Grant-Free Access via Bilinear Inference for Cell-Free MIMO With Low-Coherence Pilots

Abstract

We propose a novel joint activity, channel and data estimation (JACDE) scheme for multiple-input multiple-output (MIMO) systems. The contribution aims to allow significant overhead reduction of MIMO systems by enabling grant-free access, while maintaining moderate throughput per user. To that end, we extend the conventional MIMO transmission framework 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-coherence 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 JACDEbased grant-free access scheme in the cell-free MIMO system setup compliant with fifth generation (5G) new radio (NR) orthogonal frequency-division multiplexing (OFDM) signaling is demonstrated by simulation results. The results are shown to 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.

User-Centric Design of Millimeter Wave Communications for Beyond 5G and 6G

“…As described in [25], this granting procedure results in a latency of around 9.5 millisecond (ms), which hinders meeting the strict latency requirements of some IoT use cases [26]. Hence, grant-free access techniques such as 𝐾-repetition [27], variants of diversity slotted ALOHA [28][29][30] and code-domain GF-NOMA [25,[31][32][33][34] have been actively investigated to meet these low-latency re-quirements. Code-domain GF-NOMA can allow more users to transmit simultaneously without the BS's granting pro-cess and can theoretically achieve the most powerefficient communications [35].…”

Joint Channel Estimation, Activity Detection and Decoding using Dynamic Message-Scheduling for Machine-Type Communications

“…Thus, exact message passing based on the sum-product rule is too complicated to implement, while the computational cost is impractical for the mMTC scenario. Thus, an alternative applied in recent years [65], [71], [72] is to approximate these quantities using loopy belief propagation (LBP) [73]. In LBP, beliefs about the random variables are exchanged between the nodes of the factor graph until they converge.…”

“…P(x, c, s) N c (x|ζ, ν q ) dx dζ,(72) and h = h P(h; ρ) N c (h|ζ, ν r ) dh (h|ρ) N c (h|ζ, ν r ) dh dζ.…”

Blind Multiple Measurement Vector AMP Based on Expectation Maximization for Grant-Free NOMA

“…The decision rule of (24) can be derived using certain approximations. First, according to (8), γ t,n can be approximated as…”

“…To overcome this difficulty, schemes exploiting the inherent sparsity in sporadic traffic have been actively investigated. For instance, approaches based on message passing mechanisms have been proposed [4]- [8], performing AUD and CE with low computational complexity. However, many of these approaches, e.g., multiple measurement vector approximate message passing (MMV-AMP) [4]- [6], require large-scale fading (LSF) coefficients of all users as prior knowledge of the wireless channels that is likely to be unavailable at the BS ahead of the uplink transmission.…”