PERFORMANCE ANALYSIS OF RIS-ASSISTED RELAY SYSTEMS

Author

Alan Devkota, Southern Illinois University CarbondaleFollow

Date of Award

8-1-2022

Degree Name

Master of Science

Department

Electrical and Computer Engineering

First Advisor

Baduge, Gayan

Abstract

Reconfigurable intelligent surface (RISs) have been viewed as a vital physical-layer transmission technology for next-generation wireless systems. While the current state-of-art wireless standard is the fifth generation (5G), the RISs have emerged as a novel option to satisfy the rising demands for higher data rates, improved network coverage, increased reliability, and energy efficiency in wireless communications for sixth-generation (6G) and beyond. The spectral andenergy efficiency requirement is one of the fundamental design elements of any communication system design. The need for greener communication technologies for wireless networks has laid the groundwork for many innovative wireless power transfer methods. Due to the fact that radio-frequency (RF) signals can convey both information and energy simultaneously, there has been much research interest in designing novel technologies for simultaneous wireless information and power transmission (SWIPT) and energy harvesting (EH). Firstly, an RIS-assisted relay system model is proposed to improve the wireless system performance. By characterizing the optimal signal-to-noise ratio (SNR) attained through intelligent phase-shift controlling, the performance of the RIS-assisted relay system is investigated. To this end, the cumulative distribution function of the optimal SNR is derived by using a tight approximation. Thereby, tight approximations for the achievable rate, outage probability, and the average symbol error rate are derived. Thereby, the impact of phase-shift errors is studied by adopting discrete phase-shift adjustments. Then, by using the Monte-Carlo simulations, the validity of our theoretical results is verified. Finally, a set of insightful numerical results has been presented to evaluate the performance gains offered by the proposed RIS-assisted relay system. Secondly, the performance of simultaneous wireless information and power transfer (SWIPT) is explored for the proposed RIS-assisted relay system. Towards this end, tight bounds for the average achievable rate and optimal harvested energy are derived in closed-form for a hybrid SWIPT protocol. Then, the achievable rate-energy trade-off is derived. Thereby, the detrimental effects of phase quantization in the optimal SNR, achievable rate, and harvested energy are explored. Furthermore, the impact of varying the time-switching factor, power-splitting factor, numbers of reflective elements, and coefficient of reflection on the performance of the proposed RIS-aided is studied. Also, the performance of linear EH models and non-linear EH models are compared via analytical and Monte-Carlo simulations results. The corresponding analysis reveals that the rate-energy trade-off is highly sensitive to the number of passive reflecting elements and the number of bits used in the RIS phase-shift quantization procedure. Furthermore, it is observed that the linear EH models overestimate the harvested energy, implying that the non-linear EH models are more appropriate than their the linear counterparts for investigating performance of SWIPT with RIS-assisted relay networks.