WO2009013432A1

WO2009013432A1 - Method for transmitting a multicarrier signal using a multiple access technique by code distribution, and corresponding device, computer program product and signal

Info

Publication number: WO2009013432A1
Application number: PCT/FR2008/051329
Authority: WO
Inventors: Laurent Cariou; Maryline Helard
Original assignee: France Telecom
Priority date: 2007-07-13
Filing date: 2008-07-15
Publication date: 2009-01-29

Abstract

The invention relates to a method for transmitting a multicarrier signal using a multiple access technique by code distribution. According to the invention, such a method comprises the following steps, for at least one user: repetition (32) of at least one data symbol representing a source data signal which is to be transmitted and is associated with the user; and application (33) of a specific dephasing to each data symbol following the repetition, in such a way as to distribute the data symbols over a set of sub-carriers forming said multiple carrier signal, the dephasing taking into account at least one allocation code associated with the user.

Description

A method of transmitting a multicarrier signal implementing a code division multiple access technique, device, computer program product and corresponding signal.

FIELD OF THE DISCLOSURE The field of the invention is that of the transmission and broadcasting of digital information, in particular at a high bit rate, over a limited frequency band.

More specifically, the invention relates to the transmission of a multicarrier signal implementing a code division multiple access technique in a mobile or fixed context.

The technique according to the invention finds particular applications in point-to-multipoint systems, for transmissions from a base station to a set of terminals for example, in downlink or multipoint-to-point, for transmissions of data. A set of terminals to a base station for example, in uplink The invention is particularly suitable for transmissions in mono or multi-cellular networks

2. Prior Art

In the applications where the spectral resources must be shared between several users, still called users, within a transmission band, the OFDM modulation may be associated with CDMA type code division multiple access techniques. «Coded Division

Multiple Access ")

According to this technique, also called MC-CDMA, the data of the different users are spread over a set of subcarriers before applying the OFDM modulation, during an inverse Fouπer transform step. The spreading sequence is thus applied. in the frequency domain, which makes it possible to take advantage of the frequency diversity of the channel

Different users can then occupy the same time-frequency space by distinguishing each user by a spreading code of his own. In other words, this technique allows a set of users to transmit simultaneously in the same frequency band.

More precisely, in relation to FIG. 1, the general structure of an MC-CDMA transmission system is described, implementing block frequency interleaving. For example, consider a multi-user system comprising NU users, at least one data symbol being associated with each user. We denote x. . the j data symbol associated with the user, with

1 </ <NM and 1 ≤ j ≤ Q, where Q is the number of data symbols transmitted by a user in an MC-CDMA symbol. After serial / parallel conversion (not shown in the figure), each data symbol x _; . . undergoes N replicas during a spreading step 1 1], 1 IQ, where N is the length of the spreading codes. Thus the same data symbol is transmitted on N different subcarriers.

If we consider for example the data symbol x _χ} , we apply a "chip" spreading (also called "chip" in French), specific to each user, each replica of the data symbol.

For example, during the spreading step 11 (1 IQ), the chip C _{1 j} of the spreading code associated with the user 1 is applied to the first replica of the data symbol, V _{1 j} , the chip C _{7 ι} is applied to the second replica of the data symbol x,, and the chip c _N , is applied to the replica / v of the data symbol X _{1 j} .

Similarly, the chip c, w of the spreading code associated with the user Nw is applied to the first replica of the data symbol x ^ _j , the chip c _{2 Nι} is applied to the second replica of the data symbol x _N ,, and the chip c _{N v}

£ 1116 is applied to the replica Ar of the data symbol x _{Nu j} .

Thus, at the output of the spreading step 1 1 1 (respectively HQ), N symbols in the frequency domain, hereinafter referred to as "spread symbols", each spreading symbol carrying a replica of each data symbol associated with a spreading chip. For example, the spread symbol X,, door the first replica of the data symbol X ₁ , associated with the chip C _{1 j} , the first replica of the data symbol Λ ^* ₂ J associated with the chip C _{1 2} , and the first replica of the data symbol x _{Nu λ} associated with the chip c _{χ Nude} The spread symbol X _{N χ} carries the replica AT of the data symbol .v _{1 j} associated with the chip C ^ ₁ , the replicate ΛT of the data symbol X _{1 χ} associated with the chip c _{N 2} , and the replicate Tv of the data symbol x ^ _j associated with the chip c _NN .

A distribution step 12, also called "mapping", is then implemented. This step makes it possible to distribute the spread symbols in the frequency domain over the set M of the available subcarriers. Each subcarrier M then carries a symbol X ₁ , X ₂ , or X _M , called "mapped symbol".

The MC-CDMA symbol from the mapping step 12 may be represented in the frequency domain or in the time domain.

Figures 2A and 2B respectively show a representation of the symbol MC-CDMA in the frequency domain and in the time domain.

As illustrated in FIG. 2A, an MC-CDMA symbol is represented in the frequency domain on the available M subcarriers, as well as in the dimension of the N codes. The symbol MC-CDMA is formed of the mapped symbols X ₁ , X ₁ -, • - -, X _M • More precisely, the mapped symbol X _χ represents the symbol transmitted on the subcarrier 1, and the mapped symbol X _M represents the symbol transmitted on the subcarrier M. The mapped symbol X, corresponds to the sum of the spread symbols transmitted on the subcarrier 1 with different spreading codes. Figure 2B shows the symbol MC-CDMA of duration Ts in the time domain. Such an MC-CDMA symbol is formed of M temporal samples 21 1, 212, ..., 21 M, each having a duration Tc, transmitted simultaneously. If we consider the transmission system of Figure 1, each time sample 21 _W , with l ≤ m ≤ M, is composed of a sum of out of phase mapped symbols, denoted xf "', Xγ ^ι , ..., X ^ ¹ .

In general, a time sample may comprise a single data symbol or a sum of several data symbols. It may be noted that when these samples each comprise a single data symbol, the PAPR (Peak to Average Power Ratio) report, characterizing the fluctuations of the instantaneous power of a signal with respect to its average power, is again the fluctuations of the complex envelope of the signal are small and equivalent to that of a single-carrier transmission.

Returning to FIG. 1, the MC-CDMA symbols resulting from the mapping step 12 then undergo an OFDM-type multi-carrier modulation implementing an inverse Fourier transform step 13 (from the frequency domain to the time domain). followed by an insertion step 14 of a guard interval of size Tg, in the time domain.

