Crest-Factor Study of MC-CDMA and OFDM

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Description/Abstract

The crest factor (CF) of M-ary PSK MC-CDMA systems is shown to be dependent upon the aperiodic cross-correlations as well as on the aperiodic autocorrelations of the spreading codes used. Based on this observation, the crest factor distributions of BPSK MC-COMA using two different binary orthogonal spreading sequences namely Walsh codes and orthogonal Gold codes - as well as two polyphase orthogonal sequences - Frank codes and Zadoff-Chu codes - are compared. against that of BPSK OFDM. In a 'lightly loaded' system the polyphase sequences and the orthogonal Gold codes are shown to have better CF distributions. However, in a 'fully loaded' system, Walsh codes exhibit the best crest factor distribution amongst the investigated codes. MC-COMA using Walsh codes was found to exhibit a peak factor reduction of 1.5 over OFDM, when the number of subcarriers Is 16 and a rate 15/16 CF reduction block code Is used, while maintaining the same spectral efficiency. We found that Zadoff-Chu spread MC-COMA showed the best BER performance amongst the considered systems in Rayleigh channels, when Joint-Detection was employed, despite Its heavier clipping than that of the Walsh spread scheme. This is due to its low multiple user interference.