, the two antennas are mounted onto a chassis of an external device such as the programmer or external device 234. In other embodiments, diversity antenna system 126 includes three or more antennas. One example of diversity antenna system 126 is discussed in U.S. Patent Application Serial No. 11/068,497, entitled "DIVERSITY ANTENNA SYSTEM FOR COMMUNICATION WITH AN IMPLANTABLE MEDICAL DEVICE," filed on February 28, 2005, which is incorporated by reference herein in its entirety.

Antenna interface circuit 328 is a specific embodiment of antenna interface circuit 128 and includes tuning circuits 342A-N and a switch circuit 344. Tuning circuits 342A-N each provide tuning for a corresponding antenna of antennas 342A-N. Switch circuit 344 provides for a controllable connection between an antenna of diversity antenna system 326 and transceiver 330 according to an antenna selection signal. This antenna is used for transmitting the outgoing signal and receiving the incoming signal. In one embodiment, switch circuit 344 substantially completes the change of the connection from between an antenna of diversity antenna system 326 and transceiver 330 to between another antenna of diversity antenna system 326 and transceiver 330 in about 50 microseconds to 1 millisecond.

Transceiver 330 is a specific embodiment of transceiver 130 and includes a modulator 346 and a demodulator 348. Modulator 346 produces the outgoing signal by modulating an RF carrier with the outgoing data frames, including outgoing data frames that form the multi-frame messages. In one embodiment, the frequency of the RF carrier for the outgoing signal is in a range of approximately 902 MHz to 928 MHz, with approximately 914 MHz being a specific example. The data transmission rate for the outgoing signal is in a range of approximately 60 kilobits per second to 500 kilobits per second, with approximately 204.8 kilobits per second being a specific example. Demodulator 348 recovers the incoming data frames by demodulating the received incoming signal. The implant telemetry circuit of implantable medical device 110 produces the incoming signal by modulating another RF carrier with the incoming data frames. In one embodiment, the frequency of the RF carrier for the incoming signal is in a range of approximately 902 MHz to 928 MHz, with approximately 914 MHz being a specific example. The data transmission rate for the incoming signal is in a range of approximately 60 kilobits per second to 500 kilobits per second, with approximately 102.4 kilobits per second being a specific example. The incoming data frames include the response frames each follow one or more of the outgoing data frames. In one embodiment, amplitude- shift-keying (ASK) is the modulation scheme used for both the outgoing signal and the incoming signal. Modulator 346 is an ASK modulator, and demodulator 348 is an ASK demodulator. Antenna control circuit 332 is a specific embodiment of antenna control circuit 132 and produces the antenna selection signal. Antenna control circuit 332 includes an incoming signal monitoring timer 350, an incoming signal monitoring circuit 352, and an antenna selector 354. Incoming signal monitoring timer 350 generates incoming signal monitoring signals based on the predetermined communication protocol. The communication protocol specifies the timing for the transmission of the outgoing data frames and timing for the receipt of the incoming data frames. The incoming signal monitoring signals each allow the quality of the incoming signal to be assessed when an incoming data frame is scheduled to be received. Incoming signal monitoring circuit 352 produces an indication of the quality of the incoming signal by detecting at least one measure of quality of the incoming signal in response to each of the incoming signal monitoring signals. Examples of the measure of quality include existence of data transmission error in the incoming signal and strength of the incoming signal. Antenna selector 354 adjusts the antenna selection signal based on the indication of quality of the incoming signal, hi one embodiment, antenna selector 354 adjusts the antenna selection signal for connecting a different antenna of diversity antenna system 326 to transceiver 330 when an expected incoming data frame is not detected within a scheduled reception time window. In another embodiment, antenna selector 354 adjusts the antenna selection signal for connecting a different antenna of diversity antenna system 326 to transceiver 330 when a transmission error is detected from the incoming signal, hi another embodiment, antenna selector 354 adjusts the antenna selection signal for connecting an antenna of diversity antenna system 326 to transceiver 330 based the strength of the incoming signal associated with each antenna of diversity antenna system 326.

FIG. 4 is a block diagram illustrating an embodiment of an antenna control circuit 432, which is a specific embodiment of antenna control circuit 332. Antenna control circuit 432 includes a nonresponsive period timer 456, an incoming signal monitoring timer 450, an incoming signal monitoring circuit 452, and an antenna selector 454.

Nonresponsive period timer 456 starts a nonresponsive period during the transmission of a multi-frame message when a response frame is expected to be received according to the predetermined communication protocol. The multi- frame message is transmitted with an active antenna of diversity antenna system 326 connected to transceiver 330. The response frame is expected to be received after several outgoing data frames of the multi-frame message are transmitted, according to the predetermined communication protocol. The nonresponsive period is the maximum time interval allowed for adjusting the antenna selection signal without resulting in a receipt of any response frame without data transmission error. If the nonresponsive period expires without finding another active antenna that provides for acceptable signal quality of the incoming signal, the transmission of the multi-frame message is repeated using a different active antenna, i.e., an antenna that is different from the one with which the transmission of the multi-frame message first started. In one embodiment, the nonresponsive period is a predetermined time interval in a range of approximately 50 milliseconds to one second, with approximately 100 milliseconds being a specific example. Incoming signal monitoring timer 450 is a specific embodiment of incoming signal monitoring timer 350 and includes a response timer 458. Response timer 458 starts a response time interval during the transmission of the multi-frame message when the response frame is expected to be received and when one or more subsequent response frames, if any, are expected to be received, according to the predetermined communication protocol. In other words, response timer 458 times response time intervals each representing a time window during which a response frame is expected to be received. In one embodiment, the response time interval is a predetermined time interval in a range of approximately 4 milliseconds to 1 second, with approximately 100 milliseconds being a specific example. In one embodiment, the response time interval is set to be equal to the interval between the end of the transmission of an outgoing data frame and the beginning of the transmission of the next outgoing data frame.

Incoming signal monitoring circuit 452 is a specific embodiment of incoming signal monitoring circuit 352 and includes a response failure detector 460. After response timer 458 starts a response time interval, response failure detector 460 detects a response failure during the response time interval. The response failure is a data transmission error associated with a response frame. Examples of such a data transmission error include a failure to receive the response frame, a failure to receive the comma of the response frame, and a CRC error in the response frame. Specific examples of response failure detector 460 are discussed below with reference to FIG. 5.

Antenna selector 454 is a specific embodiment of antenna selector 354. During the nonresponsive period, antenna selector 454 adjusts the antenna selection signal for connecting a different antenna of diversity antenna system 326 to transceiver 330 if the response failure is detected during the predetermined response time interval. If the nonresponsive period expires without any successful receipt of an incoming data frame during the nonresponsive period, antenna selector 454 adjusts the antenna selection signal for connecting a different antenna of diversity antenna system 326 to transceiver 330. Here the different antenna refers to an antenna that is different from the one with which the transmission of the multi-frame message started.

FIG. 5 is a block diagram illustrating an embodiment of a response failure detector 560, which is a specific embodiment of response failure detector 460. hi one embodiment, as illustrated in FIG. 5, response failure detector 560 includes a lack-of-response detector 562, a lack-of-comma detector 564, and a CRC failure detector 566. In various other embodiments, response failure detector 560 includes any one or more of lack-of-response detector 562, lack-of- comma detector 564, and CRC failure detector 566. The response failure is detected when any one or more of lack-of-response detector 562, lack-of-comma detector 564, and CRC failure detector 566 detect a data transmission error associated with the response frame. Lack-of-response detector 562 detects the response frame during the response time interval. The response failure is detected if no response frame is detected during the response time interval. Lack-of-comma detector 564 detects a comma indicative of a receipt of the response frame during the response time interval. The response failure is detected if the comma is not detected during the response time interval. CRC failure detector 556 detects a CRC failure from the incoming signal during the response time interval. The response failure is detected if the CRC failure is detected during the response time interval. hi other embodiments, response failure detector 560 includes one or more error detectors detecting data transmission errors of types other than the lack of response, the lack of comma, and the CRC failure. Generally, response failure detector 560 detects any predetermined type data transmission error in response frames, with the predetermined type data transmission error representing the response failure.

FIG. 6 is a block diagram illustrating an embodiment of an antenna control circuit 632, which is another specific embodiment of antenna control circuit 332. Antenna control circuit 632 includes a test frame generator 668, incoming signal monitoring timer 450, incoming signal monitoring circuit 452, and an antenna selector 654.

Test frame generator 668 produces an antenna test signal prior to the transmission of a multi-frame message. The antenna test signal causes transceiver 330 to transmit a test frame that is an outgoing data frame. Response timer 458 starts the response time interval following the transmission of the test frame. Response failure detector 460 detects the response failure during the response time interval. Antenna selector 654 is another specific embodiment of antenna selector

354. Antenna selector 654 adjusts the antenna selection signal for connecting a different antenna of diversity antenna system 326 to transceiver 330 when the response failure is detected during the response time interval. In one embodiment, test frame generator 668 produces the antenna test signal for transmitting the test frame using different antennas of diversity antenna system until an antenna associated with an acceptable quality of the incoming signal is found. In another embodiment, if a response failure is detected from an incoming signal received using an antenna, that antenna is deselected as the only antenna associated with an unacceptable quality of the incoming signal. If the response failure is caused by a null encountered by the antenna, the probability that a different antenna also encounters a null is minimal. In one embodiment, test frame generator 668 produces the antenna test signal for transmitting the test frame using different antennas of diversity antenna system until the quality of the incoming signal is assessed for all the antennas of diversity antenna system 326.

