US RE41107 E1 Abstract A method of parallel suppression of interference, and corresponding stage and receiver is disclosed according to the invention, parallel suppression of interference is carried out starting from the signals selected by a maximum likelihood criterion based on the calculation of a metric and the search for the smallest possible metric.
Claims(18) 1. A method of receiving CDMA signals with parallel interference suppression in which:
a composite signal (r(t)) is received comprising a plurality of K signals corresponding to information symbols which have been spread in frequency by K different pseudo-random sequences,
these K signals are correlated using said K sequences,
the corresponding K symbols are estimated,
the K signals correlated in frequency are reconstructed by despreading re-spreading said estimated symbols through using the corresponding pseudo-random sequences,
the contributions of the other remaining (K−1) signals are subtracted from a despread spread signal to provide K new signals, spread in frequency but cleared, at least in part of the interference,
this method being characterized in that:
all the possible hypotheses on the signs of the NK correlated signals are formulated, where N is a whole number equal to 1 or to a few units greater than zero,
for each hypothesis, one calculates the distance metric between the group of correlated signals undergoing processing and the corresponding signals before processing,
the hypothesis for which the metric is the smallest is retained, being the hypothesis which has a maximum likelihood,
only those signals corresponding to this maximum likelihood hypothesis are reconstructed.
2. A parallel interference suppression stage implementing the method according to
K inputs receiving signals correlated in frequency,
K means of for estimating (ES_{1}, . . . , ES_{k}, . . . , ES_{K}) K symbols corresponding to these K signals,
K means of for reconstructing (R_{1}, . . . , R_{k}, . . . , R_{K}) signals respread in frequency using the corresponding pseudo-random sequences,
means of for parallel interference suppression comprising K channels in parallel capable of subtracting from one despread spread signal, the contributions of the other remaining (K−1) despread respread signals,
K outputs supplying K signals spread in frequency, cleared, at least in part of the interference,
this stage being characterized in that it comprises:
means (M), placed between the estimation means (ES_{1}, . . . , ES_{k}, . . . , ES_{K}) and the reconstruction means (R_{1}, . . . , R_{k}, . . . , R_{K},) and capable of for formulating all the possible hypotheses on the signs of NK correlated signals, where N is a whole number equal to 1 or to a few units greater than zero, and of for calculating, for each hypothesis, the distance metric (M_{j}) between the group of for correlated signals undergoing processing and the corresponding signals before processing, and of retaining the hypothesis (j) for which the metric (M_{j}) is the smallest, the hypothesis which offers a maximum likelihood.
3. Stage according to
means of for formulating two hypotheses on the sign to be assigned to the amplitude of the signals supplied by the means of for estimation,
means of for calculating all the differences Z_{0}(k)−Z_{i}(k)_{j}, where Z_{0}(k) represents the signal at the output from the k^{th }matched filter of the input stage and Z_{i}(k)_{j }the signal at the output from the k^{th }matched filter of the stage of row i, the signal being allocated the sign corresponding to each hypothesis j,
means of for calculating the square of these differences, or (Z_{0}(k)−Z_{i}(k)_{j})^{2}, means of for calculating the sum of these squares for all NK values of the signals, which leads, for each hypothesis (j), to the metric (M_{j}).
4. A receiver of CDMA signals that implements the method of
a general input (E) suitable for receiving a composite signal (r(t)) formed from a plurality of K signals corresponding to information symbols which have been spread in frequency by K different pseudo-random sequences,
an input stage with K channels in parallel each comprising filters (F_{01}, . . . , F_{0k}, . . . , F_{0K}) to correlate in frequency the composite signal (r(t)) through one of the K pseudo-random sequences, this stage supplying K signals correlated in frequency,
at least one parallel interference suppression stage (V_{1}, V_{2}, . . . ,),
filter stages positioned between the parallel interference suppression stages and comprising K filters (F_{1k}, F_{2k}, . . . ) matched to the pseudo-random sequences,
an output circuit (S) comprising K decision circuits (D_{1}, . . . , D_{k}, . . . , D_{K})
this receiver being characterized in that at least one of the parallel interference suppression stages is a stage according to claims 2 or 3.
