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In this paper, we investigate the impact of channel aging on the performanceof cell-free (CF) massive multiple-input multiple-output (MIMO) systems withboth spatial correlation and pilot contamination. We derive novel closed-formuplink and downlink spectral efficiency (SE) expressions that take imperfectchannel estimation into account. More specifically, we consider large-scalefading decoding and matched-filter receiver cooperation in the uplink. Theuplink performance of a small-cell (SC) system is derived for comparison. TheCF massive MIMO system achieves higher 95\%-likely uplink SE than the SCsystem. In the downlink, the coherent transmission has four times higher95\%-likely per-user SE than the non-coherent transmission. Statistical channelcooperation power control (SCCPC) is used to mitigate the inter-userinterference. SCCPC performs better than full power transmission, but thebenefits are gradually weakened as the channel aging becomes stronger.Furthermore, strong spatial correlation reduces the SE but degrades the effectof channel aging. Increasing the number of antennas can improve the SE whiledecreasing the energy efficiency. Finally, we use the maximum normalizedDoppler shift to design the SE-improved length of the resource block.Simulation results are presented to validate the accuracy of our expressionsand prove that the CF massive MIMO system is more robust to channel aging thanthe SC system.

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