Abstract:
A process which adjusts the transmission and reception chains of the paths formed by a base station of a system for radio communication between mobiles which after calibration of the antenna base adjusts the reception chains relating to each path by distributing a first specified adjustment signal synchronously over each of the reception chains and calculating an equalization filter which inverts the transfer functions related to each reception chain. The process next adjusts the transmission chains related to each path by distributing a second specified adjustment signal synchronously over each of the transmission chains, by extracting from each of the paths a part of a transmission signal before the transmission signal is sent to the antenna base so as to re-inject that part of the transmission signal into the reception chains, and by calculating an equalization filter which inverts the transfer functions related to each transmission chain. Finally, the process receives via the equalization filters previously calculated for the reception chains reception signals originating from the antenna base, or transmits via the equalization filters previously calculated for the transmission chains the transmission signals originating from the multipath transmission/reception device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process and an implementation device for adjusting the transmission and reception chains of the paths formed by a multipath transmission/reception device of a base station of a radiocommunications system, using in particular a multipath transmission/reception technique known by the abbreviation SDMA standing for “Space-Division Multiple Access”. 
     2. Discussion of the Background 
     Systems for radiocommunication with mobiles are mostly based on the cellular network concept in which a base station communicates with a number of mobiles within the cell for which it is responsible. 
     For transmitting and receiving signals, base stations currently use only simple antenna systems, either of the single-sensor or multi-sensor type, with RF combination to ensure complete coverage, or else of the two-sensor type with selection to obtain space diversity in reception. 
     The concept of SDMA, now widely reported in the literature, is aimed at increasing the capacity of a base station by performing multipath transmission and reception in an adaptive and intelligent manner. 
     However, the setting up of this technique runs up against problems of implementation and of control of the distortions in the transmission and reception chains. In particular, the most disturbing phenomenon is the phase and amplitude dematching of the paths. During reception, this phenomenon will prevent a correct location fix on the mobile and hence the allocation of an SDMA frequency. During transmission, this problem may cause the transmission of a signal in an undesired direction. 
     The techniques for alleviating these problems are well known in respect of reception, since they are implemented in radio direction finders and consist in performing a calibration of the antenna base followed by frequent adjustment of the reception paths. 
     During transmission, the problem is more complex since it involves adjusting the transmission paths (after calibration of the antenna base) while being independent of the structure of the antenna base. 
     SUMMARY OF THE INVENTION 
     The aim of the invention is to alleviate the aforesaid drawbacks. 
     To this end, the subject of the invention is a process allowing the regular adjusting of the transmission and reception chain of each of the paths formed by a base station of a system for radio communication with mobiles, including a transmission/reception antenna base and a multipath transmission/reception device, characterized in that it consists after a step of calibration of the antenna base: 
     within a step for adjusting the reception chains relating to each path, in distributing a first specified adjustment signal, synchronously over each of the reception chains, in calculating an equalization filter inverting the transfer functions relating to each reception chain, 
     within a step for adjusting the transmission chains relating to each path, in distributing a second specified adjustment signal, synchronously over each of the transmission chains, in extracting from each of the paths a part of the transmission signal before it is sent to the antenna base so as to re-inject it into the reception chains, and in calculating an equalization filter inverting the transfer functions relating to each transmission chain, 
     in performing the reception of the signals originating from the antenna base via the equalization filters previously calculated for the reception chains, or the transmission of the signals originating from the multipath transmission/reception device via the equalization filters previously calculated for the transmission chains. 
     The present invention has the advantage of adjusting the transmission and reception paths of a radiocommunication system utilizing an SDMA technique, making it possible to ensure the control of the wavefront of the signals received and transmitted by the system by using adjustment signals which avoid the phenomena of coupling between antennas. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     Other advantages and characteristics of the present invention will emerge more clearly on reading the description which follows given in conjunction with the appended figures which represent: 
     FIG. 1, a radiocommunication system incorporating a device for implementing the process according to the invention, 
     FIG. 2, a schematic diagram of the process according to the invention, 
     FIG. 3, a functional diagram of a device for implementing the process according to the invention, 
     FIG. 4, a frequency and time representation of the signal for adjusting the transmission chains, 
     FIG. 5, a first solution for dividing the carriers inside the useful band, 
     FIG. 6, a second solution for dividing the carriers inside the useful band, 
     FIGS. 7 a  and  7   b,  respectively the linear zone of the amplification of the transmission chains, and the effect of phase distortion as a function of the amplitude of the modulating envelope, and 
     FIG. 8, a functional diagram corresponding to the stage of pre-correction of the amplification of the transmission chain. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A device  1  implementing the process according to the invention is interfaced between an SDMA transmission/reception device  2  and an antenna base  3 , as indicated in FIG.  1 . 
     Consequently it is independent of the SDMA processing performed by the ad hoc device and implements as many transmission paths and reception paths as antennas. 
     FIG. 2 illustrates a schematic diagram of the process according to the invention. 
     After a first step  4  of calibration of the antennas of the antenna base, the process according to the invention consists within a second step  5 , in adjusting the reception chains relating to each path, and then within a third step  6 , in adjusting the transmission chains relating to each path. 
     Having performed the adjustment, within a fourth step  7 , the reception or transmission of the signals respectively received by the antenna base  3  or transmitted by the transmission/reception device  2  is performed. 
