Team MMSE Precoding with Applications to Cell-free Massive MIMO

Abstract

This article studies a novel distributed precoding design, coined team minimum mean-square error (TMMSE) precoding, which rigorously generalizes classical centralized MMSE precoding to distributed operations based on transmitter-specific channel state information (CSIT). Building on the so-called theory of teams, we derive a set of necessary and sufficient conditions for optimal TMMSE precoding, in the form of an infinite dimensional linear system of equations. These optimality conditions are further specialized to cell-free massive MIMO networks, and explicitly solved for two important examples, i.e., the classical case of local CSIT and the case of unidirectional CSIT sharing along a serial fronthaul. The latter case is relevant, e.g., for the recently proposed radio stripe concept and the related advances on sequential processing exploiting serial connections. In both cases, our optimal design outperforms the heuristic methods that are known from the previous literature. Duality arguments and numerical simulations validate the effectiveness of the proposed team theoretical approach in terms of ergodic achievable rates under a sum-power constraint. I. INTRODUCTIONInter-cell interference is a major limiting factor of wireless communication systems capitalizing on aggressive spectrum reuse and network densification to increase capacity. To mitigate this effect, future generation systems are expected to implement advanced cooperative communication techniques, in particular by letting geographically distributed base stations jointly serve their users. However, the practical deployment of cooperative wireless networks is currently prevented by the severe scalability issue arising from network-wide processing [2]. Specifically, the excessive amount of data and channel state information (CSI) to be timely A preliminary version of this work has been submitted to IEEE SPAWC 2021 [1].

Cell-Free Massive MIMO for 6G Wireless Communication Networks

“…Maximum ratio transmission (MRT) precoding [10] Enhanced MRT precoding [35] Zero-forcing (ZF) precoding [36] Local ZF precoding [37] Team precoding [38] Cooperative precoding [39] Power control and allocation…”

“…In Ref. [38], a novel distributed precoding design, which generalizes the classical centralized MMSE precoding to distributed processing in cell-free massive MIMO, was proposed. Based on the theory of teams, a set of necessary and sufficient conditions for optimal team MMSE (TMMSE) precoding was derived.…”

Team Precoding Towards Scalable Cell-free Massive MIMO Networks

“…On top of being heuristic, this approach is often not general enough, in the sense that it can be applied only to very specific CSI sharing patterns similar to the fully distributed setup in [1]. This long-lasting problem has been solved only recently in [12] using the so-called team MMSE (TMMSE) method. Building on powerful multi-agent control theoretical tools, and focusing on simple achievable rate bounds [5], [6], the TMMSE method provides rigorous yet practical guidelines for optimal distributed precoding design under partial CSI sharing.…”

“…Building on powerful multi-agent control theoretical tools, and focusing on simple achievable rate bounds [5], [6], the TMMSE method provides rigorous yet practical guidelines for optimal distributed precoding design under partial CSI sharing. As an important application, [12] derives the closed-form optimal local precoders for the fully distributed setup studied in [1], improving upon previously known heuristics especially in the presence of pilot contamination and/or line-of-sight (LoS) components. Furthermore, [12] also derives the closed-form optimal precoders assuming that CSI is shared unidirectionally along a serial fronthaul, leading to an efficient recursive algorithm suitable for cellfree massive MIMO networks deployed using a so-called radio stripe [13], [14].…”

Team-Optimal MMSE Combining for Cell-Free Massive MIMO Systems

“…In addition, utilizing Team Theory to model robust coordination, the authors of [15] designed the decentralized MIMO precoding in wireless networks. Very recently, the authors of [16] derived the optimal downlink precoding of CF massive MIMO systems based on the Team Theory. To the best of our knowledge, however, no analysis has been done for the uplink combiner design of CF massive MIMO systems with the Team Theory.…”