EP1925105A1

EP1925105A1 - Method for transmitting a multicarrier spectrum-spread signal, reception method, corresponding transmitting, receiving device and signal

Info

Publication number: EP1925105A1
Application number: EP06793536A
Authority: EP
Inventors: Jean-Yves Baudais
Original assignee: Centre National de la Recherche Scientifique CNRS; Universite de Rennes 1
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite de Rennes 1
Priority date: 2005-09-14
Filing date: 2006-09-14
Publication date: 2008-05-28
Also published as: CA2622774A1; WO2007031568A1; US20080260003A1

Abstract

The invention concerns a method for transmitting a multicarrier spectrum-spread signal, using a plurality of spreading codes. The invention is characterized in that the method includes a step of allocating a power and/or a rate to each of the spreading codes, based on an information representing the noise and/or on an information representing the quality of the link, said allocating step taking into account a target rate.

Description

A method of transmitting a multicore and spread spectrum signal, receiving method, transmitting device, receiving device and corresponding signal.

FIELD OF THE DISCLOSURE The field of the invention is that of multi-carrier signals, and in particular signals combining multi-carrier and code division access modulation.

More specifically, the invention presents a technique for transmitting such a multicarrier signal (for example according to a modulation of OFDM type, in English "Orthogonal Frequency Division Multiplex"), and to spread spectrum (for example of type access code division multiple CDMA, in English "Code

Division Multiple Access ").

In other words, the invention relates to the allocation of source data for forming such multi-carrier and spread-spectrum signals such as MC-CDMA (Multi Carrier Code Division Multiple Access) signals.

The invention finds particular applications in all fields implementing transmission and broadband communication techniques.

The invention applies mainly, but not exclusively, to communications in cable networks, such as xDSL type networks (Digital Subscriber Line), powerline communications (home automation, electrical distribution network, ...), intra-vehicle connections, etc.

Assuming static or quasi-static transmission channels, the invention also finds applications in wireless communications, such as radiocommunications inside buildings, communication beams, etc.

2. Prior Art

Current wired transmission techniques are based on DMT (Digital MultiTone) technology. According to this technique, a modulation is determined to be applied to each carrier of a multicarrier signal to distribute the source data, based on the quality of the link (quality of the propagation channel) and the desired link budget.

However, a major disadvantage of this technique is that the signals thus shaped are not very resistant to electromagnetic jammers.

In addition, since the data recovery is carried out carrier by DMT, the OFDM multiplex carriers have a low link budget (ie a signal-to-noise ratio which is too low to transmit data). bits of information) can not be exploited. We can not transmit information about these carriers.

Other data allocation techniques in a system employing multicarrier modulation have also been described in the articles cited in Appendix 1, which forms an integral part of the present description. 3. Objectives of the invention

The invention particularly aims to overcome these disadvantages of the prior art. More precisely, an object of the invention is to propose a transmission technique of a multicarrier and spread spectrum signal making it possible to optimize the distribution of the source data on the spreading codes.

In particular, an object of the invention is to provide such a technique for attributing an optimum power and / or rate to each of the spreading codes.

The invention also aims to implement such a technique to optimize a noise margin of the transmission system for a given spreading code length. This noise margin corresponds in particular to the maximum possible difference between the real performance of the transmission system, operating with a certain bit error rate, and the theoretical performance of the transmission system, defined by the Shannon limit.

Yet another object of the invention is to provide such a transmission technique having better performance compared to the techniques of the prior art, and in particular a better resistance to electromagnetic jammers. 4. Presentation of the invention

These objectives, as well as others that will appear later, are achieved at using a method of transmitting a multicarrier and spread spectrum signal, implementing a plurality of spreading codes.

According to the invention, such a method comprises a step of assigning a power or an energy and / or a flow rate to each of the spreading codes, as a function of information representative of the noise and / or information representative of the quality of the link, said allocation step taking into account a target bit rate (overall bit rate).

Thus, the invention is based on an entirely new and inventive approach to the distribution of source data, intended to form a signal, for example of the MC-CDMA type, on the carriers and the spreading codes associated with such a signal.

