EP0629059B1 - Spread spectrum digital transmission system with low frequency pseudorandom coding of the useful information and method for spectrum spreading and compressing used in such a system - Google Patents

Description

The field of the invention is that of modem transmission of digital signals and in particular that of spread spectrum modems. More specifically, the The present invention relates to a transmission system with spread of spectrum between a transmitter and a receiver digital signals where spread spectrum is obtained by pseudo-random coding of information useful to transmit. The invention is particularly applicable in wireless telecommunications in the military field.

In the military field, a spreading operation is generally used in ECCM (Electronic Counter-CounterMeasures) and consists of multiplying the signal useful to transmit by a code, called code or sequence spreading, from a pseudo-random generator whose clock signal frequency is much higher than the maximum frequency of the wanted signal. Number of bits of useful information transmitted by Hz is therefore very low.

Figure 1 shows a timing diagram for understand the principle of spectrum spreading by a spreading sequence.

A useful SAT signal to be transmitted, here coded in two +1 and -1 levels following NRZ coding, is multiplied by a SE cyclic spreading sequence, also coded on two levels. The signal resulting from the multiplication is the ST signal transmitted from the transmitter to a receiver after modulation. The modulated ST signal transmission medium generally consists of a radio link. To the reception, after demodulation, signal multiplication received ST with the same spreading sequence SE (same phase and same frequency) makes it possible to reconstitute the useful signal SAT.

Sequence spread spectrum transmission direct is usually used to give the signal transmitted better discretion, resistance to ECM (Electronic CounterMeasures) interference and a resistance to selective fading.

The ratio between the chip time and bit time, the chip time corresponding to the duration of a bit of the spreading sequence and the time bit to that of the wanted signal. The greater this spreading gain higher, the more the transmitted signal is able to be transmitted discreetly and therefore resist ECM devices intended to detect it and, possibly, to jam it. An essential step in ECM analysis is to determine the spreading hazard of the signal received because this step allows you to penetrate the information content of the signal received, ie to reconstitute the useful signal.

The main disadvantage of spread spectrum by direct sequence is that the sequence generator direct must operate at the transmission frequency of chips, or at a frequency of the order of several MHz. he it is therefore necessary to install this generator in an ASIC, which increases the hardware complexity and the cost of hardware development.

The present invention aims in particular to overcome this drawback.

More specifically, one of the objectives of the invention is to provide a signal transmission system digital, where spread spectrum is implemented, this system that does not require a hazard generator operating at the chip frequency. It is therefore more simple to make and less expensive, while allowing large spread of the useful signal spectrum intended for resist ECM devices.

Another object of the invention is to provide a such a system where spectrum spreading is carried out from orthogonal sequences, for example using M-type sequences (also known as maximum length or Hadamard), well known in the art field of digital signal transmission.

An additional objective is to provide a process transmission of spread digital signals spectrum where the spreading is carried out at the bit frequency and not at the chip frequency.

• l'émetteur comporte successivement :
  • des moyens de codage recevant ce signal numérique et fournissant, pour chaque bloc de k bits du signal numérique, un échantillon codé prenant une valeur entière comprise dans l'intervalle [0, N-1], chaque valeur entière étant représentative des k bits du bloc dont elle est issue ;
  • des moyens de combinaison des échantillons codés avec des échantillons issus d'un générateur d'aléas de phase pseudo-aléatoire, les moyens de combinaison fournissant un entier compris dans l'intervalle [0, M-1] pour chaque combinaison d'un échantillon codé et d'un échantillon d'aléa de phase issu du générateur d'aléas de phase, M étant supérieur à N ;
  • des moyens de génération de signaux fournissant, pour chaque entier compris dans l'intervalle [0, M-1], une suite de q nombres entiers correspondant à cet entier, les différentes suites étant orthogonales ou quasi-orthogonales entre elles ;
  • des moyens d'émission des suites de q nombres entiers à l'attention du récepteur, les moyens d'émission comprenant un modulateur à décalage de phase dont le nombre d'états est égal à M ;
• le récepteur comporte successivement :
  • des moyens de réception restituant les suites de q nombres entiers ;
  • des moyens de traitement recevant d'une part les suites de q nombres entiers des moyens de réception et d'autre part des échantillons d'aléas de phase issus d'un générateur d'aléa de phase synchronisé avec le générateur d'aléas de phase de l'émetteur, les moyens de traitement assurant une démodulation des suites de q nombres entiers et effectuant une opération inverse de celle des moyens de combinaison pour restituer les échantillons codés ;
  • des moyens de décodage restituant le signal numérique à partir des échantillons fournis par les moyens de traitement.

