Wireless Communications and Mobile Computing

Volume 2017, Article ID 5120538, 13 pages

https://doi.org/10.1155/2017/5120538

## An Optimal Joint User Association and Power Allocation Algorithm for Secrecy Information Transmission in Heterogeneous Networks

Chongqing Key Laboratory of Mobile Communications Technology, Chongqing University of Posts and Telecommunications, Chongqing, China

Correspondence should be addressed to Mingxue Chen; moc.qq@8572613601

Received 4 November 2016; Accepted 14 February 2017; Published 6 March 2017

Academic Editor: Ernestina Cianca

Copyright © 2017 Rong Chai 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

In recent years, heterogeneous radio access technologies have experienced rapid development and gradually achieved effective coordination and integration, resulting in heterogeneous networks (HetNets). In this paper, we consider the downlink secure transmission of HetNets where the information transmission from base stations (BSs) to legitimate users is subject to the interception of eavesdroppers. In particular, we stress the problem of joint user association and power allocation of the BSs. To achieve data transmission in a secure and energy efficient manner, we introduce the concept of secrecy energy efficiency which is defined as the ratio of the secrecy transmission rate and power consumption of the BSs and formulate the problem of joint user association and power allocation as an optimization problem which maximizes the joint secrecy energy efficiency of all the BSs under the power constraint of the BSs and the minimum data rate constraint of user equipment (UE). By equivalently transforming the optimization problem into two subproblems, that is, power allocation subproblem and user association subproblem of the BSs, and applying iterative method and Kuhn-Munkres (K-M) algorithm to solve the two subproblems, respectively, the optimal user association and power allocation strategies can be obtained. Numerical results demonstrate that the proposed algorithm outperforms previously proposed algorithms.

#### 1. Introduction

In the past few decades, radio access technologies have experienced rapid development. Various types of radio access networks, such as cellular networks, wireless local area networks (WLANs), and worldwide interoperability for microwave access (WiMAX), have been deployed widely to provide Internet access services to users. In certain regions, these heterogeneous access technologies may coexist, resulting in heterogeneous networks (HetNets) [1], in which user equipment (UE) with multiple interfaces is allowed to associate with the base stations (BSs) of different networks in order to achieve information interaction. The design of user association schemes in HetNets is of particular importance as different user association strategies may result in various user quality of service (QoS) as well as network transmission performance due to the different channel characteristics between UE and BSs and the heterogeneity of access networks especially in terms of available network resource and resource management schemes.

In recent years, some research works have considered user association or cell association problem in HetNets. In [2], the authors study the cell association problem of a multitier HetNet and propose a unified distributed cell association algorithm which maximizes the sum utility of long-term rate and minimizes global outage probability. In [3], a distributed optimal user association algorithm is proposed for HetNets which maximizes the utilization of BSs. The authors in [4] stress the user association problem of a HetNet consisting of low power pico-BSs (PBSs) and high power macro-BSs (MBSs). Aiming at minimizing system blocking rate as well as offering interference-free communication to the edge UE of the PBSs, the authors formulate the joint optimization problem of user association and almost blank subframes (ABS) as a combinatorial optimization problem and propose to solve the optimization problem based on Hungarian matching algorithm and iterative algorithm. The authors in [5] examine the impacts of mobile backhaul networks on fiber-wireless enhanced LTE-A HetNets and propose a backhaul-aware user association algorithm to achieve intercell load balancing and network performance enhancement in terms of transmission delay and service block probability.

In the case that user association strategies have been designed for HetNets, the transmit power of wireless transmitters, that is, BSs or UE, plays an important role in affecting the transmission performance of users. To achieve reliable and efficient user transmission, optimal power allocation strategies are highly desired. References [6, 7] stress the power allocation problem of HetNets. The authors in [6] study the downlink power allocation problem of HetNets consisting of femto-BSs (FBSs) and MBSs and formulate the power allocation problem of the FBSs as a noncooperative game model under the constraint of the outage probability of macro-UE (MUE). Through solving the Nash equilibrium solutions of the game model, the transmit power strategies can be obtained. In [7], the authors jointly consider time domain and power domain optimization of a two-tier macro-pico-HetNet. Time domain performance optimization is achieved by applying an adaptive ABS configuration scheme which dynamically matches network resources to the real-time load of the network. To further enhance network performance and achieve the performance tradeoff between the two tiers, a utility function maximization based power control and scheduling scheme is proposed.

