DIGITAL RADIO AND DIGITAL RADIO COMMUNICATION SYSTEM

Abstract: The digital radio (2) can transmit return signalling, and/or transmit interrupt signalling. In order to calculate the times for transmitting this signalling, the digital radio uses (20) knowledge of both the delay of a repeater in the radio communications system (2-8), and the settling time for its own transitions between receive and transmit modes of operation. The invention also comprises a digital radio communications system (2-8) with a repeater (8) which braodcasts information about signal delay within it. The information minimises information loss caused by return signalling or transmit interrupt signalling in a digital radio communications system (2-8).

Description: Digital Radio and Digital Radio Communication' System Technical Field The present invention relates to the field of digital radios and digital radio communication systems. Background Two-way' digital portable and mobile radios may be arranged either to communicate with one another via base stations, or directly with one another in 'direct mode'. The communication will typically either be over a digital simplex communication channel, or over a digital semi-duplex communication channel. Figure 1 illustrates the general scheme of a personal mobile radio (PMR) 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 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 direct mode operation, then they may communicate directly with one another or with other radios, without the communication link passing through base station 8. Henceforth in this description, the term 'mobile station' (MS) will be used for both portable- and mobile digital radios. A mobile station may employ a regular payload framing structure. This structure consists of a structure of fixed time points at which parts of the communication begin and end. Such a payload framing structure may allow either 'transmit interrupt' and/or 'reverse signalling' to occur. Transmit interrupt and reverse signalling time periods are periods when a transmitting radio briefly stops transmitting, thereby allowing the transmitting radio to receive a signal from another radio. Henceforth in this description, the term 'return signalling' will be used for 'transmit interrupt' and/or 'reverse signalling . Prior art radio systems have various arrangements that allow receipt of return signalling from a listening radio. The problem to be solved by this invention may occur, in general, with any radio system that needs to allow return signalling. However, the problem is explained below by reference to the Transmitter Interrupt / Reverse Signalling scenario for a DIIS radio system. In particular, the invention is explained in relation to 'Peer to Peer' DIIS systems employing a re- clocking repeater. The invention is however not limited to the DIIS system. The system considered below consists of mobile stations. In particular, MS 'A' and MS 'B' are in communication together through a repeater. The repeater is referred to as 'Re' in the figures. The mobile stations and the repeater may either be in transmit mode, that is referred to as Tx' in the figures, or be in receive mode, that is referred to as 'Rx' in the figures. In figure 2, mobile station A is in the process of transmitting a payload of information to mobile station B via a re-clocking repeater Re, with regular gaps for possible Transmitter Interrupt (Tl) or Reverse Signalling (RS). Assumptions made in figure 2 are as follows: • An MS sending Reverse Signalling should not lose any of the payload, and these transmissions should not cause loss of the payload to other listeners. • An MS sending transmitter interrupts for Emergency purposes does not care if it misses the payload, it only wishes to maximise the probability of the transmitter interrupt. • An MS using transmitter interrupts for Non-Emergency purposes does not generally wish to lose payload, and can therefore be treated exactly the same as an MS sending reverse signalling. • Re-clocking repeaters have a "propagation delay" of up to 7ms. In the future, this delay is likely to be reduced by improved technology and implementation processes, but will never be eliminated. • The time allowed for Transmitter Interrupt/Reverse Signalling is 2 timeslots: 40ms. • It is desirable that Reverse Signalling and Transmitter Interrupt signalling telegrams are of a standard 107 symbol length and are preceded by a Full Synchronisation sequence (FSS) and 7 pilot symbols. • Maximum Path delay for radio transmission over 75km is 0.25ms • Ramping and settling times specified in the working assumptions: Tx Ramping up and Down - 6 symbols Rx. Settling - 1 symbol Rx to Tx & Tx to Rx settling time, including ramping, - 5.5ms The operation of a mobile station with this arrangement