These objectives, as well as others which will appear subsequently, are achieved by means of a system for transmitting a digital signal between a transmitter and a receiver, characterized in that:

• the transmitter successively comprises:
  • coding means receiving this digital signal and providing, for each block of k bits of the digital signal, a coded sample taking an integer value included in the interval [0, N-1], each integer value being representative of the k bits of the block from which it came;
  • means for combining the coded samples with samples from a pseudo-random phase generator, the combining means providing an integer in the interval [0, M-1] for each combination of a sample coded and of a phase random sample from the phase random generator, M being greater than N;
  • means for generating signals providing, for each integer included in the interval [0, M-1], a sequence of q integers corresponding to this integer, the different sequences being orthogonal or quasi-orthogonal to each other;
  • means for transmitting sequences of q whole numbers for the attention of the receiver, the transmission means comprising a phase shift modulator whose number of states is equal to M;
• the receiver successively comprises:
  • reception means restoring the sequences of q whole numbers;
  • processing means receiving on the one hand the sequences of q whole numbers of the reception means and on the other hand samples of phase hazards originating from a phase hazard generator synchronized with the phase hazard generator of the transmitter, the processing means ensuring a demodulation of the sequences of q whole numbers and performing an inverse operation to that of the combination means for restoring the coded samples;
  • decoding means restoring the digital signal from the samples supplied by the processing means.

The M sequences of q integers are preferably made up of Hadamard sequences.

• la figure 1 représente un chronogramme permettant de comprendre le principe de l'étalement de spectre par une séquence d'étalement ;
• la figure 2 est un schéma synoptique d'un émetteur du système de transmission de la présente invention ;
• la figure 3 est un schéma synoptique d'un récepteur des signaux numériques transmis par l'émetteur de la figure 2.

Other characteristics and advantages of the invention will appear on reading the following description of a preferred embodiment, given by way of illustration and not limitation, and of the appended drawings in which:

• FIG. 1 represents a timing diagram making it possible to understand the principle of spectrum spreading by a spreading sequence;
• Figure 2 is a block diagram of a transmitter of the transmission system of the present invention;
• FIG. 3 is a block diagram of a receiver of the digital signals transmitted by the transmitter of FIG. 2.

Figure 1 has been described previously with reference to the state of the art.

Referring to FIG. 2, the digital signal to be transmitted SN is applied, here via a serial access, to coding means 21 which supply, for each block of k bits of the signal SN, a coded sample E _c taking an integer value included in the set {0, ..., N-1}, each integer value being representative of the k bits of the corresponding block. The coding means 21 can for example be constituted by a simple binary-decimal converter and the bit rate leaving the coding means is then k times lower than the bit rate entering.

The coding means 21 can optionally also interleave the bits of the SN signal.

The coded samples E _c are applied to means 22 for combining these samples with samples E _a originating from a pseudo-random generator 23, which will subsequently be called phase random generator. In general, the combination means 22 comprise a transformation algorithm which transforms each coded sample E _c into an integer s included in the set {0, ..., M-1}, with M integer greater than N. We at : s = f (E vs , E at ) where f is any function taking its values in {0, ..., M-1} and E _has a sample of phase randomness.

où

désigne l'addition modulo M pouvant aussi s'écrire : s = (Ec + Ea) mod M The combination means 22 can for example be constituted by a simple modulo M adder, as shown and providing:

or

denotes the addition modulo M which can also be written: s = (E vs + E at ) mod M

This modulo M addition, apart from the fact that it can be implemented by a very simple algorithm to implant, provides optimal resistance performance at ECM interference.

