Degrees of Freedom of Ray-Based Models for mm-Wave Wideband MIMO-OFDM

In this paper we analyze the number of degrees of freedom needed to accurately capture and model wideband MIMO-OFDM channels. Two modeling approaches are considered, either examining each coefficient of the conventional input-output MIMO channel matrix or investigating the components constituting the physical ray-based propagation between the antenna arrays. As our analysis accounts for wideband channels, a time domain decomposition of MIMO channels into tapped delay lines is performed in each case. The efficiency of each approach is then evaluated in terms of number of taps to determine the most appropriate representation between either the conventional antenna-based one or the propagation-based one. This analysis is performed analytically using the Saleh-Valenzuela model which is recognized for its consistency for wideband as well as spatial modeling. We exploit the second-order statistics of the Channel Impulse Response to derive the taps weights.