5. A receiver according to
6. A receiver according to
7. A receiver according to
8. An apparatus, comprising:
a receiver to receive a composite signal (r(t)) comprising a plurality of K signals corresponding to information symbols which have been spread in frequency by K different pseudo-random sequences, said receiver further to correlate said K signals using said K sequences, an estimation means to estimate the corresponding K symbols, a reconstruction means to reconstruct the K signals correlated in frequency by re-spreading said estimated symbols using the corresponding pseudo-random sequences, wherein the contributions of the remaining (K−1 ) signals are to be subtracted from a spread signal to provide K new signals, spread in frequency but cleared, at least in part of the interference, wherein: all the possible hypotheses on the signs of the NK correlated signals are to be formulated, where N is a whole number greater than zero, for each hypothesis, the distance metric is to be calculated between the group of correlated signals undergoing processing and the corresponding signals before processing, the hypothesis for which the metric is smallest is to be retained, being the hypothesis which has a maximum likelihood, only those signals corresponding to this maximum likelihood hypothesis are to be reconstructed. 9. An apparatus, comprising:
at least one spread-spectrum parallel interference suppression processing stage to receive a received signal comprising K spread-spectrum signals spread with K codes, to despread and demodulate the K spread-spectrum signals in parallel to obtain K extracted symbols, and, for each of the K extracted symbols, to generate a new spread-spectrum signal using the corresponding one of the K codes, and further to obtain an estimate of each of the K spread-spectrum signals by subtracting a quantity based on the remaining K−1 new spread-spectrum signals from a previous input signal; wherein said processing stage includes: at least one estimator to provide amplitude estimates corresponding to the K extracted symbols; and at least one metric calculator to calculate at least one metric based on the amplitude estimates and to choose a set of K maximum likelihood symbols based on the at least one metric, the set of K maximum likelihood symbols to be used as the K extracted symbols to generate the new spread-spectrum signals. 10. The apparatus according to
a set of K parallel despreaders to receive the received signal and to provide K parallel inputs to a first one of said at least one processing stage. 11. The apparatus according to
12. The apparatus according to
at least one reliability testing device to test of each of the K extracted symbols to determine that a number, Q, of the K extracted symbols are reliable, wherein said metric calculator is to calculate at least one metric only on the K-Q extracted symbols not determined to be reliable. 13. The apparatus according to
at least one weighting circuit to weight at least one of the extracted symbols and a corresponding at least one of the set of K maximum likelihood symbols; and at least one adder to add the resulting corresponding weighted symbols to obtain at least one of the K extracted symbols to be used to generate the new spread-spectrum signals. 14. A method, comprising:
processing a received composite code-division multiple-access (CDMA) signal, comprising K spread-spectrum signals spread using K codes, said processing comprising despreading and demodulating the K spread-spectrum signals in parallel to obtain K extracted symbols, and, for each of the K extracted symbols, generating a new spread-spectrum signal using the corresponding one of the K codes, and said processing further comprising obtaining an estimate of each of the K spread-spectrum signals by subtracting a quantity based on the remaining K−1 new spread-spectrum signals from a previous input signal; wherein said processing includes: providing amplitude estimates corresponding to the K extracted symbols; and calculating at least one metric based on the amplitude estimates and choosing a set of K maximum likelihood symbols based on the at least one metric, the set of K maximum likelihood symbols to be used as the K extracted symbols to generate the new spread-spectrum signals. 15. The method according to
16. The method according to
performing said processing at least twice, each processing an initial processing utilizing results generated by the previous processing. 17. The method according to
determining a reliability of each of the K extracted symbols to determine that a number, Q, of the K extracted symbols are reliable, and calculating at least one metric only on the K-Q extracted symbols not determined to be reliable. 18. The method according to
weighting at least one of the extracted symbols and a corresponding at least one of the set of K maximum likelihood symbols and adding the resulting weighted symbols to obtain at least one of the K extracted symbols to be used to generate the new spread-spectrum signals. Description Notice: More than one reissue application has been filed for the reissue of U.S. Pat. No. 6,621,856. The reissue applications are application Ser. No. 11/229,448 (the present application) and application Ser. No. 11/497,504 (which was a continuation of the present application, and which has been abandoned). The subject of this invention is a method of receiving CDMA signals with parallel interference suppression, a corresponding stage and a corresponding receiver. It finds application notable in radiocommunication with mobiles. The technology of spectrum spreading by a