     In step  8 , in the case in which the frequencies of the reception and transmission signals are different, the reception chains of each of the paths are adjusted to the frequency of the transmission signal. 
     To alleviate the non-linearity of the amplification chains on transmission, a step  9  allows a pre-correction of these chains. 
     An adjustment device for implementing the process according to the invention is illustrated by the functional diagram of FIG.  3 . 
     It is interfaced between the multipath transmission/reception device  2 , especially of SDMA type, and the antenna base  3 . To each antenna, not represented, there corresponds a path formed in respect of transmission and reception. In the figure, three paths i with i=1 to 3 are represented respectively by three superimposed planes. 
     Each path i includes: 
     a transmission  10   i  and reception  11   i  chain, 
     an equalization filter  12   i  for the transmission signal input-coupled to the transmission chain  10   i,    
     an equalization filter  13   i  for the reception signal output-coupled to the reception chain  11   i,    
     an input switch  14   i  and  15   i  and output switch  16   i  and  17   i  for the transmission chain  10   i  and the reception chain  11   i  respectively enabling the device to enter the various modes of operation: reception adjustment, transmission adjustment, transmission/reception, 
     a coupler  18   i  placed at the output of the transmission chain  10   i  after the output switch  16   i  of the transmission chain  10   i  making it possible to extract a part of the transmission signal before it is sent to the antenna base  3  so as to inject it at the input of the reception chain  11   i  via the input switch  15   i  of the reception chain  11   i.    
     The transmission equalization filter  12   i  is placed at the input of path i, before the input switch  14   i  of the transmission chain  10   i,  and the reception equalization filter  13   i  is placed at the output of path i, after the output switch  17   i  of the reception chain 
     The device next includes in common for all the paths i: 
     a first generator  19  of a signal for adjusting the transmission chains  10   i  which is coupled to a first distribution module  20  distributing a first specified adjustment signal over the transmission chains  10   i  of each of the paths i by way of the input switch  14   i  of the transmission chain  10   i,  placed at the output of the equalization filter  12   i.  The adjustment signal can be generated directly in the multipath transmission/reception device. 
     a second generator  21  of a signal for adjusting the reception chains  11   i  which is coupled to a second distribution module  22  distributing a second specified adjustment signal over the reception chains of each of the paths i by way of the input switch  15   i  of the reception chain  11   i,  and 
     an analyser of the adjustment signals  23  allowing analysis of the various adjustment signals so as to adapt the respective equalization filters  12   i  and  13   i  to the transmission  10   i  and reception  11   i  chains. 
     The device can furthermore include a stage for pre-correction  24  of the transmission chains which is placed between the equalization filter  12   i  and the transmission chain  10   i  and is represented with dashed lines in the figure. 
     The adjustment analyser  23  likewise in this case allows analysis of the correction signals. 
     A detailed description of the operation of the adjustment device for implementing the process according to the invention is given below: 
     The adjustment  5  of the reception chains  11   i  is aimed at correcting the drifting of the transfer functions of the reception chains  11   i.  This drifting is due mainly to the variations in temperature inside the equipment. 
     The equalization filters  13   i  at the output of the reception chains  11   i  have transfer functions, the alterations in which must consequently be followed, by performing an adjustment  5  cyclically. 
     The adjustment  5  is performed by injecting an adjustment signal whose characteristics are assumed to be known, and by making measurements whereby it is possible to determine the transfer functions so that they can be corrected. To simplify the measurements, the adjustment signal is made up of several carriers which make it possible to perform frequency discretization of the transfer functions. 
     The processing operations performed in a processor of the SDMA transmission/reception device being sensitive to the differential gaps from one path i to another, it is not the absolute transfer functions which must be corrected but merely the differential gaps from one path i to another. 
     Denoting by H i (f) the transfer function of path i, and m i  the vector of measured complex gains, we obtain:                      m   _     i          (   k   )       =             H   i          (     f   k     )           H   1          (     f   k     )            k     =   1       ,   …   ,   N           (   1   )                                
     path  1  serving as reference, f k  being the frequencies used to make the measurements and N the number of carriers used. 
     Measurement of the responses of the equalization filters  13   i  to the various frequencies can be performed by FFT, standing for “Fast Fourier Transform” or DFT, standing for “Discrete Fourier Transform”. It is preferable to perform them in succession and to use a DFT to perform the measurement, so as to avoid any problem of intermodulation. In the case of a measurement by FFT, the frequencies used must be located orthogonally, that is to say with a frequency gap equal to the inverse of the duration of the FFT measurement. 
     It is therefore necessary to calculate a set of p coefficients of an FIR filter, w i,  standing for “Finite Impulse Response Filter” for each of the reception chains  11   i,  in such a way that:                  (         H   i          (     f   k     )           H   1          (     f   k     )         )     ·       H     w   i            (   f   )         ≈   1           (   2   )                                
     for every f belonging to the useful band. 
     The transfer function H wi (f) is obtained as follows:                  H     w   i            (   f   )       =       ∑     l   =   1     p                           w   _     i          (   l   )       ·     e     2      jπfl                   (   3   )                                
     Since the differential transfer functions are known only for a discrete collection of frequencies {f k }, a simple way of obtaining w i  consists in minimizing in the least squares sense the error between the desired transfer function and that of an FIR filter with p coefficients, for all the measured frequencies, i.e.:                min       w   _     i       ∥         m   i   ′     _     -     F   ·       w   _     i              ∥   2             (   4   )                                
     with:              m   i   ′     _          (   k   )       =     1         m   _     i          (   k   )                                
     and F a p×N matrix such that: 
     
