Design, Implementation and Characterization of a Cooperative   Communications System
  Cooperative communications is a class of techniques which seek to improve reliability and throughput in wireless systems by pooling the resources of distributed nodes. While cooperation can occur at different network layers and time scales, physical layer cooperation at symbol time scales offers the largest benefit in combating losses due to fading. However, symbol level cooperation poses significant implementation challenges, especially in synchronizing the behaviors and carrier frequencies of distributed nodes. We present the implementation and characterization of a complete, real-time cooperative physical layer transceiver built on the Rice Wireless Open-Access Research Platform (WARP). In our implementation autonomous nodes employ physical layer cooperation without a central synchronization source, and are capable of selecting between non-cooperative and cooperative communication per packet. Cooperative transmissions use a distributed Alamouti space-time block code and employ either amplify-and-forward or decode-and-forward relaying. We also present experimental results of our transceiver's real-time performance under a variety of topologies and propagation conditions. Our results clearly demonstrate significant performance gains (more than 40x improvement in PER in some topologies) provided by physical layer cooperation, even when subject to the constraints of a real-time implementation. We also present methodologies to isolate and understand the sources of performance bottlenecks in our design. As with all our work on WARP, our transceiver design and experimental framework are available via the open-source WARP repository for use by other wireless researchers. 
Patrick Murphy and Ashutosh Sabharwal
cs.IT cs.NI math.IT