direct sequence consists, schematically of multiplying an information symbol (for example a binary element) by a pseudo-random sequence (also called a code) made up of a sequence of elements called “chips”. This operation has the effect of spreading the spectrum of the signal. On reception, the received signal is processed by correlation (or matched filtering) with a pseudo-random sequence identical to that of the transmission, which has the effect of reducing (or correlating) the spectrum. The signal correlated in this way, is processed in order to recover the information symbol. This technique allows several users to access a single radiocommunications system, with the condition that they use distinct codes. One is then speaking of “Code Division Multiple Access” or CDMA for short. Despite offering numerous advantages, communications by spectrum spreading with code division multiple access are of limited capacity in terms of the number of users. This limitation is due to interference occurring between signals coming from different users. The more numerous the users are, the more important this interference phenomenon becomes. Various solutions have been proposed to remedy this disadvantage and, notably, the suppression (or at the very least the reduction) of interference. Hence, in American patent U.S. Pat. No. 5,218,619, for example, sequential suppression of the interference is recommended proceeding by decreasing order of power of the signals from the various users. In American patent U.S. Pat. No. 5,363,403 contrary to this, parallel suppression of these interference signals is recommended. As this invention again takes up this latter technique, we can break off there and illustrate the general structure of a receiver of this type. The receiver illustrated in the appended The receiver firstly comprises an input stage with means 101, 102, 103 capable of receiving the composite signal and of supplying a signal correlated by the code C_{1}, C_{2 }or C_{3 }appropriate to each channel; these means can consist of a correlator or a matched filter. Next the receiver comprises a parallel interference suppression stage 100 which comprises :
After the parallel interference suppression stage, there are three matched filters 201, 202, 203 working respectively with the codes C_{1}, C_{2}, C_{3 }and correlating the signals r_{1}, r_{2}, r_{3 }then an output stage 200 with three decision circuits 211, 212, 213 supplying the three data S_{1}, S_{2 }and S_{3}. Although giving satisfaction in certain regards, such receivers do not eliminate the risks of error. The suppression of interference, if it is carried out without precautions, can even increase this risk. The purpose of this invention is precisely to reduce this risk (in other words to reduce the bit error rate), by improving the means of reconstructing the signals before the actual interference suppression itself. With the invention, a single parallel interference suppression stage offers better performance than the traditional two suppression stages. In order to obtain this result in the interference suppression stage and to estimate the received data, the invention provides for the use of a particular criterion which is called “The Maximum Likelihood” criterion. This criterion is known of itself in CDMA techniques. One may find a description for example in the work by J. G. PROAKIS entitled “Digital Communications” McGRAW-HILL Inc., 3^{rd }edition, 1995, Chapter 5-1-4. However, in the prior art, this criterion is used in an ordinary receiver, and not in a means of parallel suppression of multiple access interference. Furthermore, in the prior art, this criterion is used with the aid of an algorithm called Viterbi's Algorithm, which allows one to find, through a lattice representing all possible configurations, a sequence of data which minimizes a quantity called the “Euclidean distance metric”. This technique, which takes into account the whole of the data transmitted by all users, is often very complex. This invention adapts this technique notably by simplifying it. Furthermore it defines a metric which is particularly suitable for the parallel suppression of multiple access interference. Put precisely, the subject of this invention is a method of receiving CDMA signals with parallel interference suppression in which:
Another subject of this invention is a parallel interference suppression stage that implements this method, this stage comprising:
Another subject of the invention is a receiver for CDMA signals that implements the method defined above and comprising:
Circuit V comprises K means ES_{1}, . . . , ES_{k}, . . . , ES_{K }for estimation of the transmitted signal which estimate the amplitude and the lag of each peak supplied by the matched filter which precedes it. The circuit next comprises means M to calculate the metrics (the precise expression for which will be given later), in order to determine the smallest metric and to supply the corresponding signal configuration, which is then the most likely. The circuit further comprises K means R_{1}, . . . , R_{k}, . . . , R_{K }to reconstruct the signals, that is to say to supply signals correlated in frequency by the pseudo-random codes. These reconstructed signals are then applied to a parallel interference suppression circuit, the structure of which is not shown but which comprises, as shown in Stage V is followed by matched filters F_{1}, . . . , F_{k}, . . . , F_{K }which permit input either to a new parallel interference suppression stage or to an output stage. In order to illustrate the operation of the means M of calculating the