WO2007114743A1

WO2007114743A1 - Received signal strength measurement compensation

Info

Publication number: WO2007114743A1
Application number: PCT/SE2006/000394
Authority: WO
Inventors: Tomas Snitting; Hans Abrahamson
Original assignee: St. Jude Medical Ab
Priority date: 2006-03-31
Filing date: 2006-03-31
Publication date: 2007-10-11

Abstract

The invention relates to a method for a base station 11 in a medical implant communications system 10. The method comprises the steps of : generating, in a signal generator, a known signal RSStest; receiving at the base station 11 the known signal RSStest in a plurality of channels; performing, in the base station 11, an RSS measurement for the channels based on the known signal RSStest; calculating an RSS compensation value for each channel; and storing the calculated RSS compensation values in a memory of the base station 11, wherein the RSS compensation values are suitable for compensating for inaccuracies of an RSS measurement. The invention is also related to a method performed by a base station and to a base station.

Description

Received signal strength measurement compensation

Field of the invention

The present invention relates generally to medical implant communication systems, and in particular to methods for a base station in such systems and to a base station comprising means for performing the methods.

Background of the invention

Medical Implant Communication System (MICS) is a system for providing digital communication between an external programmer or control transceiver and a medical implant transceiver placed in a human body. The frequency band allocated for MICS is 402-405 MHz, but is shared with other users besides MICS device users. The MICS provides bi-directional radio communication with a pacemaker or other medical implant, and the maximum transmit power is very low in order to reduce the risk of co-interference among the different users. The standard is defined by the FCC (Federal Communications Commission) in the United States and by ETSI (European Telecommunications Standards Institute) in Europe.

The MICS standard is to be used in future radio frequency

(RF) medical telemetry applications. One requirement that will be introduced when taking this standard into use is compulsory signal strength measurements before initiation of communication. This is required in order to minimize the possibility of disturbance among MICS devices and to other users of the band. More specifically, a measurement of the signal strength must be performed before initiating communication and a channel having the lowest received ^"signal ^"strength ^"(^"RSS^")^~"~is^"~tc5^~"]5e^'"""cho^'se^"iϊ^""a^'S ^"the^"" communication channel. In the MICS standard this procedure is called listen before talk (LBT) .

Summary of the invention

In order to be able to compare the RSS (received signal strength) measurements of the different channels and properly chose communication channel, it is important to obtain as accurate RSS measurements as possible. The ETSI standard, EN 301 839 (in particular chapter 10.1-10.6), defines a precision of RSS measurements that have to be accomplished; the precision is required to be better than 3 dB. In view of this requirement and the related precision, it is realized that accurate measurements are necessary.

However, there are several factors affecting the accuracy of measured signal strength and the obtained measurement values. For example, a typical receiver chain will have a ripple in its gain, which will give uncertainties regarding the measured value. A filter may process a signal differently for different channels and the RSS values for different channels may thus be misleading and may in fact be non-comparable. That is, the filter affects the channels differently, which may result in that a channel having the lowest signal strength is not chosen as the communication channel. Further yet, nonlinearities of the components of a receiver circuit may also cause measurement errors.

It would therefore be desirable to provide, in a medical telemetry system, an improved method for properly choosing communication channel by providing accurate received signal strength measurements. In particular, it

^"wou^'-td^" be- ^"desirable -to- provi-de -^■sueh---meth©d-—in--a—medi-Ga-l- telemetry system for fulfilling the requirements put on a telemetry system by the MICS standard.

It is an object of the present invention to provide a method and apparatus for choosing communication channel based on accurate signal strength measurements within a medical telemetry system. More specifically, an object is to provide a reliable and accurate method in a medical telemetry system for choosing a channel before initiating communication on it.

Another object of the present invention is to provide an efficient method for performing such measurements and enabling the fulfillment of the requirements that are being introduced in medical telemetry systems.

These objects, among others, are achieved by a method as claimed in claim 1, by a method as claimed in claim 7 and by a base station as claimed in claim 10.

