WO2005048551A1

WO2005048551A1 - Blind modulation detection

Info

Publication number: WO2005048551A1
Application number: PCT/GB2004/004559
Authority: WO
Inventors: Mark Wentworth Rayne; Peter William Stephens; Martin Popple
Original assignee: Sepura Limited
Priority date: 2003-10-28
Filing date: 2004-10-28
Publication date: 2005-05-26
Also published as: GB2407951A8; GB2407951A; GB0423995D0

Abstract

A radio receiver includes a down-converter (5), a synchronisation mechanism (6), a demodulator (7), and a data sink (8). The receiver uses training sequences included in a transmission to determine the modulation scheme being used for the transmission by carrying out a correlation between the expected, known training sequence used for a given modulation type and the training sequence in the received signal, to see which training sequence produces the best correlation. The modulation scheme of the training sequence that produces the best correlation is then taken to be the modulation scheme being used for the transmission. In another preferred embodiment, the receiver analyses the phase shifts present in the received signal to determine the modulation scheme being used.

Description

BLIND MODULATION DETECTION

The present invention relates to communications systems, and in particular to communications systems that can use more than one form of modulation for their transmissions . It is becoming increasingly common for communications systems to support more than one modulation scheme. For example, in TETRA (TErrestrial Trunked RAdio) systems, it is proposed to extend the existing TETRA standard (which will be referred to herein as TETRA1) to use new, additional modulation types. This extended version of TETRA is known as TETRA Release 2 (or TETRA2) . TETRA1 uses, as is known in the art, π/4-DQPSK (differential quaternary phase shift keying) modulation. Figure 1 shows the constellation diagram for π/4-DQPSK modulation. The modulation is differential, with each pair of bits entering the modulator defining a phase change in the complex vector representing each symbol . The allowed phase changes (shown by the arrows in Figure 1) are ±π/4, ±3π/4. After a sequence of bits has been passed through the modulator, in all 8 points in the constellation may have been used. The mapping of bits to phase change is based on the following equation:

S(k) = S(k - V) - e^{υ k} with

S(0) = 1

where S represents a symbol, B a binary bit, and k = 1, 2, ... n, where n is the number of symbols in a burst. The bit to phase mapping for TETRA1 using π/4-DQPSK modulation is as follows :

Table 1: Phase shift and bit definition π/4-DQPSK

In a TETRA2 system, the existing TETRA1 π/4-DQPSK modulation scheme will still be used, but additional, new modulation schemes will be available. An example of such a new modulation scheme proposed for TETRA2 is π/8-shifted differential 8 phase shift keying (which will be referred to as π/8-D8PSK hereafter) . This modulation scheme is described in the Applicants UK Patent No. 2380103 filed 28 June 2002. Figure 2 shows the constellation diagram for π/8-D8PSK modulation. The modulation again is differential, but 3 bits are used to define the phase changes, with the allowed phase changes being ±π/8, ±3π/8, ±5π/8 and +7π/8. After a sequence of bits has been passed through the modulator, in all, 16 points in the constellation may have been used. The bit to phase mapping for TETRA2 using π/8-D8PSK modulation is as follows :

Table 2: Phase shift and bit definition π/8-D8PSK In a TETRA2 system using π/4-DQPSK modulation and another modulation scheme, such as π/8-D8PSK, information is transmitted in timeslots consisting of 255 symbols which may be modulated using either π/4-DQPSK or the alternative modulation scheme such as π/8-D8PSK. In such a system, a base station, for example, may transmit a sequence of timeslots in π/4-DQPSK modulation, or a sequence of timeslots in, e.g., π/8-D8PSK modulation, or may mix timeslots of either modulation type. Each timeslot may, as is known in the art, be intended for the same or different receivers . In communications systems that can use more than one modulation type, it is often desirable for the system to be able to select and change in use the modulation type being used for a given transmission, for example as transmission conditions or requirements change. For example, in a TETRA2 system it is desirable for the transmitting station to be able to adjust its modulation according to the properties of the transmission channel. For example, in a reliable channel it might be preferred to transmit using a higher data-rate modulation scheme, such as π/8-D8PSK, as that would give a greater data throughput. However, π/8-D8PSK modulation has a shorter range than π/4-DQPSK modulation because it provides less energy per information bit for the same transmit power. Thus if the transmitter moves to an area of less favourable radio propagation conditions, a better throughput of error-free data may be achievable by switching to π/4-DQPSK modulation. Equally, when transmission conditions improve, it may be desirable to switch back to using π/8-D8PSK modulation. As will be appreciated by those skilled in the art, ' when the transmitter switches its modulation scheme in use, the receiver must correspondingly switch to using the new modulation scheme. This could be achieved, e.g., by the transmitter (e.g. base station) transmitting signalling information to the receiver (e.g. mobile station) to instruct the receiver to switch to receiving and/or transmitting with a different modulation scheme, so that the mobile and base station receivers know what method to use for demodulating subsequent signalling. In another known technique that is used in, e.g., TETRA2, each transmission timeslot includes a field as part of its header that indicates the modulation scheme being used for that timeslot. This slot-header is well-protected and is always transmitted using the same, predetermined modulation scheme that is not changed in use. For example, in the case of TETRA2 using QAM modulation, it is intended that the slot-header will always be modulated using 4-QAM modulation, even if the rest of the timeslot uses, e.g. 16-QAM or 64-QAM. The receiver can accordingly always demodulate this slot-header using the predetermined modulation scheme and thereby read the contents of the slot-header and determine what method it should use to demodulate the rest of the timeslot. An advantage of this latter arrangement is that it avoids the need for there to be an initial exchange of signalling between the transmitter and receiver, before the transmitter can change its modulation scheme. Thus it facilitates, for example, a transmitter selecting its modulation scheme on a slot-by-slot basis, e.g., depending on its perception of current channel conditions, and any feedback it may be getting from the receiving party. In arrangements that use a slot-header transmitted using a predetermined modulation scheme to allow the receiver to identify the modulation scheme used for the remainder of the timeslot, it is, as will be appreciated by those skilled in the art, important that the slot-header is transmitted using a reliable modulation method, so that it can still be decoded even in the worst transmission channel conditions. This generally means that the slot-header information about the modulation used in the rest of the timeslot must be encoded across several bits, and preferably across several symbols, so as to provide a high level of redundancy. This accordingly consumes a proportion of the capacity of the timeslot that would otherwise be available for other signalling. The Applicants believe therefore that there remains scope for new techniques for allowing a receiver to identify the modulation scheme being used by a transmitter in a communications system. Thus, according to a first aspect of the present invention, there is provided a method of determining the modulation scheme being used for a transmission in a communications system that can use two or more different modulation schemes, the method comprising: a receiver of the transmission determining the modulation scheme being used for the transmission by assessing a characteristic of the transmission. According to a second aspect of the present invention, there is provided an apparatus for determining the modulation scheme being used for a transmission in a communications system that can use two or more different modulation schemes, the apparatus comprising: means for determining the modulation scheme being used for the transmission by assessing a characteristic of the transmission. According to a third aspect of the present invention, there is provided a method of operating a receiver in a communications system, the method comprising:~— ^~ - the receiver determining that the modulation scheme being used for a transmission has changed by assessing a characteristic of the transmission. According to a fourth aspect of the present invention, there is provided an apparatus for use in a receiver of a communications system, the apparatus comprising: means for determining that the modulation scheme being used for a transmission has changed by assessing a characteristic of the transmission. In the present invention, the modulation scheme being used for the transmission is determined by assessing a characteristic of the received transmission. The Applicants have recognised that it is not necessary to include data (information) in the transmission (such as a slot-header or other signalling) whose purpose it is to inform the receiver of the modulation scheme being used, but that the receiver can in fact identify the modulation scheme being used by assessing one or more particular characteristics or properties of the transmission. The present invention accordingly facilitates the identification of the modulation scheme being used (and accordingly the ability to change modulation schemes in use) without the need, e.g., for the transmitter to include specific data, for example in a slot-header, in the transmission whose purpose it is to identify the modulation scheme being used. The elimination of the need to use specific data, e.g., a slot-header, in this way, accordingly makes more of the capacity of the transmission (e.g. timeslot) available for other signalling. The characteristic or characteristics of the transmission that is assessed to determine the modulation scheme being used can be selected as desired. It should be a characteristic of the transmission that can be used ^" to determine the modulation scheme being used. Thus it is preferably a property of the transmission or of a portion of the transmission which is transmitted with the modulation scheme in question that can be used to determine the identity of the modulation scheme used for the (that portion of the) transmission. In a preferred embodiment, a portion or field or part (e.g. particular information or data portion or field) of the transmission that is included for another purpose in the transmission (i.e. not specifically to identify the modulation scheme being used) is assessed.

