EP1118164A1

EP1118164A1 - Method for increasing the capacity of a orthogonal waveform multiple access network, and associated units

Info

Publication number: EP1118164A1
Application number: EP00949653A
Authority: EP
Inventors: Hikmet Sari
Original assignee: Alcatel CIT SA; Alcatel SA
Current assignee: Alcatel CIT SA; Alcatel Lucent SAS
Priority date: 1999-07-08
Filing date: 2000-07-06
Publication date: 2001-07-25
Also published as: CN1317172A; WO2001005051A1; JP2003504936A; AU6294400A

Abstract

The invention concerns a method for increasing by M, M being any integer not less than 1, the number N, N being an integer not less than 2, communications set up between a central unit (15) and remote units (201-20N; 211-21M) of an orthogonal waveform multiple access network. The first N communications are multiplexed by orthogonal waveform multiple access in accordance with a first technique from the set of multiple access techniques consisting of TDMA, OCDMA, OFDMA and MC-OCDMA. The M communications additional to said N established communications are multiplexed by orthogonal waveform multiple access in accordance with a second technique from the set of multiple access techniques consisting of TDMA, OCDMA, OFDMA and MC-OCDMA, said second access technique being different from the first access technique.

Description

Method for increasing the capacity of an orthogonal waveform multiple access network, and associated units

The present invention relates generally to a method for increasing the capacity of an orthogonal waveform multiple access network, as well as units, central and / or remote, for the implementation of this method. An orthogonal waveform multiple access network (OWMA) for Orthogonal Waveform Multiple Access in Anglo-Saxon literature) shares an available frequency band, or spectrum, given between N user communications (N is the ratio between the total spectrum required for N communications is the spectrum required for a single communication) and is characterized in that when the total number of communications is less than or equal to N, the interference between any two communications from any users is substantially zero. On the other hand, this interference becomes prohibitive for at least one of the N communications established if an additional communication is added. There is therefore a brutal degradation of the quality for at least one of the communications beyond a threshold defining the maximum authorized number of communications.

This type of network is to be opposed to a network for example of multiple access type by distribution of PN waveform code (pseudo-noise in Anglo-Saxon literature) in which the quality of each communication using a particular PN sequence is degrades significantly in proportion to the number of communications using other PN sequences. There is therefore a gradual deterioration in the quality of the communications, from a number of communications equal to two, which is a function of the total number of communications sharing the given frequency band.

TDMA (Time Division Multiple Access), OFDMA (Orthogonal Frequency Division Multiple Access), OCDMA (Orthogonal Code Division Multiple Access) and MC-OCDMA (MultiCarrier Orthogonal Code Division Multiple Access) techniques are multiple access techniques in the form of orthogonal wave. In the following description, the terms “fashion” or “technique” will be used interchangeably to designate a given access technique. For example, for the TDMA technique, the N communications share the available spectrum by arranging each and in turn during a respective time window of the entire available spectrum. In the OCDMA technique, the N communications share the entire available spectrum by disposing each of these spectrum simultaneously, and by providing a code per communication which distinguishes each communication from the other communications.

The main drawback of each of these techniques, called multiple access with orthogonal waveform, is that they are defined by a limit beyond which no more new users can establish a communication because the N resources are used . It is therefore not possible to add a call without causing the loss of at least one call in progress. In a radiocommunication system, the existing solutions remedy this problem, for example by reducing the size of the cell covered by a base station, thereby reducing the number of potential users of the resources available for this cell. This solution then requires oversizing the system.

Another solution described in a patent application, not published to date, in the name of the present applicant consists in using Walsh-Hadamard type sequences for the first N communications in an OCDMA network (Orthogonal Code Division Multiple Access) and PN sequences for the following M communications as soon as the N Walsh-Hadamard type sequences are already used by N established communications.

The disadvantage of such a solution is that with regard to each of the M communications established using a PN code or sequence, there are two types of interference on this communication: the interference produced by the N communications at Walsh-Hadamard sequences and the interference produced by (M-1) other established communications using other PN sequences.

Signal processing to remove these two types of interference can then require a relatively long time for processing and convergence of the algorithm.

The present invention therefore aims to provide a method and associated transmitters and receivers for a multiple access network with orthogonal waveform, for which the problem of sudden degradation of already established communications is avoided when establishing a additional communication and for which the problem of the signal processing convergence time mentioned above is eliminated. According to the invention, schematically and simplified, the addition of a communication only leads to a “graceful” type of degradation of other communications (“graceful degradation” in Anglo-Saxon literature).

