  Hybrid analog/digital precoding allows millimeter wave MIMO systems to
leverage large antenna array gains while permitting low cost and power
consumption hardware. Most prior work has focused on hybrid precoding for
narrow-band mmWave systems. MmWave systems, however, will likely operate on
wideband channels with frequency selectivity. Therefore, this paper considers
frequency selective hybrid precoding with RF beamforming vectors taken from a
quantized codebook. For this system, a low-complexity yet near-optimal greedy
algorithm is developed for the design of the hybrid analog/digital precoders.
The proposed algorithm greedily selects the RF beamforming vectors using
Gram-Schmidt orthogonalization. Simulation results show that the developed
precoding design algorithm achieves very good performance compared with the
unconstrained solutions while requiring less complexity.
