How Much Do Downlink Pilots Improve Cell-Free Massive MIMO?

TL;DR

This paper evaluates the impact of downlink pilots on cell-free massive MIMO systems, showing significant improvements in low-density networks and moderate gains in high-density scenarios through analytical derivations and numerical simulations.

Contribution

It provides an approximate achievable downlink rate formula considering channel estimation errors and compares performance with and without downlink training across different network densities.

Findings

Downlink pilots significantly improve performance in low-density networks.

Performance gains are moderate in high-density networks.

Max-min fairness power control enhances downlink throughput.

Abstract

In this paper, we analyze the benefits of including downlink pilots in a cell-free massive MIMO system. We derive an approximate per-user achievable downlink rate for conjugate beamforming processing, which takes into account both uplink and downlink channel estimation errors, and power control. A performance comparison is carried out, in terms of per-user net throughput, considering cell-free massive MIMO operation with and without downlink training, for different network densities. We take also into account the performance improvement provided by max-min fairness power control in the downlink. Numerical results show that, exploiting downlink pilots, the performance can be considerably improved in low density networks over the conventional scheme where the users rely on statistical channel knowledge only. In high density networks, performance improvements are moderate.