The transmission system of FIG. 1 then provides a multi-carrier, code division multiple access signal.

This MC-CDMA transmission technique has good performance since it makes it possible to exploit the frequency diversity related to the total band, that is to say to all the sub-carriers available, as illustrated in FIG. 2A. . However, this technique also has drawbacks, particularly related to the use of an OFDM multi-carrier modulation.

In particular, the MC-CDMA technique suffers from strong fluctuations in the envelope of the emitted signal. Indeed, it is recalled that in the MC-CDMA system illustrated in FIG. 1, each temporal sample 2 \ _m of an MC-CDMA symbol is composed of a sum of the out of phase mapped symbols Xf ^m , X ^ '",. .., X ^.

By taking up the temporal representation of an MC-CDMA symbol of FIG. 2B, the first time sample 21 j of length Tc is therefore equal to 2 ^M ^ X • _>

M the second time sample 212 to] TX -, and the time sample ha. 21 M is also equal to] TX ^{φ M}

In other words, since the temporal samples of the symbol MC-CDMA are constituted, as in OFDM, by a sum of the M map symbols transmitted on the M subcarriers, the envelope of the MC-CDMA signal 15 undergoes strong fluctuations, corresponding to at a high PAPR

The MC-CDMA technique also has a relatively high emission numerical complexity, because of the spreading operations (implementing, for example, a Hadamard transform) and OFDM modulation operations (for example implementing a reverse Fouπer transformation). ). Other transmission techniques have been proposed to obtain a multicarrier signal having a PAPR close to that obtained for a single carrier signal, also called single carrier signal.

Unfortunately, these techniques are based on multiple frequency division access techniques. The frequency representation of such a signal shows that each user exploits only the frequency diversity linked to a limited band, since it transmits information only on a reduced number of subcarriers. This technique of transmission of the The prior art is therefore suboptimal, and maximum exploitation of frequency diversity is not possible. 3. Presentation of the invention

The invention proposes a new solution which does not have all of these disadvantages of the prior art, in the form of a method for transmitting a multicarrier signal implementing a multiple access distribution technique. of codes. According to the invention, such a method comprises the following steps, implemented for at least one user repeating at least one data symbol representative of a source data signal to be transmitted, associated with said user; applying a specific phase shift to each of said symbols of data after repetition, so as to distnbute the data symbols out of phase on a set of subcarriers forming said multi-carrier signal, said phase shift taking into account at least one spreading code associated with said user The invention thus proposes a new transmission technique based on code division multiple access.

More specifically, the transmission method according to the invention makes it possible to generate a multicarrier signal having a low PAPR (that is to say a signal whose envelope is substantially constant), which leads to a reduction in the consumption an issuer implementing such a process

To do this, the transmission method according to the invention implements steps of repetition and phase shift of the data symbol or symbols associated with the different users, making it possible to perform code division multiple access. This multiple access allows each user to transmit data symbols on all available subcarriers forming the multicarrier signal. This makes optimal use of the frequency diversity.

Thus, the code division multiple access technique according to the invention offers advantages in terms of performance, since each user can transmit information on all the subcarriers available, and in terms of robustness to the 'interference

In addition, the frequency structure of the multi-carrier signal obtained allows a high robustness to multipath, and reception by a low complexity receiver. In the rest of this document, this new FI-CDMA transmission technique is called. frequency mterleaved code division multiple access ", in French" code division multiple access with interleaving Irequentiel ")

According to a specific characteristic of the invention, the phase shift also takes account of a frequency shift depending on said data symbol. In other words, the phase shift applied to each of the data symbols after repetition changes according to the data symbol considered, and / or to the user with which this symbol is associated. This frequency offset corresponds to a number of sub-carriers. embodiment of the invention, the steps of repetition and application of a phase shift are implemented in the time domain

The multi-carrier signal in the time domain is thus constructed by data repetition and phase shift operations. In particular, the invention according to this embodiment has a reduced number of transmission complexity, close to that obtained for the transmission of data. a mono carrier signal

Indeed, the invention makes it possible to dispense with the implementation of an inverse Fouπer transform step for the OFDM multicarrier modulation, thanks to the repetition step

This construction in the time domain makes it possible to generate a multicarrier signal having a PAPR comparable to that of a single carrier signal The fluctuations of the multicarrier signal according to the invention are therefore negligible, which allows a reduction in the consumption in transmission According to a second embodiment of the invention, the steps of repetition and application of a phase shift are implemented in the frequency domain In this case, the method comprises a step of mathematical transformation of the frequency domain to the time domain of a signal obtained at the output of the step of applying a phase shift For example, the repetition step is implemented in the form of a step of spreading the data

According to a particular characteristic, the transmission method according to the invention comprises a preceding step of cyclic rotation of the data symbols associated with a user, the order of said rotation depending on said user. This rotation makes it possible to perform the multiple access, that is to say to distribute the data symbols to assign them a spreading code.

According to a first variant of the invention, an identical spreading code is assigned to each of the data symbols associated with a user. In this case, a so-called FI-CDMA grouped technique is defined.

In a second variant, a separate spreading code is assigned to each of the data symbols associated with a user.

In this case, a so-called distributed FI-CDMA technique is defined.

In particular, each of the time samples of said discretized multi-carrier signal comprises a sum of at most Q possibly out of phase data symbols, with Q <M, where Q is the maximum number of data symbols associated with a user, among all users, and M corresponds to the number of subcarriers of said set.

In particular, according to an exemplary implementation of the invention, each of the time samples of said discrete carrier signal comprises a single data symbol, possibly out of phase.

The complex envelope of the multicarrier signal transmitted according to this example is therefore constant and equal to 1 (after normalization). The PAPR is therefore minimum. According to a particular aspect of the invention, the transmission method comprises a step of inserting a guard interval. This guard interval can be any, or implemented as a cyclic prefix.

In particular, the insertion of a guard interval allows, in reception, to achieve a simple equalization in the frequency domain. This step of inserting a guard interval is of course optional.

The transmission method according to the invention can in particular be implemented in a cellular network comprising a single cell, or a plurality of cells. Another aspect of the invention relates to a program product computer downloadable from a communication network and / or records on a computer readable medium and / or executable by a processor, comprising program code instructions for implementing the transmission method decπt previously In another mode of embodiment, the invention relates to a device for transmitting a multicarrier signal implementing a code division multiple access technique.