In accordance with the invention, a method for a base station in a medical implant communications system is provided. The method comprises the steps of: generating, for example in a signal generator, a known signal; receiving at the base station the known signal in a plurality of frequency channels on which communication with a medical implant device is to be performed; performing, in the base station, an RSS measurement for all channels based on the known signal; calculating an RSS compensation value for each channel; and storing the calculated RSS compensation values in a memory of the base station. The invention provides a method for compensating for differences in received signal strength indicators for different channels. The obtained RSS compensation values axe- suitable -for compensating for inaccuracies of an RSS measurement. By means of the invention the ripple of a receiver chain as well as other factors affecting the accuracy of measurements, is taken into account and compensated for. An efficient and easily implemented method for fulfilling the requirements to be introduced into a MICS system is thereby provided. Further, a fault tolerant base station is provided, taking into account the characteristics of components in the specific base station in question and compensating for them. Further, by means of the invention the performance of a medical telemetry system can be optimized in that the transmission power utilized within the system can be minimized.

In accordance with an embodiment of the invention, the step of calculating an RSS compensation value comprises establishing a relative relationship between at least two channels and calculating an RSS compensation value for each respective channel based on this relationship. This provides a very accurate method for compensating for differences in RSS measurements between different channels. A channel having a too low RSS measurement value is preferably adjusted upward and it is ensured that all RSS measurement values are indeed comparable with each other.

In accordance with an alternative embodiment the step of calculating an RSS compensation value comprises comparing the measured RSS values to expected RSS values and calculating an RSS compensation value for each channel based on the differences between the expected values and the obtained values. This gives a somewhat less accurate method than the previous, but still gives a compensation mechanism for adjusting for differences between RSS

-measurement values—for . different, -channels... In accordance with an embodiment of the invention, the RSS values are calculated and stored within the base station at manufacturing of the base station. Each individual base station is thereby provided with a unique compensation table comprising RSS compensation values, which gives a user-friendly solution, not requiring the user to arrange for a compensation table.

In accordance with an embodiment of the invention, the step of generating a known signal comprises generating this known signal in the centre of each channel. A satisfactory RSS compensation value is thereby provided, adequate for adjusting the RSS measurement values for each channel.

In accordance with yet another embodiment of the invention, the known signal is transmitted at different signal strengths and RSS compensation values are thereby provided for each channel for different signal levels.

The RSS compensation values may be stored in a compensation table implemented as a matrix for easy retrieval when needed.

In accordance with the invention, a method at a base station in a medical implant communications system is also provided. The method comprises the steps of: monitoring at the base station a plurality of frequency channels used within the medical implant communication system; performing, in the base station, an RSS measurement for the different channels, whereby an RSS measurement value is calculated for each channel; retrieving, from a compensation table comprising calculated RSS compensation values for each channel, a compensation value for each channel; and adjusting, for each channel, the RSS measurement value by means of the retrieved RSS compensation values.

The invention is further related to a base station comprising means for performing the methods and utilising the innovative compensation table. Advantages corresponding to the above are thereby provided.

Brief description of the drawings

Figure 1 shows a typical gain and noise vs. frequency curve for a receiver chain.

Figure 2 is a schematic illustration of a receiver chain of a base station in a medical telemetry system.

Figure 3 is a schematic illustration over the steps included in the method in accordance with the invention.

Figure 4 is a schematic illustration of a medical telemetry system in which the invention may be implemented.

Detailed description of preferred embodiments

In the above mentioned standards the term "ultralow power active medical implant" (ULP-AMI) is used to denote a medical implant within a medical implant communication system or medical telemetry system. In the following description the terms implantable medical device and medical implant are used interchangeably to denote such ULP-AMI. Further, in the standard a device communicating with such ULP-AMI is called a periphery or ULP AMI-P, where the "P" stands for periphery. In the following description a device communicating with a medical implant is called a base station, and is intended to denote such ULP AMI-P. Wand and periphery unit are other commonly used equivalent terms for denoting such ULP AMI-P.

As mentioned in the introductory part of the description, a receiver chain has some degree of ripple in its gain, inter alia due to varying component performance. For example, an ideal filter would have a completely flat passband and would completely attenuate all frequencies outside the passband, but in practice no filter is ideal. Undesired frequencies may be attenuated but not rejected entirely and thus affecting the measurements. Further, a specific filter may attenuate different frequency channels (hereinafter "channel") in different ways and also differently at different signal strengths. This may lead to misleading received signal strength (RSS) comparisons resulting in that a channel not having the lowest signal strength is chosen as the channel to communicate on. Further yet, it is realized that each intermediate stage of the receiver chain adds some degree of noise to the signal, thereby affecting the accuracy. There are even further additional factors affecting the accuracy. For example, nonlinearities of the components of a receiver circuit, i.e. the signal processing components, may also introduce measurement errors; variations in power supply voltage and operating temperatures may also affect the accuracy of an RSS measurement value.

