Enhanced connectivity through double RIS uplink MIMO and advanced modulation

Abstract

Reconfigurable Intelligent Surfaces (RISs) have garnered significant attention in the development of Internet of Things (IoT) networks and vehicular communication systems, offering improved spectral efficiency with reduced power consumption and complexity overhead. However, employing a single RIS (S-RIS) may limit certain application scenarios and prove insufficient for ensuring reliable communication between transmitters and receivers. Consequently, the use of multiple RISs is recommended to further enhance coverage, capacity, and spectral efficiency. In this paper, we introduce the concept of Index Modulation (IM) into the double RIS (D-RIS) uplink Multiple-Input Multiple-Output (MIMO) communications, utilizing the antenna indices at the base station (BS). IM is realized through a combination of Space-Shift Keying (SSK) and Spatial Modulation (SM) techniques. We present an analytical model for the Double RIS based on SSK/SM methods and obtain a closed-form expression for the theoretical bit error probability (BEP), employing a greedy detector. To validate our theoretical BEP derivations for d-RIS-SSK/SM, we compare them with simulated BEP results. Our simulation data reveals an excellent match between the derived BEP and the simulated BEP, especially at lower Signal-to-Noise Ratios (SNRs), specifically -34 dB for SSK and -24 dB for SM. Furthermore, we evaluate the performance of the proposed d-RIS-aided IM methods by contrasting them with conventional d-RIS and single RIS-aided IM systems. Our simulation results demonstrate that the d-RIS-assisted IM outperforms the conventional d-RIS and single RIS-aided IM, achieving lower BEP at lower SNRs. d-RIS aided IM saves 15 dB in SNR compared to d-RIS without IM. Moreover, it saves 9 dB and 7 dB in SNR compared to single RIS-SSK and single RIS-SM, respectively when N = 64 and saves 3.5 dB and 1 dB in SNR with N = 128.