FR2861927A1 - Evaluation of radio telephone transmission channel includes production of set of coefficients at receiving end, transmitted back to transmitter to generate channel profile - Google Patents

Abstract

The evaluation of a propagation channel is carried out at the receiving end of the transmission. This evaluation is projected on to a function base in one or two dimensions of frequency and time dimension, in order to produce a set of projection coefficients which can be relayed to the transmitting station. The procedure for evaluation of at least one propagation channel, between a receiver and a transmitter includes evaluation of the channel at the receiving end of the transmission. This evaluation is projected on to a function base with at least one frequency or time dimension, in order to produce a set of projection coefficients. Data representative of this set of projection coefficients is transmitted to the transmitter. This data comprises at least one index referring to a table of possible coefficient values, stored by the transmitter and the receiver. The transmitter is then able to reconstruct a profile for the transmission channel, as it is seen from the receiving end of the channel. The function base may be two dimensional in the dimensions of time and frequency.

Description

Method for transmitting an estimate of at least one channel of

  propagation of a receiver to a corresponding transmitter, transmitter and receiver.

  FIELD OF THE DISCLOSURE The field of the invention is that of radiocommunications. More specifically, the invention relates to a technique for raising, from a receiver to a transmitter, information representative of an estimation of a transmission channel, as well as various operations in the transmitter, this information.

  The invention applies more particularly in the context of a mobile radio system, between a base station and one or more user terminals. It applies in particular, but not exclusively, in the context of an OFDM downlink (in English "Orthogonal Frequency Division Multiplexing" for "orthogonal frequency division multiplexing").

  2. Solutions of the Prior Art There are several configurations in which it is advantageous for the transmitter to know an estimate of the transmission channel made by a receiver. Indeed, the transmitter can thus adapt the data that it transmits to the receiver according to the characteristics of the channel.

  There is currently no efficient technique in multicarrier systems that allows the base station to know precisely the state of the channel seen by the terminals. One approach could be that the receiver transmits to the transmitter the complete estimate of the transmission channel that it has made, in the form for example of all the channel coefficients in the time / frequency plane. In the case where the channel varies rapidly in time and / or frequency, it would be necessary for such a transmission to be performed frequently and regularly.

  In some Universal Mobile Telecommunication Standard (UMTS) systems, information regarding channel quality (reception power estimates, signal-to-noise ratio estimation, etc.) is however reported. to the base station.

  According to other techniques, the base station may have access to information concerning the pre-processing to be performed before the data is sent to the receiver.

  3. Disadvantages of the Prior Art A disadvantage of the present methods is that one goes back to the base station only an indication of the quality of the channel, which is insufficient, or restrictive, especially for the OFDM type systems ( in English "Orthogonal Frequency Division Multiplexing").

  If it were planned to transmit a complete channel estimation from the receiver to the transmitter, this would be costly in terms of resources, particularly bandwidth. Indeed, the set of channel coefficients in the time / frequency plane represents a quantity of important information.

  Such a cost is all the more important as the channel varies, and it is therefore necessary to transmit a new channel estimate at frequent and regular intervals.

  However, the need for communication resources, and in particular bandwidth, is becoming more pressing, and current radio systems must meet the bandwidth requirements of an ever increasing number of users.

  4. OBJECTIVES OF THE INVENTION The object of the invention is notably to overcome these disadvantages of the prior art.

  More precisely, an objective of the invention is to provide a technique that makes it possible to transmit information relating to channel estimation from a receiver to a transmitter, while being little resource-intensive, and especially in bandwidth.

  Another objective of the invention is to propose such a technique, which is simple and inexpensive to implement.

  It is another object of the invention to provide such a technique which is particularly well suited to systems implementing multi-carrier modulation of the OFDM type.

  Another object of the invention is to propose such a technique that makes it possible to improve the total cellular flow rate of a cellular radiocommunications system compared to the techniques of the prior art. More precisely, an object of the invention is to provide such a technique that makes it possible to simultaneously satisfy network access requests from a larger number of users.

  Another object of the invention is to provide such a technique which makes it possible to improve the performance of a radio link in the context of a system using several transmission antennas, and in particular which maximizes the signal-to-noise ratio at reception. for all terminals served.

