Abstract:
A transmitter outputs a radio signal formed from a first baseband signal incorporating a synchronization signal. The recipient obtains a second baseband signal from the received radio signal, detects the synchronization signal in a portion of the second baseband signal in order to compute synchronization parameters and/or parameters for estimating a radio transmission channel, and uses the computed parameters to demodulate another portion of the second baseband signal and extract the transmitted information from it. The synchronization signal contains a synchronization pattern selected by the transmitter on the basis of signalling information to be supplied to the second station. The receiver searches several patterns in the second baseband signal in order to obtain the signalling information depending on the detected pattern.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to the signalling methods used in radio communication networks.  
           [0002]    Within a radio communication system, the information to be transmitted is often accompanied by a signal, known in advance to the transmitter and the receiver. Detection of this signal enables the receiver to synchronise with the transmitter (in time and/or frequency). In certain systems, this detection also enables parameters of the radio propagation channel between the transmitter and the receiver to be detected, whereby a coherent demodulation can be applied to the signal in order to extract the information transmitted.  
           [0003]    To perform such detection, the receiver computes a time correlation function between the received signal and the known pattern of the transmitted signal. The time synchronisation is characterised by the position of the maximum of the modulus of this correlation function. The correlation function or the value of its maximum modulus provides an observation of the channel response and can be used to estimate this response. Performing correlation computations on differential signals avoids any problems due to possible carrier frequency discrepancies between the transmitter and the receiver, and makes it possible to evaluate these discrepancies in order to correct them.  
           [0004]    To transmit signalling information, most systems use control channels on which this information is transmitted or broadcast.  
           [0005]    Sometimes, some of the signalling information is supplied by reserving a portion of the signal transmitted on a dedicated communication channel (a few information bits). This method may be used in a “frame stealing” mechanism, enabling a logical signalling channel to be inserted in the radio resource supporting a traffic channel. This is used in the GSM system, in particular, to insert a fast associated control channel (FACCH) in a traffic channel. A disadvantage of this method is its adverse effect on the transmission rate available on the traffic channel due to the rate consumed by the reserved signal portion (overhead).  
           [0006]    An object of the present invention is to propose a simple method of communicating signalling information in a system using synchronisation patterns between the transmitter and the receiver.  
         SUMMARY OF THE INVENTION  
         [0007]    Accordingly, the invention proposes a signalling method in a radio communication system, wherein a first station transmits, toward at least one second station, a radio signal formed from a first baseband signal including a synchronisation signal containing at least one synchronisation pattern selected on the basis of signalling information to be supplied to the second station, wherein the second station obtains a second baseband signal from a received radio signal, detects the synchronisation signal in a portion of the second baseband signal to compute synchronisation parameters and/or parameters for estimating a radio transmission channel and uses the computed parameters to demodulate another portion of the second baseband signal and extract transmitted information therefrom, whereby the second station performs a search of several patterns in the second baseband signal to obtain said signalling information on the basis of the detected pattern.  
           [0008]    The signalling information is sent at the same time as the synchronisation patterns, which enhances transmission capacity. Several synchronisation patterns are defined, and the sending of one of them provides the recipient with the signalling information. A method of this type is described in WO99/00927.  
           [0009]    If the number of synchronisation patterns is 2 N , the method enables N bits of signalling information to be transmitted whenever a pattern occurs in the signal. Accordingly, N bits of signalling information can be transmitted by means of N synchronisation patterns successively transmitted, each transmitted pattern being chosen from two predetermined patterns.  
           [0010]    The different synchronisation patterns which may be used must have good auto-correlation and cross-correlation properties in order to limit the risks of detection errors. In other words, the auto-correlation function AC x (t) of each pattern x has a pronounced maximum for a zero time offset (AC x (0)&gt;&gt;AC x (t) for t≠0 and the cross-correlation function CC x,x =(t) of two distinct patterns x, x′ is lower than the auto-correlation of any one of these patterns by at least one order of magnitude (CC x,x ,(t)&lt;&lt;AC x (0)).  
           [0011]    According to the invention, the first baseband signal is a complex signal and the synchronisation pattern is selected from a set of patterns containing a first pattern x, whose real and imaginary parts xr, xi have substantially identical auto-correlations and a cross-correlation substantially lower than said auto-correlations (CC xr,xi (t)&lt;&lt;AC xr (0)≈AC xi (0) for any t) and a second complex pattern x* which is conjugate to the first pattern.  
           [0012]    The fact of having two synchronisation patterns where one is a conjugate complex of the other permits a simplification of the means needed at the receiver to extract the signalling information. Indeed, the computations of the elementary correlation functions of the real and imaginary parts of the second baseband signal with the real and imaginary parts of one of the two conjugate patterns, needed to evaluate the complex correlation of the second baseband signal with this pattern, can also be applied directly in the evaluation of the correlation with the other pattern. The auto- and cross-correlation properties of the real and imaginary parts of the synchronisation pattern ensure that this complex pattern and its conjugate will also have good auto- and cross-correlation properties.  