FIG. 7 is a block diagram illustrating an embodiment of an antenna control circuit 732, which is another specific embodiment of antenna control circuit 332. Antenna control circuit 732 includes incoming signal monitoring timer 450, incoming signal monitoring circuit 452, and an antenna selector 754. Response timer 458 times the response time interval when a response frame is expected to be received according to the predetermined communication protocol. Response failure detector 460 detects the response failure during the response time interval.

Antenna selector 754 is another specific embodiment of antenna selector 354. Antenna selector 754 adjusts the antenna selection signal for connecting a different antenna of diversity antenna system 326 to transceiver 330 after the transmission of a multi-frame message if the response failure is detected during the transmission of that multi-frame message. Response failure detector 460 detects the response failure during the transmission of the multi-frame message. When multiple response frames are expected to be received during the transmission of the multi-frame message, a single response failure associated with any of the response frames causes antenna selector 754 to adjust the antenna selection signal for connecting the different antenna of diversity antenna system 326 to transceiver 330. The adjustment of the antenna selection signal occurs following the end of the transmission of the multi-frame message.

FIG. 8 is a block diagram illustrating an embodiment of an antenna control circuit 832, which is another specific embodiment of antenna control circuit 332. Antenna control circuit 832 includes a transmission period timer 870, incoming signal monitoring timer 450, incoming signal monitoring circuit 452, and an antenna selector 854.

Transmission period timer 870 times transmission periods each including a predetermined number of the outgoing data frames. The transmission period is determined based on the predetermined communication protocol and is specified by the number of the outgoing data frames. The transmission period is long enough such that at least one response frame is expected to be received during each transmission period. For example, if up to five consecutive outgoing data frames may be transmitted without expecting a response frame, the transmission period is determined to include at least six outgoing data frames. Response timer 458 times the response time interval when a response frame is expected to be received according to the predetermined communication protocol. Response failure detector 460 detects the response failure during the response time interval.

Antenna selector 854 is another specific embodiment of antenna selector 354. Antenna selector 854 adjusts the antenna selection signal for connecting a different antenna of diversity antenna system 326 to transceiver 330 at the end of each transmission period if the response failure is detected during that transmission period.

FIG. 9 is a block diagram illustrating an embodiment of an antenna control circuit 932, which is another specific embodiment of antenna control circuit 332. Antenna control circuit 932 includes an incoming signal monitoring timer 950, an incoming signal monitoring circuit 952, and an antenna selector 954.

Incoming signal monitoring timer 950 is another specific embodiment of incoming signal monitoring timer 350 and includes a signal strength calculation timer 972. Signal strength calculation timer 972 generates a signal strength calculation signal prior to the transmission of a multi-frame message.

Incoming signal monitoring circuit 952 is another specific embodiment of incoming signal monitoring circuit 352 and includes a signal strength detector 974, a signal strength storage circuit 976, and a signal strength calculator 978. Signal strength detector 974 measures strength parameters each representing the strength of the incoming signal associated with one antenna of diversity antenna system 326. In one embodiment, signal strength detector 974 measures the strength parameter associated with an antenna each time when that antenna is selected to be the active antenna. Signal strength storage circuit 976 stores the measured strength parameters. Each stored strength parameter is time stamped to indicate when the last measurement of that strength parameter is performed. Signal strength calculator 978 calculates a signal strength index for each antenna of diversity antenna system 326 in response to the signal strength calculation signal. The signal strength index is a function of a stored strength parameter and a weighting factor. The weighting factor is a function of the time elapsed since the last measurement of the strength parameter. A more recently detected strength parameter is given more weight in the calculation of the signal strength index.

Antenna selector 954 is another specific embodiment of antenna selector 354. Antenna selector 954 adjusts the antenna selection signal for connecting an antenna of diversity antenna system 326 to transceiver 330 based on the strength indices calculated for all the antennas of diversity antenna system 326 in response to the signal strength calculation signal. In one embodiment, Antenna selector 954 adjusts the antenna selection signal to select the antenna associated with the highest strength index.

In various embodiments, antenna control circuit 332 may combine one or more of antenna control circuits 432, 632, 732, 832, and 932, as determined by one skilled in the art upon reading and understanding this document. In general, an antenna control circuit assesses the quality of the incoming signal using any viable method and selects an antenna of the diversity antenna system for being the active antenna based on the quality of the incoming signal assessed for this antenna or for a different antenna. In various embodiments, because the probability that two or more antennas of the diversity antenna system simultaneously encounter a null is negligible, when one antenna is indicated to be associated with unacceptable quality of the incoming signal, one or more different antennas are deemed to be associated with acceptable quality of the incoming signal.

FIG. 10 is a flow chart illustrating an embodiment of a method 1000 for operating a telemetry system communicating with an implantable medical device. In one embodiment, the telemetry system includes telemetry system 322. An outgoing signal is transmitted, and an incoming signal is received, using a diversity antenna system including two or more antennas at 1010. This includes transmitting outgoing data frames to the implantable medical device by modulating the outgoing signal and receiving incoming data frames from the implantable medical device by demodulating the incoming signal. In various embodiments, the transmitted outgoing signal is modulated by multi-frame messages each including a plurality of outgoing data frames. In various embodiments, the multi-frame messages each include two or more outgoing data frames transmitted consecutively without intervening response frames.

An active antenna is selected according to an antenna selection signal at 1020. The active antenna is the antenna that is currently used to transmit the outgoing signal and/or receive the incoming signal. The antenna selection signal specifies which antenna is active.

The antenna selection signal is adjusted based on a measure of quality of the incoming signal at 1030. An incoming signal monitoring signal is generated based on a predetermined communication protocol at 1032. This predetermined communication protocol specifies timing of the transmission of the outgoing data frames and timing of the receipt of the incoming data frames. The incoming signal monitoring signal indicates a time window within which the quality of the incoming signal is to be assessed. An indication of quality is produced for the incoming signal associated with an antenna of the diversity antenna system at 1034, in response to the incoming signal monitoring signal. This indication of quality is produced by detecting at least one measure of quality of the incoming signal, such as presence of a transmission error in the incoming signal and strength of the incoming signal. The antenna selection signal is adjusted based on the indication of quality of the incoming signal at 1036. If the quality of the incoming signal is indicated to be unacceptable, the antenna selection signal is adjusted to select a different antenna of the diversity antenna system to be the active antenna. Exemplary specific embodiments of step 1030 are discussed below with reference to FIGS. 11 - 15.

FIG. 11 is a flow chart illustrating an embodiment of a method 1100 for adjusting the antenna selection signal. Method 1100 is a specific embodiment of method 1000. In a specific embodiment, method 1100 is performed by antenna control circuit 432. A predetermined nonresponsive period is started at 1110, during the transmission of a multi-frame message when a response frame is expected to be received according to the predetermined communication protocol. The transmission of the multi-frame message is performed using a first antenna of the diversity antenna system. The response frame is expected to be received after two or more outgoing data frames of the multi-frame message are transmitted using the first antenna. In one embodiment, the predetermined nonresponsive period is in a range of approximately 50 milliseconds to 1 second, with 100 milliseconds being a specific example.

A predetermined response time interval is started at 1120, during the transmission of the multi- frame message when the response frame is expected to be received and when each of one or more subsequent response frames, if any, is expected to be received. In one embodiment, the predetermined response time interval is in a range of approximately 4 milliseconds to 1 second, with approximately 100 milliseconds being a specific example. In one embodiment, the predetermined response time interval is limited by the time interval between the end of the transmission of an outgoing data frame and the beginning of the transmission of the next outgoing data frame.

A response failure is detected during the predetermined response time interval at 1130. The response failure is a data transmission error associated with a response frame. In one embodiment, the response frame is detected during the predetermined response time interval. The response failure is detected if the response frame is not detected during the predetermined response time interval. In another embodiment, a comma indicative of a receipt of the response frame is detected during the predetermined response time interval. The response failure is detected if the comma is not detected during the predetermined response time interval. In another embodiment, a CRC failure in the response frame is detected during the predetermined response time interval. The response failure is detected if the CRC failure is detected during the predetermined response time interval. In another embodiment, one or more of the response frame, the comma, and the CRC failure are detected. The response failure is detected if the response frame is not detected, or if the comma is not detected, or if the CRC failure is detected, during the predetermined response time interval. The antenna selection signal is adjusted for selecting a different antenna of the diversity antenna system to be the active antenna at 1140, if the response failure is detected during the predetermined response time interval. When two or more response frames are expected to be received during the transmission of the multi-frame message, the predetermined response time interval is started when each response frame is expected to be received, and the antenna selection signal is adjusted for selecting a different antenna if the response failure is detected during the predetermined response time interval.

