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Top1. Introduction
Joseph Mitola’s Cognitive Radio (CR) enables the unlicensed/secondary users (SRs) to occupy the spectrum assigned to the licensed/primary users (PUs) and stands as a well-established solution to spectrum underutilization problem in research community (Mitola, 2009). Primarily, three spectrum sharing modes have been widely discussed in this context: namely, Overlay, Underlay and Interweave mode. Among the above three, the Underlay spectrum sharing has the advantage of allowing the SUs to directly occupy the licensed spectrum, however, it faces the key challenge of secondary performance enhancement due to strict interference and transmit power constraints (Das et al., 2017) This obligation on the SUs in terms of transmit power control degrades the secondary communication performance and even makes it impossible to occur in the worst-case scenarios, such as heavy fading on the direct path. Fortunately, CR blended with cooperative communication stands as a fascinating combination for coverage area extension in addition to the other benefits such as enhanced data rates and interference mitigation (Ozcan & Gursoy, 2012). Cooperative communication is based on the concept of introducing multiple nodes, commonly known as relays which jointly cooperate and behave like a virtual antenna array to forward the message received from the source. In the literature, several relaying protocols exist in this context, however, the most commonly employed among all the existing protocols are Amplify-and-forward (AF) and Decode-and-Forward (Elsaadany & Hamouda, 2017).
As mentioned above, a major challenge faced by a relay-assisted cognitive radio network (RCRN) operating in an Underlay environment is the interference constraint of the PU. Thus, in attempting to satisfy the interference threshold, there might be significant power reduction at the intermediate relays resulting in performance drop at the secondary network. However, sophisticated signal processing techniques exist to mitigate the aforementioned issue and Relay Selection (RS) stands among the acknowledged methodologies in the research groups (Hu, Gross & Schmeink, 2014). RS in an Underlay scenario selects a single best relay or a best subset of relays keeping in view the privilege of the PUs. The existing RS schemes are broadly classified into Opportunistic Relay Selection (ORS) and Partial Relay Selection (PRS) (Sharma & Upadhyay, 2016). ORS relies on the perfect knowledge of channel-state information (CSI) and decides about the feasibility of each candidate relay to become a partner in communication based on the SNR of both source-relay and relay-destination hops. The later assumes fixed gain relays and makes decision based on SNR of either source-relay or relay-destination link.
Some selected contributions in RS, either opportunistic or partial, have been highlighted below in the context of Underlay sharing. The selection is based on the similarity of the system models. All cited contributions assumed single source-destination pair assisted by two-hop RCRN in a Rayleigh flat-fade scenario with all single antenna terminals. (Wang et al., 2015; Ezzeldin, Sultan & Youssef, 2014; Moualeu, Hamouda &Takawira, 2016; Li, 2011) proposed Best Relay Selection (BRS) schemes in the absence of direct path between secondary end nodes. The network performance was optimized taking into consideration the interference from the secondary network towards the PU. Similar scenario has been considered by Xu. et al (Xu et al., 2011) to carry out Multiple Relay Selection (MRS). However, there are relatively limited contributions in the area of MRS. In (Elshaarany et al., 2014), the authors catered the interference from the secondary source also and proposed MRS technique in the presence of direct path. (Hu, Gross & Schmeink, 2014) considered the absence of direct path in the secondary network and performed MRS while ignoring the interference from the secondary source towards the PU.