           [0013]    If the first baseband signal is a complex signal, generated in response to the flow of digital symbols by a phase modulation such that the modulated phase is anti-symmetrical as a function of the symbols of the flow, a modality or a variant of the above-described embodiment consists in selecting a synchronisation sequence from a predetermined set containing at least one first sequence, giving rise after modulation to a synchronisation pattern whose real and imaginary parts have substantially identical auto-correlations and a cross-correlation substantially lower than said auto-correlations, and a second sequence opposite the first sequence.  
           [0014]    This latter method is exactly the same as the first if the synchronisation patterns are transmitted in isolation.  
           [0015]    If they are immediately preceded or followed by transmitted information symbols, the synchronisation patterns will not be exactly conjugate complexes of each other if the modulation and/or the propagation channel generates a memory effect causing inter-symbol interference. Nevertheless, they are approximately conjugate complexes and the auto-correlation and cross-correlation properties remain satisfactory if the length of the synchronisation sequences is substantially longer than the memory of the channel and of the modulation. In this case, the receiver may conserve its simplicity by seeking conjugate patterns that are approximately equal to the synchronisation patterns inserted by the transmitter.  
           [0016]    To implement the method, the invention proposes, as the “second station”, a radio communication receiver comprising means for generating a complex baseband signal from a received radio signal, means for detecting a synchronisation signal including at least one synchronisation pattern in a portion of said baseband signal, for computing parameters for synchronising with a transmitter and/or parameters for estimating a radio transmission channel between the transmitter and the receiver, and means for demodulating another portion of said baseband signal with the aid of the computed parameters to extract transmitted information therefrom, the detection means being arranged to search a plurality of synchronisation patterns of a predetermined set of patterns in said baseband signal and obtain signalling information in response to the detection of a pattern of the set and as a function of the detected pattern, said set of patterns containing a first pattern whose real and imaginary parts have substantially identical auto-correlations and a cross-correlation substantially lower than said auto-correlations, and a second pattern which is a conjugate complex of the first pattern.  
           [0017]    To implement the method, the invention proposes, as a “first station”, a radio communication transmitter comprising means for transmitting, toward at least one receiver, a radio signal formed from a complex baseband signal including a synchronisation signal to enable the receiver to compute synchronisation parameters and/or parameters for estimating a radio transmission channel, means for selecting at least one synchronisation pattern from a predetermined set of patterns on the basis of signalling information to be supplied to the receiver, to include the selected pattern in the synchronisation signal, said set of patterns comprising a first pattern whose real and imaginary parts have substantially identical auto-correlations and a cross-correlation substantially lower than said auto-correlations, and a second pattern which is a complex conjugate of the first pattern.  
           [0018]    Alternatively, the radio communication transmitter according to the invention comprises modulation means for generating a complex baseband signal in response to a flow of digital symbols, according to a phase modulation such that the modulated phase is anti-symmetrical as a function of the symbols of the flow, means for transmitting, toward at least one receiver, a radio signal formed from said baseband signal, and means for selecting a synchronisation sequence from a given set of sequences on the basis of signalling information to be supplied to the transmitter, the selected sequence being included in said flow of symbols to generate a synchronisation signal included in said baseband signal to enable the receiver to compute synchronisation parameters and/or parameters for estimating a radio transmission channel, said set of sequences containing at least one first sequence giving rise to a synchronisation pattern whose real and imaginary parts have substantially identical auto-correlations and a cross-correlation substantially lower than said auto-correlations, and a second sequence opposite the first sequence.  
           [0019]    Another aspect of the present invention relates to a radio signal repeater comprising means for generating a first baseband signal from a received radio signal, means for detecting a synchronisation signal containing at least one synchronisation pattern generated in response to at least one sequence of synchronisation symbols in a portion of the first baseband signal, to compute parameters for synchronising with a transmitter and/or parameters for estimating a radio transmission channel from said transmitter, means for demodulating another portion of the first baseband signal with the aid of the computed parameters, to generate a flow of demodulated symbols, means for generating a second baseband signal from the flow of demodulated symbols and each sequence of synchronisation symbols detected, means for transmitting, toward at least one receiver, another radio signal formed from the second baseband signal with a frame structure corresponding to a frame structure of the received radio signal, and signalling means for transmitting to the receiver information indicating that the repeater is of the type outlined above, i.e. that it performs demodulation and remodulation of the repeated radio signals without controlling the frame structure of the signals.  
           [0020]    These signalling means may operate by inserting signalling symbols at given locations reserved for this purpose in the flow of demodulated symbols. However, these signalling means preferably operate in accordance with the method outlined above, i.e. the means for generating the second baseband signal comprise means for transforming at least one detected sequence of synchronisation symbols, and a modulator to generate the second baseband signal in response to the flow of demodulated symbols with the transformed sequence. Alternatively, the means for generating the second baseband signal may comprise a modulator to generate an intermediate baseband signal in response to the flow of demodulated signals containing the detected sequence, and means for obtaining the second baseband signal by transforming the intermediate baseband signal within a period corresponding to a synchronisation pattern.  
