RELAY SELECTION APPROACHES FOR WIRELESS COOPERATIVE NETWORKS 1 Relay Selection Approaches for Wireless Cooperative Networks Tauseef Jamal, Paulo Mendes UITC, University Lusofona Campo Grande, 376, Lisbon, Portugal {tauseef.jamal, paulo.mendes}@ulusofona.pt Abstract—Wireless networks are characterized by having lim- ited resources accessed by a large number of mobile stations with distinct capabilities. In such challenged environment the dynamic control of resources is of major importance to mitigate the limitations of wireless networks, such as the impact of low data rate stations and wireless channel oscillations. Such augmented usage of wireless resources can be implemented based upon cooperative relaying schemes, which have the potential to support the desired system performance and network lifetime. However, the introduction of cooperative relay raises several problems such as the issue for relay selection and resource allocation. Due to the significant number of different cooperative relaying approaches, this article aims to provide a systematic analysis and classification of major relay selection procedures, and to identify open research directions as well as the most suitable evaluation methods for an efficient analysis of different approaches. Index Terms—Cooperative Communication, Relay Selection, Relaying, Routing, Performance I. I NTRODUCTION N EXT generation wireless networks are expected to pro- vide services that need high performances as well as bandwidth efficiency. This means that as the number of wire- less terminals increases, higher system capacity is needed to provide the required data rate levels. However, although wire- less networks provide easy connectivity and fast deployment, they still present low performance levels. The major limitation of wireless networks comes from the shared medium, limited- resources devices, and unstable wireless channels. Channel conditions in wireless networks are subjected to fading vari- ations, including interference that can affect both throughput and reliability. As a result, receivers may get multiple copies of the transmitted signal, each having travelled through a different path. Such multipath fading increases the number of errors in the transmission, requiring additional re-transmissions that decreases the network throughput. Nevertheless, the effects of multipath fading can be mitigated by using cooperative communications, which aims to take advantage of wireless diversity to provide efficiency levels equivalent to wired line communications and similar to wireless Multiple-Input Multiple-Output (MIMO) systems. The application of cooperative communications ranges from ad-hoc self organizing networks to vehicular networks, sensor networks and dynamic spectrum management. Technological challenges increase when nodes have intermittent access to a network infrastructure, which can happen in the presence of low data rate stations and in mobile scenarios. In the former case, network performance may decrease since low-data rate devices may grab the radio spectrum for long periods of time. In such situation high-data rate devices may act as relays, helping low-data rate devices to release the spectrum earlier, contributing to increase the overall system performance. In the case of mobile scenarios, two examples can be provided to show the benefits of applying cooperative commu- nications: vehicular networks and pedestrian networks. In the former example, cars may often leave the range of an Access Point (AP) without terminating their communication sessions. With cooperative communications, cars can relay data to the AP via another car, minimizing packet losses [1] and increasing coverage. In the case of pedestrian mobile networks, mobile devices may perform ping-pong movements at the edge of an AP with high probability, where devices will spend too much time performing handovers between neighbor APs, leading to performance degradation. To avoid such situation, another device can act as relay allowing the moving node to stay always associated to the same AP, avoiding handovers. These examples show the advantages of deploying cooper- ative communications to improve the utilization of wireless spectrum while providing higher network performance. In this paper, we focus on the application of cooperative communications, namely relaying, to increase spectrum and power efficiency, network coverage as well as to reduce outage probabilities. With the introduction of cooperative relaying, the relay selection process requires special attention since it has a strong impact on network and transmission performance. This article provides an analysis of relay selection proce- dures proposed in recent years. The goal is to identify the most suitable relay selection mechanism to support the design of cooperative MACs and cooperative routing strategies. This study includes the creation of a taxonomy and the performance analysis of the most prominent proposals. This article ends with an analysis of open research issues. II. RELATED WORK ON RELAY SELECTION Independently of operating only at the link layer or in combination with cooperative diversity schemes at the phys- ical layer, the performance of cooperative relaying strongly depends upon the efficiency of the process used to select one or more relays. In what concerns relay selection mechanisms, the basic mechanism proposed in [2] defines an opportunistic behavior