Open Access Femtocells Research Articles

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.

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.

Base-Station Sleep Management in Open-Access Femtocell Networks

In this paper, we focus on enhancing the energy efficiency in open-access femtocell networks where many femto base stations (BSs) are deployed in a large public area such as airports, shopping malls, etc. In those areas, the femtocells are overlapped and underutilized during most of the operation time because femto BSs are densely deployed to support the peak traffic load. Therefore, if we properly coordinate the user association (UA) with BSs and put the femto BSs having no associated users to sleep, the energy consumption can be saved remarkably. In this paper, we propose a scheme that jointly decides the operation modes (e.g., active or sleep) of femto BSs and the association between users and the active BSs to minimize the energy consumption in the network. Simulation results show that the proposed scheme can significantly reduce the total energy consumption in the open-access femtocell networks.

Combined power and rate allocation in self-optimized multi-service two-tier femtocell networks

In this paper a novel utility-based game theoretic framework is proposed towards simultaneously allocating users’ uplink transmission power and rate in multi-service two-tier open-access femtocell networks. Each user is associated with an appropriately designed utility function, depending on both transmission powers and rates, while accounting for the tier each user belongs to (i.e. Macro User (MU) and Femto User (FU)), and the type of service he requests (i.e. Real Time (RT) or Non-Real Time (NRT)). Towards mitigating the intercell and cross-tier interference caused by FUs and giving higher priority to the MUs, a convex pricing mechanism with respect to FUs’ bit energy to interference density ratio is also incorporated. The overall resource allocation problem is formulated as a Utility-based uplink Power and Rate Allocation game in Multi-Service two-tier Femtocell networks (UPRA-MSF), where each user aims selfishly at maximizing his utility-based performance. The existence and uniqueness of UPRA-MSF game's Nash equilibrium with respect to both uplink transmission power and rate is analytically shown. A distributed algorithm is proposed towards obtaining the corresponding equilibrium point and its convergence is proven. Finally, the operational effectiveness and superiority of the proposed framework is evaluated through modeling and comparative simulation.

Aggregate interference modelling and static resource allocation in closed and open access femtocells

Femtocell has appeared as a solution to increase both the coverage and the capacity of cellular networks. However, interference problem between the macrocell and the femtocell must be solved before any deployment. Since femtocells are deployed by home users in random locations, decentralised interference management schemes remain worth of investigation. In this study, the author deal with the problem of aggregate interference modelling and static resource allocation in orthogonal frequency division multiple access (OFDMA)-based two-tier femtocell networks for both closed and open access femtocells. It is assumed that for macrocell users (MUs), spectrum allocation is accomplished through fractional frequency reuse. The author's objective is to maximise the femtocell user's throughput while maintaining as low as its possible impact on the MUs' performance. Their proposed algorithm is decentralised, and based on some measurements, needs to be performed only when femtocell access point is plugged in. Simulation results show the accuracy of their model and the effectiveness of the proposed scheme compared to the previous methods.

Cell selection for open-access femtocell networks: Learning in changing environment

This paper addresses the problem of cell selection in dynamic open-access femtocell networks. We model this problem as decentralized restless multi-armed bandit (MAB) with unknown dynamics and multiple players. Each channel is modeled as an arbitrary finite-state Markov chain with different state space and statistics. Each user tries to learn the best channel that maximizes its capacity and reduces its number of handovers. This is a classic exploration/exploitation problem, where the reward of each channel is considered to be Markovian. In addition, the reward process is restless because the state of each Markov chain evolves independently of the user action. This leads to a decentralized restless bandit problem. To solve this problem, we refer to the decentralized restless upper confidence bound (RUCB) algorithm that achieves a logarithmic regret over time for the MAB problem (proposal 1). Then, we extend this algorithm to cope with dynamic environment by applying a change point detection test based on the Page–Hinkley test (PHT) (proposal 2). However, this test would entail some waste of time if the change-point detection was actually a false alarm. To face this problem, we extend our previous proposal by referring to a meta-bandit algorithm (proposal 3) to solve the dilemma between Exploration and Exploitation after the change-point detection occurs. Simulation results show that•our proposal come close to the performance of opportunistic method in terms of capacity, while fewer average number of handovers is required.•The use of a change point test and meta-bandit algorithm allows better performance than RUCB in terms of capacity particularly in a changing environment.

