Abstract:
A spread spectrum data transmission apparatus, suitable for use in radiotelephony utilizing Code Division Multiple Access (CDMA), includes a receiver for receiving data organized in symbols having a duration T s  These symbols are spread by a spreading code having a period T c  prior to transmission over a transmission channel that has a plurality of main broadcasting paths, each broadcasting path producing a delay. The receiver includes a Rake receiving circuit having a plurality of branches for processing data relating to a path delay, an estimation circuit for received symbols, and an interference cancelling circuit having a long duration covering various T s  and which is adapted to perform the following operations:          ∑     k   =   1       k   =   K              d     -   k       ·     (       ∑     l   =   1       1   =   L                w   *          (     τ   1     )       ·     w        (       τ   1     -     kT   s       )           )                               
     where w( . . . ) represents a channel estimation, w*( . . . ) represents the conjugate of the channel estimation, and d −k  represents previously estimated symbols.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a spread-spectrum data transmission apparatus comprising a data receiver for receiving data organized in symbols of duration T s  and spread by means of a spreading code having period T c  before a transmission channel is borrowed which channel has various broadcasting paths each producing delays, which receiver comprises: 
     a receiving circuit called Rake circuit formed by &lt;&lt;L&gt;&gt;branches for processing data relating to a path delay, 
     an estimation circuit for the received symbols. 
     The invention also relates to a method of processing data featuring a spectrum spreading. 
     BACKGROUND OF THE INVENTION 
     Such apparatus are well known and find many applications notably in the field of portable telephones. On this subject European patent EP 0-851 600 can be consulted. This known apparatus comprises a receiver, which combines signals delayed by one symbol period or more, with the aim to diminish the influence of interference which occurs in time periods that exceed this symbol period. 
     SUMMARY OF THE INVENTION 
     The present invention also proposes to eliminate the interference which occurs in periods which exceed various times this symbol period but by utilizing other methods. 
     For this purpose, such a receiver is characterized in that it further includes a long-duration interference canceling circuit which covers various T s . 
     These and other aspects of the invention are apparent from and will be elucidated, by way of non-limitative example, with reference to the embodiment(s) described hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 shows a receiver in accordance with the invention, 
     FIG. 2 is a timing diagram explaining the symbol periods and the duration of the spreading element, 
     FIG. 3 shows in more detail the receiver in accordance with the invention, and 
     FIG. 4 shows in detail the structure of the interference canceling circuit included in the receiver shown in FIG.  3 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In FIG. 1 is represented a transmission apparatus  1  co-operating with a base station  2  which transmits, while using QPSK modulation (Quadrature Phase Shift Keying), data in complex form according to the CDMA method. This apparatus is in the form of a data transmitter  3  and a data receiver  4 , which use an antenna  8  in common. 
     The receiver  4  notably comprises a demodulator  12 , which demodulates the data received from the antenna  8  after processing them, and a data processing element  14 , which incorporates a Rake receiving circuit referenced  15 . This circuit permits to improve the reception for radio links which take place over various paths Pth 1 , Pth 2 , . . . , Pth n , so that the data d −k , which are available at terminal  20 , are of the best possible quality. 
     According to the invention an interference canceling circuit referenced  25  is further provided in this receiver. 
     In FIG. 2 the line A shows a symbol to be transmitted, which is represented in two states to make matters easier. T s , is the duration of this symbol. This symbol is subjected to a modulation via a spread-spectrum code of which each element has a duration T C . The ratio T S /T C  provides the spread-spectrum factor. This spreading code is in fact in the form of a first code C sp  relating to the user, and a second code C SC  relating to the base station that transmits the code. Thus the composite spreading code permits to find back, by means of correlations, the symbols transmitted at the level of the receiver. 