More precisely, the invention makes it possible to determine the number U of spreading codes required, the power or energy E ₁₁ assigned to each of these codes (where the power corresponds to the energy E ₁₁ per unit time), and / or the rate R ₁₁ allocated to each of these codes, as a function of information representative of the noise, in particular of the signal-to-noise ratio, and / or information representative of the quality of the link, that is, that is, the estimation of the transmission channel, and a target bit rate R.

The quality of the link is in particular a function of the estimation of the coefficients h _t of the transmission channel, and the variance N ₀ of the noise, supposed white Gaussian.

Thus, taking into account the signal-to-noise ratio (link budget) and / or the estimation of the transmission channel (link quality), and a target rate R, the invention makes it possible to optimize the margin of noise γ of the system, by optimizing the distribution of the energy E ₁₁ and / or the flow rate R ₁₁ assigned to each of the U spreading codes.

This noise margin γ corresponds in particular to the maximum difference between the real performance of the transmission system and the theoretical limit performance, as defined by the Shannon theorem. Thus, according to the invention, for a desired quality of QoS service (for example a BER of 10 ^"7 ), the bit error rate must remain lower than this quality, even in the presence of noise.

The target rate R is determined in particular according to the desired application. By way of example, in the context of an ADSL link, the target bit rate R to be reached can be 512 bits per OFDM symbol.

More precisely, this target bit rate R corresponds to the sum of the bit rates R ₁₁ allocated to each of the U spreading codes during the attribution step:

U

Σ R _U = R u = l

It can thus be noted that the transmission technique according to the invention based on an optimal allocation of resources is implemented on the basis of an algorithm having a linear structure, unlike the noise margin maximization algorithms in the context of the invention. DMT, which have an iterative structure.

The allocation step can also take into account a desired QoS quality of service, determined from a bit error rate (BER) to be respected, the coding gain provided by the channel coding and the various impairments of the transmission and reception system that can be taken into account in the noise margin F, etc., hence the necessary optimization of resources to ensure the best possible service under the performance constraints required in reception.

The allocation step may further take into account an overall power spectral density. This overall power spectral density, which can in particular be defined by a standardization organization, defines a power mask that the MC-CDMA signal must not exceed. From this spectral density of power and the bandwidth of a sub-carrier, it is possible to define a power or global energy E to be distributed between the different codes. It is recalled that a multi-carrier signal is formed of a temporal succession of symbols consisting of a set of data elements, each of the data elements modulating a carrier frequency of the signal, one of the carrier frequencies modulated at a given instant. by one of the data elements being called subcarrier.

This global energy E corresponds to the sum of the energies E ₁₁ assigned to each of the U spreading codes during the attribution step:

U Σ E _U = E u = l

Preferably, the step of allocating a bit rate comprises, for each of the spreading codes, a step of selecting a modulation for at least some, and in particular all, subcarriers of the signal. For example, the source data to be transmitted are modulated according to a quadrature amplitude modulation MAQ, such as MAQ4, MAQ16, MAQ64,

MAQ 256, etc.

The symbols X _{11 o <u} <u resulting from the quadrature amplitude modulation are then spread using the spreading codes, to form the MC-CDMA signal:

^{C "X} = ( ^c i, u) θ <i kk, o <u UU ^• [ ^X h -, ^χ u] with C the spreading matrix representative of the spreading codes. spreading thus makes it possible to collectively exploit the sub-carriers grouped together by each of the codes, which makes it possible to improve the robustness of the transmission system in noisy environments.

Advantageously, the allocation step comprises the following sub-steps: verification of the feasibility of the target rate R; if the target rate is achievable: o determination of the rate to be assigned to each of the spreading codes:

If the target bit rate R is strictly less than twice the length k of the spreading codes, assigning two bits on each of R / 2 codes;

• otherwise assignment of | _ /? / fcJ bits on each of k - (R ~ lR / k] k) first codes, and | _i? / k J + 1 bits on each of R - \ _R / k] k second codes; with LJ the whole part.

If the target rate R is not feasible, that is to say if Theorem 2 presented in Appendix 2 is not respected, it is desirable to modify the desired QoS quality of service and / or the target rate R desired, in order to respect this theorem 2.

