Quasi-Large Sparse-Sequence CDMA: Approach to Single-User Bound by Linearly-Complex LAS Detectors

TL;DR

This paper introduces quasi-large sparse-sequence CDMA, demonstrating that with sparse sequences and high spreading factors, near single-user performance is achievable with reduced complexity, approaching theoretical bounds in high SNR regimes.

Contribution

It proposes a novel sparse-sequence CDMA scheme that maintains near-optimal performance while significantly reducing detection complexity compared to dense sequences.

Findings

Sparse sequences achieve near single-user bounds at high SNR.

Reduced complexity with as few as 16 nonzero chips.

Performance approaches dense sequence CDMA with fewer active chips.

Abstract

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 alpha = K/N bits/s/Hz. The linearly-complex LAS detectors detect the transmitted bits. We have obtained that as B tends to infinity, if alpha < 1/2 - 1/(4ln2), 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 >= 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.