           [0021]    The signalling information coded by the received synchronisation pattern therefore provides the receiver with information as to whether such a repeater was or was not inserted in the transmission chain.  
           [0022]    In other applications of the method, the transmitted signalling information may identify logical channels sharing the same radio resource or else modulation schemes applied to the transmitted signal frames.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIGS. 1 and 2 are respective block diagrams of a radio communication transmitter and receiver according to the invention.  
         [0024]    [0024]FIG. 3 is a block diagram of another embodiment of the transmitter.  
         [0025]    [0025]FIG. 4 is block diagram of a module for detecting synchronisation patterns which may be used in the receiver illustrated in FIG. 2.  
         [0026]    [0026]FIGS. 5 and 6 are respective block diagrams of a repeater according to the invention and a corresponding receiver terminal.  
         [0027]    [0027]FIGS. 7 and 8 are respective block diagrams of a radio communication transmitter and receiver according to another embodiment of the invention.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0028]    The radio transmitter illustrated as an example in FIG. 1 comprises a digital modulator  1  generating a complex baseband signal s n  from a flow F of discrete symbols. The complex baseband signal, sampled at a frequency F e , has a real part (I) and an imaginary part (Q) applied to a radio stage  2  which forms the radio signal sent by the transmitter via the antenna  3 . The radio stage  2  converts the real and imaginary parts of the complex digital signal s n  into analog and uses them to modulate two carrier waves in quadrature of a same frequency. The resultant modulated waves are summed, filtered and amplified to form the transmitted radio signal.  
         [0029]    The flow F is made up of discrete symbols, for example binary (±1) or M-ary (±1, ±3, . . . ±(M−1)), namely information symbols output by a source  4  and synchronisation signals output by a synchronisation sequence generator  5 .  
         [0030]    The symbol source  4  illustrated by a diagrammatic block in FIG. 1 issues coded information symbols which contain the traffic information (voice, data, image, . . . ), the nature and shape of which will depend on the radio communication system to which the transmitter belongs, as well as various items of signalling information used to operate the system. In a conventional manner, the source  4  may have a channel coder for one or more types of information, which adds redundancy to the symbols to be transmitted and/or introduces time interleaving in order to protect these symbols from transmission errors.  
         [0031]    The sequence generator  5  outputs synchronisation symbols which the receiver uses as it detects them to obtain synchronisation information and/or estimate parameters of the radio propagation channel.  
         [0032]    Each synchronisation sequence is constructed in a conventional manner, taking account of the resulting statistical properties of the synchronisation pattern in the complex signal s n  after modulation.  
         [0033]    These statistical properties may be expressed in various ways. For example, if x=(x 0 , X 1 , . . . , x L−1 ) denotes the pattern of L complex samples at the frequency F e  generated by the modulator  1  in response to a sequence of symbols issued by the generator  5 , the sequence will be constructed so that the auto-correlation function  
           Ac   x          (   n   )       =              ∑     k   =   0       L   -   1              x     k   +   n       ·     x   k   *              2                           
 
         [0034]    (|.| denoting the modulus of a complex number) has a pronounced peak for a shift n=0(AC x (0)&gt;&gt;AC x (n) for n≠0). In the expression of the auto-correlation function AC x (n) given above, x k+n  for n+k&lt;0 (respectively n+k≧L) denotes the known value of the signal s n+k  if the synchronisation sequence is not immediately preceded (respectively followed) by information symbols. If the synchronisation sequence is immediately preceded or followed by information symbols that are unknown a priori, x k+n  for n+k&lt;0 or n+k≧L denotes a random variable having the same probability density as the signal s n  and the properties AC x (0)&gt;&gt;AC x (n) for n≠0 is verified statistically, with the observation that the samples x 0 , . . . , x K−1  may depend on information symbols if the modulation and the channel have a global memory of K samples with K≧1.  
         [0035]    A multiplexer  6  assembles the flow F from the symbols output by the source  4  and the generator  5 , in the form of frames in periodic succession and each of which comprises a given number of symbols.  
         [0036]    Some frames may contain a relatively long synchronisation sequence (typically several tens of symbols) enabling the receiver to acquire an initial synchronisation with the transmitter. Other frames transmitted more frequently may contain shorter synchronisation sequences (for example less than 20 symbols), enabling the receiver to refine and track the synchronisation. If the receiver applies coherent demodulation, these known synchronisation sequences are also used to estimate the response of the propagation channel (impulse response or simply attenuation and phase shift if the memory of the modulation and the channel is small compared with the time of a symbol).  