References [8–10] jointly consider user association and resource allocation problem in HetNets. The authors in [8] consider the downlink transmission in HetNets and propose a two-stage joint user association and power allocation algorithm which maximizes the minimum data rate of the UE. Reference [9] considers the joint design of transmit power and user association strategies in a downlink HetNet. A utility function defined as the logarithm function of user data rate is maximized to obtain the optimal power allocation and user association strategy. The authors in [10] study user association and resource allocation problem of scalable video coding multicast transmission over HetNets and propose a similarity-based negotiation algorithm to obtain the optimal user association strategies and a dynamic programming based algorithm to optimally design the transmission profile of video signals.

In previous research works [8–10], to maximize network throughput or the formulated utility function, the maximum transmit power should be applied in general. However, this may result in large power consumption and low energy efficiency, which are highly undesired. To stress the tradeoff between user transmission performance and power consumption, the energy consumption and the energy efficiency of HetNets should be considered in designing resource allocation schemes.

References [11–16] investigate the energy consumption issues in HetNets. In [11], the authors aim at minimizing the power consumption of an orthogonal frequency-division multiplexing (OFDM) based HetNet under the data rate constraints of the users. The authors in [12] investigate the energy efficient BS deployment and transmit power allocation strategies for both macrocells and femtocells in HetNets. A network energy consumption minimization problem is formulated under the constraint of coverage performance, and the optimal deployment density and the transmit power of MBSs and FBSs are obtained through solving the optimization problem. In [13], the authors consider a two-tier HetNet consisting of macrocells and small cells and propose to deploy the small cells around the edge of the macrocells. To increase the energy efficiency and spectral efficiency while reducing cochannel interference, the authors propose a location-based power control mechanism for small cell UE.

In [14], the authors present an architecture for multimedia transmission over HetNets and propose an energy efficient multimedia transmission scheme to optimize the energy efficiency of the BSs by exploiting user behavior characteristics. Reference [15] considers a downlink multiuser orthogonal frequency-division multiple access (OFDMA) system and proposes an optimal power and subcarrier allocation scheme to maximize the sum energy efficiency of users under the total transmit power constraint. The authors in [16] consider spectrum handoff and resource allocation problem of secondary users (SUs) in heterogeneous cognitive radio networks and propose an optimal joint handoff channel selection and transmit power allocation scheme which aims at maximizing the energy efficiency of all the SUs.

In previous research works [2–16], secure communication environment is considered where no information eavesdropping exists. However, as the network architecture of HetNets is more open and diverse compared to traditional cellular systems, information exchange in HetNets is more susceptible to eavesdropping; therefore, the problem of secure information transmission becomes extremely important in HetNets. References [17–19] study secure communication problem in HetNets. Reference [17] studies physical layer security in a multitier HetNet where BSs, legitimate users, and eavesdroppers are all randomly located. The authors propose an access threshold-based secrecy mobile association policy in which users are associated with the BSs offering the maximum received signal power.

In [18], the authors consider a HetNet scenario where a number of eavesdroppers tend to eavesdrop on the information of legitimate users. A resource allocation algorithm which jointly considers secure information transmission, cross-tier interference, and optimal power and subcarrier allocation is proposed in order to maximize the achievable secrecy sum rate of the network. The authors in [19] investigate secure communications in a two-tier downlink HetNet comprised of one macrocell and multiple femtocells. Assuming that an eavesdropper attempts to eavesdrop on the information of MUE, the authors propose a secure transmit beamforming scheme to maximize the secrecy rate of the MUE.

While secure information transmission in HetNets has been considered in [17–19], user association and resource allocation issues have not been studied extensively. In particular, in the case where there exist eavesdroppers who aim at eavesdropping on user information, traditional optimal user association and resource allocation strategies such as user data rate maximization based algorithm may not be feasible, as high data rate may require large transmit power, resulting in severer information leakage at the eavesdroppers, which is highly undesired.

In our previous work in [20], we study user association and power allocation problem for HetNets with eavesdroppers and propose an optimal user association and transmit power strategy for multiuser case. In this paper, we extend our previous work in [20]. Jointly considering user association and power allocation problem of HetNets where the information transmission from BSs to legitimate users is subject to the interception of eavesdroppers, we propose a joint radio resource management architecture, based on which a joint user association and power allocation algorithm is designed for both single user case and multiuser case. Aiming at achieving data transmission in a secure and energy efficient manner, the concept of joint secrecy energy efficiency of the network is introduced and is defined as the ratio of secrecy transmission rate and the power consumption of the BSs. An optimization problem is formulated which maximizes the joint secrecy energy efficiency under the power constraint of the BSs and the minimum data rate constraint of the UE. By transforming the optimization problem equivalently into two subproblems, that is, power allocation subproblem of each BS-UE pair and user association subproblem for all the users, and applying iterative method and Kuhn-Munkres (K-M) algorithm to solve the two subproblems, respectively, the optimal user association and transmit power allocation strategies can be obtained.

