| Multi-User Detection(MUD) technique is the most effective anti-MAI techniquein CDMA system. It is a method of anti-interference based on the receiver's design.It can demodulate the desired user's signal reliably by making full use of otherusers' information. While space-time processing reduces the interference in virtueof the signal's spacial property obtained from the antenna array. Combining thetwo techniques into space-time MUD (ST-MUD) is heatedly discussed in recentstudies. Compared with FDMA and TDMA, CDMA is superior in high capacity,anti-interference, frequency application, low transmitting power and high privacy,etc., which makes CDMA become the technical foundation of IMT-2000. However,in CDMA system, all users share the same time and frequency band and thereceiving signal is the sum of all users' signal. Different users' signal is correlated,because of asynchronous and multipath spreading of wireless signal. Although theMAI produced by individual user is small, it will become the primary interference,with the increase of the users' number and the signal's power. The existence ofMAI limits the capacity of CDMA, and affects other aspects of its superiority.Hence the study of various anti-MAI techniques has great significance. Optimum ST-MUD based on Viterbi algorithm can't be implemented in realtime because its computational complexity increases in exponent with the increaseof the users' number, which restricts its application and some sub-optimumST-MUDs appear. MMSE ST-MUD and Decorrelation ST-MUD are two typicalsub-optimum linear ST-MUDs. Their computational complexity lessens greatlywhile their performances are close to optimum ST-MUD's. However, decorrelationST-MUD involves computing the inverse of a matrix, and the operation amount isrelatively great. In addition, the operation of decorrelation makes the ambientnoises strong. MMSE ST-MUD compromises between eliminating MAI andbuilding up no more ambient noises, but it needs to estimate the signal amplitude, 66ABSTRACTand the performance of "near-far" resistance is not so good as DecorrelatingST-MUD and the computational complexity is close to it. Take the above problems into consideration, the following researches are carriedout: 1. Studying the decorrelating ST-MUD technique and proposed a de-biasingdecorrelating ST-MUD, whose computational complexity is comparatively smallwhile performances are superior to the ordinary decorrelating ST-MUD. In thispaper, DS-CDMA (Direct Sequence Spreading CDMA) system is adopted. Signalspreads in multipath channel and it is assumed the multipath delay of each symbolis confined in one symbol's period. Under this assumption, the information of eachsymbol is in the period of three continuous symbol centered on it, i.e. ISI onlycomes from these three bits. The de-biasing decorrelating ST-MUD takes theefficient statistics of three continuous symbol and decorrelating them, and thentakes the middle bit as the estimation. If we take these three bits as the estimationand don't take the effect of error into account, a biasing decorrelating ST-MUD isobtained. In addition, the burden of work out the inverse of a matrix is heavy, sowe adopted the Gauss-Seidel iteration algorithm which is suitable for the matrix inthis paper and is superior in the convergence. Therefore we obtained a de-biasingdecorrelating ST-MUD which is simple-structured and low computationalcomplexity. The emulation experiments which have been done are as follows: 1) Comparing the BER (bit error rate) of each user when he adopt the followingthree detectors relatively: biasing decorrelating ST-MUD, de-biasing decorrelatingST-MUD and space-time matched filter. In the experiment two low-powered usersare recorded because they can test the detectors' performances more efficiently.Through analyzing of the results of the experimental we draw the conclusion thatthe de-biasing decorrelating ST-M...
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