MC-CDMA Key Technologies Research In Mobile Communication Systems

MC-CDMA which combines the strongpoints of CDMA and OFDM has the advantages of high frequency efficiency and excellent capacity to resist frequency selective shadow. MC-CDMA has widely application value in mobile systems, wireless access, satellite communications and UWB communications . And now, people have considered MC-CDMA as the effective solution scheme in the 4G communication systems. The key technologys such as the resistence of high PAPR,channel estimation and system synchronization are deeply researched in this thesis.First of all, the three multiple access schemes based on a combination of code division and multi-carrier techniques were introduced in this thesis. The transmitter model,channel model and the receiver modes were detaily expatiated and the frequency efficiency,bit error rate(BER) performance and system capacity were analyzed.Secondly, the thesis investigated the techniques for PAPR reduction. Based on the model of high power amplifier (HPA), the in-band distortion and out-of-band noise introduced by HPA nonlinearity were analyzed and the influences on the system performances by high PAPR were described. Three kinds of PAPR reduction methods in universal OFDM systems and two methods aiming at MC-CDMA are introduced and their advantages and disadvantages were also analyzed. The correlation expressions of PAPR and user spreading sequence were deduced and the PAPR of MC-CDMA with and without virtual carriers were researched. From the conclusion, the uplink and downlink of MC-CDMA has better PAPR performance and simple system structure when using binary Golay complementary sequence. Contraposing the shortcoming of traditional Golay sequence extension methods, a fastly construction method of binary orthogonal complementary Golay sequence sets was presented. The theoretical proof of the method and computer simulation were also put forword.Thirdly, the thesis investigated the channel estimation algorithms in MC-CDMA. The wireless channel characteristics and the MC-CDMA channel model were analyzed. A modified LS channel estimation algorithm based on virtual carriers was presented. the The advantage of low complexity of LS algorithm was retained and the channel estimation performance was remarkablely improved. The blind MC-CDMA channel estimation has the disadvantage of high compution complexity and slow convergence. According to the fact that the dimension of signal subspace is far more less than that of noise subspace in MC-CDMA systems, a blind channel estimation algorithm based on extented PASTd signal subspace tracking was presented in this thessis. The channel estimation precision was remarkablely increased through reorthogonalizing the result of PASTd subspace tracking before channel estimation and therefore the practical value of the blind channel estimation algorithm was improved.At last, the thesis investigted the timing and carrier synchronization algorithms. The effects resulting from the timing and frequency offset were analyzed. Several traditional timing and frequency synchronization algorithms were discribed and their performances were analyzed. A system synchronization algorithm based on user's spreading sequence was presented. In the algorithm, the integer frequency offset was estimated through dividing the frequency into a number of blocks according to the time offset and correlating the received FFT sequences with the user's spreading sequence. The precise timing was obtained through correlating the receiver sequences after frequency offset compensation with the conference sequences in time domain. The system synchronization algorithm has high data transmission efficiency and the synchronization performance was validated by computer simulation.In the last section, the main contribution of the thesis was summarized and the further research was pointed out.