Method for obtaining full channel state information for RF beamforming

Millimeter wave (mmWave) and massive MIMO systems will be characterized by the use of a large number of antennas at the base stations. In order to reduce costs and power consumption, the number of radio frequency (RF) chains (transceivers/baseband units) should be minimized for these arrays. One option to achieve this goal is with RF beamforming where the control of the gain and phase of each element in the array is done at RF instead of at baseband in typical digital beamformers. However obtaining the full channel knowledge for RF beamformers is difficult because sounding each antenna separately incurs a power loss. In this paper we propose using basis functions to create orthogonal beams which the base station array can sound in order to obtain the full transmit CSI between the transmit array and the UE array. We also explore a method of feedback reduction where the amount of CSI is reduced by feeding back CSI separately for the azimuth and elevation dimensions of the array. Simulation results at mm Wave show that the use of the basis functions can obtain performance very close to ideal channel knowledge and that feeding back CSI separately for azimuth and elevation is effective as long as the array is manifold calibrated.