     FIG. 3 shows in more detail the receiver according to the invention. It is in the form of a Rake receiving circuit  15  which receives a complex signal y(t) formed by four branches, for example BR 1  to BR 4 , for processing the signals coming from various paths leading to respective delays τ 1  to τ 4 . These branches comprise a sampler ER 1  to ER 4  respectively, which samples the real part (y) of the received signal at the frequency  1 /T C  and another sampler EI 1  to EI 4  for the imaginary part ℑ(y) of this signal. The sampling time of these various samplers is shifted by the periods τ 1  to τ 4  for each of the branches. A corrector CR 1  to CR 4  provides the correlation of the real part of the sampled signal via the various samplers ER 1  to ER 4  with the spread-spectrum code formed from the locally processed parts C SP  and C SC . A corrector CI 1  to CI 4  provides these same correlations for the imaginary parts of this received signal. The signal on the output of this corrector is corrected by the estimate of the channel used. This estimate based on the processing of the channel impulse response is effected by a channel estimator  50 . This type of estimator forms part of the state of the art and one may find a description thereof in the article by R. PRICE and P. E. GREEN, entitled: “A Communication Technique for Multipath Channels”, published in Proceedings of IRE, vol. 46, March 1958. Notably this estimate is processed based on a channel called BCCH (see UMTS standards). Various multipliers M 11 , M 12 , M 13 , M 14  and M 15  (the latter by its multiplication by −1 gives the conjugate part of the output signal of the set of correlators CR 1  and CI 1 ), which makes it possible to modify the amplitude and the phase of the output signals of the correlators CR 1  and CI 1 ; it will be the same for the other branches. The adders A 11  and A 12  finally produce the corrected signal. The adders ARF and AIF produce the combination of the signals processed by the various branches of the circuit. 
     In accordance with the invention, before applying these signals to the decision circuits SR and SI, the contribution of the interference, which is evaluated by the interference canceling circuit  25 , is subtracted by means of the adders ARI and AII. 
     This circuit  25  comprises a part  25   a  relating to the calculations and a part  25   b  intended to store the various data estimated by the decision circuits SR and SI, which thus form an estimation circuit for symbols received and rendered available at terminal  20 . The calculation circuits thus use these various stored data and also various estimation samples of the channel produced by the estimator  50 . 
     FIG. 4 shows the structure of the canceling circuit  25 . This circuit is in the form of K intercorrelation blocks IntC 1  to IntCK, where K defines the time during which the interference may be suppressed. These blocks receive signals Epth 1  to Epth 4  which either or not validate the contribution of the branches BR 1  to BR 4  assigned to the paths Pth 1  to Pth 4 . Various multipliers N 11  to N 15  for the block IntC 1 , in co-operation with the adders PR 1  to PI 1 , perform similar operations to the preceding operations, but which operations relate to the decided data d −1  up to d −k  for the block IntCK. Two adders FRI and FII combine the contribution of all the blocks to produce the calculated interference signals (ISI) and (ISI). 
