Wireless Communications and Mobile Computing

Volume 2017 (2017), Article ID 9727360, 10 pages

https://doi.org/10.1155/2017/9727360

## Optimized Power Allocation and Relay Location Selection in Cooperative Relay Networks

^{1}School of Communication Engineering, Hangzhou Dianzi University, Hangzhou 310018, China^{2}National Mobile Communications Research Laboratory, Southeast University, Nanjing 210096, China

Correspondence should be addressed to Bin Jiang

Received 4 April 2017; Revised 2 July 2017; Accepted 11 October 2017; Published 9 November 2017

Academic Editor: Haiyu Huang

Copyright © 2017 Jianrong Bao 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.

#### Abstract

An incremental selection hybrid decode-amplify forward (ISHDAF) scheme for the two-hop single relay systems and a relay selection strategy based on the hybrid decode-amplify-and-forward (HDAF) scheme for the multirelay systems are proposed along with an optimized power allocation for the Internet of Thing (IoT). Given total power as the constraint and outage probability as an objective function, the proposed scheme possesses good power efficiency better than the equal power allocation. By the ISHDAF scheme and HDAF relay selection strategy, an optimized power allocation for both the source and relay nodes is obtained, as well as an effective reduction of outage probability. In addition, the optimal relay location for maximizing the gain of the proposed algorithm is also investigated and designed. Simulation results show that, in both single relay and multirelay selection systems, some outage probability gains by the proposed scheme can be obtained. In the comparison of the optimized power allocation scheme with the equal power allocation one, nearly 0.1695 gains are obtained in the ISHDAF single relay network at a total power of 2 dB, and about 0.083 gains are obtained in the HDAF relay selection system with 2 relays at a total power of 2 dB.

#### 1. Introduction

Recently, multiple-input multiple-output (MIMO), as a milestone in the development of wireless communications, brought an efficient transmission rate and reliability. To put it into practice, a cooperative communication scheme was then proposed in time [1], and it had been widely used and rapidly developed. In cooperative communications, the diversity gain was obtained, when the relay node forwarded messages and the destination node combined the received signals from both the source and relay nodes. According to different strategies for processing signals at the relay nodes, there are mainly three cooperation schemes, such as the amplify-and-forward (AF) [1], the decode-and-forward (DF) [2], and the coded cooperation (CC) [3]. To solve the deficiency of AF relay amplifying both the noises and signals, causing the incorrect DF relay decoding and also the error propagation phenomenon, an incremental relay protocol [4], accompanied by a hybrid decode-amplify-and-forward (HDAF), was proposed [5]. For the shortage of the incremental relay protocol, the incremental selection amplify-and-forward (ISAF) [6] was investigated, which selected the proper occasion to retransmit the messages in the source according to the channel estimation, when the direct transmission between the source and destination was failed. But the noise amplification still remained. Then, an incremental selection hybrid decode-amplify forward (ISHDAF) scheme was proposed in [7], where the HDAF scheme was combined with incremental selection strategy. Compared with the aforementioned ISAF scheme, the ISHDAF scheme had a significant improvement in bit error rate (BER) and outage probability, since both the BER and outage probability of a cooperative transmission system in the DF strategy were lower than those in the AF strategy. According to the principle of incremental relay, the average spectral efficiency of the ISHDAF scheme was also higher than that of the HDAF scheme. However, all gains were obtained under equal power allocation of the source and relay nodes for the low systematic complexity, which caused the deficiency of only few performance improvements. To improve the spectral efficiency of the system, there was also a signal-to-noise ratio (SNR) based incremental hybrid decode-amplify forward (IHDAF) protocol proposed in [8]. Furthermore, the SNR thresholds, the power allocation schemes, and the relay locations were studied to optimize the outage probability and BER performance.

