Hybrid classical relay and advanced RISs for performance enhancement of IoT sensor networks with impaired hardware

Abstract

In this paper, we propose a combination of traditional relay and advanced reconfigurable intelligent surfaces (RISs) in an Internet of things (IoT) sensor network. In particular, due to the long-range transmission from IoT sensor to receiver, a traditional relay is deployed between them. To greatly enhance the reliabilities of IoT-relay and relay-receiver communications, two RIS groups are exploited where each RIS group has multiple RISs distributed in different areas. Since the IoT, relay, and receiver are often low-cost devices, the hardware impairments (HI) are taken into account in the proposed system. We derive the closed-form expressions of average symbol error rate (SER) and capacity of the proposed hybrid RISs and relay aided IoT sensor system affected by HI and with direct IoT-relay and relay-receiver links (called as the hybrid RIS-R-HI-Wi system). Then, the performance of the hybrid RIS-R-HI-Wi system is compared with that of the relevant systems such as the hybrid RIS-R-HI system without direct links (the RIS-R-HI-Wo system), the RIS-R-ideal hardware (ID) systems with and without direct links (the RIS-R-ID-Wi and RIS-R-ID-Wo systems), and the traditional relay systems with HI and ID (the R-HI and R-ID systems). The theoretical expressions are validated through Monte-Carlo simulations. Numerical illustrations show the huge benefits of utilizing multi-RIS and relay in the hybrid RIS-R-HI-Wi system. Therefore, the performance of the RIS-R-HI-Wi system is dramatically higher than that of the RIS-R-HI-Wo and R-HI systems. Moreover, a significant performance loss caused by HI and high carrier frequencies is determined. Thus, the SER and capacity of the RIS-R-HI-Wi, RIS-R-HI-Wo, and R-HI systems are saturated in the high transmit power regime. In this circumstance, various effective methods such as reducing the distortion noises caused by the HI, using the suitable carrier frequencies, and choosing the appropriate locations of the RISs should be applied in the hybrid RIS-R-HI-Wi system for improving the performance significantly.