metrics, the simple case of a stage with two channels (therefore with two users) will be considered. It is also assumed that there are several parallel interference suppression stages, each marked by an index i, these stages following an input stage to each of which the index o has been allocated. In the stage with index i, the two means of estimating the amplitude of the transmitted signal, supply two signals marked Z_{i}(1) for the first channel and Z_{i}(2) for the second, while the two matched filters of the input stage supply signals Z_{o}(1) and Z_{o}(2). The circuit M considers the absolute value of the amplitudes of these signals, or |Z_{i}(1)| and |Z_{i}(2)| and formulates two hypotheses on the sign that can be allocated to these values, namely + or −. There are therefore 2_{2}=4 hypotheses for the groups of two signals taken with their sign, these four hypotheses (designated (Hyp)_{j}) being labeled with an index j that goes from 1 to 4. The four configurations corresponding to these four hypotheses are the following:
According to classical notation, each group of two signals can be considered as the two components of a vector designated ({right arrow over (Z)}_{i}). Therefore there are four possible vectors according to the retained hypothesis, namely:
The invention uses a Euclidean distance metric, afterwards referred to as the metric, of the form
The following four metrics, corresponding to the four formulated hypotheses, will therefore be calculated:
The smallest of these metrics corresponds to the configuration closest to the configuration at the output from the input stage and hence to the most likely configuration. If, for example, the smallest metric is the third one M_{3}, the most likely configuration will be:
The means M will then supply the signals −|Z_{i}(1)| and +|Z_{i}(2) | and the two reconstitution circuits which follow it will spread these signals using the two appropriate pseudo-random sequences. The traditional means of parallel interference suppression will then receive the spread signals of maximum likelihood and will then be able to correct these signals in an optimum way. In a general way, the means M for a stage of row i calculates the quantity
When N =1, there are only K components to be processed (the case referred to as a single symbol block) and the number of hypotheses to be formulated is 2^{K}. With NK components, this number rises to 2^{NK}. To prevent too much complexity, N is limited to a few units, for example, less than 5. In order to illustrate the variations in value taken by the metric as a function of the hypotheses made on the signs, we may consider the case of three users, each using pseudo-random sequences each with 63 elements or chips, the modulation employed being differential type modulation with quaternary phase modulation (DQPSK) with two channels per user, namely one channel in phase (called I) and one channel in phase quadrature (called Q). There are therefore 6 channels in parallel, or 2^{6}=32 possible hypotheses on the signs of a single symbol block. These 32 hypotheses or configurations are labeled by their row in the diagram in The method and the receiver that have just been described assume, for the totally general case, that 2^{NK }hypotheses are formulated. The complexity of the method can naturally be reduced by reducing the block of data with K data (a single symbol block mentioned above). However this complexity can be further reduced, in the method of seeking the maximum likelihood, by only taking into account those signals for which the estimation is judged to have little reliability or to put it another way by excluding from the method those signals judged to be reliable. Assuming that Q signals are reliable, only K−Q signals will be retained for the calculation of the metrics, which corresponds to 2^{K−Q }hypotheses. Means of measuring reliability are described and claimed in French patent application No. 98 09782 filed by the present applicant on the Jul. 30^{th }1998. However other criteria of reliability can be used, such as those which are described in patent U.S. Pat. No. 5,644,592. Naturally, several of these simplified stages can be cascaded, as for FIG. 3. In another particular embodiment, the signals supplied by the matched filters can be linearly combined before they are addressed to the output stage. One can see in The weighting coefficients can be fixed or variable. Such a technique is described in U.S. Pat. No. 5,553,062. One can also improve the reconstructions and estimations of the signals by using the reliability thresholds in order to reconstruct or not to reconstruct (or to only partially reconstruct) certain signals. Such a technique is described and claimed in the French patent application No. 98 03586 filed on the Mar. 24^{th }1998 by the present applicant. A technique of this kind is also described in the patent U.S. Pat. No. 5,644,592. The performance of a receiver according to the invention has been simulated by the applicant. To do this, certain hypotheses have been formulated for the pulse response of the propagation channel. Firstly, one can consider an ideal pulse response which would be formed by a single peak, which would correspond to an absence of multiple paths. However, one can also choose a more realistic hypothesis, illustrated in
It can be seen that the invention leads to a significant improvement in performance. In particular, a single stage of parallel interference suppression (in the simplified version) offers better performance than the traditional two stages. Patent Citations
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