If the target rate R is achievable, the value of the target rate R must be compared with the length of the spreading codes. If this value is strictly less than twice the length of the spreading codes, the distribution is expressed by the relation R - - x 2, this

which means that only 2 bits are assigned to each of the R / 2 codes, which corresponds to a quadrature amplitude modulation MAQ4. Preferably, when said target rate R is greater than or equal to twice the length k of the spreading codes, the assignment step is defined by the equations: R = (k - (R -IR / kjk)) xR / kj + (R-lR / kjk) x (12); with:

R the target rate;

R ₁₁ the flow rate assigned to the spreading code u; k the length of the spreading code; LJ the whole part. Advantageously, the allocation step also comprises a substep of determining the energy E ₁₁ representative of the power, or directly of the power (energy per unit of time), to be assigned to each of the spreading codes. , expressing itself in the form:

Eu = _v ^{2 U ~ l} E (11)

Σ V ^{Ru ~} D u = l with:

E a global energy representative of the overall power spectral density;

R ₁₁ the rate assigned to said spreading code u;

U the number of spreading codes. Thus, for a global energy E, a target rate R, and a code length k, the preceding equations (12) and (11) give the distribution of the information R ₁₁ and the energies E _{11 in} order to optimize the margin of noise γ of a system using an MC-CDMA waveform.

The invention also relates to a device for transmitting a signal implementing the transmission method described above.

The invention also relates to a method for receiving a multi-carrier and spread spectrum signal MC-CDMA, comprising a step of demodulating a signal transmitted according to the transmission method described above, as well as a reception device corresponding. The invention finally relates to a multi-carrier and spread spectrum signal MC-CDMA, transmitted by a transmission device and / or received by a reception device as described.

5. List of Figures Other features and advantages of the invention will appear more clearly on reading the following description of a preferred embodiment, given as a simple illustrative and nonlimiting example, and the accompanying drawings, among which: FIG. 1 presents an MC-CDMA transmission chain implementing the transmission technique based on the allocation of information according to the invention; FIGS. 2A to 2D illustrate the noise margin γ as a function of the length k of the codes, for two target rates R and two lengths L of the ADSL channel, in a transmission chain according to FIG. 1; FIGS. 3A to 3D illustrate the optimal length k of the codes as a function of the length L of the ADSL channel for four target rates R; Figures 4A to 4D show the performance of the invention compared to the performance of the techniques of the prior art.

6. Description of an embodiment of the invention

The general principle of the invention is based on the allocation of source data intended to form a MC-CDMA multi-carrier and spread spectrum signal, based on the determination of a number of spreading codes, a distribution of source data on codes, and a distribution of energies or powers

(energy / time) attributed to these codes.

In other words, the invention presents an information allocation algorithm applied to multi-carrier waveforms and using spread spectrum.

Thus spreading codes are allocated to groups of carriers, and the spreading code associated with each group of carriers is optimized to obtain a target bit rate R. Thus, according to the invention, for a desired QoS quality of service (eg a BER of 10 ^"7 ), the bit error rate must remain below this quality, even in the presence of noise. 6.1 General Principle of MC-CDMA Systems

MC-CDMA systems are well known, and are described in particular in documents 1 and 2 cited in Appendix 1.

According to these documents, an MC-CDMA signal can be seen as the inverse Fourier transform of a CDMA signal.

This digital CDMA signal is denoted by CX, with C = (q _M ) θ <i k k, where u ^is a spreading matrix applied to the vector of complex symbols X = [Xι, ..., Xj _j ] , and X _{11 where <u} <u are the symbols resulting from a quadrature amplitude modulation.

Conventionally, the length k of the spreading codes is equal to the number of subcarriers used. The spreading codes are orthogonal codes that can be extracted from Hadamard matrices of dimensions k x k. The number of codes used is U ≤ k.

6.2 General principle of the invention

FIG. 1 shows a simplified representation of an MC-CDMA transmission chain comprising a transmitter 11, a transmission channel 12, and a receiver 13, according to a preferred embodiment of the invention.

It may be noted in particular that the channel coding and decoding functions, which do not form part of the invention, have not been represented in this figure. In transmission, a bit stream 111, composed of the source data to be formatted, enters a quadrature amplitude modulation block MAQ 112. The order of the modulation to be applied to each of the carriers carrying the source data is in particular determined. from a centralized allocation block 14, according to this preferred embodiment of the invention. The symbols X _uo <u≤U i ^ssus of the QAM modulation block 112 are then multiplied by the spreading matrix C = (C /, _M ) O </ <£, _O < _M <£ / in the block of spread CDMA 113: ^{c • x} = ( ^c i, u) θ <i k k, o <u UU ^• [i i, ..., Q Qy].

