GB2354910A - Re-clocking repeater facilitating transmission of overhead signalling in specific timeslots - Google Patents

Abstract

In a mobile radio system including a plurality of mobile radios A, B and a re-clocking repeater R, processing delays associated with the use of the repeater may cause message signals 51-53, 55-59 to overrun into signalling timeslots 35, 36, leaving insufficient time for a mobile to transmit overhead signalling 61 (e.g. late entry telegrams (LE), forward channel signalling (FCH), reverse channel signalling (RCH) and transmit interrupts (TI)) preceded by a synchronization signal. Thus, in order to facilitate the transmission of signalling 61 in specific timeslots 35, 36, the re-clocking repeater R of the invention is adapted to insert a sync sequence, pulse or known transmit pattern S2 into its transmissions. As described, mobile A sends a message 50 in slot 34 which is retransmitted 52 by the repeater R to mobile B, if mobile B does not send a signal 55,56, the repeater realises this within a certain time period and begins transmitting its own synchronisation signal S2. Since mobile A has not received any message from mobile B in slot 35, it transmits signalling 61 but does not need to transmit a synchronization preamble since repeater R appends it own synchronization S2 to the signalling 63 for transmission. Relevant to low cost digital professional mobile radio (PMR) standard known as DIIS.

Description

2354910 MOBILE RADIO Field of the Invention This invention relates to

mobile radio. In particular, it relates to a digital mobile radio system and preferably, but not exclusively, to a professional mobile radio (PMR) system.

Background to the Invention The European Telecommunications Standard Institute (ETSI) is currently developing a low cost digital PMR standard known as DHS. This system proposes a super-frame structure which is the format by which data is sent once a call has been established. Typically, a cycle of 36 time slots is proposed, as shown in Figure 1. It is necessary to take into account several factors in the design of the frame structure. These are symmetry, minimum signalling overhead and flexibility in the choice of vocoder. Referring to Figure 1, coded voice (i.e. data) is transmitted in slots 1 to 16 and 19 to 34. Slots 17/18 and 35/36 are used for signalling overhead. Each slot is of 20 ms. The types of signals which may be required to be transmitted in the signalling overhead slot 17/18 and 35/36 may be, for example, late entry telegrams (LE), Forward Channel Signalling (FCH), Reverse Channel Signalling (RCH) and Transmit Interrupts (TI). A late entry telegram allows radios, which were out of range during the initial establishment of a group call, for example, to be brought into communication as they move into the coverage area. Reverse channel signalling (RCH) has several uses, including, for example, instructions to a radio to vary its transmission power. Forward channel signalling (FCH) may be used in response to a power control's instruction. Transmit interrupts (TI) may allow a listening radio to interrupt the current transmission.

In an analogue system, with continuous tone code squelch (CTCSS) the "late entry" time is, typically, 200 to 250 ms. In the proposed digital system, if late entry were transmitted only once in a super-frame of length 720 ms (36 x 20 ms), then the average time for late entry would be 360 ms. By sending the late entry telegram LE twice, this time reduces to 180 ms, better than for an analogue system. If there is no interrupting transmitter (an occurrence which will normally be relatively infrequent), then the second LE telegram is sent in slot 36.

Radios may operate on simplex channels, in which the transmission for a first radio A is directly received at a second radio B. Alternatively, they may work through a repeater in which the signal from station A is received by the repeater and then retransmitted by the repeater from where it is received by second radio B. The repeater may be a simple repeater or a re-clocking repeater, as is known in the art.

For radios operating on simplex channels or working through a simple repeater, the time available for switching and symbol detection during the two slot, 40 ms signalling periods is adequate for transmission of the signalling telegrams. However, a problem arises if a re-clocking repeater is used.

If a re-clocking repeater is used, then there is an additional processing delay in the repeater of 5 ms due to the need to demodulate the partial response 4GFSK modulation scheme and this leaves insufficient time (10. 5 ms) in which to send the LE telegram, which must be preceded by a 5 ms synchronisation sequence, in the second slot (i.e. slot 17 or slot 36).