Most preferably the assessment is of a part or portion of the transmission (e.g. timeslot) that is (always) modulated using the modulation scheme that it is desired to determine (i.e. the modulation scheme being used for the transmission (e.g. timeslot) that the receiver needs to identify). (This should be contrasted with, e.g., the above-discussed slot-header technique where the slot-header may be modulated using a predetermined modulation scheme that differs from the modulation scheme used for the rest of the timeslot.) In a particularly preferred embodiment, the receiver uses an assessment of a "training sequence" (or sequences) included in the transmission to determine the modulation scheme being used. As is known in the art, in many (mobile) communications systems, including TETRA systems, predetermined or known signal sequences commonly referred to as training sequences are transmitted at predetermined positions in the signal to assist a receiver of the signal to synchronise to the transmission. The training sequences used for such synchronisation purposes typically comprise, as is known in the art, predetermined bit or symbol patterns that a receiver can recognise and are, as discussed above, usually located in identifiable positions in the signal transmission, such as at the beginning of each timeslot . In order to synchronise to the transmission using the transmitted training sequence, the receiver usually carries out a correlation between reference training sequence vectors that it stores and the received signal over a range of timing positions in the received signal to see if it can "match" a reference training sequence to a signal portion of the received signal. This correlation is usually of the form:

where C_j is the correlation coefficient at position j , t is the i^th training sequence symbol, r _+j is the complex conjugate of the received symbol at position i + j , and the summation is over n training sequence symbols. The correlation function, c_j; is typically calculated over a range of relative timing positions. The correct timing position is identified by the presence of a peak in the correlation function, and the receiver can then synchronise to the transmission using that timing position. The Applicants have recognised that an assessment of such training sequences can be used by a receiver to determine the modulation scheme being used. This is because the training sequence or sequences used in the transmission (e.g. timeslot) will typically be dependent upon the modulation scheme being used for the transmission (e.g. timeslot) , and/or can be designed so as to differ from modulation scheme to modulation scheme, such that accordingly, by determining the training sequence that is included in the transmission, the modulation scheme being used can be determined. Thus, for example, in a TETRA2 system, the transmission is preferably-analysed to determine whether it includes a π/4-DQPSK training sequence, or another training sequence (e.g. a π/8-D8PSK training sequence) , with the modulation scheme being used then being determined accordingly. It is believed that this arrangement may be new and advantageous in its own right. Thus, according to a fifth aspect of the present invention, there is provided a method of determining the modulation scheme being used for a transmission in a communications system, the method comprising: a receiver of the transmission determining the modulation scheme being used for the transmission on the basis of a training sequence included in the transmission. According to a sixth aspect of the present invention, there is provided an apparatus for determining the modulation scheme being used for a transmission in a communications system, the apparatus comprising: means for determining the modulation scheme being used for the transmission on the basis of a training sequence included in the transmission. The training sequence included in the transmission is preferably determined by the receiver on the basis of an assessment of the correlation properties of the training sequence included in the transmission. Most preferably the receiver attempts to correlate the received signal with each known or expected form of training sequence for the communications system in question. The training sequence giving the best correlation would then be taken to indicate the training sequence included in the transmission (and accordingly the modulation scheme being used) . Thus, for example, in a TETRA2 system, the received transmission would preferably be correlated using the known or expected π/4- DQPSK training sequence (or sequences) and the known or expected, e.g., π/8-D8PSK training sequence (or sequences), and the training sequence giving the ireslr^' " " correlation then taken to indicate the modulation scheme being used. Thus, the present invention also extends to the use of a training sequence included in a transmission (and preferably of the correlation properties of the training sequence) to determine the modulation scheme used for the transmission (and/or to distinguish different modulation schemes) . The use of a training sequence to identify the modulation scheme being used is advantageous because it uses a portion of the transmission that would be present in any event (for time and frequency synchronisation purposes) and which can be used for modulation scheme identification without the need to use any additional signalling bits . The training sequence will also tend always use the same modulation scheme as the rest of the timeslot (transmission) . Where an assessment of the training sequence is to be used to identify the modulation scheme, it is preferred that the training sequences used for each different modulation scheme are chosen so as to be different, preferably as far as is possible, from each other, as that facilitates the identification of a training sequence specific to a given modulation scheme. Thus, in a particularly preferred embodiment, the training sequences for the different modulation schemes are chosen so as to provide better, preferably good, correlation properties (e.g. a high correlation peak) with their intended modulation scheme (i.e. good "auto-correlation" properties) , and so as to provide poorer, preferably poor, correlation properties (e.g. a "flat" correlation curve) with one or more and preferably all of the other modulation schemes that can be used in the communications system (i.e. to exhibit poor cross-correlation with the other modulation schemes) . This then facilitates identifying the particular training sequence included in a transmission using its correlation properties in the dif erent possible modulation schemes . Thus, for example, in the case of a TETRA2 system that uses π/4-DQPSK and π/8-D8PSK modulation, the training sequences used for the π/8-D8PSK modulated timeslots (or at least the central training sequences included therein) are preferably chosen so as to provide good auto-correlation properties (i.e. when correlated with a (reference) copy of themselves, i.e. using the correct training sequence and modulation scheme) , and poor cross-correlation properties when correlated using π/4-DQPSK modulation (i.e. when correlated with the existing or intended π/4-DQPSK training sequence (s) ) . Most preferably the training sequences are selected such that they do not exhibit peaks in excess of 0.15 within 3 lags either side of the (central) correlation peak when auto-correlated. Preferably, the auto-correlation amplitude does not exceed 0.15 for any lag except for the lag 0 position. Similarly, the training sequence preferably has a maximum cross-correlation peak of 0.4 (i.e. when correlated with a different training sequence, e.g. used for another modulation scheme) . Most preferably, the maximum cross-correlation peak for any lag is less than 0.35. The Applicants have found that π/8-D8PSK modulation training sequences including the following sequences of (relative) phase shifts fulfill these requirements particularly well:

π/8-D8PSK Training Sequence 1:

-5π/8, +3π/8, +5π/8, -5π/8, +π/8, -5π/8, +7π/8, -3π/8, -7π/8, +π/8, +π/8

π/8 -D8^"PSK^" Training Sequence 2:

+5π/8, -3π/8, -5π/8, +5π/8, -π/8, +5π/8, -7π/8, +3π/8, +7π/8, -π/8, -π/8

(These sequences show the relative phase shifts between successive symbols transmitted for the training sequence. As is known in the art, the absolute phase values are unimportant; it is the phase shifts between consecutive symbols that are important . ) As will be appreciated from the discussion of TETRA π/8-D8PSK modulation and in particular Table 2, above, these symbol/phase shift sequences correspond, using the mapping shown in Table 2 above, to the following bit sequences :

π/8-D8PSK Training Sequence 1:

(1,1,1), (0,0,1), (1,0,1), (1,1,1), (0,0,0), (1,1,1), (1,0,0), (0,1,1), (1,1,0), (0,0,0), (0,0,0)

π/8-D8PSK Training Sequence 2:

(1,0,1), (0,1,1), (1,1,1), (1,0,1), (0,1,0), (1,0,1), (1,1,0), (0,0,1), (1,0,0), (0,1,0), (0,1,0) (As will be appreciated by those skilled in the art, depending on the mapping of bit combinations to symbols (phase shifts) , other bit sequences could, of course, be used to generate the above phase shift sequences) . These training sequences are a conjugate pair. The above bit sequences for use as or in training sequences contain 33 bits. Thus they would be used alone where a 33 bit training sequence is required. Some TETRA bursts provide room for 36 bits in their training sequence field. In that case, the above bit sequences can be supplemented by appending a 3 bit sequeϊiue^~~(0, 0, 0) (a +π/8 phase shift) for padding at the end of the (bit/phase shift) sequence. These training sequences (and in particular symbol/phase shift sequences) for π/8-D8PSK modulation are believed to be particularly advantageous, as they have been found to have particularly good auto-correlation properties, plus a low cross-correlation with each other and the already defined π/4-DQPSK TETRA training sequences. Thus, the present invention also extends to these training sequences (and phase shift/symbol sequences and/or bit sequences) for π/8-

