EP1695473A1

EP1695473A1 - Method and device for increasing the capacity of non-spread transmission systems

Info

Publication number: EP1695473A1
Application number: EP04804594A
Authority: EP
Inventors: Marc THALES Intellectual Property CHENU-TOURNIER
Original assignee: Thales SA
Current assignee: Thales SA
Priority date: 2003-11-28
Filing date: 2004-11-26
Publication date: 2006-08-30
Also published as: US20070071135A1; JP2007513545A; WO2005053212A1; CA2547555A1; FR2863122B1; FR2863122A1

Abstract

A method for increasing the capacity of signal transmission systems comprising NT users and a monobloc receiver receiving a mixture of signals from NT users, consisting of the following steps: determination of qualitative information Info(Qs) relating to estimated symbols for each NT user; transmission of said information Info(Qs) to a processing unit which receives information a priori and which is adapted in order to generate quality information Info(Qbs) on the bits making up said symbols; transmission of the Info(Qbs) to a decoding stage in order to obtain qualitative information on the coded bits and Info(Qbu) on the useful bits.

Description

METHOD AND DEVICE FOR INCREASING THE CAPACITY OF NON-SPREAD TRANSMISSION SYSTEMS

The invention relates in particular to a method making it possible to increase the capacity of the transmission systems by multiplying the number of simultaneous transmitters in the same frequency band and making it possible to separate the users in particular through the use of iterative steps. Methods are known from the prior art allowing the simultaneous transmission of different users. They are generally based on the use of spreading codes, such as CDMA (Anglo-Saxon abbreviation of Code Division Multiple Access), MCCDMA (Anglo-Saxon abbreviation of Multicarrier Code-Division-Multiple-Access) and / or the use of multiple antenna receivers. The method according to the invention is notably based on a new approach which exploits the independence of the bit streams (signals from the different transmitters), the channel coding and the difference of the majority of the propagation channels.

The invention relates to a method for increasing the capacity of signal transmission systems comprising Nj users, a monobloc receiver receiving the mixture of signals from the N _τ users. It is characterized in that it comprises at least the following steps: a) determining qualitative information Info (Qs) of the symbols estimated for each of the N _τ users, b) transmitting this information lnfo (Qs) to a processing block receiving a priori information and suitable for generating quality information on the bits constituting the lnfo symbols (Qbs), c) transmit the lnfo (Qbs) to a decoding step to obtain qualitative information lnfo (Qbs) on the coded bits and lnfo (Qbu) on the useful bits. The method according to the invention allows in particular: • to increase the speed of transmission systems using existing standards for user stations by only modifying the access point. • to simply separate the different bit streams by exchanging information between the demodulation block and the decoding block. • increase the capacity of transmission systems by multiplying the number of transmitters without using multi-antenna receivers and without using spectrum spreading techniques, as part of normal operation. Other advantages and characteristics of the invention will appear better on reading the following description of a detailed example, given by way of illustration and in no way limiting, appended to the figures which represent: • Figure 1 the overall diagram of the process according to the invention, and FIG. 2 the detailed generic diagram of the steps of the method according to the invention.

Figure 1 shows schematically the different steps of the method according to the invention used in a communication or transmission system comprising several users or NT transmitters, and a receiver consisting for example of a single sensor R. The different transmitters transmit the symbols simultaneously in the same frequency band, for example. Since communications are generally disturbed by a propagation channel, channel coding is conventionally used. The method uses, for example, this coding to perform the demodulation. FIG. 2 represents the generic diagram of an example of a single-sensor receptor. It comprises a module 1 making it possible to receive the mixture of the signals emitted by the N _τ users or transmitters, to separate the different users and to provide qualitative information, lnfo (Qs), symbols estimated for each of the NT users (for example a probability of having received such a symbol). Module 1 can be a detector in the sense of maximum a posteriori (MAP) which provides a probability of the symbols transmitted for the different NT transmitters by relying on a priori information. The information on the estimated symbols lnfo (Qs) is then transmitted to a processing block which will deduce therefrom quality information on the bits constituting the symbols lnfo (Qbs). This information lnfo (Qbs) is then transmitted to the decoding block 4i (a deinterlacing procedure can be applied before) which, in turn, will produce qualitative information lnfo (Qbs) on the coded bits and lnfo (Qbu) on the useful bits. The information on the coded bits lnfo (Qbs) can be reused in order to re-estimate information on the symbols as described above. The information on the useful bits is deduced from the information on the bits coded for example by the decoding procedure. Prior processing of the information transmitted to the different blocks may prove necessary for the proper functioning of the process. For example, in the example described below, the information previously used to estimate new qualitative information on a bit is subtracted in order to bring only real new information to the block receiving it. These steps are repeated, either a fixed number of times, or until a criterion is verified (for example, the qualitative information no longer changes). The operation of the method is described below as an example for the user Ni. Information on the probability of symbols emitted P (a ¹ Nu | yi), Info

