WO1996025804A1

WO1996025804A1 - Wireless telephone line extender

Info

Publication number: WO1996025804A1
Application number: PCT/US1996/002037
Authority: WO
Inventors: Gregory D. Soderling; Kenneth Yu
Original assignee: Glenayre Electronics, Inc.
Priority date: 1995-02-17
Filing date: 1996-02-15
Publication date: 1996-08-22
Also published as: IL117156A0; AU4982596A

Abstract

A radio telephone communication system (15) includes an exchange end transceiver (20) that is coupled to a public switch telephone network and a remote end transceiver (22) that is coupled to a remote telephone handset (24). Each transceiver includes a list of radio frequency channels on which the telephone signals can be transmitted. To transmit the telephone signals, the transceiver includes an analog-to-digital converter that converts the analog telephone signals to a digital format and a vocoder digital signal processor that reduces the number of bits occupied by the digital signal. Each transceiver transmits the telephone signals using time division duplex to simulate a duplex telephone conversation on a single radio frequency channel.

Description

WIRELESS TELEPHONE LINE EXTENDER

Field of the Invention The present invention relates to telephone and radio communication systems. Background of the Invention In a conventional telephone system, the majority of telephone signals are carried over copper or fiber-optic landlines. These landlines are generally routed on telephone poles or buried underground throughout a geographic area. Such landlines can be laid economically because there are a sufficient number of users to recoup the cost of installation. However, in many remote areas the cost to install a telephone landline is prohibitive given the low number of potential customers who will receive telephone service. Therefore, many rural areas have never received telephone service by landline.

To provide communications service to rural areas, radio systems are often used. However, known radio communication systems suffer from several problems that make their use considerably less convenient and efficient than a conventional landline telephone system. First, many radio communications systems are simplex so that only one user can speak at a time. If duplex communication is desired, it has been necessary to transmit the call on a pair of radio frequency channels. Because it is difficult to obtain a license for channel pairs, few duplex radio communication systems are placed in the field. Finally, prior art radio communication systems are complex to operate than a conventional telephone and often require a user to perform special procedures or to operate specialized radio equipment.

Given the shortcomings with prior art radio communication systems, there is a need for a communication system that can efficiently bring telephone service to remote areas. The system should be relatively inexpensive, should provide duplex communications so that a user is unaware that the conversation is being transmitted via radio, and should utilize a minimum of radio frequency spectrum. Additionally, the system should operate with a conventional telephone handset without requiring a user to perform any special operations in order to place or receive a call.

Summary of the Invention The present invention is a radio telephone communication system for transmitting a telephone call to a remotely located telephone. The system includes at least one pair of matched radio transceivers. An exchange end transceiver is connected through a telephone line interface or direct access arrangement (DAA) to a conventional public switched telephone network. A remote transceiver is coupled through a subscriber line interface circuit (SLIC) to a conventional telephone handset. The pair of radio transceivers simulate a duplex communication channel by alternately transmitting and receiving a digitized representation of the telephone signal on a single, narrow bandwidth radio channel using voice compression and high-speed wireless modem techniques.

Each radio transceiver has an associated modem to convert an analog telephone signal for transmission to a compressed digital signal that modulates a radio frequency carrier signal. To produce the compressed digital signal, the analog telephone signal is first sampled and digitized with an analog-to-digital converter. The digitized telephone signal is then compressed using a vocoder circuit that is running a CELP algorithm. The compressed digital signal is then encoded into a series of symbols. A processor circuit within the modem calculates the transition between symbol values. A waveform table within the modem defines a series of phase change values that are supplied to a phase accumulator of a direct digital synthesis FM modulator. The FM modulator produces a frequency modulated signal that is representative of the transitions between symbol values. The frequency modulated signal is supplied to a radio transmitter where it is transmitted over a radio channel.

At the same end of the radio telephone system, a radio receiver receives the telephone data signals that are transmitted from the remote transceiver. The associated modem converts the received data signals into a corresponding digital signal. The processor circuit within the modem calculates a maximum likelihood sequence estimation in order to determine the most probable sequence of received symbols. With the most probable sequence of received symbols determined, the processor circuit converts the series of symbols to a compressed digital signal. The vocoder circuit expands the digital signal and supplies it to a digital-to-analog converter where the expanded digital signal is converted to an analog telephone signal. The analog telephone signal is supplied through the telephone interface to a telephone handset.

During the course of a conversation, the transceiver at each end alternately sends a packet of the compressed voice data and listens for a packet to be transmitted from the other end. Both packets are sent over a single radio channel, but the alternating send-and-receive operation, combined with the compression- decompression action of the modems, results in continuous two-way communications as would occur over a duplex landline. Brief Description of the Drawings

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: FIGURE 1 is a diagrammatic view of a radio telephone communication system according to the present invention;

FIGURE 2 is a functional block diagram of the radio telephone communication system according to the present invention;

FIGURES 3 and 4 are block diagrams showing different arrangements of the radio telephone communication system according to the present invention;

FIGURE 5 is a block diagram of a digital radio that transmits and receives telephone communications according to the present invention;

FIGURE 6 is a timing diagram showing the contents of a signaling and a voice data packet transmitted and received by the radio telephone communication system according to the present invention;

FIGURE 7 is a block diagram of a modem incorporated within the digital radio shown in FIGURE 5;

FIGURES 8A-8D are graphs and a table showing how a frequency modulated signal that encodes the packets is created by the modem shown in FIGURE 7; FIGURES 9A-9B are graphic representations showing a trellis diagram and a maximum likelihood sequence estimation used by the modem to decode a received data packet;

FIGURE 9C is a flow chart of the steps performed by the modem to decode a received radio signal; and FIGURES 10A-10D are flow charts showing how a telephone call is established between remote and exchange ends of the radio telephone communication system of the present invention.