       
           F ( m,n )=exp(−2. j.π.f   m   .n )  (5) 
       
     
     where m and n respectively represent the mth row and nth column of the matrix, f m  being the mth normalized adjustment frequency. 
     This leads to the solution: 
     
       
           w   i   =[F   +   .F]   −1   .F   +   .m′   i   (6) 
       
     
     where“ + ” corresponds to the operation of conjugate transposition. 
     The attraction of such an approach lies in the fact that the matrix [F + .F] −1 .F + (of dimension p×N) is constant and hence can therefore be calculated once and for all, this making it easy and inexpensive in terms of computation power to obtain w i . 
     A compromise is then apparent with regard to the number of coefficients p. A large value of the latter (that is to say close to N) leads to a small quadratic error but to a large instability in the transfer function between the measured frequencies f k . 
     An optimal value lies between N/2 and N/3, this ensuring a correction to within 0.50° and 0.1 dB. 
     The filter lii must also be convolved with a band-pass filter so as to ensure attenuation outside the useful band. 
     The main problem posed in respect of the adjustment of the transmission chains  10   i  is to render the device  1  independent of the antenna base  3  to which it is connected. This independence makes it possible to use various kinds of antenna base  3  for the SDMA function (pentagonal, linear antenna base etc.). 
     The step  6  of adjusting the transmission paths of the process according to the invention consists in performing beforehand a calibration of the antenna base at the transmission frequency fe so as to obtain a calibration table. Having performed this calibration, adjustment is performed in a regular manner (as for reception) so as to follow the variations in the mismatching of the transmission chains  10   i  over time. 
     To do this, the transmission signal is extracted, with the aid of the coupler  18   i,  situated at the output of the device  1 . This signal is re-injected into the previously adjusted reception chain  11   i.    
     In the case of duplex operation on different frequencies for transmission fe and reception fr, it is necessary to allow the reception, by the reception chains  11   i,  of the transmission frequency fe, and to adjust the reception chains  11   i  to this frequency fe. 
     In order to avoid any problem of coupling between the antennas of the antenna base  3 , which may impair the measurement (since the antenna base  3  is not disconnected), the same adjustment signal must not be injected into each of the transmission chains  10   i  since this would lead to the reception on a path i of the signal transmitted by the other paths i, via the antenna base  3 . 
     A solution according to the invention consists in dispatching, on each of the transmission chains  10   i  different adjustment signals including carriers located orthogonally: one carrier at the frequency f 1  for the transmission chain  10   1  of path  1 , one carrier at the frequency f 2  for the transmission chain  10   2  of path  2 , and one carrier at the frequency f N  for the transmission chain  10   N ; the gap between the carriers is fixed by the duration of observation T of the measurement. These signals are transmitted synchronously over each of the paths i so as to allow accurate phase measurement. This synchronism demands: 
     a digital adjustment generator  19  common to the various paths i, 
     analogue/digital converters, not represented, using the same sampling clock, and 
     a common frequency reference distributed over each of the paths i in respect of the transposition functions. 
     As represented in FIG. 4, the principle of orthogonality consists in selecting frequencies 1/T such that their frequency gap is inversely equal to the duration of observation T. Thus, if detection of the carriers by DFT or FFT is performed, with no weighting window, the frequency response is such that the other carriers lie at the minima of this response (sinx/x). 
     Two solutions can be envisaged, depending on the level of distortion created by the transmission chain  10   i.  If the latter creates only a fixed phase shift for the whole of the useful band, the first solution is to generate as many carriers as paths i, while locating them as close as possible to the centre of the useful band. 
     This first solution is illustrated by FIG.  5 . 
     Let m i  be the differential complex gain measured for path i, then the equalization filter  12   i  exhibits a complex gain equal to: 
       m′   i =1 /m   i   (7) 
     If the transmission chains  10   i  cause large phase and amplitude distortions within the useful band, the equalization filter  12   i  performs a filtering operation. The method used is therefore the same as for the adjustment  5  on reception, but this time with measurements performed in sub-ranges, but still with different frequencies for each of the transmission paths. 
     This second solution is illustrated by FIG.  6 . 
     Let m i  be the vector of differential complex gains measured for each of the sub-ranges of path i, then the equalization filter  12   i  of path i is an FIR filter whose vector of coefficients can be written according to the following equation: 
     
