GB2304501A

GB2304501A - Simulcast system and method of routing a block divided signal

Info

Publication number: GB2304501A
Application number: GB9516718A
Authority: GB
Inventors: Kristian G Pedersen; Tommy Hjordt; Keld Andersen
Original assignee: Motorola AS
Current assignee: Motorola Solutions Danmark AS
Priority date: 1995-08-15
Filing date: 1995-08-15
Publication date: 1997-03-19
Anticipated expiration: 2015-08-15
Also published as: GB2304501B; GB9516718D0

Abstract

A block divided signal is routed between an input 23 and an output 21, 22 of a switching device 15. Blocks of data are received at the input having input block boundaries and these are output with output block boundaries. A discontinuation of the blocks of data at the input is detected and dummy blocks (220,221,222, 223,Fig.3) of data are output, shorter than the blocks of data at the input. Dummy blocks are continuously output until there is a continuation of the blocks of data at the input whereupon the blocks of data from the input are again output. The arrangement is suitable for use in a simulcast radio system because it keeps the base stations keyed during gaps in the signal.

Description

SThwIÀST SYSTEM AND METHOD OF ROUTING A BLOCK DIVIDED SIGNAL Field of the Invention This invention relates to an improved method of routing a block divided signal and, separately and in addition, it relates to an improved simulcast radio system.

Background of the Invention In "conventional" (i.e. non-trunked) wide area radio systems, it is usual to employ a comparator, that is to say a signal selecting and switching device. In an analog system, for example, a "Motorola" (trade mark) "Spectratac" comparator can be used to switch between one of a number of base stations and a console for dispatch communication between a controller and mobile radios in the field. The comparator performs voting, selection and emergency pre-emption and resumption.

With the development of digital communication, voice and data communication takes place in block divided signals, where each block includes voice coding and/or error correction and a complete block must be buffered and communicated from end-to-end for any portion of the block to be usable at the destination. This requires buffering of the data which introduces a degree of delay. Delay in the communication is inconvenient and irritating to the user and must be kept at a minimum.

In a digital simulcast system it is vital to keep the base stations keyed during gaps caused either by resumptions or gaps in the incoming signal.

In existing comparators there is a considerable delay between receiving blocks at the input and outputting these blocks. A gap is not recognized as having occurred until it has ended. At the output, the gap is filled out with the correct number of dummy blocks based upon the time the gap lasted. Due to this approach, gaps that exceeded the size of the buffers (i.e. the delay within the comparator) could not be handled. Besides this, it was very difficult to calculate the time the gap had lasted.

There is a desire to reduce the delay between the input and output of a comparator. Reducing this delay enlarges the problem of handling gaps.

There is a need for an improved method of handling block divided signals and an improved simulcast radio system.

Summary of the Invention According to a first aspect of the invention, a method of routing a block divided signal between an input and an output of a switching device is provided comprising the steps of: receiving blocks of data at the input having input block boundaries and outputting the blocks of data at the output with output block boundaries, identifying a discontinuation of the blocks of data at the input; outputting dummy blocks of data at the output with output block boundaries, where the dummy blocks are shorter than the blocks of data at the input and continuing to output dummy blocks until there is a continuation of the blocks of data at the input; buffering the first block of data at the input; waiting to the next block boundary at the output and recommencing outputting of the blocks of data from the input.

In this manner continuity of output blocks is achieved without undue delay.

According to a second aspect of the invention, a simulcast radio system is provided comprising a switching device having an input and a plurality of outputs and a base station connected to each output. The switching device has an input buffer connected to the input and an output buffer connected to each output and a control circuit connected to the input buffer and the output buffers, which is arranged to sense the absence of a data block in the input buffer and to directly load in each of the output buffers a dummy data block and to send the dummy data block simultaneously to each base station, independent of the next occurrence of a data block at the input buffer.

A preferred embodiment of the invention is now described, by way of example only, with reference to the drawings.

Glossarv of Terms DIU Digital Interface Unit GPS - Global Positioning System HDLC - High Level Data Link Control LDU - Link Data Unit TDM - Time Division Multiplex VSELP - Variable Slope Excitation Linear Prediction Brief Description of Drawings FIG. 1 shows, in outline, a simulcast radio communications system in accordance with the preferred embodiment of the invention.

FIG. 2 is a timing diagram illustrating the frame structure of link data units transfer in the system of FIG. 1.

FIG. 3 is a timing diagram illustrating operation of a communications system in accordance with the preferred embodiment of the invention.

FIG. 4 is a block diagram illustrating the construction of the comparator of FIG. 1 in greater detail.

Detailed Descrintion of the Drawings FIG. 1 is an overall block diagram of a simulcast radio communications system having a signal selecting system. The radio system is a wide area system and is a "conventional" system in the sense that it is not a trunked radio system, i.e. it does not have a central trucking controller.

The radio system 10 comprises two transceiving base stations 11 and 12, each having a GPS receiver 13 and 14 respectively for accurate timing information, a comparator 15 (also having a GPS receiver 16), a DIU 17 and a console 18.