The antenna selection signal is adjusted for selecting a second antenna of the diversity antenna system at 1150, when the predetermined nonresponsive period expires without successful receipt of any response frame. The second antenna is an antenna different from the first antenna. That is, if none of the incoming data frames is received without the data transmission error during the predetermined nonresponsive period, a different antenna is selected for repeating the transmission of the multi-frame message or performing the transmission of another multi-frame message.

FIG. 12 is a flow chart illustrating an embodiment of a method 1200 for adjusting the antenna selection signal. Method 1200 is another specific embodiment of method 1000. In a specific embodiment, method 1200 is performed by antenna control circuit 632. An antenna test signal is produced at 1210, prior to the transmission of the multi-frame message. The antenna test signal cause a test frame to be transmitted to the implantable medical device, which transmits a response frame after receiving the test frame. The predetermined response time interval is started at 1220, following the transmission of the test frame. The response failure is detected during the predetermined response time interval at 1230. The antenna selection signal is adjusted for selecting a different antenna of the diversity antenna system to be the active antenna when the response failure is detected at 1240. In one embodiment, method 1200 is repeated for all the antennas of the diversity antenna system. In another embodiment, method 1200 is repeated with another antenna only if the response failure is detected with the currently active antenna, m another embodiment, if the response failure is detected with the currently active antenna, a different antenna is assumed to be associated with acceptable quality of the incoming signal and is therefore selected.

FIG. 13 is a flow chart illustrating an embodiment of a method 1300 for adjusting the antenna selection signal. Method 1300 is another specific embodiment of method 1000. In a specific embodiment, method 1300 is performed by antenna control circuit 732. The predetermined response time interval is started at 1310, during the transmission of the multi-frame message when a response frame is expected to be received. The response failure is detected during the predetermined response time interval at 1320. The antenna selection signal is adjusted for selecting a different antenna of the diversity antenna system to be the active antenna after the transmission of the multi-frame message if the response failure is detected during the transmission of the multi-frame message at 1330. When two or more response frames are expected to be received during the transmission of the multi- frame message, a single response failure associated with any of the response frames causes the antenna selection signal to be adjusted for selecting the different antenna to be the active antenna.

FIG. 14 is a flow chart illustrating an embodiment of a method 1400 for adjusting the antenna selection signal. Method 1400 is another specific embodiment of method 1000. In a specific embodiment, method 1400 is performed by antenna control circuit 832. A transmission period is timed at 1410. The transmission period includes a period during which a predetermined number of the outgoing data frames are transmitted. The transmission period is timed repeatedly while the outgoing data frames are transmitted to the implantable medical device. The number of the outgoing data frames in the transmission period is determined based on the predetermined communication protocol. At least one response frame is expected to be received during each transmission period. The predetermined response time interval is started at 1420, during the transmission period when a response frame is expected to be received. The response failure is detected during the predetermined response time interval at 1430. The antenna selection signal is adjusted for selecting a different antenna of the diversity antenna system to be the active antenna at 1440, at the end of the transmission period if the response failure is detection during the transmission period.

FIG. 15 is a flow chart illustrating an embodiment of a method 1500 for adjusting the antenna selection signal. Method 1500 is another specific embodiment of method 1000. hi a specific embodiment, method 1500 is performed by antenna control circuit 932.

Strength parameters each associated with an antenna are measured at 1510. The strength parameters each represent strength of the incoming signal received by one antenna of the diversity antenna system. Examples of the strength parameter include amplitude and power of the incoming signal. The strength parameters are stored at 1520. Each stored strength parameter is time stamped to indicate when that strength parameter was measured. Prior to the transmission of a multi-frame message, a signal strength calculation signal is generated at 1530. hi response to the signal strength calculation signal, signal strength indices are calculated at 1540. The signal strength indices are each associated with one antenna of the diversity antenna system and is calculated based on the strength parameter and a weighting factor. The weighting factor is a function of the time elapsed since the measurement of the strength parameter. The antenna selection signal is adjusted for selecting an antenna of the diversity antenna system to be the active antenna based on the signal strength indices at 1550. hi one embodiment, the antenna selection signal is adjusted for selecting the antenna associated with the highest signal strength index, i.e., associated with the strongest incoming signal strength as recently measured. Methods 1100, 1200, 1300, 1400, and 1500 are exemplary specific embodiments illustrating how the assessment of the quality of the incoming signal is timed and performed and how the antenna selection signal is adjusted based on the assessed quality of the incoming signal, hi various embodiments, these specific embodiments may be combined, and other specific embodiments may be employed, as determined by one skilled in the art upon reading and understanding this document. In general, the quality of the incoming signal is assessed using any viable method, and an antenna of the diversity antenna system is selected for being the active antenna based on the quality of the incoming signal assessed for this antenna or for a different antenna.

FIG. 16 is a block diagram illustrating an embodiment of an external telemetry system 1622, which is a specific embodiment of external telemetry- system 122. External telemetry system 1622 includes diversity antenna system 326, antenna interface circuit 328, transceiver 330, and an antenna control circuit 1632.

Antenna control circuit 1632 is a specific embodiment of antenna control circuit 132 and includes a fading detector 1650, an antenna selector 1652, and an antenna switching timing circuit 1654. Fading detector 1650 detects a transmission failure deemed to be associated with a null. Exemplary specific embodiments of fading detector 1650 are discussed below with reference to FIGS. 4-8. Antenna selector 1652 adjusts the antenna selection signal for connecting a different antenna of diversity antenna system 326 to transceiver 330 in response to a detection of the transmission failure. Antenna switching timing circuit 1654 holds the antenna selection signal while a frame of the outgoing data frames is being transmitted or a frame of the incoming data frames is being received. Thus, after antenna selector 1652 adjusts the antenna selection signal, the change of connection between diversity antenna system 326 and transceiver 330 occurs when no data frame is being transmitted or received. This prevents a potential data transmission error from being resulted from the change of connection, i.e., switching from one antenna to another. Li one embodiment, antenna switching timing circuit 1654 delays the adjustment of the antenna selection signal by antenna selector 1652 until an ongoing transmission or reception of a data frame is completed. In another embodiment, antenna switching timing circuit 1654 keeps the adjusted antenna selection signal from being applied to switch circuit 344 until an ongoing transmission or reception of a data frame is completed. In one embodiment, antenna switching timing circuit 1654 holds the antenna selection signal while a data frame is being transmitted or received only if the switching time of switch circuit 344 for switching from one antenna to another is not substantially higher than the time required for transmitting one data bit. In another embodiment, external telemetry system 1622 includes an error protection circuit to prevent data transmission errors caused by an operation of switching circuit 344 in response to a change in the antenna selection signal. The error protection circuit corrects a detected error in received incoming data frames by executing an error correction algorithm.

FIG. 17 is a block diagram illustrating an embodiment of an external telemetry system 1722, which is a specific embodiment of external telemetry system 1622. External telemetry system 1722 includes diversity antenna system 326, antenna interface circuit 328, transceiver 330, and an antenna control circuit 1732.

Antenna control circuit 1732 is a specific embodiment of antenna control circuit 1632 and includes an incoming frame failure detector 1750 and an antenna selector 1752. Incoming frame failure detector 1750 is a specific embodiment of fading detector 1650 and detects an incoming frame failure as the transmission failure from the incoming signal. The incoming frame failure includes a data transmission error in at least one of the incoming data frames, hi one embodiment, a single data transmission error in one incoming data frame constitutes the transmission failure deemed to be associated with a null. Incoming frame failure detector 1750 includes a CRC failure detector 1756 and/or a lack-of-comma detector 1758. CRC failure detector 1756 detects a

CRC failure from the incoming signal and indicates the incoming frame failure if the CRC failure is detected. Lack-of-comma detector 1758 detects a comma indicative of a receipt of an incoming data frame during a predetermined time window and indicates the incoming frame failure if the comma is not detected during the predetermined time window, hi other embodiments, incoming frame failure detector 1750 includes one or more error detector detecting data transmission errors of types other than the CRC failure and the lack of comma. Generally, incoming frame failure detector 1750 detects any predetermined type data transmission error in at least one of the incoming data frames and indicates the incoming frame failure when the predetermined type data transmission error is detected.

Antenna selector 1752 is a specific embodiment of antenna selector 1652 and adjusts the antenna selection signal for connecting a different antenna of diversity antenna system 326 to transceiver 330 when the incoming frame failure is indicated by incoming frame failure detector 1750. In one embodiment, antenna selector 1752 adjusts the antenna selection signal in response to the detection of either a CRC failure or a lack of comma. In response to the adjusted antenna selection signal, switch circuit 344 connects the different antenna to transceiver 330 for receiving the incoming signal and transmitting the outgoing signal until the detection of another incoming frame failure is indicated.

FIG. 18 is a block diagram an embodiment of an external telemetry system 1822, which is another specific embodiment of external telemetry system 1622. External telemetry system 1822 includes diversity antenna system 326, antenna interface circuit 328, transceiver 330, and an antenna control circuit 1832.