Meanwhile, power allocation in cooperation communications had always been one of the research hot-spots. In wireless uplink transmissions, successive interference cancellation (SIC) was combined with the power allocation method to obtain the optimal power allocation ratio for efficient resource allocation [9]. For relay forwarding systems, two power allocation methods, by the Lagrange multiplier method and the differential algorithm, respectively, were also proposed for the lower bound of symbol error rate (SER) in the HDAF relay cooperative networks [10]. Xiao and Ouyang in [11] developed a two-source-destination-pair cooperative network with the HDAF protocol. And a closed-form expression of the outage probability was derived and thus a minimal total power was obtained under the constraints of the supposed outage probability. Similarly, for the two-source-destination-pair system, there was also a parallel shift water filling algorithm proposed for the power allocation [12]. The advantage of it over the conventional ones was the reduced complexity by just eliminating the iterative searching process. However, the cost is a little performance decrease. In [13], a multirelay selection scheme with joint power allocation was proposed, featured with the significantly decreased complexity of computation. To achieve this effect, it used a simple power reallocation during the multirelay selection process. Also a joint relay selection and power allocation scheme for cooperative wireless sensor networks was proposed in [14]. It adaptively chose the proper relays and their transmission power to maximize the signal-to-noise ratio (SNR) at the destination by the channel state information (CSI). Then a SNR-based relay selection for IHDAF cooperative diversity protocol was also proposed in [15], and the closed-form expressions of average channel capacity and outage probability were derived simultaneously. Also a swarm intelligence-based power allocation and relay selection algorithm could be used for wireless cooperative networks [16]. It could not only reduce the computational complexity effectively, but also select the optimal relay nodes to solve the nonlinear optimization problems by a fast global search with low cost. In [17], with a AF protocol based two-hop multiple energy-harvesting relays network, an improved power allocation was investigated to improve the whole outage performance. The innovation in the proposed scheme lied in the fact that it jointly maximized the transmit power under the constraints of limited individual relay energy. Also the power allocation and relay selection strategies in both dual-hop and multihop scenarios in cognitive relay networks were researched [18]. They achieved the good features of both minimal total transmit power and maximal entire network capacity. For the relay selection optimization, there had been a dynamic strategy to choose the best relay node and path under the constraints of total power and power allocation for each relay [18]. In addition, an improved relay selection strategy of HDAF scheme was proposed to improve the BER and outage probability [19]. It can adaptively select the AF or DF forward strategy for all relays according to the channel quality. Then the best relay was chosen to forward signals. However, it still used the equal power allocation to reduce the complexity. In [20], a power allocation algorithm by the lowest average bit error was proposed for these infrastructure-less networks using unbalanced communication links. To investigate the influence of the links on the system performance, the location of the relay node with respect to the source and destination nodes was also studied. Unfortunately, only the effect of certain node locations, rather than the optimal relay location, was determined. Subsequently, a much more detailed study about the optimal relay location was presented in [21], but under simply fixed ratio nodes power.

In this paper, by analyzing a two-hop single relay cooperative network with an ISHDAF scheme and the multirelay selection strategy with a HDAF scheme, an optimized power allocation is proposed. The main contributions are summarized as follows:(1)The power allocation is optimized in both the ISHDAF single relay and the HDAF multirelay systems. In the case of link status change, the preferred links are allocated with much more power for transmission according to the well-known water filling principle in information theory, which reduces the entire power consumption under the same system performance. The proposed scheme also provides a new hybrid automatic repeat request (ARQ) retransmission and relay forward mechanism, where the source node can retransmit messages to the destination node when the first direct transmission failed. It differs from the source node sending new messages to the destination node directly in the incremental relaying protocol. So it can be more suited for all kinds of the multiple relay channel status and obviously improves the systematic outage probability without any complexity increase.(2)The approximate closed-form expression of the systematic outage probability with relation to the node power and channel coefficients is derived by the equivalent infinitesimal replacement of the probability distribution function at high SNR. And it can be taken as the objective function of the optimization. Then, the minimization is achieved under fixed total power by Lagrange multiplier method, and the objective function is related to the power of the source node and the relay nodes. The power allocation coefficients between the source and the relay nodes are then obtained to achieve optimized power allocation. Moreover, the power allocation changes the location selection of the relay nodes, which can be calculated indirectly from the above closed-form expression. It can adaptively satisfy the link conditions to optimize the entire system performance.(3)By introducing the path loss factor, the powers of the source and the relay nodes are modeled as the objective function related to the distance among all nodes. According to both the property of the objective function and the related numerical analyses, the relationships of the varied power to the distance of the nodes are obtained. Then, the optimized node positions are obtained to improve the power efficiency with minimized systematic outage probability. Also the power allocation of all relay nodes with respect to their relative location to the source and destination nodes can be clearly and quantitatively analyzed by this model. Therefore, the link status associated with the proposed relay position obviously affects the selection of the cooperative schemes, which can be adopted in practice.

This paper is organized as follows. In Section 2, a single relay cooperation system with the ISHDAF scheme is introduced. Section 3 presents a multirelay selection strategy based on the HDAF scheme. Subsequently, the analytical expressions of the outage probability of both the ISHDAF and the HDAF relay system are derived in Section 4. In this section, an optimized power allocation using Lagrange Method is also proposed to minimize the outage probability. Simultaneously, the close-form analytical expression of the optimal relay location of the proposed algorithm is given to manifest the relationship between the relay location and the outage probability. After that, in Section 5, the simulation results and analyses are presented to verify the good outage probability and optimal power allocation brought by the proposed algorithm. The optimal relay location by the proposed method is also given and tested to be effective. Finally, Section 6 concludes the whole paper.

#### 2. Single Relay Model and ISHDAF Mutual Information Evaluation

For a classic three-node relay model shown in Figure 1, it consists of a source node S, a relay node R, and a destination node D. Equipped with a single omnidirectional antenna, all nodes communicates with each other. The ideal channel state information (CSI) can be obtained through channel training. For the independent links S-D, S-R, and R-D, their channel gains, that is, , , and , are subject to the exponential distribution with channel parameters as , , and , respectively. At a flat Rayleigh fading channel, the noise is an additive white Gaussian noise (AWGN), with zero mean and variance .