Future tactical networks will be complex, with severe constraints on energy and bandwidth, operating in dynamic and unpredictable environments. By exploiting the broadcast nature of the wireless medium and spatially dispersed nodes, some of these advantages can be realized through cooperation among single-antenna nodes in a network.

In cooperative systems, a group of nodes can transmit together as a virtual antenna array to obtain diversity gains. Cooperation techniques can significantly improve the performance of wireless networks with increased transmission range, improved energy and bandwidth efficiency, and more reliable and longer lasting network connectivity. Some of these techniques are:

  • Decentralized Relay-Selection and Transmission

    For any given number of columns in the underlying space-time block coding (STBC) matrix, the optimized discrete randomized schemes can achieve almost the same performance as the continuous randomized scheme, as long as the number of potential relays is much larger than the number of columns.

  • Power-Efficient Relay Selection

    In this strategy, only a small amount of local information is required to optimize the relay selection at each hop, which could be applied to either two-hop or multihop decode-and-forward (DF) networks.

  • Power Allocation

    Two ad-hoc, yet efficient, power-allocation strategies were developed for decentralized distributed STBC. The first strategy requires some control information and is near-optimal; the second strategy requires no control signaling, but is sub-optimal.

  • Network Lifetime

    One source and one selected relay cooperate to transmit source messages to the destination. When a Best-Select strategy is used, the forwarding relay is selected as the node with the best instantaneous or mean channel gain to the destination. To maximize the network lifetime while incurring the minimum overhead, an efficient relay-selection strategy using only the mean channel state information (CSI) and the residual energy information could be used.

  • Network Coding for Cooperation with Multiple Source-Destination Pairs

    Using mean-CSI-based, Best-Select cooperation, the selected best relay performs network coding on the correctly decoded information from all the sources by transmitting a random linear combination of the columns in an underlying full-rate, full-diversity, perfect STBC.

  • Hop-by-Hop Routing Using Mean Channel Gains

    A novel optimization metric was proposed to select the best relay on a per-hop basis by only utilizing the local mean channel gain of the current hop.

  • Cooperative Routing

    An optimal routing strategy was proposed to minimize the end-to-end outage, which requires the instantaneous CSI of all the links and serves as a performance bound. TV-hop routing, where a joint optimization is performed every N hops, can achieve a good complexity-performance tradeoff.

  • Fairness and Cooperation

    Fairness can actually bring significant throughput gains by using a price-aware cooperation protocol, where the residual energy information of each node is exploited to shape the relay set.

Future tactical networks will be deployed in highly dynamic environments, with severe constraints on energy and bandwidth. The concept of cooperative networking will have the most impact precisely in these applications. Through cooperation, the average power used in the network (per node) can be significantly reduced, and the reliability and connectivity can be dramatically increased.

This work was done by Leonard J. Cimini of the University of Delaware for the Air Force Research Laboratory. AFRL-0130

This Brief includes a Technical Support Package (TSP).
Cooperative Single-Antenna Node Networks

(reference AFRL-0130) is currently available for download from the TSP library.

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This article first appeared in the October, 2009 issue of Defense Tech Briefs Magazine.

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