         [0037]    The receiver illustrated in FIG. 2 has a radio stage  8  which regenerates a complex baseband signal r n  from the radio signal picked up by the antenna  10 . The radio stage  9  essentially applies analog filtering, combines two quadrature waves at the carrier frequency, applies filtering to retain the baseband components of the combined signals and digitises the combined and filtered signals to produce the real part (I) and the imaginary part (Q) of the complex baseband signal r n .  
         [0038]    This complex signal r n  is processed firstly by a module  11  which detects synchronisation patterns and secondly by a demodulator  12  used to estimate the information symbols supplied by the source  4  of the transmitter.  
         [0039]    In a conventional manner, the module  11  uses the presence of the synchronisation sequences in the transmitted signal to extract time synchronisation symbols ST which can be used by the demodulator  12  and optionally frequency synchronisation parameters SF applied to the radio stage  9  of the receiver in order to adjust the frequency of its oscillator to tune to the carrier frequency used by the transmitter.  
         [0040]    By way of example, the time synchronisation information ST may indicate that the shift n for which the normalized correlation function:  
                 IC     r   ·   x            (   n   )       =                ∑     k   =   0       L   -   1              r     k   +   n       ·     x   k   *              2         (       ∑     k   =   0       L   -   1                   r     k   +   n            2       )     ·     (       ∑     k   =   0       L   -   1                   x   k          2       )                 (   1   )                               
 
         [0041]    is maximum. In the case of continuous phase modulation (CPM), the parameter SF may represent a frequency discrepancy to be corrected in the radio stage  9 , given for example by  
             F   e       2      π          Arg                   (       ∑     n   =   0       L   -   1              r   n   *     ·     r     n   +   1           )       ,                         
 
         [0042]    where Arg(.) denotes the argument of a complex number. It should be pointed out that numerous other synchronisation methods may be used by the detection module  11 . In particular, the frequency variance to be corrected may be evaluated more finely by methods using a frequency transform (see U.S. patent application Ser. No. 09/654,800 filed on Sep. 5, 2000).  
         [0043]    For the value of n which maximises the function IC r,x (n) or in the vicinity of this value, the complex correlation  
         Γ   n     =       ∑     k   =   0       L   -   1              r     k   +   n       ·     x   k   *                               
 
         [0044]    provides observations which, in a known manner, also enable the propagation channel between the transmitter and the receiver to be estimated. If the demodulator  12  operates coherent demodulation, it receives the corresponding estimations RC from the detection module  11 .  
         [0045]    The synchronisation information ST locks the demodulator  12  on the transmitted time structure of the frames and samples, enabling it to estimate the symbols output by the source  4  in periods located between the synchronisation sequences, and the channel estimation RC enables it to apply coherent demodulation.  
         [0046]    [0046]FIG. 4 is a schematic illustration of an example of an embodiment of the module  11  used to detect synchronisation patterns. The complex samples x k  of the synchronisation pattern x are stored in a memory  15  of the module  11  (0≦k&lt;L).  
         [0047]    The numerator of the correlation function IC r,x (n) as defined in equation (1) is given by |Γ n | 2 =(A n −D n ) 2 +(B n +C n ) 2 , where:  
         A   n     =       ∑     k   =   0       L   -   1            Re                     (     r     k   +   n       )     .                               
 
         [0048]    Re(x k ) is the correlation, calculated by a multiplier  16  and an accumulator  21  integrating on L samples, between the real part Re(r n ) of the baseband signal and the real part Re(x k ) of the synchronisation pattern, stored in the memory  15 ;  
         B   n     =       ∑     k   =   0       L   -   1            Re                     (     r     k   +   n       )     ·                               
 
         [0049]    Re(x k ) is the correlation between the imaginary part Im(r n ) of the baseband signal and Re(x k ) (multiplier  17  and accumulator  22 );  
         C   n     =       ∑     k   =   0       L   -   1            Re                     (     r     k   +   n       )     .   Im                     (     x   k     )                               
 
         [0050]    Im(x k ) is the correlation between Re(r n ) and the imaginary part Im(x k ) of the pattern, stored in the memory  15  (multiplier  18  and accumulator  23 ); and  
         D   n     =       ∑     k   =   0       L   -   1            Im                     (     r     k   +   n       )     ·                               
 
         [0051]    Im(x k ) is the correlation between Im(r n ) and Im(x k ) (multiplier  19  and accumulator  24 ). The numerator |Γ n | 2  of the correlation function is applied by the unit  28  which sums the squares of the quantities A n −D n  and B n +C n  respectively provided by the subtractor  26  and the adder  27 .  
         [0052]    The first term of the denominator in equation (1), the second term of which is constant and therefore does not need to be computed, is obtained by a unit  29  which computes the square modulus of the received complex signal r k+n  on the window in question and by the accumulator  30  which produces the sum of the square moduli on a window of L samples.  