  Yet another object of the invention is to propose such a technique which advantageously exploits the signal structures provided for by the UMTS standard, in the case of a radiotelephone system operating according to this standard.

  5. ESSENTIAL CHARACTERISTICS OF THE INVENTION These objectives, as well as others which will appear subsequently, are achieved by means of a method of transmitting an estimate of at least one propagation channel of a receiver. to an issuer.

  According to the invention, said receiver implements the following steps: estimation of said transmission channel; projecting said estimate on a function basis to at least one frequency or time dimension, delivering a set of projection coefficients; transmitting data representative of said set of projection coefficients to said transmitter, so that said transmitter can reconstruct a profile of said transmission channel as viewed by said receiver.

  Thus, the invention is based on an entirely new and inventive approach to the transmission, from a receiver to a transmitter, of an estimate of the propagation channel. Indeed, it is not necessary to carry out an expensive transmission in terms of bandwidth of the complete estimate of the channel realized by the receiver, in the form of all the channel coefficients in the time / frequency plane.

  On the contrary, after estimating the channel at the positions of the pilot symbols, according to a technique which will be described in more detail later in this document, the receiver proceeds to a projection of this estimate on the basis of functions, also known to the transmitter. This base of functions can be, for example, a base of functions with a frequency dimension, with a temporal dimension, or a base of functions with two dimensions time / frequency.

  It then transmits to the transmitter only data representative of the set of corresponding projection coefficients, which is much less expensive in time and bandwidth.

  The transmitter can, from these data and the base of functions that it knows, regenerate the channel profile seen and estimated mobile side (or more generally receiver side).

  Preferably, said base of functions is a base of two-dimensional functions time / frequency.

  The representative data includes at least one index referring to a table of possible coefficient values stored by said transmitter and receiver.

  The table of values stored jointly by the transmitter and the receiver then lists a set of projection coefficients associated with a set of predetermined channel profiles, to which the index transmitted by the receiver refers.

  Advantageously, such a method comprises a step of comparison between the projection coefficients and sets of coefficients stored in said table, so as to identify the set of coefficients closest to said set of projection coefficients.

  Thus, after estimating the channel and making the projection, the receiver determines, in the table of values that it stores, the projection coefficients that come closest to the projection coefficients that it has actually calculated. It then sends the issuer the index that corresponds in the table to these closest coefficients.

  Preferably, said base of functions is a base of flattened spheroidal functions (DPSS), which, unlike other bases, constitutes the best representation of the signal in its bandwidth. It can also be any other basis of two-dimensional functions time / frequency, such as for example the basis of the orthogonal frequencies obtained by FFT (for "Fast Fourier Transfom", "Fast Fourier Transform") or the base of orthogonal polynomials.

  Advantageously, said receiver is a terminal and said transmitter is a base station in a radiotelephone system.

  Preferably, said radiotelephone system implements OFDM modulation.

  Advantageously, said data representative of said projection are transmitted using the bits of the Channel Quality Indicator (CQI) on the HS-DPCCH (High Speed Dedicated Physical Control CHANNEL channel). "in French" high speed dedicated physical control channel ") HSDPA link link (in English" High Speed Data Packet Access ", in French" high speed data packet access ").

  According to the current specifications of the UMTS-HSDPA system, each mobile has 20 bits every TTI (for "Transmission Time Interval", ie every 2 ms.

  In an advantageous embodiment of the invention, the receiver estimates at least two distinct transmission channels, corresponding to as many transmit antennas separate from said transmitter.

  Such an embodiment is in the UMTS standard Closed Loop Diversity Scheme to improve the performance of a radio link over a time-varying multipath propagation channel. , in the case where several antennas are used in transmission on the side of the base station.

  Preferably, the signal received by the receiver being a multicarrier signal, comprising a plurality of carrier frequencies, said transmission channels are estimated for each of said carrier frequencies, or for subsets of said carrier frequencies.

  Indeed, the channel may have different characteristics depending on the frequencies used, and may in particular have fading at certain frequencies.

  According to an advantageous application of the invention, said base station implements a step of allocating at least one communication frequency with a terminal, as a function of said one or more channel estimates, so that the allocated frequencies correspond , on average, to areas in which the corresponding channel estimate has characteristics in accordance with at least one predetermined criterion.

