WO2002037714A1

WO2002037714A1 - Method and apparatus to synchronize mobile radios

Info

Publication number: WO2002037714A1
Application number: PCT/EP2001/012583
Authority: WO
Inventors: Richard Charles Lucas; Jonathan Alastair Gibbs
Original assignee: Motorola Inc
Priority date: 2000-11-06
Filing date: 2001-10-29
Publication date: 2002-05-10
Also published as: GB0027062D0; GB2369267A; EP1334574A1; GB2369267B; AU2002219067A1

Abstract

A mobile radio (2) is adapted for direct two-way peer-to-peer radio communication with one or more other portable radios (4,6). The radio (2) may be a PMR radio, a Bluetooth radio, a 3G or 4G radio, or another radio communicating on an ad-hoc basis.The radio (2) comprises means (20) for entering into and awakening from a sleep mode, and means (20, 22, 24,26) for generating and transmitting synchronisation signals in a sleep mode. The synchronisation signals are received by one or more other radios (4,6) whereby the radios may become synchronised. When synchronised, the radio (2) may vary the mark: space ratio of its receive period, and may vary the frequency with which it transmits synchronisation signals. The variation may be dependent on the rate at which the radio (2) receives synchronisation signals from other radios (4,6).

Description

METHOD AND APPARATUS TO SYNCHRONIZE MOBILE RADIOS

Field of the invention

The present invention relates to mobile radios and a method of communication between such radios.

Background of the invention

Units for use in mobile communications are known in the art and referred to herein as mobile radios or mobile stations. The terms ^Λmobile radio' and mobile station (MS) ' are intended to include within their meaning mobile (including vehicle mounted) and portable radiotelephones and mobile radio communications units and the like. Systems and methods which provide radio communication between mobile radios are referred to herein as mobile radio communications systems and methods.

Mobile radios may be arranged to communicate with one another via base stations. This is known as trunked or cellular communication. Alternatively, or in addition (in an alternative operation mode) , such radios may be arranged to communicate with one another directly without the communication passing through a base station. This latter mode of communication is termed ^Λpeer to peer' or ^λdirect mode of operation' (DM0) .

Figure 1 of the accompanying drawings illustrates the general method of operation of a known radio system 10. Mobile radios 2 , 4 and 6 can communicate with a base station 8. Each of these radios can communicate through the base station 8 with one or more other mobile radios (not shown) . If the radios 2, 4 and 6 are capable of peer-to- peer operation, then they may communicate directly with one another or with other radios, without the communication link passing through the base station 8.

Mobile radios need to have appropriate size, weight, range and battery life. Prior art analogue two-way radio receivers have needed to be active continuously. This is in order to ensure that when a transmission from another radio commences, the receiver will capture the signal. However, such operation provides a significant ongoing load on the battery of the radio receiver.

Mobile radios are nowadays being designed to provide communications in digital, rather than analogue, form. For radios which operate in a peer-to-peer mode this provides the possibility of introducing sleep periods to save battery power consumption. Sleep is the time when a mobile radio is switched on but is powered down and not able to receive incoming calls, even for radios in direct communication with one another.

For mobile radios operating in a trunked or cellular system the system infrastructure can control the length of a sleep period, i.e. when a radio needs to be awakened to receive a call. Suitable sleep period control is not currently available in mobile radios which operate in a mode in which they communicate directly with one another. The purpose of the present invention is to address this shortcoming.

Summary of the Invention

According to the present invention in a first aspect there is provided a mobile radio adapted for direct two-way peer- to-peer radio comiaunica'ion with one or more other mobile radios, the radio including means for entering into and awakening from a sleep mode, and means for generating and transmitting synchronisation signals, wherein the means for generating and transmitting synchronisation signals is operable: (i) to transmit in the sleep mode synchronisation signals in randomly selected time slots, until the radio becomes synchronised with at least one other mobile radio; and then (ii) to transmit in the sleep mode synchronisation signals in time slots selected from those time slots when the at least one other mobile radio is in a receive mode.

The radio may become synchronised with at least one other radio when it receives a synchronisation signal from a radio in a pre-defined talk group to which it belongs or when another radio in that group receives a synchronisation signal from that radio.

The radio according to the first aspect may be further adapted to operate for an initial period after switch-on of the radio in a receive mode of continuously only receiving signals, and not transmitting synchronisation signals. This is to allow receipt of a synchronisation signal being sent from any other radio in the appropriate talk group . The initial period may suitably be between 10 and 30 seconds.

