  Considering the expensive radio frequency (RF) chain, huge training overhead
and feedback burden issues in massive MIMO, in this letter, we propose a
mixed-timescale per-group hybrid precoding (MPHP) scheme under an adaptive
partially-connected RF precoding structure (PRPS), where the RF precoder is
implemented using an adaptive connection network (ACN) and M analog phase
shifters (APSs), where M is the number of antennas at the base station (BS).
Exploiting the mixed-time stage channel state information (CSI) structure, the
joint-design of ACN and APSs is formulated as a statistical
signal-to-leakage-and-noise ratio (SSLNR) maximization problem, and a heuristic
group RF precoding (GRFP) algorithm is proposed to provide a near-optimal
solution. Simulation results show that the proposed design advances at better
energy efficiency (EE) and lower hardware cost, CSI signaling overhead and
computational complexity than the conventional hybrid precoding (HP) schemes.