The term "comparator" is used in the art of private mobile radio to refer to a signal selecting and switching device.

It will, of course, be understood that base stations in addition to base stations 11 and 12 can be included in the system.

The comparator 15 has a number of ports, of which three are shown (ports 21-23). Ports 21 and 22 of the comparator 15 are connected to base stations 11 and 12 via telephone lines or microwave links 24 and 25 respectively. Port 23 is connected to a digital interface unit 17 which is in turn connected to console 18.

Some of the elements shown in FIG. 1 are optional. For example, not all systems include console 18.

In operation, the comparator 15 receives signals from the DIU 17 and it sends these signals to the base stations 11 and 12 which simultaneously transmit them to a mobile radio 30, under synchronization timing received from the GPS receivers 13 and 14. It also receives signals from the base stations and passes one or a combination of these signals to the DIU 17 and on to the console 18. Signals also arrive from one base station, e.g. base station 11 for retransmission by one or more other base stations.

The comparator 15 sets up an uplink channel for passing signals from one of the base stations 11 and 12 to the DIU 17 and it sets up a downlink channel for passing signals from the DIU 17 or from one of the base stations to the base stations or the other base station(s) respectively.

The comparator 15 compares received signals from the base stations 11 and 12 and selects one of these signals for passing, i.e. routing, to the console 18.

It may be noted that variations in the comparator can be made which allow for diversity combining of different signals from the base stations 11 and 12.

Referring to FIG. 2, uplink and downlink signals passing through the comparator 15 are passed as data blocks, called link data units (LDUs).

FIG. 2 shows two LDUs 100 and 101, which together form a single frame of data. Each LDU has a duration of 180msec. The frame has a duration of 360msec. Each LDU comprises 6 segments 110 to 115 of coded voice. An example of voice coding is VSELP coding. Each segment 110 to 115 comprises an embedded signalling part 120 and a data part 121. The embedded signalling part of the first segment 110 contains a synchronising pattern and a network ID 122. The remaining embedded signalling for the other segments of the first LDU 100 contains link control information. In the second LDU 101 of the frame, frame encryption and signalling is sent.

This frame structure allows continuous re-entry (every 180 msec), continuous ID (every 360msec), continuous encryption synchronisation (every 360msec), enhanced signalling, embedded signalling (available every 360msec), signalling without voice truncation and received unit ID display (refreshed every 360msec).

Uplink and downlink signals passing through the comparator 15 are buffered, so that complete blocks can be passed uplink or downlink.

Buffering of the data introduces a degree of delay, which must be kept at a minimum.

Referring to FIG. 3, two signals are shown labelled A and B.

Signals A is being received at input port 23 (although it can be received at a different port such as port 22 or 24).

Signal B represents the presentation of the signal A at the output port or ports such as ports 22 and 24 of the comparator in accordance with the present invention.

Inbound and outbound arbitration points are shown in FIG.3. An inbound arbitration point exists where the beginning of an LDU arrives at the comparator 15. An outbound arbitration point is a new feature and exists at a predetermined time in advance of an LDU being sent out from the comparator 15. The predetermined time is sufficient for a selecting decision to be taken and for switching and buffer control operations to be carried out.

Signal A shows different LDUs 211,212,214,215 and 216 (labelled LDU1, LDU2....LDU6) of inbound frames from base station 11. There is a discontinuity in the input data after LDU 212.

Signal B shows LDUs of outbound frames from comparator 15 to the base stations 11 and 12. The LDUs 211,212,214,215 and 216 are seen in the outbound signal with four dummy LDUs 220-223 between blocks 212 and 214.

Each outbound LDU contains a launch time calculated from a time stamp received from GPS receiver 16. The comparator adds an offset to the time stamp and sends this increased time to the base stations 11 and 12.

The base stations 11 and 12 transmit the LDU at the designated time, as indicated by GPS receivers 13 and 14. In this manner, accurate synchronization of transmission is achieved.

It is important that there is minimum delay between sending an LDU from the comparator and receiving it at a base station, otherwise the launch time may be overrun. An unacceptable delay would occur if one of the base stations keyed-down and had to key-up again. Unless the data to the base stations is continuous, the base stations will key-down and key-up again on recommencing of the data.

To avoid this, the comparator identifies the absence of the next first LDU segment 110 containing the synchronizing pattern and a network ID 122.

Upon identifying the absence of the first LDU segment 110, comparator 15 commences outputting dummy LDUs, each with a first LDU segment having the synchronising pattern and a network ID. The dummy LDUs are much shorter than the other LDUs (about one quarter of the duration of a regular LDU).

The comparator 15 continues outputting dummy LDUs until a new LDU is identified at the input. The new LDU is identified by its synchronizing pattern. When this is received, the comparator waits until the end of the current dummy LDU 223 (in fact it waits until a predetermined time in advance of the end of the current dummy LDU) and it commences outputting the next received LDU which is LDU 214.

