On the performance of distributed space-time coded cooperative relay networks based on inter-relay communications

Le-Quang-Vinh Tran 0 Olivier Berder 0 Olivier Sentieys 0 0 INRIA/IRISA, Universite de Rennes 1 , 6 rue de Kerampont, Lannion 22300, France A new protocol, called fully distributed space-time coded (FDSTC) protocol having information exchange between relays, is proposed and compared with the conventional distributed space-time coded (DSTC) protocol using non-regenerative relays (NR-relays) and regenerative relays (R-relays). Closed-form error probabilities are derived to verify the simulations. In terms of error performance, the FDSTC protocol gets significant average signal-to-noise ratio (SNR) gains (3.7 dB for NR-relays and 18.1 dB for R-relays). In addition, the impact of the relative distance of relays on the required SNR is reduced up to 70%. The system diversity order using the FDSTC protocol is higher than that using the DSTC protocol (especially, the FDSTC protocol obtains full diversity with NR-relays). As a result, at the same spectral efficiency, FDSTC has better performance in terms of outage probability in high SNR regions. In terms of energy efficiency, the FDSTC protocol is shown to outperform DSTC for long-range transmissions. 1 Introduction In the last years, cooperative relay has been identified as a core technique to overcome challenges in wireless environments. Based on the transmission protocol performed at relays, we can classify them into two main categories: repetition-based and distributed space-time code-based (DSTC-based) cooperative relay techniques [1]. In repetition-based cooperative relay techniques, each relay, after receiving signal from the source, will consecutively forward the signal to destination. Each transmitted signal (including the original one transmitted from source to destination) is passed through multiple independent paths, and thus, the probability that the signal fails to reach the destination is significantly reduced. In contrast to repetition-based cooperative relay techniques, in DSTC-based cooperative relay techniques, the relays will take advantage of DSTC techniques to simultaneously forward the signal to the destination, and hence, the spectral efficiency of the system is increased. The initial work on using DSTC protocols over relay channel is considered in [1-4]. We denote them as the conventional DSTC protocols, in which the whole communication consists of only two phases. Firstly, the source transmits signals to relays and destination. Secondly, the non-regenerative or regenerative relays (NR-relays or R-relays, respectively) use the DSTC protocol to forward signals to the destination. 1.1 Related work Recently, data exchanges among the relays, or interrelay communications, have been considered in [5-9]. In [5], an inter-relay cooperation scheme allowing message exchanges among the relays was presented in order to mitigate the impact of error propagation of R-relays. A hybrid TDMA-FDMA-based protocol with inter-relay communication for Nakagami-m fading channel was investigated in [6]. The lower bound on outage probability of the interrelay communication-based protocol was mathematically derived and validated by simulations [7]. Besides, the novel idea of inter-relay traffic cooperative network-coded hybrid ARQ was proposed as a smart way to improve R-relays in [8]. Error probability of DSTC protocols was derived for NR-relays in [4,10-13]. An approximate formula of average symbol error probability (ASEP) for a DSTC system based on multi-user cooperation was found in [4]. On the other hand, performance of the DSTC protocol in regenerative relay networks was derived in [3,14-17]. In [3], a regenerative DSTC protocol is applied for source and relay with optimal allocation of transmit power in order to minimize the average bit error rate (BER) at final destination. In [17], Anghel et al. proposed two error-aware distributed space-time (EADST) protocols to overcome worse BER performance of R-relays induced by decoding errors at the relays. However, these two protocols require feedback from destination. Recently, an extension of the DSTC protocols for more than two hops was considered in [18]. Besides BER performance consideration, in [2], the DSTC protocol was shown to be effectively used for higher spectral efficiency and to achieve full spatial diversity in some specific cases. In [1], the performance of cooperative protocols was considered in terms of outage probability. The authors showed that, except for fixed decode-andforward (DF) protocol, all of their cooperative protocols are efficient in the sense of achievement of full diversity. In [13], the DSTC system was shown to have the same diversity as a multiple-antenna system when the coherence interval is greater than the number of relays. With NR-relays, the diversity of the DSTC system was shown to depend on the scaling factor of the relays [12]. In [19], a diversity-multiplexing trade-off (DMT) analysis was considered in multi-hop multiple-input and multiple-output (MIMO) relay networks. A DMT analysis was also considered recently for an AF two path half-duplex relaying scheme in [20]. Wicaksana showed that DMT is achievable for finite codeword lengths with a careful choice of coding strategy. Cooperative techniques have recently been potential candidates to reduce energy consumption in wireless networks. In [21], using cooperative MIMO systems, a tremendous energy saving was shown for long transmission distances in comparison with single-input and singleoutput (SISO) systems. Besides, the energy efficiency of different cooperative relaying techniques was investigated. In LTE-advanced networks, using relay nodes, it was shown that energy saving up to 15.6% is possible in the two hop schemes, and up to 8.5% are possible with the multicast cooperative scheme [22]. In [23], a transparent relay with cooperative strategy was shown to save about 60% power consumption as compared to the transparent relay in IEEE 802.16j under given simulation configurations. Inter-relay communications have been considered in the repetition-based cooperative relay protocol and are shown to be able to increase its performances [5-8]. The present paper associate inter-relay communications with the DSTC-based cooperative relay protocol to further increase its performance. The fully distributed space-time coded (FDSTC) protocol in an inter-relay communication phase is added for cooperative relay systems. FDSTC was originally mentioned in [9] with NR-relays jointly with a lower bound on its ASEP. In this work, the FDSTC protocol is thoroughly considered in terms of error probability, outage probability, diversity order, and energy efficiency. Firstly, a closed-form ASEP of non-regenerative relays based on FDSTC is considered. Moreover, for regenerative relays, a closed-form expression of error probability, conditioned to the channel, is also derived. With the help of data exchange between relays in the FDSTC protocol, the SNR at relays is increased. This leads to a higher SN (...truncated)