of return signalling is shown in appended figure 2. In figure 2, signalling from MS A, received at MS B, is delayed by a maximum of 2 X the path delay plus the repeater delay. This is a total of 7.5 ms, See point A on figure 2. If no signal is to be lost, then this is the earliest point at which MS B can start to switch from Rx to Tx, which takes 5.5ms. The first symbol from MS B will now be delayed by the repeater and path delays of 7.5ms before it reaches MS A receiver, 20.5ms after the start of the TI/RS period. See point B on figure 2. The length of the transmission from B to A is governed by the need for MS A to switch back to Transmit in order to resume its payload transmission. This must happen no later than 5.5ms before the end of the TI/RS period. Therefore the maximum transmission length is 40- (20.5 +5.5)ms = 14ms which equals 84 symbols. If we now deduct the 30 symbol FSS and 7 symbol pilot sequence from these 84 possible symbols, we have 84-30-7 = 47 symbols only available for signalling. The above total of 47 symbols of signalling falls well short of the desired 107 symbols. This invention therefore addresses this problem, as can be seen from the detailed description which follows Other prior art arrangements are known. For example, the down-link channels in the GSM and Tetra systems provide advance timing parameters to subscribers. This is controlled by the base-station. A need exists to alleviate the problems of the prior art. Summary of the Invention In accordance with the invention, there is provided a digital mobile station. The digital mobile station is for use in a digital radio communications system that allows return signalling and incorporates at least one repeater. The digital mobile station of the invention comprises: means for signal transmission; means for signal reception; means for calculating a time range for transmitting return signalling, the means for calculating the time range being adapted to: (i) receive or derive information concerning a signal delay in the repeater; and (ii) receive or derive information concerning the settling times of the digital mobile station between transmit (Tx) and receive (Rx) operation, and between receive (Rx) and transmit (Tx) operation; (iii) calculate the time range so as to minimise loss of information. The means for calculating the time range for transmitting return signalling may be adapted to calculate, as the earliest possible time for transmitting return signalling, the time at which the mobile station transmitting a payload is just able to receive the last symbol of the TI/RS prior to commencing the transition between receive (Rx) and transmit (Tx) operation. The means for calculating the time range for transmitting return signalling may be adapted to calculate, as the latest possible time for transmitting return signalling, the time at which the digital mobile station can just receive the last symbol of the payload prior to commencing the transition between receive (Rx) and transmit (Tx) operation. The means for calculating the time range for transmitting return signalling may assume a worst-case value for the settling times, between transmit (Tx) and receive (Rx) operation, and between receive (Rx) and transmit (Tx) operation, of the mobile station transmitting the payload. Further in accordance with the invention, there is provided a digital radio communications system. The digital radio communications system allows return signalling, and comprises: (i) a plurality of mobile stations; and (ii) at least one repeater, the repeater broadcasting information about the signal delay in the repeater; one or more of the mobile stations being adapted to use the information received from the repeater about the signal delay in the repeater to calculate the timing of return signalling. The invention optimises transmit interrupt signalling and/or reverse signalling in a digital radio and a digital radio communications system. Brief description of the drawings Figure 1 illustrates an arrangement of portable and/or mobile radios and a base station. Figure 2 illustrates a signalling sequence which may occur in a digital radio system. Figure 3 illustrates a mobile station in accordance with the present invention. Figures 4-6 illustrate signalling sequences which may occur with mobile stations and digital radio communication systems in accordance with the invention. Detailed description of the preferred embodiment In the broadest sense, the invention solves the problems of prior art digital radios and radio systems employing Transmitter Interrupt / Reverse Signalling, by allowing the MS that is sending the Transmitter Interrupt / Reverse Signalling, MS B, to: (i) compensate for the additional timing delay introduced by the repeater; (ii) accommodate future improvements