  Thus, the base station takes into account one or more channel estimates when allocating the communication frequencies, both to the terminals already served by the network and to the new terminals requesting access.

  It could also be envisaged that the base station would implement such a frequency allocation technique, regardless of how it became aware of the channel estimate. In other words, such a frequency allocation technique could be implemented by the base station even if the receiving terminal has not communicated to it the channel estimate by way of return (for example in the case where the base station would have made the channel estimate by itself even).

  When a terminal wishes to establish a new communication within the network, the base station is no longer satisfied with determining whether there is at least one free communication frequency: it also takes into account one or more estimate (s) channel of which it is aware to choose whether to accept the request of the incoming terminal, and determine the frequency band that must be allocated. As will be seen in more detail in the remainder of this document, such a technique therefore makes it possible to increase both the total bit rate of each cell and the total number of users that the base station can satisfy.

  In addition, taking into account the channel estimates during the allocation of communication frequencies makes it possible to improve the overall quality of the communications, and avoids the unwanted communication impairments induced by the random frequency hopping technique (referred to above). after).

  Advantageously, said allocation step selects, for a given terminal, one or more available frequencies and for which the channel estimate or estimates have a maximum level and / or greater than a predetermined threshold.

  Such a level may for example be expressed in terms of signal-to-noise ratio, or take into account fading phenomena affecting the channel. The base station thus selects, among the available frequencies that it has identified, those that will establish the best quality of communication, and therefore best meet the expectations of the user.

  Preferably, said allocation step implements a step of frequency hopping, said frequency hopping taking into account said channel estimates so as to allocate frequencies to each communication so as to limit, as far as possible, the effect fading of the channel on these communications.

  Instead of performing a random frequency jump, the invention proposes to determine in advance whether the new frequency band that the base station wishes to allocate to the terminal is affected or not fading. For example, the base station can, from the channel estimate, identify that the new frequency band it intended to allocate to the terminal to improve its performance is actually more strongly affected by fading, and choose to maintain the initial frequency band, or search for another available higher quality bandwidth.

  According to an advantageous characteristic of the invention, said allocation step implements one of the operations belonging to the group comprising: - acceptance or refusal of a new communication, a communication being refused if it does not exist at least a frequency band, formed of at least one frequency, available for which the channel estimate (s) has (s) a level greater than a predetermined threshold; - waiting for at least one calling terminal; determining a quality of service of at least one service requested by a terminal; - allocating, to a new communication, a band of at least one frequency, previously allocated to a communication terminal for which said channel estimate has a level below said predetermined threshold.

  Thus, in the case where the number of requests for access to the network is greater than the available resources, the base station can defer the transmission to one or more receiving terminals in favor of a new incoming terminal, if the transmission channel of the first is very degraded. This amounts to prioritizing, among other parameters, the quality of the downstream transmission channel, both for users already served by the network and for new entrants.

  More specifically, the base station can choose to accept a request for communication of a new terminal if the corresponding channel estimate is of good quality, in order to be able to interchange the frequency bands allocated to this new terminal on the one hand. , and a terminal whose propagation channel is very degraded on the other hand. After inverting the frequencies allocated to each of these two terminals, the base station can indeed better meet the demand of the mobile terminal degraded channel. In addition, the total number of users is increased.

  The base station can also take into account the service to which a terminal wishes to access (voice communication, data transmission, etc.) to determine the level of quality required, and therefore the frequency band that it is possible for it to allocate, according to the associated channel estimate.

  Preferably, said base station implements at least two transmitting antennas, and such a method comprises a weighting step in which the respective weight associated with each of said antennas for a given communication is a function of the corresponding channel estimates.

  The operation of the radio system considered is then said diversity mode, which improves the performance of radio links in the context of time-varying multipath paths. From the channel estimates, the base station knows the state of reception of the signals from the two antennas, and can apply a phase shift, possibly accompanied by a gain, to the two antennas, in order to compensate for the difference between the two antennas. received signals, due to the characteristics of each of the propagation channels associated with the antennas.

  According to an advantageous variant of the invention, the signal transmitted towards said terminal being a multicarrier signal formed of a plurality of carrier frequencies, said weighting step selectively assigns distinct complex weights to each of said carrier frequencies, or to subsets. carrier frequencies.

