GB2378872A

GB2378872A - Communications systems

Info

Publication number: GB2378872A
Application number: GB0215021A
Authority: GB
Inventors: Mark Wentworth Rayne; Diana Margaret Ball
Original assignee: Sepura Ltd
Current assignee: Sepura Ltd
Priority date: 2001-06-29
Filing date: 2002-06-28
Publication date: 2003-02-19
Anticipated expiration: 2022-06-28
Also published as: WO2003003645A1; GB2378872B; GB2378872C; EP1400050A1; GB0116019D0; ES2389933T3; EP1400050B1; GB0215021D0

Abstract

A communication system e.g. TDMA (GSM or TETRA) in which signalling information and other traffic such as user information between a mobile station and a base station, or between a first mobile station and a second mobile station is transmitted in variable bit-length messages. The transmitter includes in its message a length indication message for indicating to the recipient the length of the message that is being transmitted. The length indication message has a basic meaning in units of octets but is multiplied in the receiver by a multiplying factor which is dependent upon the modulation method used for the transmission to determine the true length of the message. Thus each indication message is ambiguous in its meaning, the ambiguity being resolved by knowledge of the modulation scheme being used. One indication message may indicate that the variable bit-length message fills the whole of a burst.

Description

1- 2378872

75891.69

COMMUNICATIONS SYSTEMS

5 The present invention relates to communications systems and in particular to such systems where transmissions of variable length messages can take place.

Many communications systems, and in particular mobile 10 communications systems such as GSM (Global System for Mobile Communications) and TETRA (Terrestrial Trunked RAdio), send their transmissions (which may be of signalling, user information, eta) in fixed length (in terms of their time period) discrete segments or bursts.

15 In time division multiple access (TDMA) systems, such as GSM and TETRA, each transmission burst is typically the length of (or a fraction of the length of) a single timeslot. 20 In the GSM system, for example, a single timeslot lasts 0.577 ms. The timeslots are arranged in repeating groups of eight timeslots, each group making up a frame" of length 4.615 ms. (See, for example, M. Mouly and M. Pautet, "The GSM System For Mobile 25 Communications", 1992, ISBN 2-9507190-0-7, Cell and Sys, 4 Rue Elisee Reclus, F-91120, PALAISEAU, FRANCE).

In TETRA, each timeslot lasts 14.167 ms and there are 4 timeslots in a repeating frame of length 56.67 ms. 18 30 such frames constitute a multiframe, and 60 multi-frames constitute a hyper-frame. A transmitting mobile station may transmit on one to four timeslots per frame, and a base station usually transmits on all four timeslots per frame (to the same or differing mobile 35 stations). On some occasions (e.g. for random access signalling), a mobile station may transmit short bursts within the first or second half of a timeslot. (See for

example, "Terrestrial Trunked Radio (TETRA); Voice plus Data (V+D); Part 2: Air Interface (AI)", EN 300 392-2, available from ETSI, F05921 Sophia Antipolis CEDEX -

FRANCE).

In such communication systems, signalling information and other traffic such as user information between a mobile station and a base station, or between a first mobile station and a second mobile station will be 10 transmitted in the fixed length (time period) transmission bursts. Such information is usually transmitted in variable bit-length messages (referred to as "protocol data units" (PDU's) in TETRA). Because these messages can have a variable length, it is common 15 to include in the communications systems signalling protocol a means of indicating to the recipient the length of a given message (e.g. PDU) that is being transmitted. This can be useful where, for example, it is possible to transmit more than one such message (e.g. 20 PDU) in a single transmission burst, as the receiver may use the length of the first message to identify the end of the first message and the start of the second message. 25 In TETRA, for example, each MAC (Medium Access Control) PDU may contain a length indication in units of octets.

A receiver of such a MAC PDU uses the length indication to locate the end of the last octet of the PDU. If the length indication was not used, then it would not be 30 possible to include more than one PDU in a single burst (typically a timeslot).

It is also known where the last indicated part (e.g. octet) of a message is not completely filled with data, 35 to fill that part (octet) up completely with an identifiable pattern of so-called "fill bits" to allow the receiver to identify the true end of the message.