The number U of spreading codes, as well as the energy E ₁₁ allocated to each of these codes, are also determined from the centralized allocation block 14.

The CDMA C • X signal thus obtained is then modulated according to a modulation OFDM in block 114, to form a MC-CDMA signal, and then converted to an analog signal in CNA block 115, according to this preferred embodiment.

According to the invention, a static or quasi-static transmission channel 12 is considered, and the OFDM component of the MC-CDMA signal adapted to the transmission channel 12 is assumed. The channel 12 can then be modeled in the frequency domain with a coefficient by sub-carrier, as proposed in document 3 cited in appendix 1.

In reception, the analog signal is converted into a digital signal in the CAN block 131, then undergoes an OFDM demodulation, using a Fourier transform and the suppression of the guard interval, in the block 132. then uses an equalizer ZF 133 (in English "zero forcing", in French

"Forcing to zero") which inverts the transmission channel 12, to equalize the signal obtained.

The equalized signal is then despread in a CDMA despreading block 134, taking into account the number U of spreading codes, and the energy E ₁₁ assigned to each of these codes, determined from the centralized allocation block 14 .

After OFDM demodulation 132, ZF equalizer 133, and 134 despreading the signal received by the u code is written as: _z k

i = l "* with: • k the length of the code u;

"X ₁₁ the symbols from the QAM modulation, O <u ≤ U;" (q _M ) the coefficients of the spreading matrix C, 0 <i ≤ k, 0 <u ≤ U; "Z _i a complex sample of background noise, supposedly white and Gaussian;" h _t the coefficients of the transmission channel. We denote by N ₀ the variance of the complex sample Z ₁ .

The signal Y ₁₁ received by each of the spreading codes is then subjected to a MAQ demodulation 135, taking into account the order of the modulation determined from the centralized allocation block 14.

According to this preferred embodiment of the invention, the allocation block 14 thus makes it possible to determine: the number U of spreading codes to be used;

"the order of the modulation to be applied to the carriers, that is to say a

R ₁₁ to be assigned to each of the spreading codes; and the energy E ₁₁ to be assigned to each of the spreading codes, based on information representative of the noise and / or information representative of the quality of the link, and of the target rate R to be reached.

The MC-CDMA system is thus sized according to this target bit rate, which makes it possible to optimize the noise margin. So we are not trying to maximize the bit rate, but to optimize the noise margin by reaching this target bit rate. More precisely, the information representative of the quality of the link depends on the quality of the estimation of the transmission channel, that is to say the parameters h _t and N ₀ .

The information representative of the noise depends in particular on the link budget, that is to say the signal-to-noise ratio at the output of the transmission system. According to this preferred embodiment of the invention, the centralized allocation block 14 takes into account a target bit rate R and a quality of service QoS (for example a BER of the order of 10) to be reached, defined depending on the intended application, and an overall power spectral density, represented by the overall energy E, not to be exceeded, defined by the standardization bodies. It is thus assumed, according to this preferred embodiment, the use of a power mask on transmission, limiting the overall power spectral density (DSP) of the transmitted signal. This constraint is important since it is within this framework that the allocation of information is carried out. The amplitude of the received signal then depends on the number of subcarriers k. Thus, according to the invention, the spread brings power, which is consistent with the constraint, not in total power transmitted, but in spectral power density.

It can also be noted that spread spectrum is well known for its robustness in scrambled environments. The use of a spreading code thus enables the carriers grouped together by the same code to be exploited collectively, contrary to the techniques of the prior art which require carrier-borne processing.

As indicated above, the invention makes it possible in particular to find the number of spreading codes, the distribution of the modulations on the codes, and the distribution of the energies attributed to these codes, under the constraint of a target bit rate and possibly under duress. a spectral density of power.

In particular, the so-called "optimal" distribution is considered to maximize the noise margin of the transmission system for a given code length, ie to maximize the difference between the actual performance of the transmission system and the theoretical performance obtained. by the Shannon boundary.

6.3 Example of application in the context of an ADSL connection

The following is an example of application of the invention in the context of an ADSL connection (in English "Asymmetric digital subscriber loop").