According to the invention, such a device comprises ^: means for repeating at least one data symbol representative of a source data signal to be transmitted, associated with a user, means for applying a specific phase shift to each said data symbols after repetition, so as to separate the out-of-phase data symbols on a set of subcarriers forming said multi-carrier signal, said phase shift taking account of at least one spreading code associated with said user

Such a transmission device is particularly suitable for implementing the transmission method described above. This is for example a base station, or a terminal type radiotelephone, laptop, personal assistant type PDA (in English "Personal Digital Assistant") It is recalled in fact that the invention finds applications for uplink or downlink transmissions

Another aspect of the invention relates to a multi-carrier, code division multiple access signal.

Such a signal is formed of a set of subcarriers each carrying a symbol of out-of-phase data, said corresponding out-of-phase data symbols, for at least one user, having repetitive data symbols to which a specific phase shift is applied, said phase shift being at least one spreading code associated with said user, said data symbols being representative of a source data signal a transmit associated with said user.

Such a signal can in particular be transmitted by the transmission method described above. This signal can of course include the various characteristics relating to the transmission method according to the invention. In particular, according to an exemplary implementation of the invention, this signal has a substantially constant PAPR, and comparable to that of a single-carrier signal

4. List of figures

Other features and advantages of the invention will emerge more clearly on reading the following description of a particular embodiment, given by way of a simple illustrative and nonlimiting example, and the appended drawings, among which: FIG. 1 , described in relation with the prior art, presents a transmission system of the MC-CDMA type according to the prior art; FIGS. 2A and 2B, also presented in relation with the prior art, provide a representation of an MC-CDMA symbol in the frequency and time domains; Figure 3 shows the main steps of the transmission method according to the invention; FIGS. 4A to 4C illustrate a first example of implementation of the technique according to the invention, and the representation of a FI-CDMA symbol obtained in the frequency and time domain; FIGS. 5A to 5C illustrate a second example of implementation of the technique according to the invention; FIGS. 6A to 6C illustrate a third example of implementation of the technique according to the invention; FIGS. 7A to 7C illustrate a fourth example of implementation of the technique according to the invention; Figure 8 shows the performance of the invention compared to the MC-CDMA technique; and Figure 9 shows the structure of a transmission device implementing a transmission technique according to a particular embodiment of the invention.

5. Description of Embodiments of the Invention 5.1 General Principle

The general principle of the invention is based on the construction and transmission of a multicarrier signal implementing a code division multiple access, such that: in the frequency domain, the data associated with each user is distributed over the set of available sub-carriers M forming the multicarrier signal, and in the time domain, a time sample of the discretized multicarrier signal comprises a sum of at most Q data symbols possibly out of phase, with Q <M. transmission technique is noted FI-CDMA.

The main steps of the transmission method according to the invention are illustrated in relation to FIG.

To do this, at least one source data signal 31 to be transmitted is associated with a user. In particular, in the context of a point-to-multipoint transmission, a single source data signal, associated for example with a base station, is considered. On the other hand, in the context of multipoint-to-point transmission, several source data signals each associated with a distinct user are considered. By user, therefore, we mean both a user of the service and a base station (point of attachment). For at least one user, the transmission method according to the invention comprises the following steps: a step 32 of repeating at least one data symbol from the source data signal associated with the user; a step 33 of applying a specific phase shift to each of the data symbols after repetition, so as to distribute the data symbols on a set of subcarriers forming the multicarrier signal, the phase shift taking into account at least one spreading code associated with said user.

This generates a multi-carrier signal 34 implementing a code division multiple access technique.

5.2 Implementation in the time domain

The following is a first embodiment of the invention in which the steps of repetition and application of a phase shift are implemented in the time domain. This generates the multi-carrier signal in the time domain.

The FI-CDMA transmission technique then consists in applying to each data symbol: a repetition of order Q; and a phase shift relative to the order of the spreading code used as well as to a frequency offset / (in number of subcarriers).

A conventional insertion step of a cyclic prefix, or any guard interval, can then be implemented. This step is of course optional.

There are also two variants, depending on whether one assigns an identical spreading code or different spreading codes to the data symbols associated with the same user.

In particular, the term "grouped FI-CDMA", or grouped FI-CDMA, is the technique of assigning an identical spreading code on the Q data symbols transmitted by a user, and "Distπbuted FI-CDMA", or FI Distributed CDMA, the technique of attenuating a different spreading code on each of the Q data symbols transmitted by a user

These two vaπantes allow a code division multiple access in the frequency domain (since the time samples of the discretized multiple carrier signal are distributed over different subcarriers), the distributing the data symbols associated with each user on all the sub-carriers available in the frequency domain, and constructing a FI-CDMA symbol transmitted in the time domain by data repetition and phase shift operations. In addition, according to a particular example of implementation of FI-

Distributed CDMA, a time sample (of duration Tc) of a symbol FI-CDMA (of duration Ts) consists only of a single data symbol, possibly out of phase. The PAPR of the multicarrier signal is therefore substantially constant, and identical to that of a single carrier signal. Moreover, the invention according to this embodiment makes it possible to dispense with the implementation of an inverse Fourier transform step for the OFDM multicarrier modulation, thanks to the repetition step.

The invention according to this first embodiment thus proposes a clever distribution of the data symbols to be transmitted. Four examples of implementation of the invention, in a single-cell, uplink or downlink context, according to the variant FI-CDMA or distributed FI-CDMA, are presented below.

A) FI-CDMA technique grouped in uplink FIG. 4A illustrates a first example of implementation of the invention, for uplink communication of the terminals of the different users to a base station. The data of all users are therefore transmitted from separate attachment points. Consider a system in which: the number of available subcarriers is M = 12; the number of users is Nu = 3; the number of data symbols associated with a user is

<2 = 3; the length of the spreading codes in the frequency domain is yv = 4. In other words, each user can transmit Q data symbols (possibly out of phase) by IF-CDMA symbol (Q = MIN).

For example, the data symbols x 1, x ₂ and x ₃ representative of a source data signal to be transmitted associated with the user i are considered.

In this exemplary implementation, a first repetition step 41 \ takes as input a vector V ₁ consisting of the data symbol X ₁ and NI null symbols: V ₁ = (x _j , 0,0,0). The position in this vector V ₁ of the data symbol x depends on the subscript of the user.