  Advantageously, said base station implements a step of pretreatment, in particular pre-equalization, of a signal transmitted to said receiver, as a function of said channel estimate (s).

  The invention also relates to a receiver comprising means for transmitting an estimate of at least one propagation channel to an emitter, comprising: means for estimating said transmission channel; means for projecting said estimate on a basis of two-dimensional time / frequency functions, delivering projection coefficients; data transmission means representative of said projection coefficients to said transmitter, so that said transmitter can reconstruct a profile of said transmission channel as seen by said receiver.

  Such a receiver is, in an advantageous variant of the invention, a radiotelephone terminal.

  The invention also relates to a transmitter comprising means for taking into account an estimate of at least one propagation channel transmitted by a receiver in the form of data representative of projection coefficients obtained by projecting an estimate of said transmission channel. transmission on a two-dimensional time / frequency function basis.

  According to the invention, such a transmitter comprises means for analyzing and processing said projection coefficients, so as to reconstruct a profile of said transmission channel as seen by said transmitter.

  In an advantageous variant of the invention, such an emitter is a base station of a radiotelephone system.

  6. List of Figures Other features and advantages of the invention will appear more clearly on reading the following description of a preferred embodiment, given as a simple illustrative and nonlimiting example, and the accompanying drawings, among others. which: Figure 1 shows a block diagram of the projection principle of the channel estimation on a basis of flattened spheroidal functions, allowing the receiver to determine a set of projection coefficients; Fig. 2 illustrates a closed-loop antenna diversity application for the UMTS standard; Figure 3 depicts an adaptation of the technique of Figure 2 to OFDM modulation; FIG. 4 illustrates an application of the invention to the "intelligent" planning of the frequency utilization by frequency hopping; FIG. 5 illustrates, in the context of FIG. 4, an example of interchangeability of the frequency sub-bands allocated to two distinct mobile terminals.

  7. DESCRIPTION OF AN EMBODIMENT OF THE INVENTION The general principle of the invention is based on the projection of an estimate of the propagation channel on the basis of functions with at least one time or frequency dimension, so that a receiver can trace, to a transmitter, information relating to the channel, in the form of data representative of the coefficients of this projection.

  This base of functions can be a base of functions with a temporal dimension, with a frequency dimension, or for example with a base of two-dimensional time / frequency functions. Throughout the rest of this document, an attempt will be made to describe an embodiment of the invention in which a base of 2D time / frequency functions is implemented. The skilled person will extend this teaching without difficulty to the case of a base of functions of any kind, one or more dimensions.

  An embodiment of such a projection of channel estimation on a DPSS basis is presented in connection with FIG.

  First of all, it is recalled that the receiver can estimate the propagation channel by means of the pilot symbols, or reference symbols, whose value and the transmission position are known to the receiver, inserted into the data stream. received.

  In order to know the value of the transfer function of the propagation channel at a particular position of the time / frequency space, the receiver divides the value of the received driver by its known value on transmission. It can also divide the received pilot symbols by a predetermined coefficient, corresponding to the ratio of the power of the pilots to the power of the useful symbols.

  To know the transfer function of the propagation channel over the entire time / frequency plane, it is then necessary to interpolate between the different values of the channel thus calculated at the pilot positions. For more information on this channel estimation technique, reference may be made, for example, to French patent document No. 2,820,574, in the name of the same Applicant as the present patent application, which is hereby incorporated by reference.

  Several methods can be used to estimate the propagation channel in the entire time / frequency plane of interest. A first method consists of interpolating between the pilots by a projection on a certain predetermined base only in time. Another method of interpolation can be to realize this projection in frequency only. Finally, a last method is to perform a joint interpolation in time and frequency.

  According to a preferred embodiment of the invention, the base used for the projection consists of discrete spheroidal discrete functions ("Discrete Prolate Spheroidal Sequences" in English) in two time / frequency dimensions, as illustrated by FIG. In this figure, we focused on the particular case where the signal received and processed by the receiver undergoes a multi-carrier modulation OFDM type. As indicated above, it is of course also possible to use a function base with a time or frequency dimension.

  The OFDM time / frequency propagation channel 10 seen by a receiver, for example a mobile terminal, of a radio communication network is considered.