The maximum number of usable subcarriers is 220, and an example of target rate R is 512 bits per OFDM symbol. Considering Document 4 quoted in Appendix 1, the maximum order modulation is 32768 MAQ.

It is therefore necessary to distribute 512 bits over at most 220 codes, with a maximum of 15 bits per code, because of the maximum order of the QAM modulation.

According to conventional techniques, an exhaustive search for optimal distribution requires testing 3 380 629 853 852 186 combinations. This method of finding the optimal distribution is therefore not possible.

According to the invention, it is considered that a calculation of capacity, or more exactly of the achievable rate, of the transmission system taking into account the receiver and the noise margin is written:

with

R the target rate;

U the number of spreading codes;

R ₁₁ the flow rate assigned to the spreading code u; k the length of the spreading code; h _τ the estimation of the coefficients of the transmission channel; γ the noise margin of the transmission system; F the noise margin of the MAQ modulations, as described in Document 3 cited in Appendix 1;

E ₁₁ the energy assigned to the spreading code u.

It should be noted in particular that the noise margin Y can also take into account the gain provided by the channel coding.

Thus, in equation (2), the unknowns are R ₁₁ , E ₁₁ , U, and it is sought to optimize the noise margin γ of the transmission system.

The constraint of a global power spectral density, that is to say a power mask or mask of the emission signal, can be written in the form of a global energy E representative of the power spectral density: U Σ E _U = E (3) u = 1 using Equation (2):

The noise margin γ is then written:

U

(2 ^ R ₁ ^u ₁ - \) (called first u = l summation), and / or to minimize the term (called second summation).

The second summation is simple to minimize, just choose H ₁ such that:

V / e [l; *], Y _/ e [1; fc], ^ -> hi

To minimize the first summation, it is possible to use Theorem 1 as presented in Appendix 2, which is an integral part of the present description: UU

Theorem 1: Under the constraint Σ R ₁₁ = R, Σ (2 ^u - 1) is minimal if and u = lu = l only if k - (R - \ _R I k \ k) values of R ₁₁ are equal to | _i? / & _ | and R - \ _R / kjk values of R ₁₁ are equal to \ _R I k J + 1.

It then remains to distribute the energy E between the codes, or between the symbols, relative to the modulation order, that is to say relative to the distribution of the bits. Using the expression of E ₁₁ equation (4), and using equation (5), we obtain:

2 ^R u _ i

Eu = -Q ^E (H)

Σ (2 * ^« - 1) u = l

Thus, for a given power spectral density (DSP) expressed by represented by a global energy E, a target rate R, and a code length k, it is possible to define the distribution of the information R ₁₁ and the energies E ₁₁ to assign to each of

U spreading codes to optimize the noise margin γ of a system using an MC-CDMA waveform.

The distribution of information is thus the following: R = (k - (R - IR / k) k)) x lR / k] + + 1) (12) with LJ the whole part.

There remains however a particular case, that of the MDP2, which can not be used in the applications, which means that the minimum number of bits allocated according to the invention is 2 and not 1.

Indeed, the distance between the Shannon limit and the bit rate that this modulation allows is greater than that of the higher order modulations. It is therefore not possible to use equation (12) when there exists R ₁₁ = 1, that is to say when [R / k \ e {0,1}, ie R <2 xk . In this particular case, the distribution is simply:

R = - x 2 (13)

that is, there are R / 2 codes carrying 2 bits, which corresponds to quadrature amplitude modulation of order 4 (MAQ 4).

It can also be noted that an odd R <2 x k rate can not be allocated.

In addition, the feasibility of the target rate R has so far been assumed.

However, it is desirable to first test whether the target rate R is achievable or not, using Theorem 2 as presented in Appendix 2 forming an integral part of this description.