During the repetition step 411, the vector V ₁ is repeated Q times. We thus obtain a vector F ₁

represented by the elements (X _{1 15} X _{j 2} , ..., X ₁ 12) in the tetragonal domain.

Similarly, if we consider a second repetition step taking as input a vector v ₂ = (X ₂₁ O ₅ O ₅ OJ, we obtain at the output of this repetition step a vector F ₂ = LT ₂₅ O ₁ O ₅ O ₅ X _{2 5} O ₁ O ₅ O ₁ X ₂₇ O ₁ O ₅ OJ, represented by the elements (X ₂ ,, I _{2 2} , ..., X _{2 12} J in the frequency domain, and if we consider a third repeating step 413 taking as an input a vector V ₃ = (X _31, Y ₁ Y ₁ Oj ₁ vector of the output is _F3 = (X ₃₁ O ₁ O ₁ O ₁ X ₃₁ O ₁ O ₁ O ₁ X ₃₁ O ₅ O ₅ OJ, represented by the elements (X ₃ ,, J, ₂ , ..., X _{3 12} j in the tetragonal domain.

We thus obtain Q = 3 vectors (V _χ , V ₂ and F ₃ ), each having a length M = 12. The index of the non-zero data symbol in the vectors V ₁ , v ₂ and V ₃ is specific to each user, in order to perform the multiple access. In this example, the subscript / user has been used as a nonzero symbol index.

This arrangement of the data may also be effected by a user-specific cyclical rotation stage performed on the given Q vectors in the repetition stage.

During a next step 42 (respectively 423) of application of a phase shift, each of the elements of the Q vectors generated at the repetition stage is multiplied by a phase shift coefficient φ equal to:

. 2π J "'• / with:

/ = ψ, ..., Q - 1! the frequency offset index of the repeated data symbol (equal in this example to the index of the data symbol); and m = -JO, ..., M - 1> the index of the vector element F ₁ , V ₂ or F ₃ at the output of the repetition step (41 \, 413), depending on the code d spreading associated with the user.

For example, the elements of the vector F ₁ are not out of phase since the frequency offset is equal to 0 for the first data symbol associated with a user (φ = e ¹² = 1). The elements of the vector F ₂ are out of phase by j - ^m φ = e ¹² because the frequency offset is equal to 1 for the second data symbol associated with a user. The elements of the vector F ₃ are out of phase by j-m * 2 φ = e ¹² , because the frequency offset is equal to 2 for the third data symbol associated with a user.

More precisely, the following vectors are obtained after phase shift: Vf = Qc ₁ , 0.0, 0, X ₁ , 0.0, 0, X ₁ , 0.0, 0)

Aπ

J-,

FJ = IX ₂ , 0, 0.0, X ₂ e, 0.0.0, I, e ³ , 0.0, and

Ff = 1 X _3, 0,0, 0, x _^3/3, 0,0,0, x _3/3, 0,0,0.

During a step 43, the three out-of-phase vectors vf, vf and vf of length M are ultimately termed. In the time domain, an IF-CDMA symbol, represented by the time samples X ₁ , is thus obtained. .., X ₁₂ .

FIG. 4B shows a representation in the frequency domain of the FI-CDMA symbol, on the M = 12 subcarriers available, as well as in the dimension of the spreading codes (c). In this variant embodiment FI-CDMA grouped, it is recalled that an identical spreading code is assigned to the Q data symbols transmitted by a user. More precisely, the element X ₁ represents the symbol transmitted on the sub-carrier 1, and the element X ₁₂ represents the symbol transmitted on the subcarrier 12 The elements X _j , X ₄ , X _j and X ^ _Q , in oblique hatching, bear the same symbol possibly out of phase, corresponding to the data symbol Y. The elements X ₂ , A ₅ , X _g and X,, in vertical hatching, bear the same symbol possibly out of phase, corresponding to the data symbol r ₂ The elements X ₃ , X, X _g and X _p , in horizontal hatch, bear the same symbol possibly out of phase, corresponding to the symbol of data x ^

FIG. 4C proposes a representation in the time domain of the symbol FI-CDMA. Such a symbol FI-CDMA is constituted of 12 time samples of duration Tc transmitted simultaneously. These 12 samples are equal to

(lπ Aπ Aπ% π \

J - J - J - J -

X ₁ H- τ ₂ + Λ3, 0, 0, 0, XJ + t2 ^{e 3} + ^x 3 ^{e 3} , 0, 0,0, X ₁ + X ₂ ^ ³ + xfi- ³ , 0,0, 0

Thus, each time sample is composed of at most a sum of

Q phase-shifted data symbols (Q non-zero symbols) It is recalled that according to the prior art, the temporal samples of the symbols MC-CDMA were composed of a sum of M independent symbols (Q <M). The ratio between the two sums is therefore equal to N

Finally, returning to FIG. 4A, it is possible to insert a guard interval (or a cyclic prefix) to the FI-CDMA symbol during a step 44.

In other words, if we consider that the user i has Q source symbols d _,; with / the frequency shift index, and that the spreading code assigned to this user i for each source symbol is the spreading code of order n _t , then it is possible to define data symbols X r _n ( with / = {θ,, ρ - l} and "= - | b,, JV - l}) such that

In particular, with reference to FIG. 4A, a single User, therefore df, = d _j The vectors V ₁ = (VJ ₁ O ₁ O ₅ OJ, = (x ₂ , 0,0,0) and v ₃ = (x ₃ , O, O, θ) are thus constructed.

The mathematical representation of the temporal samples of the symbol FI-CDMA group, denoted x _m for m = | θ,, M - 1 \, is then expressed in the following form π

= 1 J-

V m - TQ τNrf ^m

Nq + n ^~ ZJ ^e ", for 9 = | θ,, Q - \] / = 0

Thus, this example of implementation offers a reduction of the complexity of the transmitter compared to the MC-CDMA technique, while preserving the advantages of the MC-CDMA technique. B) Fl-CDMA technique distributed in the rising channel

FIG. 5A illustrates a second exemplary implementation of the invention, for uplink communication of the terminals of the different users to a base station

Consider again a system in which - the number of available subcarriers is A / - 12, the number of users is Nu = 3, the number of data symbols associated with a user is 2 = 3, the length of spreading codes in the frequency domain is N = 4

In other words, each user can transmit Q data symbols (possibly out of phase) by IF-CDMA symbol (Q = M I N)

The data symbols such Consider A _1, V ₂ representative ^e t ^r i a source data signal issue associated to the user 1 These data symbols and N - Q null symbols form an initial vector v = Ix ,, X ₂ , X-, O j of size N

The initial vector v can undergo a cyclic rotation 51 of an order specific to each user in order to perform the multiple access. One can notably use the index 1 of the user as a rotation order. For example, if we consider the user / = 0, step 51 does not modify the initial vector If we consider the user / - 2, step 51 delivers the vector

).

