Study On The Key Technology Of Cellular Mobile Communication System Based On OFDM

Data transmission reliably with high-speed rate in execrable wireless environmentis the goal of the future cellular mobile communication system. As a multicarriercommunication technology, Orthogonal Frequency Division Multiplex (OFDM) isprovided with excellent performance of antimultipath, and then can provide very highdata throughput. The complexity for realizing transmitter and receiver decreases greatlysince taking advantage of Discrete Fourier Transform (DFT). Multicarrier-CodeDivision Multiple Access (MC-CDMA) is the best combination of OFDM and CDMA(Code Division Multiple Access), thus advantages of both techniques are exploited, andthat capacity and performance of mobile communication system essentially depend onthe choice of the multiple access scheme. In MC-CDMA system, each data of thesubscriber is modulated by OFDM after spreading in domain, and then the data symbolis transmitted in parallel on all subcarriers, hence resulting in the advantage offrequency diversity. The cellular mobile communication system based on OFDM takeson well prospect for future business application. The significant research results from this dissertation manifest that CDMA isorthogonal code division multiplex technique, at the same time, OFDM is orthogonalfrequency division multiplex technique. Both of the transmitted radio waves areGaussian white noise approximately which is best signal meeting Shannon theorem inGaussian channel and multi-path fading channel. This thesis investigates theperformance of MC-CDMA system in Rayleigh and Rician channel respectivelyadopting Equal Gain Combination (EGC) technique and Maximum Ratio Combination(MRC) technique respectively, afterwards a general matrix analyzing model and noveltheoretical bit error rate (BER) formulas of MC-CDMA is proposed. For engineering application, OFDM system must solve two pivotal issues, thePeak-to-Average Power Ratio (PAPR) problem and the synchronization problem. Afterdefinition, three ways to reducing PAPR is studied, Amplitude Clipping, SelectedMapping, Partial Transmit Sequences. Subsequently, synchronization in time domainand in frequency domain is discussed due to the specialty of Cyclic Prefix (CP) addedinto system. And then a synchronous method in time domain based on statistics isintroduced. Another specialty of MC-CDMA system is selecting spread spectrum code to III重庆大学博士论文reducing PAPR. In this paper, a preferred method for selecting spread spectrumsequence is proposed, that is considering crest factor (CF), correlation function andspectrum correlation function all together. By reason of the characteristics of Golaysequence, CF less than 6dB, poor correlation and poor spectrum correlation, this thesiswill improve the Golay sequence making use of m sequence and Gold sequence.MC-CDMA system based on the rule put forward above have the advantage of lowcomputational complexity, therefore is suitable for practicality.