Antenna control circuit 1832 is a specific embodiment of antenna control circuit 1632 and includes a response failure detector 1850 and an antenna selector 1852. Response failure detector 1850 is a specific embodiment of fading detector 1650 and detects a response failure as the transmission failure from the incoming signal. The response failure includes a data transmission error in at least one response frame of the incoming data frames. The response frame is an incoming data frame produced and sent by implantable medical device 110 in response to an outgoing data frame transmitted to implantable medical device 110. In one embodiment, a single data transmission error in one response frame constitutes the transmission failure deemed to be associated with a null. Response failure detector 1850 includes a CRC failure detector 1856 and/or a lack-of-response detector 1858. CRC failure detector 1856 detects a CRC failure from the incoming signal and indicates the response failure if the CRC failure is detected. Lack-of-response detector 1858 detects a response frame indicative of a receipt of the outgoing data frame transmitted to implantable medical device 110 and indicates the response failure if no response frame is detected during a predetermined time window starting from the transmission of the outgoing data frame. Antenna selector 1852 is a specific embodiment of antenna selector 1652 and adjusts the antenna selection signal for connecting a different antenna of diversity antenna system 326 to transceiver 330 when the response failure is indicated. In one embodiment, antenna selector 1852 adjusts the antenna selection signal in response to the detection of either a CRC failure or a lack of response. In response to the adjusted antenna selection signal, switch circuit 344 connects the different antenna to transceiver 330 for receiving the incoming signal and transmitting the outgoing signal until the detection of another response failure is indicated. FIG. 19 is a block diagram illustrating an embodiment of an external telemetry system 1922, which is another specific embodiment of external telemetry system 1622. External telemetry system 1922 includes diversity antenna system 326, antenna interface circuit 328, transceiver 330, and an antenna control circuit 1932. Antenna control circuit 1932 is a specific embodiment of antenna control circuit 1632 and includes a signal strength failure detector 1950 and an antenna selector 1952. Signal strength failure detector 1950 is a specific embodiment of fading detector 1650 and detects a signal strength failure as the transmission failure from the incoming signal. The signal strength failure as detected by signal strength failure detector 1950 is deemed to be caused by a null. Signal strength failure detector 1950 includes a signal strength detector 1960, a signal strength averaging circuit 1961, a threshold generator 1962, and a comparator 1964. Signal strength detector 1960 measures a strength parameter being a measure of the strength of the incoming signal. In one embodiment, the strength parameter is a power measured in dBm (decibel ratio (log 10) of watts (W) to one milliwatt (1 mW)), and signal strength detector 1960 includes a signal power detector. In another embodiment, the strength parameter is an amplitude measured in volts, and signal strength detector 1960 includes a signal amplitude detector. Signal strength averaging circuit 1961 calculates an average value for the measured strength parameter. In one embodiment, signal strength averaging circuit 1961 calculates the average value for the strength parameter over a predetermined period of time. In another embodiment, signal strength averaging circuit 1961 calculates an average value for the strength parameter over a predetermined number of frames. In a specific embodiment, the predetermined number is in a range of 4 to 50 frames, with approximately 6 frames being a specific example. Threshold generator 1962 dynamically produces a threshold strength based on the average value for the measured strength parameter. In one embodiment, the threshold strength is produced by subtracting a predetermined margin from the average value for the measured strength parameter. In one specific embodiment, the predetermined margin is in a range of approximately 10 dBm to 30 dBm, with approximately 20 dBm being a specific example. Comparator 1964 includes an input to receive the average value for the measured strength parameter, another input to receive the dynamically produced threshold strength, and an output indicative of the signal strength failure when the average value for the measured strength parameter falls below the dynamically produced threshold strength.

Antenna selector 1952 is a specific embodiment of antenna selector 1652 and adjusts the antenna selection signal for connecting a different antenna of diversity antenna system 326 to transceiver 330 when comparator 1964 indicates the signal strength failure. Li one embodiment, antenna selector 1952 adjusts the antenna selection signal immediately in response to an indication of the signal strength failure. In another embodiment, antenna selector 1952 adjusts the antenna selection signal if the signal strength failure is indicated for a predetermined time interval in a range of approximately 20 milliseconds to 200 milliseconds, with approximately 50 milliseconds being a specific example. In response to the adjusted antenna selection signal, switch circuit 344 connects the different antenna to transceiver 330 for receiving the incoming signal and transmitting the outgoing signal until the detection of another signal strength failure is indicated.

FIG. 20 is a block diagram illustrating an embodiment of an external telemetry system 2022, which is another specific embodiment of external telemetry system 1622. External telemetry system 2022 includes diversity antenna system 326, antenna interface circuit 328, transceiver 330, and an antenna control circuit 2032.

Antenna control circuit 2032 is a specific embodiment of antenna control circuit 1632 and includes a signal strength failure detector 2050 and an antenna selector 2052. Signal strength failure detector 2050 is another specific embodiment of fading detector 1650 and detects a signal strength failure associated with the outgoing signal as the transmission failure. Signal strength failure detector 2050 includes a signal strength receiver 2065 and/or an acknowledgement signal receiver 2066. Signal strength receiver 2065 receives a reporting frame being an incoming data frame produced and sent by implantable medical device 110, which detects the signal strength failure in receiving the outgoing frames. Signal strength receiver 2065 indicates the signal strength failure when the reporting frame includes data indicating a signal strength failure associated with the outgoing signal. Acknowledgement signal receiver 2066 receives an acknowledgement frame of the incoming data frames and indicates the signal strength failure if the acknowledgement frame is not received within a predetermined time interval after a transmission of an outgoing frame to implantable medical device 110. The acknowledgement frame is indicative of a successful receipt of the outgoing frame by implantable medical device 110.

Antenna selector 2052 is a specific embodiment of antenna selector 1652 and adjusts the antenna selection signal for connecting a different antenna of diversity antenna system 326 to transceiver 330 when the signal strength failure is indicated. In one embodiment, antenna selector 2052 adjusts the antenna selection signal in response to either a receipt of a data frame indicative of a signal strength failure in outgoing signal as received by implantable medical device 110 or a lack of the acknowledgement frame, hi response to the adjusted antenna selection signal, switch circuit 344 connects the different antenna to transceiver 330 for receiving the incoming signal and transmitting the outgoing signal until the detection of another signal strength failure is indicated.

FIG. 21 is a block diagram an embodiment of an external telemetry system 2122, which is another specific embodiment of external telemetry system 1622. External telemetry system 2122 includes diversity antenna system 326, antenna interface circuit 328, transceiver 330, and an antenna control circuit 2132.

Antenna control circuit 2132 is a specific embodiment of antenna control circuit 1632 and includes a signal strength failure detector 2150 and an antenna selector 2152. Signal strength failure detector 2150 is another specific embodiment of fading detector 1650 and detects signal strength failures associated with the incoming signal and the outgoing signal. Signal strength failure detector 2150 includes an incoming signal strength failure detector 2168 and an outgoing signal strength failure detector 2170. Incoming signal strength failure detector 2168 detects a signal strength failure associated with the incoming signal as the transmission failure and indicates an incoming signal strength failure when the signal strength failure is detected. In one embodiment, incoming signal strength failure detector 2168 is substantially identical or similar to signal strength failure detector 1950. Outgoing signal strength failure detector 2170 detects signal strength failures associated with the outgoing signal as the transmission failure and indicates an outgoing signal strength failure when the signal strength failure is detected, hi one embodiment, outgoing signal strength failure detector 2170 is substantially identical or similar to signal strength failure detector 2050.

Antenna selector 2152 includes a receiving antenna selector 2172 and a transmitting antenna selector 2174. Receiving antenna selector 2172 adjusts the antenna selection signal for selecting a different antenna of diversity antenna system 326 for receiving the incoming signal when a detection of the incoming signal strength failure is indicated. Transmitting antenna selector 2174 adjusts_^ the antenna selection signal for selecting a different antenna of diversity antenna system 326 for transmitting the outgoing signal when a detection of the outgoing signal strength failure is indicated. In one embodiment, the antenna selection signal allows selection of two different antennas: one for receiving the incoming signal and the other for transmitting the outgoing signal. In another embodiment, the antenna selection signal allows selection of either one antenna, or two different antennas, for receiving the incoming signal and transmitting the outgoing signal. hi various other specific embodiments, fading detector 1650 selectively includes one or more elements of incoming frame failure detector 1750, response failure detector 1850, signal strength failure detector 1950, signal strength failure detector 2050, and signal strength failure detector 2150. In one exemplary embodiment, fading detector 1650 includes incoming frame failure detector 1750 (or portions thereof) and signal strength failure detector 1950 (or portions thereof). Antenna selector 1752 adjusts the antenna selection signal in response to the detection of either an incoming frame failure or a signal strength failure. In another embodiment, fading detector 1650 includes incoming frame failure detector 1750, response failure detector 1850, and signal strength failure detector 2150. Antenna selector 1752 adjusts the antenna selection signal in response to the detection of any of an incoming frame failure, a response failure, an incoming signal strength failure, and an outgoing signal strength failure. Other embodiments involving such embodiments will become apparent to those skilled in the art upon reading and understanding this document. FIG. 22 is a block diagram illustrating an embodiment of an external telemetry system 2222, which is another specific embodiment of external telemetry system 122. External telemetry system 2222 includes diversity antenna system 326, antenna interface circuit 328, transceiver 330, and an antenna control circuit 2232.