3 - In such a case, a "fill-bit indication" is usually also transmitted (in TETRA by setting a flag in the PDU header) to the receiver to indicate to the receiver that fill bits are present. In TETRA, where fill bits are 5 used, the first unused data bit in the octet is set to "1" and the remaining bits are set to ''0". A receiver seeing the fill bits indication searches backwards from the end of the last indicated octet (which it knows from the transmitted length indication) through the fill bits 10 until it finds a "1" (which will be the first bit after the end of the message's useful data). It then knows that it has found the end of the useful data in the message. 15 The length indications allowed for in existing communications protocols are naturally chosen to match possible message lengths that can be transmitted. Thus in TETRA, for example, a length indication of 6 bits is provided, which can be used to indicate message length 20 up to 61 (or sometimes of 47) octets (the full range of "6-bit" length values is not available because some of the indication values are given special pre-assigned meanings). This is satisfactory for standard TETHER, since using the normal TETRA n/4 DQPSK modulation, the 25 maximum length of PDU' s such as MAC-DATA, MAC-RESOURCE or MAC-END is 263 bits or 33.5 octets.

However, new developments in signal processing are making it possible to increase the number of bits of 30 information transmitted in a given time, for example, by increasing the modulation rate or level of the signalling or by increasing the bandwidth of the transmission, or both. For example, although standard TETRA uses n/4 DQPSK modulation which encodes 2 bits per 35 modulation symbol, it would now be possible to use 16-QAM modulation which encodes 4 bits per modulation symbol. This would double the amount of data which can

- 4 be transmitted in a given time period (such as a single timeslot). Such enhanced data transmission rates are becoming 5 increasingly desirable for communications systems and so it is desirable to enhance an existing communications systems by adding an increased rate of signalling and data transmission. However, it is also important in such circumstances to maintain backward compatibility 10 with the communications system's existing transmission format and protocol. This means in practice that it is preferred to still use the existing transmission burst structure and protocols, etc. when using the enhanced data transmission rate.

In general it would normally be satisfactory to use the existing transmission burst structure and protocol, etc. with an enhanced data transmission rate. However, the Applicants have recognized that where message length 20 indications are used by the communications system, then the existing length indications may not have sufficient capacity to indicate the maximum quantity of data which could be contained in a transmission message (e.g. PDU) when a new, higher transmission rate is being employed.

25 For example, in the TETRA system, using 16-QAM modulation as discussed above would allow a Protocol Data Unit such as MAC-DATA, MAC-RESOURCE or MAC-END to have a length up to, say, 67 octets. However, as noted above, the maximum octet length that the standard 6-bit 30 TETRA length indication can indicate is 61 octets.

This problem could be avoided by changing the size of (i.e. the number of bits available for) the length indication to allow for longer length messages. For 35 example, in TETRA the length indication could be made 7-

bits long rather than 6-bits long. However, the Applicants have recognised that changing the size of the

- 5 length indication is undesirable, as it would make the signalling protocol for the enhanced data rate arrangement incompatible with the protocol of the existing communications system. This would, for 5 example, mean that communications units having the enhanced data rate capability would need to be programmed with both signalling protocols (to allow backwards compatibility), which would be inconvenient and expensive.

According to a first aspect of the present invention, there is provided a method of operating a communications system in which discrete messages of a variable length can be transmitted, comprising transmitting to the 15 receiver an indication of the length of such a message when it is to be transmitted, wherein the same length indication message can indicate different length messages. 20 According to a second aspect of the present invention, there is provided a data transmitting apparatus which can transmit discrete messages of a variable length, comprising: means for transmitting to the receiver of a message 25 an indication of the length of the message, wherein the same length indication message can indicate different length messages.

In the present invention, the same length indication 30 message can be used to indicate different length messages, i.e. the meaning of an individual length indication message varies. This allows increased length messages to be indicated using existing available length indication messages and without the need to change the 35 basic length indication protocol (e.g. length indication size).

- 6 As the same length indication message can have different meanings, it is necessary for both the transmitter and receiver to know the intended meaning of the length indication currently being transmitted. This can be 5 achieved as desired, but in a particularly preferred embodiment, the meaning of the length indication message is dependent upon one or more predefined criteria.

Thus, according to a third aspect of the present lo invention, there is provided a method of operating a communications system in which discrete messages of a variable length can be transmitted, comprising: when such a message is to be transmitted, transmitting to the receiver an indication of the length 15 of the message, wherein the meaning of a given length indication message varies in accordance with one or more predefined criteria.