In this example of implementation, we are interested in the transmission scheme for the user / = 0 in uplink.

The next step of repeating 52 takes as input the vector v, and the repeated Q times is thus obtained a vector F = (X _{1 5} A ₂ _J ^ O ₅ X ₁₅ X ₂₅ X ₃₅ O ₅ XJ X _{2 1} ₅ X ₁₅ OY represented by the elements (X ₁ J ₅ X _{1 2} ,., X _{1 12} J in the frequency domain

A vector V of length M = 12 is thus obtained. During a subsequent step 53 for applying a phase shift, each of the elements of the vector V at the output of the repetition step 52 is multiplied by a phase shift coefficient φ. equal to :

/ - ni f Φ = e ^QN , where: - / = | θ,, Q - 1 j the frequency shift index of the repetitive data symbol (equal in this example to the index of the data symbol); and m = jθ,, M - 1! the index of the vector element V at the output of the repetition step 52

For example, the first element of vector V is not out of phase

₇ ^ 0 * 0 _/ ^ l 1 * 2 (φ = e ^{] 2} = 1) The last element of vector V is out of phase with φ = e ¹²

More precisely, the following vector is obtained after phase shift: f jπ / 2π j5π j 3π / \ 0π ^ = \ x _h x ₂ e ^, x ₃ e ³ , 0, x _h x ₂ e ⁶ , x ₃ , 0, τ ₁ , x ₂ e ² ^ _e ³ , 0

In the time domain, a FI-CDMA symbol is thus obtained

(T _{1 1} X ₂ ,., X _W ) Figure 5 B shows a representation in the frequency domain of the FI-CDMA symbol, on the M == 12 subcarriers available, as well as in the dimension of the spreading codes. (c) In this variant embodiment FI-CDMA distributed, it is recalled that a separate spreading code is assigned to each of the Q data symbols transmitted by a user.

More precisely, the element X ₁ represents the symbol transmitted on the sub-carrier 1, and the element X ₁₂ represents the symbol transmitted on the subcarrier 12. The elements X ₁ , X ₄ , X ₇ and X ₁₀ , in oblique hatching, bear the same symbol possibly out of phase, corresponding to the data symbol X ₁ . The elements X ₂ , X ₅ , X ₈ and X _n , in vertical hatch, bear the same symbol possibly out of phase, corresponding to the data symbol Jt ₂ - The elements X ₃ , X ₆ , X ₉ and X ₁₂ , in horizontal hatchings , bear the same symbol possibly out of phase, corresponding to the data symbol X ₃ .

Figure 5C provides a time-domain representation of the FI-CDMA symbol. Such an IF-CDMA symbol consists of 12 time samples of duration Tc transmitted simultaneously. These 12 samples are equal to f jπ j2π j5π j3π j ^ Oπ ^ x _h x ₂ e ⁶ , x ₃ e ³ , 0, x _b x ₂ e ⁶ , x ₃ , 0, x _h x ₂ e ² , x ₃ e ³ , 0

In particular, in the case where N = Q, each time sample is composed of a single data symbol, possibly out of phase. The complex envelope of the multi-carrier signal obtained is therefore constant, and equal to 1 (after normalization).

Finally, returning to FIG. 5A, it is possible to insert a guard interval (or a cyclic prefix) to the FI-CDMA symbol during a step 54.

In other words, if we consider that the user / has Q source symbols d _,; with / the frequency shift index, and a spreading code of nj distinct order is assigned to each source symbol of this user /, then we can define data symbols x _f (with / = <0 , ..., Q - 1 J- and n = | θ, ..., iV - lj) such that:

The mathematical representation of the temporal samples of the symbol FI-CDMA distributed, noted x _m for m = -JO, ..., M - 1 j-, is then expressed in the following form

^J % N ^fm 1 Xm ^{~ X} Nq + n ^{~ e ~} 7j ^X n, pOUT q = {θ, ..., £> - lj

As illustrated in FIG 5A, is chosen for example to associate a spreading code sequence ri _j = (f + /) modN to the source symbol _uk

Thus, this example of implementation offers a reduction of the complexity of the transmitter compared to the MC-CDMA technique, while preserving the advantages of the MC-CDMA technique. In addition, this technique allows a maximum reduction of the PAPR since each time sample of a FI-CDMA symbol corresponds to a single data symbol possibly out of phase.

C) Downlink FI-CDMA Technique Figure 6A illustrates a third exemplary implementation of the invention, for downlink communication from a base station to the terminals of the different users. The data of all the users is thus transmitted from the same point of attachment, namely the base station We consider again a system in which the number of sub-carriers available is M = 12, the number of users is Nu = 3; the number of data symbols associated with a user is

£? = 3; the length of the spreading codes in the frequency domain is

N = A

In other words, each user can transmit Q data symbols (possibly out of phase) by IF-CDMA symbol (Q = M J N).

There is x ley data symbol associated with the user i, with

1 <z <Nude and 1 <j <Q

Consider for example the input of the system v _π data symbols, v _{1 2} and T _{March 1st} associated with user 1, _{JC-,! ?} X _{1 1} and x _{2 i} associated with the user 2, and X _{3 1} , V ₃₂ and Y _{3 3} associated with the user 3

In this exemplary implementation, a first repetition step 61] takes as input a vector V ₁ consisting of a data symbol of each of the

Naked users and null N-vectors, with Nu ≤ N: V ₁ = (x _j ^ x ^ x ^ O). During the repetition step 61 1, the vector v is repeated Q times. We thus obtain a vector V _x = (X _{1 1 5} X ₂₁₅ X ₃₁₅ O ₁ X ₁₁₅ X ₂₁₅ X _{31 5} O ₅ X _{1 1} , ^r 2 _b ^λ 31 _> °) _> represented by the elements (X _{1 [} , I _{1 2} _,, X _{1 ι2} J in the _frequency domain

