Performance Study Of Synchronous And Quasi-Synchronous MC-CDMA Systems Based On The Walsh Sequence

As one of the combining schemes of Orthogonal Frequency Division Multiplexing(OFDM) and Code Division Multiple Access(CDMA) techniques, multicarrier CDMA (MC-CDMA) applies OFDM technique to CDMA systems so as to decrease the data rate of CDMA systems and enhance the capability of high data rate service processing. Due to the advantages inherited from OFDM and MC-CDMA techniques, MC-CDMA technique exhibits unique advantages in the resistance of frequency selective channel and inter-symbol interference as well as the high spectral efficiency. In addition, in MC-CDMA system, because the multicarrier modulation and demodulation can be implemented by means of Inverse discrete Fourier transform(IDFT) and DFT, the system complexity is reduced significantly. As a result, MC-CDMA is widely considered as one of the most competitive techniques of the future 4G wireless communication systems.Firstly, the fundamental principles and advantages/advantages of DS-CDMA and OFDM techniques are introduced. Then three solutions of OFDM-CDMA techniques are elaborated and analyzed. On the basis of the analyzing and comparing results of these three solutions, it can be concluded that MC-CDMA is superior to the other systems in terms of the overall performance, which can be used as the reference of the practical utilization of OFDM-CDMA technique.Secondly, the system model of the downlink MC-CDMA system imploying Walsh sequence, including the transmitter, multipath fading channel and receiver, is investigated, on the basis of which, the bit error rate(BER) performance of the system is presented in terms of both numerical and simulation results.Moreover, the system model of Quasi-Synchronous(QS) MC-CDMA system is presented, based on which, the BER performance analyzing results of the MC-CDMA system employing WH sequence are provided. By discussing the spectral correlation properties of Walsh Hadamard (WH) and Gold sequences, it shows that because the WH code subsets exhibit low spectral correlation zone(LSCZ) property, the multiple access interference(MAI) in QS MC-CDMA system can be suppressed effectively and as a result the system error performance can be improved.