Local Partial Zero-Forcing Combining for Cell-Free Massive MIMO Systems

TL;DR

This paper analyzes scalable combining schemes for cell-free massive MIMO systems, deriving closed-form spectral efficiency expressions and comparing their performance and complexity under various system parameters.

Contribution

It introduces and evaluates partial and local zero-forcing combining methods using channel statistics, providing new closed-form SE expressions and insights into their scalability and performance.

Findings

LRZF yields the highest spectral efficiency.

PWPFZF is advantageous with many pilot sequences and few antennas per AP.

PWPFZF with fractional power control improves weak user performance.

Abstract

Cell-free massive multiple-input multiple-output (MIMO) provides more uniform spectral efficiency (SE) for users (UEs) than cellular technology. The main challenge to achieve the benefits of cell-free massive MIMO is to realize signal processing in a scalable way. In this paper, we consider scalable fullpilot zero-forcing (FZF), partial FZF (PFZF), protective weak PFZF (PWPFZF), and local regularized ZF (LRZF) combining by exploiting channel statistics. We derive closed-form expressions of the uplink SE for FZF, PFZF, and PWPFZF combining with large-scale fading decoding over independent Rayleigh fading channels, taking channel estimation errors and pilot contamination into account. Moreover, we investigate the impact of the number of pilot sequences, antennas per AP, and APs on the performance. Numerical results show that LRZF provides the highest SE. However, PWPFZF is preferable when the number of pilot sequences is large and the number of antennas per AP is small. The reason is that PWPFZF has lower computational complexity and the SE expression can be computed in closed-form. Furthermore, we investigate the performance of PWPFZF combining with fractional power control and the numerical results show that it improves the performance of weak UEs and realizes uniformly good service for all UEs in a scalable fashion.