BACKGROUND TO THE INVENTION In conventional simplex operation one transceiver of a radio system transmits while another receives. Both single and dual frequency simplex systems exist. In conventional duplex operation two transceivers are able to transmit and receive at the same time. Two frequencies are generally required although some time division systems which use a single frequency have been proposed. In some cases the time division systems simply allocate half the bandwidth of an available channel to each transceiver at all times, but this reduces the quality of received signals. Others allocate bandwidth according to voice activity but suffer reduced quality caused by interruptions at those times when voice activity is present at each transceiver.

SUMMARY OF THE INVENTION It is an object of the present invention to provide new methods for duplex communication between mobile radios. In general terms the invention provides a method of radio communication in which a voice signal is transmitted from a first transceiver to a second transceiver, using a relatively low compression rate in the absence of a voice signal

transmitted from the second transceiver, and alternatively the voice signal is transmitted from the first transceiver to the second transceiver, using a relatively high compression rate in the presence of a voice signal transmitted from the second transceiver.

In one aspect the invention may broadly be said to consist in a method of operating a radio transceiver during a call to another radio transceiver including: mode A: (i) transmitting an outgoing voice signal at relatively low compression while receiving a synchronisation signal from the other transceiver, mode B: (i) alternately transmitting and receiving outgoing and incoming voice signals at relatively high compression while not receiving a synchronisation signal from the other transceiver, mode C: (i) receiving an incoming voice signal at relatively low compression while transmitting a synchronisation signal to the other transceiver.

In another aspect the invention may be said to consist in a method of synchronising a call between two radio transceivers including: transmitting an outgoing signal while receiving a synchronisation signal from the other transceiver, and receiving an incoming signal while transmitting a synchronisation signal to the other transceiver.

In a further aspect the invention may also be said to consist in a method of operating a radio transceiver including: transmitting an outgoing signal in a series of bursts, and generating each burst with a start portion derived from an end portion of the preceding burst.

The invention also consists in a radio transceiver which implements a method according to any one of the preceding aspects, and a system of radio transceivers which implement a method according to any one of the preceding aspects.

The invention may most broadly be said to consist in any alternative combination of parts or features mentioned in the specification or shown in the accompanying drawings. All equivalents of these parts or features are deemed to be included.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention will be described with respect to the drawings, of which: Figures 1 and 2 schematically show communication between two mobile radio units using single and dual frequency operation respectively, Figure 3 indicates a system having modes of apparently simplex and duplex communication between the radios, Figure 4 is a block diagram showing components of a radio that might implement the system shown in Figure 3, Figures 5 and 6 indicate how the radios transmit signals using relatively high and low compression rates respectively, Figure 7 indicates a waveform that may be used as a clock signal to synchronise transmission between the radios, Figures 8,9 and 10 indicate transmissions during setup, simplex and duplex communications, Figures 11 and 12 indicate how simplex and duplex communications with relatively high and low compression take place in accord with the clock, and Figure 13 indicates how two radios are synchronised by the clock signal.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring to these drawings it will be appreciated that the invention may be implemented in many forms within the scope of the claims, to enable quasi-duplex operation of radio transceivers. These preferred embodiments are described by way of example only. Many details relating to the operation of transceivers will be known to a skilled person and have not been given here.

Figure 1 shows two mobile radios A, B in direct communication by transmission over a single channel at frequency fl. Figure 2 alternatively shows two radios in communication

through a repeater station which transmits on a second frequency f2. The units are may be part of a larger radio system such as a trunked system having many mobile radios, with one or more dispatcher and repeater stations which automatically share multiple channels among users. They may be communicating voice or data signals. Each mobile radio incorporates transmitter and receiver circuits which share a common antenna. In conventional simplex operation, the transceiver at one radio transmits while the other receives, and the users take turns to transmit, perhaps by way of push-to-talk (PTT) switches. When pushed and held, a PTT switch temporarily transfers power and access to the antenna from the receiver to the transmitter. In conventional duplex operation, both transceivers are able to receive and transmit at the same time by way of a duplexer which allows both the receiver and transmitter to use the antenna together at different frequencies.