Each integer is then supplied to means 24 of generation of signals providing, for each integer s, a corresponding SQ sequence of q samples, each sample q being an integer. 24 generation means of signals transform each integer s into a SQ sequence, this transformation being bi-unequivocal, that is to say that a given integer s corresponds to a single sequence SQ and reciprocally.

We can write : SQ = b s 0 b s 1 b s 2 .... b s q-1 where bs / i is an integer between 0 and L-1.

The signal generator can for example be consisting of a transcoding table. We will refer usefully to French patent n ° 2,337,465 in the name of COMPAGNIE IBM FRANCE ™ which describes sequences called CAZAC which are periodic pseudo-random sequences of complex numbers that have an autocorrelation function periodic whose only the first coefficient is not zero and of which all complex numbers have an amplitude constant. The generation of such sequences can be generalized to obtain sequences made up of whole numbers, these sequences being orthogonal between them, that is to say having properties optimal autocorrelation. We can also mention Gold sequences which are quasi-orthogonal, like those of Kasami, or those called polyphases.

In a preferred embodiment, the means 24 generate sequences SQ which are substantially orthogonal to one another. By way of example, the signal generation means 24 can transform each integer s into a sequence SQ of q bits (samples each taking a value in {0,1}) according to table 1 below. Input value s SQ Suite generated 0 0000000 1 1110100 2 0111010 3 0011101 4 1001110 5 0100111 6 1010011 7 1101001

In this configuration, M = 8 and q = 7. Each suite of q bits is produced by circular shifts of a sequence of maximum length of length 7, excluding the first suite always consisting of zeros. These suites have quasi-orthogonality properties, say that for two different and arbitrary sequences, the sum of the EXCLUSIVE OUs of each term is equal to 4.

It is possible to generalize this principle of generation of quasi-orthogonal SQ signals for all M power of 2. For this, after determining a sequence of maximum length of period M-1 (by one of the well-known methods in the field of treatment signals), the M sequences of M-1 bits are obtained by circular shifts of the initial sequence, with the exception of the first always consisting of zeros.

Another class of usable sequences, perfectly orthogonal, is that constituted by Hadamard sequences. An example of such sequences is illustrated in table 2, for samples also constituted by bits. Input value s SQ Suite generated 0 11111111 1 10101010 2 11001100 3 10011001 4 11110000 5 10100101 6 11000011 7 10010110

The length of these sequences or sequences is 8.

The preceding description shows that each block of k bits of signal SN has been transformed into a sequence Corresponding SQ, each SQ suite comprising a pseudo-random component. Useful information is coded in this SQ suite and the different suites are orthogonal or quasi-orthogonal to each other. Since then that M and q are large before k or before N, we understand that this coding operation consisted in increasing by importantly the number of samples to be transmitted and that we have therefore spread the signal spectrum useful SN using hazards provided at low frequency.

The main advantage of the invention lies precisely in this coding which is carried out at the bit frequency and not not at the chip frequency (where the spectrum spread is performed by direct sequence). The working frequency of means described so far can be very weak, around 16 Kbits, compare with 10 Mchips in the case spread spectrum by direct sequence.

It should be noted that the samples may take larger values, depending on the modulation used in transmission means 25 to which SQ suites are provided.

These transmission means 25 supply a signal STR transmitted to the attention of the receiver. They can be from any type, analog or digital.

In the embodiment shown, the means 25 are digital and have a phase shift modulator 28. This modulator 28 is by example of type MPSK (Multiple Phase Shift Keying) where M corresponds here to the number of possible values of samples q of the sequences SQ and therefore the number of states of phase of the modulated signal STR. It is for example possible to perform a BPSK modulation if the SQ suites are exclusively made up of bits, QPSK modulation if the integers of the SQ suites are each included in the set {0, 1, 2, 3}, and a 64-PSK modulation if the whole SQ suites are each included in the set {0, 1, ..., 63}. The phase shift modulator 28 can also be of the QAM type. It provides a modulated signal noted SM.

