Novel Precoded Relay-Assisted Algorithm for Cellular Systems

Hindawi Publishing Corporation EURASIP Journal on Wireless Communications and Networking Volume 2010, Article ID 414657, 10 pages doi:10.1155/2010/414657 Research Article Novel Precoded Relay-Assisted Algorithm for Cellular Systems Sara Teodoro, Adão Silva, João M. Gil, and Atı́lio Gameiro Instituto de Telecomunicações, Universidade de Aveiro, 3810-193 Aveiro, Portugal Correspondence should be addressed to Sara Teodoro, Received 14 July 2010; Accepted 11 November 2010 Academic Editor: Richard Kozick Copyright © 2010 Sara Teodoro 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. Cooperative schemes are promising solutions for cellular wireless systems to improve system fairness, extend coverage and increase capacity. The use of relays is of significant interest to allow radio access in situations where a direct path is not available or has poor quality. A data precoded relay-assisted scheme is proposed for a system cooperating with 2 relays, each equipped with either a single antenna or 2-antenna array. However, because of the half-duplex constraint at the relays, relaying-assisted transmission would require the use of a higher order constellation than in the case when a continuous link is available from the BS to the UT. This would imply a penalty in the power efficiency. The simple precoding scheme proposed exploits the relation between QPSK and 16-QAM, by alternately transmitting through the 2 relays, achieving full diversity, while significantly reducing power penalty. Analysis of the pairwise error probability of the proposed algorithm with a single antenna in each relay is derived and confirmed with numerical results. We show the performance improvements of the precoded scheme, relatively to equivalent distributed SFBC scheme employing 16-QAM, for several channel quality scenarios. 1. Introduction Multiple-input, multiple-output (MIMO) wireless communications are effective in mitigating channel fading, thus improving the cellular system capacity [1]. However, there is significant correlation between channels in some environments, and using an antenna array at the user terminal (UT) may not be feasible due to size, cost, and hardware limitations. Cooperative systems are promising solutions for wireless systems to overcome such limitations, when the direct link does not have good transmission conditions [2]. It can be achieved through cooperation of terminals (either dedicated or user terminals acting as relays), which share their antennas and thereby create a virtual antenna array (VAA) or a virtual MIMO (VMIMO) system [3]. These allow single antenna devices to benefit from spatial diversity without the need for colocated additional physical antenna arrays. Several cooperative diversity protocols have been proposed and analysed to demonstrate the potential benefits of cooperation [4, 5]. In [6], a theoretical diversitymultiplexing trade-off study is presented regarding a cooperative system with 1 and 2 antennas in a single-relay scheme. Furthermore, in [7] the Rayleigh performance of a single-relay cooperative scenario with multiple-antenna nodes is investigated, deriving pairwise error probability (PEP) expressions. In order to get higher diversity orders, one can also consider the use of multiple-relay nodes [8]. However, increasing the number of relays reduces the bandwidth efficiency of the system, as the source uses only a fraction of the total available degrees of freedom to transmit the information. In [9], the idea of space-time coding devised for pointto-point colocated multiple antenna systems is applied for a wireless relay network with single antenna nodes and PEP of such a scheme is derived. The authors show that in a relay network with a single source, a single destination with R single antenna relays, distributed spacetime coding (DSTC) asymptotically approaches the diversity of a colocated multiple antenna system with R transmit antennas and one receiving antenna. More recently, in [10], distributed orthogonal space-time block codes (DOSTBCs) achieving single-symbol decodability have been introduced for cooperative networks. The authors considered a special class of these codes (row monomial DOSTBCs), deriving upper-bounds on the maximum symbol-rate. In [11], singlesymbol decodable (SSD) DSTBCs have been studied when the relay nodes are assumed to know the corresponding 2 EURASIP Journal on Wireless Communications and Networking channel phase information. An upper bound on the symbol rate for such a set up is shown to be one half of the rate of the direct link transmission when this link is available, which is independent of the number of relays. In [12], a semi-orthogonal precoded DSTBC was proposed, wherein the source performs precoding on the information symbols before transmitting it to all the relays. Several relays are considered all equipped with a single antenna. A systematic construction of these codes is presented for more than 3 relays and the constructed codes are shown to have higher rates than those of DOSTBCs. Although achieving full diversity, these distributed orthogonal algorithms cannot achieve full spectral efficiency, since they use 2 phases for transmission. For this reason, these cooperative systems achieve half of the bandwidth efficiency of the equivalent non-cooperative systems. Other works were developed with the objective of increasing capacity or diversity order of cooperative systems, using non-orthogonal protocols for cooperative systems with 2 or more relays, as in [13, 14]. In [13] a generalised non-orthogonal amplify-and-forward protocol is proposed with a low decoder complexity, achieving better error performances than in [9], depending on the coding. In [14] coding strategies are studied for non-orthogonal cooperative channels, using one or more designed space-time precoders, in a protocol where inter-relay communication is allowed, but again having no full spectral efficiency. In these nonorthogonal algorithms, transmission via an existing direct path is required. Thus, in situations with poor direct link conditions, performance is significantly degraded and in case of outage of one relay some information can be lost. Further along the development of cooperative systems, some relay precoder designs were also proposed, however with different goals [15, 16]. In [15], the precoder maximizes the capacity between the source and destination nodes in a non-regenerative relay system, with a single relay node, considering all the nodes with multiple antennas. In [16], MIMO relay provides robustness against imperfect channel state information (CSI), for a multipoint-to-multipoint communication through the use of a relay precoder design. Our previous work included a distributed spacefrequency block coding (SFBC) scheme, designed for orthogonal frequency-d (...truncated)