If the radio has not received any synchronisation signal from another radio at the end of this initial period, the radio may proceed to operate in a receive mode within a sleep cycle, in which signals are received from other radios in receive periods which are interrupted only in order to allow transmission of synchronisation signals by the radio, until the radio becomes synchronised with at least one other radio.

The radio according to the first aspect of the invention may be operable whereby, when the radio has become synchronised with at least one other radio, the means for entering into and awakening from a sleep mode causes the radio to enter into sleep times of variable duration.

The radio according to the first aspect of the invention may be operable whereby the means for entering into and awakening from a sleep mode:

(i) raises the proportion of time that is sleep time if the radio is receiving synchronisation signals from at least one other radio at a rate determined by the radio to be comparatively frequently; and

(ii) lowers the proportion of time that is sleep time if the radio is receiving synchronisation signals from at least one other radio at a rate which the radio determines to be comparatively inf equently; whereby the length of the receive period is dynamically variable dependent on the rate of reception of synchronisation signals from one or more other radios.

The radio according to the first aspect of the invention may be operable whereby, when the radio becomes synchronised with at least one other radio, the means for generating and transmitting synchronisation signals causes the radio to transmit synchronisation signals less often than prior to the establishment of synchronisation.

The means for generating and transmitting synchronisation signals may vary the probability that the radio will transmit a synchronisation signal in any one receive period for the at least one other radio, dependent on the rate at which the rate at which the radio receives synchronisation signals from the at least one other radio.

The radio according to the first aspect of the invention may be operable whereby, at times when the radio is synchronised with at least one other radio, the means for generating and transmitting synchronisation signals reduces the probability that the radio will transmit a synchronisation signal in any one receive period for the at least one other radio, as the rate of rate at which the radio receives synchronisation signals from the at least one other radio rises, thereby reducing the probability of the synchronisation signals from the radio and at least one other radio clashing.

The radio may become synchronised with at least one other radio when it receives a synchronisation signal from a radio in a predefined talk group to which it belongs or when a radio in that group receives a synchronisation signal from that radio.

In the radio according to the first aspect of the invention the means for entering into and awakening from a sleep mode may comprise a suitably programmed controller, e.g. digital signal processor. Such a controller may also be included in the means for generating and transmitting a synchronisation signals .

According to the present invention in a second aspect there is provided a method of communication between mobile radios wherein the mobile radios are radios according to the first aspect. Synchronisation by such radios may be achieved in a sleep mode by the radios transmitting synchronisation signals in randomly selected time slots until the radios becomes synchronised with one another; and then at least one one of the radios transmitting synchronisation signals in time slots selected from those time slots when at least one other of the radios is in a receive mode.

The invention provides a mechanism for peer-to-peer mobile radios to synchronise their sleep cycles by having individual radios sporadically transmit synchronisation signals. This beneficially enables savings in battery power consumption and battery life to be obtained by allowing shrinking of a required ^λawake and receiving window' when the mobile radio is active without significantly lengthening call set-up times. The invention is applicable in any digital mobile communications systems in which there is no pre-designated master device, and battery life is important. This is likely to have significant potential impact in all self-forming peer-to-peer networks. The invention may be applied in mobile radios for use in accordance with the known DIIS standard protocol. It may also useful in connection with mobile radios operable according to other standards such as Bluetooth (a standard procedure providing short range radio connectivity) and in the future 3G and 4G (third and fourth generation) mobile units .

Brief description of the drawings

Figure 1 is an illustration of a known arrangement of mobile radios and a base station communicating with one another .

Figure 2 is a block circuit diagram in simplified form illustrating a mobile radio embodying the present invention.

Figures 3 to 8 are signal timing graphs illustrating operation of a method embodying the present invention.

Figures 9A and 9B are respectively receive and transmit process flowcharts for a method embodying the present invention. Figure 10 are signal timing graphs for receiving and transmitting of signals by three radios using a method embodying the present invention.

Detailed description of embodiments of the invention

Figure 2 illustrates a mobile radio operable in accordance with an embodiment of the present invention. The radio 2 of ' Figure 2 can for example transmit signals representing speech from a user of the radio. The radio 2 includes a microphone 34 which provides an input signal for conversion into a corresponding RF signal for transmission by the radio. The signal from the microphone 34 is converted into a suitable RF signal by a transmission circuit (Tx) 22. The transmission circuit 22 transmits via a switch 24 and an antenna 26.