(Qs), is transmitted to a device 2ι (or de-mapping) having in particular the function of providing information on the probability of the bits transmitted L _D (c _k ¹ ) by the user N1 Info (Qbs). This information is for example sent in a 3 ^ deinterlacer then to a BCJR algorithm (coding block 4i) in order to obtain the probability of the coded bits L _c (c _k ¹ ) (qualitative information on the coded bits lnfo (Qbs) and the useful bits, Info This last information (L _c (c _k ¹ )) is subtracted from the first probability information L _D (cκ ¹ ) on the bits (quality information on the bits constituting the symbols Info (Qs)) before pass through the deinterlacer. It is also sent to an interleaver 5ι then to a device 6 ₁ having a mapping function, before being reinjected into device 1 which uses this information lnfo (Qs) at the step of obtaining the probability of the symbols emitted. The mapping, de-mapping, interleaver and deinterleaver devices are devices known to those skilled in the art which are not detailed in the present description. invention, the following example is don born in the case of OFDM transmitters (Anglo-Saxon abbreviation of orthogonal frequency division multiplexing) synchronized in frequency. For this so-called multi-carrier or parallel waveform, the various symbols are transmitted simultaneously on orthogonal subcarriers. . In this exemplary embodiment, the different transmitters use a convolutional code as in the Hiperlan / 2 or IEEE802.11a standard. The receiver conventionally performs a discrete Fourier transform (TFD) over a determined time interval to estimate the symbols transmitted. In the case of multiple transmissions synchronized in frequencies and sufficiently synchronized in time to avoid inter-symbol interference, the signal received by the receiver after the Fourier Transform is given by: (1) with • y the received signal represented by a vector N Jxl with N the number of subcarriers, • a is the vector of dimension containing the sy ^J mboles transmitted by the N transmitters. The first N elements are the symbols transmitted on the first subcarrier. • F 1 = F 1 ®IN is the matrix performing the DFT on transmission with IN the identity matrix of dimension N and the operator <8> the product of Kronecker.

• I PC = ï PC ®IN is the matrix of dimension NT (N lï_ + N DFT) xN TN W. which performs the insertion of the cyclic prefix (specific to OFDM) • H is the matrix of the samples representing the propagation channel, of dimension NT \ [NW + N DFT) \ + NH) xN ri N Λ + N CP with N the maximum length of the propagation channels. • I_ = Ï_ ®I is the matrix which carries out synchronization and removes CP CP N _τ the cyclic prefix • F is the matrix which carries out TFD at the level of the receiver • b is the vector of dimension N χl containing the samples of SP noise considered in this example as temporally blank. The matrix K defined below is a circulating block and as such it can be written as:

with G a diagonal block matrix and F and F TFD matrices As I_HI is circulating block, the received signal can be written as: y = Ga + b (3)

with G a diagonal block matrix with blocks of size lχN

So for the subcarrier i the vector observation y. can be written y = G a + b like: • ^'''<^'< ^* (4)

where G i contains the elements of the frequency response of the channel.

Here, as we only use a single receiver, ^G is a vector of size ^{lx N} r.

Thus the observation ^y <is scalar and is written: y _t = ^' ∑fha _l + b _l (5) i = l

In this case, the detector within the meaning of the MAP provides the following probabilities: (qualitative information of the estimated symbols - probability of the symbols emitted for the different emitters)

where σ ² is the variance of the noise and A a ^k is defined by: A a ^k contains the vectors of symbols a which have the symbol a at position k. These probabilities are then used to calculate the probability of the bits making up the symbols:

with A ⁺ the set of symbols where bit c is 1 and A ^" the set of symbols where bit c is 0.

These quantities are then used to calculate: L _D (c) = L (c) -L _c (c) (9) which is supplied to the decoding block. In the figure, equation (9) is represented by the indices L _D (c _k ') = L (c) - Lc (c'). The term L _c (c) (L _c (c _k ') in fig. 2) corresponds to the information, a priori, from the previous decoding. At the first iteration, L _c (c) = 0. These values L _D (c) (L _D (c) in fig. 2) are the inputs of the flexible decoder which, in the example, is a type algorithm BCJR, described for example in the document by L. Bahl, J. Cocke, F. Jelinek, and J. Raviv, entitled "Optimal decoding of linear codes for minimizing symbol error rate," IEEE Trans. Inform. Theory, pp. 284 -287, Mar. 1974. This block is not described in more detail, this decoder provides both a probability of the useful bits (before coding) and a probability of the coded bits which constitute the symbols.

The method is used for example for BPSK (Anglo-Saxon abbreviation for Bit Phase Shift Keying) or QPSK (Anglo-Saxon abbreviation for Quadrature Phase Shift Keying) modulations.

Claims

1 - Method for increasing the capacity of signal transmission systems comprising N _τ users, a monobloc receiver receiving the mixture of signals from N _τ users, characterized in that it comprises at least the following steps: a) determining qualitative information Info (Qs) of the symbols estimated for each of the N _τ users, b) transmit this information lnfo (Qs) to a processing block receiving a priori information and adapted to generate quality information, lnfo (Qbs), on the bits constituting the symbols, c) transmitting the info (Qbs) to a decoding step to obtain qualitative information on the coded bits and Info (Qbu) on the useful bits.

2 - Method according to claim 1 characterized in that step a) is carried out using a MAP detector (Maximum a Posteriori).

3 - Method according to claim 1 characterized in that steps a) to c) are repeated until the qualitative information is substantially constant. 4 - Use of the method according to one of the preceding claims for transmitters using one of the following modulations: BPSK, QPSK, OFDM.