How to Solve the Problem of Bad Performance of Cooperative Protocols at Low SNR

Hindawi Publishing Corporation EURASIP Journal on Advances in Signal Processing Volume 2008, Article ID 243153, 7 pages doi:10.1155/2008/243153 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, 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. 1. 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 selection takes all the channel links into account. Outage probability calculations and simulation results prove that the new adaptive AF and DF protocols perform asymptotically better than their corresponding AF and DF protocols, and more interesting, solve the problem of poor performance of cooperation protocols at low SNR. 2. SYSTEM MODEL We consider N + 1 terminals who want to transmit messages to the same destination D. The channel is shared in a TDMA manner, so each terminal is allocated a different time slot and the system can be reduced to a relay channel with one source, N relays, and one destination (Figure 1). The N + 1 terminals play the role of the source in succession while the others are used as relays. In the next sections, we will use the notation given in Figure 1. The channel coefficient of the link between source 2 EURASIP Journal on Advances in Signal Processing R1 R2 h1 g1 h2 g2 g0 S hn Rn D gn Figure 1: System model : a relay channel with one source, N relays, and one destination. (b) SISO case: only direct link is used, symbols are sent over the source-destination link in a noncoded manner, at a rate of 1 symb. pcu. (c) NLOS case: only nonline-of-sight (NLOS) link is used, in a first phase symbols are sent over the source-relay link in a noncoded manner and forwarded by the relay in a second phase. The rate is then 1/2 symb. pcu. Therefore in order to have the same spectral efficiency of 1 symb. pcu case as in the other cases, we need to use a larger constellation. For example, if the other protocols use a 16-QAM constellation, the NLOS scheme must use a 256-QAM. The principle of this new adaptive AF strategy is to evaluate the qualities of the three schemes (SISO, AF, and NLOS) and to select the best of them. Generalization to the N-relay case S and destination D is g0 , the one between source S and relay Ri is hi , and the one between relay Ri and destination D is gi . We consider a half-duplex channel; each terminal, and in particular the relays, cannot receive and transmit at the same time. The channel links are Rayleigh, slow fading, so we can consider their coefficients as constant during the transmission of at least one frame. We suppose that all terminals are equipped with only one antenna; the MIMO case is not considered in this work. We focus here on the protocol. So, for simplicity, we assume a uniform energy distribution between source and relays, with the total power kept constant. We will see in the following (see Sections 3.3 and 6.2) that channel state information needs to be known only at destination. 3. NEW SELECTION FOR AF PROTOCOLS AF protocols proposed in literature [3, 4, 6] bring diversity at high SNR, but their performance at low SNR is poorer than that of the noncooperative scheme. To solve this issue, we introduce the adaptive AF strategy where the choice of a transmission scheme is based on the channel links quality. 3.1. Presentation of the adaptive AF The idea leading to the definition of the adaptive AF strategy is to consider all possible transmission schemes and decide which one to select. In order to better understand this strategy, the one-relay case is detailed, before the generalization to the N-relay case. One-relay case There are only three possible transmission schemes as follows (Figure 2). (a) AF case: full cooperation scheme is used, symbols are sent using the AF protocol. In case of a full rate protocol (NAF [9]), the symbol rate is 1 symbol per channel use (1 symb. pcu). This selection can be generalized quite easily to a higher number of relays. For example, for 2 relays R1 and R2 , the number of possible schemes is 7: (1) full cooperation: symbols are sent using the AF protocol for 2 relays. With a full rate protocol, the symbol rate is 1 symb. pcu; (2 (...truncated)