Measurement and Characterization of the Temporal Behavior of Fixed Massive MIMO Links

Measurements of fixed Massive MIMO links at a carrier frequency of 2.6GHz are reported. The temporal fading at the individual antenna elements is characterized in terms of K-factors and coherence times. We observe ??5dB larger K-factors during night-time as well as ??10dB higher values for the line-of-sight location. The analysis of the fading's cross-correlation between the different antenna elements shows a stronger correlation in the vertical domain than in the horizontal one. However, a horizontal lambda/2-spacing still provides a significant amount of cross-correlation (i.e., values above 0.5 in 6 out of 10 locations), and yields the smallest orthogonality between channel vectors of different locations. The channel hardening effect mitigates the temporal channel gain variations for a 64-antenna link to less than 1dB. The dominant temporal fluctuation of a channel vector is the drift of its subspace, which significantly deteriorates the performance of nullspace-based spatial multiplexing schemes such as zero-forcing.