in repeater delays; and (iii) prioritise the impact of positioning its signalling in such a way that it either misses the last part of the payload, or the MS that is transmitting the payload misses the last part of its TI/RS signalling. In doing so, this invention can also ensure interoperability with the MS transmitting the payload, by assuming the worst case Rx to Tx & Tx to Rx settling times for the MS transmitting the payload. The invention provides a mechanism for minimising the information loss resulting from the combined effect of a repeater delay and mobile station Rx to Tx & Tx to Rx settling time, when transmitting Transmitter Interrupt / Reverse Signalling within a Peer to Peer type Network employing a regular Superframe Structure. The invention relies on the repeater broadcasting its delay, and the mobile station transmitting the TI/RS being aware of its own Rx to Tx & Tx to Rx settling time. Based on these two figures, the Mobile Station transmitting the TI/RS then calculates a range across which it may advance the TI/RS transmission in order to control and minimise the information loss. The earliest possible position corresponds to the point at which the Mobile transmitting the Payload is just able to receive the last symbol of the TI/RS, prior to it commencing the Rx-Tx transition. The latest possible position corresponds to the point at which the mobile station transmitting the TI/RS is just able to receive the last symbol of the payload, prior to it commencing the Rx-Tx transition. In order to improve interoperability, the Rx to Tx & Tx to Rx settling time of the mobile station transmitting the payload is here assumed to be "worst case". For worst case at one extreme, the mobile station transmitting the TI/RS will miss the last part of the payload. For DIIS, this is 10ms. For worst case at the other extreme, the mobile station transmitting the payload will miss the last part of the TI/RS. For DIIS, this is also 10ms. Within these two extremes, the mobile station transmitting the TI/RS is able to optimise the trade-off between loss in payload and potential for success of the TI/RS. This takes into account the relative importance of the payload vs. TI/RS integrity, and the ability to mitigate the losses to some extent by the use of techniques such as interleaving, redundancy and erasures etc. Importantly, an advantage of this invention is that it allows the mobile station to optimise the trade-off between payload and TI/RS in line with any future improvements in repeater and mobile station performance. So, if a repeater with a lower delay time becomes available in future, the invention will be able to take advantage of this. Figure 3 illustrates a mobile station in accordance with the present invention. The mobile station of figure 3 may be either a portable- or a mobile digital radio. The radio 2 of figure 3 can transmit speech from a user of the radio. The 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. Radio 2 also has a controller 20 and a read only memory (ROM) 32. 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 3 also comprises a display 42 and keypad 44, which serve 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. 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 would provide a signal comprising data for transmission by transmitter circuit 22, switch 24 and antenna 26. In general terms, the digital radio communication system of the invention has the same overall configuration as the system shown in Figure 2. The repeater of the invention may broadly resemble the base station shown in Figure 1 as element 8, although a repeater may not have all the features of a full base-station. The operation of an embodiment of a digital mobile station with transmit interrupt signalling is shown in appended figure 4. Further signalling schemes in accordance with the invention are shown in appended figures 5 and 6. The invention is described below with further reference to the DIIS example cited in the background portion of the description. The rules below can best be recognised by making reference to appended figures 4-6. In a DIIS arrangement, the invention incorporates the following rules: 1. All re-clocking repeaters will broadcast information about their delay. For DIIS this is in symbol periods. 2. An MS will be required to "know" its own Tx-Rx-Tx delay parameters. 3. Maximum Path Delay will always be assumed. For DIIS, this is 0.25ms. 4. An MS making an emergency Transmitter Interrupt will calculate the latest possible time to start transmission, from the repeater broadcast delay parameter and its own Tx-Rx-Tx delay parameters. See Figure 4, point A. This will both ensure no impact on other listeners on the channel, and ensure the minimum loss of payload and the maximum probability of success for the Tl. 5. The default for Reverse Signalling is the same as for Transmitter Interrupts See Figure 4. 