Thus, when the end of a current output LDU (real or dummy) is imminent, the comparator checks its input buffer to see if a new LDU has been received. If it has, the new LDU is output. If it has not, a dummy LDU is output.

Thus there has been described an improved method of handling discontinuities in block divided input data in a simulcast system.

Referring to FIG. 4, details of the comparator 15 are shown. It is seen that the comparator 15 comprises eight wireline interface boards 501 to 508. One of these wireline interface boards, e.g. board 505 is connected to the DIU 17 via port 23. One of the wireline interface boards is connected to each of the base stations 11 and 12 via respective ports 21 and 22. The other wireline boards 504 and 505 to 508 are surplus to the system illustrated in FIG. 1, but can be used for system expansion. Connected to the wireline interface boards 501 to 508 via an HDLC bus 510 and a TDM bus 511 is a station control module 512. Each wireline interface board, e.g. board 501, has an input line 520 and an output line 521, for uplink and downlink traffic respectively.

Wireline board 505 is shown as having an input buffer 555. Wireline interface boards 501 and 502 are shown as having output buffers 551 and 552 respectively. The various boards have other input and output buffers which need not be described in detail.

Digital analog traffic is communicated between wireline interface boards over the TDM bus 511. Digital encoded voice, plus control information for control of the wireline interface boards is conducted via the HDLC bus. The station control module 512 has the following inputs or outputs; RSS terminal 531, external speaker output 532, local speaker output 533 and handset connection 534.

In operation, signals received by the comparator 15 are received at a wireline interface board input, e.g. input 520 and are sent to the station control module 512 via the HDLC bus 510.

The station control module 512 performs selecting between signals from different receivers, according to known criteria. One such criterion is relative bit error rate. The bit error rates of the various uplink signals are compared to identify the best possible signal. Another feature is the provision of signalling in each signal, which includes signal priority information. Different signal priority values of different inbound signals are compared and the highest priority signal is identified.

The station control module 512 performs the routing of signals. The "selected" signal is routed back to the selected wireline interface board for onward communication (the particular board depending upon the direction of the communication and whether the signal is being repeated by the base station 11 or is being passed from the mobile radio 20 to the console 35 etc.). At the destination wireline interface board, e.g. board 502, the signal is buffered in buffer 552 until the station control module 512 instructs it to be sent out on the output line, e.g. line 523.

In normal operation, the station control module 512 instructs the wireline interface board 505 to send the contents of its buffer 555 to the wireline interface boards 501 and 502, upon the occurrence of the next outbound arbitration point.

The station control module 512 initiates the outputting of dummy blocks by instructing the wireline interface boards 501 and 502 to output a dummy block from their buffers 551 and 552 when there is no input block at wireline interface board 505. This instruction is sent over the HDLC bus 510. In this manner, the control module 512 senses the absence of a data block in the input buffer 555 and directly loads in each of the output buffers 551 and 552 a dummy data block causes the wireline interface units 501, 502 to send the dummy data block simultaneously to each base station, independent of the next occurrence of a data block at the input buffer 555.

This continues until a new data block is received at the input buffer 555.

Claims

1. A method of routing a block divided signal between an input and an output of a switching device, comprising the steps of: receiving blocks of data at the input having input block boundaries and outputting the blocks of data at the output with output block boundaries, identifying a discontinuation of the blocks of data at the input; outputting dummy blocks of data at the output with output block boundaries, where the dummy blocks are shorter than the blocks of data at the input and continuing to output dummy blocks until there is a continuation of the blocks of data at the input; buffering the first block of data at the input; waiting to the next block boundary at the output and recommencing outputting of the blocks of data from the input.

2. A method according to claim 1, wherein each block has a synchronization pattern and the step of identifying a discontinuation of the blocks comprises identifying the absence of a synchronization pattern.

3. A method of routing a block divided signal between an input and an output of a switching device, comprising the steps of: a) receiving blocks of data at the input having input block boundaries and outputting the blocks of data at the output with output block boundaries, b) identifying a discontinuation of the blocks of data at the input; c) outputting dummy blocks of data at the output with output block boundaries, where the dummy blocks are shorter than the blocks of data at the input, d) waiting until a predetermined time in advance of the next output block boundary; e) determining whether the blocks of data at the input have recommenced; f) repeating steps c, d and e until there is a continuation of the blocks of data at the input; g) buffering the next block of data at the input; ; h) waiting to the next block boundary at the output and g) recommencing outputting of the blocks of data from the input.

4. A simulcast radio system comprising a switching device having an input, a plurality of outputs and a base station connected to each output, the switching device having an input buffer connected to the input and an output buffer connected to each output and a control circuit connected to the input buffer and the output buffers, which is arranged to sense the absence of a data block in the input buffer and to directly load in each of the output buffers a dummy data block and to send the dummy data block simultaneously to each base station, independent of the next occurrence of a data block at the input buffer.