Theorem 2: R flow is achievable if and only if:

R ≤ _{k (2} * / k-1K / k] _ ₁₎ | _{+ Jt} | _ςR _{/ Jt} j _{with% = Uog2} i ₊ i

i = l \ K Thus, according to the preferred embodiment of the invention, the centralized allocation algorithm 14 has the following structure:

1. Input: R, T, E, k, A ₁ , N ₀ ;

2. Check the feasibility of the target bit rate: if the bit rate is not feasible, change the quality of service QoS (T) or the target bit rate R to comply with Theorem 2;

3. If i? <2k, then assign 2 bits (MAQ4) on each of the R / 2 codes;

4. Otherwise, assign | _ /? / A; J bits on each of k - (R - \ _R / k) k) codes, and \ _R I k \ + 1 bits on each of R - \ _R / k] k other codes;

5. Calculate the energy distribution according to equation (11); 6. Output: U, R ₁₁ , E ₁₁ .

For example, consider an ADSL link having the transfer function:

with L the length of the line (in meters), and i _j - e [35; 64 [u] 64; 255] the index of the subcarrier, noting ie [θ; 22θ [indices corresponding to the indices if. Let R = 512 bits / symbols, T = 4, 04 (which corresponds to a symbol error rate of the order of 10 ^"3 ), E = -39 dBm / Hz, k = 100, and N ₀ = -140 dBm / Hz.

So, for a line length L of 3000 meters, we obtain with the help of theorem 1 and equation (11) a number of codes U = 100, with 88 codes such that R ₁₁ = 5 and E _n = 8, 898.10 ^"3 E, and 12 codes such that R _n = 6 and E _n = 1, 808. 10 ^{- 2} E.

In this case, the noise margin is γ = 45.5, ie 16.6 dB of additional noise margin. In particular, it could be considered that the MC-CDMA system does not, in itself, make it possible to obtain a better noise margin than that obtained with the DMT systems of the prior art. But added to the noise margin, the spreading gain gives the system greater robustness.

Thus, results have already shown the advantage of the MC-CDMA system in scrambled environments even in the absence of optimization of the allocation, as indicated in documents 6 and 7 presented in appendix 1.

The invention makes it possible to further improve these results.

FIGS. 2 to 4 show a few simulation results of an exemplary application of the invention in the ADSL context. FIGS. 2A to 2D illustrate in particular the noise margin γ as a function of the length k of the codes, for two target rates R and two lengths L of the ADSL channel.

Thus, FIG. 2A shows the evolution of the noise margin γ as a function of the length k, for a rate R of 512 bits / symbols and a channel length L of 2000 meters, FIG. 2B for a flow rate R of 512. bits / symbols and an L-channel length of 3000 meters, Figure 2C for a R-bit rate of 1024 bits / symbols and an L-channel length of 2000 meters, and Figure 2D for a R-bit rate of 1024 bits / symbols and a L-channel length of 3000 meters.

It can thus be observed that there is an optimum value of k for each configuration that makes it possible to optimize the noise margin. For example :

for the configuration of FIG. 2A, the optimum value of k is about 130; for the configuration of FIG. 2B, the optimum value of k is about 90; For the configuration of FIG. 2C, the optimum value of k is about 175; and for the configuration of FIG. 2D, the optimum value of k is about 125.

These values are also given in relation with FIGS. 3A to 3D, which illustrate the optimal length k of the codes as a function of the length L of the ADSL channel for four target rates R (304 bits / symbols - FIG. 3A, 512 bits / symbols - Figure 3B, 1024 bits / symbols - Figure 3C, and 2048 bits / symbols - Figure 3D).

Finally, FIGS. 4A to 4D illustrate the performances of the invention in an MC-CDMA system implementing an allocation of a bit rate and / or a power to each of the spreading codes according to the invention, compared to the performance of the techniques of the prior art in a DMT type system.

In particular, FIGS. 4A to 4D illustrate the margin of the systems in dB as a function of the length of the ADSL channel for four R target rates (304 bits / symbols - FIG. 4A, 512 bits / symbols - FIG. 4B, 1024 bits / symbols - FIG. 4C, and 2048 bits / symbols - FIG. 4D). So :

curve 1 (+) presents the noise margin of a transmission system according to the DMT technique as a function of the length L of the channel; curve 2 (4) presents the noise margin of a transmission system according to the MC-CDMA technique with a fixed code length k (k = 64) as a function of the length L of the channel; and

curve 3 (V) presents the noise margin of a transmission system according to the MC-CDMA technique with an optimal code length k for each length of the channel, with the noise margin γ in full lines, and the margin of noise combined with the spreading gain in broken lines.

It can be seen in these figures that the invention makes it possible to optimize the noise margin γ of the system, thanks to an optimal distribution of the energies E ₁₁ and flow rates R ₁₁ on the U spreading codes.

