Hybrid Access Femtocells Research Articles

Rate Maximization for Hybrid Access Femtocell Networks With Outage Constraints Based on Pricing Incentive Mechanism

Wireless services are developing rapidly and user requirements also are becoming more demanding. Femtocell networks have great potential in many aspects, especially in expanding the capacity and increasing the coverage of traditional cellular networks. However, it is challenging to motivate femtocell base stations (FBSs) to voluntarily adopt a hybrid access strategy to provide services and access to macrocell users (MUEs). In order to maximize the transmission rate of users, we propose an incentive mechanism in two-tier femtocell networks including a macrocell and multiple femtocells that share the spectrum. In the proposed mechanism, the macrocell base station (MBS) attempts to provide femtocells with a reward by pricing the transmission rates of the accessed MUEs, which are guaranteed by the FBSs. A Stackelberg game is developed to maximize the utility of the MBS and FBSs, and to guarantee the quality of service (QoS) of different users. An uncertain channel gain is formulated in the optimization problem and the uncertainty is restricted by a probabilistic constraint. Furthermore, a power control and nonuniform price bargaining algorithm is proposed to satisfy practical operation and obtain an optimal solution. Finally, numerical results validate the effectiveness of the proposed algorithm in terms of transmission rate improvement and outage reduction.

Robust power control based on hierarchical game for hybrid access femtocell networks

Femtocell network is regarded as the potential and effective technique to improve the capacity and coverage of traditional cellular networks. One of the challenges is how to access the network and manage the interference among different users. Compared with other access strategies, hybrid access strategy allows femtocell base stations (FBSs) to provide preferential access to femtocell users (FUEs) while other users can access nearby FBS with specific restrictions. In this paper, a robust downlink power control scheme is studied in two-tier femtocell networks, where femtocells share the same frequency with macrocell. A hierarchical game framework that takes the different users' requirements into consideration is constructed. In addition, as the link gains are actually uncertain in dynamic environment, probabilistic constraints are used to describe the uncertainty. Then two sub-problems are obtained to maximise the sum rate of macrocell and femtocells, respectively and guarantee the quality of service (QoS) of different users. To tackle the nonlinear and nonconvex optimization problem, successive convex approximation is introduced. And the practical iterative power allocation algorithm is provided. Finally, numerical results show that the proposed scheme is effective in aspect of energy saving and QoS guarantee under dynamic environment.

Dynamic Resource Management for LTE-Based Hybrid Access Femtocell Systems

Hybrid access femtocells for long term evolution (LTE)-based cellular networks provide a tradeoff between closed and open access femtocells whereby all subscribers are granted access albeit with priority given to closed access subscribers. Due to the need to accommodate both closed and open access subscribers, quality of service (QoS) provisioning for LTE-based hybrid access femtocells has become more challenging. This paper addresses this issue and proposes a new dynamic resource management scheme for such hybrid architectures. In particular, the proposed scheme first classifies and performs lexicographic admission control on the incoming traffic data flows using an optimal greedy algorithm. A suboptimal delay-bounded packet scheduling algorithm and a dual decomposition-based power allocation algorithm are developed to solve the non-convex maximization problem such that the weighted sum rate of each femtocell is maximized, subject to bounded packet delays and power constraints. Simulation results show that the proposed scheme can significantly outperform existing schemes in terms of QoS, throughput and fairness.

Improving data rate of indoor users through time sharing access of femtocell service in underlay heterogeneous network

This Letter proposes a cooperation policy between a macrocell and a hybrid access femtocell in an underlay heterogeneous network to improve the achievable data rate of an indoor macrocell user (MU) through time sharing access of the femtocell service together with mitigating cross-tier interference through price-based power control. Through numerical examples it is proved that the authors’ proposed cooperation policy is more beneficial to the macrocell not only for providing a higher achievable data rate for its MU but also for earning a higher revenue compared with the incentive-based cooperation policy proposed earlier without much degradation of the achievable data rate performance of any associated femtocell user.

