Quasi-large sparse-sequence CDMA: Approach to single-user bound by linearly-complex LAS detectors

We have proposed a quasi-large random- sequence (QLRS) CDMA where K users access a point through a common channel with spectral spreading factor N. Each bit is extended by a temporal spreading factor B and hopped on a BN-chip random sequence that is spread in time and frequency. Each user multiplexes and transmits B extended bits and the total channel load is a = K/N bits/s/Hz. The linearly-complex LAS detectors detect the transmitted bits. We have obtained that as B rarr infin, if alpha < 1/2 - 1/(41n2), each transmitted bit achieves the single-bit bound in BER in high SNR regime as if there was no interference bit. In simulation, when bit number BK ges 500, each bit can approach the single-bit bound for alpha as high as 1 bit/s/Hz. In this paper, we further propose the quasi- large sparse-sequence (QLSS) CDMA by replacing the dense sequence in QLRS-CDMA with sparse sequence. Simulation results show that when the nonzero chips are as few as 16, the BER is already near that of QLRS- CDMA while the complexity is significantly reduced due to sequence sparsity.