Crest-factor study of MC-CDMA and OFDM
B.J. Choi, Ee-Lin Kuan, L. Hanzo · 1999
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-CDMA 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 (1972) 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-CDMA 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-CDMA 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.