Antenna control circuit 2232 is another specific embodiment of antenna control circuit 132 and includes a selection sequence generator 2276 and a switching timer 2278. Selection sequence generator 2276 produces the antenna selection signal for selecting an antenna of diversity antenna system 326 to be connected to transceiver 330 in response to an antenna switching timing signal. The antenna is used for receiving the incoming signal and transmitting the outgoing signal. In one embodiment, selection sequence generator 2276 includes a random sequence generator that dynamically generates a random sequence and produces the antenna selection signal to select an antenna of diversity antenna system 326 in response to the antenna switching timing signal according to the dynamically generated random sequence. In another embodiment, selection sequence generator 2276 includes a predetermined sequence generator that produces the antenna selection signal to select an antenna of diversity antenna system 326 in response to the antenna switching timing signal according to a predetermined sequence such as a built-in sequence or a programmed sequence. Switching timer 2278 generates the antenna switching timing signal according to a predetermined schedule specifying times at which a new antenna is to be selected. In one embodiment, the predetermined schedule includes a predetermined period being in a range of approximately 50 milliseconds to 500 milliseconds, with approximately 200 milliseconds being a specific example. Switching timer 2278 generates the antenna switching timing signal on a period basis using this predetermined period. In one embodiment, switching timer 2278 also includes a switching holding circuit that holds the antenna switching timing signal while a frame of the outgoing data frames is being transmitted or a frame of the incoming data frames is being received. This prevents possible data transmission errors caused by switching from one antenna to another while a data frame is being received or transmitted. In one embodiment, switching timer 2278 delays the generation of the antenna switching timing signal until an ongoing transmission or reception of a data frame is completed. In another embodiment, switching timer 2278 keeps any change in the antenna selection signal from being applied to switch circuit 344 until an ongoing transmission or reception of a data frame is completed.

FIG. 23 is a block diagram illustrating an embodiment of an external telemetry system 2322, which is another specific embodiment of external telemetry system 122. External telemetry system 2322 includes diversity antenna system 326, an antenna interface circuit 2328, a transceiver 2330, and an antenna control circuit 2332.

Antenna interface circuit 2328 is a specific embodiment of antenna interface circuit 128 and includes tuning circuits 342 A-N. Tuning circuits 342 A- N each provide tuning for a corresponding antenna of antennas 342A-N. Transceiver 2330 is a specific embodiment of transceiver 130 and includes a plurality of receiving modules 2380A-N and a switch circuit 2382. Receiving modules 2380A-N each have an input coupled to a corresponding antenna of antennas 342 A-N through a corresponding tuning circuit of tuning circuit 342 A-N. Switch circuit 2382 connects the output of one receiving module of receiving modules 2380A-N to external system controller 124 according to a receiving path selection signal. That is, the incoming signal processed by one of receiving modules 2380A-N is selected for use by external system controller 124 according to the receiving path selection signal.

Antenna control circuit 2332 is a specific embodiment of antenna control circuit 132 and produces the receiving path selection signal. Instead of selecting one active antenna as in external telemetry systems 1622-2222, two or more of antennas 340A-N are active through a telemetry session. Antenna control circuit 2332 selects an effective antenna by selecting a receiving path including that antenna. Each receiving path includes an antenna of antennas 340A-N, a corresponding tuning circuit of tuning circuits 342A-N, and a corresponding receiving module of receiving modules 2380A-N. For example, one of the receiving paths includes antenna 340A, tuning circuit 342A, and receiving module 2380A. Selecting a receiving path includes connecting the output of one receiving module of receiving modules 2380A-N to external system controller 124. Antenna control circuit 2332 includes a signal quality assessment circuit 2384, a receiving path selector 2386, and a path switching timing circuit 2388.

Signal quality assessment circuit 2384 produces an indication of quality for the incoming signal processed by each of receiving modules 2380A-N. In the illustrated embodiment, signal quality assessment circuit 2384 includes an incoming frame failure detector 2390 and a signal strength detector 2392. Incoming frame failure detector 2390 detects incoming frame failures from the incoming signal processed by each of receiving modules 2380A-N. In one specific embodiment, incoming frame failure detector 2390 includes a CRC failure detector that detects a CRC failure from the incoming signal processed by each of receiving modules 2380A-N and indicates the incoming frame failure for any receiving module in which the CRC failure is detected. In another specific embodiment, incoming frame failure detector 2390 includes a lack-of-comma detector adapted to detect a comma indicative of a data frame from the incoming signal processed by each of receiving modules 238 OA-N during a predetermined time window and indicates the incoming frame failure for any receiving module in which the comma is not detected. Signal strength detector 2392 measures a strength parameter being a measure of strength of the incoming signal processed by each of receiving modules 2380A-N. In one embodiment, signal strength detector 2392 includes an incoming signal strength failure detector that detects an incoming signal strength failure associated with the incoming signal processed by each of receiving modules 2380 A-N. In a specific embodiment, the incoming signal strength failure detector is substantially identical to similar to signal strength failure detector 1950.

Receiving path selector 2386 adjusts the receiving path selection signal based on the indications of quality for the incoming signal produced for receiving modules 2380A-N. The indications of quality include one or both of the incoming frame failure and the incoming signal strength failure. Receiving path selector 2386 adjusts the receiving path selection signal to deselect any of the receiving paths in which at least one of the incoming frame failure and the incoming signal strength failure is detected. In one embodiment, receiving path selector 2386 adjusts the receiving path selection signal based on the measured strength parameters for the receiving paths.

Path switching timing circuit 2388 holds the receiving path selection signal while a frame of the outgoing data frames is being transmitted or a frame of the incoming data frames is being received. This prevents possible data transmission error caused by switching from one receiving path to another while a data frame is being received or transmitted. In one embodiment, path switching timing circuit 2388 delays the adjustment of the receiving path selection signal until an ongoing transmission or reception of a data frame is completed. In another embodiment, path switching timing circuit 2388 keeps any change in the receiving path selection signal from being applied to switch circuit 2382 until an ongoing transmission or reception of a data frame is completed.

FIG. 24 is a flow chart illustrating a method for operating a telemetry system communicating with an implantable medical device. In one embodiment, the method is performed by external telemetry system 1622, including of its specific embodiments discussed in this document.

Signals are transmitted and received using a diversity antenna system including two or more antennas at 2400. This includes transmitting an outgoing signal to the implantable medical device and receiving an incoming signal from the implantable medical device. The outgoing signal includes an RF carrier signal modulated with outgoing data frames by the telemetry system communicating with the implantable medical device. The incoming signal includes another RF carrier signal modulated with incoming data frames by the implantable medical device. The incoming data frames are recovered by demodulating the received incoming signal. In one embodiment, ASK is the modulation scheme used to modulate the outgoing signal and the incoming signal.

An active antenna is selected according to an antenna selection signal at 2410. The active antenna is an antenna that is currently used for transmitting and/or receiving signals. The antenna selection signal controls which one or more antennas of the diversity antenna system are active.

A transmission failure deemed to be associated with a null is detected at 2420. The null is known to cause such a transmission failure. Examples of the transmission failure include an incoming frame failure, a response frame failure, and a signal strength failure. One or more types of such transmission failures are detected as an indication of a null by the telemetry system communicating with the implantable medical device. An incoming frame failure is detected, for example, by detecting a CRC failure or a lack of comma from the incoming signal. A response frame failure is detected, for example, by detecting a CRC failure or a lack of response frame from the incoming signal. The response frame is sent from the implantable medical device in response to an outgoing data frame sent to the implantable medical device. The lack of response frame is detected within a predetermined period after the outgoing data frame is sent to the implantable medical device. The signal strength failure is detected, for example, by detecting a sudden drop in a strength parameter, such as amplitude or power, of the outgoing signal and/or the incoming signal. The strength parameter is measured by the telemetry system communicating with the implantable medical device, by the implantable medical device, or both. In one embodiment, a transmitting antenna is selected for transmitting the outgoing signal to the implantable medical device based on the signal strength of the outgoing signal as measured and reported by the implantable medical device, and a receiving antenna is selected for receiving the incoming signal from the implantable medical device based on the signal strength of the incoming signal as measured by the telemetry system communicating with the implantable medical device.

When the transmission failure is detected, the antenna selection signal is adjusted to select a different active antenna at 2430. In other words, when a null is deemed to be encountered, the telemetry system switches from the currently used antenna to a different antenna. It is very unlikely that the null is encountered with two antennas of the telemetry system at the same time.