The radio 2 also includes a controller 20 and a read only memory (ROM) 32. The controller 20 may be a digital microprocessor. ROM 32 is a permanent memory, and may be a non-volatile Electrically Erasable Programmable Read Only Memory (EEPROM) .

The radio 2 of Figure 2 also includes a display 42 and keypad 44, which serve as part of the user interface components of the radio 2. Functions of the radio 2 may be activated by the user using the keypad 44 in a known manner. Voice activation of the radio, or other means of interaction with a user, may be employed in an alternative embodiment.

Incoming RF signals received by the antenna 26 of the radio 2 from another radio (e.g. radio 4 or 6 shown in Figure 1) are routed by the switch 24 to receiving circuitry (Rx) 28. From receiving circuitry 28, the received signals are routed to the controller 20 and audio processing circuitry 38. A loudspeaker 40 is connected to the audio processing circuitry 38. The loudspeaker 40 forms a further part of the user interface.

A data terminal 36 may be provided. Terminal 36 provides an interface by which signals comprising data (e.g. text data) for transmission by transmitter circuit 22, switch 24 and antenna 26 can be input.

The radio 2 is adapted for direct two-way peer-to-peer radio communication with one or more other mobile radios having a similar construction and operation. These may be radios 4 or 6 shown in Figure 1 communicating in a direct mode rather than via the base station 8.

The controller 20 allows the radio 2 to enter into, and to awake from, a sleep mode. In the sleep mode, the radio cannot receive signals from other radios. However, this sleep mode will substantially reduce the overall power consumption of the radio, thereby increasing the battery life.

The controller 20 controls the generation in the sleep mode of synchronisation signals which are sent in RF form by the transmission circuit 22 via the switch 24 and antenna 26. In operation these signals are generated and sent as follows:

(i) synchronisation signals are sent in randomly selected time slots, until radio 2 becomes synchronised with at least one of the other radios (e.g. 4 or 6); and then

(ii) synchronisation signals are further sent in time slots selected randomly from those time slots when the at least one other radio is in a receive mode.

This operation is illustrated further in Figures 3 to 5 explained as follows . In Figure 3, the operation of radio 2 is shown plotted against time t. For an initial, extended period of time, the radio only receives signals. Having not received any synchronisation signals from other radios, at time tl the radio ceases receiving and transmits a synchronisation signal. Immediately afterwards, the radio 2 returns to receive mode .

By time t2, the radio 2 has still not received a synchronisation signal from another radio. Therefore the radio transmits another synchronisation signal, at time t2.

In Figure 3, the radio 2 has not received any synchronisation signal from another radio at the end of the initial period, and so at tl has entered a receive mode signals, with interruptions to the periods of receiving signals only in order to allow the radio 2 to transmit synchronisation signals. This procedure will last until the radio becomes synchronised with at least one of the other portable radios .

In Figure 4 however, a different scenario is illustrated. The upper two traces in Figure 4 show the function of a radio ^ΛA' . The upper of these two traces, marked ^λRx , shows the receive activity of the radio A. Below it is a trace showing the transmission activity of radio A, marked as ^λTx' .

Radio A in Figure 4 is in receive mode for an initial extended period, until time tl, similarly to the operation shown in Figure 3. However, during this time period, radio A receives a synchronisation signal from a second radio, radio ^λB' . The transmission of this synchronisation signal is shown on the lowest trace of Figure 4, which illustrates the transmission activity of radio B. Radio B transmits the synchronisation signal at time t3.

When radio A reaches the end of the initial period at time tl, it reacts differently from the operation shown in Figure 3. Radio A transmits a synchronisation signal at time t4. Radio A has derived the time t4 from the synchronisation signal received from radio B. Time t4 corresponds to a period when radio A determines that radio B will be in receive mode.

Also shown in the upper trace of Figure 4 are the receive periods for radio A after time tl. These receive periods are separated by sleep periods . The receive periods coincide with periods when radio A determines that radio B may be sending synchronisation signals. Conversely, the sleep periods in the upper trace of Figure 4 are periods when radio A determines that no receiving synchronisation signals from radio B are expected.

Clearly radios A and B, whose operation is illustrated in Figure 4, become synchronised. Furthermore, during the sleep periods for radio A battery power is being saved.

