GB2369267A

GB2369267A - Radio communication system

Info

Publication number: GB2369267A
Application number: GB0027062A
Authority: GB
Inventors: Richard Charles Lucas; Jonathan Alastair Gibbs
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2000-11-06
Filing date: 2000-11-06
Publication date: 2002-05-22
Anticipated expiration: 2020-11-06
Also published as: GB2369267B; AU2002219067A1; GB0027062D0; EP1334574A1; WO2002037714A1

Abstract

A portable radio (2) is adapted for direct two-way peer-to-peer radio communication with one or more other portable radios (4, 6). Portable 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 portable radio (2) comprises means (20) for entering into and awakening from a sleep mode, and means (20, 22, 24, 26) for transmitting synchronisation signals. The synchronisation signals are received by one or more other radios (4, 6), whereby the radios become synchronised. When synchronised, the portable 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 portable radio (2) receives synchronisation signals from other radios (4, 6).

Description

2369267. À..

Radio Communication System Technical Field

5 The present invention relates to the field of radio communication systems. In

particular, the invention relates to digital radio communications systems comprising a plurality of mobile or portable radios.

Background

Two-way' digital portable and mobile radios may be arranged to communicate with one another via base stations. Alternatively or in addition, the radios may be arranged to communicate directly with one another without the communication passing through a base station. This latter mode of 15 communication is termed 'Peer to peer'.

Figure 1 illustrates the general scheme of a PIER radio system 10. Portable radios 2, 4 and 6 of figure 1 can communicate with a base station 8. Radios 2, 4 and 6 could equally well be mobile radios mounted in vehicles. Each of the 20 radios shown in figure 1 can communicate through base station 8 with one or more other radios. If 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 base station 8.

25 Portable and mobile radios need to have appropriate size, weight, range & 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 30 of the radio receiver.

Recently, the 'Bluetooth' standard has introduced a class of radio device with the capability of short range communication. Devices communicating in accordance with the Bluetooth standard may communicate typically over a 35 range of up to 10 metros. These devices may in fact do more than simply transmit speech, and may vary enormously in function. For example, household and industrial electronic appliances might be equipped with Bluetooth two-way radio communication links. These devices might typically exchange data over the Bluetooth link, for example, about their operation or

r À ,, position. Here Bluetooth is understood to be an evolving standard, and the present invention is not limited in its applicability to devices using the

current Bluetooth standard.

5 Finally, future'3G' end '4G' radio communication devices may be capable of direct communication, without the communication link passing through fixed infrastructure.

Henceforth, the term 'portable radio' will be used to describe both mobile 10 and portable- radios, devices which are able to communicate via the Bluetooth standard, 3G and 4G radio communication devices, and other ad-hoc' radio systems.

Turning to existing cellular telephone and paging system receivers, these 15 receivers do not need to be constantly in operation. This is because the infrastructure of these systems can predict when a receiver will be active.

Hence significant improvements in battery life are possible for these products. 2 0 Two-way portable radios are now being built to provide digital, rather than analogue, communication. This digital communication may now provide a possibility of introducing synchronized 'sleeping patterns', sleep being the time when a portable radio is powered down and not able to receive incoming calls, even for portable radios in direct communication with one 2 5 another. Such synchronization might be analogous to the way that cellular and paging systems operate. This operation would provide a trade-off, providing longer battery life due to the reduction of power consumption during 'sleep' intervals, at the cost of longer call set-up times.

30 There are particular problems associated with any form of 'sleep' mode in the infrastructure-less scenario of portable radios communicating directly with one another. The present invention addresses some of these issues.

Prior art cellular and paging systems synchronise the sleep cycles of mobile

3 5 telephones and pagers with the use of broadcasts. These broadcasts come from the infrastructure, consisting typically of a central controller and base stations. However, portable radio units do not have dedicated infrastructure to act as a synchronization master when they are in direct communication with one

_, r r r I, another. In addition, there may be different numbers of units in communication, or in a group, from one time to the next. This is different from cellular telephone systems, where communication is normally between only two telephones, and is normally via the infrastructure.

