Hybrid Double-RIS and DF-Relay for Outdoor-to-Indoor Communication

Reconfigurable intelligent surface (RIS) is a promising solution to enhance the energy and spectral efficiency of future wireless networks. The system with single RIS might not be able to provide sufficient line-of-sight (LoS) links for outdoor-indoor communication, which often limits the signal transmission distance and coverage range. This paper first proposes a novel hybrid double-RIS and full-duplex (FD) decode-and-forward (DF) relay aided system to eliminate the impact of building occlusion and penetration loss in outdoor-indoor communication. Expressions of the upper bound for the ergodic spectral efficiency (ESE) are derived and match the simulation results well. To maximize the ESE, two algorithms are proposed for passive beamforming of the RIS: optimal phase shifts (OPS) and generalized cross-entropy (GCE) algorithm. Results show that the OPS can provide a power gain of the square of the RIS elements’ number. The proposed GCE-based discrete phase shifts algorithm can achieve a good performance on the energy efficiency. An optimal power allocation (OPA) scheme between the source and relay is also proposed to maximize the spectral efficiency. Numerical results demonstrate the performance advantage of the OPA over the equal power allocation (EPA).