Mobile Information Systems

Volume 2016, Article ID 7364154, 11 pages

http://dx.doi.org/10.1155/2016/7364154

## Fairness-Aware and Energy Efficiency Resource Allocation in Multiuser OFDM Relaying System

Department of Telecommunication and Information Engineering, Jiangsu Key Lab of Wireless Communications, Key Lab on Wideband Wireless Communications and Sensor Network Technology of Ministry of Education, Nanjing University of Posts and Telecommunications, Nanjing, Jiangsu 210003, China

Received 11 September 2015; Revised 30 March 2016; Accepted 21 April 2016

Academic Editor: Lin Gao

Copyright © 2016 Guangjun Liang 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

A fairness-aware resource allocation scheme in a cooperative orthogonal frequency division multiple (OFDM) network is proposed based on jointly optimizing the subcarrier pairing, power allocation, and channel-user assignment. Compared with traditional OFDM relaying networks, the source is permitted to retransfer the same data transmitted by it in the first time slot, further improving the system capacity performance. The problem which maximizes the energy efficiency (EE) of the system with total power constraint and minimal spectral efficiency constraint is formulated into a mixed-integer nonlinear programming (MINLP) problem which has an intractable complexity in general. The optimization model is simplified into a typical fractional programming problem which is testified to be quasiconcave. Thus we can adopt Dinkelbach method to deal with MINLP problem proposed to achieve the optimal solution. The simulation results show that the joint resource allocation method proposed can achieve an optimal EE performance under the minimum system service rate requirement with a good global convergence.

#### 1. Introduction

Recently, orthogonal frequency division multiple (OFDM) cooperative communication systems have been widely used to overcome the limitations of the users’ space constraints, because the relays can adopt the frequency diversity technique to deal with channel fading. In multiple frequency channels, subcarrier pairing was first devised independently for a single-user communication environment in [1, 2], by matching the subcarriers in OFDM relaying networks. Further in multiuser relaying networks [3, 4], both the relay and users are sharing all the channel. Owing to the complexities and drastic variations in the channel conditions for different users, appropriate channel-user assignments can potentially enable significant improvements in the spectral efficiency (SE). Recently, energy efficiency (EE) [5–9] has emerged as one of the most promising solutions to resolve issues such as the rapidly increasing energy consumption and the carbon emissions caused by the escalating growth of wireless data traffic in next-generation networks. The authors in [10–13] have conducted a preliminary research on EE multiuser relaying resource allocation (RA). In such OFDM relaying networks, an optimal system performance should take into account three problems jointly: the subcarrier pairing, channel-user assignment (the assignment of subcarrier pairs to users), and the power allocation that have a strong correlation among them. However, the combinatorial solution of these three problems entails the mixed-integer linear programming problem (MILP) which generally has an excessive computational complexity. Besides, if we further consider the energy efficiency resource allocation problem, the optimization problem will be a mixed-integer nonlinear programming (MINLP) problem that in general is computationally undesirable because of its combinatorial nature which could adopt branch-and-bound method to solve [14]. Previous attempts to optimize the performance of the multiple users OFDM relaying networks have usually considered only a subset of three problems regarding the SE [15–22] or a subset of three problems regarding the EE [5–12] or have adopted a suboptimal approach [13, 23].

Based on single-user amplify-and-forward (AF) OFDM relaying networks, [15] shows that the subcarrier pairing method according to the instantaneous channel gain that matches the incoming and outgoing subcarriers is sum-rate optimal. The authors in [16, 17] propose a joint power allocation and subcarrier pairing scheme in the same scenarios with [15] for the AF and the decode-and-forward (DF) networks, respectively. However, the works mentioned above only address the maximization of the end-to-end SE without jointly considering the direct passing path. For the AF and the DF, respectively, [18, 19] consider power allocation and subcarrier pairing scheme jointly, which could be obtained by the Lagrange means, in the single-user OFDM relaying network when the passing path directly is available. An equal power allocation policy in [23] exploits an optimal subcarrier-user allocation method which maximizes the system throughput. For DF relaying networks, [21] provides an asymptotically optimal scheme to jointly allocate power and assign subcarrier user. The same in DF relaying networks, [20] considers all the three problems with the total power constraint. However, this approach is suboptimal because of separately applying the power allocation, subcarrier pairing, and subcarrier-user allocation. The authors in [22] address a maximized throughput problem by joint considering power allocation, subcarrier pairing, and subcarrier pair-user allocation. Based on usual relaying strategies, the optimal resource allocation problem can be obtained by Lagrange means.