To this end, a method for increasing M, M being any integer greater than or equal to 1, the number N, N being an integer greater than or equal to 2, of communications established between a central unit and distant units of an orthogonal waveform multiple access network, according to which said N first communications are multiplexed by orthogonal waveform multiple access according to a first technique from the set of multiple access techniques consisting of access techniques

TDMA, OCDMA, OFDMA and MC-OCDMA, is characterized according to the invention in that said M communications additional to said N established communications are multiplexed by multiple access with orthogonal waveform according to a second multiple access technique among the set multiple access techniques consisting of TDMA, OCDMA, OFDMA and MC-

OCDMA, this so-called second technique being different from the first technique.

Advantageously, according to the invention, - a first interference is generated which is generated by all the communications according to the first access technique on each communication according to the second access technique, - said first interference subtracted from all communications using the first access technique is subtracted from each communication using the second access technique,

a first interference generated by all the communications according to the second access technique is synthesized on each communication according to the first access technique, and

- We subtract this first interference generated by all communications according to the second access technique from each communication according to the first access technique.

In order to improve the quality of the received signal, the method described above can be repeated up to a rank P. Thus, for any rank P greater than or equal to 2,

(1) - in a first sub-step, we synthesize a P ^th respective interference that generates on each communication according to the second technique all the communications according to the first technique, using decisions on symbol values of these communications according to the first technique which are taken during a second sub-step of previous rank (P-1), then we subtract this P ' ^ee interference from the respective communication according to the second technique, and we decide the values of symbol of each communication according to the second technique after subtracting this P ^th interference from said each communication according to the second technique;

(2) - in a second sub-step, we synthesize a P ' ^th respective interference that generates on each communication according to the first technique all the communications according to the second technique, using decisions on symbol values of these communications according to the second technique which are taken during the first sub-step (1) above, then this P ^{ιe e} interference is subtracted from the respective communication according to the first technique, and the symbol values of each communication according to the first technique after subtraction of this P ^th interference from said each communication according to the first technique. A telecommunications network is thus obtained in which are established:

- N first communications between first remote units and a central unit (15), these N first communications being multiplexed by multiple access with orthogonal waveform according to a first technique among the set of multiple access techniques consisting of the techniques d 'access

TDMA, OCDMA, OFDMA and MC-OCDMA, and

- at least one additional communication to said N communications is established by multiple access in orthogonal waveform between at least a second remote unit and said central unit (15) according to a second multiple access technique among all the techniques of multiple access consisting of TDMA, OCDMA, OFDMA and MC-OCDMA techniques, this so-called second multiple access technique being different from the first multiple access technique. Typically, a unit of such a network comprises: a first synthesizer of a first interference which generates all the communications according to the first access technique on each communication according to the second access technique,

a subtractor for subtracting said first interference generated by all the communications according to the first access technique from each communication according to the second access technique,

another synthesizer of a first interference generated by all the communications according to the second access technique on each communication according to the first access technique, and

another subtractor for subtracting this first interference generated by all the communications according to the second access technique from each communication according to the first access technique.

By generalizing, P being any integer greater than or equal to 2, the unit includes, for any rank P,: - for a first sub-step, a P ^th synthesizer to synthesize a P ^th respective interference that generates on every second mode communication every first mode communication, using decisions on symbol values of these communications according to the first mode which are taken during a second substep of previous rank (P-1), a subtractor to subtract this P ^{, th} interference from the respective communication of the second mode, and decision means for deciding the symbol values of each communication according to the second mode after subtracting this P ^th interference from said each communication of the second mode;

- for a second sub-step, a P ^th synthesizer for synthesizing a p ^th respective interference that generates on each communication of the first mode all the communications of the second mode, using decisions on symbol values of these communications according to the second mode which are taken during the first sub-step above, a subtractor for subtracting this P ' ^th interference from the respective communication of the first mode, and a decision means for deciding the symbol values of each communication according to the first mode after subtracting this P ^{'è e} interference of said each communication of the first embodiment.

