On the "Critical Rate" in Ricean MIMO Channels

We analyze the outage characteristics of correlated Ricean fading, coherent multiple-input multiple-output (MIMO) channels. In particular, we establish the notion of a "critical rate", below which communication at zero outage is possible and above which the channel appears as Ricean fading. The critical rate is shown to depend on the "angle" between the subspace spanned by the Ricean component of the channel matrix and the subspace spanned by the correlation matrix of the Rayleigh fading component. A nonzero critical rate is possible only if the correlation matrix of the Rayleigh fading component is rank- deficient. Finally, we provide a complete characterization of the optimum diversity-multiplexing tradeoff for correlated Ricean fading MIMO channels taking into account the existence of the critical rate and thereby establishing the notion of a critical multiplexing gain.