Outage Probability versus Fairness Trade-off in Opportunistic Relay Selection with Outdated CSI
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Research Article Outage Probability versus Fairness Trade-off in Opportunistic Relay Selection with Outdated CSI Jose Lopez Vicario, Albert Bel, Antoni Morell, and Gonzalo Seco-Granados Group of Signal Processing for Communications and Navigations (SPCOMNAV), Autonomous University of Barcelona, 08193 Bellaterra (Cerdanyola del Valles), Barcelona, Spain Correspondence should be addressed to Jose Lopez Vicario, [email protected] Received 1 July 2008; Revised 18 November 2008; Accepted 20 January 2009 Recommended by Alagan Anpalagan We analyze the existing trade-offs in terms of system performance versus fairness of a cooperative system based on opportunistic relay selection (ORS) and with outdated channel state information (CSI). In particular, system performance is analytically evaluated in terms of outage probability, and the fairness behavior is assessed based on the power consumption at the different relays. In order to improve the fairness behavior of ORS while keeping the selection diversity gain, we propose a relay selection mechanism where the relay with the highest normalized signal-to-noise ratio (SNR) is selected for relaying the source’s information. The proposed strategy is compared with existing relay selection strategies by adopting a novel graphical representation inspired by expected profit versus risk plots used in modern portfolio theory. As shown in the paper, this strategy allows operating the system in more favorable points of the outage versus fairness region. Copyright © 2009 Jose Lopez Vicario et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
1. Introduction Cooperative diversity has been shown to be an efficient way to combat wireless impairments using low-complexity terminals [1–4]. Basically, these schemes allow for the exploitation of spatial diversity gains without the need of multiantenna technology. Different spatial paths are provided by sending/receiving the information to/from a set of cooperating terminals working as relays. By doing so, most of the advantages of multiple-input multiple-output (MIMO) techniques [5] can be extracted while keeping the complexity of the individual terminals reduced. Indeed, the benefits captured by cooperative communications are well extended in the research community, and standardization groups are considering the inclusion of cooperative techniques in practical systems. For instance, the IEEE 802.16j Relay Task Group [6] is involved in the incorporation of relaying mechanisms in the standard adopted by the new wireless system WiMAX [7]. Among the set of cooperative techniques, opportunistic relay selection (ORS) is a useful strategy for practical implementation [8]. This is because ORS is a low-complexity
strategy consisting only in activating the best relay (in accordance to a given performance metric). Apart from the inherent simplicity of the proposed technique, this strategy av
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