Detailed Description of the Preferred Embodiment The present invention provides a radio telephone communication system that can transmit duplex telephone signals to a remote location on a single, narrow bandwidth radio channel. As will be explained in further detail below, the radio communication system transmits the telephone signals such that a user does not detect that a radio channel is being used to carry the conversation. Because the radio communication system uses narrow bandwidth radio frequency channels, the system provides improved radio spectrum utilization and increased capacity and efficiency on a non-interfering basis with historical allocations of various portions of the radio frequency spectrum.

Turning now to FIGURE 1, a conventional telephone system includes a public switched telephone network (PSTN) 10 that routes duplex telephone signals on a copper or fiber-optic landline 12 to a plurality of telephone handsets 14, 16, and 18. To provide telephone service to remote or outlying areas, a radio telephone communication system 15 according to the present invention is located at a terminal end of the telephone landline 12. An exchange end radio transceiver or "wireless line extender" (WiLE) 20 converts the telephone signals carried on the landline 12 into a radio frequency signal that is transmitted to a remotely located WiLE transceiver 22. The remote WiLE transceiver 22 converts the radio signal back into a telephone signal and supplies it to a conventional telephone handset 24. With the WiLE transceivers 20 and 22, a user of the remote telephone handset 24 is able to send and receive duplex telephone signals as if the telephone handset were directly connected to the PSTN 10 via the telephone landline 12.

In operation, the radio signals transmitted by the exchange end WiLE transceiver 20 may be received by the remote WiLE transceiver 22 at a distance of up to 75 km. Therefore, the present invention allows telephone communication services to be provided to remote or inaccessible areas where it is not cost effective to extend the telephone landline 12.

A functional block diagram of the radio telephone communication system 15 according to the present invention is shown in FIGURE 2. As indicated above, the radio telephone communication system includes at least one pair of matched WiLE transceivers 20 and 22. The WiLE transceiver 20 is referred to as an exchange end unit because it is coupled to the PSTN. The WiLE transceiver 22 is referred to as a remote unit. The WiLE transceivers are substantially identical. The difference between the exchange end unit and the remote unit is the particular type of interface that connects the unit to a telephone. The exchange unit 20 is coupled to the PSTN through a conventional direct access arrangement interface or DAA 24, such as CHI 817 Module, available from CERMETEK Microelectronics Corp. of Sunnyvale, California. The DAA detects signals on the telephone landline 12, such as a ringing signal, and converts the telephone signals to be compatible with the digital circuit components of a digital radio 25 that is a part of the WiLE transceiver 20.

The digital radio 25 within the remote WiLE transceiver 22 is coupled to a remote telephone handset 26 through a conventional subscriber line interface circuit or SUC integrated circuit 27. The SLIC integrated circuit converts the signals produced by the digital radio 25 so that remote telephone 26 operates as if connected to a conventional telephone landline. A representative SLIC integrated circuit is part no. MC341 available from Motorola Semiconductor of Austin, Texas. To transmit simultaneous telephone conversations, the radio telephone communication system of the present invention may include more than one pair of WiLE transceivers. In the example shown in FIGURE 3, two exchange end WiLE transceivers 30 and 32 are coupled to the public switch telephone network through a pair of telephone landlines 31 and 33, respectively. Each exchange end WiLE transceiver has a corresponding remote WiLE transceiver associated with it. For example, the WiLE transceiver 30 is associated with a remote WiLE transceiver 34 in order to transmit telephone signals to a remote telephone handset 36. Similarly, the exchange end WiLE transceiver 32 has a remote WiLE transceiver 38 associated with it in order to transmit telephone signals to a remote telephone handset 40. In the arrangement shown in FIGURE 3, each pair of WiLE transceivers only transmits telephone signals to its associated remote unit on a preassigned frequency fj or f₂.

An alternative configuration of the radio telephone communication system according to the present invention is shown in FIGURE 4. Here there are more remote WiLE transceivers than exchange end WiLE transceivers. A pair of exchange end WiLE transceivers 42 and 44 are coupled to the public switch telephone network via a pair of telephone landlines 43 and 45. Each of the exchange end WiLE transceivers 42 and 44 is capable of transmitting telephone signals to any of the remote WiLE units 46, 48, or 50. The WiLE transceivers are frequency agile such that they can transmit and receive telephone calls on a number of authorized radio frequency channels. Each WiLE transceiver initiating a call searches a number of the authorized radio frequency channels to find an available channel on which the call can be made. For example, assume that it is desired to connect a remote telephone handset 54 to the telephone landline 43. In that case, the exchange end WiLE transceiver 42 scans its list of authorized radio frequency channels f_j and i_- If the radio frequency channel fj is available, the telephone call is carried on that channel. If a subsequent telephone call is made from the remote telephone handset 56, the remote WiLE transceiver 50 will scan its list of authorized radio frequency channels, determine that the channel fj is busy, and place the call on the channel f₂. With both exchange end WiLE transceivers 42 and 44 busy, telephone calls cannot be made to or from the remote telephone handset 52. Referring now to FIGURE 5, a WiLE transceiver includes a digital radio 25 that receives analog telephone signals and converts such signals to a corresponding digital signal that is used to modulate a radio frequency carrier to transmit the digital signal to a remotely located WiLE transceiver.

Controlling the digital radio 25 is a suitably programmed microprocessor or CPU 122 that is inter&ced with an analog-to-digital converter 100, a vocoder digital signal processor 104 and a modulator/demodulator (modem) digital signal processor 108. Coupled to the CPU 122 is a read only memory 126 that stores a computer program that controls the operation of the CPU as well as a list of authorized frequencies on which the WiLE transceiver can transmit or receive telephone signals.