  This propagation channel 10 is projected mathematically on a basis of flattened spheroidal functions, of which only the first eight elements 11 to 18 have been represented in the time / frequency space. The channel 10 can thus be expressed in the form of a linear combination of the different DPSSs 11 to 18, the coefficients of which are the projection coefficients C 1 to C 8 listed in the table 19.

  Instead of returning the channel coefficients in the time / frequency plane (which constitutes a fairly high amount of information), the receiver can then simply send the transmitter information related to these coefficients C, C8 used at the base station (transmitter) to regenerate the seen and estimated channel profile mobile side (receiver).

  More generally, the set of projection coefficients determined by the receiver may comprise between 5 and 20 coefficients, depending on the severity of the propagation channel considered. The more complex the channel is to describe, the higher the number of projection coefficients must be.

  However, this information to be transmitted requires more bits than the FBI ("FeedBack Information"), which is a reserved field for this purpose in the UMTS-HSDPA system.

  In a preferred embodiment of the invention, the bits of the CQI ("Channel Quality Indicator") on the HS-DPCCH channel of the HSDPA uplink are therefore used for this transmission. According to the current specifications, each mobile has 20 bits all TTI ("Transmission Time Interval", "transmission time interval" with a duration of 2 ms).

  These bits can index an array of predefined values, stored on both sides (i.e. sender side and receiver side) that includes the projection coefficients of different channel realizations (or any other values related to them). In this case, the receiver identifies the set of coefficients of this table that best approximates the coefficients C1 to C8 actually found by the projection, and sends the index of this set in the CQI.

  Since the CQI field comprises 20 bits, the memorized coefficient table on the transmitter and receiver sides can contain up to 220 fields, corresponding to 220 sets of pre-recorded projection coefficients.

  The receiver then compares the projection coefficients C 1 to C 8 with all or part of these 220 sets of coefficients of the table to identify the set with the greatest resemblance to the set C 0 to C 8 of the coefficients actually calculated. It then sends the transmitter the index coded on 20 bits referencing in the table the set of coefficients identified.

  The knowledge of the information relating to the propagation channel on the OFDM downlink makes it possible to improve the performances concerning the total cellular rate. According to the invention, two main applications are envisaged: the first application concerns the diversity of antennas with feedback (in closed loop mode); the second application concerns a strategy for managing multiple access.

  These two cases are for uses where the speed of the terminal is limited, which makes it possible to assume a slow variation of the channel over time, compared to the TTI.

  First of all, an attempt is made to describe an application of the invention to diversity in closed-loop mode, in relation with FIGS. 2 and 3.

  According to the UMTS standard, diversity is a means of improving the performance of a radio link on a time-varying multipath ("Delay Spread-Doppler Channel") channel. One of the techniques is based on the use of several antennas 21, 22 to the emission, on the side of the base station 20: this number is limited to two for the current specifications of UMTS. A single antenna 23 is provided at the reception side of the mobile 24. Two modes of antenna diversity are used in the UMTS: - the first mode is called "diversity in open loop mode" (in English "Open Loop Diversity Scheme "). According to this first mode, which is not the subject of the present invention, there is no feedback from the receiver (mobile) to the transmitter (the base station). This first mode notably covers the following techniques: Space Time Transmit Diversity (STTD), Time Switched Transmit Diversity (TSTD) and Orthogonal Transmit Diversity (OTD). ; the second mode is called "Closed Loop Diversity Scheme" and provides feedback from the receiver to the transmitter. This information relates to the state of reception of the signals from the two antennas 21, 22. The base station can then take into account the information transmitted by the mobile 24 to generate 26 weights W1, W2 applicable to each of the two antennas These weighting W ,, W2 may consist of applying a simple phase shift between the two antennas 21, 22 (mode 1), or to jointly apply a phase shift and a gain (mode 2).

  Figure 2 illustrates this principle for the UMTS standard.

