While studying the effect of channel aging in massive MIMO, I could find the fact that a larger spatial correlation can reduce the impact of channel aging. Specifically, a smaller angular standard deviation (ASD) means a strong spatial correlation [1]. I could understand that a large ASD leads to a large multipath gain, so its spectral efficiency is also high. However, in Figure 9 of [1], it is hard for me to understand why the small channel aging effect can decrease spectral efficiency. To my knowledge, the spectral efficiency may deteriorate as channel aging deepens because the channel aging effect varies channel responses in the coherence interval. If so, why is the spectral efficiency higher in weak channel aging than in strong channel aging? Thank you for your valuable responses in advance.
[1] J. Zheng, J. Zhang, E. Björnson and B. Ai, "Impact of Channel Aging on Cell-Free Massive MIMO Over Spatially Correlated Channels," in IEEE Transactions on Wireless Communications, vol. 20, no. 10, pp. 6451-6466, Oct. 2021, doi: 10.1109/TWC.2021.3074421.
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