  Hybrid analog/digital precoding architectures can address the trade-off
between achievable spectral efficiency and power consumption in large-scale
MIMO systems. This makes it a promising candidate for millimeter wave systems,
which require deploying large antenna arrays at both the transmitter and
receiver to guarantee sufficient received signal power. Most prior work on
hybrid precoding focused on narrowband channels and assumed fully-connected
hybrid architectures. MmWave systems, though, are expected to be wideband with
frequency selectivity. In this paper, a closed-form solution for
fully-connected OFDM-based hybrid analog/digital precoding is developed for
frequency selective mmWave systems. This solution is then extended to
partially-connected but fixed architectures in which each RF chain is connected
to a specific subset of the antennas. The derived solutions give insights into
how the hybrid subarray structures should be designed. Based on them, a novel
technique that dynamically constructs the hybrid subarrays based on the
long-term channel characteristics is developed. Simulation results show that
the proposed hybrid precoding solutions achieve spectral efficiencies close to
that obtained with fully-digital architectures in wideband mmWave channels.
Further, the results indicate that the developed dynamic subarray solution
outperforms the fixed hybrid subarray structures in various system and channel
conditions.