Figure 3 summarises operation of a system which permits apparent duplex operation by two radio transceivers A, B according to the invention, although neither radio physically transmits and receive signals at the same time. Operation depends on whether the radios detect voice activity by their respective users. The bandwidth of a single channel can be dynamically allocated as required by the voice acivity. Once a call between the radios has been set up, the voice signals are generally compressed at a high or low rate depending on whether one or both users are talking. A radio which does not detect voice activity by the respective user generally transmits a standard simple signal which serves to assist synchronisation. A radio which does detect speech activity by the local user, transmits a compressed voice signal, with the level of compression generally determined by whether or not the other radio is transmitting a synchronisation signal. A compressed voice signal is transmitted in bursts between either a synchronisation signal from the other radio, or between bursts of a compressed voice signal from the other radio. When both radios detect voice activity the operation involves alternate transmission and reception of bursts and can be performed sufficiently well to appear as a duplex communication to the users. When neither radio detects local voice activity, that with most recent voice activity, which has most recently been transmitting a compressed voice signal, continues to receive the synchronisation signal from the other, until both radios time out and the call is terminated.

A system as described in relation to Figure 3 therefore generally involves at least two rates or levels of compression for transmission of voice or data signals over a single radio channel. A relatively low rate is generally required for one-way or simplex communication, typically about 4/5 so that a voice frame of 100ms is transmitted from a radio in an interval of about 80ms, for example. This leaves an interval of about 20ms for alternate receipt of a synchronisation signal from the other radio using the single channel.

The quality of the voice signal after decompression is quite good. A relatively high rate of compression is generally required for two-way or duplex communication, typically about 2/5 so that the voice frame would be transmitted from one radio in about 40ms, for example. This leaves an interval of about 60ms for alternate receipt of a compressed voice frame from the other radio. Transitions between these modes of operation are preferably triggered by the presence or absence of a synchronisation signal or a related signal of some kind. During simplex operation the synchronisation signal is preferably transmitted by the radio which is receiving a voice or data signal from the other. During duplex operation it is possible to omit the synchronisation signal so long as timing accuracy can be reasonably maintained until a period of simplex operation. An overlap between compressed frames can be introduced to reduce the effect of a gradual loss of synchronisation during duplex operation, and also to generally reduce the likelihood of audio artefacts.

Figure 4 schematically shows parts of a radio transceiver which allows communication as described in relation to Figure 3. A central controller 10 is responsible for overall operation of the radio. A microphone 11 and speaker 12 enable audio interaction with a local user.

Amplifiers 13 and 14 amplify voice signals from the microphone and to the speaker respectively. A receiver circuit 15 and transmitter circuit 16 enable interaction with another similar radio and are connected to a common antenna 17. A synthesiser 18 is operated by the controller to generate a carrier frequency signal for the transmitter or receiver as required. The transmitter generally modulates the carrier signal with an outgoing voice related signal for transmission at the antenna, while the receiver mixes the carrier signal with a signal received at the antenna to recover an incoming voice related signal. Various arrangements are possible in this part of the radio and are only described here in a

simplified way. The radio may operate with single or dual frequencies for transmission and reception as mentioned above in relation to Figures 1 and 2. A call initialisation switch 19 can be manually operated by the user to set up or clear down a call with another radio.

A clock 20 generates a timing signal for various actions by the controller, particularly in relation to compression and decompression of voice signals.

In Figure 4, speech by a local user of the radio is picked up at the microphone 11 and passed through the amplifier 13. The controller monitors output from the amplifier through a detector 21 and thereby determines voice activity by the user. A voice signal from the amplifier is compressed at relatively high or low rates by compressor circuits 22 or 23 respectively. The controller operates switches 24 and 25 to engage either compressor, depending on whether or not a remote user of the other radio is also speaking. A signal generator 26 is used by the controller to provide a synchronisation signal in the absence of voice activity by the user. The controller operates switch 27 so that either a compressed voice signal or a synchronisation signal is passed by the transmitter 16 to the antenna 17 during a call to the remote user.

In Figure 4, an incoming signal from the antenna is processed by receiver circuits 15 and decompressed at relatively high or low rates by expandor circuits 28 and 29. The controller operates switches 30 and 31 to engage either expandor, depending on whether or not both users are speaking. A decompressed voice signal is passed to the speaker 12. A signal detector 32 is used by the controller to determine the existence of a synchronisation signal at the output of the receiver and thereby whether or not the remote user is speaking. In this example, both users are speaking and switches 24,25 and 30,31 are set for the high rate compressor 22 and expandor 28 respectively. No synchronisation signals are normally generated by either radio in this condition.

Figure 5 indicates a voice signal in uncompressed, compressed and decompressed states during a call between two radios, each generally of the kind described in relation to Figure 4. In this example, just one user is speaking, so the compression rate is relatively low.

