Coded Orthogonal Modulation for the Multi-Antenna Multiple-Access Channel

TL;DR

This paper introduces COMMA, a low-complexity coded orthogonal modulation scheme for multi-antenna multiple-access channels, improving spectral efficiency and robustness with theoretical bounds and practical FFT-based implementation.

Contribution

It proposes COMMA, a novel low-complexity coding and decoding scheme for multi-antenna multiple access, with theoretical analysis and FFT-based receiver design.

Findings

COMMA outperforms linear multiuser detection with Gaussian signaling.

Derived bounds and scaling laws relate antennas, users, SNR, and spectral efficiency.

FFT-based implementation offers low complexity and robustness to offsets.

Abstract

This study focuses on (traditional and unsourced) multiple-access communication over a single transmit and multiple ($M$) receive antennas. We assume full or partial channel state information (CSI) at the receiver. It is known that to fully achieve the fundamental limits (even asymptotically) the decoder needs to jointly estimate all user codewords, doing which directly is computationally infeasible. We propose a low-complexity solution, termed coded orthogonal modulation multiple-access (COMMA), in which users first encode their messages via a long (multi-user interference aware) outer code operating over a $q$-ary alphabet. These symbols are modulated onto $q$ orthogonal waveforms. At the decoder a multiple-measurement vector approximate message passing (MMV-AMP) algorithm estimates several candidates (out of $q$) for each user, with the remaining uncertainty resolved by the single-user outer decoders. Numerically, we show that COMMA outperforms a standard solution based on linear multiuser detection (MUD) with Gaussian signaling. Theoretically, we derive bounds and scaling laws for $M$, the number of users $K_a$, SNR, and $q$, allowing to quantify the trade-off between receive antennas and spectral efficiency. The orthogonal signaling scheme is applicable to unsourced random access and, with chirp sequences as basis, allows for low-complexity fast Fourier transform (FFT) based receivers that are resilient to frequency and timing offsets.