Cooperative communications has attracted significant recent attention as a transmission strategy for future wireless networks. Cooperative communications efficiently take advantage of the broadcast nature ofwireless networks to allownetwork nodes to share theirmessages and transmit cooperatively as a virtual antenna array, thus providing diversity that can significantly improve systemperformance. Cooperative communications can be applied in a variety of wireless systems and networks. In the research community, a considerable amount of work has been done in this area for networks such as cellular, WiFi, ad hoc/sensor networks, and ultrawideband (UWB). These ideas are alsoworking their way into standards; e.g., the IEEE 802.16  (WiMAX) standards body for future broadbandwireless access has established the 802.16jRelay Task Group to incorporate cooperative relayingmechanisms into this technology.Most existingwork on cooperative communications concentrates on the physical (PHY) and medium access control (MAC) layers of wireless networks, examining issues such as capacity improvement, power control, and relay selection. The impact on the higher layers, such as routing in the network layer, has not been fully investigated. With cooperative communications, link-level performance can be improved by using relays. So, optimal route selection depends not only on the nodes of the direct links but also on the relays as well. In this chapter, we examinemethods for improving traditional routing schemes by considering the use of cooperative diversity over a variety of wireless systems such as ad hoc, sensor, and general wireless networks.
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