One of the effective techniques of improving the coverage and enhancing the capacity and data rate in cellular wireless networks is to reduce the cell size (i.e., cell splitting) and transmission distances. Therefore, the concept of deploying femtocells over macrocell has recently attracted growing interests in academia, industry, and standardization forums. Various technical challenges towards mass deployment of femtocells have been addressed in recent literature. Interference mitigation between neighboring femtocells and between the femtocell and macrocell is considered to be one of the major challenges in femtocell networks because femtocells share the same licensed frequency spectrum with macrocell. Further, the conventional radio resource management techniques for hierarchical cellular system is not suitable for femtocell networks since the positions of the femtocells are random depending on the users' service requirement. In this article, we provide a survey of the different state-of-the-art approaches for interference and resource management in orthogonal frequency-division multiple access (OFDMA)-based femtocell networks. A qualitative comparison among the different approaches is provided. To this end, open challenges in designing interference management schemes for OFDMA femtocell networks are discussed.

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Interference management in OFDMA femtocell networks: issues and approaches

Increase in system capacity and data rates can be achieved efficiently in a wireless system by getting the transmitter and receiver closer to each other. Femtocells deployed in the macrocell significantly improve the indoor coverage and provide better user experience. The femtocell base station called as Femtocell Access Point (FAP) is fully user deployed and hence reduces the infrastructure, maintenance and operational cost of the operator while at the same time providing good Quality of Service (QoS) to the end user and high network capacity gains. However, the mass deployment of femtocell faces a number of challenges, among which interference management is of much importance, as the fundamental limits of capacity and achievable data rates mainly depends on the interference faced by the femtocell network. To cope with the technical challenges including interference management faced by the femtocells, researchers have suggested a variety of solutions. These solutions vary depending on the physical layer technology and the specific scenarios considered. Furthermore, the cognitive capabilities, as a functionality of femtocell have also been discussed in this survey. This article summarises the main concepts of femtocells that are covered in literature and the major challenges faced in its large scale deployment. The main challenge of interference management is discussed in detail with its types in femtocells and the solutions proposed over the years to manage interference have been summarised. In addition an overview of the current femtocell standardisation and the future research direction of femtocells have also been provided.

Interference Management in Femtocells

Improvement of cell coverage and network capacity are two major challenges for the evolving 4G cellular wireless communication networks such as LTE-Advanced networks. In this context, hierarchical layering of cells with macro base stations coexisting with low-power and shortrange base stations (corresponding to picocells or femtocells) in a service area is considered to be an efficient solution to enhance the spectralefficiency of the network per unit area. Also, such a hierarchical cell deployment, which is referred to as a heterogeneous network, or Het- Net, provides significant improvement in the coverage of indoor and cell edge users and ensures better QoS to the users. Interference mitigation between different layers is one of the key issues that needs to be resolved for successful deployment of HetNets. To this end, fast frequency response, FFR, is considered to be an efficient intercell interference coordination technique for OFDMA-based HetNets. This article focuses on evaluating three state-of-the-art FFR deployment schemes: strict FFR, soft FFR, and FFR-3 schemes for OFDMA-based two-tier HetNets comprising macrocells overlaid with femtocells. Also, a variation of the FFR-3 scheme, which is referred to as the optimal static FFR (OSFFR) scheme, is proposed. A broad comparison among all these FFR schemes is performed by using Monte Carlo simulations considering performance metrics such as outage probability, average network sum rate, and spectral efficiency. Simulation results show that, the average gains in spectral efficiency (b/s/Hz) of the network are significantly higher for the proposed scheme when compared to the strict FFR, soft FFR, and FFR-3 schemes.

Fractional frequency reuse for interference management in LTE-advanced hetnets

Support of femtocells is an integral part of the Long Term Evolution - Advanced (LTE-A) system and a key enabler for its wide adoption in a broad scale. Femtocells are short-range, low-power and low-cost cellular stations which are installed by the consumers in an unplanned manner. Even though current literature includes various studies towards understanding the main challenges of interference management in the presence of femtocells, little light has been shed on the open issues of mobility management (MM) in the two-tier macrocell-femtocell network. In this paper, we provide a comprehensive discussion on the key aspects and research challenges of MM support in the presence of femtocells, with the emphasis given on the phases of a) cell identification, b) access control, c) cell search, d) cell selection/reselection, e) handover (HO) decision, and f) HO execution. A detailed overview of the respective MM procedures in the LTE-A system is also provided to better comprehend the solutions and open issues posed in real-life systems. Based on the discussion for the HO decision phase, we subsequently survey and classify existing HO decision algorithms for the two-tier macrocell-femtocell network, depending on the primary HO decision criterion used. For each class, we overview up to three representative algorithms and provide detailed flowcharts to describe their fundamental operation. A comparative summary of the main decision parameters and key features of selected HO decision algorithms concludes this work, providing insights for future algorithmic design and standardization activities.