Any algorithm to provide for'sleep periods' for radio units therefore needs to: (i) be able to function without synchronization provided by a fixed infrastructure; (ii) be robust to changes in the number of units from power-up; and (iii) allow a radio unit to be powered-up at a time when existing units are already 10 synchronized and spending a significant proportion of time sleeping.

Summary of the Invention

In accordance with a first embodiment of the invention, a portable radio 15 comprises the features of appended claim 1.

In accordance with a second embodiment of the invention, a portable radio comprises the features of appended claim 11.

20 Portable radios of the invention my comprise the features of dependent claims 2-10 and 12-14.

Brief description of the drawings

25 Figure 1 illustrates an arrangement of portable and/or mobile radios and a base station. Figure 2 illustrates a portable radio in accordance with the present invention.

30 Figures 3-8 illustrate signal timing diagrams for embodiments of the present invention. Figures 9a and 9b show respectively possible receive and transmit flowcharts for the present invention.

Figure 10 shows the results of a computer simulation of three radios in accordance with the invention.

lo; Detailed description of the preferred embodiment

In its broadest form, the invention is a mechanism for peer to peer radios to synchronise their sleep cycles by having individual units transmit sporadically.

5 This enables savings in battery life by shrinking the required 'awake receiving window' without lengthening call set up times. The preferred ways of achieving this are outlined below. Notably, the invention requires neither an infrastructure, such as base stations, nor does it require that one particular radio be designated as a 'master' radio, having authority over other radios.

Figure 2 illustrates a radio in accordance with the present invention. The radio of figure 2 is a portable radio as defined in the 'Background' section above.

Portable radio 2 of figure 2 can transmit speech from a user of the radio. The 15 radio comprises a microphone 34 which provides a signal for transmission by the radio. The signal from the microphone is transmitted by transmission circuit 22. Transmission circuit 22 transmits via switch 24 and antenna 26.

The transmitter 2 also has a controller 20 and a read only memory (ROM) 32.

20 Controller 20 may be a 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 comprises a display 42 and keypad 44, which serve 25 as part of the user interface circuitry of the radio. At least the keypad 44 portion of the user interface circuitry is activatable by the user. Voice activation of the radio, or other means of interaction with a user, may also be employed.

Signals received by the radio are routed by the switch to receiving circuitry 28.

30 From there, the received signals are routed to controller 20 and audio processing circuitry 38. A loudspeaker 40 is connected to audio circuit 38.

Loudspeaker 40 forms a further part of the user interface.

A data terminal 36 may be provided. Terminal 36 provides a signal comprising 35 data for transmission by transmitter circuit 22, switch 24 and antenna 26.

... - n , , .,I,. 5 _ The portable radio of the invention is adapted for direct two-way peer-to-peer radio communication with one or more other portable radios. These may be portable radios 4 or 6 shown on Figure 1.

5 Means 20 allow the portable radio to enter into, and 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.

10 Elements 20, 22, 24, 26 of portable radio 2 allow transmission of synchronisation signals, whereby, in operation: (i) means 20-26 transmit synchronisation signals in randomly selected time slots, until portable radio 2 becomes synchronized with at least one of the other portable radios; and then 15 (ii) means 20-26 transmit synchronisation signals in time slots selected randomly from those time slots when the at least one other portable radio is in a receive mode.

This function can be seen from appended figures 3-5.

In figure 3, the operation of portable 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 t1 the radio ceases receiving and transmits a synchronisation signal. Immediately 2 5 afterwards, the radio returns to receive mode.

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

30 In figure 3, portable radio 2 has not received any synchronisation signal from another radio at the end of the initial period, and so at t1 has entered a mode of receiving signals, with interruptions to the periods of receiving signals only in order to allow the radio to transmit synchronisation signals. This mode will last until the portable radio becomes synchronized with at least one of the other 35 portable radios.

In figure 4 however, a different scenario is illustrated. The upper two traces on figure 4 show the function of a radio 'A'. The upper of these two traces, marked

r r Rx', shows the receive activity of radio A. Below it is a trace showing the transmission activity of radio A, marked as 'Tx'.