The invention also provides a unit, central or remote, of a telecommunications network, which comprises a first transmitter / receiver operating according to one of the techniques from among the set of multiple access techniques consisting of the TDMA, OCDMA techniques, OFDMA and MC- OCDMA and a second transmitter / receiver using all or part of the same frequency band as the first technique and operating according to another of the techniques among the set of multiple access techniques consisting of the TDMA, OCDMA, OFDMA techniques and MC-OCDMA. When it is mentioned that the transmitter / receiver uses all or part of the same frequency band as that of the first technique, this means that the communications established respectively according to the first and second technique use at least partly the same spectral band. The terms “in part” result from the consideration that the N communications can occupy in practice a wider spectrum than the M communications, the latter being less than N. It is nevertheless possible, for example for the mode OCDMA, spread these M communications in a voluntarily wider spectrum which coincides with the spectrum of N communications. In all cases, and according to the invention, it is important that the spectrum occupied by the M additional communications is at most limited by the upper and lower bounds of the spectrum of the N communications, so that the telecommunications operator is not forced to use a spectral band other than that allocated to N communications.

A central unit of the type proposed above belonging to a telecommunications network and communicating with a plurality of remote units, according to the invention comprises a resource allocator for allocating a resource or according to a first technique among all the techniques access consisting of TDMA, OCDMA, OFDMA and MC-OCDMA access techniques, or alternatively according to a second technique among all of the access techniques consisting of TDMA, OCDMA, OFDMA and MC-OCDMA access techniques using all or part of the same frequency band as the first technique, depending on the availability of N resources of the first access technique.

Other characteristics and advantages of the present invention will appear more clearly on reading the description which follows, with reference to the corresponding appended drawings in which:

- Figure 1 is a schematic representation of a multiple access network;

- Figures 2 and 3 each show a representation of power distribution as a function of time for two respective orthogonal waveform access techniques;

- Figure 4 is a block diagram of a reception circuit of two sets of communications carried respectively by two types of orthogonal distinct waveforms between them, according to a first variant of the invention; - Figure 5 is a block diagram of a circuit for receiving two sets of communications carried respectively by two types of orthogonal waveforms distinct from each other, according to a second variant of the invention; and - Figure 6 shows in the form of a block diagram a remote unit and a central unit according to the invention communicating with each other.

With reference to FIG. 1, the present invention applies to a multiple access network with orthogonal waveform. In such a network, (N + M) communications are established between (M + N) remote units 20 20 _N and 21 _r 21 _M respectively and a central unit 15. For example and for information only, the units 20 _r 20 _N and 21 21 _M are fixed terminations or mobile terminals, and the central unit 15 is a base station of a radiocommunication network. The network can also be a wired network, or a satellite network. The network is said to have multiple access because several communications between the remote units and the central unit are multiplexed in a given spectral band. The multiplexing can be temporal according to the TDMA technique or else frequency according to the OFDMA technique or even by code according to the OCDMA technique. According to the invention, the first N respective communications with the units

20 20 _N and the central unit 15 are established according to a first technique from among the set of multiple access techniques consisting of the TDMA, OCDMA, OFDMA and MC-OCDMA techniques. When no resource (time interval in TDMA mode, code in OCDMA mode, etc.) using this first access technique is available to establish a communication additional to these N already established communications, a new communication is established using a different access technique (which nevertheless uses the same spectrum or a portion of it) from that available for the first access technique. Thus, for the following M units 20 20 _M , M being any integer greater than or equal to 1 and substantially less than N for the reasons given below, M respective additional communications can be established according to a second multiple access technique among all the multiple access techniques consisting of the TDMA, OCDMA, OFDMA and MC-OCDMA techniques, this second multiple access technique being different from the first technique. These M additional communications do not then induce interference capable of causing the loss of the N established communications.

Figure 2 shows the power distributions for two communications carried respectively by two distinct orthogonal waveforms between them which are the TDMA (Time Division Multiple Access) and

OCDMA (Orthogonal Code Division Multiple Access) according to an exemplary embodiment. In this figure 2, T _c represents the duration of a “chip”, constituting the elementary coding symbol used by the OCDMA technique, T _s = NT _c is the duration of an elementary symbol of the established communication, typically a bit , or binary element. It is recalled that according to the OCDMA technique, in transmission, the symbols of each communication are multiplied by one of a plurality of OCDMA codes orthogonal to one another. Each of these codes is for example a so-called Walsh-Hadamard sequence, of the type presented in American patent US-A-5103459, included by reference in the present application. This sequence is defined by a series of binary elements having a rate much higher than the rate of the symbols of the communication. This then results in spreading each communication over a much wider spectral band than that theoretically required for the transmission of the communication symbols in baseband. On reception, a reproduction of the coding sequence used in transmission is then used to separate each communication which is spread in transmission over a spectral band corresponding to N times the band required by each communication, N corresponding to the bit rate ratio between the bit rate of the communication symbols and the coding sequence rate.