In the same way, if we consider a second repetition step taking as input a vector v ₂ = (X ₁₂ , X ₂₂ , X ₃₂ , O) _5, we obtain at the output of this repetition step a vector V ₂ = (x _12, x _22, x _32, 0 ₅ X _12, X _22, x ^ _2, 0, ^τ _l 2 ^'τ _22> ^x ₃ _2> 0), represented by the elements [X _{2 χ,} X _{2 2,.} ., X _{2 12} ) in the frequency domain, and if we consider a third repetition step 613 taking a vector input, v ₃ - (x ₁₃ , x ₂₃ , x ₃₃ , 0), a vector of length is output M ^ ₃ = (X ₁₃₅ X ₂₃₅ X ₃₃₅ O ₅ X ₁₃₅ X ₂₃₅ X ₃₃₅ O ₇ X ₁₃₅ X ₂₃₇ X ₃₃₅ O), represented by the elements ⁽ X ₃ p ^ ₃ 2 v »- ^ _{3 12} J in the frequency domain.

We thus obtain Q = 3 vectors (V _χ , V ₂ and V ₃ ), each having a length M = 12.

The index of the data symbol in the vectors vj, vj and V3 entering the repetition stage is specific to each user, in order to perform the multiple access In this example, the index 1 of the user

During a subsequent step 621 (respectively 623) of application of a phase shift, each element of the Q vectors generated at the repetition stage is multiplied by a phase shift coefficient φ equal to:

2π I nor f

Φ = e ^JQ "\ with:

/ = -JO ,. The frequency shift index of the repeated data symbol (equal in this example to the index of the data symbol), and m - | 0,, M - \ \ the index of the element of the data symbol; vector V _χ , V ₂ or V ^ at the output of the repetition step (61, 613). For example, the elements of the vector V, are not out of phase because the frequency offset is equal to 0 for the first data symbols of each

J-m * 0 user (φ = e ^{i 2} - I) - The elements of the vector V ₂ are out of phase

2π 2π j - m / - m

^ = E 12 _e 12 = φ _{^ since]} _e frequency shift is equal to 1 for the second data symbols associated with each user. The elements of the vector V-, j - m * 2 are out of phase with φ = e ¹² , since the frequency offset is equal to 2 for the third data symbols associated with each user.

More precisely, the following vectors are obtained after phase shift:

°) e ^t

! π) - - - 11 -! £ .. 1

Vf = ^v l 3 ' ^Λ 2 3 ^e ' ³ ^ 3 3 ^{e 3} ^^ 1 3 ^{e 3} > * 2,3 ^{e 3} ^ 3 3 ' ⁰ ' ^Λ l 3 ^{e 3} ^ 2 3 ^^^ ³ > °

During a step 63, the three out-of-phase vectors vf, vf and ff, of length M, are ultimately termed.

In the time domain, a FI-CDMA symbol is thus obtained. Figure 6B shows a frequency-domain representation of the FI-CDMA symbol, the available M-12 subcarriers, and the size of the spreading codes (c). In this variant implementation FI-CDMA grouped, it is recalled that an identical spreading code is assigned to the Q data symbols transmitted by the same user. More precisely, the element X ^ represents the symbol transmitted on the subcarrier 1, and the element

X ₁₂ represents the symbol transmitted on the subcarrier 12. For example, the subcarrier 1 carries the first phase shift data symbol associated with the user 1, noted, vf ,, with the spreading code associated with the user 1, the first out of phase data symbol associated with user 2, denoted by Λ, ,, with the spreading code associated with user 2, and the first out of phase data symbol associated with user 3, noted xf ,, with the spreading code associated with the user 3.

Figure 6C provides a representation in the time domain of the FI-CDMA symbol. Such an IF-CDMA symbol consists of 12 time samples of duration Tc transmitted simultaneously. These 12 samples are defined by the term-term sum of the three phase-shifted vectors Vf, Vf and Vf.

Thus, each time sample is composed of a sum of at most Q out of phase data symbols. It is recalled that according to the prior art, the temporal samples of the symbols MC-CDMA were composed of a sum of M independent symbols. The ratio between the two sums is therefore equal to

N. It can also be noted that the lower the value of Q, the lower the PAPR associated with the multicarrier signal at the output of the transmission system.

Finally, returning to FIG. 6A, it is possible to insert a guard interval (or a cyclic prefix) to the FI-CDMA symbol during a step 64.

In other words, if we consider that the user / has Q source symbols d, with / the frequency shift index, and that the spreading code assigned to this user / for each source symbol is the spreading code of order " _z -, then we can define data symbols x _f (with

/ = {θ, ..., ρ-l} and "= {θ, ..., N - l}) such that: xf, n = ^d f ,, ^{if n} = ⁿ ι The mathematical representation of temporal samples of the symbol

Combined FI-CDMA, denoted x _m for m - ψ, ..., M - \, then expresses itself in the following form:

IQ j-) m xm = ^x + n = Σ N q ^{e QN x} fn 'F ^or <? = {Ô .-. Ô - 1}

If we choose n _t = /, this expression becomes:

Thus, in addition to reducing the complexity of the transmitter compared to the MC-CDMA technique, this implementation example allows a reduction significant of the PAPR since the temporal samples of the IF-CDMA symbols are constituted by a sum of Q independent symbols, with Q <M. D) FI-CDMA technique distributed in a downlink Figure 7A illustrates a fourth example of implementation of the invention for downlink communication from a base station to the terminals of the different users.

Consider again a system in which the number of available subcarriers is M = 12, the number of users is Nu = 3, - the number of data symbols associated with a user is

2 = 3, the length of the spreading codes in the frequency domain is N = 4.

We denote x ley data symbol associated with; user, with

1 <; <Nu and 1 ≤ j ≤ Q

For example, at the input of the system, the data symbols are considered

X _xj , τ _{t 2} and X _{| 3} associated with the user 1, forming with N - Q null symbols a first initial vector v \, X _{2 x} , X _{2 2} ^{and χ} ₂₃ associated with the user 2, forming with

N - Q null symbols a second initial vector vj, and x _{3 x} , X ₃₂ and X _{3 3} associated with the user 3, forming with N - Q null symbols a third initial vector V3

These initial vectors can undergo a cyclic rotation 71 j, 7I3, of an order specific to each user in order to perform the multiple access. It is possible in particular to use the index / of the user as order of rotation.