As illustrated in Figure 4, radio B is able to receive the synchronisation signal transmitted at time t4 by radio A, aiding in the accuracy of radio B's synchronisation. Notably, the radios A and B have achieved a mutual synchronisation without intervention by fixed infrastructure, such as base stations. The timing of the receive slots used by radios A and B may in fact bear no relation to the timing used by parts of any communications infrastructure to which radios A and B may otherwise be permitted access, so radios A and B are acting independently. A mobile radio may be considered to have become synchronised with at least one other mobile radio when it receives a synchronisation signal from a radio in a talk group that it belongs to. The composition of such a talk group may have been predetermined between its members . The radio 2 may be programmed to operate in a known manner whereby signals from other members of the designated group , and only those signals, are recognised by the radio.

The procedure described with reference to Figure 4 relates to the achievement of synchronisation between two radios. However, a similar procedure can be used to provide synchronisation between more than two radios. This is illustrated later for example with reference to Figure 8. These radios will all then have synchronisation between their receive periods.

The reason for achieving synchronisation between radios is to allow the radios to be able to initiate speech or data communications to one another. Any request for a communication link from one radio to another can be made only when the radios to communicate are in a mutual 'receive' period. This is a period that a radio which initiates a call set-up request determines that the one or more radios with which it is synchronised will all be able to receive the request .

As can be seen in the upper trace of Figure 4 and in Figure 3, radio 2 is adapted to operate in a mode of continuously only receiving signals, and not transmitting synchronisation signals, for an initial period after switch-on of the radio. This initial period is preferably be of predetermined duration. This initial period allows the radio to listen for any synchronisation signals from other radios, particularly those related to an existing state of synchronisation that may already have been achieved by the talk group of that radio. Indeed, time tl may be selected to correspond generally to a time when it is likely that a radio will have detected any synchronisation that has already been established by other radios .

The initial period, shown in Figures 3 and 4 as the period prior to time tl, is preferably in the range of between 10 and 30 seconds.

Figure 5 shows another sequence of operation which may be applied in accordance with an embodiment of the present invention. Here radio A completes the initial receive only period at tl, without having received a synchronisation signal from any other radio. However, as radio A transmits its first synchronisation signal, a second radio, radio B, is in receive mode. Radio B may for example have just been switched on, and may have been switched on after radio A. In this case, radio B receives the synchronisation signal from radio A, and will then synchronise its timing to that of radio A. This is shown at the right hand side of Figure 5, where the radios have settled into mutual synchronisation. At the right hand side of Figure 5, radio A has a relatively short receive period, with radio A in sleep mode before and after the receive mode, and radio B sends a synchronisation signal in what it knows to be the receive period of radio A.

Dynamic sleep periods

The three sleep periods between Rx signals that are shown at the right hand side of the series of signals in the upper trace of Figure 4 are of a constant length determined by the controller 20 (Figure 1) . These periods may alternatively be of variable duration. Figures 6 and 7 show variation in the length of the receive period of a mobile radio that has established a state of synchronisation with at least one other mobile radio.

In Figure 6, the receive period has a length or duration M. The sleep period has a duration S. The total time M+S can be considered as the length of one 'sleep cycle' .

In Figure 7, the receive period has become shorter than that in Figure 6, and the sleep period has become longer. Hence the 'mark: space' ratio M/S has been reduced, although the total sleep cycle duration is the same as that in Figure 6.

Figures 6 and 7 illustrate a general principle of how the receive duration may be varied. In accordance with an embodiment of the present invention, the mobile radio may operate with a reduced receive period length at times when it is receiving synchronisation signals relatively frequently. This could correspond to a time when the radio is in synchronisation with a large number of other radios. When the radio is in synchronisation with fewer radios and receives fewer synchronisation signals, it may increase the length M of its receive period.

In particular therefore, the controller 20 which controls entering into and awakening from a sleep mode in the radio 2 embodying the present invention may be arranged to: (i) raise the proportion of time that is 'sleep' if the radio is receiving synchronisation signals from other radios comparatively frequently; and

(ii) lower the proportion of time that is 'sleep' if the radio is receiving synchronisation signals from other radios comparatively infrequently; whereby the mark: space ratio M/S of the receive periods is dynamically variable dependent on the rate of reception of synchronisation signals from other radios. Timing of transmissions when synchronised

When radio 2 becomes synchronised with at least one other radio 4 or 6, the controller 20 controlling the transmission of synchronisation signals may adjust the rate of issue of synchronisation signals, causing the radio 2 to transmit synchronisation signals less often than prior to the establishment of synchronisation.

