Uplink Transmission Design for Crowded Correlated Cell-Free Massive MIMO-OFDM Systems

TL;DR

This paper proposes novel uplink transmission strategies for crowded cell-free massive MIMO-OFDM systems with correlated channels, focusing on pilot design and power control to improve channel estimation and spectral efficiency.

Contribution

It introduces an APSP set allocation scheme and a max-min power control algorithm tailored for crowded environments with limited pilots and correlated channels.

Findings

Significant reduction in MSE-CE with proposed APSP scheme

Enhanced minimum spectral efficiency through power control

Effective handling of pilot contamination in crowded scenarios

Abstract

In cell-free massive multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems, user equipments (UEs) are served by many distributed access points (APs), where channels are correlated due to finite angle-delay spread in realistic outdoor wireless propagation environments. Meanwhile, the number of UEs is growing rapidly for future fully networked society. In this paper, we focus on the uplink transmission design in crowded correlated cell-free massive MIMO-OFDM systems with limited number of orthogonal pilots. For the pilot transmission phase, we identify active UEs based on non-orthogonal pilot phase shift hopping patterns and non-orthogonal adjustable phase shift pilots (APSP). We derive a closed-form expression of mean square error of channel estimation (MSE-CE) and obtain an optimal condition for minimizing MSE-CE. According to this condition, the APSP set allocation scheme is proposed. Furthermore, for the data transmission, the max-min power control algorithm is devised to maximize the minimum spectral efficiency (SE) lower bound among active UEs. Simulation results indicate significant performance gains in terms of MSE-CE for the proposed APSP set allocation scheme. The proposed power control scheme can further improve the minimum SE among active UEs. Hence, they are crucial for crowded correlated cell-free massive MIMO-OFDM systems.