Although not shown, each transceiver includes an appropriate power supply. Power may be provided by a conventional AC line or a solar powered battery if a transceiver is located where electricity is not available. When battery power is used, the transceivers are able to operate in an "active" and an "idle" state. In the idle state, a transceiver will only monitor its authorized radio frequency channels to detect whether a call is being made. The receiver monitors each radio frequency channel for a "gather tone" to be transmitted. Once the gather tone has been detected, the CPU 122 within the transceiver that causes the transceiver to "wake up" and provide power to all circuits with the transceiver. The analog-to-digital converter 100 samples the analog telephone signal received from the telephone interface (not shown) at a rate of 8K samples second and converts the samples to a corresponding digital signal. The digital signal is supplied to the vocoder digital signal processor 104 in order to compress the digital signal. The vocoder digital signal processor is a high speed microprocessor that is programmed to reduce the number of bits occupied by the digital signal supplied by the analog-to-digital converter by removing redundancy in the voice signal. In the presently preferred embodiment of invention, the vocoder digital signal processor is programmed to implement a CELP compression algorithm that was developed by AT&T. The CELP algorithm achieves over a 10-to-l compression of the digital signal with minimal loss of signal quality. The CELP algorithm used is a variation of the public domain 4800 bps DOD CELP FED-FTD 1016. The AT&T version of the CELP algorithm used in the vocoder is described in one or more of the following U.S. Patents: 4,133,976; 4,220,819; 4,472,832; 4,701,954; 4,827,517; 4,910,781; 5,267,317 and Re 32580, which are herein incorporated by reference. Although the CELP algorithm is the preferred compression scheme, other compression algorithms could be used, such as the improved multiband exciter technology (IMBE) developed by DVSI of Boston, Massachusetts.

The compressed digital signal produced by the vocoder is stored in a memory buffer 106. The modem 108 periodically reads a portion of the compressed digital signal from the memory buffer 106 and creates a voice data packet that is transmitted from the digital radio 25. As will be discussed in fiirther detail below, the modem 108 creates an frequency modulated signal that is applied to a radio transmitter/power amp 110 to be broadcast to the corresponding remote WiLE transceiver. The frequency modulated carrier signal is supplied through a transmit/receive switch 112 to an antenna 116, which broadcasts the frequency modulated carrier signal. To receive telephone signals from a remote WiLE transceiver, the transmit- receive switch 112 is placed in a receive position by the transmit/receive clock signal produced by the CPU 122. The radio signals collected by the antenna 116 are supplied to a radio receiver 120 that filters the received signals and converts the carrier frequency to a base-band signal. A signal strength detect circuit (not shown) within the radio receiver measures the strength of the received radio signals. If the received radio signals are weak, the receiver 120 can change the position of an antenna switch 114. The antenna switch 114 connects a diversity antenna 118 to the radio receiver. The diversity antenna is located at least several feet away from the antenna 116 and may provide the receiver with a better signal. The receiver 120 is a quadrature demodulator that produces two signals. An

"I" signal is in phase with the received radio signal and a "Q" signal is ninety degrees out of phase with the received radio signal. The details of the receiver 120 are considered well known to those of ordinary skill in the radio communication arts and therefore will not be discussed in further detail. The I and Q signals produced by the radio receiver are applied to the modem 108 where they are decoded into a corresponding received digital signal. The received digital signal is thai applied to the vocoder digital signal processor 104 that expands the digital signal, preferably using the same algorithm (CELP) described above. The expanded digital signal is then applied to a digital-to-analog converter 128 that converts the expanded, digital signal to a corresponding analog telephone signal. The output of the digital-to-analog converter 128 is supplied to the telephone interface (i.e. the DAA or SLIC) to be received by a telephone handset.

As indicated above, one of the problems with prior art radio telephone systems is that they only operated in a simplex mode (i.e., only one person could speak at a time). If duplex communication was desired, it was necessary to provide separate radio channels. The radio frequency communication system of the present invention achieves duplex communication on a single, narrow bandwidth radio frequency channel using time division duplexing. The WiLE transceivers 20 and 22 simulate a bi-directional communication link by alternately transmitting and receiving signal packets via a high-speed modem. In the presently preferred embodiment of the invention, the WiLE transceivers produce the transmit/receive clocking signal that switches the transmit/receive switch 112 such that data is transmitted for 25 milliseconds and then received for 25 milliseconds. It is important that the transmit/receive (Tx Rx) clock signal remain as stable as possible despite variations in component aging and temperature of the transceivers. A stable crystal 124 provides the CPU 122 with an accurate timing signal. As will be described below, the modem 108 produces a clock drift signal that adjusts the Tx/Rx clock signal based on a received packet. Furthermore, the CPU can also adjust the Tx Rx clock if another, closely located, WiLE transceiver is handling a telephone call. For users of the system, it appears that simultaneous conversations are carried in both directions over the single radio channel.

As can be seen in FIGURE 6, the WiLE transceivers alternately transmit and receive data packets on the radio frequency channel. A data packet may be of two types. A signaling packet is used to set up and tear down the communication link over which the telephone signals are transmitted. A voice data packet contains the compressed telephone signal to be transmitted to the remote receiver. To ensure that a signaling packet is received correctly by a WiLE transceiver, each signaling packet is transmitted a plurality of times, preferably three times. For example, if a signaling packet is to be transmitted from the exchange end WiLE transceiver 20 to the remote WiLE transceiver 22, the data portion of the signal packet is transmitted three times. The redundancy provided by transmitting the data portion of the signal packet three times improves the chances that the packet will be received correctly. The details of the signaling and voice data packets are described in further detail below.

