Spatial and Temporal Variations of MIMO Channels and Impacts on Capacity

This paper presents an analysis of spatial and temporal variations of multiple-input-multiple-output (MIMO) channels at the mobile. Doppler spread, angular spread and Rician K factor are evaluated based on the temporal correlation of field components. This approach is implemented to process the narrowband measurement results at 2.11 GHz in Manhattan, New York, with 16 transmitting antennas and 16 receiving antennas. Results show that the angular distribution is not uniform in 360 degrees in azimuth as the Jakes model suggests. The median rms angular spread at the mobile was found to be 22.5 and 25.5 degrees for horizontally and vertically polarized antennas, respectively, with the antenna array facing one side of the street. Analysis of the correlation coefficient indicates the presence of small coherent component with K factor less than 0.5. The small angular spread and presence of coherent power result in higher correlation distance and time than the predicted values from Jakes model. The measured coherence time is at least a few seconds for stationary channels, and 90 ms for mobile pedestrian speed of 3 km/hr. The impact of the angular spread and Rician K factor on the MIMO channel capacity is also evaluated. Although the correlation increases due to the angular spread and Rician component, the degradation of the overall link capacity is small. Theory and measurements show that, for a 4 by 4 system with half wavelength antenna spacing at the mobile and uncorrelated antennas at the base, at least 90% of channel capacity can be achieved with the rms angular spread larger than 20 degrees and K less than 1.