D8PSK modulated transmissions, the use of these training sequences (and phase shift/symbol sequences and/or bit sequences) in a TETRA communications system and in a TETRA π/8-D8PSK modulated timeslot, and suitable apparatus for achieving such use. Similarly, according to another aspect of the present invention, there is provided a method of generating a training sequence for use with π/8-D8PSK modulation in a TETRA communications system that can use π/4-DQPSK and π/8-D8PSK modulation schemes, comprising: selecting the training sequence to use with π/8- D8PSK modulation on the basis of the correlation properties of the training sequence with π/8-D8PSK modulation and with π/4-DQPSK modulation. According to another aspect of the present invention, there is provided an apparatus for generating a training sequence for use with π/8-D8PSK modulation in a TETRA communications system that can use π/4-DQPSK and π/8-D8PSK modulation schemes, the apparatus comprising: means for selecting the training sequence to use with π/8-D8PSK modulation on the basis of the correlation properties of the training sequence with π/8-D8PSK modulation and with π/4-DQPSK modulation. According to a further aspect of the present invention, there is provided a method of generating a training sequence for use with a first modulation scheme in a communications system that can use the first modulation scheme and a second, different modulation scheme, comprising: selecting the training sequence to use with the first modulation scheme on the basis of the correlation properties of the training sequence with the first modulation scheme and with second modulation scheme. According to another aspect of the present invention, there is provided an apparatus for generating a training sequence for use with a first modulation scheme in a communications system that can use the first modulation scheme and a second, different modulation scheme, the apparatus comprising: means for selecting the training sequence to use with the first modulation scheme on the basis of the correlation properties of the training sequence with the first modulation scheme and with second modulation scheme . As discussed above, the training sequence is preferably selected so as to provide good (auto-) correlation properties with the first modulation scheme (e.g. π/8-D8PSK modulation), and poor (cross-) correlation properties with the second modulation scheme (e.g. π/4-DQPSK modulation) . In another preferred embodiment, the determination of the modulation scheme is based on an assessment of a portion of the transmission after the transmission has been demodulated. The assessment can then be (and preferably is) used to determine the modulation scheme being used for (or in) the demodulated portion of the transmission. In such an arrangement, the receiver preferably demodulates the transmission using more than one modulation scheme (and preferably using each modulation scheme that it knows can be, or expects to be, usedr^~in the communications system) , and then determines the modulation scheme being used (in that portion of the transmission) by analysing the so-demodulated versions of the transmission. It is again believed that this arrangement may be new and advantageous in its own right. Thus, according to a seventh aspect of the present invention, there is provided a method of determining the modulation scheme being used for a transmission in a communications system, the method comprising: a receiver of the transmission demodulating at least a portion of the transmission using two or more different modulation schemes, and determining the modulation scheme being used for the transmission by assessing a characteristic of the so-demodulated portions of the transmission. According to an eighth aspect of the present invention, there is provided an apparatus for determining the modulation scheme being used for a transmission in a communications system, the apparatus comprising: means for demodulating at least a portion of the transmission using two or more different modulation schemes, and for determining the modulation scheme being used for the transmission by assessing a characteristic of the so-demodulated portions of the transmission. The portion of the transmission that is used in this arrangement can be selected as desired. It is preferably a portion of the transmission that will readily indicate whether the transmission has been correctly demodulated. Thus, in a particularly preferred embodiment, an error correction part or indicator included in the transmission (timeslot) is analysed. Most preferably a cyclic redundancy check (CRC) included in the transmission is analysed (with a valid CRC then being taken to indicate that the transmission has been correctly demodulated (and accordingly the modulation scheme being used) ) . It may also, for example, be possible to use a "fraτtιe^"che^~ck sequence" in some communications systems. Thus, in a TETRA2 system, for example, the received timeslot would preferably be demodulated twice, once assuming π/4-DQPSK modulation, and once assuming, e.g., π/8-D8PSK modulation, checking in each case to see if the CRC in the demodulated timeslot is valid. Thus, the present invention also extends to the use of a cyclic redundancy check field included in a transmission to determine the modulation scheme being used for the transmission (and/or to distinguish different modulation schemes) . In another particularly preferred embodiment the modulation scheme being used for the transmission is determined by assessing whether the transmitted signal has a particular characteristic of a given modulation scheme or schemes. The Applicants have recognised that many modulation schemes will have characteristic properties to their signals that can be relatively rapidly identified and will distinguish them from signals of other modulation schemes. Thus assessing whether the transmitted signal has the relevant particular characteristic gives an assessment of the likelihood that the signal has been transmitted using that modulation scheme. It is again believed that this arrangement may be new and advantageous in its own right. Thus, according to a ninth aspect of the present invention, there is provided a method of determining the modulation scheme being used for a transmission in a communications system that can use two or more different modulation schemes, the method comprising: a receiver of the transmission determining the modulation scheme being used for the transmission by assessing whether the transmitted signal has a particular characteristic of a or^of more than one of the modulation schemes that can be used by the communications system. According to a tenth aspect of the present invention, there is provided an apparatus for determining the modulation scheme being used for a transmission in a communications system that can use two or more different modulation schemes, the apparatus comprising: means for determining the modulation scheme being used for the transmission by assessing whether the transmitted signal has a particular characteristic of a or of more than one of the modulation schemes that can be used by the communications system. According to an eleventh aspect of the present invention, there is provided a method of operating a receiver in a communications system, the method comprising: the receiver determining that the modulation scheme being used for a transmission has changed by assessing whether the transmitted signal has a particular characteristic of a selected modulation scheme. According to a twelfth aspect of the present invention, there is provided an apparatus for use in a receiver of a communications system, the apparatus comprising: means for determining that the modulation scheme being used for a transmission has changed by assessing whether the transmitted signal has a particular characteristic of a selected modulation scheme. The characteristic (s) of the modulation scheme or schemes for which the transmitted signal is assessed can be selected as desired. For each modulation scheme, it should be a characteristic of the modulation scheme that can be used to distinguish it from other modulation schemes . It should also preferably not take too long to assess . More than one characteristic can be considered if desired. Preferably, the characteristic of the transmitted signal is assessed, and the observed signal characteristics then compared with the expected characteristics for the modulation scheme, e.g. to see how well they correspond, to assess whether the transmitted signal has the required characteristic. In a particularly preferred embodiment, the assessment of whether the received signal has a particular characteristic of a given modulation scheme comprises an assessment of whether the signal exhibits particular phase shift characteristics, e.g. between consecutive symbols. This could be assessed, e.g., by analysing the phase shifts between successive selected samples or portions, e.g. symbols, in the signal and comparing the observed phase shifts with the expected phase shifts for the known or expected modulation schemes, e.g. to see how well they correspond, to assess which of those particular forms of modulation the signal carries . The Applicants have recognised that many modulation schemes have particular phase shift characteristics that can be assessed relatively quickly. Thus considering the phase shifts exhibited by the received signal can give a good assessment of its modulation scheme. For example, π/4-DQPSK modulation as used in TETRA has the particular characteristic that there is a +/- π/4 or +/- 3π/4 radians phase shift between each transmitted symbol. On the other hand, π/8-D8PSK modulation as proposed for use in TETRA2 exhibits a +/- π/8, +/- 3π/8, +/- 5π/8, or a +/- 7π/8 radians phase shift between each transmitted symbol . Thus, an assessment of the phase shifts between consecutive symbols in the received signal can be used to distinguish between the use of π/4-DQPSK or π/8-D8PSK modulation. For example, the transmitted signal could be assessed to see whether the phase shifts it exhibits cluster around +/- π/4 or +/- 3π/4 (thereby indicating π/4-DQPSK modulation), or cluster around ^■■^■*/ =■ π/8, +/- 3π/8, +/- 5π/8 and +/- 7π/8 (thereby indicating π/8- D8PSK modulation) . Similarly, the GSM system which uses GMSK (Gaussian Minimum Shift Keying) modulation, employs phase shifts of +/- π/2 radians relative to a steady RF (radio frequency) carrier. In this case a phase shift of zero is permitted, so a characteristic of GMSK modulation is the presence of phase shifts of either zero radians or π radians between successive symbols (more specifically a +π is always followed by either 0 or -π, and no +π may follow +π and any subsequent O's until a -π has occurred; a -π is always followed by either 0 or +π, and no -π may follow -π and any subsequent 0 ' s until a +π has occurred) . It is again believed that these arrangements may be new and advantageous in their own right. Thus, according to a thirteenth aspect of the present invention, there is provided a method of determining the modulation scheme being used for a transmission in a communications system that can use two or more different modulation schemes, the method comprising: a receiver of the transmission determining the modulation scheme being used for the transmission by assessing phase shifts exhibited by the transmission. According to a fourteenth aspect of the present invention, there is provided an apparatus for determining the modulation scheme being used for a transmission in a communications system that can use two or more different modulation schemes, the apparatus comprising: means for determining the modulation scheme being used for the transmission by assessing phase shifts exhibited by the transmission. According to a fifteenth aspect of the present invention, there is provided a method of operating a receiver of a communications system, the method comprising: the receiver determining that the modulation scheme being used for a transmission has changed by assessing phase shifts exhibited by the transmission. According to a sixteenth aspect of the present invention, there is provided an apparatus for a receiver of a communications system, the apparatus comprising: means for determining that the modulation scheme being used for a transmission has changed by assessing phase shifts exhibited by the transmission. The present invention accordingly also extends to the use of phase shifts exhibited by a transmission for determining the modulation scheme being used for the transmission (and/or for distinguishing between different modulation schemes) . Characteristics other than phase shifts, such as frequency shifts and/or amplitude shifts, could instead or also be used, and may, e.g., be more appropriate for particular modulation schemes. For example, other modulation schemes (such as 16QAM) may have other characteristics which can be detected in an appropriate manner. For example, a four-level FM modulation scheme will exhibit FM modulation with transitions between four different modulating frequencies, and so will show characteristic frequency shifts. To determine the type of QAM modulation which is being used, phase and amplitude shifts could be assessed simultaneously. The analysis of the phase (or frequency, etc.) shifts can be carried out as desired. Thus, in a TETRA system, for example, the phase shifts between each successive symbol received during a given time period could be measured. In the TETRA system, each symbol has a duration of 56 microseconds, so up to 90 symbol phase shifts could be examined in the 5 ms period (or up to 18 in a 1 ms period) . The measured phase (or frequency, etc.) shifts can be used as desired to assess whether the signal has the particular modulation scheme characteristic. For example, the actual values of the phase shifts could be considered. The assessment could, for example, comprise looking at whether appropriate values and/or sequences of phase shifts are observed. In another arrangement the measured phase shifts for the symbols could be mapped to a common (e.g. the first) quadrant (i.e. have their modulation induced phase differences removed) to obtain a mean phase shift value and that value compared with the expected values for the modulation schemes used by the communications system. The variation in the values could also be considered. In a particularly preferred embodiment, the assessment of whether or not the received signal has the particular characteristic comprises allocating a probability that the signal has that characteristic based on the assessment of the signal. Preferably three levels of relative probability, low, medium and high are used. Thus preferably the fit of the received signal to the desired characteristic is measured on a probability scale and the probability of a match, e.g. the probability that it carries the particular form of modulation recorded. For example, the deviation of the mean phase shift from the expected value if the signal were to carry a particular form of modulation and/or the proportion of measured phase shift values having the value of a particular modulation scheme, could be used to classify how likely the signal is to have been modulated using that particular