From this Figure 2, can be deduced, at least theoretically, on a scale standardized at time "chip", the theoretical interference that generates each communication using a TDMA mode on a communication using an OCDMA mode, and conversely the theoretical interference generated by each communication using an OCDMA mode on a communication using a TDMA mode, when these OCDMA and TDMA communications are carried simultaneously in the same band frequencies.

It is clear that due to the use of a so-called orthogonal waveform, TDMA or OCDMA, the interference that communications generate between them is zero for each given mode, TDMA or OCDMA. We can also write for simplification that each communication in OCDMA mode is defined by:

- the continuous occupation over time (T _s ) of a frequency band equal to N times the frequency band occupied by the communication in baseband, N being the processing gain (“processing gain” in English literature) Saxon) of the OCDMA code, and each communication in TDMA mode is defined by:

- the only occasional occupation during a periodic time window (T _c ) of a frequency band equal to N times the frequency band occupied by the communication in baseband (it is assumed that the TDMA frame is suitable for carrying up to to N communications for the simplification of this demonstration), and that the average ratio between the transmission power of a communication in mode

TDMA; which exists only during a time interval, and the transmission power of a communication in OCDMA mode which is transmitted continuously is given by N, the energy transmitted by symbol being identical in OCDMA mode and in TDMA mode.

For N communications established in OCDMA mode using length codes N bits per symbol, and M communications established in TDMA mode, the interference between these two types of communications is as follows:

INTERFERENCE OF M TDMA COMMUNICATIONS ON 1 OCDMA COMMUNICATION The interference caused by a single communication in TDMA mode on a communication in OCDMA mode is theoretically given by 1 / N, N being the spreading factor, or "processing gain", between the spectrum of the communication in OCDMA mode and the baseband communication spectrum. Each communication in TDMA mode generates interference effect only on 1 / Nth of the time of the communication in OCDMA mode. For M communications in TDMA mode, the interference generated on a communication in OCDMA mode is therefore M / N. It is therefore this M / N ratio which limits the total authorized number of the following units 20 ₁ -20 _M which can establish additional communications in TDMA mode, M therefore having to be very significantly less than N to avoid communications in OCDMA mode are lost due to excessive interference.

INTERFERENCE OF N OCDMA COMMUNICATIONS ON 1 TDMA COMMUNICATION

The interference caused by a communication in OCDMA mode on a communication in TDMA mode is also theoretically given by 1 / N, N being the spreading factor, or "processing gain", between the spectrum of the communication in OCDMA mode and the communication spectrum in baseband (TDMA mode). Each communication in OCDMA mode, spread over a spectral band equal to N times the spectral band in baseband, in fact generates interference in the time interval occupied by the communication in TDMA mode with a power having a ratio 1 / N compared to the power of communication in TDMA mode because the power of communication in TDMA mode is precisely concentrated on a single time interval of the TDMA frame. For N communications in OCDMA mode, the interference generated on a communication in TDMA mode is therefore total and equal to N / N = 1. Likewise, in Figure 3, the power distributions for two communications shown by two distinct orthogonal waveforms are shown, which are OFDMA and MC-OCDMA techniques. The above considerations concerning the interference of M TDMA communications on an OCDMA communication, and the interference of N OCDMA communications on a TDMA communication lead, according to the invention, to a receiver diagram which first of all subtracts the interference induced by the communications. in OCDMA mode on each communication in TDMA mode, because, as we have just seen, each communication in TDMA mode being defined by a signal to noise ratio equal to 1, no processing makes it possible to extract the interference from the useful signal in communication in TDMA mode.