For a downlink OFDM system, [5] presents a bandwidth allocation scheme based on energy efficiency to maximize the number of bits per Joule energy consumption. Reference [6] offers a global optimal energy efficiency scheme to solve the optimal energy efficiency of the system that decomposes the joint optimization problem into three subproblems. However, [5, 6] do not consider the EE of the OFDM relaying networks which are traditional OFDM networks without relays. Considering a sum-rate constraint for a cooperative OFDM DF relaying network, [7] develops a sum-power minimized RA algorithm without the direct source-destination link. In [8], based on AF relaying networks with a frequency selective channel, a suboptimal two-step power allocation and subcarrier pairing strategy is raised to maximize the energy efficiency when the direct source-destination link is unavailable. In a downlink cooperative multiuser OFDM relaying network, the authors in [9] present a joint subcarrier pairing and power allocation scheme to maximize the EE of the system with the proportional fairness.

In [10–13], multiple users relaying networks are considered. Considering only the power allocation without considering the subcarrier pairing and the channel-user assignment, [10] presents a multiple users AF relaying model. Reference [11] formulates energy efficiency optimization problem for power allocation with fairness in cooperative multiple users fading channels. In [12, 13], maximizing the EE problem is proposed which is formulated as the ratio of the SE over the total power dissipation and joint power and subcarrier allocation in a multiple users OFDM relaying network. In [12], the optimization problem is solved under a constraint of providing the minimum required spectral efficiency in the OFDM relaying networks.

In [13], the objective function (OF) is proven to be quasiconcave and can adopt Dinkelbach method to obtain the optimal solution by solving a sequence of subtractive concave problems using the dual decomposition approach. However, [12, 13] jointly consider only power allocation and subcarrier pairing without considering channel-user assignment.

In this work, we focus on the design of a fairness-aware resource allocation scheme in a cooperative OFDM network. Compared with traditional OFDM relaying networks, the source is permitted to retransfer the same data transmitted by it in the first time slot, further improving the system capacity performance. The problem which maximizes the energy efficiency (EE) of the system with total power constraint and minimal spectral efficiency constraint is formulated into a mixed-integer nonlinear programming (MINLP) problem which has an intractable complexity in general. The optimization model is simplified into a typical fractional programming problem which is testified to be quasiconcave. Thus we can adopt Dinkelbach method to deal with MINLP problem proposed to achieve the optimal solution. The simulation results show that the joint resource allocation method proposed can achieve an optimal EE performance under the minimum system service rate requirement with a good global convergence.

The main contributions of the work are summarized as follows:(i)We propose a new energy-efficient maximizing method in a multiple users relaying system with direct passing path and total power constraints which jointly optimizes the subcarrier pairing, the power allocation, and the channel-user assignment. We can also adopt Lagrangian method and continuity relaxation to solve the joint optimal problem. But the three-dimensional assignment problem is its key subproblem which is nondeterministic polynomial time (NP-hard) but has time complexity.(ii)Compared with traditional OFDM relaying networks, the source is permitted to retransfer the same data transmitted by it in the first time slot, further improving the system capacity performance. Because of both the new relay cooperation protocol and the subcarrier pairs-user three-dimensional assignment algorithm, the proposed joint resource allocation method could achieve optimal EE performance under a minimum system data rate requirement with a good global convergence which can be proved by theoretical derivation and simulation analysis.(iii)The optimization model is simplified into a typical fractional programming problem which is testified to be quasiconcave. Thus we can adopt Dinkelbach method to deal with MINLP problem proposed to achieve the optimal solution.

The remained of this paper is organized as follows. In Section 2, the system model and the resource allocation problem are described. In Section 3, the problem is formulated and reformulated by the convex optimization technique. In Section 4, a resource allocation iteration algorithm will be adopted and solved by a dual decomposition method. In Section 5, both the analysis and the simulation results are presented to compare the performances of the different resource allocation schemes. Section 6 gives a conclusion for this paper.

#### 2. System Model

In this section, we first introduce the adopted system model and the performance measure. Then, the design of the resource allocation and scheduling is formulated as an optimization problem.

##### 2.1. System Model

Considering an OFDM single relay multiusers system as in Figure 1, source transfers the information to all the users with the help of one relay. In this paper, we focus on the joint resource allocation problem in a scenario including one source, one relay, and users. equal-bandwidth orthogonal channels haves been divided from the available frequency spectrum, accessible by the relay and all the users. Without loss of generality, we assume that all the channels occupy the same bandwidth and experience independent frequency selective fading. It is also assumed that all pieces of the channel state information (CSI) are available at the source, the relay, and the users [18]. The half-duplex transmission process can be divided into two phases. The source transmits the information data to all the other nodes including the relay and all the users in the first phase. The relay decodes the received signals in the first phase and retransmits them to all the users in the second phase. Compared with traditional OFDM relaying networks [22], the source is permitted to retransfer the same data transmitted by it in the first time slot, further improving the system capacity performance [24].