Grant-Free Access via Bilinear Inference for Cell-Free MIMO with Low-Coherent Pilots

TL;DR

This paper introduces a joint activity, channel, and data estimation scheme for cell-free MIMO systems that enables grant-free access with low overhead, leveraging Bayesian message passing and structured low-coherent pilots to improve performance.

Contribution

It presents a novel Bayesian inference-based JACDE scheme with low-coherent pilots for grant-free cell-free MIMO, reducing overhead while maintaining reliable detection.

Findings

Significantly outperforms existing methods in simulations.

Approaches the performance of ideal genie-aided schemes.

Effectively detects active users and estimates channels without spreading data.

Abstract

We propose a novel joint activity, channel, and data estimation (JACDE) scheme for cell-free multiple-input multiple-output (MIMO) systems compliant with fifth-generation (5G) new radio (NR) orthogonal frequency-division multiplexing (OFDM) signaling. The contribution aims to allow significant overhead reduction of cell-free MIMO systems by enabling grant-free access, while maintaining moderate throughput per user. To that end, we extend the conventional MIMO OFDM protocol so as to incorporate activity detection capability without resorting to spreading informative data symbols, in contrast with related work which typically relies on signal spreading. Our method leverages a Bayesian message passing scheme based on Gaussian approximation, which jointly performs active user detection (AUD), channel estimation (CE), and multi-user detection (MUD), incorporating also a well-structured low-coherent pilot design based on frame theory, which mitigates pilot contamination, and finally complemented with a detector empowered by bilinear message passing. The efficacy of the resulting JACDE-based grant-free access scheme without spreading data sequences is demonstrated by simulation results, which are shown to significantly outperform the current state-of-the-art and approach the performance of an idealized (genie-aided) scheme in which user activity and channel coefficients are perfectly known.