Cooperative space-time codes with network coding

Because of the gains like diversity, throughput, and the coverage extension provided by the multiple-input multipleout (MIMO) systems, it has been part of the various wireless communications standards. However, to reap the benefits offered by MIMO systems in size-constrained nodes, used in wireless communications, has been a challenging issue. To address this challenge, cooperative communications has been introduced as an implementable way to exploit the multi-antenna (MIMO) gains in small size nodes. Various schemes or protocols have been devised to address the way by which cooperative communications actually takes place. In this article, we propose and investigate a novel scheme which provides a way to combine the benefits of space-time codes and network coding for cooperative communications. In this scheme, the cooperating users using space-time codes are assisted by the availability of a fixed relay node proposed for the future wireless networks. We compare our scheme with cooperative communications using Alamouti scheme and non-cooperative communications. With analytical results, we prove that the proposed scheme offers diversity gain of order three while cooperative communications using Alamouti scheme offers two and non-cooperative communications offers diversity order of one only. Moreover, the proposed relay assisted scheme outperforms by 15% in terms of multiplexing gain, the cooperative communications scheme being compared. Introduction Cooperative communications has been a hot topic for research since its introduction by Sendonaris et al. [1-3] in 1998. The reason for the popularity of the idea is that it gives a way to exploit the multiple-input multiple-out (MIMO) benefits in size-constrained nodes, like used in Ad hoc, sensor or a cellular network. It is well known that the wireless channel, while giving us independence of movement, also introduces an unreliability in the message being transmitted [4]. This unreliability is due to the inherent characteristics of the wireless channel like scattering, reflection, refraction, and diffraction. To overcome this problem, MIMO was introduced to use the spatial dimension, another degree of freedom, available in the wireless channel. MIMO uses multiple transmit and multiple receive antennas for transmission of the message and introduces independent fading paths between each transmit and receive antenna of theMIMO systems. In this way, *Correspondence: [email protected] 1Information Technology Centre, Sindh Agriculture University, Tandojam, Pakistan Full list of author information is available at the end of the article the receiver is provided with multiple copies of the same message from various statistically independent paths; this introduces spatial diversity gain in the wireless channel. As for as point to point communications is concerned, this technique ofMIMO systemworked and has been part of many standards like UMTS [5], WiMAX [6], WLAN [7], and LTE-Advanced [8]. But due to the space problem, same happened to be challenging in size-constrained nodes. Cooperative communications is a concept to implement MIMO in the situation where it is not feasible to install multiple antennas on the nodes due to their size and the cost incurred per node. Cooperative communications is a communications strategy where transmitting users use each others’ antennas to realize MIMO gains. In cooperative communications when the transmitters send their messages to the destination, the free users present in their surrounding also receive that message. The users after detecting those messages, forward some additional information on behalf of the transmitting users to the destination. In this way, the receiver is provided with multiple copies of the same © 2012 Das Menghwar et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Das Menghwar et al. EURASIP Journal onWireless Communications and Networking 2012, 2012:205 Page 2 of 8 http://jwcn.eurasipjournals.com/content/2012/1/205 message transmitted and the spatial diversity gain is introduced in the channel [2,3,9]. On the other hand, in a relay channel [10], a dedicated node is present between the transmitter and the receiver which is used to forward the second copy of the message transmitted by the user to the destination. The way by which a relay node in a relay channel and the partner user in cooperative communications forward information to the destination is known as protocol. Some of the basic relaying protocols introduced so far are Decode-and-Forward, Compress-and-Forward, and Amplify-and-Forward [11,12]. In Decode-and-Forward relaying protocol, the relay node decodes and then reencodes the message received from the transmitting user and forwards it to the destination. In Amplify-andForward protocol, the relay node amplifies and forwards the signal received from the transmitting user to the destination. Finally, in Compress-and-Forward protocol, the relay node forwards the compressed version of the message received from the transmitting user to the destination. We propose a scheme which is based on Decode-andForward protocol. Here the transmitting users, also called the partner users, act as relay nodes for each other. In addition, these cooperating users also exploit the availability of a dedicated relay node to cope with various adverse effects of the wireless channel and achieve the MIMO gains. Information theoretic outage probability and diversity multiplexing tradeoff are used as performance measures and the proposed scheme is compared with Alamouti scheme based cooperative communications and non-cooperative communications. The results achieved show that the proposed scheme offers higher diversity gain than non-cooperative as well as cooperative communications using Alamouti scheme. The rest of the article is structured as follows. In Section System model, we describe our system model. Section Mutual information and the outage probability deals with the mutual information and the resulting outage probability of the proposed scheme. On the basis of the outage probability results in Section Mutual information and the outage probability, the diversity-multiplexing tradeoff perspective of the proposed scheme is presented in Section Diversity-multiplexing tradeoff. Finally, in Section Conclusion we conclude our article. Systemmodel To better explain our scheme, we take a cellular scenario with two mobile users u1 and u2 transmitting to a common destination, that is base station in this case. These transmitting users are considered to use space-time coding based Alamouti scheme for cooperation. In addition to that, we also introduce a dedicated relay node in the surrounding of the transmitting users and the scenario is depicted in Figure 1. The dedicated relay node is used to send network coded bits of the information received from the cooperating users to the destination. The network coded bits are formed by taking Exclusive-OR of the information bits received from the transmitting users.a For better understanding, first we briefly discuss here the cooperative communications setup using Alamouti scheme only, this is depicted in Figure 2. The transmission of the message is accomplished in two phases. The first phase is the broadcast phase where mobile station users transmit their messages s1 and s2, respectively to the destination. Due to the broadcast nature of the wireless channel, these messages, in addition to the intended destination, are also received by each other and the scenario is illustrated in Figure 2a. The second phase is called the multiple-access phase. In this phase, the mobile station users, after detecting the messages from each other, send −s∗2 and s∗1, respectively to the destination. The messages sent in the multiple-access phase are formed by following well known space-time coding based Alamouti scheme [13]. The second phase of the transmission is shown in Figure 2b. After having discussed this, now we explain our proposed scheme which we call as cooperative communications using Alamouti scheme with network coding. Again the whole process of communication spans two phases, i.e., the broadcast phase and the multiple access phase. This time in the first phase, the transmitted messages are not only received by the cooperating mobile station users and the intended destination but at a dedicated relay node,