FIGURE 7 is a block diagram showing the functions performed by the modem digital signal processor 108. The modem 108 includes its own processor 150 that controls the encoding and decoding of data packets. As represented in FIGURE 6, a signal packet begins with a bit sync pattern 180 that informs the receiving modem when a received radio signal should be sampled to accurately decode a transmitted symbol. Following the bit sync pattern is a frame sync pattern 182. The frame sync pattern informs the modem where the data portion of the transmitted signaling packet begins. Following the frame sync pattern is the data portion 184 of the signaling packet. The data informs the receiving WiLE transceiver about the status of the communication link, such as dialed digits, remote identification number, special functions, and others. A series of forward error correction check bits 186 are included in the signal packet to allow a receiver to correct for errors that may have occurred during transmission of the signal. The frame sync 182, data bits 184, and the forward end connection bits 186 are transmitted three times to maximize the likelihood that they are received correctly. Finally, a guard space 188 is inserted after the forward error bits to allow the receiving transceiver sufficient time to start its transmitter before a new packet must be transmitted. To assemble a voice data packet, a portion of the compressed voice signal is read from the memory buffer 106 and assembled into a string of binary digits. At the beginning of the voice data packet is a sync code 190 that informs the receiving modem when to sample the received radio signal. Following the sync code is a frame sync pattern 191 that marks the beginning of the compressed voice data 192. A series of forward error correction bits 194 follow the voice data to allow correction of errors that may have occurred during transmission. Finally, a guard space 196 allows the receiving transceiver sufficient time to start its transmitter before a radio signal must be transmitted.

Once the processor 150 has assembled a packet (either a signaling or a voice data packet) as a string of bits, the bit string is encoded as a series of symbols. In the present embodiment of the invention, the modulation scheme used to transmit the data packet is a constant envelope, four-level phase modulation scheme wherein a radio frequency signal is modulated with four different values. FIGURE 8 A shows how the processor 150 encodes the bit string as a series of four different symbols. First the string of binary digits is divided into groups of two bits. Each bit is either a logic 1 or a logic 0. With a two bit grouping, there are four transitions between bits: a transition from a logic 0 to a logic 0, a transition from logic 0 to a logic 1, a transition from logic 1 to a logic 0, and a transition from logic 1 to a logic 1. Each bit transition is a symbol. Each symbol is represented as a raised cosine pulse of varying amplitude. The raised cosine pulse has a shape defined by the equation:

^{p(t) =} {2EΪ l^{1 ~ cos} (IT) j ^0≤t≤Lτ> ° ^{other ise} C¹⁾ where L is the duration of two bits and 1 T is the baud rate of the transmitted signals.

As shown in FIGURE 8B, the symbols have a relative amplitude of +3, +1, -3 and -1.

Once the data packet has been divided into a series of symbols, the modem digital signal processor creates a frequency modulated signal thai, encodes the transitions between the four possible symbols. FIGURES 8C and 8D show how this is accomplished by the modem digital signal processor. As indicated above, each symbol has a duration that is twice the length the of bits that define the symbol. Because of the symbol length, each symbol overlaps its adjacent symbols by 50%. For example, FIGURE 8C shows an analog data signal, V, that represents a bit string 01001100. This bit string is encoded as four symbols +1, +3, -1, and +3. The analog data signal, V, has a magnitude that is defined by the sum of these individual symbols as they occur in time. The analog data signal, V, represents the modulating signal that varies the frequency of a sinusoidal signal produced by a direct digital synthesis FM modulator. For a modulation scheme having four symbols, there are a total of sixteen possible transitions between symbols. To produce a frequency modulated signal that encodes the transition between two symbols, a series of phase changes are stored in a waveform table 154 that resides in a read-only memory within the modem 108 (FIGURE 7). The phase change values are read sequent-ally from the waveform table and stored in a phase accumulator register of a direct digital synthesis (DDS) FM modulator 156 (also shown in FIGURE 7). The desired frequency modulated signal to be transmitted is described by the equation: t s(t) = A cos [2π f t + j θ(t)dt] (2) ^c o where A is a fixed magnitude, ζ. is a desired center frequency of a modulated signal produced by the FM modulator. The signal θ(t) is given by the equation: θ(t) = 2πh∑d_np(t - nT) (3) where h is a constant of 0.5, d„ is the amplitude of the symbols (+3, +1, -1, -3) as they are summed together as shown in FIGURE 8C, and p(t-nT) is the raised cosine shape set forth in Equation 1. The instantaneous frequency of the frequency modulated signal described by Equation 2 is equal to: 2πf_c +θ(t) (4)

To calculate that values for the phase changes that are supplied to the phase accumulator register in order to encode a symbol transition, a mathematical model of a signal θ(t) is created on a computer system.

The mathematical representation of the signal θ(t) is sampled at a plurality of time intervals which are used to compute the phase value changes.

As indicated above, the DDS FM modulator produces a constant frequency, sinusoidal signal. The FM modulator 156 has two inputs. The first input receives a clocking signal from the processor 150 with a frequency of F_s. The output frequency, F of the DDS FM modulator is given by the equation:

F_s xΔφ Fo = "^ (5) where F_s is the frequency of a clocking signal supplied to the FM modulator, Δφ is the value of the phase accumulator register and 2^N is the number of possible bit patterns that can be stored in the phase accumulator register.

Each sample of the θ(t) signal described above indicates the frequency deviation of the desired frequency modulated signal to be transmitted. Therefore, the instantaneous phase change value required to produce the desired frequency can be determined by setting Equation 4 equal to the right side of Equation 5 and solving for Δφ . Specifically, a value of Δφ is calculated by the results of the equation:

where θ(t) is each sample described above. The second term in Equation 6 is stored in the waveform table.