For example, if we consider the user; = 0, step 71 1 does not modify the first initial vector If we consider the user 1 = 2, step 7I3 delivers the vector v ' ₃ = (r _{3 3} , 0, x ₃ ,, r _{3 2} )

In this implementation example, a first repetition step 72] takes as input the first initial vector V ₁ = Mr ₁ ^ x _{1 2} , ^χ ι _j , ®) During the repetition step 72j, the vector v, Q is repeated. We thus obtain a vector F ₁ -

3> θ), represented by the elements (X ₁ ^, X _{1 2} , ..., X _{j 12} ) in the frequency domain.

In the same way, if we consider a second repetition step taking as input a vector v ' ₂ = (θ, x _2, 1, ^ 2.2 ^ 2 _, 3)' ^we obtain at the output of this repetition step a vector F ₂

\ Represented by the elements ^(X ₂ X ₂ ^ _2, ---, X ₂ 12) ^years ^ ^ ^e frequency domain, and if one considers a third repeating step 723 taking as an input a vector V ^ = (Jr _{March 35} O ₁ X _{3 15} X _{3 2} ), a vector F ₃ 0, X ₃ J ₅ X _{3 2} , X ₃ is obtained at the output _; 3, O, x ₃ 1, X _{3 2} , x _{3 3} , O, x _{3 j} , x _{3 2} ), represented by the elements (X _{3 j} , X _{3 2} , ..., X _{3 12} J in the frequency domain.

We thus obtain Q = 3 vectors (F ₁ , V ₂ and F ₃ ), each having a length M = M.

During a subsequent step 731 (respectively 733) of application of a phase shift, each of the elements of the Q vectors generated in the repetition step is multiplied by a phase shift coefficient φ equal to:

2π I ni. t

Φ = e ^QN \ with f - ψ, ..., Q - \> the frequency shift index of the repeated data symbol (equal in this example to the index of the data symbol); and m = | θ, ..., M - 1> the index of the vector element F ₁ , F ₂ or F ₃ at the output of the repetition step (72, 723).

For example, the first element of the vector F ₁ is not out of phase

2π 2π j - 0 * 0 d - 1 1 * 2

(φ = e ¹² = 1). The last element of the vector F ₃ is out of phase with φ = e ^u s after phase shift: 3 / r 10; r> v- JTΓ ^j -r _! ' ^X h2 ^{e 2} ' ^x l, 3 ^{e 3} O

During a step 74, one-to-one amount the three-phase V ^ vectors F ^e t V $ _2, of length M. In the time domain, thus obtaining an IF-CDMA symbol.

FIG. 7B shows a representation in the frequency domain of the FI-CDMA symbol, on the M = 12 subcarriers available, as well as in the dimension of the spreading codes (c). In this distributed variant embodiment FI-CDMA, it is recalled that a separate spreading code is assigned to each of the Q data symbols transmitted by the same user.

More specifically, the element X ₁ represents the symbol transmitted on the sub-carrier 1, and the element X ^ ₂ represents the symbol transmitted on the subcarrier 12. For example, the subcarrier 1 carries the first symbol of shift data associated with the user 1, noted xf _h with the spreading code associated with the first symbol and the user 1, the first phase shift data symbol associated with the user 2, note X _{2 I} ' ^with I ^e spreading code associated with the first symbol and the user 2, and the first out of phase data symbol associated with the user 3, denoted X _{j 1} , with the spreading code associated with the first symbol and the user 3 Figure 7C provides a time-domain representation of the FI-CDMA symbol. Such an IF-CDMA symbol consists of 12 time samples of duration Tc transmitted simultaneously. These 12 samples are defined by the term-term sum of the three phase-shifted vectors fy, Vj and Vf.

Thus, each time sample is composed of a sum of at most Q data symbols possibly out of phase (non-zero symbols). Recall that according to the prior art, the temporal samples of the symbols MC-CDMA were composed of a sum of M independent symbols. The relationship between two sums is therefore equal to N. It may also be noted that the lower the value of Q, the smaller the PAPR associated with the multicarrier signal at the output of the transmission system.

Finally, returning to FIG. 7A, it is possible to insert a guard interval (or a cyclic prefix) to the FI-CDMA symbol during a step 75.

In other words, if we consider that the user; has Q source symbols d. , and that a spreading code of order nf is assigned to each source symbol d, associated with the user i, then it is possible to define data symbols X _n (with n - ψ, ..., N - l |) such that: xn, ι = ^d j, < ^{S1 n} = ⁿ )

As illustrated in FIG. 7A, for example, it is chosen to associate a spread code of order ", - = (/ + j) modτV with the source symbol d.

The mathematical representation of the temporal samples of the distorted FD-CDMA symbol, denoted x _m for m - ψ, ..., M - 1 j, is then expressed in the following form:

. ,. 2π

^ Fn -1 J ι ^m x _m = λyv _{o + n} ^e ^~ Σ ^ ^{N x} _{n, ι} ^'with f _{n, ι} ^ ^e frequency shift associated to the data symbol X _n, and q = | θ, Q - 1 J-

Thus, in addition to reducing the complexity of the transmitter compared to the MC-CDMA technique, this implementation example allows a significant reduction of the PAPR since the time samples of the IF-CDMA symbols consist of a sum of Q symbols. non-zero independent, with Q <M.

It may be noted that in these four implementation examples, a user can transmit more than Q data symbols (possibly out of phase) per IF-CDMA symbol. Under these conditions, the maximum number of users Nu is reduced.

It can also be noted that these four implementation examples are unchanged in a multi-cellular context

In particular, with a frequency reuse coefficient of 1, the FI-CDMA technique according to the invention takes advantage, without additional processing at the reception, a gain of spreading to fight against intercellular interference.

In a second embodiment of the invention, the steps of repetition and application of a phase shift are implemented in the frequency domain. In this case, the transmission method according to the invention comprises a step of mathematical transformation of the frequency domain to the time domain of the signal obtained at the output of the step of applying a phase shift. For example, this mathematical transformation is a transform of

Fourier inverse.

5.3 Performance curves

Hereinafter, in relation with FIG. 8, the performances in terms of PAPR of the FI-CDMA technique according to the invention (curve 81) are compared with the solutions of the prior art of the MC-CDMA type (curve 82). ).