According to a fourth aspect of the present invention, 20 there is provided a data transmitting apparatus which can transmit discrete messages of a variable length, comprising: means for transmitting to the receiver of a message an indication of the length of the message, wherein the 25 meaning of a given length indication message varies in accordance with one or more predefined criteria.

According to a fifth aspect of the present invention, there is provided a method of determining the size of a 30 portion of data which has been received through a communications link, comprising: receiving a length indication message indicating the length of the data portion, wherein the same length indication message can indicate different message 35 lengths, and the received length indication message is interpreted to determine the length of the data portion in accordance with one or more predefined criteria.

- 7 - According to a sixth aspect of the present invention, there is provided a receiving apparatus for a communications system in which discrete messages of a variable length can be transmitted, the apparatus S comprising: means for receiving a length indication message indicating the length of a transmitted message; wherein the same length indication message can indicate different message lengths, and the apparatus 10 further comprises means for interpreting the received length indication message in accordance with one or more predefined criteria to determine the length of a received message.

15 While the predetermined criteria that govern the meaning and interpretation of the length indication message can be selected as desired, in a particularly preferred embodiment the meaning and interpretation of the length indication message is dependent on one or more 20 characteristics of the data being transmitted and/or of the communications link between the transmitter and receiver. These characteristics are, for example, convenient to use, because for any transmission between a transmitter and a receiver along a communication link, 25 the transmitting and receiving parties must both know and understand the characteristics of the communication link and data transmission.

Thus according to a seventh aspect of the present 30 invention, there is provided a method of determining the size of a portion of data which has been received through a communications link, comprising: receiving a length indication message indicating the length of the data portion; and 35 interpreting the received length indication message to determine the length of the data portion on the basis of one or more characteristics of the communications

8 - link and/or of the data transmission.

According to an eighth aspect of the present invention, there is provided an apparatus for determining the size 5 of a portion of data which has been received through a communications link, comprising: means for receiving a length indication message indicating the length of the data portion; and means for interpreting the received length 10 indication message to determine the length of the data portion on the basis of one or more characteristics of the communications link and/or of the data transmission.

The characteristic of the communications link and/or 15 data transmission used to determine the meaning of the length indication message can be selected as desired.

In a particularly preferred embodiment they are the data rate of the transmission and/or the modulation method being used for the transmission (communication link).

20 Other modulation characteristics such as channel coding type, interleaving depth, etc. could also or instead be used. In all these cases, the transmitter and receiver would 25 know or be able to determine the relevant characteristic(s). For example, where both low and high data rates can be used, the receiving party may be able to determine the data rate or modulation method being used by analysing the incoming transmission.

Additionally or alternatively, the transmission characteristics, etc. will have been previously arranged and indicated to the receiver and transmitter, such that both will know the characteristic in question before the 35 transmission takes place. For example, the communications system's signalling protocol will probably require the transmitting unit to use an agreed

- 9 - method for choosing the characteristic (e.g. modulation level) of the transmission.

For example, in the TETRA arrangement discussed above, 5 it may be that the normal n/4 DQPSK modulation is used by default and/or on start-up, with a switch to a higher level of modulation, such as 16-QAM, only taking place on a specific instruction from, e.g. a base station, or once agreed by a signalling exchange between 10 communications units. The same could apply to the change back to n/4 DQPSK modulation (or to another modulation rate). In these cases the characteristics of the transmission would be prearranged between the transmitter and receiver (and before any characteristic 15 change takes place). Thus in a preferred embodiment the meaning and interpretation of the length indication message is based on a prearranged characteristic of the communications link and/or transmission.

20 The way that the meaning of the length indication varies can be selected as desired. In one preferred embodiment, each length indication could be ascribed a first meaning (e.g. its normal, standard meaning) for a first set of criteria (e.g. standard data rate 25 transmissions), and each length indication ascribed the same meaning, such as "maximum length transmission", for another set of criteria (e.g. higher data rate transmissions). In this latter case, where all length indications are interpreted as indicating a "maximum 30 length" transmission, the effect would be that the length indications are effectively ignored and the receiver instead assumes that the message fills the transmission burst (i. e. occupies the maximum possible space available to it). In this case the receiver could 35 assume that the entire burst is the message, or, preferably, go to the end of the transmission burst and search backwards to find the true end of the message.