Interference Avoidance Challenges in Densely Deployed OFDMA Hybrid Access Femto Cellular Networks

: Femtocell is a low powered small base station that operates on licensed frequency band. Femtocells are deployed by users to improve indoor coverage. Femtocells and the outer macrocell can co-exist with dedicated spectrum allocation or shared spectrum allocation strategy to avoid interference. The dedicated spectrum allocation strategy is not cost-effective as spectrum resource is limited. However the shared spectrum allocation strategy where the femtocells share the same frequency bands with the macrocell is an efficient solution, but demands specialized inter-cell interference management techniques. This paper investigates the characteristics of dense femtocells and its various access control modes and provides an overview of resource allocation, open issues and challenges for interference avoidance in dense OFDMA hybrid access femtocells.

Channel resource allocation and availability prediction in hybrid access femtocells

In this paper, three channel assignment models are proposed for channel resource allocation in femtocells. The models proposed are based on the Markov chain process. According to the access mechanisms in femtocells, the Third-Generation Partnership Project (3GPP) has described two kinds of users in the femtocell application; open users and the closed subscriber group (CSG). In hybrid access mechanism for femtocells, both CSG users and open users are usually referred as subscribers and non-subscribers respectively. So, in this work, for all the proposed models, the CSG and open users are categorized into two groups; the subscriber group (SG) and the non-subscriber group (NSG) respectively. The proposed models provide priority based channel resource allocation strategies between the SG and NSG. Furthermore, the focus of this research is to provide variable channel resource sharing among the SG and NSG to keep the wastage of channel resources minimum for better quality of service (QoS). The analysis is conducted in terms of channel resource blocking management for all the models and to validate the analysis, simulations are performed at the end of this paper. Further, channel resource blocking prediction, based on the blocking probability results for SG and NSG users, is also provided at the end. The prediction is done through risk analysis using the @ Risk tool. The simulations are provided in two parts; 1) the probability curves for SG and NSG against the total number of channels and 2) the risk analysis results for blocked channels prediction using the @ Risk tool.

A Novel Resource Allocation and Power Control Mechanism for Hybrid Access Femtocells

LTE Small cells like Femto cells are being deployed in enterprises and residential buildings to improve data rates of indoor users who experience low Signal-to-Interference plus Noise Ratio (SINR) from Macro Base Stations (MBSs). Deployment of Femto cells inside a building can lead to signal leakage at the edges/corners of the buildings. This causes cross-tier interference and degrades the performance of users in High Interference Zone (HIZone) around the building area, who are connected to one of the MBSs in LTE Heterogeneous Networks (HetNets). Hybrid Access Femto cells (HAFs) can ensure QoS for paid Subscriber Group (SG) users by giving them preferential access to resource blocks over non-SG (NSG) users and also improve the throughput of LTE HetNet system by serving nearby NSG users. In this work, we address various challenges involved in deployment and operation of HAFs in indoor environments by proposing an Optimal Placement of Femto cell (OPF) model, a dynamic Bandwidth Allocation (BWA) mechanism for splitting resource blocks between SG and NSG users, a dynamic power control mechanism to mitigate co-tier and cross-tier interference in HetNets and an Enhanced Priority (EP) scheduling mechanism to give more priority to SG users over NSG users.During peak traffic load scenarios, HAFs may not be able to guarantee QoS of both indoor and HIZone users connected to them. As HAFs are primarily meant for indoor users, HAFs employ an Optimal Power Control (OPC) mechanism to tune their transmit powers so that HIZone users are offloaded to near-by MBSs. Since the OPC is a Mixed Integer Non-linear Programming (MINLP) problem, we put forth a Sub-Optimal Power Control (SOPC) mechanism. The SOPC mechanism boosts the throughput of MBSs by almost 62% over the traditional 3GPP proposed enhanced Inter-Cell Interference Co-ordination (eICIC) mechanism for 300 users in the HetNet system. Also, the proposed EP scheduling mechanism maintains Jain’s fairness index of 0.99 for both SG and NSG users while providing a 40% higher per user throughput than that obtained with the legacy proportional fair and Priority Set schedulers.