As will be appreciated, the value of f-. is a constant and is changed depending on the channel on which the transceiver is transmitting and receiving. This offset is added to the second term of Equation 6 before the sum is loaded into the phase accumulator register of the DDS FM modulator.

The combination of the compression provided by the vocoder circuit and the production of the frequency modulated signal using the DDS FM modulator allows the telephone signals to be digitized and transmitted on a single radio channel having a bandwidth of 25 KHz. This narrow bandwidth allows improved radio spectrum utilization and increased capacity and efficiency on a non-interfering basis with historical allocations of various portions of the radio frequency spectrum to be used for transmitting telephone signals.

In addition to producing the analog data signal that modulates the radio frequency carrier signal to be transmitted, the modem 108 also demodulates received data packets. The demodulator coherently demodulates a base band signal using a maximum likelihood sequence estimation (MLSE technique). For a continuous phase modulation scheme such as that used in the present invention, a phase trellis can be generated to describe all the possible phase states that the modulation can take on. A MLSE decoder chooses the most likely transmitted sequence by finding a path through the phase trellis such that the summation of all its branches is maximized.

With reference to FIGURE 7, the I and Q signals produced by the radio receiver are supplied to an analog-to-digital converter 160 within the modem circuit 108. The analog-to-digital converter 160 samples the received I and Q signals and converts them to a corresponding digital format. The samples are stored in a random access memory 162 where they can be read and analyzed by the processor 150. It should be noted that not all the decoding of received signals is performed using the MLSE technique, specifically, the processor 150 may use a correlation detection algorithm to detect the arrival of a bit sync pattern. FIGURES 9A and 9B show a trellis diagram and a maximum likelihood sequence estimation calculation performed by the modem to decode the sequence of transmitted symbols. An optimal decoder requires a 16-phase state trellis. In order to reduce the complexity of the decoder, a reduced complexity MLSE decoder is used. The phase pulse for the decoder is truncated to L=l. As a result, the number of phase states reduces to four.

FIGURE 9A is a four-phase state trellis diagram. In the continuous phase modulation scheme used in the present invention, each symbol (+3, +1, -1, -3) is transmitted as a change of phase in the radio frequency carrier. Beginning at any known phase state, a transition to another state can represent one of two possible symbols. For example, a transition from phase state 0 to phase state — can represent the symbol -3 or +1. Alternatively, a phase change from phase state 0 to phase state — can represent the symbol -1 or +3. A phase change from π to — can represent the symbol -1 or 3, while a phase change from π to — can represent the symbol -3 or

+1.

To decode a series of received symbols, the processor 150 within the modem calculates a metric for the received symbol. A metric is a measure of how closely the received signal matches a reference signal. Each metric depends upon the previous symbol transmitted. Therefore, a total metric for each phase state is stored. As a new metric is calculated for each possible symbol transmitted, the metric is added to the total metrics for the previously allowed phase states. The symbol producing largest metric is selected as the most likely symbol transmitted and a record of that symbol is stored in a symbol string within the random access memory 162 that is part of the modem. After the last symbol has been transmitted, the symbol string associated with the largest total metric is selected as the most likely sequence of symbols transmitted. Once the symbol string has been selected, the symbols that comprise the string are decoded back into binary digits. In the presently preferred embodiment of the invention, the incoming I and Q signals are sampled at a rate of 48 KHz by the analog-to-digital converter 160. Because the preferred symbol transmission rate is 8 ksps, the analog-to-digital converter samples the I and Q signals six times per symbol. The processor circuit then computes a value for the following equation:

(n+nr .; *„)= j ( ∞sWM + θ.]₊Q(t)-ύn[6 t;d.)₊ θ.])dt (7) nT

where I(t) and Q(t) are the I and Q signals and

θ(t;d =2πhd_I1p(t -nT) (8)

where, as described above, d_n is the symbol amplitude (+3, +1, -1, -3), h is a constant 0.5, and p(t-nT) is the raised cosine shape described in Equation 1. Equation 7 is calculated for each of the four possible phase states and for each of the four symbol amplitudes. Therefore, 16 metrics are calculated for each received symbol. FIGURE 9C is a flow chart of the steps performed by the processor 150 to decode a string of transmitted symbols. Beginning at a step 200, the processor 150 analyzes the received signals to detect a bit sync or dotting pattern 180 (FIGURE 6) that begins every transmitted data packet. In the presently preferred embodiment of the invention, the dotting pattern is represented as a series of alternating +3 and -3 symbols. By correlating the received signals with the dotting pattern, the microprocessor is able to reset the sampling time of the analog-to-digital converter such that the converter samples received radio signals at the proper time at step 204. Once the sampling time has been reset, the I and Q signals are sampled and stored in the random access memory 162 (shown in FIGURE 7) at a step 210. At a step 212, the metrics for each possible symbol and phase state are computed. As indicated above, in the currently preferred embodiment of the invention, there are four phase states and four possible symbols that can be transmitted in each phase state. Therefore, there are 16 possible metrics that are calculated according to Equation 7 described above. At a step 214, the microprocessor analyzes the metrics calculated for each phase state. As can be seen in FIGURE 9 A, for phase state 0, there are two possible previous phase states. A phase change from — to phase state 0 can represent a symbol -1 or +3. Alternatively, a phase change from — to phase state 0 can represent symbol -3 or +1. To select the most likely symbol transmitted, the processor adds the metric computed for symbols -1 and +3 to the total metric computed for phase state — . Then the processor calculates the metrics computed for the symbols -3 and +1 to the total metric computed for phase state — . The processor then selects the largest of these four values as the most likely symbol transmitted. The symbol is stored in the random access memory for the symbol string maintained for the phase state 0 and the new total metric for the phase state is stored.