In accordance with the invention ripple in the gain as well as other factors are taken into consideration and compensated for. More particularly, the effects of ripple, as well as other factors, are minimized by implementing a compensation table or look-up table, as will- -be described- later . _ Zigure _1 show.s__a . jtypical__gain and noise vs. frequency curve, the gain being defined as the ratio of output signal to input signal and expressed in decibels. The upper graph, indicated by I, shows the gain as function of frequency and the lower graph, indicated by II, shows the noise as a function of frequency. In the figure it can be seen that the ripple, i.e. the variation of gain with frequency, measured in a typical receiver chain is approximately 2 dB (peak-to- peak) .

The gain of a receiver chain may differ for different, individual base stations even when the base stations are manufactured utilizing the same process and having same or similar receiver chains comprising the same type of components. The gain will also generally differ for different channels for a specific base station. The result of RSS measurements will therefore vary in a base station for different channels. Therefore, when comparing the RSS measurements in order to determine which channel to communicate on, the comparisons may actually be misleading and a "wrong" channel be utilized, i.e. resulting in that a channel actually having the lowest received signal strength is not utilized.

Figure 2 illustrates a typical receiver chain 1 of a base station used in a telemetry system. A signal is received by means of an antenna 2 and passed on to a matching network 3, in which the signal is processed in a conventional manner, well known to a person skilled in the art. That is, the input and output impedances are preferably matched so as to maximize the power transfer and minimize noise. Thereafter the signal is filtered in a bandpass filter 4 and amplified in a low noise amplifier (LNA) 5, as is well known within the field. Ne-xt, -a ^local oscillator . gensrates _a_ he_a_t_ s_ignal_. _This_ oscillator signal is injected into a mixer 6 along with the signal from the antenna 2 in order to effectively change the antenna signal to produce a signal which will be at the intermediate frequency and which can be handled by an intermediate frequency (IF) filter 7 and detector, all in a conventional manner. Finally, RSS circuitry 8, 9 is provided for generating an RSS signal, the circuitry comprising in this embodiment a filter 8 and an RSS detector 9, for example a power detector. The RSS signal may be used for providing a received signal strength indicator (RSSI) . It is realized that the RSS circuitry may include other components (not shown) . The generation of the RSS signal may be conventional in practice, for example it may be derived from the IF-stage 7 of the receiver chain 1.

Each of the components included and described above affect the received signals and may introduce errors to a measurement. Errors may occur both when altering signal power and frequency of operation.

In accordance with the invention a compensation table or look-up table is implemented in the transceiver of a base station in a RF medical telemetry system. The compensation table is used in order to compensate for ripple in the gain of the receiver as well as for other factors affecting the measurements. In accordance with the invention, a known signal is transmitted and measured in each channel that is to be used within the system. The signal used is a well known signal, hereinafter called

RSStest- The characteristics of this signal RSStest ^are suitably chosen and thus known, for example having a well defined transmission power and/or a well known amplitude.

The known signal RSStest is preferably applied in the

-ee-nt-re- -o-f--eaG-h--f-r-eq-uen-cy—band-, (channel.) Thereafter.,., an...

RSS measurement is performed in a microcontroller based on this known signal RSS_test and for each channel. A received signal strength indicator (RSSI) may be calculated and used in preceding calculations. Ideally, the RSS measurement for each channel should yield the same result, but this will generally not be the case, for example due to the above described shortcomings of the included components. The RSS measurement values for the different channels are used in order to calculate a compensation table for future reference. The compensation table comprises an RSS compensation value for each channel, and these RSS compensation values may be stored in a memory for later retrieval and use.

