Impact of Channel Aging on Cell-Free Massive MIMO Over Spatially Correlated Channels

TL;DR

This paper analyzes how channel aging affects cell-free massive MIMO systems with spatial correlation and pilot contamination, deriving spectral efficiency expressions and comparing performance with small-cell systems.

Contribution

It provides novel closed-form spectral efficiency formulas accounting for imperfect channel estimation and channel aging, and compares CF massive MIMO with small-cell systems under these conditions.

Findings

CF massive MIMO outperforms small-cell systems in uplink spectral efficiency.

Coherent downlink transmission yields four times higher per-user SE than non-coherent.

Channel aging reduces spectral efficiency, but CF massive MIMO remains more robust than small-cell systems.

Abstract

In this paper, we investigate the impact of channel aging on the performance of cell-free (CF) massive multiple-input multiple-output (MIMO) systems with both spatial correlation and pilot contamination. We derive novel closed-form uplink and downlink spectral efficiency (SE) expressions that take imperfect channel estimation into account. More specifically, we consider large-scale fading decoding and matched-filter receiver cooperation in the uplink. The uplink performance of a small-cell (SC) system is derived for comparison. The CF massive MIMO system achieves higher 95\%-likely uplink SE than the SC system. In the downlink, the coherent transmission has four times higher 95\%-likely per-user SE than the non-coherent transmission. Statistical channel cooperation power control (SCCPC) is used to mitigate the inter-user interference. SCCPC performs better than full power transmission, but the benefits are gradually weakened as the channel aging becomes stronger. Furthermore, strong spatial correlation reduces the SE but degrades the effect of channel aging. Increasing the number of antennas can improve the SE while decreasing the energy efficiency. Finally, we use the maximum normalized Doppler shift to design the SE-improved length of the resource block. Simulation results are presented to validate the accuracy of our expressions and prove that the CF massive MIMO system is more robust to channel aging than the SC system.