This process repeats for the remaining phase states, — , π and — (FIGURE 9B).

At a step 216, the processor determines whether all symbols transmitted have been analyzed. If not, the processor loops back to step 210 and the next symbol is analyzed. Once all the symbols have been analyzed, the processor selects the phase state with the greatest total metric at step 218. The symbol string for the phase state selected is retrieved at a step 220. Finally, the symbol string is converted back to a series of binary digits at a step 222. Once the string of binary digits has been computed, the processor transmits the string to the vocoder where the string is expanded (decompressed), preferably using the CELP algorithm described above. FIGURES 10A-10D set forth the steps performed by a WiLE transceiver to transmit a telephone call. FIGURES 10A and 10B detail the steps performed when a telephone call is received by an exchange end transceiver.

Beginning at a step 250, an exchange end WiLE transceiver detects a ringing signal from the DAA interface that is coupled to the public switched telephone network. At a step 252, the WiLE unit monitors the received signal strength circuit. At a step 254, the transceiver determines whether the radio frequency channel is busy. If the channel is busy, the microprocessor then selects the next channel from its list of authorized channels at step 258 and loops back to step 252 to determine if the channel is available. This process continues until an open channel is located.

Once an open radio frequency channel has been found, the transceiver determines at step 262 whether any other WiLE transceiver is transmitting radio frequency signals. As indicated above, in order to avoid interference, all co-located transceivers must coordinate the transmit and receive periods. Therefore, if another WiLE transceiver is transmitting, the WiLE transceiver will reset its transmit/receive clock to be in sync with the transmit/receive clock produced by another WiLE transceiver. The first WiLE transceiver to begin transmitting assumes the role of the master clock source. Any other co-located WiLE transceivers will be slaves and will reset their transmit/receive clocks to be in sync with the master. At a step 264, the WiLE generates a "gather tone" for N consecutive transmit time slots (N=4 in the present embodiment of the invention). The gather tone is a simple tone and is not a digitized packet. The gather tone is created by retrieving the phase changes associated with the symbol transition of +3 to -3 from the waveform table described above. The gather tone serves to indicate to the remote transceiver that a telephone call is being initiated and that the transceiver should "wake up." At a step 266, the transceiver transmits a "connect request" packet that indicates which remote transceiver is to respond to the telephone call. Included within the data portion of the connect request packet is an internal identification number (ANI) that uniquely identifies the intended remote WiLE transceiver. FIGURE 10B shows the steps performed by the remote WiLE transceiver in responding to the gather tone. At a step 290, the remote transceiver periodically monitors its radio frequency channel to determine whether a gather tone is transmitted. If no gather tone is detected, the remote transceiver tunes its receiver to the next channel in the remote transceiver's list of authorized channels at step 292. This process continues until the remote transceiver detects a gather tone. Once a gather tone has been detected, the remote transceiver wakes up and again monitors the selected radio frequency channel at a step 294. The remote transceiver determines whether a "connect request" packet has been received at a step 300. If the packet is not received, the remote unit loops back to step 294 and monitors the channel until a connect packet is received. If a connect request packet is detected, it is determined at a step 302 whether the receiving unit's internal identification number (ANI) matches the ANI number transmitted in the connect request packet. If not, the remote transceiver knows that it is not the intended destination for the telephone call being transmitted and therefore the remote transceiver monitors the next channel in its list of authorized channels. If the remote transceiver's ANI matches the ANI transmitted in the connect request packet, the remote unit transmits a "connect confirm" packet at a step 304.

Returning now to FIGURE 10 , the exchange end transceiver determines at a step 268 whether the remote transceiver has transmitted the connect confirm packet. If a response is received, the transceiver assumes that a connection has been established and a communication takes place by alternately transmitting and receiving voice data packets to and from the remote transceiver at step 270.

If a response to the connect request packet is not received at step 268, the exchange end transceiver determines whether the connect request packet has been transmitted for "M" alternate time slots at a step 272. The value "M" is determined during system initialization. In the presently preferred embodiment of the invention, the connect request packet is transmitted five times. If the connect request packet was transmitted M times and no response was received, the exchange end transceiver goes to the next radio channel at step 274 and the process starts over.

Once a call has been established, the telephone signals are transmitted using the time division duplex scheme described above. This process continues until the remote transceiver detects an "on hook" condition at its telephone interface. At this time a disconnect request packet is transmitted to the exchange transceiver that receives the packet and responds with a disconnect confirm packet. At this time, the radio frequency channel is cleared. When a telephone call is received by an exchange end transceiver, the remote transceiver will synchronize to the exchange end's transmit/receive clock because the remote unit synchronizes itself with each packet transmitted. When a call is initiated from the remote transceiver, there is no guarantee that the remote transceiver's transmit/receive clock will be synchronized with the exchange unit's transmit receive clock. However, it is not possible to resynchronize the exchange end's transmit/receive clock to the remote transceiver's transmit receive clock because one or more exchange end transceivers may already be transmitting at the time the call is initiated from the remote transceiver. Therefore, even if the telephone call is initiated at the remote transceiver, the remote transceiver will resynchronize itself to the transmissions from the responding exchange end transceiver. FIGURES IOC and 10D describe the steps taken by the radio communication system of the present invention when a call is initiated from the remote WiLE transceiver. Beginning at a step 320, the remote WiLE transceiver detects that a user has placed a remote telephone "off-hook." At a step 322, the remote transceiver reads the signal strength detect circuit for the current radio frequency channel selected. At a step 324, the remote transceiver determines if the current radio frequency channel is busy. If the channel is busy, the remote transceiver proceeds to step 325 where it is determined if all the channels are busy. If all the radio frequency channels are busy, the remote transceiver will cause a distinctive busy tone to be generated at the SLIC interface at step 326 thereby informing the user that no telephone calls can be made at that time. If the answer to step 325 is no, the next radio channel in the remote transceiver's list of authorized channels is selected (step 327).