The stored values are compensation values for each respective channel to be used when a required listen- before-talk procedure is to be performed. For example, the relative values are calculated, whereby the channel having the highest RSS value is set to be 100% and needing no compensation. The other, remaining channels are set in relation to this. Thus, a channel having an RSS value of 95% of the highest RSS value should be compensated for in appropriate manner, i.e. compensated up to 100%. This ensures that all RSS values are indeed comparable .

The compensation values can thus be a percentage value as described above. That is, the RSS measurement values obtained are always compensated in the manner prescribed by the compensation table. However, other compensation values fulfilling the same purpose may of course be utilized. The idea is to provide RSS compensation values compensating for shortcomings in the receiver chain for a specific channel. In an alternative, since the

-characteristics- - of—the si-gnaJ E.S5-_test- — are _ _knojwn_ . jthe. measured RSS value can be compared to an expected RSS value for this signal. The difference between the measured values and expected value, if any, may then be used for compensating the RSS values in all future RSS measurements. It is realized that a compensation mechanism utilizing the relative relationship (such as percentage values) between the different channels would yield a more accurate result.

When the base station is taken into use and an RSS measurement is performed, the obtained RSS measurement value is adjusted accordingly by means of the compensation table, which thus is utilized for accessing the corresponding RSS compensation value for that specific channel.

The measured RSS values and the corresponding calculated RSS compensation values are representative for the performance of the particular base station in question, i.e. taking into account differences in for example device components in that base station. The measured RSS compensation values for each respective frequency band or channel are preferably stored in a memory in the form of a compensation table.

The measurements of RSS for the known signal RSStest applied in the different frequency bands are preferably performed in the manufacturing process, i.e. when producing the device. A unique compensation table is thus provided for each device.

Figure 3 illustrates schematically a flow chart over the steps included in the method 100 in accordance with the invention. In the first step, step 110, a suitable signal generator is utilized for generating a known signal RSS_test, preferably ±τi the centre of each -frequency band- of- interest. The RSS_test signal preferably has a known and well defined signal strength and is received at the base station that is being manufactured. Thereafter, in step 120, an RSS measurement is performed in the base station for each channel based on this known signal RSS_test- This step yields an RSS measurement value for each channel. Next, in step 130, RSS compensation values are calculated. The measured RSS values for each channel are utilized for calculating these RSS compensation values. For example, the RSS measurement values can be compared with each other and a relative relationship be established. The RSS compensation values can then be calculated based on this relative relationship. Finally, in step 140, the compensation values are stored in a compensation table.

When the base station is taken into use, RSS measurement values, that are ultimately used in determining which channel to communicate on, are compensated for by retrieving a correction value from the compensation table. Thereby the RSS measurement values for each respective channel are indeed comparable ^■ and the most suitable channel can be chosen for communication.

In an alternative embodiment a more general compensation table is used, in which embodiment the compensation table is based on statistical outtakes. Such look-up table comprises a compensation mechanism where each channel is compensated in a fashion where the channel showing the lowest value is up-compensated and the channel showing the largest value is down-compensated. This is done in order to keep maximum dynamic range of the RSS measurement.

In an alternative embodiment the compensation table is implemented as a matrix, and- the signal generator transmits a known signal at different signal levels or signal strengths. The performance of the receiver chain, for example affected by ripple in gain as a function of frequency, may thus be measured for different signal levels. An even higher accuracy is thereby provided.

Figure 4 illustrates a medical implant communications system or medical telemetry system 10 in which the above described method may advantageously be implemented. The system 10 comprises a base station 11 for use in communication between a programmer 12 and a medical implantable device 13. The programmer 12 comprises conventionally included means for enabling programming and/or monitoring of a patient related device 13. For example, the programmer 12 is provided with input and/or output means for transmitting programming instructions to an implantable medical device 13, and/or for outputting patient related data. A physician is thereby able to easily see such patient related data received from the implantable medical device. Further, other conventional means such as RX electronics and micro controller may also be included, as is obvious for a person skilled in the art. The base station 11 may be an integrated part of the programmer 12 or an external device.

