Downlink Spectral Efficiency of Cell-Free Massive MIMO with Full-Pilot Zero-Forcing

TL;DR

This paper derives a closed-form expression for downlink spectral efficiency in cell-free Massive MIMO systems using full-pilot zero-forcing precoding, demonstrating significant performance improvements over traditional schemes without additional fronthaul costs.

Contribution

It introduces a new analytical expression for downlink spectral efficiency with fpZF precoding in cell-free Massive MIMO, considering imperfect CSI and pilot contamination.

Findings

fpZF outperforms maximum ratio transmission in spectral efficiency

The analysis includes max-min fairness DL power optimization

Significant performance gains achieved without increasing fronthaul overhead

Abstract

Cell-free Massive multiple-input multiple-output (MIMO) ensures ubiquitous communication at high spectral efficiency (SE) thanks to increased macro-diversity as compared cellular communications. However, system scalability and performance are limited by fronthauling traffic and interference. Unlike conventional precoding schemes that only suppress intra-cell interference, full-pilot zero-forcing (fpZF), introduced in [1], actively suppresses also inter-cell interference, without sharing channel state information (CSI) among the access points (APs). In this study, we derive a new closed-form expression for the downlink (DL) SE of a cell-free Massive MIMO system with multi-antenna APs and fpZF precoding, under imperfect CSI and pilot contamination. The analysis also includes max-min fairness DL power optimization. Numerical results show that fpZF significantly outperforms maximum ratio transmission scheme, without increasing the fronthauling overhead, as long as the system is sufficiently distributed.