  In the context of cell-free massive multi-input multi-output (mMIMO),
zero-forcing precoding (ZFP) requires the exchange of instantaneous channel
state information and precoded data symbols via a fronthaul network. It causes
considerable propagation and processing delays, which degrade performance. This
letter analyzes the impact of channel aging on the performance of ZFP in
cell-free mMIMO. The aging effects of not only user mobility but also phase
noise are considered. Numerical results in terms of per-user spectral
efficiency are illustrated.