The transmission means 25 may also include means 26 for spreading spectrum by spreading sequence. The spreading sequence SE is generated by a spreading sequence generator 27. In the embodiment shown, it is assumed that the bits of the sequences SQ take their values in {0,1} and that the chips of the sequence d spreading SE also take their values in {0,1}. Each sample bs / i produced by the signal generation means 24 is added modulo L to G hazards e _s belonging to the set {0, 1, ..., L-1} and coming from the generator 27, where G represents the spread gain by direct sequence. The increase in bit rate caused by this processing is equal to G. In the case of a spread of spectrum by direct sequence, it is therefore the output signal of the means 26, denoted SQE, which is applied to the modulator 28.

Each sample a _i of an SQE sequence takes its value in {0, 1, ..., L-1}. In the case where no spreading by direct sequence is implemented, G = 1 and e _s = 0, that is to say that this operator is transparent.

où g est la fonction de mapping réalisée par le modulateur 28, Ts le temps symbole et h_e(t-iTs) le filtrage émission. A titre d'exemple:

• en modulation BPSK, L = 2 et on a g(0) = -1 et g(1) = 1

The STR signal sent to the receiver is of the form:

where g is the mapping function performed by the modulator 28, Ts the symbol time and h _e (t-iTs) the emission filtering. For exemple:

• in BPSK modulation, L = 2 and we ag (0) = -1 and g (1) = 1

avec α_i = 0 ou 1

• en modulation QPSK, L = 4 et g(0) = 1, g(1) = j,
     g(2) = -1 et g(3) = -j
• en modulation 8PSK, L = 8 et g(k) = e^2jkπ/8

In this case the relation 1 is written:

with α _i = 0 or 1

• in QPSK modulation, L = 4 and g (0) = 1, g (1) = j,
  g (2) = -1 and g (3) = -j
• in 8PSK modulation, L = 8 and g (k) = e ^{2jkπ / 8}

Generally, in MPSK modulation, L = M and g (k) = 2 ^{jkπ / M.}

lorsqu'un étalement par séquence directe est mis en oeuvre (G > 1).Note that the mapping function g of the modulator must respect the relationship:

when a spread by direct sequence is implemented (G> 1).

et

The impulse response h _e of the emission filter is assumed such that:

and

The means 26 of spectrum spreading by sequence direct are of course optional in the invention and are therefore shown in broken lines.

The transmission means 25 can also include frequency escape means 29, 30, also optional and therefore shown in broken lines, suitable to modify the carrier frequency of the signal transmitted to the receiver. The frequency escape is to change frequently of carrier frequency in order to further broaden the spectrum of the signal transmitted to the receiver. The modulated signal SM, in baseband or in intermediate frequency, is applied to a multiplier 29 receiving a signal from carrier frequency of a generator 30.

We can see that the phase 23 generator allows low-frequency coding of the signal to be transmitted and allows to modify in a pseudo-random way the phase of the signal transmitted when the modulation is of MPSK type. We can thus consider that the generator 23 and the means of combination 22 provide a phase escape function performed at low frequency. Amplitude modulation, also pseudo-random, the signal to be transmitted comes combine with this phase escape when the modulation is of the QAM type (modification of the phase and the amplitude of the transmitted signal). This is how the system transmission of the invention provides a high resistance to ECM interference.

The output signal STR of the transmission means 28 is transmitted over the air to receiver 31 whose diagram synoptic is given in figure 3.

The receiver 31 receives a corresponding signal STRr to the STR signal noisy by the transmission medium. he includes reception means generally referenced by 40 restoring the sequences SQ of q whole numbers, noted SQr at the receiver. Means of reception 40 here include means 32 for removing the carrier frequency controlled by a local oscillator 33. The means 32 conventionally comprise two mixers controlled by quadrature clock signals and we obtains at the output of these means two quadrature signals. When a frequency escape is used at the transmitter 20, the local oscillator 33 operates in synchronism with that of the transmitter, referenced 30. This synchronization can be obtained by known means. The output signal of the means 32 is noted SMr and corresponds to the transmitter SM signal.