- 10 The fill-bits technique discussed above could be used in this arrangement to fill the burst from the true end of the message to the end of the available data portion and to allow the receiver to more easily identify the true 5 end of the message by searching backward from the maximum possible size of the message (indicated end of the data portion) until the first fill-bit is located, as discussed above.

10 One disadvantage of this arrangement is that it could preclude sending multiple messages (e.g. PDU's) in a single transmission burst (e.g. timeslot), since when two or more messages are included in a single burst, a receiver may then be unable to identify the end of the 15 first message.

In a preferred embodiment of this arrangement therefore, each length indication could have an ascribed value (its normal value) for a first set of criteria (e.g. normal 20 data rate transmissions) and some but not all of the length indication messages would retain that meaning for a second set (and other sets) of criteria (e.g. higher data rate transmissions) but with one or more length indication messages (values), and preferably one only, 25 and most preferably the longest available length indication, being ascribed the meaning "maximum possible length'' for the second set (and/or other sets) of criteria. This would then allow the length of messages up to a particular value to be indicated properly for 30 (e.g.) all criteria, with only longer messages at e.g. higher data rates using the indication "maximum possible length". This would then allow two shorter messages at least to be included in a single transmission burst, which is still useful, as the ability to include 35 multiple messages in a single burst is most useful where each individual message is one half or less of the maximum message (e.g. transmission burst) length.

- 11 Indeed, this arrangement is believed to be advantageous in its own right. Thus, according to a further aspect of the present invention, there is provided a method of transmitting data in a communications system in which 5 messages of a variable length can be transmitted in discrete transmission bursts, comprising: transmitting to a receiver of the message a indication of the length of the message, wherein at least one value of the length indication message 10 indicates that the receiver should locate the message starting with the assumption that the message fills the entire space available to it in the transmission burst.

According to a further aspect of the present invention, 15 there is provided an apparatus for transmitting data in a communications system in which messages of a variable length can be transmitted in discrete transmission bursts, comprising: means for transmitting to a receiver of the message 20 a indication of the length of the message, wherein at least one value of the length indication message indicates that the receiver should locate the message starting with the assumption that the message fills the entire space available to it in the transmission burst.

It is preferred in these aspects of the invention to use a fill-bits technique as discussed above to pad out the end of the transmission burst with an identifiable pattern of fill bits. In that case, a fill-bit 30 indication should also be transmitted, where appropriate, and the receiver will locate the message by assuming that it fills the burst and then removing the fill-bits as appropriate.

35 In a particularly preferred embodiment, the variable meaning and interpretation of the length indication message is provided by using a multiplying factor for

- 12 the burst length indication value, which multiplying factor varies according to the predefined criteria, such as preferably, communication link or transmission characteristics, such as the data rate or modulation 5 method being used. In this case, each length indication value preferably indicates a number of length "units" (e.g. octets of bits as discussed above), and the length of the message is found by multiplying the number of length units indicated by the appropriate multiplying 10 factor. In other words, the unit of length for the length indication messages varies according to (and is dependent upon) the predefined criteria.

The multiplying factor can be selected as desired, e.g. 15 on the basis of the maximum size of message that can be transmitted under the relevant transmission characteristics. Thus, for example, in the TETRA example discussed above, given a fixed-size length indication, the multiplying factor would be set to ''one" 20 for the n/4 DQPSK modulation, and to "two" when 16-QAM modulation was in use. This would allow the same 6-bit length indications to behave correctly for "standard" TETRA, yet have a capacity of 94 octets when the higher modulation rate was in use. If a yet higher level 25 modulation scheme, e.g. 64-QAM, was introduced later, a new multiplier value "three" could be introduced, and so on. In this arrangement it is possible that, depending on 30 the multiplier value, there may be one or more fewer useful data "units" (e.g. octets) in the message than is actually indicated by the length indication. In that case, the unused "part" (octets) of the length indication are preferably filled with fill-bits as 35 discussed above, and the receiver locates the end of the useful part of the message by searching back through the fill- bits from the end of the message as indicated by

the length indication.

In another preferred embodiment, the length indication message values are predetermined and stored, e.g. in a 5 look-up table, by the transmitter and receiver. The stored value of each length indication message will depend upon the predefined criteria, as appropriate.

