Multi-Keyholes and Measure of Correlation in MIMO Channels

Keyhole MIMO channels were predicted theoretically and also observed experimentally. However, they are not often encountered in practice since the assumption of a single propagation eigenmode is only a rough approximation of real propagation environments. This paper presents an extension to the single-keyhole channel model, termed a "multi-keyhole channel", which includes a number of statistically independent keyholes. Correlated full-rank and rank-deficient multi-keyhole channels are considered in detail. Under some general conditions the full-rank multi-keyhole channel is asymptotically Rayleigh fading if the number of keyholes is large. When the number of both Tx and Rx antennas is large, the capacity of a rank-deficient multi-keyhole channel is a sum of the capacities of the equivalent single-keyhole channels. The outage capacity distribution of both full-rank and rank-deficient multi-keyhole channels is asymptotically Gaussian. Based on the asymptotic capacity analysis, full ordering scalar measures of MIMO channel correlation and power imbalance are introduced