This receiver diagram is now given, according to a first embodiment, with reference to Figure 4. A receiver of the type shown in Figure 4 is for example included in the central unit 15 and each of the remote units 20., - 20 _N and 21, -21 _M to authorize bidirectional communications between central unit 15 and remote units 20 ₁ -20 _N and 21 _r 21 _M. For such two-way communications, uplink communications, on the one hand, and downlink communications, on the other hand, typically use two separate frequency bands. For rigorous terminology, it should be noted that if the terms TDMA (Time Division Multiple Access), OFDMA (Orthogonal Frequency Division Multiple Access), OCDMA

(Orthogonal Code Division Multiple Access) and MC-OCDMA (MultiCarrier Orthogonal Code Division Multiple Access) are appropriate for uplink, the corresponding appropriate terms for downlink are TDM (Time Division Multiplex), OFDM (Orthogonal Frequency Division Multiplex) respectively. , OCDM (Orthogonal Code Division Multiplex) and

MC-CDM (MultiCarrier Code Division Multiplex), the downlink channels do not strictly speaking give rise to multiple access. The receiver comprises, for the N communication channels in OCDMA mode, N correlators 1, -1 _N , N first threshold detectors 2, -2-, a first interference synthesis circuit 3, N subtractors 4, -4 „ , N second threshold detectors 5, -5 _N , a second interference synthesis circuit 6. It also comprises, for the M channels of communications in TDMA mode, M first subtractors 7, -7 _M , M first threshold detectors 8, -8 _M , an interference synthesis circuit 9, M second subtractors 10, -10 _M , and M second threshold detectors 11, -11 _M. The term “correlator” designating each of the elements 1, −1 _N is used in the present description not only to designate a circuit ensuring the correlation function proper (ie a circuit detecting the coincidence between a received signal and a local OCDMA code sequence by detecting a peak), but also the auxiliary circuits to demodulate the received OCDMA communication signal accordingly (ie the circuits possibly providing a multi-path signal recovery function and ensuring a demodulation function of the received signal, as a function of the local OCDMA code sequence detected as appropriate). Such circuits are known from the state of the art for CDMA technology. Each of the N correlators demodulates in OCDMA mode a respective communication received to produce a communication signal in OCDMA demodulated baseband which is applied to a respective one of the N first detectors with threshold 2, -2 _N. Each threshold detector 2, at 2 _N is actually a circuit which, receiving the demodulated baseband communication signal OCDMA, produces symbols from a limited set of authorized symbols, the symbols produced being the symbols retained by the detector at threshold 2, at 2 _N as having the greatest probability of being the symbols actually transmitted by the remote transmitter. Each series of symbols produced by a respective one of the threshold detectors 2, at 2 _N is applied to a respective input of the first interference synthesis circuit 3. The latter implements an interference evaluation algorithm which generates the set of signals

OCDMA on each TDMA communication channel. For example, the algorithm is as follows:

Let _{l (l = 1ι 2,, N)} . N symbols respectively transmitted by N remote units in OCDMA mode at the same given instant, and w, _k ^th "chip" of the Walsh-Hadamard sequence associated with the remote unit F ^me OCDMA mode. The theoretical interference of the N remote units on the communication from the remote JTE _m ^ιeme _a TDMA mode for the signal portion coinciding with the / V ^th chip is written:

The synthesis of the interference of N remote units on the communication from the m ^{th e} remote unit in TDMA mode is obtained by replacing in the expression (1) the terms a, the determination, denoted â "actually taken by the respective threshold detectors 2, -2 _N.

Regarding the interference of M communications in TDMA mode on each of the N communications in OCDMA mode, it is of the form:

ι n _n - = "* N" + - f - ^wrr n, i (2)

the symbols a _{N +} , _{(l =} , _{2 M)} , designating M symbols respectively transmitted by M remote units in TDMA mode at the same given instant. The asterisk designates the conjugate complex and disappears from expression if the OCDMA spreading sequences used are real.

As before, the synthesis of the interference of the M remote units on the communication of the ^nth remote unit in OCDMA mode is obtained by replacing in expression (2) the terms a _{N + l} by the decisions, denoted by _{N +1} , actually taken by the respective threshold detectors 8, -8 _M.

In order to obtain a communication signal of a given mode having an ever lower level of interference, it is possible, according to the invention, to P iterations of the interference synthesis described above, as shown below.

It should be noted that the interference evaluation algorithm can be implemented in partially or totally hardware and / or software form.

The M subtractors 7, -7 _M each subtract, from a respective one of the M communication signals received in TDMA mode, the interference synthesized by the circuit 3. The outputs of the M subtractors 7, -7 _M are applied to inputs respective of the M threshold detectors 8, -8 _M , which each produce a series of symbols of a respective TDMA mode communication, in which the interference from the communications in OCDMA mode is significantly reduced. The respective outputs of the M threshold detectors 8, -8 _M are applied to inputs of the interference synthesis circuit 9, which produces on each of its N respective outputs, an interference signal to be deduced from the respective one of the OCDMA demodulated baseband communication signals, by means of one of the N subtractors 4, -4 _N. The outputs of the N subtractors 4, -4 _N are applied to the respective inputs of the threshold detectors 5, -5 _N. Each threshold detector produces a series of symbols of a communication in mode

Respective OCDMA, in which interference from TDMA mode communications is significantly reduced.