modulation scheme. As discussed above, the present invention facilitates the changing by a transmitter of the modulation scheme that it is using without the need to include in its transmission some form of information signalling or data to tell the receiver of the modulation scheme change (i.e. allows the transmitter^" to make "unannounced" modulation scheme switches) . Thus, in use of the present invention, the transmitter can decide in use to switch modulation schemes, for example because of a change in radio propagation conditions, and the receiver will detect that change and switch to using the new modulation scheme based on an assessment of a characteristic of the received transmission. Thus, according to a seventeenth aspect of the present invention, there is provided a method of operating a transmitter in a communications system in which the transmitter can transmit using more than one modulation scheme, the method comprising: the transmitter transmitting to a receiver using a first modulation scheme; the transmitter changing the modulation scheme it is using for transmission and transmitting using the new modulation scheme without including in its transmission an information portion for informing the receiver of the new modulation scheme being used. According to an eighteenth aspect of the present invention, there is provided an apparatus for operating a transmitter in a communications system in which the transmitter can transmit using more than one modulation scheme, the apparatus comprising: means for transmitting to a receiver using a first modulation scheme; means for changing the modulation scheme being used for transmission and for transmitting using the new modulation scheme without including in the transmission an information portion for informing the receiver of the new modulation scheme being used. In these aspects and embodiments of the invention, the transmitter changes the modulation scheme it is using without, e.g., either first signalling to the receiver that the change is going to take place, or including in its transmissions a signal portion (such as a slot-header) that includes data for indicating the modulation scheme being used. In other words, the transmitter does not use an information portion nor include data in its transmission solely for the purpose of allowing the receiver to identify the modulation scheme being used. Thus it does not, for example, use a specific indicator of the modulation type in a known format . According to a ninteenth aspect of the present invention, there is provided a method of operating a communications system in which a transmitter can transmit using more than one modulation scheme, the method comprising: a transmitter of the communications system transmitting to a receiver using a first modulation scheme; the transmitter changing the modulation scheme it is using for the transmission; and a receiver of the transmission determining that the modulation scheme being used for the transmission has changed by assessing a characteristic of the transmission. According to a twentieth aspect of the present invention, there is provided a communications system in which transmitters can transmit using more than one modulation scheme, the system comprising: one or more transmitters that include means for transmitting to a receiver using a first modulation scheme, and means for changing the modulation scheme being used for a transmission and transmitting using the new modulation scheme; and one or more receivers that include means for determining that the modulation scheme being used for a transmission has changed by assessing a characteristic of the transmission. In these aspects and embodiments of the invention, the transmitter again preferably changes the modulation scheme it is using without, e.g., either first signalling to the receiver that the change is going to take place, or including in its transmissions a signal portion (such as a slot-header) that includes data for indicating the modulation scheme being used. The present invention is applicable to communications systems generally, and is, as discussed above, particularly suited to use in mobile communications systems, and particularly in digital communications systems . It is also particularly suitable for use in communications systems that use timeslot divided transmission (i.e. time division multiple access (TDMA) communications systems) , particularly where the modulation scheme being used for transmission can be changed between successive timeslots (bursts) . The present invention is particularly intended for use in the TETRA2 communications system, although it is, of course, not restricted to use in such a system. The present invention is applicable to communications systems that use different modulation schemes but of the same type, such as both phase modulation (such as is the case for TETRA2 as discussed above) or both QAM (Quadrature Amplitude Modulation) , but it can also be used where the different modulation schemes are of different types . The present invention is particularly suited for use where there are two different modulation schemes, but it can also be used where the communications system uses more than two modulation schemes. In that case, the same principles as discussed above can be applied to determine the modulation scheme being used for the transmission. The present invention also extends to communications devices, such as transmitters, receivers, terminals, base stations, mobile stations, etc., that use the methods of the present invention, and/or that include apparatus in accordance with any one or more aspects of the invention. As will be appreciated by those skilled in the art, all of the above aspects and embodiments of the invention can include, as appropriate, any one or more or all of the preferred and optional features of the invention described herein. For example, it would be possible to use more than one of the particular techniques discussed above to determine the modulation scheme being used, for example by considering both the training sequences and the phase shifts exhibited by the transmission. The methods in accordance with the present invention may 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 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 comprising such software which when used to operate a communications system or communications device comprising data processing means causes in conjunction with said data processing means said system or device to carry out the steps of the methods 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 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 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 . The present invention may accordingly suitably be embodied as a computer program product for use with a computer system. Such an implementation may comprise a series of computer readable instructions either fixed on a tangible medium, such as a computer readable medium, for example, diskette, CD-ROM, ROM, or hard disk, or transmittable to a computer system, via a modem or other interface device, over either a tangible medium, including but not limited to optical or analogue communications lines, or intangibly using wireless techniques, including but not limited to microwave, infrared or other transmission techniques. The series of computer readable instructions embodies all or part of the functionality previously described herein. Those skilled in the art will appreciate that such computer readable instructions can be written in a number of programming languages for use with many computer architectures or operating systems. Further, such instructions may be stored using any memory technology, present or future, including but not limited to, semiconductor, magnetic, or optical, or transmitted using any communications technology, present or future, including but not limited to optical, infrared, or microwave. It is contemplated that such a computer program product may be distributed as a removable medium with accompanying printed or electronic documentation, for example, shrink-wrapped software, pre-loaded with a computer system, for example, on a system ROM or fixed disk, or distributed from a server or electronic bulletin board over a network, for example, the Internet or World Wide Web . A number of preferred embodiments of the present invention will now be described by way of example only¹ and with reference to the accompanying drawings, in which: Figure 1 is a constellation diagram for π/4-DQPSK modulation; Figure 2 is a constellation diagram for π/8-D8PSK modulation; Figure 3 shows schematically the basic elements of a digital communications system; Figure 4 shows schematically the timeslot structure used in the TETRA communications system; Figure 5 shows schematically a TETRA training sequence for use with π/4-DQPSK modulation; Figure 6 shows the auto- and cross-correlation performance of known TETRA π/4-DQPSK training sequences; Figure 7 shows auto- and cross-correlation performance of preferred examples of TETRA π/8-D8PSK training sequences; Figure 8 compares the auto-correlation amplitudes of the π/8-D8PSK and π/4-DQPSK training sequences the subject of Figures 6 and 7; and Figure 9 shows the cross-correlation amplitudes of the π/8-D8PSK and π/4-DQPSK training sequences the subject of Figures 6 and 7. Figure 3 shows schematically the basic elements of a digital communications system. The transmitting side includes a data source 1 which provides a binary bit sequence to a modulator 2. The binary bit sequence has a structure to it, and contains with it one or more training sequences as well as other data. The modulator 2 converts the binary bit sequence to a symbol sequence, represented as a vector on the complex plane, as is known in the art. Each symbol is defined by one or more bits at a time depending on the modulation type employed by the modulator 2. The modulator 2 typically also includes filtering and interpolation and produces a base-band version of the signal to be transmitted. The signal to be transmitted is then frequency shifted by an up-converter 3 and amplified by an amplifier 4 before being transmitted. At the receiver, the converse operation takes place. Thus the receiver includes a down-converter 5, a synchronisation mechanism 6, a demodulator 7, and a data sink 8. As discussed above, a training sequence is included in the transmission by the transmitter, and is used by the synchronisation mechanism 6 (typically by carrying out a correlation between known training sequences and the received signal) to allow the receiver to identify the relevant parts of the transmission. The operation of a digital communications such as that shown in Figure 3 in accordance with the present invention will now be described. As discussed above, the present invention is applicable to communications systems that use two or more different modulation schemes. The TETRA2 system is an example of such a communications system, and uses, e.g., π/4-DQPSK and π/8- D8PSK modulation for its transmissions. As discussed above, in a TETRA2 system the transmitter may wish to switch from using one modulation scheme to the other during its transmission. The present invention provides a means whereby a receiver can identify such a modulation scheme switch without the transmitter needing to include in its transmission data "announcing" that the modulation scheme switch has been made. The present embodiments will be described with reference to a TETRA2 digital mobile communications system, although, as will be appreciated by those skilled in the art, the present invention is not limited to such a system. A first preferred embodiment of the present invention will be^" described with reference to Figures 4 to 9. In this embodiment, the receiver uses a training sequence included in the transmission to determine the modulation scheme being used. TETRA2 uses training sequences for synchronisation purposes . The training sequences form part of the signal burst and allow a receiver to synchronise to the transmission and to demodulate the signal, as is known in the art. In a continuous TETRA transmission, a training sequence is placed in the middle of each timeslot and a training sequence is also placed in between timeslots (a so-called "inter-slot" training sequence) . Figure 4 illustrates this signal structure of TETRA transmissions . Figure 4 shows two timeslots 9, 10 from a sequence of slots transmitted. Each timeslot contains a leading half training sequence 11, 17, a data field 12, a central training sequence 13, another data field 14 and finally a trailing half training sequence 15. The trailing and leading half training sequences 15, 17 between successive timeslots together define an inter-slot training sequence. The contents of each timeslot, including the training sequences, are defined, as is known in the art, as a pattern of bits which are converted to symbols by a modulator for transmission. (As will be appreciated by those skilled in the art, variations to the timeslot structure shown in Figure 4 are possible (and indeed may be common) . For example, the data fields may contain further divisions, and the leading and trailing half training sequences may not be exactly of the size of half a full training sequence, but may only be approximately half the size of a full training sequence, etc.). When using the timeslot structure illustrated in Figure 4, when a receiver wishes to receive the second timeslot 10, it begins its reception at where it expects the trailing training sequence 15 in the first slot 9 to begin, i.e. at the time 16 as shown in Figure 4. This allows the receiver to synchronise to the second timeslot^" 10 using the trailing half training sequence 15 from the first timeslot 9 and the leading half training sequence 17 from the second time slot 10 (which half training sequences are designed to be combined for this purpose) . This allows the receiver to obtain early synchronisation for the second timeslot 10. Figure 5 shows a π/4-DQPSK training sequence already defined in the current TETRA1 standard. This training sequence is a sequence of 11 symbols generated by a π/4-DQPSK modulator according to the binary bit sequence defined for the training sequence. In Figure 5, the symbols are represented as vectors on the complex plane (constellation diagram) . The training sequence symbol positions shown in Figure 5 are produced by, as is known in the art, providing to the modulator an appropriate bit sequence that will result in that sequence of symbol positions. Thus, in the case of this π/4-DQPSK training sequence, a sequence of bit pairs: (1,0), (1,1), (0,1), (1,1), (0,0), (0,0), (0,1), (1,0), (1,0), (1,1), (0,1) is provided to the modulator. In fact, for TETRA π/4-DQPSK modulation, there are two main training sequences of interest, TS1 and TS2, for normal and stolen bursts respectively. These training sequences are a conjugate pair, i.e. π/4TS2 = π/4TSl* . For the π/8-D8PSK modulation to be used in this exemplary TETRA2 system, two corresponding training sequences will be desirable (once again for normal and stolen bursts) . In the present embodiment, the following exemplary, preferred training sequences (in terms of their (relative) phase shifts between consecutive symbols) are used:

π/8-D8PSK Training Sequence 1: -5π/8, +3π/8, +5π/8, -5π/8, +π/8, -5π/8, +7π/8, -3π/8, -7π/8, +π/8, +π/8

π/8-D8PSK Training Sequence 2: +5π/8, -3π/8, -5π/8, +5π/8, -π/8, +5π/8, -7π/8, +3π/8, +7π/8, -π/8, -π/8

As will be appreciated from the discussion of TETRA π/8-D8PSK modulation and in particular Table 2, above, these phase shift sequences correspond to the following bit sequences:

π/8-D8PSK Training Sequence 1: (1,1,1), (0,0,1), (1,0,1), (1,1,1), (0,0,0), (1,1,1), (1,0,0), (0,1,1), (1,1,0), (0,0,0), (0,0,0)

π/8-D8PSK Training Sequence 2: (1,0,1), (0,1,1), (1,1,1), (1,0,1), (0,1,0), (1,0,1), (1,1,0), (0,0,1), (1,0,0), (0,1,0), (0,1,0)

These training sequences are again a conjugate pair. As can be seen, in the case of the π/8-D8PSK training sequence, a sequence of groups of three bits is provided to the modulator. The above training sequences contain 33 bits. As is known in the art, some TETRA bursts provide room for 36 bits in their training sequence field. In that case, the above training sequences can be supplemented by appending a 3 bit sequence (0,0,0) for padding at the end of the bit sequence. (As will be appreciated by those skilled in the art, depending on the mapping of bit combinations to symbols (phase shifts) , other bit sequences could be used to generate the above symbol/phase shift sequences) . These preferred π/8-D8PSK training sequences have been found to have good auto-correlation properties (i.e. when correlated with "themselves") plus a low cross-correlation (i.e. when correlated with a different training sequence to themselves) with each other and with the original π/4-DQPSK training sequences. They are accordingly sufficiently different from the π/4-DQPSK training sequences that the training sequences can be distinguished by attempting to correlate with each training sequence . As will be appreciated by those skilled in the art, other π/8-D8PSK training sequences are possible. Generally speaking, it is preferred for the π/8-D8PSK training sequences (i.e. the bit sequence when modulated by π/8-D8PSK modulation) to have the following correlation properties (viewing the correlation results over 7 lags either side of the 0 lag position) : the auto-correlation amplitude should not exceed 0.15 for any lag except the lag 0 position; the cross-correlation amplitude for the bit sequence (when modulated by π/8- D8PSK modulation) when cross-correlated with its complex conjugate should not exceed 0.30 for any lag; and the cross-correlation amplitude for the bit sequence (when modulated using π/8-D8PSK) when cross-correlated with each π/4-DQPSK training sequences (i.e. the π/4-DQPSK training sequence bit sequences when modulated using π/4- DQPSK modulation) should not exceed 0.35 for any lag. Figures 6 to 9 illustrate the π/8-D8PSK auto-correlation properties of the above preferred π/8- D8PSK training sequences and their cross-correlation performance with the π/4-DQPSK training sequences 1 and 2 (and hence the ability to discriminate between these four training sequences) . The π/4-DQPSK training sequence 1 and 2, auto- and cross-correlation performance is illustrated in Figure 6. A number of figures of merit can be determined from this Figure, which are as follows: • The auto-correlation amplitude ±2 symbols of the peak should desirably be <0.15. • The maximum auto-correlation amplitude (other than the central peak) should desirably be <~0.3. • The maximum cross-correlation amplitude should desirably be <~0.4