A possible solution to this problem would be to increase the length of the time slot. This is, however, greatly undesirable because of constraints dictated by the need for flexibility in choice of vocoder and other system based issues. An alternative possible solution is to provide three slots for the signalling. This is again greatly undesirable because of the increase in overhead.

The present invention arose in an attempt to provide a solution to the above problems which does not reduce flexibility and does not result in an increase in overheads.

Brief Summary of the Invention According to the present invention in a first aspect there is provided a mobile radio system, including a plurality of mobile radios and a re- clocking repeater, wherein the repeater is adapted to insert a sync sequence or pulse into its transmissions.

According to the present invention in a second aspect there is provided a re-clocking repeater for a mobile radio system, which is adapted to insert a sync sequence or a known pseudo-random pattern or word.

According to the present invention in a further aspect there is provided a mobile radio system, including a plurality of mobile radios, a reclocking repeater wherein the re-clocking repeater comprises means for generating a sync sequence or pulse if it detects that a mobile radio has not transmitted a signal within one or more predetermined time slots.

According to the present invention in a further aspect there is provided a method for enabling signalling to be transmitted within predetermined time slots of a mobile radio system, comprising enabling a re-clocking repeater to generate sync sequences or pulses if it detects that a mobile radio has not generated a transmission within a certain time slot.

Description of the Drawin2s

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows the super-frame structure of a professional mobile radio system; Figure 2 shows part of a radio system; Figure 3 shows schematically a portable mobile radio; Figure 4 shows transmission and reception sequences; and Figure 5 shows a block diagram of a repeater. 5 Detailed Description of Preferred Embodiments of the Invention

A professional mobile radio system according to the DIIS standard has been developed within an ETSI sub-committee and comprises a plurality of mobile radios, of which only two are shown in the Figure, A and B. These communicate via a base station or repeater R which acts as a booster. The repeater R receives the signal from A for example at a first frequency and then retransmits it at a second frequency for reception by radio B. Similarly, when radio B. transmits, the signal is received by repeater R and then transmitted to radio A.

Repeater R may be a simple repeater or may be a re-clocking repeater. In embodiments of the invention, the repeater is a re-clocking type repeater.

Figure 3 illustrates a mobile 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.

The radio transmitter 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 Programmable Read Only Memory (EEPROM). ROM 32 contains various different regions of memory, which are explained below in connection with Figures 3 and 4. RAM 33 is a random access memory which is used for the short-term storage of data.

The radio 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 will generally be provided. Terminal 36 provides a signal comprising data for transmission by transmitter circuit 22, switch 24 and antenna 26.

A repeater may have the following construction:

An antenna 86 receives signals at frequency fl, which are fed by a duplexer 84 to a receiver 88. Signals at frequency f2 are generated by a transmitter 82 which are fed to the antenna through the duplexer 84. The output of the receiver 88 is connected to a controller 80 which, in turn, controls a switch 85. In one position of the switch the receiver 88 is connected to the transmitter for retransmission of the received signal. In the other position of the switch, the output of the controller 80 is connected to the transmitter 82 to insert a sync sequence or pulse into the retransmissions.

The controller 80 is coupled to an interface 84 as well as a read only memory 82 and a random access memory 83.

Transmission and reception sequences in embodiments of the invention are shown by way of example only in Figure 4. This shows slots 35 and 36 of a super-frame, together with the tail end of the previous slot 34.

Assume first that mobile radio A is transmitting. In slot 34, radio A transmits a message 50. This is received at the repeater as received message 51 after a propagation delay of 0.25 ms. The message is subsequently retransmitted by the repeater at 52 after a processing delay shown as 5.3 ms. The message is finally received by radio B at 53 after a ftnther propagation delay of about 0.25 ms. Note that although radio A stopped transmitting at the beginning of slot 35, it is well into slot 35 until radio B receives the end of the transmission.

There are now two alternatives. Either mobile station B commences transmitting within the signalling slots, or it does not. Assuming first that it does begin transmitting, then, after a delay 54 of about 5 ms to switch between reception and transmission modes, station B begins transmitting first a standard sync sequence or pulses 55. It subsequently transmits voice or other data 56. The repeater receives the sync information 55 as received sync message 56 after a propagation delay. After a processing delay in the receiver, it retransmits this at 57 and this sync message is subsequently received by transmitter A at 58.