As a result of the above processing, at the output of the threshold detectors 5, - 5 _N , on the one hand, and of the threshold detectors 8, -8 _M , on the other hand, are obtained series of communication symbols in mode OCDMA, on the one hand, and in TDMA mode, on the other hand, for which the interference level is significantly reduced.

It is possible to repeat, at least for one of the modes of communications, the processing operations described above. This is achieved in the diagram of Figure 4 by means of the interference synthesis circuit 6, subtractors 10, -10 _M and threshold detectors 1 1, -1 1 _M. The interference synthesis circuit 6 receives the respective outputs of the N threshold detectors 5, -5 _N. Circuit 6 implements an interference assessment algorithm generated by all of the OCDMA signals on each TDMA communication channel. The M subtractors 10, - 10 _M each subtract the interference synthesized by the circuit 6 from a respective one of the M communication signals received in TDMA mode, in which a first interference subtraction processing has already been carried out. The outputs of the M subtractors 10, -10 _M are applied to respective inputs of the M threshold detectors 11, -11 _M , which each produce a series of symbols of a communication in respective TDMA mode, in which the interference from the communications in OCDMA mode is still significantly reduced.

The following steps are thus implemented:

- We synthesize a first interference generated by all the communications of a first mode, here OCDMA, on each communication of a second mode, here TDMA. This first interference is then subtracted from this second mode TDMA communication. Then, we synthesize a first interference generated by all second mode communications, namely TDMA, on each OCDMA first mode communication. This interference is then subtracted from each communication in OCDMA mode.

In order to increase the quality of the communication received, iteratively for a rank P = 2, the following sub-steps are implemented:

(1) - in a first substep, one synthesises (6) a 2 ^-th respective interference caused on each communication according to the second technical all communications according to the first technique, using decisions (5, -5 _N ) on symbol values of these communications according to the first technique which are taken during a second substep of rank preceding 1, then we subtract (10, -10 _N ) this 2 ^nd interference from the respective communication according to the first technique, and we decide (1 1, -1 1 _M ) of the symbol values of each communication according to the second technique after subtraction (10, -10 _N ) of this 2 ^nd interference from said each communication according to the second technique;

(2) - in a second sub-step, we synthesize (synthesizer not normally represented downstream of the threshold detectors (11, -11 _N )) a 2 ^nd respective interference that generates on each communication according to the first technique all communications according to the second technique, using decisions (11, -11 _N ) on symbol values of these communications according to the second technique which are taken during the first substep (1) above, then the this 2 ^nd interference is subtracted from the respective communication according to the first technique, and the symbol values of each communication according to the first technique are decided after subtracting this 2 ^nd interference from said each communication according to the first technique; and so on for interference of any rank.

Thus, for any rank P strictly greater than 2,

(1) - in a first sub-step, a respective P ^ιe interference is synthesized that generates on each communication according to the second technique all the communications according to the first technique, using decisions on symbol values of these communications according to the first technique which are taken during a second sub-step of previous rank (P-1), then this P ^ιeme interference is subtracted from the respective communication according to the first technique, and the values of symbol of each communication according to the second technique after subtracting this P ^th interference from said each communication according to the second technique;

(2) - in a second sub-step, we synthesize a P ' ^th respective interference that generates on each communication according to the first technique all the communications according to the second technique, using decisions on symbol values of these communications according to the second technique which are taken during the first sub-step (1) above, then this P ' ^eme interference is subtracted from the respective communication according to the first technique, and the symbol values of each communication according to the first technique after subtracting this P ^th interference from said each communication according to the first technique.

According to the invention, the N first respective communications between the central unit and the units 20, -20 _N are established according to a first technique

OCDMA. As soon as no resource using this first technique is available to establish an additional communication to these N already established communications, a new communication is established using a different access technique, TDMA according to Figure 4, which nevertheless uses the same spectrum as that of the first technique, or a portion of it. It is therefore necessary according to the invention to implement a resource allocation algorithm, for example by the central unit 15. For this, a resource, for example a two-way communication channel using the first OCDMA technique is, according to an embodiment, dedicated to signaling request and resource allocation according to solutions known from the state of the art.

