Distributed Iterative ML and Message Passing for Grant-Free Cell-Free Massive MIMO Systems

TL;DR

This paper introduces a distributed iterative machine learning and message passing framework for grant-free cell-free massive MIMO systems, addressing activity detection, channel estimation, and pilot contamination in IoT scenarios.

Contribution

It proposes a novel distributed algorithm combining iterative ML activity detection with PP-VB-EP for joint data detection and channel estimation, improving convergence and robustness.

Findings

Enhanced convergence of PP-VB-EP compared to conventional VB-EP.

Reduced sensitivity to initialization in the proposed algorithm.

Distributed pseudo prior computation via VL-EP algorithm.

Abstract

Cell-Free (CF) Massive Multiple-Input Multiple-Output (MaMIMO) is considered one of the leading candidates for enabling next-generation wireless communication. With the growing interest in the Internet of Things (IoT), the Grant-Free (GF) access scheme has emerged as a promising solution to support massive device connectivity. The integration of GF and CF-MaMIMO introduces significant challenges, particularly in designing distributed algorithms for activity detection and pilot contamination mitigation. In this paper, we propose a distributed algorithm that addresses these challenges. Our method first employs a component-wise iterative distributed Maximum Likelihood (ML) approach for activity detection, which considers both the pilot and data portions of the received signal. This is followed by a Pseudo-Prior Hybrid Variational Bayes and Expectation Propagation (PP-VB-EP) algorithm for joint data detection and channel estimation. Compared to conventional VB-EP, the proposed PP-VB-EP demonstrates improved convergence behavior and reduced sensitivity to initialization, especially when data symbols are drawn from a finite alphabet. The pseudo prior used in PP-VB-EP acts as an approximated posterior and serves as a regularization term that prevents the Message Passing (MP) algorithm from diverging. To compute the pseudo prior in a distributed fashion, we further develop a distributed version of the Variable-Level Expectation Propagation (VL-EP) algorithm.