The antenna selection signal is held while a frame is being transmitted or received at 2440. To prevent data transmission errors, the antenna selection signal causes actual antenna switching when no data frame is being transmitted or received. This is particularly important when the time required to complete an antenna switching is not substantially shorter than the time required for transmitting or receiving a data bit. In one embodiment, an antenna switching is permitted while a data frame is being transmitted or received when the time required to complete the antenna switching is substantially shorter than the time required for transmitting or receiving a data bit. In another embodiment, an antenna switching is permitted while a data frame is being transmitted or received when an error correction algorithm is executed to correct potential data transmission errors.

The different active antenna is selected according to the adjusted antenna selection signal at 2450. This newly selected active antenna is to be used for transmitting and/or receiving data frames until another transmission failure is detected. During a telemetry session, steps 2420-2450 are repeated to continuously monitor for the transmission failures and switch to a different antenna when a transmission failure is detected.

FIG. 25 is a flow chart illustrating another method for operating a telemetry circuit communicating with the implantable medical device. In one embodiment, the method is performed by external telemetry system 2222.

Signals are transmitted and received using a diversity antenna system including two or more antennas at 2500. This includes transmitting the outgoing signal to the implantable medical device and receiving the incoming signal from the implantable medical device, as discussed above for step 2400.

An active antenna is selected according to an antenna selection signal at 2510. The active antenna is an antenna that is currently used for transmitting and/or receiving antennas. The antenna selection signal controls which one or more antennas of the diversity antenna system are active.

The antenna selection signal is produced for selecting a new active antenna based on a predetermined schedule at 2520. In other words, the antenna selection signal is adjusted at times specified by the predetermined schedule. The antenna selection signal is adjusted according to an antenna selection sequence chosen to reduce or minimize the probability for the telemetry system to encounter a null. Depending on how the antenna selection sequence is generated, the new active antenna may be the same antenna that is currently used or a different antenna. In one embodiment, the new active antenna is always different from the antenna that is currently used. In one embodiment, the antenna selection signal is produced based on a dynamically generated random sequence. In an alternative embodiment, the antenna selection signal is produced based on a predetermined sequence. In one embodiment, a new active antenna is selected on a periodic basis. In a further embodiment, the antenna selection signal is held while a frame is being transmitted or received.

. During a telemetry session, steps 2510-2520 are repeated to adjust the antenna selection signal at predetermined times, such as the predetermined periodic basis. The probability for the telemetry system to encounter a null is reduced because both the location of the nulls and the location of the active antenna vary while the telemetry system communicates with the implantable medical device.

FIG. 26 is a flow chart illustrating another method for operating a telemetry circuit communicating with the implantable medical device. In one embodiment, the method is performed by external telemetry system 2322.

Signals are transmitted and received using a diversity antenna system including two or more antennas at 2600. This includes transmitting the outgoing signal to the implantable medical device and receiving the incoming signal from the implantable medical device, as discussed above for step 2400. The two or more antennas are used to receive the incoming signal simultaneously.

The incoming signal is processed through a plurality of processing paths at 2610. In one embodiment, the plurality of processing paths each include a processing modules each coupled to one antenna of the diversity antenna system to process the incoming signal received by that antenna. An indication of quality for the incoming signal processed by each processing path is detected at 2620. The detection results in indications of quality each associated with one of the processing paths. Examples of the indication of quality include presence of incoming frame failures and incoming signal strength failures. Such indications of quality are indicative of a possibility that the telemetry system has encountered a null. One or more types of such indications of quality are detected by the telemetry system. An incoming frame failure is detected, for example, by detecting a CRC failure or a lack of comma from the incoming signal. The incoming signal strength failure is detected, for example, by detecting a sudden drop in a strength parameter, such as amplitude or power, of the incoming signal.

A processing path is selected based on the detected indications of quality at 2630. In one embodiment, the selection becomes effective while no data frame is being transmitted or received. During a telemetry session, steps 2620- 2630 are repeated to continuously monitor for the indication of quality and to switch to a different processing path when the indication of quality suggests that the telemetry system has encountered a null.

It is to be understood that the above detailed description is intended to be illustrative, and not restrictive. For example, the implantable medical device can be any implantable medical device capable of communicating with an external system or device via RF telemetry. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

Claims

What is claimed is:

1. A system for communicating with an implantable medical device, the system comprising: a diversity antenna system including a plurality of antennas configured to transmit an outgoing signal to the implantable medical device and to receive an incoming signal from the implantable medical device; a transceiver adapted to transmit outgoing data frames by modulating the outgoing signal and to receive incoming data frames by demodulating the incoming signal; an antenna interface circuit coupled between the diversity antenna system and the transceiver, the antenna interface circuit including a switch circuit adapted to connect an antenna of the diversity antenna system to the transceiver according to an antenna selection signal; and an antenna control circuit adapted to produce the antenna selection signal, the antenna control circuit including: an incoming signal monitoring timer adapted to generate incoming signal monitoring signals based on a predetermined communication protocol; an incoming signal monitoring circuit coupled to the incoming signal monitoring timer, the incoming signal monitoring circuit adapted to produce an indication of quality of the incoming signal by detecting at least one measure of quality of the incoming signal in response to each of the incoming signal monitoring signals; and an antenna selector coupled to the incoming signal monitoring circuit and the switch circuit, the antenna selector adapted to adjust the antenna selection signal based on the indication of quality of the incoming signal.

2. The system of claim 1, wherein the switch circuit is adapted to connect the antenna of the diversity antenna system to the transceiver for transmitting the outgoing signal and receiving the incoming signal.

3. The system of claim 1, wherein the transceiver is adapted to transmit multi- frame messages each including a plurality of data frames of the outgoing data frames.

4. The system of claim 3, wherein the incoming signal monitoring timer comprises a response timer to start a predetermined response time interval when a response frame is expected to be received according to the predetermined communication protocol, the response frame being one of the incoming data frames and responding to one or more frames of the outgoing data frames transmitted to the implantable medical device, and wherein the incoming signal monitoring circuit comprises a response failure detector adapted to detect a response failure during the predetermined response time interval, the response failure being a data transmission error associated with the response frame.

5. The system of claim 4, wherein the response failure detector comprises a laclc- of-response detector adapted to detect the response frame during the predetermined response time interval, and the response failure is detected if the response frame is not detected during the predetermined response time interval.

6. The system of claim 4, wherein the response failure detector comprises a lack- of-comma detector adapted to detect a comma indicative of a receipt of the response frame during the predetermined response time interval, and the response failure is detected if the comma is not detected during the predetermined response time interval.

7. The system of claim 4, wherein the response failure detector comprises a cyclic redundancy check (CRC) failure detector adapted to detect a CRC failure in the response frame during the predetermined response time interval, and the response failure is detected if the CRC failure is detected during the predetermined response time interval.

8. The system of claim 4, wherein the antenna control circuit further comprises a nonresponsive period timer to start a predetermined nonresponsive period during a transmission of one of the multi-frame messages with a first antenna of the diversity antenna system connected to the transceiver when the response frame is expected to be received, and wherein the response timer is adapted to start the predetermined response time interval during the transmission of the one of the multi-frame messages when the response frame is expected to be received and when each of one or more subsequent response frames is expected to be received, the subsequent response frame each being one of the incoming data frames and responding to one or more frames of the outgoing data frames transmitted to the implantable medical device.

9. The system of claim 8, wherein the antenna selector is adapted to adjust the antenna selection signal for connecting a different antenna of the diversity antenna system to the transceiver if the response failure is detected during the predetermined response time interval and adapted to adjust the antenna selection signal for connecting a second antenna of the diversity antenna system to the transceiver when the predetermined nonresponsive period expires if none of the incoming data frames is received without the data transmission error during the predetermined nonresponsive period.

10. The system of claim 4, wherein the antenna control circuit further comprises a test frame generator to produce an antenna test signal causing the transceiver to transmit a test frame of the outgoing data frames prior to a transmission of one of the multi- frame messages, and wherein the response timer is adapted to start the predetermined response time interval following the transmission of the test frame.

11. The system of claim 10, wherein the antenna selector is adapted to adjust the antenna selection signal for connecting a different antenna of the diversity antenna system to the transceiver if the response failure is detected during the predetermined response time interval.

12. The system of claim 4, wherein the response timer is adapted to start the predetermined response time interval during a transmission of one of the multi-frame messages when the response frame is expected to be received.

13. The system of claim 12, wherein the antenna selector is adapted to adjust the antenna selection signal for connecting a different antenna of the diversity antenna system to the transceiver after the transmission of the one of the multi-frame messages if the response failure is detected during the transmission of the one of the multi-frame messages.

14. The system of claim 4, wherein the antenna control circuit further comprises a transmission period timer to time transmission periods each including a period during which a predetermined number of data frames of the outgoing data frames are transmitted, and wherein the response timer is adapted to start the predetermined response time interval during each of the transmission periods when the response frame is expected to be received.

15. The system of claim 14, wherein the antenna selector is adapted to adjust the antenna selection signal for connecting a different antenna of the diversity antenna system to the transceiver at an end of the each of the transmission periods if the response failure is detected during the each of the transmission periods.