Thus, for example, in the TETRA example, when standard n/4 DQPSK TETRA is in use, each length indication 10 message could have its standard meaning, but when an enhanced data rate is in use, the length indication message meanings could be looked up in a (different) stored predefined table. This may allow more flexibility than just using a multiplier, as, for 15 example, the first few values in the modified meaning table could increase in single steps of length, and then the lengths increase more steeply. For example, the lengths indicated by the length indication message could be: 1, 2, 3,..., 29, 30, 32, 34, 36, 38, 40, 42, 44, 46, 20 48, 50, 58, 66, 74, 82, 90, 98, 102. This example coding would allow for a data rate of up to three times that available in standard ( /4 DQPSx) TETRA.

The same, single modified length indication message 25 value table could be used for all enhanced data rates, etc., or one or more different such tables could be used (with there being, e.g., a different table for each modulation level and/or data rate).

30 The length indication message of the present invention can be sent as desired, but is preferably included with the message itself, as is known in the art.

It is preferred to use "fill bits" techniques as 35 discussed above to pad out messages where necessary when the true end of the message does not match exactly the available length indication messages. In such a case, a

- 14 fill bits indication message, which is preferably included in the message itself, is preferably transmitted to the receiver, as is known in the art.

5 The present invention is applicable to communications systems generally, but is particularly applicable to mobile communications systems such as TETRA and GSM.

Thus the present invention also extends to a mobile station and a base station of a mobile communications lo system incorporating any or all of the above aspects and preferred features of the present invention.

It will be appreciated by those skilled in the art that the present invention can be implemented in a removable 15 memory module for a communications unit (such as a SIM (Subscriber Identity Module)). Thus the present invention also extends to such a memory module incorporating any or all of the above aspects and preferred features of the present invention. Such a 20 memory module may comprise not only a memory for storing data, but also any necessary processing means for carrying out the invention.

The methods in accordance with the present invention may 25 be implemented at least partially using software e.g. computer programs. It will thus be seen that when viewed from further aspects the present invention provides computer software specifically adapted to carry out the methods hereinabove described when installed on 30 data processing means, and a computer program element comprising computer software code portions for performing the methods hereinabove described when the program element is run on data processing means. The invention also extends to a computer software carrier 35 comprising such software which when used to operate a system comprising data processing means causes in conjunction with said data processing means said system

- 15 to carry out the steps of the method of the present invention. Such a computer software carrier could be a physical storage medium such as a ROM chip, CD ROM or disk, or could be a signal such as an electronic signal 5 over wires, an optical signal or a radio signal such as to a satellite or the like.

It will further be appreciated that not all steps of the method of the invention need be carried out by computer 10 software and thus from a further broad aspect the present invention provides computer software and such software installed on a computer software carrier for carrying out at least one of the steps of the methods set out hereinabove.

A number of preferred embodiments of the present invention will now be described by way of example only.

The present invention will be described with reference 20 to the TETRA system. However, as noted above, the present invention is equally applicable to other communications systems.

Table 1 below shows the standard TETRA uplink MAC-END 25 PDU (Protocol Data Unit) message.

Information Length Value Remark Element 11 PDU type 2 012 MAC-END or MACFRAG 12 PDU subtype 1 1 MAC-END 5 13 Fill bit 1 0 No fill bits present indication 1 Flll bi t(s) present 14 Length 6 oooooO2 Reserved indication _ 0Oo00l2 Reserved OOOO1O2 Length of MAC PDU in octets 10. etc.. etc. 15 l000lO2 Longest MAC PDU l ooOl12 Reserved etc....etc. 1011112 Reserved 15 llxxxx2 Reservation requirement element 16 TM-SDU varies Table 1

20 Such a MAC-END Protocol Data Unit is used to carry the last portion of a higher layer Protocol Data Unit which has been fragmented into two or more portions which will fit into individual TETRA timeslots. This last portion of the fragmented higher-layer Protocol Data Unit can 25 therefore be of any length. The last fragment or data portion of the higher-layer Protocol Data Unit is contained in the TM-SDU information element 16 of the Protocol Data Unit shown in Table 1. (The TM-SDU is defined as the SDU (Service Data Unit) from the layer 30 above the MAC (Medium Access Control)).

