Hybrid RIS With Sub-Connected Active Partitions: Performance Analysis and Transmission Design

TL;DR

This work examines hybrid RIS architectures that combine SC-active and FC-active elements to balance spectral efficiency and energy consumption in RIS-assisted networks. It develops a diagonal beamforming model that accounts for amplifier sharing, derives asymptotic SNR expressions for both Rayleigh and LoS channels, and demonstrates that hybrid designs can approach the performance of fully active RIS with substantial EE gains. The optimization framework leverages fractional programming and block coordinate ascent to jointly design transmit precoding and RIS beamformers under BS and RIS power constraints, with extensive numerical results showing favorable EE/SR trade-offs across deployment scenarios. Overall, the study provides design insights and practical guidelines for deploying energy-efficient, scalable hybrid RIS systems in multi-user MISO downlink scenarios.

Abstract

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.

Figures (9)

• Figure 1: System setup: A BS equipped with $M$ antennas serves $K$ single-antenna users on a single time-frequency resource with the aid of a hybrid RIS having $N$ elements. The RIS is partitioned into two RSs, namely, RS1 and RS2, with $N_1$ and $N_2$ elements, respectively.
• Figure 2: Hybrid RIS architectures. In this example, each RIS has $N = 16$ elements divided into two RSs with $N_1=N_2=8$ elements each. Thus, in FC-active/passive RIS, there are $8$ power amplifiers, each one feeding its own element. In SC-active/passive RIS, in turn, we note that the SC-active RS is partitioned into 2 groups with $4$ elements each. Hence, this RIS structure utilizes $2$ power amplifiers, with each one being shared among $4$ elements. Likewise, FC-active/SC-active RIS makes use of $8+2 = 10$ power amplifiers. Finally, in SC-active/SC-active RIS, RS2 employs $4$ groups of $2$ elements each, thereby resulting in $2+4 = 6$ power amplifiers. This example shows that the various hybrid RIS architectures differ in the number of power amplifiers and the level where amplification control is applied.
• Figure 3: Each RS independently adopts the passive, FC-active, or SC-active RIS architecture. All these RIS structures provide phase shift control per element. The active RIS variants provide also amplification control. Specifically, FC-active RIS offers amplification control per element, since each element is connected to its own reflect-type power amplifier. SC-active RIS, on the other hand, enables amplification control per partition, since each group of elements shares a common power amplifier. This implies that each element in FC-active RIS imposes its own phase shift and amplitude to the incident RF signal, while the elements in an SC-active RIS partition induce different phase shifts but the same amplitude to it. Furthermore, from a circuit implementation perspective, we note that in an SC-active RIS partition, each element (patch) individually phase shifts the incident RF signal. Then, the outputs of the phase shift control circuits are added together and this sum is amplified. In the reverse signal path towards the patch, where the signal will be eventually reflected to the direction dictated by the phase shifts, the combined signal power is inevitably re-distributed to the RIS elements in the partition.
• Figure 4: SNR of active/passive vs. active or passive RIS as we vary the fraction of active elements in the former, for different operation regimes: (a) standard regime, (b) large number of RIS elements, and (c) large transmit power budget.
• Figure 5: SNR of active/active vs. active or active/passive RIS as we vary the number of RSs in active/active RIS or the fraction of active elements in active/passive RIS, for different operation regimes: (a) standard regime, (b) large transmit power budget, and (c) large reflect power budget.