Can Dynamic TDD Enabled Half-Duplex Cell-Free Massive MIMO Outperform Full-Duplex Cellular Massive MIMO?

TL;DR

This paper proposes a dynamic TDD enabled half-duplex cell-free massive MIMO system that outperforms full-duplex cellular massive MIMO in spectral efficiency, using novel algorithms for AP scheduling and pilot contamination mitigation.

Contribution

It introduces a low-complexity greedy AP scheduling algorithm with an optimality guarantee and a novel pilot contamination minimization method for CF-mMIMO systems.

Findings

CF-mMIMO with dynamic HD AP scheduling outperforms FD-mMIMO in spectral efficiency.

The proposed algorithms effectively manage pilot contamination and AP scheduling.

Simulation results confirm the theoretical advantages of the proposed system.

Abstract

We consider a dynamic time division duplex (DTDD) enabled cell-free massive multiple-input multiple-output (CF-mMIMO) system, where each half-duplex (HD) access point (AP) is scheduled to operate in the uplink (UL) or downlink (DL) mode based on the data demands of the user equipments (UEs), with the goal of maximizing the sum UL-DL spectral efficiency (SE). We develop a new, low complexity, greedy algorithm for the combinatorial AP scheduling problem, with an optimality guarantee theoretically established via showing that a lower bound of the sum UL-DL SE is sub-modular. We also consider pilot sequence reuse among the UEs to limit the channel estimation overhead. In CF systems, all the APs estimate the channel from every UE, making pilot allocation problem different from the cellular case. We develop a novel algorithm that iteratively minimizes the maximum pilot contamination across the UEs. We compare the performance of our solutions, both theoretically and via simulations, against a full duplex (FD) multi-cell mMIMO system. Our results show that, due to the joint processing of the signals at the central processing unit, CF-mMIMO with dynamic HD AP-scheduling significantly outperforms cellular FD-mMIMO in terms of the sum SE and 90% likely SE. Thus, DTDD enabled HD CF-mMIMO is a promising alternative to cellular FD-mMIMO, without the cost of hardware for self-interference suppression.