As can be seen from Table 1, the MAC-END PDU has two information elements, 11 and 12, which together define the meaning of the Protocol Data Unit (in this case as a

- 17 MAC-END PDU). It also includes both a length indication portion 14 and fill bit indication portion 13.

If the Protocol Data Unit (including the TM-SDU) does 5 not make up a complete number of octets, the last octet is padded with fill bits, in the pattern 1, 0, 0,..., 0 up to the end of that octet. The fill bit indication element 13 is then set to "l" to indicate that fill bits have been used.

The length indication information element 14 indicates the total length of the MAC-END PDU in units of octets (bytes). The length indicated includes the variable length TM-SDU information element, and all the preceding 15 information elements, up to and including the PDU type information element 11. It also includes any fill bits which may have been appended to the end of the Protocol Data Unit.

20 The length indication element 14 has 6 bits, but not all of those bits are used to represent the length of the message (PDU). Firstly, values of the length indication element 14 starting with "11" have been assigned for another purpose. Furthermore, since the minimum 25 possible length of the Protocol Data Unit (which would be with a zero length TM-SDU element 16) is 10 bits, length indication values OOOOOO2 and 0000012 have been reserved for some undefined future use. Similarly, as the maximum possible length of a MAC-END PDU is limited 30 by the TETRA burst length and channel characteristics to 268 bits or 33.5 octets, as discussed above,values 1000112 to 1011112 have been reserved for some undefined future use.

35 When the receiver receives the MAC-END PDU, it reads the length indication value 14 and also checks whether the fill bit indication element 13 indicates that fill bits

- 18 have been used. If fill bits have not been used, the receiver identifies the end of the TM-SDU element 16 of the message by counting octets from the start of the message until it either reaches the number of Octets 5 indicated in the length indication element 14 or the end of the data in the overall transmission. If fill bits have been used, the receiver locates the end of the message as above, but then searches back through the fill bits until it finds a "1". The last bit of the 10 TM-SDU element 16 will then be the bit preceding this 111 11.

The case where a TETRA system is modified to support longer (in terms of bit content, i.e. greater data 15 capacity) transmission bursts, such as would be available to enhanced TETRA systems using higher modulation levels or wider RF frequency bandwidths, or both, will now be considered. In these embodiments, it is assumed that the enhanced TETRA system may use the 20 standard TETRA n/4 DQPSK modulation, giving 2 bits per symbol, 16-QAM giving 4 bits per symbol, or 64-QAM, giving 6 bits per symbol. 16QAM and 64-QAM therefore represent data rate increases of 2 times and 3 times the rate available with n/4 DQPSK modulation. Using 16-QAM 25 thus increases the number of bits available to the MAC-END (uplink) PDU to 536 (67 octets), and using 64-QAM increases the number of bits available to the MAC-END (uplink) PDU to 804 (100.5 octets) (assuming that the duration of a transmission burst or timeslot 30 remains unaltered and that the number of overhead symbols remains the same).

However, the maximum length which can be signalled in an unmodified TETRA length indication for a MAC-END PDU is 35 47 octets, since values 11000O2 and above have been assigned for a different purpose. Even if those values had not been so assigned, the length indication would be

- 19 limited to 63 octets. However, with 16-QAM it is necessary to signal lengths up to 67 octets, and with 64-QAM, lengths up to 101 octets.

5 In a first preferred embodiment of the present invention to allow the TETRA system to cope with this possibility, the length indication value 1011112 is ascribed the meaning "length shall be deemed to be the remaining capacity of the burst", and fill bits (if indicated) are 10 used to find the true length of the message. All lower values of the length indication element 14 would be used to indicate the number of octets as for standard TETRA.

As noted above, one disadvantage of the above 15 arrangement is that it would not be possible to include more than one MAC-PDU in a single transmission burst if any but the last PDU is longer than 46 octets.

Therefore, in another preferred embodiment of the present invention, the meaning of the length indication 20 element is modified. This is illustrated in Table 2 below, which shows a preferred embodiment of a modified TETRA MAC-END PDU in accordance with the present invention that can be used for a TETRA system employing higher data rates.

