Massive Unsourced Random Access: Exploiting Angular Domain Sparsity

TL;DR

This paper presents a novel unsourced random access scheme leveraging angular domain sparsity in MIMO channels, removing redundancies in coding to enhance spectral efficiency and detection accuracy.

Contribution

It introduces an uncoupled transmission protocol exploiting angular domain information and a new clustering decoder with a slot-balanced K-means algorithm for improved activity detection.

Findings

Achieves better error performance at high spectral efficiency.

Effectively exploits angular domain sparsity for activity detection.

Outperforms existing CCS-based URA schemes in simulations.

Abstract

This paper investigates the unsourced random access (URA) scheme to accommodate numerous machine-type users communicating to a base station equipped with multiple antennas. Existing works adopt a slotted transmission strategy to reduce system complexity; they operate under the framework of coupled compressed sensing (CCS) which concatenates an outer tree code to an inner compressed sensing code for slot-wise message stitching. We suggest that by exploiting the MIMO channel information in the angular domain, redundancies required by the tree encoder/decoder in CCS can be removed to improve spectral efficiency, thereby an uncoupled transmission protocol is devised. To perform activity detection and channel estimation, we propose an expectation-maximization-aided generalized approximate message passing algorithm with a Markov random field support structure, which captures the inherent clustered sparsity structure of the angular domain channel. Then, message reconstruction in the form of a clustering decoder is performed by recognizing slot-distributed channels of each active user based on similarity. We put forward the slot-balanced K-means algorithm as the kernel of the clustering decoder, resolving constraints and collisions specific to the application scene. Extensive simulations reveal that the proposed scheme achieves a better error performance at high spectral efficiency compared to the CCS-based URA schemes.