Experimental Characterization of the Outdoor MIMO Wireless Channel Temporal Variation

Time-variant multiple-input multiple-output (MIMO) channels are measured in an outdoor campus environment at 2.45 GHz with directional patch arrays and omnidirectional monopole arrays. A number of useful metrics are proposed for quantifying time variation in MIMO channels: eigenvalue level crossing rate, eigenvector angular deviation, and capacity loss for delayed transmit and receive channel state information (CSI). Measurements in four different environments confirm the strong correlation between angular spread of multipath and MIMO channel time variability. The rate of time variation is also strongly influenced by the type of array, indicating that directional elements may be advantageous for highly mobile environments. The proposed metrics indicate that although the physical communication layer may need to update CSI several times per wavelength, the required rate of adaptation in transmit rate, modulation, and power allocation is much less severe.