| Code Division Multiple Access system has multiple access capability, the stability of the decline, the strong anti-jamming capabilities and can achieve soft switching, and other characteristics through using the technology of launching the broad-band signal which is expanded by the narrow-band signal multiplied by spread spectrum signal. So it has been widely paid attention to and successfully applied to the second generation of mobile communication. But because of the MAI and the ISI, it is difficult for CDMA to achieve high-speed transmission. With the development of mobile communications, business needs develop from a single voice service to the high-speed multimedia services and at this time the traditional CDMA technology will not be competent. Based on this, we must find a new solution which is more suitable for high-speed data transmission and has stronger anti-jamming capability. OFDM make frequency balance achieved more easily by using the characteristics of the multi-path effect in the frequency domain and can effectively fight against frequency selective fading.MC-CDMA is in this context that came into being. It combines OFDM and CDMA and own both advantages. In the transmission of high-speed data, the performance of resisting symbol interference is better than the traditional single-carrier CDMA. In the transmitter and receiver of the system, it is very easy to produce and the restoration of the multi-carrier signals by using Fast Fourier Transform (FFT) without increasing the complexity of the transmitter and receiver. And because the sub-carrier spectrums overlap, it make MC-CDMA have high efficiency on the use of the spectrum.This paper is to explore the multi-user detection technology in the MC-CDMA system which is mainly used for resisting multiple access interference (MAI). The previous studies pay more attention to the inhibition of the MAI in the downlink. Actually using the multi-user detection technology will increase the complexity of the receiver. So in the mobile terminal side it is rarely used due to the limited of the equipment. The actual value of the study in the downlink of the system is not high. However in the uplink, because of the faultiness of the power control and the non-synchronous transmission, MAI is much serious. So it should be paid more attention to. In this paper, we take the technical difficulties of the uplink into account, consider the multi-user detection algorithm combined with the sequential hypothesis testing theory, and apply it to the receiver of the DS-CDMA system to detect and judge the signals, inhibit the MAI. At the same time we could detect the useful signal more accurately and make the bit error rate of the system lower. Through in-depth analysis of the characteristics of the signals which passed through the multi-path fading channel, we make full use of the interference user information which have already been mastered by the base stations, get the expression of the detected statistics variable, identify the executive property of the sequential likelihood ratio test and the two thresholds for the judgement of the signals, detect and judge the symbols of the signals reached the receiver with unfixed length, at last resume the final user information. After the systemic simulation in Matlab, we get better BER performance so that in the next step apply it to the MC-CDMA system. In the MC-CDMA system, considering the special nature of multi-carrier modulation for the data sources in the transmitter, revise the expression of the detected statistics variable, the executive property of the sequential likelihood ratio test and the two thresholds, then simulate it in the Matlab and get the graph of the BER of system. Compared with other multi-user detection algorithms, the BER performance of this algorithm gets improved and no apparent increase in system complexity. It has made a very good balance between the algorithm performance and the complexity of the system and has a certain value both in theory and application. |
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