  The coefficients W. represent the coefficients related to the propagation channel. The computation of these coefficients W, and W2 to be applied can be done in the mobile 24, in order to maximize a certain quantity linked to the two propagation channels (associated respectively with each of the two antennas 21, 22), or, in a mode of preferential embodiment of the invention, in the base station, from the projection coefficients communicated by the mobile 24. It should be noted that the coefficients W, and W2 to be applied will take concrete values according to the information contained in the FBI bits . The multiplication by the weights of the two antennas 21, 22 serves to make the sum of the two signals, emitted by the two antennas 21, 22 and received at the mobile level 24 by the antenna 23, the most coherent possible. The values of W1 and W2 are calculated from the projection coefficients of the estimated propagation channel on the pilot symbols on the chosen basis. These projection coefficients, which are used to calculate W, and W2 are stored in an array, indexed by FBI bit values.

  The present invention applies in particular in the context of the application of the "diversity in closed-loop mode" mode to multicarrier modulations, and in particular to OFDM modulations.

  Indeed, UMTS ("Universal Mobile Telecommunications Standard", 30 "universal mobile telecommunications standard") being a single-carrier transmission system, the coefficients of the propagation channel can be considered constant over the entire frequency band.

  In a multicarrier system such as OFDM for example, the profile of the propagation channel changes with frequency, which makes the multiplication of each transmission antenna obsolete by a single coefficient W; The correction must therefore be made individually for each subcarrier. The implementation of this technique, illustrated in FIG. 3, can be done in frequency before the IFFT 30 ("Inverse Fast Fourier Transform", "Fast Fourier Transform") by multiplying each subcarrier d, to dN by a complex coefficient W, ', where the index j represents the index of the transmitting antenna 21, 22, and where i is the index of the subcarrier considered.

  Since these coefficients W, 'are intended to compensate for the difference, induced by the propagation channels, between the two signals received on the receiving antenna 23 of the mobile 24, it is conceivable that only the coefficients W2 associated with the antenna 22 are complex. The other coefficients W 'associated with the transmitting antenna 21 may be real, or even the identity.

  The base station 20 having reconstructed an estimate of the channels corresponding to each antenna 21, 22, it has all the freedom to apply the algorithm of its choice for calculating the coefficients W ', and W2 ;. Since the pilot symbols are treated on transmission as the free symbols, the information sent by the terminal 24 relates to either the propagation channel, the previously applied coefficients, or a combination of the propagation channel and the W, and W2 coefficients previously applied. The base station 20 then knows to find the information of the channel.

  The base station can thus distribute the power between several subcarriers for each antenna inversely proportional to the corresponding transmission channel, which it knows thanks to the data transmitted by the associated receiver.

  For example, if Hk ,, is the channel seen by the subcarrier k emitted by the antenna 1, the power Pk ,, assigned to the antenna 1 for the subcarrier k can then be Pm / 12, Pm being the average power. The symbol transmitted on the sub-carrier k will then for example be multiplied by the quantity H * k ,, / IHk.I 12.

  It will be noted in FIG. 3 the possible presence of blocks 311, 312 associated with each of the two antennas 21, 22, intended to add a guard interval in the transmitted symbol stream, and this, in a conventional manner, in order to reduce interference between symbols.

  In addition to the closed-loop diversity, the invention can also be applied to the management of multiple access in a cellular radio network, as described in more detail below with reference to FIGS. 4 and 5.

  This technique of managing multiple access can be implemented during a normal transmission (without antenna diversity); it aims at increasing the total rate of the cell, by controlling the transmission of the packets according to the following strategy: - during the initialization of the OFDM link, the receivers (or UEs) trace the information concerning their respective transmission channels to the station basic; the base station then makes a comparison between the different propagation channels for which it has information from the receiver and tries to find the best placements in the time / frequency plan of the packets (which are in the form of blocks at two time / frequency dimensions), so that the signal-to-noise ratio is maximized at reception for all the UEs served. This technique can be called "Closed Loop Frequency Hopping" and is illustrated in Figure 4.

  FIGS. 4A and 4C relate to the frequency sub-band 41 allocated to a first mobile terminal UE1. FIGS. 4B and 4D relate to the frequency subband 42 allocated to a second mobile terminal UE2.

  FIGS. 4A and 4B illustrate these two frequency sub-bands 41m 42B, and in particular the possible fadings which affect them, at a first instant t = 0. FIGS. 4C and 4D illustrate these two frequency sub-bands 41c, 42D after time evolution, at a time t = 100 ms.