The described method for providing an individual compensation table for each base station 11 in a medical implant communications system 10 enables a good way for choosing a communication channel. More specifically, the base station 11 monitors, as prescribed by the MICS standard, the channels used within the system 10 and performs an RSS measurement for all channels. The base station 11 comprises circuitry for calculating an RSS

- measurement-- -value-—for—eac-h—-channeJ -The .αhtainecL_ESS_ measurement values are thereafter adjusted by means of RSS compensation values stored in the compensation table comprising calculated RSS compensation values for each channel. The adjusted RSS measurement values are thereafter preferably compared to each other, and the most suitable channel is chosen for communication with a medical implant device. In general the most suitable channel is the channel having the lowest RSS-value, but it is realised that other criteria can be used for allocating a communication channel.

The base station 11 comprises means for performing the above-mentioned determination of communication channel. More particularly, the base station 11 comprises means for monitoring the channels used within the system 10, means for performing an RSS measurement, a compensation table comprising calculated RSS compensation values for each channel, means for retrieving, from the compensation table, a compensation value for each channel, and also means for compensating, for each channel, the RSS measurement by means of the RSS compensation value. If the adjusted RSS measurement values are to be compared, means for this end is also provided.

In summary, by taking into account the ripple of a receiver chain and compensate for it, the present invention provides an efficient and easily implemented method for fulfilling the requirements to be introduced into a MICS system. Further, a fault tolerant transceiver device, preferably a base station, is provided, taking into account the characteristics of components in the specific device in question and compensating for them. Further, by means of the invention the performance of a medical telemetry system can be optimized in that the transmission- power utilized wi-t-hin the- system _ can be minimized.

Claims

1. A method for a base station (11) in a medical implant communications system (10) , said method comprising the steps of:

5 - generating a known signal (RSS_test) _<

- receiving in said base station (11) said known signal (RSStest) iⁿ a plurality of frequency channels,

performing, in said base station (11), an RSS measurement for said plurality of channels based on said 10 known signal (RSS_test) _r whereby an RSS measurement value is obtained for each channel,

- calculating, based on said RSS measurement values, an RSS compensation value for each channel, and

- storing the calculated RSS compensation values in a 15 memory of said base station (11), said RSS compensation values being suitable for compensating for inaccuracies of an RSS measurement.

2. The method as claimed in claim 1, wherein said step of calculating an RSS compensation value comprises

20 establishing a relative relationship between at least two channels and calculating an RSS compensation value for each respective channel based on this relationship.

3. The method as claimed in claim 1, wherein said step of calculating an RSS compensation value comprises comparing

25 the measured RSS values to expected RSS values and calculating an RSS compensation value for each channel based on said differences between said expected values

^■-- and- said obtained -values-.--

4. The method as claimed in any of the preceding claims, wherein said RSS values are calculated and stored within said base station at manufacturing.

5. The method as claimed in any of the preceding claims, wherein the step of generating a known signal (RSS_tes_t) comprises generating said known signal in the centre of each channel.

6. The method as claimed in any of the preceding claims, wherein said known signal is transmitted at different signal strengths and said RSS compensation values are stored in a compensation table implemented as a matrix, whereby RSS compensation values are provided for each channel for different signal levels.

7. A method in a base station (11) of a medical implant communications system (10), said method comprising the steps of:

- monitoring at said base station (11) a plurality of frequency channels used within said system (10),

performing, in said base station (11) , an RSS measurement for said plurality of channels, whereby an RSS measurement value is calculated for each channel,

- retrieving, from a compensation table stored in said base ' station (11) and comprising calculated RSS compensation values for each channel, a compensation value for each channel, and

- adjusting, for each channel, the RSS measurement value by means of said RSS compensation value.

8. The method as claimed in claim 7, further comprising the step of comparing the RSS measurement values .

9. The method as claimed in claim 8, further comprising the step of selecting a channel having the lowest RSS value for communication with a medical implant device.

10. A base station (11) for a medical implant communication system (10), said base station (11) comprising:

- means for monitoring a plurality of frequency channels used within said system (10),

means for performing an RSS measurement for said plurality of channels, whereby an RSS measurement value is obtained for each channel,

a compensation table comprising calculated RSS compensation values for each channel,

- means for retrieving, from said compensation table, a compensation value for each channel, and

- means for compensating, for each channel, the RSS measurement by means of said RSS compensation value.

11. The base station (11) as claimed in claim 10, further comprising means for comparing said compensated RSS measurement values .

12. The base station (11) as claimed in claim 11, further comprising means for selecting a channel having the lowest RSS value for communication with a medical implant device .