Exploring MIMO Correlation Matrix Distance for Cell-free V2I Channel Characterization

This letter investigates the stationarity characteristics of vehicle-to-infrastructure (V2I) channels in a cell-free context, based on 5.89 GHz field measurements. The Correlation Matrix Distance (CMD) metric is used to quantify the stationarity time of both individual access point (AP)-to-user channels and the collective channel formed by stacking multiple APs. Results show that with two APs, the collective channel's stationarity time is similar to that of the most non-stationary AP. However, when the number of APs increases to four, the collective channel's stationarity time decreases significantly, falling below that of any individual AP. This demonstrates that increasing spatial distribution in cell-free architectures amplifies the impact of local non-stationarities, especially near APs. These findings highlight the importance of thoroughly characterizing stationarity in cell-free systems, particularly as the network scales.