  As shown in FIG. 4A, the sub-band 41A assigned to the first receiver UE1 is of poor quality at t = 0. On the other hand, the sub-band 42B assigned to the receiver UE2 at the same time is of satisfactory quality, so that it is easy to satisfy, with a good quality, the requests of the user of UE2.

  To predict the evolution, and therefore the possible degradation, of the frequency subtrack 41A allocated to UE1, it is conceivable to perform a frequency hopping, so as to assign a new frequency sub-band to the receiver UE1. According to the techniques of the prior art, this was done by performing a random jump of frequency 40. However, as illustrated in FIG. 4C, such a random jump can lead to assigning the receiver UE1 a frequency sub-band. 43 fading similar to that of the sub-band 41c of UE1, before frequency hopping.

  According to the invention, thanks to the knowledge of the channel estimation performed by the mobile terminals, the base station can carry out an intelligent frequency planning, so as to allocate to the receiver UE1 a new frequency subband 44 which does not it is not affected by fading (or in any case less than the band 41c which the receiver UE1 would have without frequency hopping).

  The choice of the new allocated frequency sub-band is based for example on the instantaneous power on a frequency, or the average power of a frequency sub-band.

  With regard to the UE2 receiver, it is not necessary to perform such an "intelligent" frequency jump, the frequency sub-band 42D being of a quality substantially similar to that of the frequency sub-band 42B.

  In the case where the number of access requests to the OFDM link is greater than the resources, the base station may defer transmission to a 30 (or more) UE (s) in favor of a new incoming UE if the first is very degraded. In this case, the total number of UEs served by the base station increases on average, as well as the total throughput of the cell. Indeed, this strategy limits the abuse of HARQ (in English "Hybrid Automatic Repeat Request"). This amounts to assigning a priority rank depending, among other parameters, on the quality of the downstream transmission channel, the allocation of this priority concerning the users already served, as well as the new entrants.

  Figure 5 illustrates in more detail the principle explained above. FIGS. 5A and 5C show the propagation channel associated with a receiver UE1 at two distinct instants t = 0 and t = 100.

  The base station may find that the frequency sub-band 51 assigned to the UE1 at t = 0 is of poor quality, and fear a deterioration of this quality in the following moments. The base station can then choose to accept serving a new receiver UE2, whose sub-frequency band at t = 0 is of satisfactory quality, in order to perform an exchange of frequency sub-bands at the instant t = 100, between the two receivers UE1 and UE2.

  Thus, at t = 100, the receiver UE1 is allocated the frequency sub-band 53, close to 200 kHz, previously allocated to the receiver UE2. Conversely, the receiver UE2 is allocated the sub-frequency band 54, close to 100 kHz, previously allocated to the receiver UE1.

  Such an exchange of frequency subbands makes it possible, on average, to maintain a satisfactory transmission quality for each of the two receivers UE1 and UE2 at t = 100. Without such an exchange, the frequency sub-band 51 allocated to the receiver UE1 would have suffered a deep fading at t = 100.

  Such frequency hopping is determined by comparison by the base station of the selection criteria of the frequencies mentioned above, such as the instantaneous power on a frequency or the average power of a frequency sub-band.

Claims (20)