Note that in the Figure towards the right-hand side there are two column bars shown for the repeater transmit (Tx) and receive (Rx). The top ones of these (1) relate to the situation in which a signal is transmitted by radio B. The bottom ones (II), to be discussed below, relate to the situation in which radio B does not transmit a signal.

Mobile station A thereby receives the synchronisation message 58 transmitted by station B and subsequently receives the message 59 (corresponding to message 56). Radio A must receive the whole of sync pattern 58 (55) before it can assume that mobile station B has commenced sending a message.

Delays such as 54 represent the time taken for a mobile radio to change from reception to transmission mode. Typically, this is around 5 ms.

All the various delay mechanisms mean that virtually all (or even more than) one time slot is inevitably used. It then becomes less tenable for A to use the slot at all if prior art systems are used since A must insert a sync message before transmitting any signalling information.

We now turn to the situation in which radio B does not begin transmitting a message in slot 35. The repeater R clearly knows, before radio A, that no message has been received from B, since A would inevitably receive the message after the repeater. Thus, if no message from transmitter B is received in slot 35, then, once the time for receiving any sync signal such as 55 has passed, the repeater knows that B has not commenced transmitting. Accordingly, it begins transmitting its own sync sequence or pulses S2. Radio A, having not received any message from radio B within first slot 35, after a delay 60 to change from reception to transmission mode, begins transmitting, say, a late entry telegram LE 61. Alternatively, this may be an FCH signal, or other appropriate signalling. It can only commence sending this because it has not received any message from B. However, radio A does not, in embodiments of the invention, have to transmit its own sync sequence. This is instead generated by the repeater as sequence S2. LE 61 is received by the repeater at 62 after a propagation delay of 0.25 ms and is, after processing delays in the repeater, retransmitted at 63. The inserted sync S2 is timed so that the repeater transmits first sync S2 and then directly follows this with the delayed LE telegram 63 as shown.

Thus, the present invention enables signalling, such as late entry telegrams, forward channel signalling, or other signalling requirement, to be effectively made within a desired two time slot period without the disadvantages of the prior art. This is achieved by enabling the repeater to generate its own sync sequence or sync pulses if it has not received an appropriate transmission from a radio station within a time slot. 10

The invention may be embodied by suitable software at the repeater, which determines whether the appropriate transmissions have been made and whether to generate and transmit the synchronisation (sync) data, perhaps by using a look-up table or similar, as will be apparent to those skilled in the art.

Claims (10)

1. A mobile radio system, including a plurality of mobile radios and a reclocking repeater, wherein the repeater is adapted to insert a sync sequence or 5 known transmit pattern into its transmissions.

2. A re-clocking repeater for a mobile radio system, which is adapted to insert a sync sequence or transmit pattern into its transmissions.

3. A mobile radio system, including a plurality of mobile radios, a reclocking repeater wherein the re-clocking repeater comprises means for generating a sync sequence or known transmit pattern if it detects that a mobile radio has not transmitted a signal within one or more predetermined time slots.

4. A mobile radio system as claimed in Claim 3, wherein the repeater is adapted to generate the sync sequence, if it does not direct a signal from a first mobile radio, for appending to the front of a signal transmission from a second mobile radio.

S. A mobile radio system as claimed in Claim 4, wherein the signal transmission from the second radio is a late entry telegram, forward channel signalling, reverse channel signalling or a transmit interrupt.

6. A method for enabling signalling to be transmitted within predetermined time slots of a mobile radio system, comprising enabling a re-clocking repeater to generate sync sequences or known transmit pattern if it detects that a mobile radio has not generated a transmission within a certain time slot. 5

7. A method as claimed in Claim 6, wherein the repeater is adapted to generate the sync sequence, if it does not direct a signal from a first mobile radio, for appending to the front of a signal transmission at a second mobile radio.

8. A method as claimed in Claim 7 wherein the signal transmission from the second radio is a late entry telegram, forward channel signalling, reverse channel signalling or a transmit interrupt.

9. A mobile radio system substantially as hereinbefore described with 15 reference to, and as illustrated by, the accompanying drawings.

10. A method of signalling using a mobile radio system substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.