First IEEE International Conference on Communications in China: Communications QoS and Reliability (CQR)
An Improved Spectrum Management Scheme for OFDMA Femtocell Networks Mingjie Feng, Da Chen, Zhiqiang Wang, Tao Jiang and Daiming Qu Wuhan National Laboratory for Optoelectronics Department of Electronics and Information Engineering Huazhong University of Science and Technology Wuhan, 430074, China Email:
[email protected] Abstract—The femtocell network is regarded as an effective approach to provide better services for indoor users with higher capacity. However, the spectrum allocation problem of the femtocell network is challenging for the wireless operator, since the added femtocell tier either requires additional spectrum resource or brings interference to the existing cellular network. In this paper, we consider the methods of spectrum allocation and spectrum sharing to improve the capacity performance of femtocell networks. We first propose a spectrum allocation strategy that utilizes the structure of the cellular network to reduce interference. Then, we propose a spectrum sharing scheme that deals with the problem of insufficient channels for femtocells. Adjacent femtocells could coordinate with each other for resource allocation by taking the instantaneous traffic demand into consideration. Finally, the proposed spectrum management methods are compared with several conventional methods and the improvements of the proposed schemes are verified by simulation results.
Index Terms — femtocell, spectrum allocation, spectrum sharing, throughput. I. I NTRODUCTION By deploying indoor base stations that are close to indoor users to shorten the transmit-receive distance, femtocell technology could effectively solve the indoor coverage problem and provide high data rates [1]. However, when femtocells utilize the same spectrum band with existing cellular networks, cross-tier interference and interference among different femtocells will be inevitable. To deal with the interference problem, power control [2] and spectrum partitioning [3], [4] were proposed. In [5], a fractional frequency reuse (FFR) scheme was proposed to reduce interference, but the reduction of interference is at the expense of decreased spectrum utilization. In [6], an adaptive fractional frequency reuse scheme was proposed, where the frequency reuse pattern is adjusted according to the specific situation. Cognitive radio was applied in [7], [8] to avoid cross-tier interference. Femtocells opportunistically utilize the available channels by recognizing the interference levels around. However, the problem of miss detection and false alarm still exist, and the optimal channel allocation strategy requires further study. Another method to handle cross-tier interference is using access control strategy [9]. In the closed access pattern, only the registered users are permitted to access to femtocell, while all the users can by served by femtocell in the open
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access pattern. The uplink capacity under different access patterns was analyzed in [10]. In [11], the author calculated the average downlink capacity in different areas with both access patterns. In this paper, we first propose a novel spectrum allocation scheme for femtocell networks. A 7-cell macrocell system is considered, and femtocells in different regions utilize the spectrum bands that generate less interference. After that, a spectrum sharing mechanism is proposed to address the problem of channel insufficiency of femtocell. When a femtocell is overloaded, or some channels of this femtocell is severely interfered, it can borrow channels from neighboring femtocells. Since the spectrum allocation strategy fully utilizes the cellular structure to reduce interference, and the spectrum sharing mechanism enables femtocells to coordinate with each other for better resource allocation patterns, our schemes offer better throughput performance compared with several conventional schemes. The rest of this paper is organized as follows. The system model of this paper is presented in Section II. The proposed spectrum allocation scheme is described in Section III. Then, the spectrum sharing strategy for each femtocell is analyzed in Section IV. The performance of the proposed scheme is verified in Section V. Finally, Section VI summarizes the conclusions. II. S YSTEM MODEL The system model of this paper is shown in Fig. 1. We consider a 7-cell cellular system, and the methods of this paper can be applied to other kinds of cellular networks. The whole spectrum band of the wireless system is divided into 7 parts, each macrocell utilizes one part of the spectrum that is different to others. Denote the radius of macrocell and femtocell as Rc and Rf , respectively. We assume that the bandwidth of a macrocell B is divided into N channels, and the bandwidth of each channel W is equal to the bandwidth required for macro-user communication. Thus, each macrouser corresponds to one channel in our model. Both femtocells and macro-users are randomly distributed in the macrocell area, and the number of femtocell users in a femtocell area follows poisson distribution with mean value λ. The channel model is considered as rayleigh fading channel with unit
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average power, and experience path loss and wall penetration loss. The wall penetration loss and the path loss exponent are denoted as L and α, respectively. The noise power is σ 2 .
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Fig. 1.
of all the macrocell users. Therefore, it is necessary to adopt different spectrum allocation strategies in different regions. Besides, if femtocells in the outer region utilize the spectrum of neighboring macrocells, the interference scheduling involves multiple macrocells, which may greatly increase the system complexity. In [14], femtocells in the inner region utilize the spectrum of another macrocell, while femtocells in the outer region utilize the spectrum of the corresponding macrocell. For femtocells in the inner region, due to the relative long distance to other macrocell, interference between femtocells and other macrocell is significantly reduced. In the outer region, the femtocell base stations are not close to macrocell base station, therefore, we can utilize spectrum sensing for femtocells to avoid interference with cell-edge macrocell users.
System model.
