  The emerging reflecting intelligent surface (RIS) technology promises to
enhance the capacity of wireless communication systems via passive reflect
beamforming. However, the product path loss limits its performance gains.
Fully-connected (FC) active RIS, which integrates reflect-type power amplifiers
into the RIS elements, has been recently introduced in response to this issue.
Also, sub-connected (SC) active RIS and hybrid FC-active/passive RIS variants,
which employ a limited number of reflect-type power amplifiers, have been
proposed to provide energy savings. Nevertheless, their flexibility in
balancing diverse capacity requirements and power consumption constraints is
limited. In this direction, this study introduces novel hybrid RIS structures,
wherein at least one reflecting sub-surface (RS) adopts the SC-active RIS
design. The asymptotic signal-to-noise-ratio of the FC-active/passive and the
proposed hybrid RIS variants is analyzed in a single-user single-input
single-output setup. Furthermore, the transmit and RIS beamforming weights are
jointly optimized in each scenario to maximize the energy efficiency of a
hybrid RIS-aided multi-user multiple-input single-output downlink system
subject to the power consumption constraints of the base station and the active
RSs. Numerical simulation and analytic results highlight the performance gains
of the proposed RIS designs over benchmarks, unveil non-trivial trade-offs, and
provide valuable insights.