Once an open radio frequency channel is located, the remote transceiver will transmit a gather tone for "N" consecutive transmit and receive time slots (step 328). Unlike the exchange end transceiver, the remote end transceiver will transmit its gather tone continuously during both the transmit and receive time slots.

FIGURE 10D discloses the steps taken by the exchange end transceiver to detect a gather tone transmitted from the remote transceiver. Beginning at a step 350, the exchange end transceiver monitors its current radio frequency channel. At a step 352, the exchange end transceiver determines whether the gather tone is detected. If not, the exchange end transceiver tunes its receiver to the next radio frequency channel on its list of authorized radio frequency channels (step 354). This process continues until the exchange end transceiver detects the gather tone.

Once a gather tone is detected, the exchange end transceiver monitors the selected radio frequency channel at a step 356. At a step 358, it is determined whether a "connect request" packet has been received. If not, the exchange end transceiver performs the steps set forth at 266 in FIGURE 10A This causes the system to behave as if a telephone call was being initiated at the exchange end rather than at the remote end. If a connect request packet has been received at step 358, the exchange end transceiver echoes back the connect request packet to the remote exchange end transceiver at a step 362. After echoing the connect request packet, the connection is established with the remote transceiver (step 364).

Returning to FIGURE IOC, once the remote transceiver has transmitted the connect request packet at a step 330, the remote transceiver monitors the selected radio frequency channel at a step 332. Because it is possible that the remote transceiver's transmit/receive clock may be out of sync with the exchange end, the remote end monitors the radio frequency channel continuously until either a connect request packet is transmitted from the exchange end or a predefined amount of time has elapsed. If the remote transceiver times out, the transceiver performs step 327 and a new attempt is made to signal the exchange end on a different channel. If a connect request packet is detected at step 334, the remote transceiver will reset its transmit/receive clock based on the time of arrival of the frame sync bits. The remote transceiver knows that the frame sync bits are always transmitted at a predefined time after the beginning of a transmit time slot. Therefore, by averaging the delay between when a signal is first received and the arrival of the frame sync bits allows the remote transceiver to reset its transmit/receive clock accurately. After resetting the transmit/receive clock at step 336, data transfer takes place by alternatively transmitting and receiving data packets as described above until the remote transceiver detects the "on-hook" condition. As indicated above, it is important that the remote unit remain synchronized to the exchange end unit. The processor within the modem of the remote unit continually monitors its Tx/Rx clock to ensure that it remains synchronized with the transmit receive clock of the exchange end transceiver. To maintain the synchronization, the modem of the remote transceiver resets an internal timer 166 (FIGURE 7) each time a frame synchronization bit pattern is received. The time is stopped at some predefined point on the remote unit's transmit/receive clock (such as a falling edge). The value accumulated in the timer allows the remote transceiver to determine whether its transmit/receive clock is drifting with respect to the transmit/receive clock of the exchange end transceiver. Any drift detected is used to set the clock drift signal (FIGURE 5) which causes the main CPU of the remote end transceiver to either increase or decrease the frequency of the Tx/Rx clock.

As can be seen from the above description, the present invention allows duplex telephone conversations to be transmitted on a single radio frequency channel over large distances. The invention provides improved radio spectrum utilization and increased capacity and efficiency with historical allocations of various portions of the radio frequency spectrum. Each WiLE transceiver interfaces with a conventional telephone handset so that the user does not have to perform any special steps and is therefore unaware that the signals are being transmitted via radio.

While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention. The scope is to be determined solely from the following claims.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A radio communication system for transmitting a duplex telephone signal on a single radio channel between a first telephone and a second telephone, comprising: a first radio transceiver coupled to the first telephone and a second radio transceiver coupled to the second telephone, each transceiver including: a) an analog-to-digital converter for sampling the telephone signal to be transmitted and for converting the sampled telephone signal into a first digital signal containing a number of binary digits that are representative of the telephone signal; b) a compression circuit for converting the first digital signal into a compressed digital signal having fewer binary digits than the first digital signal; c) a modem circuit for assembling the compressed digital signal into a plurality of data packets to be transmitted; d) a radio transmitter for transmitting the plurality of data packets; e) a radio receiver for receiving a transmitted data packet; f) means for alternately selecting the radio transmitter and the radio receiver to transmit and receive data packets such that the duplex telephone signal is transmitted on the single radio channel; g) a decoder circuit for converting a received data packet into a received digital signal containing a number of binary digits that are representative of a received telephone signal; h) a decompression circuit for creating an expanded digital signal having a greater number of binary digits than the received digital signal; and i) a digital-to-analog converter circuit for converting the expanded digital signal into a corresponding analog signal representative of the received telephone signal.

2. The radio communication system of Claim 1, wherein the modem circuit includes: a processor circuit for assembling a portion of the compressed digital signal into a string of binary digits that comprise the data packet and for dividing the string of binary digits into a series of symbols, wherein each symbol is representative of two or more binary digits; a digital memory having a waveform table stored therein, said waveform table including a plurality of phase changes each associated with a transition between sequential symbols; a direct digital synthesis FM modulator including a phase accumulator register, wherein said processor circuit reads the series of symbols to determine a symbol transition and sequentially loads each phase change associated with the symbol transition into the phase accumulator register such that the direct digital synthesis FM modulator produces a frequency modulated analog data signal that is representative of the symbol transition.