         [0053]    The detection module  11  has a decision unit  32  which determines the maximum of the ratio of variables respectively applied by the units  28  and  30  and which decides that the receiver is synchronised when this maximum exceeds a given threshold. In this case, the time position of the maximum is coded in the synchronisation information ST applied to the demodulator. If the latter is applying coherent demodulation, the complex number Γ n =(A n −D n )+j.(B n +D n ) may be used to estimate the propagation channel.  
         [0054]    The transmitter proposed by the invention, illustrated in FIG. 1, has a control unit  7  which selects the synchronisation sequence output by the generator  5  on the basis of signalling information Z to be supplied to the receiver. The generator  5  is therefore capable of producing several different synchronisation sequences, the selected sequence being representative of the information Z.  
         [0055]    These different sequences must of course have good cross-correlation properties. Secondly, their respective auto-correlations must be of the same order so that they have similar detection probabilities, i.e. AC x (0)≈AC x ,(0) for the synchronisation patterns x,x′ respectively generated by the modulator  1  in response to two of these sequences. Good cross-correlation properties mean, for example, that the cross-correlation function  
           CC     x   ,     x   ′              (   n   )       =              ∑     k   =   0       L   -   1              x     k   +   n       ·     x   k   *              2                           
 
         [0056]    is lower than the auto-correlations AC x (0) and AC x ,(0) by at least one order of magnitude.  
         [0057]    In order to identify the synchronisation pattern present in the received signal r n  and hence identify the transmitted signalling information Z, the means for detecting synchronisation patterns may incorporate several modules such as that described above, operating in parallel on the basis of different possible sequences. Detection of a sequence by one of these modules will then enable the synchronisation to be achieved and the information Z to be extracted.  
         [0058]    A known way of generating sequences having the properties described above is to use error correction codes. This will reduce the complexity of detection processing due to the algebraic structure of these codes. For example, a first order Reed-Muller code may be used, which, because of its regular structure, will reduce the computing power needed in the receiver. Several sequences which may be used by the transmitter are derived from an initial synchronisation sequence by multiplying the initial sequence by several codewords belonging to the relevant correction code. However, this method can only be applied in the case of linear signals and binary symbols.  
         [0059]    In the embodiment of the transmitter illustrated as an example in FIG. 3, the generator  5  outputs a single synchronisation sequence which gives rise to a pattern x at the output of the modulator  1 . This pattern x=xr+j.xi has real and imaginary parts xr, xi, whose auto-correlations are substantially identical and whose cross-correlation is substantially lower than said auto-correlations (CC xr,xi (n)&lt;&lt;AC xr (0)≈AC xi (0) for any n). A unit  8  selectively inverts the sign of only the imaginary part xi (or in an equivalent manner of only the real part xr) of the pattern x to form the complex baseband signal s n  in the regions corresponding to the pattern x. This sign inversion will occur only if an information signalling bit Z has a given value (hereafter Z=1), the baseband signal s n  being obtained directly at the output of the modulator when Z=0.  
         [0060]    The synchronisation pattern inserted in the signal s n  is then either the pattern x or its conjugate pattern x*=xi−j.xr. Taking account of the correlation properties of the real sequences xr and xi, the two complex patterns x and x* have satisfactory cross-correlation properties.  
         [0061]    Either one of these two patterns x, x* may be detected by the module  11  illustrated in FIG. 4, in which elements  33 - 35  represent a negligible additional complexity. The adder  33  computes the sum A n +D n  whilst the subtractor  34  computes the difference B n −C n . The unit  35  then computes the value |Γ′ n | 2 =(A n +D n ) 2 +(B n −C n ) 2  which represents the numerator of the correlation function IC r,x* (n) in accordance with equation (1). The denominators of the correlation functions IC r,x (n) and IC r.x* (n), on the other hand, are identical for the two conjugate patterns x, x*. Consequently, the decision unit  32  merely has to choose the larger of the two variables |Γ n | 2 , |Γ′ n | 2  issued by the units  28  and  35  when examining the synchronisation criterion. If this criterion is satisfied, the synchronisation is achieved and the unit  32  will deliver the information bit Z corresponding to that of the patterns for which the decision variable is the higher (Z=0 if |Γ n | 2 &gt;|Γ′ n | 2  and Z=1 if |Γ n | 2 &lt;|Γ′ n | 2 ).  
         [0062]    The method outlined above may be generalised for transmitting a number N&gt;1 of signalling bits Z. If the synchronisation pattern x is sub-divided into N successive sub-patterns, contiguous or not, N bits can be transmitted by selectively replacing each of the sub-patterns with its conjugate pattern.  
         [0063]    If the patterns or sub-patterns are transmitted continuously, it is desirable to guarantee the continuity of the modulated signal s n . The same detection module  11  may be used for this purpose without the synchronisation patterns or sub-patterns being exact conjugates one of the other. This is the case in particular for modulations with a constant envelope having a memory effect, for example GMSK (Gaussian Minimum Shift Keying). This may easily be applied if the duration of the memory effect is small relative to the duration of the synchronisation pattern or sub-pattern.  