Furthermore, the controller 20 may vary the probability that the radio 2 will transmit a synchronisation signal in any one of the 'receive' periods of the radio 2, dependent on the rate at which the radio 2 receives synchronisation signals from other radios.

The controller 20 may reduce the probability that the radio 2 will transmit a synchronisation signal in any one 'receive' period for the other radios, as the rate at which the radio 2 receives synchronisation signals from other radios rises, thereby reducing the probability of the synchronisation signals from two or more radios clashing, i.e. being co-incident.

Figure 8 illustrates some aspects of the timings of receive periods and synchronisation transmissions in a 'steady state' . This is a state in which synchronisation has been achieved between a given group of radios, without any further radios being in the act of joining the group, or any having left the group recently.

Figure 8 shows the receive periods and instants of transmission of synchronisation signals for four radios A, B, C, D. The four traces correspond respectively to the radios A,B,C and D as labelled in Figure 8. As shown in Figure 8, all four radios have synchronised the timing and width of their receive periods .

As shown in Figure 8, Radio D transmits a synchronisation signal at time t5. This synchronisation signal will be received by radios A, B and C. Radios A, B and C may use this synchronisation signal in a calculation to determine the width of future receive periods .

At time t6 in Figure 8, radio A transmits a synchronisation signal. The other radios will receive this.

Notably, the timings of synchronisation signals at t5 and tβ may be entirely random. Here 'random' means that the timing of the synchronisation signals is selected to fall within a receive period, but that the particular receive period is chosen at random.

As shown in Figure 8, all four radios operate with a probability of transmitting a synchronisation signal in any one receive window that is small enough to ensure a very low probability of two or more of the four radios transmitting synchronisation signals in the same receive window. This reduces the chances of a clash of two synchronisation signals.

However, the radios must still transmit synchronisation signals sufficiently often for the four radios A, B,C and D to maintain synchronisation. The frequency of transmission required for this depends on, mainly:

(i) the rate of drift of the timing of each radio;

(ii) the width of each receive period.

If a radio determines that it has lost synchronisation, it may extend significantly the duration of each receive period. This increase in the mark: space ratio M/S will increase the drain on the battery. However, the radio will then be able to detect a synchronisation pulse from another of the radios, if they are still in range and transmitting synchronisation signals . A radio may be adapted to decide that it has lost synchronisation when it has not received any synchronisation pulses for a predetermined period of time.

As the number of synchronised radios in a group rises, the number of opportunities for any one radio to re-synchronise by receiving a synchronisation signal rises . Thus the radios can achieve a tight synchronisation, using only a narrow receive window.

If the group of four radios whose operation is illustrated in Figure 8 were expanded to include, say, ten radios, then the probability of a clash of synchronisation signals would rise. Thus, to maintain a constant probability of clashes occurring, each radio can (by its controller corresponding to controller 20 shown in Figure 2) reduce the frequency with which it transmits synchronisation signals as the number of synchronised radios in the group increases. Each radio is aware that the number of radios is increasing, because it detects more synchronisation signals.

Numerical example for PIIS

An embodiment of the invention is described as follows, with numerical examples of signals timings. The embodiment is described with reference to the DIIS protocol for digital PMR radios. However, the invention is applicable to other radio systems, as explained above, for which other signal timings would be substituted for those given below. The timings below enable a quantitative appreciation of the advantages of the invention. The DIIS protocol provides a channel status signalling opportunity at least every 1440 ms . This may be as often as once per 360 ms . Radios embodying the present invention can use this signalling mechanism. This signalling mechanism allows some degree of trade-off between sleep duty cycle and call set-up times.

Assume that a full 20 ms time slot needs to be monitored by a radio in receive mode. In this case, typical savings would be of the order of:

(i) 22/1440 M 65:1 for a 1.44 second maximum call setup. This would correspond to an average call set up time of 720 ms. (ii) 22/720 * 33:1 for a 720 ms maximum call set-up. This would correspond to an average call set up time of 360 ms.

(iii) 22/360 « 16:1 for a 360 ms maximum call set-up. This would correspond to an average call set up time of 180 ms.

