Measured capacity gain using water filling in frequency selective MIMO channels

We analyze the channel capacity of multiple-input multiple-output (MIMO) systems in frequency selective channels, with channel knowledge at the transmitter side. An optimum transmission scheme uses Shannon´s principle of water filling jointly in the frequency and the spatial domain. However, we show that the largest gain by using water filling resides in the spatial domain, and that the capacity gain from the frequency domain is very small for MIMO systems. We quantify the gain for a theoretical Rayleigh fading channel model and for microcellular channels as measured in an enclosed courtyard.