Enhancing Favorable Propagation in Cell-Free Massive MIMO Through Spatial User Grouping

TL;DR

This paper explores how network configuration and user grouping strategies can enhance favorable propagation in Cell-Free Massive MIMO systems, addressing spatial correlation issues to improve overall network performance.

Contribution

It formulates a user grouping and scheduling optimization problem leveraging spatial diversity and proposes a semidefinite relaxation approach to improve propagation in CF Massive MIMO.

Findings

Numerical results show significant performance improvements.

User grouping based on spatial diversity enhances propagation.

The optimization approach effectively mitigates spatial correlation effects.

Abstract

Cell-Free (CF) Massive multiple-input multiple-output(MIMO) is a distributed antenna system, wherein a large number of back-haul linked access points randomly distributed over a coverage area serve simultaneously a smaller number of users. CF Massive MIMO inherits favorable propagation of Massive MIMO systems. However, the level of favorable propagation which highly depends on the network topology and environment may be hindered by user' spatial correlation. In this paper, we investigate the impact of the network configuration on the level of favorable propagation for a CF Massive MIMO network. We formulate a user grouping and scheduling optimization problem that leverages users' spatial diversity. The formulated design optimization problem is proved to be NP-hard in general. To circumvent the prohibitively high computational cost, we adopt the semidefinite relaxation method to find a sub-optimal solution. The effectiveness of the proposed strategies is then verified through numerical results which demonstrate a non-negligible improvement in the performance of the studied scenario.