For a group of mobile radios, there could be a small but significant penalty in terms of transmitting the synchronisation patterns in the manner described earlier. This would be up to 20ms transmission time, "Tx" mode, and 20ms extra receive time every 15 seconds, approximately. This may reduce the gains given above to 50:1 for a 1.44 second maximum call set-up (720 ms average), 30:1 for a 720 ms maximum call set-up (360 ms average) and 15:1 for a 360 ms maximum call set-up (180 ms average) .

For the purposes of applying this embodiment of the present invention, a channel status message may have one or both of two functions in a sleep cycle: (i) sleep cycle synchronisation. (ii) call set-up .

In accordance with this embodiment of the invention, a DIIS mobile radio in a peer-to-peer two-way radio network is allowed to send sleep cycle synchronisation signals. However, in order to allow call set-ups from other radios, the radio must listen immediately prior to sending. If anything is heard then the radio suspends transmission of the sleep cycle synchronisation, and the portable radio receives instead.

Algorithm flowcharts for receive Rx and transmit Tx modes of this DIIS embodiment are illustrated in Figures 9A and 9B respectively.

Referring to Figure 9A, from the time when the radio is switched on, the radio goes into receive-only mode continuously for several seconds in the manner described earlier. This may be of the order of 15 to 30 seconds. The receive mark: space ratio is 1:1. If, during this period, the radio receives no synchronisation signal (channel-status) from another radio within the correct group or fleet, then the receiver timing is not changed. If the receiving radio receives a synchronisation signal from another radio its receive and transmit timing sequences are adjusted to match that of the received signal. The on-time mark: space ratio of the receive window is reduced, and the radio continues to receive these broadcasts from the other radio. The rate of arrival of synchronisation signals (Rx arrival rate) from other radios is monitored. If the mark: space ratio is now determined to be too low it is increased. If an incoming call detected as a call set-up request signal is detected the process stops and active call set-up begins. If not the radio continues in a sleep mode monitoring incoming synchronisation signals and if appropriate adjusting the receive period mark-space ratio.

Reference is now made to Figure 9B. At the end of the initial (15-30 second) receive-only period, whether synchronised or not, the radio begins to transmit synchronisation signals for its group/fleet. This transmission is initially at a very low rate. This transmission occurs randomly, using the radio's nominal timing. The transmission is such as to have a low probability of transmission during any particular time period. If the radio is already synchronised, then the radio will transmit the synchronisation signals at the same time as other units are expecting them to arrive.

If multiple signals are transmitted by more than one radio, they may interfere. If this occurs, the signals from all radios will probably be lost. Therefore, if a radio does not receive synchronisation messages from its group/fleet for several frames, then it could be as a result of either too many competing synchronisation signals being sent from various radios, or insufficient synchronisation signal being sent.

The radio therefore monitors the number of received synchronisation messages and keeps a record of how many other radios are sending synchronisation messages, in order to estimate how many radios are in the group/fleet. In this way it can make a decision whether its contribution of synchronisation signals is at an appropriate rate or whether the rate needs adjustment.

If a rate adjustment is required, then a gradual adjustment, either up or down, may be made by the radio to the probability of transmitting. The aim is to ensure that both an individual radio, and other radios, will receive sufficient synchronisation signals to ensure that they maintain their synchronisation within acceptable limits over time. This must be done sufficiently accurately to take into account oscillator drift, temperature, Doppler and propagation range, whilst minimising battery drain and thereby maximising the battery life of all radios.

If insufficient synchronisation signals are received, then it will be necessary to broaden the receive period (receiver on-time) in order to ensure that call set-ups when sent will be received by the radio. This will, of course, unfortunately reduce the potential savings in battery life.

The radio will listen for a 'synch suspend' message. Such a message will be sent when another radio issues a call set-up request signal. When such a signal is received the sleep mode and transmission of synchronisation signals is suspended and the radio goes into active receive mode to establish receipt of a call. If such a message is not sent, the radio continues in sleep mode by transmitting synchronisation signals as earlier in the process shown in Figure 9B.

Computer simulation

Figure 10 illustrates the results of a computer simulation of three radios operating in accordance with an embodiment of the invention, in a 'live' communications scenario in the field.

The upper three traces in Figure 10 show the receive modes of three radios A, B and C. Radios A, B and C are initially unsynchronised. The lower three traces in Figure 10 show the transmit modes of the three radios A, B and C.