         [0064]    In the case of continuous phase modulation where the phase varies linearly as a function of the symbols of the flow F, the control unit  7  (FIG. 1) may then issue a command to the generator  5  to output either a given synchronisation sequence or the opposite sequence, depending on a signalling bit Z. The phase modulation being linear (or more generally anti-symmetrical) as a function of the symbols, replacing a sequence with the opposite sequence is tantamount to replacing the corresponding synchronisation pattern x with the conjugate pattern x*, except at the edges of the synchronisation pattern, over a period corresponding to the modulation memory.  
         [0065]    Accordingly, if the length L of the synchronisation patterns is long compared with the length K of the modulation memory, the two patterns obtained are almost conjugate of each other and have satisfactory correlation properties.  
         [0066]    For the shorter sequences, generating synchronisation patterns whose length L is of the same order of magnitude as the memory K of the modulation, a transmitter of the type illustrated in FIG. 3 may be used. In this case, in order to avoid phase discontinuities, the synchronisation sequence may be preceded and followed by known symbols in the flow F over a period corresponding to the modulation memory. The disadvantage of this is that it lengthens the time needed by the synchronisation sequences. In order to eliminate this drawback, it is possible to select several sequences which will generate the best statistical properties once the synchronisation patterns have been modulated and to arrange the transmitter as illustrated in FIG. 1. In this latter case, the means used to detect the synchronisation patterns in the receiver must perform several computations of correlations in parallel, which is not too demanding in terms of computing resources given that, in theory, the synchronisation sequences are short.  
         [0067]    The signalling method described above may be used in a radio communication system in which mobile terminals are able to communicate directly with one another in direct mode and/or via repeaters. By way of example, the repeaters may be of three types:  
         [0068]    type 1: a simple repeater which does not perform demodulation of received signals. Such repeater may essentially consist of an amplifier and a frequency transposing device to retransmit the radio signals picked up;  
         [0069]    type 2: a repeater with clock regeneration which performs demodulation of the received signal and remodulation of the flow of demodulated digital symbols to form the radio signal retransmitted on a transposed frequency. The flow of demodulated symbols is not subjected to channel decoding to correct any transmission errors which might have affected the received signal;  
         [0070]    type 3: a corrector repeater which performs demodulation of the received signal, channel decoding (de-interleaving, correction decoding), channel recoding from the decoded information and remodulation to produce the radio signal retransmitted on a transposed frequency.  
         [0071]    Repeaters of types 1 and 2 operate in slave (or asymmetrical) mode. The transmitter generating the radio signal is the master, particularly since it sets the frame structure of this signal, and the repeater copies this frame structure in the signal repeated on another frequency, with a processing delay in the case of a repeater of type 2 because of the demodulation and remodulation operations performed.  
         [0072]    The repeater of type 3 is set up to operate in master (or symmetrical) mode, whereby the correction repeater controls the frame structure on the one hand toward the transmitter terminal and on the other hand toward the receiver terminal. This repeater is considerably more complex than those of types 1 and 2. In particular, the frame structure of the repeated signal is generated separately from that of the received signal. This is due to the relatively long processing delay imposed by the correction repeater, in particular because of the signal interleaving used to improve the efficiency of the correction decoder. In view of the periods typically used in the radio frames (several tens of milliseconds), this processing delay does not allow the frame structure of the received signal to be re-used).  
         [0073]    The transmitter and receiver terminals of a system organised as described above may be used to implement the present invention to supply a destination receiver terminal with signalling information indicating whether or not the radio signal was repeated by a repeater of type 2. This information may be provided in particular by means of the long synchronisation pattern initially sent by the transmitter to achieve the first synchronisation of the receiver.  
         [0074]    In order to illustrate how this information is used by the receiver, we will look at an example in which the signals are transmitted using a super-frame structure generated by the master station, each super-frame comprising periods of 320 ms for sending traffic signals from the transmitter to the receiver, alternating with periods of 40 ms seconds during which the signalling signals are sent, every second 40 ms period being dedicated to forward signalling, i.e. from the transmitter to the receiver, and the other 40 ms periods being dedicated to backward signalling, i.e. from the receiver to the transmitter. The receiver has no difficulty in sending backward signalling if the radio signal is received in direct mode or repeated mode with a repeater of type 1: with a 5 ms interval for switching frequency at the start and end of the 40 ms period, the receiver returns the signalling in alignment with the frame structure imposed by the transmitter. There is no difficulty in the case of a repeater of type 3 either, since it is the correction repeater which imposes the frame structure for communicating with the receiver.  
         [0075]    The case of the repeater of type 2 is more difficult because the modulation and demodulation operations performed in the two directions take time (typically about 10 ms for each direction) so that the transmitter might miss a part of the signalling returned to it since it risks receiving the latter outside the 40 ms period which it set and at the end of which it still has to reserve about 5 ms in order to switch its frequency.  