16. The system of claim 3, wherein the incoming signal monitoring timer includes a signal strength calculation timer adapted to generate a signal strength calculation signal prior to a transmission of one of the multi-frame messages.

17. The system of claim 16, wherein the incoming signal monitoring circuit comprises: a signal strength detector to measure strength parameters each representative of strength of the incoming signal associated with one antenna of the diversity antenna system; a signal strength storage circuit to store the strength parameters; and a signal strength calculator to calculate signal strength indices each associated with one antenna of the diversity antenna system based on the strength parameters and weighting factors each associated with one of the strength parameters and related to a timing of the measurement of that one of the strength parameters in response to the signal strength calculation signal.

18. The system of claim 17, wherein the antenna selector is adapted to adjust the antenna selection signal for connecting an antenna of the diversity antenna system to the transceiver based on the signal strength indices.

19. A system for communicating with an implantable medical device, the system comprising: a diversity antenna system including a plurality of antennas configured to transmit an outgoing signal to the implantable medical device and to receive an incoming signal from the implantable medical device; a transceiver adapted to transmit outgoing data frames by modulating the outgoing signal and to receive incoming data frames by demodulating the incoming signal; an antenna interface circuit coupled between the diversity antenna system and the transceiver, the antenna interface circuit including a switch circuit adapted to connect an antenna of the diversity antenna system to the transceiver according to an antenna selection signal; and an antenna control circuit adapted to produce the antenna selection signal, the antenna control circuit including: a fading detector coupled to the transceiver, the fading detector adapted to detect a transmission failure deemed to be associated with a null; an antenna selector coupled to the fading detector, the antenna selector adapted to adjust the antenna selection signal for connecting a different antenna of the diversity antenna system to the transceiver in response to a detection of the transmission failure; and an antenna switching timing circuit coupled between the antenna selector and the switch circuit, the antenna switching timing circuit adapted to hold the antenna selection signal while a frame of the outgoing data frames is being transmitted or a frame of the incoming data frames is being received.

20. The system of claim 19, wherein the switch circuit is adapted to connect the antenna of the diversity antenna system to the transceiver for transmitting the outgoing signal and receiving the incoming signal.

21. The system of claim 19, wherein the fading detector comprises an incoming frame failure detector adapted to detect an incoming frame failure from the incoming signal, the incoming frame failure being a data transmission error in at least one frame of the incoming data frames, and wherein the antenna selector is adapted to adjust the antenna selection signal for connecting the different antenna of the diversity antenna system to the transceiver when the incoming frame failure is indicated.

22. The system of claim 21, wherein the incoming frame failure detector comprises a cyclic redundancy check (CRC) failure detector adapted to detect a CRC failure from the incoming data frames and to indicate the incoming frame failure when the CRC failure is detected.

23. The system of claim 21, wherein the incoming frame failure detector comprises a lack-of-comma detector adapted to detect a comma indicative of a receipt of a frame of the incoming data frames during a predetermined time window and to indicate the incoming frame failure if the comma is not detected during the predetermined time window.

24. The system of claim 19, wherein the fading detector comprises a response failure detector adapted to detect a response failure being a data transmission error in at least one response frame of the incoming data frames, the at least one response frame responding to a transmission of a frame of the outgoing data frames, and wherein the antenna selector is adapted to adjust the antenna selection signal for connecting the different antenna of the diversity antenna system to the transceiver when the response failure is indicated.

25. The system of claim 24, wherein the response failure detector comprises a lack- of-response failure detector adapted to detect the at least one response frame of the incoming data frames during a predetermined time interval starting with the transmission of the frame of the outgoing data frames and to indicate the response failure if the response frame is not detected.

26. The system of claim 24, wherein the response failure detector comprises a cyclic redundancy check (CRC) failure detector adapted to detect a CRC failure in the at least one response frame of the incoming data frames following the transmission of the frame of the outgoing data frames and to indicate the response failure if the CRC failure is detected.

27. The system of claim 19, wherein the fading detector comprises a signal strength failure detector adapted to detect a signal strength failure of at least one of the incoming signal and the outgoing signal, and wherein the antenna selector is adapted to adjust the antenna selection signal for connecting the different antenna of the diversity antenna system to the transceiver when the signal strength failure is indicated.

28. The system of claim 27, wherein the signal strength failure detector comprises: a signal strength detector to measure a strength parameter being a measure of strength of the incoming signal; a threshold generator, coupled to the signal strength detector, to dynamically produce a threshold strength based on the measured strength parameter; and a comparator having a first input to receive the measured strength parameter, a second input to receive the dynamically produced threshold strength, and an output to indicate the signal strength failure when the measured strength parameter falls below the dynamically produced threshold strength.

29. A system for communicating with an implantable medical device, the system comprising: a diversity antenna system including a plurality of antennas configured to transmit an outgoing signal to the implantable medical device and to receive an incoming signal from the implantable medical device; an external system controller; a transceiver adapted to transmit outgoing data frames by modulating the outgoing signal and to receive incoming data frames by demodulating the incoming signal, the transceiver including: a plurality of receiving modules each having an input coupled to one antenna of the diversity antenna system and an output; and a switch circuit to connect the output of one receiving module of the plurality of receiving modules to the external system controller according to a receiving path selection signal; and an antenna control circuit adapted to produce the receiving path selection signal, the antenna control circuit including: a signal quality assessment circuit coupled to the transceiver, the signal quality assessment circuit adapted to produce an indication of quality for the incoming signal processed by each receiving module of the plurality of receiving modules; and a receiving path selector coupled to the transceiver, the receiving path selector adapted to adjust the receiving path selection signal based on the indications of quality for the incoming signal produced for the plurality of receiving modules.

30. The system of claim 29, wherein the signal quality assessment circuit comprises an incoming frame failure detector adapted to detect an incoming frame failure from the incoming signal processed by each receiving module of the plurality of receiving modules, and the receiving path selector is adapted to deselect a receiving module of the plurality of receiving modules when the incoming frame failure is indicated for that receiving module.

31. The system of claim 30, wherein the incoming frame failure detector comprises a cyclic redundancy check (CRC) failure detector adapted to detect a CRC failure from the incoming signal processed by each receiving module of the plurality of receiving modules and to indicate the incoming frame failure for the each receiving module when the CRC failure is detected.

32. The system of claim 30, wherein the incoming frame failure detector comprises a lack-of-comma detector adapted to detect a comma indicative of a data frame from the incoming signal processed by each receiving module of the plurality of receiving modules during a predetermined time window and to indicate the response failure for the each receiving module if the comma is not detected during the predetermined time window.

33. The system of claim 29, wherein the signal quality assessment circuit comprises a signal strength detector adapted to measure a strength parameter being a measure of strength of the incoming signal processed by each receiving module of the plurality of receiving modules.

34. The system of claim 33, wherein the receiving path selector is adapted to adjust the receiving path selection signal based on the measured strength parameters for the plurality of receiving modules.

35. The system of claim 34, wherein the signal strength detector comprises an incoming signal strength failure detector adapted to detect an incoming signal strength failure associated with the incoming signal processed by each receiving module of the plurality of receiving modules, and wherein the receiving path selector is adapted deselect a receiving module of the plurality of receiving modules when the incoming signal strength failure is indicated for that receiving module.

36. The system of claim 29, wherein the antenna control circuit further comprises a path switching timing circuit coupled between the receiving path selector and the switch circuit, the path switching timing circuit adapted to hold the receiving path selection signal while a frame of the outgoing data frames is being transmitted or a frame of the incoming data frames is being received.

37. The system of claim 36, wherein the outgoing signal and the incoming signal are each modulated by amplitude-shift-keying (ASK), and the transceiver comprises an

ASK modulator to modulate the outgoing signal with the outgoing data frames and an ASK demodulator to recover the incoming data frames by demodulating the incoming signal.

38. A method for operating a telemetry system communicating with an implantable medical device, the method comprising: transmitting an outgoing signal to the implantable medical device and receiving an incoming signal from the implantable medical device using a diversity antenna system including a plurality of antennas, wherein transmitting the outgoing signal includes transmitting outgoing data frames by modulating the outgoing signal, and receiving the incoming signal includes receiving incoming data frames by demodulating the incoming signal; selecting an antenna of the diversity antenna system to be an active antenna according to an antenna selection signal, the active antenna used for transmitting the outgoing signal and receiving the incoming signal; generating incoming signal monitoring signals based on a predetermined communication protocol; producing an indication of quality of the incoming signal by detecting at least one measure of quality of the incoming signal in response to each of the incoming signal monitoring signals; and adjusting the antenna selection signal based on the indication of quality of the incoming signal.

39. The method of claim 38, wherein transmitting the outgoing data frames comprises transmitting multi-frame messages each including a plurality of data frames of the outgoing data frames.

40. The method of claim 39, wherein generating the incoming signal monitoring signals comprises starting a predetermined response time interval when a response frame is expected to be received according to the predetermined communication protocol, the response frame being one of the incoming data frames and responding to one or more frames of the outgoing data frames transmitted to the implantable medical device, and wherein detecting the at least one measure of quality of the incoming signal comprises detecting a response failure during the predetermined response time interval, the response failure being a data transmission error associated with the response frame.