Although in the above description, the preferred embodiment refers to the combination of the OCDMA and TDMA techniques, in practice, the first N communications are multiplexed by multiple access to orthogonal waveform according to any first technique among the set of multiple access techniques consisting of the TDMA, OCDMA, OFDMA and MC-OCDMA techniques and the M communications additional to these N established communications are multiplexed by multiple access with orthogonal waveform according to a second multiple access technique among the 'set of multiple access techniques consisting of TDMA, OCDMA, OFDMA and MC-OCDMA techniques, this second access technique being different from the first multiple access technique. Thus, for example, with reference to FIG. 5, the first access technique uses the TDMA mode and the second access technique uses the technique

OCDMA. In this case, the receiver includes M correlators 1, -1 _M for M OCDMA communications. As shown in Figure 6, each remote unit 20 _r 20 _N and 2l 2i _M comprises a first transmitter / receiver 33 operating in a first mode and a second transmitter / receiver 34 operating in a second mode. A management unit 35 in the remote unit is responsible, inter alia, for producing resource allocation request requests in order to allow the remote unit to access the telecommunication system. It produces for this a resource allocation request which is for example transmitted to the transmitter / receiver 33, to be transmitted by this transmitter in a particular channel of a bidirectional communication channel using the first OCDMA technique, this channel or channel then being dedicated to signaling request and resource allocation according to solutions known from the state of the art. This request is received by the management unit 32 in the central unit 15 through the transmitter / receiver 30, which, depending on the availability of the N resources of the first access technique, returns an allocation message resource, which allocated resource uses either the OCDMA technique or the TDMA technique. The central unit 15 also includes this first transmitter / receiver 30 operating in a first mode and a second transmitter / receiver 31 operating in a second mode. The management unit 32 includes a resource availability table for each of the two modes, for example OCDMA and TDMA. The unit 32 uses this table, in order to return, depending on the availability of the N resources of the first access technique, a resource allocation message, which allocated resource uses either the OCDMA technique or the TDMA technique.

The reader will note that, on the one hand in the central unit 15, between the first transmitter / receiver 30 operating in the first mode and the second transmitter / receiver 31 operating in the second mode and, on the other hand in the unit distant, between the first transmitter / receiver 33 operating in a first mode and a second transmitter / receiver 34 operating in a second mode, links as described with reference to Figures 4 or 5 are established between receivers. It should be noted that the receivers of Figures 4 or 5, although being diagrammed each by circuits, for example the interference synthesis circuits 3 and / or 6 and / or 9, operating "in series", only one and even a physical circuit could carry out the operations implemented in these interference circuits 3 and / or

6 and or 9. The same applies to the subtraction functions by the various subtractors, threshold detectors. In this case, these circuits operate "in parallel". Likewise, all these circuits can be produced in totally or partially software form. Finally, a person skilled in the art will agree that although the invention has been described for a preferred embodiment relating to the combined OCDMA / TDMA modes, it can be modified, by simple application of knowledge of the state of the art, to cover any combined modes using two modes from the set of TDMA, OCDMA, OFDMA and MC-OCDMA modes, the two modes being different from each other.

Claims

CLAIMS i - Method for increasing by M, M being any integer greater than or equal to 1, the number N, N being an integer greater than or equal to 2, of communications established between a central unit (15) and remote units ( 20,-20N; 21,-21M) of an orthogonal waveform multiple access network, according to which said N first communications are multiplexed by orthogonal waveform multiple access according to a first technique among all of the multiple access techniques consisting of TDMA, OCDMA, OFDMA and MC-OCDMA access techniques, characterized in that said M additional communications to said N established communications are multiplexed by multiple access with orthogonal waveform according to a second technique of multiple access among the set of multiple access techniques consisting of TDMA, OCDMA, OFDMA and MC-OCDMA techniques, this said second multiple access technique being different from the first multiple access technique and using all or part of the frequency band used by the first technique. 2 - Method according to claim 1, characterized in that: - we synthesize (3) a first interference that generates all communications according to the first access technique on each communication according to the second access technique, - l 'we subtract (7,-7M) said first interference generated by all the communications according to the first access technique from each communication according to the second access technique, - we synthesize (9) a first interference which generates all the communications according to the second access technique on each communication according to the first access technique, and - we subtract (4,-4N) this first interference generated by all the communications according to the second access technique from each communication according to the first access technique. 3 - Method according to claim 2, characterized in that, iteratively, for any rank P greater than or equal to 2,