         [0076]    In order to ensure orderly transmission of the backward signalling signals if a repeater of type 2 is on the path, it is of advantage to use a systematic code for channel coding, preferably with a high rate. It is therefore judicious to use a systematic block code. However, this block code will not be optimum for the transmission of the backward signalling in the case of direct mode or in situations where the signals are relayed by a repeater of type 1 or 3. In this case, it is preferable to use a convolutional code, of a lower rate (for example 1/2), with a soft decoding to enhance the performance.  
         [0077]    Accordingly, two different transmission formats may be provided for transmitting the backward signalling. In order to know what format to use, the receiver must have identified the presence of a repeater of type 2, if any, in the transmission chain of the signals it has received, hence the use of the above-mentioned signalling information.  
         [0078]    The signals prepared by the original transmitter, or at least some of them, contain synchronisation patterns reflecting synchronisation sequences inserted in the flow of digital symbols. These synchronisation patterns are not modified by repeaters of type 1 or 3. However, one or more of these patterns (in particular the initial synchronisation pattern) are modified by the repeater of type 2, which means that the receiver is able to detect the presence of this repeater of type 2 once it detects the modified synchronisation pattern in place of the original synchronisation pattern.  
         [0079]    Block diagrams of such a repeater of type 2 and a corresponding receiver terminal are illustrated in FIGS. 5 and 6, respectively, the same reference numerals being used as those used above to denote elements of the same nature.  
         [0080]    The receiving part of the repeater has a radio stage  9  which generates a baseband signal r′ n  from the radio signal picked up by the antenna  10  from the original transmitter. The module  11  detects the (original) synchronisation pattern to supply the parameters needed by the demodulator  12  and optionally by the radio stage  9 . The demodulator  12  generates a flow of demodulated symbols from the portions of the baseband signal r′ n  located outside the synchronisation patterns.  
         [0081]    This demodulator  12  replaces the symbol source  4  illustrated in the transmitter diagram of the FIG. 1.  
         [0082]    The transmitting part of the repeater of FIG. 5 further has a symbol sequence generator  5 , which generates the modified synchronisation sequence characteristic of the repeater of type 2, a multiplexer  6  which reassembles the flow of symbols F with the modified sequence, a modulator  1  and a radio stage  2  feeding the antenna  3 .  
         [0083]    In an alternative embodiment, the generator  5  produces the same sequence as the original transmitter (where this sequence is obtained by demodulating the signal r′ n ) and the change in the synchronisation pattern is carried out by changing the sign of the imaginary part only (or the real part only) of the baseband signal s n  applied to the radio stage  2 , as indicated by block  8  shown in broken lines in FIG. 5. In this latter case, the repeater behaves like a transmitter of the type illustrated in FIG. 3, the information Z indicating whether it is of type 2 or of type 1 or 3.  
         [0084]    [0084]FIG. 6 illustrates a terminal incorporating a receiver similar to that of FIG. 2, the detection module  11  being capable of extracting the information bit Z defined by the synchronisation sequence which it has detected. This module  11  is of the type illustrated in FIG. 4, for example. For the transmitting part, the station has a symbol source  40 , diagrammatically illustrated on a single block in FIG. 6 and which generates the symbols to be modulated with the exception of those relating to backward signalling. These latter symbols are generated from a signalling information source  41  by redundant coding.  
         [0085]    Depending on the bit Z extracted by the detection module  11 , this redundant coding is applied either by a systematic block coder  42  (if Z indicates detection of the modified synchronisation pattern) or by a convolutional coder  43  (if Z indicates detection of the original pattern). A multiplexer  44  inserts the symbols issued by the coder  42  or  43  in those output by the source  40 , complying with the frame structure with which the detection module  11  has acquired the synchronisation. The modulator  45  generates a baseband signal s′ n  from the flow of symbols generated by the multiplexer  44  and this signal s′ n  is processed by the transmitting part of the radio stage  9 , which forms the radio signal retransmitted by the terminal.  
         [0086]    In the embodiment illustrated in FIGS. 7 and 8, the signalling information Z communicated to the receiver by means of the synchronisation patterns is used firstly to multiplex the logical channels on the same physical transmission channel and secondly to identify the modulation scheme used to generate the radio signals.  
         [0087]    The combined modulator  1  of the transmitter illustrated in FIG. 7 is arranged to use one of two modulation schemes.  
         [0088]    It comprises a continuous phase modulator (CPM)  50 , which, depending on the M-ary symbols a i  fed at the rate 1/T, produces for p.T≦t&lt;(p+1).T a waveform  
           S        (   t   )       =     exp              [   2                 j                 π                   h   ·     (       ∑     i   =     -   ∞       p              a   i     ·   q                     (     t   -     i   ·   T       )         )           ]                         
 
         [0089]    where h is the modulation index and q(t) the phase pulse of the modulation, with q(t)=1 for t≧K.T, K being the memory of the modulation expressed as a number of M-ary symbols. The waveform S(t) is sampled at the frequency F e  to shape the baseband signal s n . In the example of FIG. 7, the symbols of the flow F are binary at values 0 or 1 and converted into quaternary symbols a i  with values ±1 and ±3 by a unit  53  at the input of the CPM modulator  50 .  
