Use of Doppler focusing to resolve spatial channels from moving platforms

Models of the spatial distribution of the scatterers that surrounds a transmission path, i.e., spatial channel models, play a crucial role in predicting the performance of multiple-input multiple-output wireless communication systems. Most conventional approaches to characterizing spatial channels based on measured channel response data require either mechanically steerable directional antennas or multiple antenna systems at both the transmitter and receiver to resolve the directions of arrival and departure. Here, we show that when either the receiving or transmitting platform is moving with a constant velocity, as in the case of high speed rail, the channel measurement system can be simplified considerably by exploiting the manner in which the signal associated with a multipath component located at a given angle with respect to the direction of travel is Doppler shifted. Further, the accuracy and resolution of the spatial channel model that is so obtained can be improved greatly by collecting the entire range-Doppler history of the returns from a given scatterer and focusing them into a single point using techniques adopted from bistatic synthetic aperture radar signal processing. Fourier methods commonly used to process SAR data are not appropriate here due to the proximity of the scatterers to the transmission path. The back projection technique, however, is entirely suitable albeit at the expense of greater computational overhead.