Line 50 represents a flow of speech by the user into the microphone 11, as detected by the controller 10 and divided into a sequence of frames of generally equal length. Two full frames are shown in this example with frame N representing speech ahead of frame N+1.

Line 51 represents the result of compression of these frames by compressor 23. The compressor operates to form extended frames by joining each compressed frame from the voice signal with a portion E from the end of the previous frame. Time intervals 55 between frames of the signal which is then transmitted allow for reception of a synchronisation signal from the other radio. Line 52 represents reception of the compressed voice signal by the other radio, and sampling of the extended frames prior to decompression. The sampling process is timed to begin during the portion E of each extended frame and finish before the end of the frame. This reduces the likelihood of audio artefacts caused by inaccurate synchronisation between the radios, or by significant discontinuities in content of filters which form part of the receiver 15. Line 53 finally represents decompression by expandor 29 and a flow of speech from speaker 12 in the other radio.

Figure 6 also indicates a voice signal in uncompressed, compressed and decompressed states during a call between two radios, but with both users speaking so the compression rate is relatively high. Line 60 represents a flow of speech by one user into the microphone 11, as detected by the controller 10 and divided into a sequence of frames. Two full frames are shown in this example with frame N representing speech ahead of frame N+1. Line 61 represents the result of relatively high compression of these frames by compressor 22. As in Figure 5, the compressor operates to form extended frames by joining each compressed frame from the voice signal with a portion E from the end of the previous frame. Longer time intervals 65 between frames of the signal which is then transmitted allow for reception of a compressed voice signal from the other radio. Line 62 represents reception of the compressed voice signal by the other radio, and sampling of the extended frames prior to decompression. The sampling process is timed to begin during the portion E of each extended frame and finish before the end of the frame. Line 63 finally represents

decompression by the relatively high rate expandor 28 and a flow of speech from speaker 12 in the other radio.

Figure 7 indicates an internal system event signal created by the clock 20 in Figure 4. The signal preferably has a non-symmetric waveform as shown, with actions such as transitions between states of the transceiver taking place as required on rising or falling edges of the pulse pairs 70. Figures 8, 9,10 indicate different stages of a call between two radios A, B such as shown in Figures 1 and 2, based on a signal of this kind. Intervals of transmission and reception are indicated as TX and RX respectively. Timing of events during the call and synchronisation between the radios is initially determined according to the event signal of the radio which requests the call. While one or other user only is speaking through their radio, synchronisation is preferably determined by transmission of a synchronisation signal from the radio of the other user, who is listening. While both users are speaking, no synchronisation signal is transmitted by either radio, at least in this embodiment. In practice, intervals of time during which both users are speaking together are usually short, so with reasonable accuracy of their respective clocks, the radios remain sufficiently synchronised until a further time when one or other user only is speaking, and a synchronisation signal is again transmitted.

In Figure 8, both radios are initially idle until the user of radio A requests that a call be setup. The user presses a respective CIS 19 in Figure 4 and radio A begins a handshaking routine with radio B. The routine preferably involves transmission by radio A of a predetermined number of phase shift keyed (PSK) bursts from the synchronisation generator 26. These are received and counted at radio B through a respective synchronisation detector 32, which transmits an acknowledgment, typically a single PSK burst, then waits for detection of a carrier signal. Setup is completed by an adjustment of clock 20 by radio B to match the clock 20 in radio A according to the timing of the bursts.

Radio B then transmits a synchronisation signal while waiting for a voice signal from radio A. The synchronisation signal is preferably a single PSK burst transmitted in the TX

interval of radio B. Eventually the call will timeout if no voice signal is received from radio A.

In Figure 9, one user is speaking and the radios are in a simplex mode of communication where only one radio is transmitting a voice related signal. In this example, the user of radio A is speaking and the voice signal is transmitted in bursts with relatively low compression. The TX intervals for radio A and corresponding RX intervals for radio B are relatively long and nearly twice the period of the waveform of the clocks. Synchronisation signals are transmitted by radio B between receipt of these bursts, each being preferably a single PSK burst. The TX intervals for radio B and corresponding RX intervals for radio A are relatively short. Rectangular boxes between TX and RX intervals indicate the time required by the change between transmit and receive operations. If the user of radio A stops speaking and there is no transmission of a voice signal, radio B continues to transmit the synchronisation signal until a timeout or until either user actively clears the call. If the user of radio A continues to speak and the user of radio B begins to speak then radio B indicates a change to duplex mode by transmitting a predetermined number of PSK bursts.