         [0090]    The modulator  1  further has a redundancy coder  51  operating on the basis of the convolutional code CC(2,1,3), for example. This coder  51  is activated upstream of the CPM modulator  50  only if switches  52  are positioned accordingly by the selection and control unit  7 . The dual mode modulator  1  processes the flow of symbols F output by the multiplexer  6 . When the convolutional coder  51  is in service, the dual mode modulator  1  applies a coded modulation (see G. Ungerboeck “Channel Coding with Multilevel/Phase Signals”, IEEE Transactions on Information Theory, Vol. IT-28, No. 1, January 1982). When the convolutional coder  51  is not in service, the modulation is not coded and the rate of the symbols in the flow F is doubled. In principle, coded modulation secures better transmission quality but at a lower transmission rate (see EP-A-0 896 443).  
         [0091]    In the example illustrated in FIG. 7, the symbol source  4  has a unit  55  transmitting on a traffic channel (TCH), with a symbol flow rate selected by the control unit  7  to comply with the type of modulation applied. The source  4  also has a unit  56  defining a fast associated control channel (FACCH), on which coded modulation is always applied. The distinction between the TCH and FACCH logical channels is performed by the control unit  7  which controls the multiplexer  6 .  
         [0092]    The synchronisation sequence generator  5  is arranged to insert two synchronisation patterns per burst of the baseband signal s n . The corresponding sequences are determined beforehand as indicated hereabove, by minimising the cross-correlations between the patterns which they generate in the complex baseband signal s n .  
                       TABLE I                       Logical ch./modulation   1 st  sequence   2 nd  sequence                   FACCH/coded               Binary sequence (F)   0 1 0 1 0 1   0 1 0 0 0 1       Quaternary sequence   −3 −1 −3 +3 −3 +3   −3 −1 −3 −1 −3 −1       (a i )       TCH/coded       Binary sequence (F)   0 0 1 0 0 1   1 1 1 0 1 1       Quaternary sequence   −3 −3 +1 −3 −1 +1   −1 −1 +3 −1 −1 +1       (a i )       TCH/non-coded       Binary sequence (F)   1 0 0 0 0 1 1 0 1 0 1 0   0 1 0 1 0 1 1 0 1 1 0 1       Quaternary sequence   −1 −3 +1 −1 −1 −1   +1 +1 +1 −1 +3 +1       (a i )                  
 
         [0093]    By way of example, if the CPM modulator  50  applies quaternary modulation with an index 1/3 described as an example in EP-A-0 936 784, the patterns may be generated from the sequences of symbols given in Table 1, the first sequence of each block being preceded by the symbols a i  set to zero (constant phase) and followed by the information symbols that are unknown a priori, whereas the second sequence of each frame is preceded by the symbols that are unknown a priori and followed by the symbols a i  set to zero.  
         [0094]    In the receiver illustrated in FIG. 8, the means  11  for detecting synchronisation patterns conduct a search in parallel of the patterns associated with the above sequences. Depending on the sequence identified (signalling information Z), the detection means  11  issue a command to:  
         [0095]    the demodulator  12 , which may operate in a conventional manner using the Viterbi algorithm, with a simple demodulation trellis  60  where modulation on the TCH channel is non-coded or with a supertrellis  61  integrating the modulation and convolutional coding states in order to obtain the gain of the coded modulation on the TCH or FACCH;  
         [0096]    the demultiplexer  62  which directs the flow of symbols output by the demodulator  12  either to the units  63  processing the traffic channel TCH or to the units  64  processing the signalling channel FACCH;  
         [0097]    the units  63  processing the traffic channel TCH to indicate what the transmission rate is on this channel.  
         [0098]    The radio communication system incorporating the transmitters illustrated in FIG. 7 and the receivers illustrated in FIG. 8 have the advantage of enabling an associated signalling channel to be inserted in a traffic channel by a “frame stealing” type of mechanism without adding overhead, i.e. without specifically reserving information symbols in the block transmitted in each burst to indicate the type of logical channel. It is also of advantage to be able to select frame by frame the modulation scheme used, which enables the instantaneous flow rate on the channel to be dynamically adapted.  
         [0099]    It should be pointed out that the signalling information likely to be transmitted between transmitters and receivers operating as explained above with reference to FIGS.  1  to  4  may be very varied in nature, the examples of applications given with reference to FIGS.  5 - 6  and  7 - 8  not being restrictive.  
         [0100]    In an alternative to the above-described embodiments, the synchronisation patterns are detected on the basis of a differential baseband signal, corresponding to r n .r n−1 *. In this case, the desired correlation properties must of course be respected with regard to the differential synchronisation patterns x k .x k−1 *.