Radio A in figure 4 is in receive mode for an initial extended period, until time t1, 5 similarly to the operation shown in figure 3. However, during this time period, radio A receives a synchronisation signal from a second radio, radio 'B'. 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 point t1, 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 15 when radio A knows that radio B will be in receive mode.

Also visible in the upper trace of figure 4 are the receive periods for radio A after time point t1. These receive periods are separated by sleep periods. The receive periods coincide with periods when radio A knows that radio B may be 2 0 sending synchronisation signals. Conversely, the sleep periods in the upper trace of figure 4 are periods when radio A does not anticipate receiving synchronisation signals from radio B. Clearly, radios A and B in figure 4 are synchronized. Furthermore, the sleep 25 periods for radio A mean that radio A is saving battery power at these times.

Radio B in figure 4 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 have achieved a mutual synchronisation without intervention by fixed 3 0 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 here.

35 A portable radio may be considered to have become synchronized with at least one of the other portable radios when it receives a synchronisation signal from a radio in a talk group that it belongs to.

' ',...

r.. : The scheme outlined above relates to the achievement of synchronisation between two radios. However, this scheme can result in synchronisation between more than two radios. This is illustrated in later figures. These radios will all then have some degree of 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 calls to one another. Any request for a communication link from one radio to another can now be made during the mutual 'receive' period. This is a period that the initiating radio knows that the 10 one or more radios with which it is synchronized will all be able to receive the request. As can be seen in the upper trace of figure 4 and in figure 3, portable radio 2 may be adapted to operate in a mode of continuously only receiving signals, 15 and not transmitting synchronisation signals, for an initial period after first switch-on of the radio. This initial period may 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 radio's talk group.

2 0 Indeed, time point t1 corresponds generally to a time point 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 up until t1, may be in 2 5 the range of 10-30 seconds.

Figure 5 shows another sequence of operation which may arise in accordance with the present invention. Here radio A completes the initial receive only period at t1, without having received a synchronisation signal from any other radio.

30 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 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 to the right of figure 5, where the radios have settled 35 into mutual synchronisation. At the right 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 radio A's . receive period.

r - At,:: -

8.... Dynamic sleep periods The three sleep periods that were shown at the upper right of figure 4 are of a 5 length determined by the means for entering into and awakening from a sleep mode 20. These periods may in fact be of variable duration.

Figures 6 and 7 show variation in the length of the receive period of a portable radio that has established a state of synchronisation with at least one other 10 portable radio.

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

15 In figure 7, the receive period has become shorter then that in figure 6, and the sleep period has become longer. Hence the 'mark:space' ratio M/S has 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 20 vary. In accordance with the present invention, the portable radio may operate with a reduced receive duration at times when it is receiving synchronisation signals relatively frequently. This would correspond to a time when the portable radio is in synchronisation with a large number of other radios. When the portable radio is in synchronisation with fewer radios and receives fewer 2 5 synchronisation signals, it may increase the length m of its receive period.

in particular therefore, the means 20 for entering into and awakening from a sleep mode in portable radio 2 of the present invention may be arranged to: (i) raise the proportion of time that is 'sleep' if the portable radio is receiving 3 o synchronisation signals from other portable radios comparatively frequently; and (ii) lower the proportion of time that is 'sleep' if the portable radio is receiving synchronisation signals from other portable radios comparatively in- frequently; whereby the mark:space ratio M/S of the receive periods is dynamically variable in dependence on the rate of reception of synchronisation signals from other 35 portable radios.

r: 7 _

,. _...

Timing of transmissions when synchronized When portable radio 2 becomes synchronised with at least one other portable radio 4 or 6, the means 2026 for transmitting synchronisation signals may 5 cause the radio to transmit synchronisation signals less often than prior to the establishment of synchronisation.

Furthermore, the means 20-26 for transmitting synchronisation signals may vary the probability that the portable radio will transmit a synchronisation signal in 10 any one of the radios' 'receive' periods, in dependence on the rate at which the portable radio receives synchronisation signals from other portable radios.