Similar performance would be desirable in a π/8-D8PSK training sequence. Figure 7 is an equivalent figure but for the π/8- D8PSK correlation characteristics whilst Figure 8 compares the π/8-D8PSK and π/4-DQPSK auto-correlation amplitudes . From these Figures it is clear that the selected π/8 training sequences have a performance equal to or for many lags better than that of the π/4-DQPSK training sequences (which is desirable) . Note that as the π/8-D8PSK sequences form a conjugate pair, their amplitude characteristics are identical. Figure 9 plots the π/8-D8PSK and π/4-DQPSK cross- correlation amplitudes (discontinuous line) . The π/4- DQPSK training sequences' 1 and 2 cross-correlation is also plotted for reference (continuous line) . This Figure illustrates that the π/8-D8PSK - π/4-DQPSK cross-correlation performance is as good as that of the original π/4-DQPSK training sequences. (The cross-correlation of 4 training sequences results in 4 cross correlation series. However as the training sequences 1 and 2 for π/4-DQPSK and π/8-D8PSK are conjugate pairs, the cross-correlation amplitude is the same in 2 cases. Therefore only two cross-correlation lines appear in Figure 9.) In this embodiment of the present invention, the above discussed training sequences are used as the central training sequences 13 (Figure 4) in π/4-DQPSK and π/8-D8PSK modulated timeslots as appropriate. The receiver then uses these training sequence (s) included in the transmission to determine the modulation scheme being used (as well as for synchronisation purposes) . It does this by analysing the training sequence in each received timeslot to determine whether it is a π/4-DQPSK or a π/8- D8PSK training sequence. This analysis is carried out by carrying out a correlation between the expected, known training sequences used for each modulation type and the training sequence in the received signal, to see which training sequence produces the best correlation. The modulation scheme of the training sequence that produces the best correlation is then taken to be the modulation scheme being used for the transmission. In a second preferred embodiment, the receiver determines the modulation scheme being used by attempting to demodulate the received timeslot twice, once assuming that it has been transmitted using π/4-DQPSK modulation, and once assuming that it has been transmitted using π/8- D8PSK modulation, and for each case then checks to see if the CRC (cyclic redundancy check) in the transmitted timeslot is valid. The demodulation scheme that produces a valid CRC is then assumed to indicate the modulation scheme that is being used. In a third preferred embodiment, the receiver analyses the phase shifts present in the received signal to determine the modulation scheme being used. This is possible in a TETRA2 system because π/4-DQPSK modulation should produce phase shifts between consecutive symbols clustered at +/- π/4, and +/- 3π/4, whereas π/8-D8PSK modulation should produce phase shifts between consecutive symbols clustered at +/- π/8, +/- 3π/8, +/- 5π/8 and +/- 7π/8. Thus by determining, e.g., the most common phase shift (s) in the received signal, an assessment of the modulation scheme being used can be made. The way that the phase shifts in the received signal are analysed can be selected as desired. In this embodiment the receiver records the digital samples received by its demodulator and using a digital signal processor, examines the recorded digital samples, looking for the phase shifts occurring at the expected symbol rate. The probability that particular phase shifts (e.g. +/- π/4, and +/- 3π/4, or +/- π/8, +/- 3π/8, +/- 5π/8, and +/- 7π/8) have been detected is then determined, and the modulation scheme being used estimated accordingly. Although the present embodiments have been described with reference to TETRA2 and π/4-DQPSK and π/8-D8PSK modulation, as discussed above the present invention is applicable to other modulation types. Generally speaking the present invention is particularly suitable when the modulation schemes are of the same type (e.g. both phase modulation, or both QAM) , but it can be used for modulation schemes of different types as well. As can be seen from the above, the present invention, in its preferred embodiments at least, provides in a communications system in which information is transmitted in timeslots (bursts) and can be so-transmitted using different modulation types, means whereby the transmitter can change the type of modulation it uses on a slot-by-slot basis without the need for prior instruction or information exchange with the receiver, and without the need for the slot to contain, e.g., a specific slot header using a fixed modulation type to indicate what modulation is used for the rest of the timeslot. This is achieved by the receiver determining how to demodulate a received timeslot by inspection of characteristics of the received timeslot (i.e. by the receiver analysing one or more properties of the received signal to determine the modulation in use) . Thus the present invention facilitates, for example, "unannounced" modulation switching, and modulation adaptation without the need to use a specific "-indicator of the modulation type in a known format. For example, in the context of a TETRA2 communications system in which information can be modulated using π/4-DQPSK or π/8-D8PSK modulation, the present invention, in its preferred embodiments at least, would allow the transmitter to be at liberty to make unannounced changes between π/4-DQPSK and π/8-D8PSK modulation at the boundary between timeslots, or when it transmits a single timeslot, and without the need for the timeslot to contain a header using a predetermined modulation for indicating what modulation is used for the rest of the timeslot .

Claims

1. A method of determining the modulation scheme being used for a transmission in a communications system that can use two or more different modulation schemes, the method comprising: a receiver of the transmission determining the modulation scheme being used for the transmission by assessing a characteristic of the transmission.

2. A method of operating a receiver in a communications system, the method comprising: the receiver determining that the modulation scheme being used for a transmission has changed by assessing a characteristic of the transmission.

3. The method of claim 1 or 2, comprising the receiver assessing a training sequence included in the transmission to determine the modulation scheme being used, or the change in the modulation scheme being used.

4. The method of claim 3, comprising the receiver assessing correlation properties of the training sequence included in the transmission.

5. The method of any one of the preceding claims, comprising the receiver assessing a portion of the transmission after the transmission has been demodulated to determine the modulation scheme being used, or the change in the modulation scheme being used.

6. The method of claim 5, comprising the receiver demodulating the transmission using more than one modulation scheme, and then determining the modulation scheme being used, or the change in the modulation scheme being used, by analysing the so-demodulated versions of the transmission.

7. The method of claim 5 or 6, comprising the receiver assessing an error correction part or indicator included in the transmission.

8. The method of any one of the preceding claims, comprising the receiver assessing whether the transmitted signal has a particular characteristic of a given modulation scheme or schemes, to determine the modulation scheme being used, or the change in the modulation scheme being used.

9. The method of claim 8, comprising the receiver assessing whether the transmitted signal exhibits particular phase, frequency and/or amplitude shift characteristics .

10. A method of determining the modulation scheme being used for a transmission in a communications system that can use two or more different modulation schemes, the method comprising: a receiver of the transmission determining the modulation scheme being used for the transmission by assessing phase, frequency and/or amplitude shifts exhibited by the transmission.