(1) - in a first sub-step, we synthesize a P ^lth respective interference that generates on each communication according to the second technique all the communications according to the first technique, using decisions on symbol values of these communications according to the first technique which are taken during a second sub-step of previous rank (P- 1), then we subtract this P' ^th interference from the respective communication according to the first technique, and we decide on the values of symbol of each communication according to the second technique after subtracting this ^P'th interference from said each communication according to the second technique;

(2) - in a second sub-step, we synthesize a P ^ιth respective interference that generates on each communication according to the first technique all the communications according to the second technique, using decisions on symbol values of these communications according to the second technique which are taken during the first sub-step (1) above, then we subtract this P ^{iè e} interference from the respective communication according to the first technique, and we decide the symbol values of each communication according to the first technique after subtracting this ^P'th interference from said each communication according to the first technique.

4 - Telecommunications network comprising at least one central unit (15) and a plurality of remote units (20,-20 _N and 21,-21 _M ), in which when N first communications are established respectively between N first remote units ( 20,-20 _N ) and the central unit (15), said N first communications being multiplexed by orthogonal waveform multiple access according to a first technique with N resources among the set of multiple access techniques consisting of the techniques TDMA, OCDMA, OFDMA and MC-OCDMA access, at least one additional communication to said N communications is established by the remote unit by orthogonal waveform multiple access between at least one second remote unit (21, -21 _M ) and said central unit (15) according to a second multiple access technique among the set of multiple access techniques consisting of TDMA techniques, OCDMA, OFDMA and MC-OCDMA, this so-called second multiple access technique being different from the first multiple access technique.

5 - Unit (15; 20,-20 _N ; 21,-21 _M ) of a telecommunications network, characterized in that it comprises a first transmitter/receiver (30, 33) operating according to one of the techniques among all multiple access techniques consisting of TDMA, OCDMA, OFDMA and MC-OCDMA techniques and a second transmitter/receiver (31, 34) using all or part of the same frequency band as the first technique and operating according to a another technique among the set of multiple access techniques consisting of TDMA, OCDMA, OFDMA and MC-OCDMA techniques.

6 - Central unit (15) of a telecommunications network which conforms to claim 5 and which communicates with a plurality of remote units (20, - 20 _N ; 21, - 21 _M ), characterized in that it comprises a resource allocator (32) for allocating a resource either according to a first technique among the set of access techniques consisting of TDMA, OCDMA, OFDMA and MC-OCDMA access techniques, or according to a second technique among the set of access techniques consisting of TDMA, OCDMA, OFDMA and MC-OCDMA access techniques using all or part of the same frequency band as that of the first technique, depending on the availability of N resources of the first access technique.

7 - Unit of a telecommunications network according to claim 5, characterized in that it comprises:

- a first synthesizer (3) of a first interference that generates all communications according to the first access technique on each communication according to the second access technique,

- a subtractor (7.-7 _M ) for subtracting said first interference generated by all communications according to the first access technique from each communication according to the second access technique,

- another synthesizer (9) of a first interference that generates all communications according to the second access technique on each communication according to the first access technique, and - another subtractor (4.-4 _N ) to subtract this first interference generated by all communications according to the second access technique from each communication according to the first access technique.

8_- Unit according to claim 7, characterized in that it comprises, P being any integer greater than or equal to 2,

- for a first sub-step, a P ^ιth synthesizer for synthesizing a P ^ιth respective interference that generates on each communication according to the second technique all the communications according to the second technique, using decisions on symbol values of these communications according to the second technique which are taken during a second sub-step of previous rank (P-1), a subtractor for subtracting this P' ^th interference from the respective communication of the second technique, and a decision means for deciding the values of symbol of each communication according to the second technique after subtraction of this P ^the interference of said each communication of the second technique;

- for a second sub-step, a ^P'th synthesizer for synthesizing a ^pth respective interference that generates on each communication according to the first technique all the communications according to the second technique, using decisions on symbol values of these communications according to the second technique which are taken during the first sub-step above, a subtractor for subtracting this P ^ιth interference from the respective communication of the first technique, and a decision means for deciding the symbol values of each communication according to the first technique after subtracting this ^P'th interference from said each communication according to the first technique.