In Figure 10, the users are speaking in a duplex mode of communication where both radios are transmitting voice related signals at relatively high rates of compression. The TX and RX intervals for each radio are equal and less than the period of the clocks, and there is no synchronisation signal from either radio. It can be seen that the compressed voice signals are not transmitted simultaneously, but rather alternately, in a manner which might be termed"half-duplex"in conventional terminology. Nevertheless because their voice signals are compressed, the users perceive duplex operation. If the user of either radio stops speaking then transmission of a synchronisation signal by that radio, instead of a voice signal, indicates a change to simplex mode in which the other user continues to speak.

The compression rate in the radio at which the single user is now speaking drops to a relatively low level as indicated in Figure 9.

Figures 11 and 12 show more detail of the simplex and duplex compression processes described in relation to Figures 5,9 and 6,10 respectively. Each figure represents the synchronisation signal produced by the event clock 20 in Figure 4, a sequence of frames or blocks N and N+1 representing speech or data input to the compressors 22 or 23, and a timing sequence for the transmit and receive intervals TX and RX. Overlap between the frames as described in relation to Figures 5 and 6 is also indicated, with part P at the end of each frame being sampled to provide part E in the previous figures.

Figure 11 represents simplex operation and the radio is transmitting a voice signal with relatively low compression. The content of frame N and an end portion P from the previous frame are compressed to be transmitted in the interval TX which is less than two periods of the clock signal. A synchronisation signal is received from the other radio in the intervals RX.

Figure 12 represents duplex operation and the radio is transmitting a voice signal with higher compression. The content of frame N and an end portion of the previous frame are compressed to be transmitted in the interval TX which is less than one period of the clock signal. A similarly compressed voice signal is received from the other radio during the intervals RX.

During simplex operation in a test embodiment, the period of the clock signal was 50ms, with Pl=5ms, P2=6ms, P3=84ms and the frames being 100ms long. The voice signal from the microphone 11 in Figure 4 was sampled at 8000/sec so that each frame produced 800 samples, and the end portions were set to include 40 samples. These 840 samples were then transmitted at 10,000/sec over P3, so that the effective compression rate was 84%.

During duplex operation, the sampling rate was lowered to achieve higher compression, with P4=34ms, P5=44ms and P6=lms being a small delay required to initiate a transmission interval. The signal from the microphone was sampled at 4000/sec and to produce 400 samples from each frame and end portions of 40 samples for the overlap.

These 440 samples were transmitted at 10, 000/sec over P6 so that the effective compression rate was 44%.

Figure 13 demonstrates synchronisation between two radios A, B during a simplex operation. Radio A transmits a compressed voice signal which is received by radio B during respective intervals TX and RX. Radio A receives a synchronisation signal transmitted by radio B during respective intervals RX and TX. Radio A expects to receive the synchronisation signal midway through the interval P2 of the clock cycle in Figure 11 as indicated by dashed line S 1. In this example the signal has been delayed as indicated by dashed line S2, so the controller 10 in radio A delays the clock to match, as indicated by arrow D.

Figure 14 is a possible embodiment of the synthesiser 18 in Figure 4, intended for use in a dual frequency transceiver. This synthesiser is based on a phase lock loop and is able to generate the two required frequencies for reception and transmission using the single loop.

A reference source 100 generates a reference frequency signal for the loop and a voltage controlled oscillator 101 generates an output signal having a frequency which is a multiple of the reference frequency. The output signal is either used as a carrier at one frequency for transmission of an outgoing signal from the transceiver, or as a local oscillator at another frequency for reception of an incoming signal from another transceiver, as required by the controller 10. The loop operates in the usual way except that the controller switches between two loop filters 102,103 in order to vary the frequency sufficiently rapidly from a carrier for the outgoing signal to a local oscillator for the incoming signal. Energy levels in the filters are thereby approximately maintained between bursts of transmission and reception during duplex and simplex operation of the transceiver. Various alternatives such as use of separate synthesisers for each of the frequencies are also possible but are more expensive in some cases.

In Figure 14, the phase lock loop also includes a generally standard synthesiser chip 104 having divider and comparator functions, and a mixer 105 that combines a compressed

voice signal with the loop signal, as required, in order to produce a phase modulated output from the VCO 101. A sequencer chip 106 is used to switch the loop between the transmit frequency filter 102 and the receive frequency filter 103, in time with changes to the action of the divider function, to avoid unintended phase shifts in the loop signal. The controller operates both the sequencer chip and the synthesiser chip so that changes between the transmit and receive frequencies are accurately coordinated.