Cooperative communications in resource-constrained wireless networks

In order to exploit the spatial diversity gains inherent in multiuser wireless systems without the need of multiple antennas at each node, cooperative communications have been proposed. The concept is to have users relay each others messages and thus forming multiple transmission paths to the destination. The advantages of cooperation in resources constrained networks can be further exploited by optimally allocating the energy and bandwidth resources among users based on the available channel state information (CSI) at each node. Cooperative communications exploit the spatial diversity in multiuser systems by allowing users with diverse channel qualities to cooperate and relay each other's messages to the destination. Instead of having users compete for channel usage, we show that resources can be utilized more efficiently by allowing sensors with highly correlated data to cooperate and transmit simultaneously in the same channel or time slot. Two features differentiate cooperative transmission schemes from conventional non-cooperative system including: the use of multiple users' resources to transmit the data of a single source and a proper combination of signals from multiple cooperating users at the destination. Cooperation techniques can also be readily extended to a large network. In dual-hop relay networks, there is an increased degree of freedom as a result of more relay nodes. In a multihop scenario, cooperative transmission system can be extended by concatenating multiples of the three-node or the dual-hop networks.