The signal SMr is applied to means 34 of spectrum compression to suppress spreading by direct sequence possibly performed at the transmitter 20. Spectrum compression means are notably described in "Digital Communications" by J.G. PROAKIS, McGraw-Hill ™, Chapter 8. Those depicted in the Figure 3 include a sampler 35 controlled at the frequency chip Fc followed by a module 36 for compression of spectrum. Module 36 includes a complex multiplier 37 followed by a summator 38. The multiplier 37 receives a direct sequence SE of a generator 39, this sequence direct SE being identical to that generated by the generator 27 of the transmitter 20. The phase timing of these two direct sequences is obtained by known means.

où e_sk est la valeur du chip à l'instant k de la séquence directe SE et * désigne le complexe conjugué. Cette sommation permet de supprimer l'étalement spectral par séquence directe. The summator 38 calculates, for each block of G consecutive samples r _k from the multiplier 37, the following sum:

where e _sk is the value of the chip at time k of the direct sequence SE and * denotes the conjugate complex. This summation eliminates spectral spreading by direct sequence.

Each sum U _k therefore corresponds to a sample α _i of the signal STR transmitted to the receiver. At the output of module 36, there are therefore suites SQr identical to the suites SQ originating from the means 24 for generating signals from the transmitter 20.

These SQr suites are applied to means 45 for processing that have the function of performing a demodulation of the received signal and removing the random phase E _is introduced at the transmitter 20 by the generator 23 hazards.

pour s = 0 à M-1.In the embodiment shown, the processing means 45 comprise correlation means 41 which calculate, for each block of Q successive sums U, the following value:

for s = 0 to M-1.

The correlation means 41 therefore receive a SR reference signal consisting of the different sequences SQ can be generated at transmitter 20, this is to say those for example represented in tables 1 or 2. The advantage of generating orthogonal sequences or quasi-orthogonal using the generator 24 of Figure 2 (and not any sequences) is that it is easy to detect a correlation of these signals.

The calculated correlations provide sums C ⁰ to C ^M-1 which each correspond to one of the integers from the combining means 22 of the transmitter 20. These sums are applied to a demultiplexer 42 receiving from a generator 43 a signal E _a identical to that generated by the generator 23 of the transmitter, and in phase with it.

The demultiplexer 42 selects N sums C ^S from M as a function of the value of the hazard E _a . In general, the demultiplexer 42 performs an inverse function f ^-1 to remove the phase hazard introduced at low frequency on transmission.

le démultiplexeur 42 fournit en sortie les signaux:

pour i = 0 à N-1 et E_a appartenant à l'ensemble {0, 1, ..., M-1}. Le démultiplexeur 42 sélectionne ainsi les échantillons C^s en fonction de l'aléa E_a.By way of example, if the combining means 22 produce:

the demultiplexer 42 outputs the signals:

for i = 0 to N-1 and E _a belonging to the set {0, 1, ..., M-1}. The demultiplexer 42 thus selects the samples C ^s as a function of the hazard E _a .

Each sample d _i therefore corresponds to a sample E _c of the emitter. These samples d _i are then applied to decoding means 44 performing an inverse operation to that of the coding means 21 of the transmitter 20. They can also carry out a deinterlacing of the decoded samples if the coding means perform an interleaving of the samples coded. The output signal SNr of the decoding means 44 then corresponds to the digital signal SN of the transmitter.

Of course, other embodiments of the processing means 45 can be envisaged. It is for example possible to calculate only the samples d _i according to the relation:

This direct calculation makes it possible not to use a fast correlation algorithm and therefore to simplify the practical implementation of the receiver. Only the useful correlations are then calculated. The processing means 45 then only comprise correlation means such as 41, receiving the signal E _a .

The present invention applies for example to transmission systems where error correcting codes are used and where an alphabet of orthogonal signals of very large size, larger than the alphabet used by the error correction code is available. The elements of the alphabet not used by the code can be used for low-frequency pseudo-random coding of signal to be transmitted, thereby improving low cost the robustness of the system with regard to interception.