Scheduling and Power Allocation for Hybrid Access Cognitive Femtocells

This paper addresses the problem of resource and power allocation for hybrid access femtocells. We introduce a refund mechanism to incentivize femtocell basestations (FBSs) to serve macrocell users (MUEs) suffering from low signal to interference and noise ratio (SINR) to enhance overall performance. Our goal is to guarantee quality of service for users, while allowing spectrum sharing between macrocell basestation (MBS) and the underlying FBSs. We exploit overheard user channel quality indicator (CQI) reports using different channel models in order to assess the interfered channel state and channel parameter distribution. We analyze the distribution of the SINR for both femtocell users and MUEs. Based on the analytical results, our solution decomposes the scheduling and power allocation problem into two sub-problems and tackles them sequentially. Using problem reduction/transformation, we convert the decomposed problems into well known reduced forms and provide solutions in accordance. Finally, we verify the presented results through simulations.

Load Balancing in Two-Tier Cellular Networks With Open and Hybrid Access Femtocells

Femtocell base station (BS) is a low-power, low-price BS based on cellular communication technology. It is expected to become a cost-effective solution for improving the communication performance of indoor users, whose traffic demands are large in general. There are mainly three access strategies for femtocell, i.e., closed access, open access and hybrid access strategies. While it has been generally known that open/hybrid access femtocells contribute more to enhancing the system-wide performance than closed access femtocells, the operating parameters of both macro and femtocells should be carefully chosen according to the mobile operator's policy, consumer's requirements, and so on. We propose long-term parameter optimization schemes, which maximize the average throughput of macrocell users while guaranteeing some degree of benefits to femtocell owners. To achieve this goal, we jointly optimize the ratio of dedicated resources for femtocells as well as the femtocell service area in open access femtocell networks through the numerical analysis. It is proved that the optimal parameter selection of open access femtocell is a convex optimization problem in typical environments. Then, we extend our algorithm to hybrid access femtocells where some intra-femtocell resources are dedicated only for femtocell owners while remaining resources are shared with foreign macrocell users. Our evaluation results show that the proposed parameter optimization schemes significantly enhance the performance of macrocell users thanks to the large offloading gain. The benefits provided to femtocell users are also adaptively maintained according to the femtocell users' requirements. The results in this paper provide insights about the situations where femtocell deployment on dedicated channels is preferred to the co-channel deployment.

A Rewarding Framework for Network Resource Sharing in Co-Channel Hybrid Access Femtocell Networks

With the explosive growth in mobile data traffic, femtocell technology is regarded as the most effective way to enhance the mobile service quality and system capacity of cellular networks. However, the major problem with femtocell deployment is finding an appropriate access control mode that mobile operators and users are willing to adopt. Among the various kinds of access control modes, the hybrid access mode is considered the most promising because it allows femtocells to give preferential access to femtocell owners, while other public users can only access femtocells with certain restrictions. Because all femtocell owners are selfish, how to provide sufficient incentives so that they will share their femtocell resources is a challenging issue. To address the problem, we propose an economic framework for mobile operator and femtocell users based on game theoretical analysis. We also exploit the concept of revenue sharing, which provides a positive cycle to sustain the femtocell service. In the framework, a femtocell game is formulated where the femtocell owners determine the proportion of femtocell resources they will share with public users, while the operator maximizes its benefit by setting the ratio of the revenue distributed to femtocell owners. We analyze the existence and uniqueness of the Nash Equilibrium of the game. The results of extensive simulations show that the proposed framework maximizes the operator’s benefit and satisfies the users’ service requirements.

Rate-Based Pricing Framework in Hybrid Access Femtocell Networks

In femtocell networks, it is a challenge to motivate femtocell access points (FAPs) to adopt hybrid access policy and provide services to accessed macrocell user equipments (MUEs) in access control mechanism investigation. In this letter, we propose an incentive framework within which the macrocell base station (MBS) provides profit to femtocells by pricing accessed MUEs' transmission rates guaranteed by FAPs. A Stackelberg game is formulated to maximize the utility functions of both the MBS and FAPs. The Stackelberg equilibrium (SE) for the proposed game is investigated and the best strategies for both tiers are obtained. Numerical results show that utility functions of FAPs can be improved significantly, which provides strong motivation for FAPs to adopt hybrid access and implement the proposed framework.