     The various calculations performed by these blocks will be explained with the following considerations: 
     In the communications in which radio broadcasting is used, the transmission channel is split up into L paths each bringing about on the one hand a delay of τ 1  and, on the other hand, a phase rotation φ 1  and an amplitude attenuation β 1 , so that the received signal y(t) coming from a transmitted signal s(t) will take these various disturbances and also the noise n(t) into account and will thus be written as:                y        (   t   )       =         ∑     l   =   1     L              β   1     ·          j                   ϕ   1                s        (     t   -     τ   1       )           +     n        (   t   )                 (   1   )                                
     The signal s(t) represents data d k  which undergo a spreading defined by a spreading code C(t-k.T s ), where T s  represents the duration of the transmitted symbol. This code is formed by values C j , so that there may be written:                C        (   t   )       =       ∑     j   =   0       j   =     K   -   1                C   j          δ        (     t   -     jT   c       )                   (   4   )                                
     where T c  represents the duration of the spreading code element and K the number of these spreading code elements. One may also write: T s  =KT c . The signal x(t) can then be written as:                y        (   t   )       =         ∑     l   =   1     L            β   1     ·          jϕ   1       ·       ∑     k   =     -   ∞         k   =     +   ∞                d     -   k       ·     C        (     t   -     τ   1     -     kT   S       )               +     n        (   t   )                 (   5   )                                
     Each branch of the Rake receiver effects a correlation, the output signal r(τ 1 ) of these branches is then written as:                r        (     τ   1     )       =         ∑     k   =     -   ∞         k   =     +   ∞                d     -   k       ·     w        (       τ   1     -     kT   S       )           +     η        (     τ   1     )                 (   6   )                                
     w(τ 1 +kT s ) is the channel impulse response at the time indicated in parentheses. This signal is split up into two parts: a useful signal r us , and a train signal r ps , which can be written as:                  r   us          (     τ   1     )       =       d   0     ·     w        (     τ   1     )                 (     7      a     )                   r     p                 s            (     τ   1     )       =       ∑     k   ≠   0              d   k          w        (       τ   1     +     kT   s       )                   (     7      b     )                                
     As the delays τ 1  are chosen in the time interval [ 0 , T s ], which thus implies that only the passed symbols will be taken into account, the equation (6) is rewritten as:                r        (     τ   1     )       =         ∑     k   =   0       k   =   K              d     -   k       ·     w        (       τ   1     -     kT   S       )           +     η        (     τ   1     )                 (   9   )                                
     K is chosen such that KT c  ≦τ L  τ L  represents the longest time which in fact is not fixed, because it depends on the physical environment. It may just be said that one may vary from a fraction of a symbol to various symbols. 
     According to certain standards (notably UMTS) τ L  is supposed to be equal to 256 T C , whence the factor K:              K   =         256   ·     T   C         SF   ·   TC       =     256   SF               (   10   )                                
     SF is the spreading factor that may be taken from the range running from 4 to 256. It should be noted that the duration of one symbol T s  for the information, which is not known a priori, may vary from 4 to 256 SF and that the duration of one symbol of a control channel, which is used for estimating the delays due to the paths, is fixed (it is equal to 256 in the case of UMTS and 128 in the case of IS95). 
     Based on the formula (9), the part r’(t) free from interference caused by already transmitted data may be evoked.                  r   1          (   t   )       =         r        (     τ   1     )       -       ∑     k   =   0       k   =   K                d   k     ·   w          (       τ   1     -     kT   S       )           +=         d   0          w        (     τ   1     )         +     η        (     τ   1     )                   (   11   )                                
     d 0  is the symbol one wishes to estimate and d −1 , d −2 , . . . d −k  are already estimated passed symbols. 
     The signal Z at the output of the adders ARI and AII, which results from the combination of the various branches of the Rake receiver and of the circuit  25 , is written as:              Z   =         ∑     l   =   1       l   =   L                w   *          (     τ   1     )       ·     r        (     τ   1     )           -       ∑     k   =   1       k   =   K              d     -   k       ·     (       ∑     l   =   1       l   =   L                w   *          (     τ   1     )       ·     w        (       τ   1     -     kT   s       )                           (   12   )               Z   =         ∑     l   =   1       l   =   L                w   *          (     τ   1     )       ·     r        (     τ   1     )           -       ∑     k   =   1       k   =   K              d     -   k       ·       R   k     .                   (   13   )                                
     where                R   k     =       ∑     l   =   1       l   =   L                w   *          (     τ   1     )       ·     w        (       τ   1     -     kT   s       )                   (   14   )                                
     The blocks IntCK perform the calculations indicated by this formula (14). 
     The various multipliers N 11 , . . . , N 15  up to NK 1 , . . . , NK 5  perform the operations situated after the minus sign of the equation (11). This formula (14) shows that an intercorrelation of the channel impulse response limited to [ 0 , T s ] and of the impulse response outside this interval (0,T s ) is effected.