_ _ Information Length Value Remark Element 21 PDU type 2 012 MAC -END or MAC-FRAG 22 PDU subtype 1 MAC- END 5 23 Fill bit 1 O No fill bits present indication 1 Fill bit(s) present 24 Length 6 oooooO2 Reserved indication ooooOl2 Reserved 1O2 Length of MAC PDU in units of y octets etc.... etc. 25 100010 2 Longest MAC PDU 1000112 Reserved I etc.. etc. | 1Ol1ll2 Reserved l xxxx2 Reservation requirement element 10 26 TM-SDU Varies Note y = 1 when the MAC-END PDU is sent using n/4 DQPSK y = 2 when the MAC-END PDU is sent using 16-QAM y = 3 when the MAC-END PDU is sent using 64-QAM Table 2

In Table 2, the length indication element 24 is modified so that its basic meaning in units of octets is multiplied by a factor y which is dependent upon a characteristic (in this case the modulation method used) 20 of the transmission. In the present embodiment, the factor y is "1" for n/4 DQPSK modulation, "2" for 16-QAM and "3" for 6s-QAM, although, of course, other multiplying factor values could be used if desired.

25 The receiver can readily determine which value of "y" is to be used to locate the end of the Protocol Data Unit

- 21 (message), since it has to know which modulation method is being used to be able to decode the incoming signal in any event.

5 This embodiment shows a TETRA uplink MAC-END PDU. A multiplier could be used in a similar manner in the length indication element in other TETRA transmission messages, such as MAC-PDUs which may be sent using a higher data rate, such as MAC-DATA, MAC-RESOURCE and 10 downlink MAC- END PDUS.

Although the present invention has been described specifically with reference to TETRA MAC-PDUs, as will be appreciated by those skilled in the art it can be 15 applied to any signalling method where the length of messages or data portions is indicated by a length indication. It may therefore, for example, be applied at different layers inside TETRA, and can also, for example, be applied to GSM, DECT, GPRS, EDGE and UMTS.

Claims

1. A method of operating a communications system in which discrete messages of a variable length cay be 5 transmitted, comprising transmitting to the receiver an indication of the length of such a message when it is transmitted, wherein the same length indication message can be used to indicate different length messages.

10 2. The method of claim 1, wherein the meaning of a length indication message is dependent upon one or more predefined criteria.

3. A method of operating a communications system in 15 which discrete messages of a variable length can be transmitted, comprising: when such a message is to be transmitted, transmitting to the receiver an indication of the length of the message, wherein the interpretation of a given 20 length indication message by the receiver varies in accordance with one or more predefined criteria.

4. A method of determining the size of a portion of data which has been received through a communications 25 link, comprising: receiving a length indication message indicating the length of the data portion, wherein the same length indication message can be used to indicate different message lengths, and the received length indication 30 message is interpreted to determine the length of the data portion in accordance with one or more predefined criteria. 5. The method of claim 2, 3, or 4, wherein the meaning of a length indication message is dependent on one or more characteristics of the data being transmitted and/or of the communications link between the

- 23 transmitter and receiver.

6. A method of determining the size of a portion of data which has been received through a communications 5 link, comprising: receiving a length indication message indicating the length of the data portion; and interpreting the received length indication message to determine the length of the data portion on the basis 10 of one or more characteristics of the communications link and/or of the data transmission.

7. The method of claim 5 or 6, wherein the characteristic of the communications link and/or data 15 transmission used to determine the meaning of a length indication message comprises the data rate of the transmission and/or the modulation method being used for the transmission.

20 8. The method of claim 5, 6 or 7, wherein the meaning of a length indication message is based on a prearranged characteristic of the communications link and/or data transmission. 25 9. The method of any one of claims 2 to 8, wherein for a first set of criteria each length indication message has one meaning, and for another set of criteria all the length indication messages have the same meaning.

30 10. The method of any one of claims 2 to 8, wherein each length indication message has one meaning for a first set of criteria, some but not all of the length indication messages retain that meaning for a second set of criteria, and one or more length indication messages 35 have a second meaning when the second set of criteria is met.

- 24 11. The method of any one of claims 2 to 8, wherein the meaning of a length indication message is determined by multiplying a basic length value indicated by the length indication message by a multiplying factor.

12. The method of claim 11, wherein the multiplying factor is dependent upon one or more predefined criteria. 10 13. The method of claim 12, wherein the multiplying factor is dependent upon one or more characteristics of the communications link and/or of the data transmission between the transmitter and receiver.