As can be seen at the time labelled t7, radio A transmits a synchronisation signal. Both radios B and C are in receive mode at time t7, since they have yet to achieve synchronisation. The receive traces for radios B and C show both radios moving into synchronisation shortly after time t7, in response to the transmission by radio A. Notably, the receive periods of radios B and C become synchronised with each other, and also become shorter.

Claims

1. A mobile radio (2) adapted for direct two-way peer-to- peer radio communication with one or more other mobile radios (4, 6), the radio (2) including means (20) for entering into and awakening from a sleep mode, and means (20, 22, 24, 26) for generating and transmitting synchronisation signals, wherein the means for generating and transmitting synchronisation signals is operable: (i) to transmit in the sleep mode synchronisation signals in randomly selected time slots, until the radio becomes synchronised with at least one other mobile radio (4, 6) ; and then (ii) to transmit in the sleep mode synchronisation signals in time slots selected from those time slots when the at least one other mobile radio (4, 6) is in a receive mode.

2. A radio (2) according to claim 1, the radio being further adapted to operate for an initial period after switch-on of the radio in a receive mode of continuously only receiving signals, and not transmitting synchronisation signals.

3. A radio (2) according to claim 2 and which is operable whereby, if the radio has not received any synchronisation signal from another radio at the end of the initial period, the radio operates in a receive mode, in which signals are received from other radios, in receive periods which are interrupted only in order to allow transmission of synchronisation signals, until the radio becomes synchronised with at least one other radio.

4. A radio (2) according to claim 2 or claim 3 and wherein the initial period is in the range of 10 to 30 seconds.

5. A radio (2) according to any one of the preceding claims and which is operable whereby, when the radio (2) has become synchronised with at least one other radio (4, 6) , the means (20) for entering into and awakening from a sleep mode causes the radio to enter into sleep times of variable duration.

6. A radio (2) according to claim 5 and which is operable whereby the means (20) for entering into and awakening from a sleep mode:

(i) raises the proportion of time that is sleep time if the radio is receiving synchronisation signals from at least one other radio at a rate determined by the radio to be comparatively frequently; and (ii) lowers the proportion of time that is sleep time if the radio is receiving synchronisation signals from at least one other radio at a rate which the radio determines to be comparatively infrequently; whereby the length of the receive period is dynamically variable dependent on the rate of reception of synchronisation signals from one or more other radios.

7. A radio (2) according to any one of the preceding claims, and which is operable whereby, when the radio (2) becomes synchronised with at least one. other radio (4, 6), the means (20, 22, 24, 26) for generating and transmitting synchronisation signals causes the radio to transmit synchronisation signals less often than prior to the establishment of synchronisation.

8. A radio (2) according to any one of the preceding claims, and which is operable whereby, at times when the radio (2) is synchronised with at least one other radio (4, 6), the means (20, 22, 24, 26) for generating and transmitting synchronisation signals varies the probability that the radio will transmit a synchronisation signal in any one receive period for the at least one other radio, dependent on the rate at which the rate at which the radio receives synchronisation signals from the at least one other radio .

9. A radio (2) according to claim 8, and which is operable whereby, at times when the radio (2) is synchronised with at least one other radio (4, 6) , the means (20, 22, 24, 26) for generating and transmitting synchronisation signals reduces the probability that the radio will transmit a synchronisation signal in any one receive period for the at least one other radio, as the rate of rate at which the radio receives synchronisation signals from the at least one other radio rises, thereby reducing the probability of the synchronisation signals from the radio and at least one other radio clashing.

10. A radio (2) according to any one of the preceding claims, and which is operable whereby the radio becomes synchronised with at least one other radio (4, 6) when it receives a synchronisation signal from a radio in a predefined talk group to which it belongs .

11. A radio according to any one of the preceding claims and wherein the means (20) for entering into and awakening from a sleep mode comprises a programmed controller which is a digital signal processor.

12. A radio according to claim 11 and wherein the controller (20) is also included in the means for generating and transmitting synchronisation signals.

13. A radio according to any one of the precding claims and which is operable according to the DIIS standard protocol.

14. A method of communication between mobile radios wherein the mobile radios are radios according to any one of the preceding claims and wherein synchronisation is achieved in a sleep mode by the radios generating and transmitting synchronisation signals in randomly selected time slots until the radios becomes synchronised with one another; and then one of the radios transmitting synchronisation signals in time slots selected from those time slots when at least one other of the radios is in a receive mode.