Frequency Selective Hybrid Precoding in Millimeter Wave OFDMA Systems

Hybrid precoding, a combination of analog and digital precoding, is an attempt to reach a compromise between complexity and performance. By exploiting more than one RF chain, hybrid precoding enables a millimeter wave (mmWave) system to take advantage of both spatial multiplexing and beamforming gain. A major challenge with hybrid precoding is its configuration in wideband systems. The reason is that the analog beamforming weights are the same across the entire band. In this paper, we propose a frequency selective hybrid precoding technique for a mmWave orthogonal frequency division multiple access system. We propose an algorithm to jointly optimize the wideband analog beamformers and the per- subcarrier digital precoders. We compare an upper bound with our solution and show that our optimized approach achieves good performance in terms of beamforming gain in the sense that it minimizes the loss caused by allocating multiple users to different subcarriers with a limited number of radio frequency chains.