15 14. The method of claim 11, 12 or 13, wherein each length indication message indicates a number of length units, and the length of the message is found by multiplying the indicated number of length units by the multiplying factor.

15. The method of any one of claims 2 to 8, wherein the unit of length for the length indication messages is dependent upon one or more predefined criteria.

25 16. The method of any one of claims 2 to 15, wherein the length indication message values are predetermined and stored by the transmitter and receiver.

17. A method of transmitting data in a communications 30 system in which messages of a variable length can be transmitted in discrete transmission bursts, comprising: transmitting to a receiver of the message a indication of the length of the message, wherein at least one value of the length indication message 35 indicates that the receiver should locate the message starting with the assumption that the message fills the entire space available to it in the transmission burst.

- 25 18. A data transmitting apparatus which can transmit discrete messages of a variable length, comprising: means for transmitting to the receiver of a message an indication of the length of the message, wherein the 5 same length indication message can be used to indicate different length messages.

19. The apparatus of claim 18, wherein the meaning of a length indication message is dependent upon one or more 10 predefined criteria.

20. A data transmitting apparatus which can transmit discrete messages of a variable length, comprising: means for transmitting to the receiver of a message an indication of the length of the message, wherein the meaning of a given length indication message varies in accordance with one or more predefined criteria.

21. A receiving apparatus for a communications system 20 in which discrete messages of a variable length can be transmitted, the apparatus comprising: means for receiving a length indication message indicating the length of a transmitted message; wherein the same length indication message can be 25 used to indicate different message lengths, and the apparatus further comprises means for interpreting the received length indication message in accordance with one or more predefined criteria to determine the length of a received message.

22. The apparatus of claim 18, 19, 20 or 21, wherein the meaning of a length indication message is dependent on one or more characteristics of the data being transmitted and/or of the communications link between 35 the transmitter and receiver.

23. An apparatus for determining the size of a portion

- 26 of data which has been received through a communications link, comprising: means for receiving a length indication message indicating the length of the data portion; and 5 means for interpreting the received length indication message to determine the length of the data portion on the basis of one or more characteristics of the communications link and/or of the data transmission.

10 24. The apparatus of claim 22 or 23, wherein the characteristic of the communications link and/or data transmission used to determine the meaning of a length indication message comprises the data rate of the transmission and/or the modulation method being used for 15 the transmission.

25. The apparatus of any one of claims 18 to 24, comprising means for determining the meaning of a length indication message by multiplying a basic length value 20 indicated by the length indication message by a multiplying factor.

26. The apparatus of claim 25, wherein the multiplying factor is dependent upon one or more predefined 25 criteria.

27. The apparatus of claim 26, wherein the multiplying factor is dependent on one or more characteristics of the communications link and/or of the data transmission 30 between the transmitter and receiver.

28. The apparatus of any one of claims 18 to 27, further comprising means for storing predetermined length indication message values.

29. An apparatus for transmitting data in a communications system in which messages of a variable

- 27 length can be transmitted in discrete transmission bursts, comprising: means for transmitting to a receiver of the message a indication of the length of the message, wherein at 5 least one value of the length indication message indicates that the receiver should locate the message starting with the assumption that the message fills the entire space available to it in the transmission burst.

10 30. A mobile station of a mobile communications system comprising the apparatus of any one of claims 18 to 29.

31. A base station of a mobile communications system comprising the apparatus of any one of claims 18 to 29.

32. A removable memory module for a communications unit comprising the apparatus of any one of claims 18 to 29.

33. A computer program element comprising computer 20 software code portions for performing the method of any one of claims 1 to 17 when the program element is run on data processing means.

34. A method of operating a communications system 25 substantially as hereinbefore described.

35. A method of determining size of a portion of data which has been received through a communications link substantially as hereinbefore described.

36. A method of transmitting data in a communications system substantially as hereinbefore described.

37. A data transmitting apparatus substantially as 35 hereinbefore described.

38. A receiving apparatus for a communications system

- 28 -

substantially as hereinbefore described.

39. An apparatus for determining the size of a portion of data which has been received through a communications 5 link substantially as hereinbefore described.

40. A mobile station of a mobile communication system substantially as hereinbefore described.

10 41. A base station of a mobile communication system substantially as hereinbefore described.

42. A removable memory mobile for a communications unit substantially as hereinbefore described.