The major contributions of this paper are summarized as follows:(i)We study user association and power allocation problem of HetNets which consists of multiple access networks. To achieve joint resource management and performance enhancement of various access networks, we propose a joint radio resource management architecture, based on which a joint user association and power allocation algorithm is designed.(ii)User association problems or resource allocation problems in HetNets have been studied separately in previous works [2–7]. In this paper, we jointly consider user association and power allocation problem in HetNets and design joint optimal strategies so that the overall performance of the networks can be maximized.(iii)While most of the previous works [2–16] consider secure communication environment in HetNets, it is highly possible that the information transmission from BSs to UE is eavesdropped on by eavesdroppers. In this paper, we study the problem of information transmission in HetNets where eavesdroppers tend to eavesdrop on user information. To stress the secure transmission requirements and the tradeoff between transmission data rate and power consumption, we introduce the concept of secrecy energy efficiency of BSs. Jointly considering the overall network performance, we examine the sum secrecy energy efficiency of all the BSs and formulate the joint user association and power allocation problem in HetNets as a secrecy energy efficiency maximization problem.(iv)Since the formulated optimization problem is a mixed-integer nonlinear optimization problem which cannot be solved conveniently, we transform the original optimization problem equivalently into two subproblems, that is, power allocation subproblem and user association subproblem, which can then be solved by applying iterative method and K-M algorithm, respectively. The rest of the paper is organized as follows. Section 2 describes the system model considered in this paper and the proposed joint radio resource management architecture. In Section 3, we present the optimization problem formulation. The solution of the optimization problem is discussed in Section 4. In Section 5, we present the simulation results and discussions on the computation complexity of the algorithms. Finally, we conclude this paper in Section 6.

#### 2. System Model and the Proposed Joint Resource Management Architecture

In this section, we describe the system model considered in this paper and propose a joint resource management architecture.

##### 2.1. System Model

In this paper, we consider the downlink transmission of HetNets, which consist of multiple overlapping access networks. The UE located in the area of the HetNets may associate with one of the networks for information interaction. We assume that each network is assigned a portion of spectrum and no spectrum sharing between networks is allowed. Within each network, orthogonal resource blocks are allocated to various UE; thus, no transmission interference exists among UE in the same network. We consider the scenario that a number of eavesdroppers located in the HetNets intend to eavesdrop on user information. Due to the heterogeneity of the access networks especially in terms of transmission schemes and resource allocation strategies, each eavesdropper can only eavesdrop on the information of the UE associated with one particular network. For convenience, we assume that each eavesdropper is associated with one network and thus is capable of eavesdropping on user information when the users access the same network.

We denote the number of access networks and the amount of UE by and , respectively. Without loss of generality, we assume that only one BS is deployed in each network. For simplicity, the BS of the th network is denoted as , , and the th UE is denoted as , . We further assume that each UE can only associate with one BS and each BS can only serve one UE on certain time-frequency resource block.

In this paper, we assume the received signal of from can be expressed aswhere and are, respectively, the transmitted data symbol and the transmit power of when transmitting to , represents the channel gain of the link from to , , where and denote the path loss and shadowing attenuation of the link between and , respectively, denotes the slow fading channel coefficient of the link, which is modeled as a Rayleigh distributed random variable, and is the additive white Gaussian noise (AWGN) with zero mean and variance .

Similarly, in the case where is associated with , the received signal of the eavesdropper within the th network can be expressed aswhere denotes the channel model of the link between and the eavesdropper, , where and denote, respectively, the path loss and shadowing attenuation of the link between and the eavesdropper, and denotes the slow fading channel coefficient of the link, which follows Rayleigh distribution.

##### 2.2. The Proposed Joint Resource Management Architecture

Due to the heterogeneity of various access networks, the convenient information interaction and resource management of users within different networks may not be feasible. To achieve efficient information interaction and resource management within each access network and between different networks, we propose a joint resource management architecture in this paper.

Figure 1 shows the proposed joint resource management architecture, in which three types of functional entities, that is, global resource management entity (GRME), local resource management entity (LRME), and user resource management entity (URME), are introduced to tackle the dynamic information of the integrated networks and to conduct joint user association and power allocation of BSs. The major roles and functions of GRME, LRME, and URME are as follows.