Relay Selection and User Scheduling in Reconfigurable Intelligent Surface Assisted Millimeter-wave D2D Communication

Abstract

Reconfigurable intelligent surface (RIS) assisted millimeter wave (mmWave) device to device (D2D) communication has recently been proposed as a viable solution to support the overwhelming data traffic in fifth-generation (5G) and beyond wireless networks. However, due to the substantial propagation and penetration losses of mmWave, a direct line of sight (LoS) link between a pair of proximity devices is required for effective communication. Static obstacles like trees and buildings can easily obstruct the direct LoS connectivity between a device pair. In such cases, RISs help to establish an indirect LoS link between an obstructed device pair by reflecting the signals in the desired direction. In Chapter 2, we propose a set cover-based RIS deployment strategy to serve the maximum number of obstructed device pairs with the minimum number of RISs. In particular, we have demonstrated that double reflections via two consecutive RISs can greatly lower the RIS density in the environment, preventing resource waste and enabling the service of more obstructed device pairs. After the RIS deployment, for information transfer, we also propose an energy-efficient RIS group selection criteria. Moreover, we prove that under some conditions, double reflections are more beneficial than single reflection, which is counter-intuitive. Numerical results show that our approach outperforms a random and a recent deployment strategy. In Chapter 3, assuming that the RISs are already deployed and the positions of the nearby users are known, we propose a double-RIS assisted multihop routing scheme for a device pair. Besides the RISs, the emphasis of this work is to make more use of the existing intermediate users (IUs), which can act as relays. Hence, the density of RIS deployment in the surroundings can be reduced, which leads to the avoidance of resource wastage. However, we cannot solely depend on the IUs because this implies complete dependence on their availability for relaying and as a result, the aspect of reliability in terms of delay-constrained information transfer may not be guaranteed. Moreover, the IUs are considered capable of energy harvesting, and as a result, they do not waste their own energy in the process of volunteering to act as a relay for other users. Numerical results demonstrate the advantage of the proposed scheme over some existing approaches, and lastly, useful insights related to the scheme design are also drawn, where we characterize the maximum acceptable delay at each hop under different set-ups. The previous chapter presented a routing scheme for a particular device pair. For multiple device pairs, a single RIS may be simultaneously requested by several devices to act as a relay for their seamless communications. To deal with such cases, in Chapter 4, we propose a priority-aware, user traffic–dependent, grouping-based multihop user scheduling scheme for RIS-assisted mmWave D2D communication under spatially correlated channels. Specifically, the proposed scheme exploits the priority of the users (based on their respective delay-constrained applications) and the aspect of spatial correlation in the narrowly spaced reflecting elements of the RISs. Here, based on the other users in the neighborhood, their respective traffic characteristics, and the already deployed RISs in the surroundings, we establish a multihop connection for energy-efficient information transfer from one of the users to its intended receiver. In this context, we take into account the impact of considering practical discrete phase shifts at the RIS patches instead of its ideal continuous counterpart. Moreover, we also claim and demonstrate that the existing classic least remaining distance based approach is not always the optimal solution. Finally, the numerical results demonstrate the advantages of the proposed strategy over the existing benchmark schemes in terms of data throughput, energy consumption, and energy efficiency. In the previous chapter, each device pair selects the best energy-efficient group, and in case of multiple pairs selecting the same best group, they are scheduled as per their priorities. However, the best group may lead to a large delay for some of the pairs. Moreover, the energy harvesting aspect is not considered there. In Chapter 5, we investigate various group selection strategies which select the k-th best group by considering self-sustainable RIS with spatially correlated channels. Specifically, we consider both power splitting and time switching configurations of the self-sustainable RIS to analyze the system performance and propose appropriate bounds on the choice of system parameters. The analysis takes into account both linear and non-linear energy harvesting models. Based on the application requirements, we propose various group selection strategies, which schedule the k-th best available group based on the end-to-end signal-to-noise ratio and also the energy harvested at a particular group. Accordingly, by using tools from high-order statistics, we derive analytical expressions for the outage probability of each selection strategy. Moreover, using extreme value theory, we investigate an asymptotic scenario where the number of groups available for selection at an RIS approaches infinity. The nontrivial insights obtained from this approach are especially beneficial in applications like large intelligent surface-aided wireless communication. Finally, the numerical results demonstrate the importance and benefits of the proposed approaches in terms of throughput and outage performance.