The means 20-26 for transmitting synchronisation signals may reduce the probability that the portable radio will transmit a synchronisation signal in any one 'receive' period for the other radios, as the rate of rate at which the portable 15 radio receives synchronisation signals from other portable radios rises, thereby reducing the chances of the synchronisation signals from two or more portable radios clashing.

Figure 8 illustrates some aspects of the timings of receive periods and 20 synchronisation transmissions in the 'steady state'. This is the state where synchronisation has been achieved between a given group of a number of radios, without any more radios being in the act of joining the group, or any having left the group recently.

25 Figure 8 shows the receive periods and instants of transmission of synchronisation signals for four radios, A, B. C, D. In figure 8, all four radios have synchronised the timing and width of their receive periods.

Radio D in figure 8 transmits a synchronisation signal at time t5. This 30 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 35 radios will receive this.

Notably, the timings of synchronisation signals at t5 and t6 may be entirely random. Here 'random' means that the timing of the synchronisation signals is

c -... -

. r selected to fall within a receive period, but that the particular receive period is chosen at random.

In figure 8, all four radios operate with a probability of transmitting a 5 synchronisation signal in any one receive window that is low 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.

10 Notably however, the radios must still transmit synchronisation signals sufficiently often for radios A-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 portable radio discovers 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 2 0 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.

Second embodiment of the Invention In a second embodiment of the invention, a portable radio 2 is adapted for direct two-way peer-to-peer radio communication with one or more other portable radios, the portable radio comprising means 20 for entering into and awakening from a sleep mode, these means being adapted to vary the proportion of time 3 0 that the portable radio spends in the sleep mode in dependence on the number of synchronisation signals received from other portable radios 4, 6.

This facet of the invention is clearly derivable from the discussion above concerning figures 6 and 7.

Means 20-26 of the portable radio 2 may transmit synchronisation signals at a rate varying in dependence on the number of synchronisation signals received from other portable radios 4, 6. This has been discussed in relation to figure 8.

_., ' -

r C The means 20 for entering into and awakening from a sleep mode may be adapted to increase the proportion of time that the portable radio spends in the sleep mode as the rate of receiving synchronisation signals from other portable 5 radios rises, and to decrease the proportion of time that the portable radio spends in the sleep mode as the rate of receiving synchronisation signals from other portable radios falls. This relationship can be understood with reference to figures 6 and 7. As the number of synchronized radios in a group rises, the number of opportunities for any one radio to re-synchronise by receiving a 10 synchronisation signal rises. Thus the radios can achieve a tight synchronisation, using only a narrow receive window.

The means for transmitting synchronisation signals 20-26 may be adapted to decrease the rate of transmitting synchronisation signals as the rate of receiving 15 synchronisation signals from other portable radios rises, and to increase the rate of transmitting synchronisation signals as the rate of receiving synchronisation signals from other portable radios falls. This can be understood in relation to figure 8. If the group of four radios in figure 8 were to expand to say, ten radios, then the chances of a clash of synchronisation signals would 2 0 rise. Thus, to maintain a constant probability of clashes occurring, each radio will 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 DIIS One embodiment of the invention is described below, with numerical examples of signals timings. The embodiment is described with reference to the DIIS 30 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.

35 The DIIS protocol provides a channel status signalling opportunity at least every 1440 ms. This may be as often as once per 360 ms. The present invention can use this signalling mechanism. This signalling mechanism

-: tI:: allows some degree of trade-off between sleep duty cycle and call set-up times. Assume that a full 20ms time slot needs to be monitored by a radio in 5 receive mode. In this case, typical savings would be of the order of: (i) 22/1440 65:1 for a 1.44 second maximum call set-up. 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.

0 (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 small fleet of portable radios, there could be a small but significant penalty in terms of transmitting the synchronization patterns. This would be 15 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 the present invention, the channel status message may have one or both of two functions in this sleep cycle: (i) Sleep cycle synchronization.