Mobility Management for Femtocells in LTE-Advanced: Key Aspects and Survey of Handover Decision Algorithms

This paper provides a tutorial on mobility-aware performance analysis of cellular networks considering both the spatially random [where base stations (BSs) are deployed according to a homogeneous point process] and deterministic grid-based network deployment topologies. We first provide a summary of users’ mobility models with no spatiotemporal correlations (e.g., random walk, random way point, and random direction), with spatial correlations (e.g., pursue mobility and column mobility), and with temporal correlations (e.g., Gauss–Markov and Levy flight). The differences among various mobility models, their statistical properties, and their pros and cons are presented. We then describe two primary approaches (referred to as trajectory-based and association-based approaches) for mobility-aware performance analysis of both random and deterministic cellular networks. For the first approach (which is more general but less tractable than the other approach), we describe a general methodology and present several case studies for different cellular network tessellations, such as square lattice, hexagon lattice, single-tier, and multi-tier models in which BSs follow a homogeneous Poisson point process (PPP). For the second approach, we also outline the general methodology and highlight some limitations/imperfections of the existing techniques and provide corrections to these imperfections. For both of these approaches, we present selected numerical and simulation results to calibrate the achievable handoff rate and coverage probability in various network settings. Finally, we point out specific emerging fifth generation (5G) cellular wireless applications where the impact of mobility would be significant and outline the challenges associated with mobility-aware analysis of those network applications.

Fundamentals of Mobility-Aware Performance Characterization of Cellular Networks: A Tutorial

Recently, Long Term Evolution (LTE) has developed a femtocell for indoor coverage extension. However, interference problem between the femtocell and the macrocell should be solved in advance. In this paper, we propose an interference management scheme in the LTE femtocell systems using Fractional Frequency Reuse (FFR). Under the macrocell allocating frequency band by the FFR, the femtocell chooses sub-bands which are not used in the macrocell sub-area to avoid interference. Simulation results show that proposed scheme enhances total/edge throughputs and reduces the outage probability in overall network, especially for the cell edge users.

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Interference management in LTE femtocell systems using Fractional Frequency Reuse

The specification of the Home Evolved NodeB (Home-eNB), which is a small base station designed for use in residential or small business environment, is currently ongoing in 3GPP LTE (Long Term Evolution) systems. One of the key requirements for its feasibility in the LTE system is the mobility management in the deployment of the numerous Home-eNBs and other 3GPP network. In this paper, we overview the characteristic of Home-eNB and also describe the mobility management issues and the related approaches in 3GPP LTE based Home-eNB systems. Keywords : Home-eNB, 3GPP LTE (Long Term Evolution), Mobility Management 1. Introduction issues of HeNB in 3GPP LTE systems and related approaches A significant interest within the telecommunications industry has recently focused on the femto-cell which is defined broadly as low-cost, low-power cellular base stations that operate in licensed spectrum to connect conventional, unmodified mobile terminals to a mobile operator’s network [1][2][3]. Femto-cell has been actively discussed in 3

https://www.koreascience.or.kr:443/article/JAKO200803544452956.pdf

Mobility Management Survey for Home-eNB Based 3GPP LTE Systems

Femtocell as home base station for extending indoor coverage is introduced recently. The most important requirement of femtocell networks is to maintain self-configuration and self-optimization. User Equipment (UE) is needed to search autonomously the femtocells to reduce costs. In this paper, we propose Physical Cell identities (PCI) reservation scheme for 3GPP LTE femtocell. Some part of the PCIs is reserved for femtocells. Using several reservation types, the different number of PCIs can be reserved in each type. A macrocell can change the reservation type depending on the number of active femtocells. Simulation results show that proposed scheme reduces the average delay to find out the detected cells. Also, it increases the network capacity within the same delay constraints.

Physical cell identity reservation for 3GPP LTE femtocell

All wireless systems require the precise timing synchronization to satisfy the higher Quality of Service (QoS) requirements. Particularly, timing synchronization between Base Stations (BSs) during soft handover is required to increase the diversity gain of soft handover. Current timing synchronization protocol (e.g. IEEE1588, GPS) updates the synchronization at fixed time duration and it causes the excess overhead. In this paper, dynamic maintenance method is proposed. Proposed maintenance method based on user's PER (Packet Error Rate) of user reduces the total triggers of synchronization and consequently shows the decreased message overhead comparison with existing synchronization methods.

Poongup Lee

Papers

Interference management in LTE femtocell systems using Fractional Frequency Reuse

Mobility Management Survey for Home-eNB Based 3GPP LTE Systems

Physical cell identity reservation for 3GPP LTE femtocell

Dynamic maintenance method of timing synchronization for soft handover