11. A method of operating a receiver of a communications system, the method comprising: the receiver determining that the modulation scheme being used for a transmission has changed by assessing phase, frequency and/or amplitude shifts exhibited by the transmission.

12. A method of operating a transmitter in a communications system in which the transmitter can transmit using more than one modulation scheme, the method comprising: the transmitter transmitting to a receiver using a first modulation scheme; the transmitter changing the modulation scheme it is using for transmission and transmitting using the new modulation scheme without including in its transmission an information portion for informing the receiver of the new modulation scheme being used.

13. A method of operating a communications system in which a transmitter can transmit using more than one modulation scheme, the method comprising: a transmitter of the communications system transmitting to a receiver using a first modulation scheme; the transmitter changing the modulation scheme it is using for the transmission; and a receiver of the transmission determining that the modulation scheme being used for the transmission has changed by assessing a characteristic of the transmission.

14. The use of a training sequence included in a transmission to determine the modulation scheme used for the transmission or to distinguish different modulation schemes .

15. The use of one or more training sequences including the following phase shift sequences :

-5π/8, +3π/8, +5π/8, -5π/8, +π/8, -5π/8, +7π/8, •3π/8, -7π/8, +π/8,^' +π/8; or +5π/8, -3π/8, -5π/8, +5π/8, -π/8, +5π/8, -7π/8, +3π/8, +7π/8, -π/8, -π/8 in π/8-D8PSK modulated transmissions.

16. The use of one or more training sequences including the following bit sequences:

(1,1,1), (0,0,1) ,'-(1,0,1) , (1,1,1), (0,0,0), (1,1,1), (1,0,0), (0,1,1), (1,1,0), (0,0,0), (0,0,0); or

(1,0,1), (0,1,1), (1,1,1), (1,0,1), (0,1,0), (1,0,1), (1,1,0), (0,0,1), (1,0,0), (0,1,0), (0,1,0), in π/8-D8PSK modulated transmissions.

17. A method of generating a training sequence for use with π/8-D8PSK modulation in a TETRA communications system that can use π/4-DQPSK and π/8-D8PSK modulation schemes, comprising: selecting the training sequence to use with π/8- D8PSK modulation on the basis of the correlation properties of the training sequence with π/8-D8PSK modulation and with π/4-DQPSK modulation.

18. The method of claim 17, wherein the training sequence is selected so as to provide good correlation properties with π/8-D8PSK modulation, and poor correlation properties with π/4-DQPSK modulation.

19. A method of generating a training sequence for use with a first modulation scheme in a communications system that can use the first modulation scheme and a second, different modulation scheme, comprising: selecting the training sequence to use with the first modulation scheme on the basis of the correlation properties of the training sequence with the first modulation scheme and with second modulation scheme.

20. The method of claim 18, wherein the training sequence is selected so as to provide good correlation properties with the first modulation scheme, and poor correlation properties with the second modulation scheme.

21. The use of a cyclic redundancy check field included in a transmission to determine the modulation scheme being used for the transmission or to distinguish different modulation schemes .

22. The use of phase, frequency and/or amplitude shifts exhibited by a transmission for determining the modulation scheme being used for the transmission or for distinguishing between different modulation schemes.

23. An apparatus for determining the modulation scheme being used for a transmission in a communications system that can use two or more different modulation schemes, the apparatus comprising: means for determining the modulation scheme being used for the transmission by assessing a characteristic of the transmission.

24. An apparatus for use in a receiver of a communications system, the apparatus comprising: means for determining that the modulation scheme being used for a transmission has changed by assessing a characteristic of the transmission.

25. The apparatus of claim 23 or 24, comprising: means for determining the modulation scheme being used for the transmission on the basis of a training sequence included in the transmission.

26. The apparatus of claim 25, comprising means for assessing correlation properties of the training sequence included in the transmission.

27. The apparatus of any one of claims 23 to^"~2^'6, comprising means for assessing a portion of the transmission after the transmission has been demodulated to determine the modulation scheme being used, or the change in the modulation scheme being used.

28. The apparatus of claim 27, comprising: means for demodulating at least a portion of the transmission using two or more different modulation schemes, and for determining the modulation scheme being used for the transmission by assessing a characteristic of the so-demodulated portions of the transmission.

29. The apparatus of any one of claims 23 to 28, comprising: means for assessing whether the transmitted signal has a particular characteristic of a given modulation scheme or schemes, to determine the modulation scheme being used, or the change in the modulation scheme being used.

30. The apparatus of claim 29, comprising means for assessing whether the transmitted signal exhibits particular phase, frequency and/or amplitude shift characteristics .

31. An apparatus for determining the modulation scheme being used for a transmission in a communications system that can use two or more different modulation schemes, the apparatus comprising: means for determining the modulation scheme being used for the transmission by assessing phase, frequency and/or amplitude shifts exhibited by the transmission.

32. An apparatus for a receiver of a communications system, the apparatus comprising: means for determining that the modulation scheme being used for a transmission has changed by assessing phase, frequency or amplitude shifts exhibited by the transmission.

33. An apparatus for operating a transmitter in a communications system in which the transmitter can transmit using more than one modulation scheme, the apparatus comprising: means for transmitting to a receiver using a first modulation scheme; means for changing the modulation scheme being used for transmission and for transmitting using the new modulation scheme without including in the transmission an information portion for informing the receiver of the new modulation scheme being used.

34. A communications system in which transmitters can transmit using more than one modulation scheme, the system comprising: one or more transmitters that include means for transmitting to a receiver using a first modulation scheme, and means for changing the modulation scheme being used for a transmission and transmitting using the new modulation scheme; and one or more receivers that include means for determining that the modulation scheme being used for a transmission has changed by assessing a characteristic of the transmission.

35. An apparatus for generating a training sequence for use with π/8-D8PSK modulation in a TETRA communications system that can use π/4-DQPSK and π/8-D8PSK modulation schemes, the apparatus comprising: means for selecting the training sequence to use with π/8-D8PSK modulation on the basis of the correlation properties of the training sequence with π/8-D8PSK modulation and with π/4-DQPSK modulation.

36. An apparatus for generating a training sequence for use with a first modulation scheme in a communications system that can use the first modulation scheme and a second, different modulation scheme, the apparatus comprising: means for selecting the training sequence to use with the first modulation scheme on the basis of the correlation properties of the training sequence with the first modulation scheme and with second modulation scheme.

37. A communications device including the apparatus of any one of claims 23 to 33.

38. A TETRA mobile communications system including the apparatus of any one of claims 23 to 33.

39. A computer program element comprising computer software code portions for performing the methods of any one of claims 1 to 22 when the program element is run on data processing means.

40. A method of operating a receiver in a communications system substantially as hereinbefore described with reference to any one of the accompanying drawings .

41. A method of operating a transmitter in a communications system substantially as hereinbefore described with reference to any one of the accompanying drawings .

42. A method of generating a training sequence substantially as hereinbefore described with reference to any one of the accompanying drawings .

43. An apparatus for use in a receiver of a communications system substantially as hereinbefore described with reference to any one of the accompanying drawings .

44. A communications system substantially as hereinbefore described with reference to any one of the accompanying drawings .