Efficiency Evaluation for Femtocell Spectrum Access in LTE-Advanced

Femtocells are considered one of the ultimate solutions for the ever increasing demand in LTE-Advance. Recently, wireless industries have resorted to femtocell networks in order to enhance indoor coverage and quality of service since macro-antennas fail to reach these objectives. In enabling indoor home or enterprise users with mobile broadband solutions, role of femtocells are crucially important. While considering low cost solutions for higher coverage and data rate, femtocells apparently have one of the best potentials for indoor users. Due to the dense self-deployment of femtocells in a limited area, serious inter-femtocell interference (IFI) may cause, which consequently results in severe performance degradation. To mitigate the IFI and utilize spectrum resource more efficiently, this paper proposes a cluster based femtocell deployments along with a capacity based cognitive resource allocation scheme .Three different channel configurations in a hybrid access femtocell network are considered for performance analysis. The results of a performance analysis of the cluster based femtocell configurations in a priority based users’ network are presented.

Utility-driven relay for hybrid access femtocells based on cognitive radio spectrum auction

Femtocells promisingly improve coverage and spectrum efficiency at the edge of macrocell. However, macro users may suffer dead-zone trouble due to strong interference coming from closed access femtocell, or compete spectrum resource with femto users in the open access model. In this paper, we propose a novel utility-driven relay scheme adopted by hybrid access femtocell (HAF) to address the above two issues, wherein the macrocell maximizes the usage of its idle spectrum bands, bided by femtocells, via cognitive radio spectrum auction; and recompense relay expense to stimulate selfish femtocells undertaking HAFs to relay the signal to the vicinal macro user, meanwhile avoiding two-tier interference. Spectrum competition among femtocells is formulated as a static Cournot game provided with global information; then necessarily considered as a dynamic Cournot game of the strategy adapting distributively according to the players previous actions. Furthermore, we introduce a protocol and propose refund factor, spectrum price dynamic adjustment algorithms to achieve the Stackelberg equilibrium, i.e., the solution of the non-cooperative game between macrocell and femtocells. We also prove the existence of equilibrium and the convergence of algorithms. Extensive simulation results show the details of games, which indicate they can match the win-win hybrid access mechanism, benefiting cell-edge macro users for better QoS and higher access opportunities for femto users.

Radio Resource Sharing Among Users in Hybrid Access Femtocells

A problem related to deployment of femtocells is how to manage access of users to radio resources. On one hand, all resources of the femtocell can be reserved for users belonging to a closed subscriber group (CSG), which is a set of users defined by a femtocell subscriber. This approach, known as closed access, however, increases interference to users not included in the CSG as those users do not have a permission to access this femtocell. Contrary, resources can be shared by all users with no priority in an open access mode. In this case, the femtocell subscriber shares radio as well as backhaul resources with all other users. Thus, throughput and quality of service of the subscriber and the CSG users can be deteriorated. To satisfy both the CSG as well as non-CSG users, a hybrid access is seen as a compromise. In this paper, we propose a new approach for sharing radio resources among all users. As in common cases, the CSG users have a priority for usage of a part of resources while rest of the resources is shared by all users proportionally to their requirements. As the simulation results show, the proposed resource sharing scheme significantly improves throughput of the CSG users and their satisfaction with granted bitrates. At the same time, throughput and satisfaction of the non-CSG users is still guaranteed roughly at the same level as if conventional sharing schemes are applied.

Resource Allocation in Hybrid Access OFDMA Femtocell Networks

Femtocell(飞小区)能够有效加强室内覆盖和系统容量。混合接入(hybrid access)方式可以兼顾授权用户和非授权用户的业务体验。针对下行混合接入方式下的正交频分复用接入(OFDMA) Femtocell网络，该文提出带权重的比例公平(WPF)调度算法，为不同类型的用户提供不同的调度权重。该文提出两种算法确定用户的调度权重，一种是自适应权重的比例公平算法，实时调整调度权重；另一种是静态权重的比例公平算法，基于WPF调度的渐近性分析得到静态最优调度权重。仿真结果表明该文提出的两种算法均能在授权用户和非授权用户间合理分配资源，保证不同用户的速率需求。