Optimization of phase sequences design for TDCS-based multiple access system

Transform domain communication system (TDCS) has been proposed to support multiuser communications through spectrum bin nulling and frequency domain spreading for overlay cognitive radio networks. In TDCS systems, specific pseudorandom spreading sequences are assigned to different users for the purpose of multiple access. However, due to the non-zero cross-correlations between any pair of TDCS signals, multiuser interference always exists and becomes a main factor of bit error rate performance degradation. To improve multiple access capability, in this paper, a novel spread phase coding scheme is introduced for multiuser TDCS systems. It is proved that the key point is the design of sequence with impulse-like correlation. In addition, a new algorithm to design unimodular sequence is proposed. Following this way, we can obtain a class of almost perfect sequences through spreading, whose autocorrelation and cross-correlations are much better than that of traditional sequences. Simulation results demonstrate that multiuser TDCS systems employing our proposed sequences can achieve a significant BER performance improvement.