To avoid inter-femtocell interference, femtocells that are close to each other could not utilize the same channel simultaneously. Therefore, we utilize the method in [6] for interfemtocell spectrum allocation. In this method, the bandwidth of each femtocell is determined by the number of femtocells in the same interference group, and each femtocell in the group is allocated with the same bandwidth. For example, if 4 femtocells are in the same interference group, which means each of the 4 femtocells brings interference to at least one other femtocell in the group, then, each femtocell is allocated with a quarter of total bandwidth of femtocell tier, and femtocells utilize different spectrum bands to avoid interference. Based on this spectrum allocation method, we will discuss the spectrum sharing mechanism later. III. F EMTOCELL SPECTRUM ALLOCATION The femtocell spectrum allocation methods in previous study can be categorized as follows. One category is that femtocells located in a macrocell area share the spectrum with this macrocell, thus, interference management is necessary to coordinate between the two tiers. For the other category, femtocells utilize the spectrum of another macrocell [12], [13], while femtocells in cell-edge area may still interfere with other macrocells. In [11], the macrocell area is divided into inner region and outer region with boundary Dth , noticing that femtocells located in the two regions have different features, and we can utilize the features to reduce interference. Actually, we can utilize the cognitive radio technology to effectively control the interference between macrocell users and femtocells since macrocell users are distributed all over a macrocell area. However, the interference between macrocell base station and femtocells may hardly be solved by cognitive radio method, especially for femtocells that are close to the macrocell base station, since macrocell base station operates in a centralized pattern that transmits and receives the signal
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The proposed spectrum allocation scheme.
In order to further improve the system capacity, we propose a novel spectrum allocation pattern that utilizes the features of the cellular network. As shown in Fig. 2, the inner region of a macrocell is further divided into 6 parts, femtocells located in the 6 areas utilize the spectrum of different neighboring macrocells. With this allocation pattern, the femtocells are separated further from the co-channel neighboring macrocell, thus, the interference is reduced. In scenarios with more compact frequency reuse pattern, such as 3-cell cellular system, the value of Dth should decrease in order to guarantee the QoS of cellular system. Based on this spectrum allocation structure, the spectrum allocated to each femtocell is according to the rules in [6]. IV. S PECTRUM SHARING Since femtocells in the outer region utilize the same spectrum with macrocell, femtocell base station (FBS)potentially interfere or are interfered by nearby macrocell users (MU). When the interference between an FBS and an MU is severe, it is necessary for the FBS to avoid using the same channel with MU, hence, the available spectrum bandwidth of FBS is reduced. Besides, when the traffic load of an FBS is heavy, while the cross-tier and inter-femtocell interference also exist, the FBS may not be able to provide enough bandwidth for
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each femtocell user (FU), thus, the QoS can not be guaranteed. Therefore, the methods to deal with the insufficiency of channels is required for femtocell network. In this paper, we propose a method that allow femtocells to borrow channels from neighboring femtocells in order to satisfy the traffic demand. A. The conditions to trigger spectrum sharing In our assumption, femtocells in the outer region may utilize uplink and downlink spectrum of the macrocell, and FBS should avoid occupying the same channel with nearby MUs. In the outer region of macrocell, the uplink transmission power of MU is high while the downlink power received by MU is low. Therefore, in order to avoid cross-tier interference, FBS first detects the interfering uplink signal and records the frequency band used by the interfering MU. Then, the FBS reports the channel information of the MU to the macrocell base station (MBS). MBS identifies the MU according to the channel information, and sends the downlink channel scheduling information to the FBS. Based on the signal strength of nearby MU, FBS could estimate the interference strength that may cause to the MU if FBS occupies the downlink channel of the MU. Finally, with the downlink scheduling information, the FBS avoids using the same channels with nearby MUs. The signal to interference and noise ratio (SINR) of an FU when interfered by a nearby MU is denoted as Pf R−α g0 (1) Pu Ld−α g0 + σ 2 The SINR of an MU when interfered by an FBS is denoted as Pc D−α g0 (2) SIN RM U = Pf Ld−α g0 + σ 2 SIN RF U =
Where Pc and Pf denote the transmission power of MBS and FBS, respectively, d is the distance between the interfering femtocell and MU, R is the distance between the FBS and FU, D is the distance between the MBS and MU. g0 is the exponentially distributed channel power with unit mean. The SINR estimated by the FBS is compared with a threshold γ. If the SINR is below γ, the communication quality of an FU or MU is unsatisfactory that may lead to outage, and it is necessary for the FBS to evacuate from the corresponding channels. Another aspect that may cause the insufficiency of channels for femtocell is the traffic load. Assume that for femtocell m, there are pm femtocells in the same interference group, and the traffic is tm . There are qm MUs that use the same channels with femtocell m around and require femtocell m to evacuate from the interfering channels. To guarantee the QoS of FUs, we also assume that the bandwidth provided to each FU should be no less than the bandwidth of an MU, and the uplink and downlink transmissions of femtocells utilize different channels (an FDD pattern). Therefore, femtocell m will have insufficient available channels when round PBm − qm W