Throughput Optimization, Spectrum Allocation, and Access Control in Two-Tier Femtocell Networks

The deployment of femtocells in a macrocell network is an economical and effective way to increase network capacity and coverage. Nevertheless, such deployment is challenging due to the presence of inter-tier and intra-tier interference, and the ad hoc operation of femtocells. Motivated by the flexible subchannel allocation capability of OFDMA, we investigate the effect of spectrum allocation in two-tier networks, where the macrocells employ closed access policy and the femtocells can operate in either open or closed access. By introducing a tractable model, we derive the success probability for each tier under different spectrum allocation and femtocell access policies. In particular, we consider joint subchannel allocation, in which the whole spectrum is shared by both tiers, as well as disjoint subchannel allocation, whereby disjoint sets of subchannels are assigned to both tiers. We formulate the throughput maximization problem subject to quality of service constraints in terms of success probabilities and per-tier minimum rates, and provide insights into the optimal spectrum allocation. Our results indicate that with closed access femtocells, the optimized joint and disjoint subchannel allocations provide the highest throughput among all schemes in sparse and dense femtocell networks, respectively. With open access femtocells, the optimized joint subchannel allocation provides the highest possible throughput for all femtocell densities.

Distribution of Downlink SINR in Heterogeneous Cellular Networks

The Signal to Interference Plus Noise Ratio (SINR) on a wireless link is an important basis for consideration of outage, capacity, and throughput in a cellular network. It is therefore important to understand the SINR distribution within such networks, and in particular heterogeneous cellular networks, since these are expected to dominate future network deployments . Until recently the distribution of SINR in heterogeneous networks was studied almost exclusively via simulation, for selected scenarios representing pre-defined arrangements of users and the elements of the heterogeneous network such as macro-cells, femto-cells, etc. However, the dynamic nature of heterogeneous networks makes it difficult to design a few representative simulation scenarios from which general inferences can be drawn that apply to all deployments. In this paper, we examine the downlink of a heterogeneous cellular network made up of multiple tiers of transmitters (e.g., macro-, micro-, pico-, and femto-cells) and provide a general theoretical analysis of the distribution of the SINR at an arbitrarily-located user. Using physically realistic stochastic models for the locations of the base stations (BSs) in the tiers, we can compute the general SINR distribution in closed form. We illustrate a use of this approach for a three-tier network by calculating the probability of the user being able to camp on a macro-cell or an open-access (OA) femto-cell in the presence of Closed Subscriber Group (CSG) femto-cells. We show that this probability depends only on the relative densities and transmit powers of the macro- and femto-cells, the fraction of femto-cells operating in OA vs. Closed Subscriber Group (CSG) mode, and on the parameters of the wireless channel model. For an operator considering a femto overlay on a macro network, the parameters of the femto deployment can be selected from a set of universal curves.

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In this paper we evaluate the economic gains of a joint deployment of femtocells and macrocells for the provision of Long Term Evolution (LTE) mobile broadband services in urban environments. Frequency bands of 2.6 GHz and 900 MHz are analyzed and different parameters related to the business model are considered for a 30% market share operator. Results show important benefits for the base case where the service is offered to fixed-broadband clients, up to 75%, for small bandwidths. It results feasible to attribute subscriber loop costs to the radio access network (RAN) costs, so that the service could be offered to non fixed-broadband clients, in both cases of closed access and open access femtocells. However, initial savings result notably reduced, up to 50% less than in the base case if closed access is adopted and up to 13% less for open access. Site reuse reduces the initial savings only in 3%.

Open vs. Closed Access Femtocells in the Uplink

Femtocells are assuming an increasingly important role in the coverage and capacity of cellular networks. In contrast to existing cellular systems, femtocells are end-user deployed and controlled, randomly located, and rely on third party backhaul (e.g. DSL or cable modem). Femtocells can be configured to be either open access or closed access. Open access allows an arbitrary nearby cellular user to use the femtocell, whereas closed access restricts the use of the femtocell to users explicitly approved by the owner. Seemingly, the network operator would prefer an open access deployment since this provides an inexpensive way to expand their network capabilities, whereas the femtocell owner would prefer closed access, in order to keep the femtocell's capacity and backhaul to himself. We show mathematically and through simulations that the reality is more complicated for both parties, and that the best approach depends heavily on whether the multiple access scheme is orthogonal (TDMA or OFDMA, per subband) or non-orthogonal (CDMA). In a TDMA/OFDMA network, closed-access is typically preferable at high user densities, whereas in CDMA, open access can provide gains of more than 200% for the home user by reducing the near-far problem experienced by the femtocell. The results of this paper suggest that the interests of the femtocell owner and the network operator are more compatible than typically believed, and that CDMA femtocells should be configured for open access whereas OFDMA or TDMA femtocells should adapt to the cellular user density.