How to Solve the Problem of Bad Performance of Cooperative Protocols at Low SNR
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Research Article How to Solve the Problem of Bad Performance of Cooperative Protocols at Low SNR Charlotte Hucher, Ghaya Rekaya-Ben Othman, and Jean-Claude Belfiore Ecole Nationale Superieure des Telecommunications, 46 rue Barrault, 75013 Paris Cedex 13, France Correspondence should be addressed to Charlotte Hucher, [email protected] Received 1 June 2007; Accepted 27 August 2007 Recommended by Ranjan K. Mallik We propose some new adaptive amplify-and-forward (AF) and decode-and-forward (DF) protocols using a selection. The new selection criterion is a function of the instantaneous capacities of all possible transmission schemes (with or without cooperation). Outage probabilities and simulation results show that the adaptive cooperation protocols solve the problem of bad performance of cooperation protocols at low SNR. Moreover, they improve the asymptotic performance of their corresponding AF and DF protocols. Copyright © 2008 Charlotte Hucher et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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INTRODUCTION
Diversity techniques have been developed in order to combat fading on wireless channels and improve the reliability of the received message. Recently, cooperation has been proposed as a new mean to obtain “space-time” or “cooperative” diversity [1, 2]. Different nodes in the network cooperate in order to form a virtual MIMO system and exploit space-time diversity even if their hardware constraints do not allow them to support several antennas. Many cooperative protocols have been proposed [3–6] which can be classified in three main families: amplify-and-forward (AF), decodeand-forward (DF), and compress-and-forward (CF). In this paper we are interested in the two first families, which are the more natural ones. AF protocols have been studied the most due to their simplicity. Indeed, the relays just amplify the received signals and forward them. DF protocols require a bit more processing: this strategy consists in decoding the received signals at the relays and then forwarding them. They have interesting performance, however, and are even essential for multihop systems. Asymptotically, both protocols bring diversity and give better performance than SISO which only uses the direct link. However, it does not match noncooperation at low SNR. We propose here a new strategy named adaptive cooperation which can be applied either to AF or to DF protocols.
This new strategy consists in choosing the best transmission scheme, based on a new selection criterion, a function of the instantaneous capacities of all these possible transmission schemes. Selection between cooperation and noncooperation has already been proposed in literature for DF protocols [5, 7], as well as relay selection [8], but never adapted to AF protocols. Moreover, the usual selection criterion of DF protocol is based only on the source-relay outage probability, while the proposed sel
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