Abstract
Cognitive Radio and the emerging Non-Orthogonal Multiple Access (NOMA) techniques hold the potential to fulfill the increasing demands of radio spectrum by 5G-and-B5G wireless systems. It is a usual practice in underlay cognitive radio networks to take assistance from intermediate relays to reach the remote destination. Putting together, NOMA based Cognitive Relay Networks (NCRNs) have recently gained tremendous research attention to improve the spectrum utilization efficiency. This article studies the application of best relay selection (BRS) in downlink scenario of NCRNs in which a base station (BS) being unable to communicate directly with the far user U2 takes assistance from the near user U1 and from the best Decode-and-Forward (DF) relay selected from the potential NCRN operating in underlay environment. Three BRS schemes are proposed selecting a relay which (i) maximizes signal-to-noise ratio (SNR) on relayU2 link, (ii) minimizes interference on relay-primary user (PU) link, (iii) maximizes the quotient of relay-U2 link's SNR and relay-PU link's interference power. Scheme 3 considers the existence of U1 -U2 link as well whereas the other two schemes do not. To characterize the performance of the proposed schemes assuming Rayleigh channel model, closed-form expressions of probability distribution function (PDF) of received SNR at U2, average number of reliable relays, outage probability and bit error rate are obtained. For insight analysis, analytical results are validated through simulations which reveal that relay selection in NCRNs is different from non-cognitive networks and is feasible in low-to-medium SNR regions. The signal from near user further improved the outage performance.
Highlights
It is predicted that by 2030, more than 125 billion wireless devices will be inter-connected exchanging about 5 zettabytes (ZB) of data per month at an individual data rate of up to 100 Gbps [1]
SIMULATION RESULTS AND DISCUSSION This section examines the performance of best relay selection (BRS) schemes proposed for two different scenarios of NOMA based Cognitive Relay Networks (NCRNs) in underlay environment
This paper presents various BRS schemes for NCRNs operating in underlay environment with DF relays
Summary
It is predicted that by 2030, more than 125 billion wireless devices will be inter-connected exchanging about 5 zettabytes (ZB) of data per month at an individual data rate of up to 100 Gbps [1] Due to such unprecedented growth rate, 5G and B5G mobile communication systems are facing the challenges of increased connection density, spectrum utilization efficiency, mobility, seamless connectivity, energy efficiency and user fairness [2]–[4]. The associate editor coordinating the review of this manuscript and approving it for publication was Yan Huo. spectrum under-utilization problems through dynamic re-use of frequency bands assigned to the PUs. The emergence of cognitive radio technology seems to provide a perfect answer to address the requirements and challenges of future wireless networks [5]. In underlay mode on the other hand, SU transmits simultaneously
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