6. Reverse Signalling is permitted to be delayed, so that some of the payload from MS A and some of the Reverse Signalling is lost, as shown in Figure 5. This may have advantages if faster repeaters and mobiles are in use, and this permits a small number of erasures being used to recover both MS A and MS B signalling. An MS is permitted to use intermediate cases between those shown in figures 4 and 5, to optimise the balance between received payload and signalling performance. 7. An MS is NOT permitted to delay the start of its transmissions later than that shown as point A in Figure 6. Performance of the Invention In the case of the DIIS example, the loss of payload/signalling is related to the MS and RE delays by: Loss (ms) = 2*(RE delay) + (MS Rx to Tx Delay) - 9.5 With "worst case" MS & RE, the loss of payload in the default situation is 10ms, and therefore highly unlikely to make the last payload timeslot recoverable. However the situation improves rapidly with improvements in RE performance. The examples in table 1 below illustrate this. Table 1 : Delay Examples Clearly example 5 predicates no loss of information. Examples 2, 3 & 4 are probably recoverable, by exploiting the properties of the error correction code. This is especially likely if the MS chooses to split the overlap into two as shown in Figure 5, or some intermediate value. Where the overlap is as per Figure 4, then the loss of payload associated with the above delays expressed as a percentage of the overall interleaving depth is as shown in table 2 below. Table 2: Loss of Payload Examples

Claims: Claims 1. A digital mobile station for use in a digital radio communications system, the digital radio communications system allowing return signalling and comprising at least one repeater, the digital mobile station comprising: means (22, 24, 26) for signal transmission; means (26, 24, 28) for signal reception; means (20) for calculating a time range for transmitting return signalling, the means for calculating the time range being adapted to: (i) receive or derive information concerning a signal delay in a repeater; and (ii) receive or derive information concerning the settling times of the digital mobile station between transmit (Tx) and receive (Rx) operation, and between receive (Rx) and transmit (Tx) operation; (iii) calculate the time range so as to minimise loss of information. 2. A digital mobile station in accordance with claim 1 , whereby the means for calculating the time range for transmitting return signalling is adapted to calculate, as the earliest possible time for transmitting return signalling, the time at which the mobile station transmitting a payload is just able to receive the last symbol of the TI/RS prior to commencing the transition between receive (Rx) and transmit (Tx) operation. 3. A digital mobile station in accordance with claim 1 or claim 2, whereby the means for calculating the time range for transmitting return signalling is adapted to calculate, as the latest possible time for transmitting return signalling, the time at which the digital mobile station can just receive the last symbol of the payload prior to commencing the transition between receive (Rx) and transmit (Tx) operation. 4. A digital mobile station in accordance with any previous claim, whereby the means for calculating the time range for transmitting return signalling assumes a worst-case value for the settling times, between transmit (Tx) and receive (Rx) operation, and between receive (Rx) and transmit (Tx) operation, of the mobile station transmitting the payload. 5. A digital radio communications system (2-8), the digital radio communications system allowing return signalling, the digital radio communications system comprising: (i) a plurality of mobile stations (2-6) ; and (ii) at least one repeater (8), the repeater broadcasting information about the signal delay in the repeater; one or more of the mobile stations (2) being adapted to use the information received from the repeater about the signal delay in the repeater to calculate the timing of return signalling. 6. A digital mobile station or a digital radio communications system substantially as hereinbefore described with reference to, or as illustrated by, any of figures 3,4,5 or 6 of the drawings.

Inventors: FENTON SHAUN RICHARD, GIBBS JONATHAN ALASTAIR, PHILLIPS CHRISTOPHER FRANK, FENTON, SHAUN, RICHARD, GIBBS, JONATHAN, ALASTAIR, PHILLIPS, CHRISTOPHER, FRANK
Applicants: FENTON SHAUN RICHARD, GIBBS JONATHAN ALASTAIR, MOTOROLA INC, PHILLIPS CHRISTOPHER FRANK, FENTON, SHAUN, RICHARD, GIBBS, JONATHAN, ALASTAIR, PHILLIPS, CHRISTOPHER, FRANK
Application year: 2001
Main classification: H04Q7
IPC codes: H04B7