RF Beamforming and Subcarrier Allocation Using Beam Squint in mmWave Systems

In a multi-user wideband millimeter wave (mmWave) communication system, the existing works optimize the hybrid precoder by assuming a priori subcarrier allocation. However, the beam squint effect in mmWave affects the gain over the subcarriers differently for different beam steer directions. Thus, the design of radio frequency (RF) precoder and subcarrier allocation are intertwined. Based on this observation, in this letter we propose a sub-array hybrid precoder design wherein we jointly estimate the RF precoder and subcarrier allocation and then design the baseband precoder. Our numerical results demonstrate the benefits of the proposed strategy compared to the existing approach wherein subcarrier allocation precedes the RF and baseband precoder design.