  A method for transmitting an estimate of at least one propagation channel from a receiver to a transmitter, characterized in that said receiver implements the following steps: estimation of said transmission channel; projecting said estimate on a function basis to at least one frequency or time dimension, delivering a set of projection coefficients; transmission of data representative of said set of projection coefficients to said transmitter, said representative data comprising at least one index referring to a table of possible coefficient values, stored by said transmitter and said receiver, so that said transmitter can reconstruct a profile said transmission channel as seen by said receiver.   2. A method of transmitting an estimate of a propagation channel according to claim 1, characterized in that said base of functions is a base of two-dimensional functions time / frequency.   3. Method for transmitting an estimate of a propagation channel according to claim 1 or 2, characterized in that it comprises a step of comparison between the projection coefficients and sets of coefficients stored in said propagation channel. table, so as to identify the set of coefficients closest to said set of projection coefficients.   4. A method of transmitting an estimate of a propagation channel according to any one of claims 1 to 3 characterized in that said function base is a base of flattened spheroidal functions (DPSS).   5. A method of transmitting an estimate of a propagation channel according to any one of claims 1 to 4, characterized in that said receiver is a terminal and said transmitter is a base station in a radio system.   6. A method of transmitting an estimate of a propagation channel according to claim 5, characterized in that said radiotelephone system implements OFDM modulation.   7. A method of transmitting an estimate of a propagation channel according to claim 6, characterized in that said data representative of said projection are transmitted using the bits of the quality indicator of the channel (CQI " Channel Quality Indicator "), on the HS-DPCCH channel of HSDPA uplink.   8. A method for transmitting an estimate of a propagation channel according to any one of claims 1 to 7, characterized in that said receiver estimates at least two distinct transmission channels corresponding to as many transmit antennas. separate from said transmitter.   9. A method for transmitting an estimate of a propagation channel according to any one of claims 7 and 8, characterized in that, the signal received by the receiver being a multicarrier signal, comprising a plurality of carrier frequencies, said transmission channels are estimated for each of said carrier frequencies, or for subsets of said carrier frequencies.   10. Method for transmitting an estimate of a propagation channel according to any one of claims 5 to 9, characterized in that said base station implements a step of allocating at least one communication frequency. with a terminal, according to said one or more channel estimates, so that the allocated frequencies correspond, on average, to areas in which the corresponding channel estimate has characteristics in accordance with at least one predetermined criterion.   11. A method for transmitting an estimate of a propagation channel according to claim 10, characterized in that said allocation step selects, for a given terminal, one or more frequencies available and for which said channel estimate or estimates present a maximum level and / or higher than a predetermined threshold.   12. A method for transmitting an estimate of a propagation channel according to claim 10, wherein said allocation step implements a step of frequency hopping, said frequency hopping taking place. counting said channel estimates so as to allocate frequencies to each communication so as to limit, as far as possible, the fading effect of the channel on these communications.   13. A method for transmitting an estimate of a propagation channel according to any one of claims 10 to 12, characterized in that said allocation step implements one of the operations belonging to the group comprising: - acceptance or refusal of a new call, a call being refused if there is not at least one frequency band, consisting of at least one available frequency, for which the channel estimate (s) present (s) a higher level at a predetermined threshold; - waiting for at least one calling terminal; determining a quality of service of at least one service requested by a terminal; - allocating, to a new communication, a band of at least one frequency, previously allocated to a communication terminal for which said channel estimate has a level below said predetermined threshold.   18. A method for transmitting an estimate of a propagation channel according to any one of claims 10 to 13, characterized in that said base station uses at least two transmitting antennas, and in that it comprises a weighting step in which the respective weight associated with each of said antennas for a given communication is a function of the corresponding channel estimates.   19. A method for transmitting an estimate of a propagation channel according to claim 14, characterized in that, the signal transmitted to said terminal being a multicarrier signal formed of a plurality of carrier frequencies, said weighting step selectively affects distinct complex weights at each of said carrier frequencies, or sub-sets of carrier frequencies.   16. A method for transmitting an estimate of a propagation channel according to any one of claims 5 to 15, characterized in that said base station implements a pre-processing step, including pre-equalization, a signal transmitted to said receiver, as a function of said one or more channel estimates.   17. Receiver comprising means for transmitting an estimate of at least one propagation channel to an emitter, characterized in that it comprises: means for estimating said transmission channel; means for projecting said estimate on a basis of two-dimensional time / frequency functions, delivering projection coefficients; data transmission means representative of said projection coefficients to said transmitter, said representative data comprising at least one index referring to a table of possible coefficient values, stored by said transmitter and said receiver, so that said transmitter can reconstruct a profile of said transmission channel as seen by said receiver.   18. Receiver according to claim 17, characterized in that it is a radiotelephone terminal.   19. Transmitter comprising means for taking into account an estimate of at least one propagation channel transmitted by a receiver in the form of data representative of projection coefficients obtained by projecting an estimate of said transmission channel on a base two-dimensional functions time / frequency, said representative data comprising at least one index referring to a table of possible coefficient values, stored by said transmitter and by said receiver, characterized in that it comprises means of analysis and processing said projection coefficients, so as to reconstruct a profile of said transmission channel as seen by said transmitter.   20. Transmitter according to claim 19, characterized in that it is a base station of a radiotelephone system.