Thus, the invention gives the communications greater robustness in environments disturbed by electromagnetic jammers.

Moreover, this transmission technique based on a debit assignment and / or energy to each of the spreading codes offers all its interest in systems multiplexing several elementary MC-CDMA modules in the frequency domain, as presented in document 8 cited in appendix 1.

This transmission technique can also be implemented for different channels (ADSL, PLC), in a point-to-multipoint or multipoint-to-point context (multi-user communication). respectively broadcast or access), especially when the multiplexing is frequency.

The overlay layer related to the multi-user context is not part of the present invention.

The invention thus provides greater robustness to communications in environments disturbed by electromagnetic jammers, and in particular to wireline communications.

Recall that the invention requires knowledge of the channel on transmission, it is commonly accepted that this type of technology is more suitable for wired communications.

But this is not exclusive: radiocommunications inside buildings, as well as communication beams can be made through static channels, relative to the flow of communications. Thus, the invention may be envisaged for certain wireless communications.

The invention can also be used in systems where the number of subcarriers is greater than the length of the codes.

Also, several blocks of subcarriers can be defined, each block forming a MC-CDMA system, the total system being known by the acronym SS-MC-MA (in English "spread spectrum multicarrier multiple access"). The invention can then be applied to each block. APPENDIX 1 N. Yee, JP. Linnartz and G. Fettweis

"Multi-carrier CDMA in indoor wireless radio networks"

In IEEE Personal, Indoor and Mobile Radio Symposia, pages 109-113, September 1993. S. Hara and R. Prasad

"Overview of multicast CDMA"

IEEE Communications Magazine, Vol. 35, No. 2, pp. 126-133, December 1997. J.M. Cioffi

"A multicarrier primer"

Report, ANSI T1E1.4 / 91-157, Committee Contribution, 1991. G992-3

Asymmetrical Digital Subscriber Line (ADSL) transceivers

International Telecommunication Union, 2002. Mr. Crussière, J-Y. Baudais and J-F. Hélard

"Robust and high-bit rate communications over PLC channels: Bit-loading multi-carrier spreadspectrum solution"

In International Symposium on Power-Line Communications and Its Applications, (Vancouver, Canada), April 2005. S. Mallier, F. Nouvel, J-Y. Baudais, D. Gardan and A. Zeddam

«Multicarrier CDMA over Unes - Comparison of performances with the ADSL System»

In IEEE International Workshop on Electronic Design, Testing and Applications, pp. 450-452, January 2002. J-Y. Baudais

«Improvement of the robustness of the ADSL system in the presence of jammers: use of MC-CDMA techniques»

In GRETSI Symposium, Signal Processing Research and Study Group, September 2003. O. Isson, JM. Brossier and D. Mestdagh

"Multi-carrier bit-rate improvement by carrier merging"

Electronics Letters, Vol. 38, No. 9, pages 1134-1135, September 2002. ANNEX 2

Theorem 1:

U U

Under the constraint ^ R ₁₁ = R, ^ (2 ^α - 1) is minimal if and only if u = lu = lk - (R - IR I k \ k) values of R ₁₁ are equal to | _i? / k J, and i? The values of R ₁₁ are equal to R 1 k J + 1.

Demonstration:

Let R = kq + r with q = \ _R I k J and k / (0) = (jfc - r) 2 ^ + rl ^{q + l} = Σ 2 ^ «(6) u = 1 with i? _M e [q, q + 1} and [J the integer part. The demonstration is made by the absurd.

Let a ≥ 1. Suppose that there exists R ₍ = q and R _j - - q such that R ₍ becomes q + a and

R ₇ - becomes ç - α, involving / (0)> f (a) = (kr - 2) 2 + 2 ^{q + a} + 2 ^{q ~ a} + r 2 ^{q + 1} .

The total number of bits is always equal to R. f {a) - / (0) = -2 x 2 ^q + 2 ^{q + a} + 2 ^{q ~ a} f (a) - / (0) = 2 ^{q ~ a} (2 ^{a + 1 i} 2 ^{a ~ 1} - 1) + 1) (7)

Like α≥l, then / (α) - / (0)> 0, which leads to a contradiction. Thus XR ₁ -> q, fa _j <q such that / (0)> / (α).

