Multi-carrier circular-shift division multiple access for multi-user wireless systems

In this paper, a multi-carrier circular-shift division multiple access (MC-CSDMA) scheme is proposed for multiuser wireless communication systems. At the transmitter, each modulated symbol is first spread onto a subset of M subcarriers in the frequency domain through simple repetition codes. The obtained signals are then converted to the time domain, where users circularly shift their respective time domain signals by different numbers of samples. Signals from multiple users are transmitted over the same frequency at the same time. The time domain circular shifting operations render a special signal structure that allows the receiver to extract the signals from different users with zero or a small amount of multiple access interference (MAI). If the number of users is no more than M, which is the number of subcarriers bearing the same modulated symbol, then there are at most L mutually interfering users at any given subcarrier, where L is the length of the equivalent discrete-time frequency selective fading channel. Furthermore, if the number of users is no more than M/L, then MAI-free communications can be achieved. Both analytical and simulation results show that full multipath diversity gain can be achieved by the proposed MC-CSDMA scheme regardless of the presence of MAI, thus it achieves a significant performance gains over conventional orthogonal frequency division multiple access (OFDMA) with the same spectral efficiency.