For each technique, the complementary distribution function of the

PAPR of a transmitted FI-CDMA or MC-CDMA symbol, denoted CCDF, is evaluated. Its calculation, for a PAPR value set at PAPR O, is the probability that the PAPR calculated on each FI-CDMA or MC-CDMA symbol is greater than PAPR O:

CCDF (PAPR O) = Pr (PAPR> PAPRJ))

The main parameters used for the uplink simulations are:

- size of the FFT: M = 1024; - size of the guard interval: 256 temporal samples;

- length of the spreading codes: N = 32;

- spreading codes used for MC-CDMA technique: Walsh-Hadamard;

number of data symbols transmitted per OFDM symbol: Q = 32; - number of users: Nu = 1;

- 16QAM modulation;

- number of OFDM symbols per simulation: 100,000.

FIG. 8 shows the CCDF of the FI-CDMA techniques distπbuee according to the invention and MC-CDMA in uplink mode with 16QAM modulation.

It can be seen from this figure that the results obtained with the distorted FI-CDMA technique are equivalent to those of a single-carrier technique, and therefore correspond to a maximum reduction of the PAPR.

For example, the gain obtained with the FI-CDMA technique distnbuée compared to the technique MC-CDMA is 21 dB for a CCDF of 10 ^" , and to 19

Thus, the technique according to the invention has many advantages in these different embodiments, such as the maximum reduction of the fluctuations of the envelope of the multicarrier signal, which can be identical to that of a single carrier signal; significantly reducing the complexity of the transmitter without increasing the complexity of the receiver compared to a conventional OFDM modulation, because of the suppression of the spreading operations (implemented in the form of a transform of

Hadamard of order N for example) and of OFDM modulation (M-order inverse Fouπer transform), the multipath robustness thanks to a spectral representation identical to that of a multicarrier modulation; the possibility, for each user, of transmitting information on all the sub-carriers available in order to optimally exploit the frequency diversity, the code division multiple access in the frequency domain, in a multicellular context with a frequency reuse factor of 1, the IFDMA technique makes it possible to benefit from a gain spreading to fight intercellular interference. 5.4 Structure of the transmission device

Finally, in connection with FIG. 9, the simplified structure of a transmission device implementing a transmission technique according to one of the embodiments described above is presented. This is for example a communication terminal or a base station.

Such a device comprises a memory 91 consisting of a buffer memory, a processing unit 92, equipped for example with a microprocessor μP, and driven by the computer program 93, implementing the transmission method according to the invention. .

At initialization, the code instructions of the computer program 93 are for example loaded into a RAM memory before being executed by the processor of the processing unit 92. The processing unit 92 receives as input at least a data symbol X _{1 j} associated with a user. The microprocessor of the processing unit 92 implements the steps of the transmission method described above, according to the instructions of the computer program 93, to innovatively distribute the data symbols. For this purpose, the transmission device comprises, in addition to the buffer memory 91, means for repeating the data symbols and means for applying a specific phase shift to each of the data symbols after repetition. These means are controlled by the microprocessor of the processing unit 92.

The processing unit 92 outputs a multicarrier signal, the data symbols being distributed over the set of subcarriers forming the multicarrier signal.

Claims

A method of transmitting a multicarrier signal implementing a code division multiple access technique, characterized in that it comprises the following steps, for at least one user repetition (32) of at least one data symbol representative of a source data signal to be transmitted, associated with said user, application (33) of a specific phase shift to each of said data symbols after repetition, said phase difference taking account of at least one associated spreading code user auditing, so as to separate the out-of-phase data symbols on a set of subcarrier Ms forming said multi-carrier signal, and so that, in the time domain, each of the samples of said discrete carrier signal includes a sum of at most Q out of phase data symbols, where Q <M, where Q is the maximum number of data symbols associated with a user, among all the utili sateurs.

2. Transmission method according to claim 1, characterized in that said phase shift also takes account of a frequency shift dependent on said data symbol.

3. Transmission process according to any one of claims 1 and 2, characterized in that said steps of repetition (32) and application of a phase shift (33) are implemented in the time domain

4. Transmission method according to any one of claims 1 and 2, characterized in that said steps of repetition (32) and application of a phase shift (33) are implemented in the frequency domain, and in that said process comprises a step of mathematical transformation of the frequency domain to the time domain of a signal obtained at the output of said step of applying a phase shift

5. Transmission method according to any one of claims 1 to 4, characterized in that it comprises a preliminary step of cyclic rotation of the data symbols associated with a user, the order of said rotation depending on said user

Transmission method according to any one of claims 1 to 5, characterized in that an identical spreading code is assigned to each of the data symbols associated with a user.

7. Transmission process according to any one of claims 1 to 5, characterized in that a separate spreading code is assigned to each of the data symbols associated with a user.

8. Transmission method according to any one of claims 1 to 7, characterized in that each of the time samples of said discrete carrier signal comprises a single phase-shifted data symbol.

9. Computer program product downloadable from a communication network and / or records on a computer-readable medium and / or executable by a processor, characterized in that it comprises program code instructions for the implementation of the transmission method according to at least one of claims 1 to 8.

10. Device for transmitting a multicarrier signal implementing a code division multiple access technique, characterized in that it comprises: - means for repetition (32) of at least one representative data symbol a source data signal to be transmitted, associated with a user; means (33) for applying a specific phase shift to each of said data symbols after repetition, said phase shift taking account of at least one spreading code associated with said user, so as to distribute the data symbols out of phase on a a set of subcarriers forming said multi-carrier signal and such that, in the time domain, each of the samples of said discrete carrier signal comprises a sum of at most Q out of phase data symbols, with Q <M, where Q is the maximum number of symbols of data associated with a user, among all users.

11. Transmission device according to claim 10, characterized in that it forms, or is integrated in, at least one of the equipment belonging to the group comprising: - a base station; a terminal.

12. Multi-carrier, code division multiple access signal, characterized in that a set of subcarriers each carrying a phase-shifted data symbol is formed in the frequency domain, and in the time domain, a set of samples each carrying a sum of at most Q out of phase data symbols, with Q <M, where Q is the maximum number of data symbols associated with a user, among all users, and M is the number of sub-carriers of said set, said corresponding phase-shifted data symbols, for at least one user, to repeated data symbols to which a specific phase shift is applied, said phase shift taking account of at least one spreading code associated with said user said data symbols being representative of a source data signal to be transmitted associated with said user.