Using the same analysis for the following cases:

and

[R; = g + 1 i → ç + 1 + a

{i? _{J =} ç i → qa ⁽⁹⁾ and

we get the same conclusion. The function / is minimal in zero, and the distribution k (2 ^u - \). If q ≠ O, the number of

M = I codes used is U = k. Theorem 2:

The flow R is achievable if and only if R ≤

Demonstration:

As this part is not the object of the present patent application, we give only the principles of the demonstration. Document 5, cited in Appendix 1, provides a more complete demonstration.

Working in the set of real numbers, and using Lagrange multipliers, we show that the maximum flow is:

SR = Jfclog ₂ I ₊ (14)

It is obvious that to attribute bits on each of the k codes leads to a feasible bitrate. But can we transmit more information? The whole problem is then to find the integer n such that the power spectral density constraint is satisfied, and that the following equations are verified:

k

₂ L * / * J + i _ Λ * - ⁽ * + i ⁾ / ₂ l 9t / * | . ^ ₀ u = l _q ui c _on this Duit e ^ ^ _(l ^ -L ^ J _-iJj.

Claims

A method of transmitting a multicarrier and spread spectrum signal, implementing a plurality of spreading codes, characterized in that it comprises a step of assigning a power and / or a rate at each of said spreading codes, based on information representative of the noise and / or information representative of the quality of the link, said allocation step taking into account a target bit rate.

2. Transmission method according to claim 1, characterized in that said allocation step also takes into account an overall power spectral density.

3. Transmission method according to any one of claims 1 and 2, characterized in that said step of allocating a bit rate comprises, for each of said spreading codes, a step of selecting a modulation for at least some subcarriers of said signal.

4. Transmission method according to any one of claims 1 to 3, characterized in that said allocation step comprises the following sub-steps: verification of the feasibility of said target rate R; if said target rate is achievable: o determining said bit rate to be assigned to each of said spreading codes:

If said target rate R is strictly less than twice the length k of the spreading codes, assignment of two bits on each of R / 2 codes;

• otherwise assignment of | _ /? / fcJ bits on each of k - (R - \ _R / kjk) first codes, and \ _R I k J + 1 bits on each of R - \ _R I second codes; with [J the whole part.

5. A transmission method according to claim 4, characterized in that, when said target rate R is greater than or equal to twice the length k of the spreading codes, said allocation step is defined by the equations:

R = (k - (R - 1R / k) k)) xR 1 / k] + (R - 1R / k) k) x (1R / k] + 1); with: R said target rate;

R ₁₁ the rate assigned to said spreading code u; k the length of said spreading code; LJ the whole part.

6. Transmission method according to any one of claims 4 and 5, characterized in that said allocation step also comprises a substep of determining an energy E ₁₁ representative of said power to be assigned to each of said codes. spreading, expressing itself in the form:

E - ^lRu - ^λ E

Σ (2 ^ -D u = 1 with:

E a global energy representative of said overall power spectral density;

R ₁₁ the rate assigned to said spreading code u; U the number of spreading codes.

7. Device for transmitting a multicarrier and spread spectrum signal, implementing a plurality of spreading codes, characterized in that it comprises power allocation means and / or a rate at each of said spreading codes, taking into account a target bit rate, based on information representative of the noise and / or information representative of the quality of the link.

A method of receiving a multicore and spread spectrum signal, implementing a plurality of spreading codes, a power and / or a rate being assigned before transmission to each of said spreading codes as a function of information representative of the noise and / or information representative of the quality of the link and taking into account a target bit rate, characterized in that it comprises a step of demodulating said signal taking account of said power and / or said flow rate attributed to each of said spreading codes

9. Device for receiving a multicarrier and spreading signal spectrum, implementing a plurality of spreading codes, a power and / or a bit rate being allocated before transmission to each of said spreading codes according to information representative of the noise and / or information representative of the quality of the link and taking into account a target rate, characterized in that it comprises means for demodulating said signal taking into account said power and / or said rate attributed to each of said spreading codes.

10. A multicarrier and spread spectrum signal, comprising a plurality of spreading codes, characterized in that each of said spreading codes is associated with a power and / or a bit rate, allocated according to a piece of information. representative of the noise and / or information representative of the quality of the link, and a target bit rate.