(ii) Call set-up.

In accordance with the invention, a DIIS portable radio in a peer-to-peer two way radio network is allowed to send sleep cycle synchronization messages. However, in order to allow call set-ups from other portable radios, the portable radio must listen immediately prior to sending. If 30 anything is heard then the portable radio suspends transmission of the sleep cycle synchronization, and the portable radio receives instead.

Possible algorithm flowcharts for Rx and Tx are illustrated in Figures 9 a) and b) respectively.

From the time when a radio is switched-on, the portable radio unit goes into receive-only mode continuously for several seconds. This may be of the order of 15-30 seconds. If, during this period, the portable radio receives a synchronization signal (channel-status) from another portable radio within

- Hi- r: : : : - - ' r the correct group or fleet, then the receiver timing is adjusted to match that of the received signal. Additionally, the on-time mark:space ratio of the receive window is reduced, and the unit continues to receive these broadcasts. At the end of the initial (15second) receive-only period, whether synchronized or not, the portable radio begins to transmit synchronisation signals for it's group/fleet. This transmission is initially at a very low rate.

10 This transmission occurs randomly, using the portable radio's nominal timing. The transmission is such as to have a low probability of transmission during any particular time period. If the portable radio is already synchronized, 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 fleet for several frames, then it could be as a result of either too many being 2 0 sent from competing units, or insufficient messages being sent.

A radio therefore monitors the number of synchronisation messages and a radio keeps track of how many radios are sending synchronisation messages, in order to get an idea of how many radios are in the group/fleet.

25 In this way it can make a decision whether it's contribution to the synchronisation task is of the correct magnitude.

If not, then a gradual adjustment in the probability of transmitting, either up or down, may be made by the radio. The aim is to ensure that both an 30 individual mobile station (MS), and other MSs, will receive sufficient timing signals to ensure that they maintain their timing within acceptable limits over time. This must be done sufficiently accurately to take into account oscillator drift, temperature, Doppler and propagation range, whilst maximising the battery life of all units.

If insufficient synchronisation signals are received, then it will be necessary to broaden the receiver on-time in order to ensure that call set-ups will be received by the MS. This will reduce the potential savings in battery life.

:.:: À. Computer simulation Figure 10 illustrates the results of a computer simulation of three radios in 5 accordance with 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 unsynchronized.

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 point labelled t7, radio A transmits a synchronization 15 signal. Both radios B and C are in receive mode at time t7, since they have yet to achieve synchronization. The receive traces for radios B and C show both radios moving into synchronization shortly after time point t7, in response to the transmission by radio A. Notably, the receive periods of radios B and C become synchronized with each other, and also become 2 o shorter.

The present invention may be applied to any digital communications systems where there is no master, and battery life is important. This has potential impact in all self-forming peer to peer networks. This invention 2 5 provides an improvement in battery life, which is one of the top required improvements in portable communications equipment.

Claims

_. ' - - Claims

1. A portable radio (2), the portable radio being adapted for direct twoway peer to-peer radio communication with one or more other portable radios (4, 6), the portable radio (2) comprising means (20) for entering into and awakening from a sleep mode, the portable radio (2) further comprising means (20, 22, 24, 26) for transmitting 10 synchronisation signals, whereby the means for transmitting synchronisation signals are adapted to, in operation: (i) transmit synchronisation signals in randomly selected time slots, until the portable radio becomes synchronized with at least one of the other portable radios (4, 6); and then 15 (ii) transmit synchronisation signals in time slots selected randomly from those time slots when the at least one other portable radio (4, 6) is in a receive mode.

2. A portable radio (2) in accordance with claim 1, the portable radio being further adapted to operate in a mode of continuously only receiving signals, and 2 0 not transmitting synchronisation signals, for an initial period after first switch-on of the radio, the initial period preferably being of predetermined duration.

3. A portable radio (2) in accordance with claim 2, whereby, if the portable radio has not received any synchronisation signal from another radio at the end of the 25 initial period, the portable radio is adapted to operate in a mode of receiving signals, with interruptions to the periods of receiving signals only in order to allow the portable radio to transmit synchronisation signals, until the portable radio becomes synchronized with at least one of the other portable radios.

