RIS-Assisted mmWave Networks With Random Blockages: Fewer Large RISs or More Small RISs?

Reconfigurable intelligent surface (RIS), which provides indirect line-of-sight (LoS) transmission paths between the receiver and its blocked transmitter, has been proposed as one of the promising technologies for network performance enhancement. Recent works indicate that both the numbers of RISs and unit cells per RIS have a significant impact on network performance. However, with a given total number of unit cells, the joint analysis of these two factors considering blockage effect has not been investigated. In this paper, the coverage probability of a three-dimensional downlink millimeter wave RIS-assisted network is analyzed. We first derive the acceptable area where a LoS RIS is capable to satisfy the signal-to-noise ratio threshold at the receiver. Next, we model the centers of building blockages and human-body blockages as two independent Poisson point processes and derive the probability that indirect LoS transmissions exist. Then, we derive and validate the analytical upper and lower bounds of the coverage probability as the functions of network parameters and blockage densities. We also derive and validate the closed-form coverage probability when RISs are much closer to UE than BS. Finally, we propose a general network cost model for RIS-assisted network. Results show that in terms of coverage enhancement, densely deployed small-scale RISs outperform sparsely deployed large-scale RISs in scenarios with dense blockages or short transmission distances, while sparsely deployed large-scale RISs are preferable in scenarios with sparse blockages or long transmission distances.