41. The method of claim 40, wherein detecting the response failure comprises detecting the response frame during the predetermined response time interval, and wherein the response failure is detected if the response frame is not detected during the predetermined response time interval.

42. The method of claim 40, wherein detecting the response failure comprises detecting a comma indicative of a receipt of the response frame during the predetermined response time interval, and wherein the response failure is detected if the comma is not detected during the predetermined response time interval.

43. The method of claim 40, wherein detecting the response failure comprises detecting a cyclic redundancy check (CRC) failure in the response frame during the predetermined response time interval, and wherein the response failure is detected if the CRC failure is detected during the predetermined response time interval.

44. The method of claim 40, further comprising starting a predetermined nonresponsive period during a transmission of one of the multi-frame messages using a first antenna of the diversity antenna system selected to be the active antenna when the response frame is expected to be received, and wherein starting the predetermined response time interval comprises starting the predetermined response time interval during the transmission of the one of the multi-frame messages when the response frame is expected to be received and when each of one or more subsequent response frames is expected to be received, the one or more subsequent response frames each being one of the incoming data frames and responding to one or more frames of the outgoing data frames transmitted to the implantable medical device.

45. The method of claim 44, wherein adjusting the antenna selection signal comprises adjusting the antenna selection signal for selecting a different antenna of the diversity antenna system to be the active antenna if the response failure is detected during the predetermined response time interval.

46. The method of claim 45, wherein adjusting the antenna selection signal further comprises adjusting the antenna selection signal for selecting a second antenna of the diversity antenna system to be the active antenna when the predetermined nonresponsive period expires if none of the incoming data frames is received without the data transmission error during the predetermined nonresponsive period.

47. The method of claim 40, further comprising producing an antenna test signal causing a transmission of a test frame of the outgoing data frames prior to a transmission of one of the multi-frame messages, and wherein starting the predetermined response time interval comprises starting the predetermined response time interval following the transmission of the test frame.

48. The method of claim 47, wherein adjusting the antenna selection signal comprises adjusting the antenna selection signal for selecting a different antenna of the diversity antenna system to be the active antenna when the response failure is detected during the predetermined response time interval.

49. The method of claim 40, wherein starting the predetermined response time interval comprises starting the predetermined response time interval during a transmission of one of the multi-frame messages when the response frame is expected to be received.

50. The method of claim 49, wherein adjusting the antenna selection signal comprises adjusting the antenna selection signal for selecting a different antenna of the diversity antenna system to be the active antenna after the transmission of the one of the multi-frame messages if the response failure is detected during the transmission of the one of the multi-frame messages.

51. The method of claim 40, further comprising timing transmission periods each including a period during which a predetermined number of data frames of the outgoing data frames are transmitted, and wherein starting the predetermined response time interval comprises starting the predetermined response time interval during each of the transmission periods when the response frame is expected to be received.

52. The method of claim 51, wherein adjusting the antenna selection signal comprises adjusting the antenna selection signal for selecting a different antenna of the diversity antenna system to be the active antenna at an end of the each of the transmission periods if the response failure is indicated during the each of the transmission periods.

53. The method of claim 39, wherein generating the incoming signal monitoring signals comprises generating a signal strength calculation signal prior to a transmission of one of the multi-frame messages.

54. The method of claim 53, wherein producing the indication of quality of the incoming signal comprises: measuring strength parameters each representative of strength of the incoming signal associated with one antenna of the diversity antenna system; storing the measured strength parameters; and calculating signal strength indices each associated with one antenna of the diversity antenna system based on the strength parameters and weighting factors each associated with one of the strength parameters and related to a timing of the measurement of that one of the strength parameters in response to the signal strength calculation signal.

55. The method of claim 54, wherein adjusting the antenna selection signal comprises adjusting the antenna selection signal for selecting an antenna of the diversity antenna system to be the active antenna based on the signal strength indices.

56. A method for operating a telemetry system communicating with an implantable medical device, the method comprising: transmitting an outgoing signal to the implantable medical device and receiving an incoming signal from the implantable medical device using a diversity antenna system including a plurality of antennas, wherein transmitting the outgoing signal includes transmitting outgoing data frames by modulating the outgoing signal, and receiving the incoming signal includes receiving incoming data frames by demodulating the incoming signal; selecting an active antenna from the diversity antenna system according to an antenna selection signal; detecting a transmission failure deemed to be associated with a null; adjusting the antenna selection signal for selecting a different active antenna of the diversity antenna system in response to a detection of the transmission failure; holding the antenna selection signal while a frame of the outgoing data frames is being transmitted or a frame of the incoming data frames is being received; and selecting the different active antenna from the diversity antenna system according to the adjusted antenna selection signal.

57. The method of claim 50, wherein selecting the active antenna comprises selecting an antenna for receiving the incoming signal and transmitting the outgoing signal.

58. The method of claim 50, wherein detecting the transmission failure comprises detecting an incoming frame failure from the incoming signal, the incoming frame failure being a data transmission error in at least one frame of the incoming data frames, and wherein adjusting the antenna selecting signal comprises adjusting the antenna selection signal for selecting the different active antenna from the diversity antenna system when the incoming frame failure is indicated.

59. The method of claim 58, wherein detecting the transmission failure comprises cyclic comprises detecting a cyclic redundancy check (CRC) failure from the incoming data frames and indicating the incoming frame failure when the CRC failure is detected.

60. The method of claim 58, wherein detecting the transmission failure comprises detecting a comma indicative of a receipt of a frame of the incoming data frames during a predetermined time window and indicating the incoming frame failure if the comma is not detected during the predetermined time window.

61. The method of claim 50, wherein detecting the transmission failure comprises detecting a response failure being a data transmission error in at least one response frame of the incoming data frames, the at least one response frame responding to a transmission of a frame of the outgoing data frames, and wherein adjusting the antenna selecting signal comprises adjusting the antenna selection signal for selecting the different active antenna of the diversity antenna system when the response failure is indicated.

62. The method of claim 61, wherein detecting the response failure comprises detecting at least one response frame of the incoming data frames during a predetermined time interval starting with the transmission of the frame of the outgoing data frames and indicating the response failure if the response frame is not detected.

63. The method of claim 61 , wherein detecting the response failure comprises detecting a cyclic redundancy check (CRC) failure detector in the at least one response frame of the incoming data frames following the transmission of the frame of the outgoing data frames and indicating the response failure if the CRC failure is detected.

64. The method of claim 50, wherein detecting the transmission failure comprises detecting a signal strength failure of at least one of the incoming signal and the outgoing signal, and wherein adjusting the antenna selecting signal comprises adjusting the antenna selection signal for selecting the different antenna of the diversity antenna system when the signal strength failure is indicated.

65. The method of claim 64, wherein detecting the signal strength failure comprises: measuring a strength parameter being a measure of strength of the incoming signal; producing a threshold strength dynamically based on the measured strength parameter; and indicating the signal strength failure when the measured strength parameter falls below the dynamically produced threshold strength.

66. A method for operating a telemetry system communicating with an implantable medical device, the method comprising: transmitting an outgoing signal to the implantable medical device and receiving an incoming signal from the implantable medical device using a diversity antenna system including a plurality of antennas, wherein transmitting the outgoing signal includes transmitting outgoing data frames by modulating the outgoing signal, and receiving the incoming signal includes receiving incoming data frames by demodulating the incoming signal; processing the incoming signal through a plurality of processing paths each coupled to one antenna of the diversity antenna system; detecting an indication of quality for the incoming signal processed by each processing path of the plurality of processing paths; and selecting a processing path of the plurality of processing paths based on the detected indications of quality associated with the plurality of processing paths.

67. The method of claim 66, wherein detecting the indication of quality comprises detecting an incoming frame failure in each processing path of the plurality of processing paths, and selecting the processing path comprises selecting a processing path in which the incoming frame failure is not detected.

68. The method of claim 67, wherein detecting the incoming frame failure comprises detecting a cyclic redundancy check (CRC) failure.

69. The method of claim 68, wherein detecting the incoming frame failure comprises detecting a lack of comma indicative of an incoming data frame of the incoming data frames.

70. The method of claim 66, wherein detecting the indication of quality comprises measuring a strength parameter being a measure of strength of the incoming signal processed by each processing path of the plurality of processing paths.

71. The method of claim 70, wherein selecting the processing path comprises selecting the processing path of the plurality of processing paths based on the measured strength parameters for the plurality of processing paths.

72. The method of claim 71, wherein measuring the strength parameter comprises detecting an incoming signal strength failure associated with the incoming signal processed by each processing path of the plurality of processing paths, and selecting the processing path comprises selecting a processing path in which the incoming signal strength failure is not detected.

73. The method of claim 66, wherein selecting the processing path comprises selecting the processing path while a frame of the outgoing data frames is not being transmitted and a frame of the incoming data frames is not being received.

74. The method of claim 73, further comprising modulating each of the outgoing signal and the incoming signal by amplitude-shift-keying (ASK).