  This paper investigates the hybrid precoding design in millimeter-wave
(mmWave) systems with a fully-adaptive-connected precoding structure, where a
switch-controlled connection is deployed between every antenna and every radio
frequency (RF) chain. To maximally enhance the energy efficiency (EE) of hybrid
precoding under this structure, the joint optimization of switch-controlled
connections and the hybrid precoders is formulated as a large-scale
mixed-integer non-convex problem with high-dimensional power constraints. To
efficiently solve such a challenging problem, we first decouple it into a
continuous hybrid precoding (CHP) subproblem. Then, with the hybrid precoder
obtained from the CHP subproblem, the original problem can be equivalently
reformulated as a discrete connection-state (DCS) problem with only 0-1 integer
variables. For the CHP subproblem, we propose an alternating hybrid precoding
(AHP) algorithm. Then, with the hybrid precoder provided by the AHP algorithm,
we develop a matching assisted fully-adaptive hybrid precoding (MA-FAHP)
algorithm to solve the DCS problem. It is theoretically shown that the proposed
MA-FAHP algorithm always converges to a stable solution with the polynomial
complexity. Finally, simulation results demonstrate that the superior
performance of the proposed MA-FAHP algorithm in terms of EE and beampattern.
