  Hybrid analog/digital precoding in millimeter-wave (mmWave) multi-input
multi-ouput (MIMO) systems is capable of achieving the near-optimal
full-digital performance at reduced hardware cost and power consumption
compared to its full-RF digital counterpart. However, having numerous phase
shifters is still costly, especially when the phase shifters are of high
resolution. In this paper, we propose a novel twin-resolution phase-shifter
network for mmWave MIMO systems, which reduces the power consumption of an
entirely high-resolution network, whilst mitigating the severe array gain
reduction of an entirely low-resolution network. The connections between the
twin phase shifters having different resolutions and the antennas are either
fixed or dynamically configured. In the latter, we jointly design the
phase-shifter network and the hybrid precoding matrix, where the phase of each
entry in the analog precoding matrix can be dynamically designed according to
the required resolution. This method is slightly modified for the fixed
network's hybrid precoding matrix. Furthermore, we extend the proposed method
to multi-user MIMO systems and provide its performance analysis. Our simulation
results show that the proposed dynamic hybrid precoding method strikes an
attractive performance vs. power consumption trade-off.