3. The radio communication system of Claim 1, wherein the means for alternately selecting the radio transmitter and the radio receiver comprises a transmit/receive switch and means for generating a transmit/receive clock signal to control the transmit/receive switch.

4. The radio communication system of Claim 3, wherein the second radio transceiver includes means for synchronizing its transmit/receive clock to the transmit/receive clock of the first radio transceiver such that when the transmit/receive switch of the first transceiver is in a transmit position, the transmit/receive switch of the second transceiver is in a receive position.

5. The radio communication system of Claim 4, wherein each data packet includes a series of frame synchronize bits that mark a beginning of the data within the data packet and wherein the means for synchronizing the transmit/receive clock includes: means for correlating the received digital signal with the frame synchronize bits; means for recording a time at which a series of frame synchronize bits are received; means for comparing the time at which the series of frame synchronize bits were received to an expected time; and means for adjusting the transmit receive clock based on the difference between the time at which the series of frame synchronization bits were received and the expected time.

6. The radio communication system of Claim 1, wherein the first and second transceivers operate in an idle state and an active state, the modem circuits in each of the first and second radio transceivers including means for transmitting a gather tone before any data packet is transmitted, the receiver of each transceiver including means for monitoring the radio frequency channel for the gather tone from the other transceiver and upon detection of the gather tone, for causing such transceiver to operate in the active state.

7. A two way radio communication system for transmitting a duplex telephone signal on a smgle radio channel, wherein the duplex telephone signal comprises a first telephone signal to be transmitted from a first telephone to a second telephone and a second telephone signal to be transmitted from the second telephone to the first telephone, the system comprising: a pair of transceivers each of which includes: means for sampling the first telephone signal and for converting the first telephone signal into a corresponding digital signal; means for compressing the digital signal to create a compressed digital signal; means for assembling the compressed digital signal into a plurality of data packets; a radio transmitter for transmitting said data packets on the single radio channel, the assembling means including means for assembling received data packets into a compressed received digital signal, and the compressing means including means for decompressing the compressed received digital signal; and a radio receiver for receiving a plurality of data packets that are transmitted from a corresponding transceiver on the single radio frequency channel; means for selecting the radio transmitter and the radio receiver such that the transceiver alternately transmits and receives packets on the single radio channel.

8. A modem circuit for transmitting a digital signal to remote location, comprising: a processor circuit for receiving a digital signal to transmitted, and for dividing the digital signal into a plurality of symbols each of which is representative of a pair of binary digits; a memory circuit having a waveform table stored therein, the waveform table including a plurality of phase change values that are associated with transitions between the sequential symbols; and a direct digital synthesis FM modulator having a phase accumulator register, wherein the processor circuit reads the series of symbols to determine a symbol transition and sequentially loads each phase change value associated with the symbol transition into the phase accumulator register such that the direct digital synthesis FM modulator produces an analog data signal that is representative of the digital signal to be transmitted.

9. The modem circuit of Claim 8, further comprising: an analog-to-digital converter for sampling an analog signal and converting the analog signal into the digital signal to be transmitted.

10. The modem circuit of Claim 9, further comprising a compression circuit that receives the digital signal to be transmitted, produces a compressed digital signal having fewer binary digits that the digital signal and feeds the compressed digital signal to the processor circuit.

11. A method of transmitting a telephone signal supplied by a telephone over a single radio channel comprising the steps of: a) sampling the telephone signal and converting the samples into a corresponding digital signal; b) applying the digital signal to a compression/decompression circuit that reduces a number of binary digits occupied by the digital signal to create a compressed digital signal; c) creating a data packet that includes a portion of the compressed digital signal; d) transmitting the packet over the single radio channel to a remote transceiver; e) receiving a packet that has been transmitted from the remote transceiver; f) converting the packet received into a received, compressed digital signal; g) applying the received, compressed digital signal to the compression/decompression circuit to create a received digital signal having an increased number of binary digits than the received, compressed digital signal; h) converting the received digital signal to a received analog signal; and i) supplying the received analog signal to the telephone; wherein the steps of a)-d) are performed alternately with the steps e)-i) such that a duplex telephone signal is carried on the single radio channel.

12. A radio communications system for two-way transmission of duplex telephone signals on a single radio channel between a first telephone and a second telephone, comprising a first radio receiver coupled to the first telephone, a second radio transceiver coupled to the second telephone, each transceiver being operable in a transmit mode and in a receive mode and, when operating in the transmit mode, including: a) an analog to digital converter for sampling a telephone signal to be transmitted and for converting the sampled telephone signal into a first digital signal representative of the telephone signal; b) a compression circuit for converting the first digital signal into a compressed digital signal; c) a buffer circuit for storing the compressed digital signal; d) a modem circuit for receiving the compressed digital signal from the buffer circuit in the form of a plurality of sequential data packets; and e) a radio transmitter for transmitting the plurality of sequential data packets at intervals spaced in time; each radio transceiver, when operating in the receive mode, including: f) a radio receiver for receiving data packets transmitted by the other transceiver; g) a decoder circuit for converting the received data packets into a compressed received digital signal; h) a decompression circuit for converting the compressed received digital signal to an expanded digital signal representative of the telephone signal transmitted from the other transceiver; and i) a digital to analog converter circuit for converting the expanded digital signal into a corresponding analog signal representative of the telephone signal; and means for controlling and coordinating operation of the transceivers such that the first transceiver is in the transmit mode when the second transceiver is in the receive mode, and vice versa, such that data packets are alternately transmitted in opposite directions by the respective transceivers.