  Dual-function-radar-communication (DFRC) is a promising candidate technology
for next-generation networks. By integrating hybrid analog-digital (HAD)
beamforming into a multi-user millimeter-wave (mmWave) DFRC system, we design a
new reconfigurable subarray (RS) architecture and jointly optimize the HAD
beamforming to maximize the communication sum-rate and ensure a prescribed
signal-to-clutter-plus-noise ratio for radar sensing. Considering the
non-convexity of this problem arising from multiplicative coupling of the
analog and digital beamforming, we convert the sum-rate maximization into an
equivalent weighted mean-square error minimization and apply penalty dual
decomposition to decouple the analog and digital beamforming. Specifically, a
second-order cone program is first constructed to optimize the fully digital
counterpart of the HAD beamforming. Then, the sparsity of the RS architecture
is exploited to obtain a low-complexity solution for the HAD beamforming. The
convergence and complexity analyses of our algorithm are carried out under the
RS architecture. Simulations corroborate that, with the RS architecture, DFRC
offers effective communication and sensing and improves energy efficiency by
83.4% and 114.2% with a moderate number of radio frequency chains and phase
shifters, compared to the persistently- and fullyconnected architectures,
respectively.
