Subset Regularized Zero-Forcing Precoders for Cell-Free C-RANs

Abstract

This paper formulates regularized zero-forcing (RZF) precoders that are both network-and user-centric for the downlink of centralized radio-access networks operating in a cellfree fashion. The transmission to every user involves a distinct subset of access points (APs), and every AP participates in the transmission to a distinct subset of users, hence the moniker subset precoders. These subsets, defined on the basis of the largescale channel gains between users and APs, capture the most relevant signal and interference contributions while disregarding those whose processing is cost-ineffective and whose associated channel estimations would incur unnecessary overheads. With that, subset precoding approaches the performance of networkwide RZF, for the power allocation strategies of interest, while being scalable.

Throughput maximization-based AP clustering methods in downlink cell-free MIMO under partial CSI condition

“…The BF vector to UE k can be obtained from the Moore-Penrose generalized inverse of  k H S . The idea of the muting method is also used in the study of cell-free MIMO in [18].…”

“…This section describes the conventional AP clustering methods proposed in [18] and [21], which will be evaluated in Sect. 5 along with the proposed methods.…”

“…Although there are many approaches, this paper focuses on a method that determines the transmitting AP groups, which is also known as clustering 1 . Methods for AP clustering each UE independently from other UEs have been proposed [9,[18][19][20][21]. In [19], an AP clustering method was proposed that determines the transmitting AP groups while taking into account the received signal power using the partially non-orthogonal ZF method as a † The authors are with the Graduate School of Science and Technology, Tokyo University of Science, Noda-shi, 278-8510 Japan.…”

“…Moreover, in [20] and [21], this method was extended to consider the interference to other UEs. In contrast, the methods proposed in [9] and [18] rely on path loss to determine the transmitting AP groups. However, independent determination of the transmitting APs for a specific UE induces interference to other UEs, leading to degradation in the system throughput.…”

Cluster Precoders for Cell-Free MU-MIMO Systems

“…Recently, scalable MMSE precoders and combiners have also been put forward to facilitate the deployment of CF systems [12]. In [13], the number of APs serving each user was reduced by forming subsets and employing the regularized ZF precoder.…”