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

Abstract

We propose a novel joint activity, channel and data estimation (JACDE) scheme for cell-free multiple-input multipleoutput (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.

Joint Device Detection, Channel Estimation, and Data Decoding with Collision Resolution for MIMO Massive Unsourced Random Access

“…We note this data-aided CE algorithm appears to be similar to the work in [14], which employs the data as soft pilots to conduct CE jointly with the real pilot sequences. However, the convergence and correctness of the estimated data cannot be verified, which may result in the propagation of errors and failure of the joint data and channel estimation.…”

“…According to the formulation in (8), the recovery of the sparse matrix X can be addressed by the CS-based methods, such as the AMP algorithm and its variants [4], [8]- [11]. Besides, the MP-based approaches [13], [14] also work well on the above issues. The Bernoulli and Rician messages are jointly updated in [13], which considers the fading channel and grant-free scenario.…”

“…The Bernoulli and Rician messages are jointly updated in [13], which considers the fading channel and grant-free scenario. This scenario is also considered in [14], which takes advantage of estimated data symbols as soft pilot sequences to perform joint channel and data estimation. In this subsection, we consider the MIMO system and derive the update rules of messages based on the BP algorithm.…”