30

4. A portable radio (2) in accordance with any of claims 2-3, wherein the initial period is in the range of 10-30 seconds.

5. A portable radio (2) in accordance with any previous claim, whereby, when the portable radio (2) has become synchronized with at least one other portable 35 radio (4, 6), the means (20) for entering into and awakening from a sleep mode causes the radio to enter into sleep periods of variable duration.

À r À ,

6. A portable radio (2) in accordance with claim 5, whereby the means (20) for entering into and awakening from a sleep mode: (i) raises the proportion of time that is 'sleep' if the portable radio is receiving synchronisation signals from other portable radios comparatively frequently; and 5 (ii) lowers the proportion of time that is 'sleep' if the portable radio is receiving synchronisation signals from other portable radios comparatively in-frequently; whereby the mark:space ratio of the receive periods is dynamically variable in dependence on the rate of reception of synchronisation signals from other 0 portable radios.

7. A portable radio (2) in accordance with any previous claim, whereby, when the portable radio (2) becomes synchronized with at least one other portable radio (4, 6), the means (20, 22, 24, 26) for transmitting synchronisation signals 15 causes the radio to transmit synchronisation signals less often than prior to the establishment of synchronisation.

8. A portable radio (2) in accordance with any previous claim, whereby, at times when the portable radio (2) is synchronised with at least one other portable 2 0 radio (4, 6), the means (20, 22, 24, 26) for transmitting synchronisation signals varies the probability that the portable radio will transmit a synchronisation signal in any one 'receive' period for the at least one other radio, in dependence on the rate at which the portable radio receives synchronisation signals from the at least one other radio.

9. A portable radio (2) in accordance with claim 8, whereby, at times when the portable radio (2) is synchronised with at least one other portable radio (4, 6), the means (20, 22, 24, 26) for transmitting synchronisation signals reduces the probability that the portable radio will transmit a synchronisation signal in any 3 0 one 'receive' period for the other radios, as the rate of rate at which the portable radio receives synchronisation signals from other portable radios rises, thereby reducing the chances of the synchronisation signals from two or more portable radios clashing.

10. A portable radio (2) in accordance with any previous claim, whereby the portable radio becomes synchronized with at least one of the other portable

- . A : radios (4, 6) when it receives a synchronisation signal from a radio in a talk group that it belongs to.

11. A portable radio (2), the portable radio being adapted for direct twoway 5 peer-to-peer radio communication with one or more other portable radios (4, 6), the portable radio (2) comprising means (20) for entering into and awakening from a sleep mode, the means (20) for entering into and awakening from a sleep mode being adapted to vary the proportion of time that the portable radio 10 spends in the sleep mode in dependence on the number of synchronisation signals received from other portable radios (4, 6).

12. A portable radio (2) in accordance with claim 11, the portable radio (2) further comprising means (20, 22, 24, 26) for transmitting synchronisation 15 signals, the means for transmitting synchronisation signals being adapted to vary the rate of transmitting synchronisation signals in dependence on the number of synchronisation signals received from other portable radios (4, 6).

13. A portable radio (2) in accordance with claim 11 or claim 12, the means (20) for entering into and awakening from a sleep mode being adapted to increase the proportion of time that the portable radio spends in the sleep mode as the rate of receiving synchronisation signals from other portable radios rises, and to decrease the proportion of time that the portable radio 2 5 spends in the sleep mode as the rate of receiving synchronisation signals from other portable radios falls.

14. A portable radio (2) in accordance with any of claims 11-13, 3 0 the means for transmitting synchronisation signals being adapted to decrease the rate of transmitting synchronisation signals as the rate of receiving synchronisation signals from other portable radios rises, and to increase the rate of transmitting synchronisation signals as the rate of receiving synchronisation signals from other portable radios falls.

15. A portable radio substantially as hereinbefore described with reference to, or as illustrated by, any of figures 2-10 of the drawings.