Performance analysis of cognitive relay networks using spectral_ and energy-efficient schemes

Abstract

Over the past few years, the wireless communication technologies have witnessed the rapid evolution to enrich the quality of our day-to-day life. This evolution may eventually necessitates the e cient utilization of limited spectral resources. In this regard, cognitive radio technology has received tremendous research interest owing to its capability for improving spectrum utilization e ciency in wireless networks. This dynamic spectrum access technology enables the unlicensed secondary users (SUs) to share the licensed spectrum of primary users (PUs) under certain constraints. With such quality-of-service restrictions from the PUs, it becomes challenging to improve the performance of SUs. For this, cooperative relaying techniques have been integrated with the cognitive radio technology to improve the overall system performance and reliability. Another important aspect of the future networks is energy e ciency. On that front, the energy harvesting (EH) techniques have been suggested for the design of energy-e cient wireless systems. To address the design objectives of the future networks, this thesis comprehensively analyzes the performance of relay-aided cognitive radio networks, referred to as cognitive relay networks (CRNs), by exploring various spectral- and energy-e cient schemes. Firstly, we consider a resource sharing CRN (RSCRN) with two-way PUs' communication. In this analytical system design, besides assisting the transmission of SUs, the relay also provides an aid for the bi-directional PUs' transmission. To further enhance the performance of SUs, the secondary system employs an opportunistic user scheduling with multiple destinations. For the performance assessment of this system, we derive novel closed-form expressions of outage and average error probabilities considering Nakagami-m fading channels. Our results illustrate that, leveraging two-way relaying for primary transmissions, the proposed RSCRN offers higher spectral e ciency. Moreover, by exploiting inherent multiuser diversity, reliability of SUs' communication can be signi cantly improved. We further investigate the performance of an underlay two-way CRN (TWCRN) with direct link in the presence of PU's interference. Hereby, we propose an adaptive link utilization scheme (ALUS) that can exploit both direct and relay links with appropriate diversity combining methods to improve the performance of SUs. For this system, we derive tight closed-form expressions for the outage probability of secondary communications for two di erent relaying strategies viz., xed relaying and incrementalrelaying. We also conduct asymptotic analysis to examine the diversity order of the considered system. Our results reveal that the full diversity for secondary system can be achieved as long as the primary interference remains limited, otherwise the performance remarkably deteriorates. Next, we analyze the performance of cognitive multi-relay network (CMRN) in the presence of hardware and channel imperfections under di erent operating conditions. Herein, we consider the hardware impairments (HIs) invoked by the non-ideal secondary transceivers and residual interference originating from channel estimation errors (CEEs). From our analysis, we nd that the detrimental impact of HIs can potentially cap the fundamental capacity of the system for the high-rate applications. We also observe that, due to CEEs, the system could not exploit the diversity, resulting in irreducible outage oors. Based on our investigations, we provide useful insights into the tolerable level of HIs for designing the practical systems. We also compare the robustness of two classical relaying schemes, i.e., amplify-and-forward and decode-and-forward, for the considered CMRN against HIs. To improve the energy e ciency of cognitive radio networks, we employ a radiofrequency based EH technique in CRNs. Speci cally, we consider a non-linear EHbased overlay CRN (EHCRN) where an energy-constrained secondary node acts as a cooperative relay to assist the transmission of PU. In return for the benign cooperation, the secondary node enjoys access to the PU's spectrum for its own information transfer. For this overall setup, we investigate the system performance by deriving the expressions for outage probability of the primary and secondary networks over Rayleigh fading channels. Further, we attempt to harness the combined advantages of multiuser diversity and cooperative diversity to improve the performance of primary network. Thereafter, we propose an improved EH-based relaying scheme which is shown to substantially enhance the performance of considered EHCRN. Above all, this thesis addresses various technical aspects and challenges for realization of CRNs and eventually provides useful insights into the practical system design. All the theoretical developments, proposed schemes, and strategies hereunder are primarily aimed at improving the spectral e ciency, energy e ciency, and reliability of the CRNs for their possible applications in the future networks.