Claims (8)

1. System for transmitting a digital signal (SN) between a transmitter (20) and a receiver (31), characterized in that:

   said transmitter (20) includes in succession:

   coding means (21) receiving said digital signal (SN) and supplying, for each block of k bits of said digital signal (SN), a coded sample (Ec) taking an integer value in the range [0, N-1], each integer value (Ec) being representative of the k bits of the block from which it is obtained;

   combining means (22) for combining said coded samples (Ec) with samples (Ea) from a pseudorandom random phase generator (23), said combining means (22) supplying an integer (s) in the range [0, M-1] for each combination of a coded sample (Ec) and a random phase sample (Ea) from said random phase generator (23), M being greater than N;

   signal generator means (24) supplying, for each integer (s) in the range [0, M-1], a sequence (SQ) of q integers corresponding to said integer (s), the various sequences (SQ) being orthogonal or quasi-orthogonal;

   transmit means (25) for transmitting said sequences (SQ) of q integers to said receiver (31), said transmit means (25) comprising a phase shift modulator using M states;

   said receiver (31) includes in succession:

   receive means (40) recovering said sequences (SQr) of q integers;

   processing means (45) receiving said sequences (SQr) of q integers from said receive means (40) and random phase samples (Ea) from a random phase generator (43) synchronized with said random phase generator (23) of said transmitter (20), said processing means (45) demodulating said sequences (SQr) of q integers and implementing an operation which is the inverse of that implemented by said combining means (22) to recover said coded samples (di);

   decoding means (44) for recovering said digital signal (SNr) from said samples supplied by said processing means (45).
2. System according to claim 1 characterized in that said M sequences (SQ) of q integers are Hadamard sequences.
3. System according to claim 1 or claim 2 characterized in that said transmit means (25) comprise spectrum spreading means (26, 27) using a spreading sequence (SE) and in that said receive means (40) comprise spectrum compression means (34) operating in synchronism with said spectrum spreading means (26, 27) of said transmit means (25).
4. System according to any one of claims 1 to 3 characterized in that said transmit means (25) comprise frequency evasion means (29, 30) adapted to modify the carrier frequency of said signal transmitted to said receiver (30) and in that said receive means (40) comprise means (32, 33) implementing a function which is the inverse of that of said frequency evasion means (29, 30), adapted to eliminate said frequency evasion introduced at said transmitter (20).
5. System according to any one of claims 1 to 4 characterized in that said coding means (21) also interleave the bits of said digital signal (SN) and in that said decoding means (44) also disinterleave the decoded samples (di).
6. System according to any one of claims 1 to 5 characterized in that said combining means (22) of said transmitter (20) supply, for each coded sample (Ec), an integer (s) equal to:

   

   where:

   s is said integer supplied by said combining means (22);

   Ec is said coded sample;

   Ea is a random phase sample from said random phase generator (23) of said transmitter (20);

   

   denotes modulo M addition, where M is an integer;

   and in that said means for eliminating said random phase of said receiver (30) supply, for each sequence (SQe) of q bits from said processing means, an integer (di) equal to:

   

   where Ea is a random phase sample from said random phase generator (43) of said receiver (31).
7. Spread spectrum method of transmitting a digital signal between a transmitter (20) and a receiver (30) characterized in that it consists in:

   at said transmitter (20):

   generating, for each block of k bits of said digital signal, a coded sample (Ec) taking an integer value in the range [0, N-1], each integer value being representative of the k bits of the respective block;

   combining said coded samples (Ec) with random phase samples (Ea) to generate an integer (S) in the range [0, M-1] for each combination of a coded sample (Ec) and a random phase sample (Ea), M being greater than N;

   generating for each integer (s) in the range [0, M-1] a corresponding sequence (SQ) of q integers, by means of a one-to-one conversion process, the various sequences (SQ) being orthogonal or quasi-orthogonal;

   transmitting said sequences (SQ) of q integers to said receiver (30);

   at said receiver (30):

   recovering said sequences (SQr) of q integers from the signal received from said transmitter (20) and, for each sequence (SQr) of q integers recovered, generating an integer by means of a conversion which is the inverse of that carried out at said transmitter (20);

   combining each integer generated with a random phase sample (Ea) identical to that used to obtain said integer at said transmitter (20), so as to recover the corresponding coded sample (di), said combination thus eliminating said random phase (Ea);

   decoding each coded sample (di) to recover said digital signal (SNr).
8. Method according to claim 7 characterized in that said sequences (SQ) of q integers are Hadamard sequences.

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