       
           w   i   =[F   +   .F]   −1   .F   +   .m′   i   (8) 
       
     
     with m′ i  the vector of inverse complex gains. 
     All the adjustment measurements are performed in the linear zone of the amplifiers, which are not represented, of the various transmission chains  10   i.  Unfortunately, the amplifiers used have linear amplification in a restricted zone of input level, as illustrated in FIG. 7 a.    
     Indeed, the amplifiers cause PM phase and AM amplitude distortion which depends only on the amplitude of the modulating envelope, as illustrated in FIG. 7 b.    
     The SDMA transmission/reception device  2  carries out the formation of beams by combining several transmissions with complex weightings which differ from one path to another. The consequence of this is that signals are input to the amplifiers of the transmission chains  10   i,  having different levels on each path i. Thus, the operating points of the amplifiers are not the same, this giving rise to a different phase distortion for each path i. 
     In order to alleviate this problem of phase and amplitude distortion, the process according to the invention performs a pre-correction  9  of the transmission chains  10   i.  This is achieved with the aid of phase and amplitude measurement of the transfer function of the amplifiers with the help of a pre-correction stage  24   i.  The measurement consists in performing detection of the instantaneous amplitude, as well as of the instantaneous phase of the modulating signal. In the case of baseband processing: 
     
       
         ρ k   ={square root over (I k   2   +Q   k   2 +L )}   (9) 
       
     
     
       
         
           
             
               θ 
               k 
             
             = 
             
               arctan 
                
               
                 ( 
                 
                   
                     I 
                     k 
                   
                   
                     Q 
                     k 
                   
                 
                 ) 
               
             
           
         
                 
         
             
         
      
     
     where 
     ρ k : instantaneous modulus of the signal 
     θ k : instantaneous phase of the signal 
     i.e.: 
     
       
           I   k =ρ k .cos(θ k )  Q   k =ρ k .sin(θ k )  (10) 
       
     
     I k  and Q k  representing respectively the complex phase and phase quadrature signals. 
     The transfer functions of the amplifiers will be called f and g: 
     
       
         ρ out   =f (ρ in ) θ out =θ in +Δθ k =θ in   +g (θ in )  (11) 
       
     
     This step  9  of pre-correction therefore consists in applying the function which is the inverse of the transfer function of the amplifier of the transmission chain  10   i,  which is stored, as represented in FIG. 8, in AM/AM and AM/PM tables. 
     
       
         ρ′ k   =f′ (ρ k ) θ′ k =θ′ k +Δθ k =θ′ k   +g′ (ρ k )  (12) 
       
     
     The pre-correction functions f′ and g′ are obtained as a function of the transfer functions f and g of the amplifier by solving the following system:              {             ρ   k   ″     =       f        (     ρ   k   ′     )       =       f        (       f   ′          (     ρ   k     )       )       =     G   ·     ρ   k                         θ   k   ″     =         θ   k   ″     +     g        (     ρ   k   ′     )         =         θ   k     +     g        (       f   ′          (     ρ   k     )       )       +       g   ′          (     ρ   k     )         =     θ   k                         (   13   )                                
     with G the fixed gain of the corrector  24   i -amplifier of the amplification chain  10   i  pair. i.e.:              {               f   ′          (     ρ   k     )       =       f     -   1            (     G   ·     ρ   k       )                       g   ′          (     ρ   k     )       =     -     g        (       f     -   1            (     G   ·     ρ   k       )       )                         (   14   )