Dionysis Xenakis

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

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.

A novel handover decision policy for reducing power transmissions in the two-tier LTE network

Femtocells are attracting a fast increasing interest nowadays, as a promising solution to improve indoor coverage, enhance system capacity, and lower transmit power. Technical challenges still remain, however, mainly including interference, security and mobility management, intercepting wide deployment and adoption from mobile operators and end users. This paper describes a novel handover decision policy for the two-tier LTE network, towards reducing power transmissions at the mobile terminal side. The proposed policy is LTE backward-compatible, as it can be employed by suitably adapting the handover hysteresis margin with respect to a prescribed SINR target and standard LTE measurements. Simulation results reveal that compared to the widely-adopted strongest cell policy, the proposed policy can greatly reduce the power consumption at the LTE mobile terminals, and lower the interference network-wide.

A Context-Aware Vertical Handover Framework Towards Energy-Efficiency

Current mobile devices are equipped with multi-standard interfaces to fully utilize both the existing and the imminent Public Land Mobile Network infrastructure. This flexibility comes with significant energy consumption overheads at the mobile terminal side and thus, the adopted interface / network selection scheme, a.k.a. Vertical Handover mechanism, should be based on both Quality of Service oriented and energy-efficiency criteria. This paper summarizes the current state of the art on energy-centric vertical handover algorithms, discusses weak aspects of existing approaches and presents a novel context-aware vertical handover framework towards energy-efficiency. Rather than a pure energy-centric vertical handover scheme, this framework is a context-awareness enabler for energy-centric vertical handover decision making.

An energy-centric handover decision algorithm for the integrated LTE macrocell-femtocell network

Femtocells are attracting a fast increasing interest nowadays, as a promising solution to improve indoor coverage and system capacity. Due to the short transmit-receive distance, femtocells can greatly lower transmit power, prolong handset battery life, and enhance the user-perceived Quality of Service (QoS). On the other hand, technical challenges still remain, mainly including interference mitigation, security and mobility management, intercepting wide deployment and adoption by both mobile operators and end users. This paper introduces a novel energy-centric handover decision policy and its accompanied algorithm, towards minimizing the power consumption at the mobile terminal side in the integrated LTE macrocell-femtocell network. The proposed policy is shown to extend the widely-adopted strongest cell policy, by suitably adapting the handover hysteresis margin in accordance with standardized LTE measurements on the tagged users neighbor cells. Performance evaluation results show that significantly lower interference and power consumption can be attained for the cost of a moderately increased number of network-wide handover executions events.

OpeNB: A framework for virtualizing base stations in LTE networks

In this paper, we present a novel framework for virtualizing the Long Term Evolution (LTE) base stations to improve the user performance and lower the downlink interference. Our proposal, referred to as “OpeNB”, is an attempt to utilize the benefits of Software Defined Networking (SDN) and OpenFlow, introducing network virtualization in LTE networks with an adaptable, highly extensible, and less invasive manner. The proposed solution consists of a novel network architecture and a mechanism that allows the virtualization of LTE base stations depending on the network state. Simulation results show that the proposed framework improves the average interference, throughput and packet loss experienced by the end users.

Energy-efficient and interference-aware handover decision for the LTE-Advanced femtocell network

Femtocells will play a key role for the wide adoption of the LTE-Advanced system, as they bring the access network closer to the end user in a cost-effective manner. This disruptive communication paradigm, however, dictates the use of advanced interference and mobility management algorithms to cope with the dense yet unplanned network layout. In this paper, we present an interference-aware handover decision algorithm for the LTE-Advanced femtocell network, which utilizes standard signal measurements to select the candidate cell that a) attains the minimum required channel gain for sustaining service continuity and b) minimizes the mean UE transmit power for a prescribed mean SINR target. The proposed algorithm attains backwards compatibility with the LTE-Advanced system, as it is deployed by using the private mechanism for non-standard use. Based on the evaluation methodology of the Small Cell forum, we validate the performance of the proposed algorithm and compare it against that of other state-of-the-art algorithms.

An SDN-based framework for elastic resource sharing in integrated FDD/TDD LTE-A HetNets

Spectrum and infrastructure sharing among multiple mobile network operators have recently drawn significant attention from the industry and the academia sectors. Recent advances in the area of Software Defined Networking (SDN) launch the design of innovative SDN-based solutions for resource sharing in multi-tier networks of macrocell and small-sized base stations, a.k.a. HetNet. In this paper, we present a novel SDN-based framework that enables efficient and elastic spectrum utilization among multiple operators in 3GPP LTE-A HetNet scenario. Assuming a multi-operator environment of Frequency Division Duplex (FDD) macrocells complemented by multi-tenant Time Division Duplex (TDD) pico cells, we present a SDN-based architecture that allows efficient resource sharing among the TDD and FDD systems in a dynamic way. A TDD frame re-configuration mechanism is also employed, to optimize the ratio of uplink and downlink slots in the TDD frame of picocells. System-level simulation results demonstrate that the combination of these two functional enhancements in the LTE-A HetNet, significantly reduces the application layer delay for both the FDD macrocell and TDD picocell systems, leading to highly efficient and dynamic spectrum sharing among multiple network operators.

ARCHON: An ANDSF-assisted energy-efficient vertical handover decision algorithm for the heterogeneous IEEE 802.11/LTE-advanced network

To address the challenging issues of energy-efficiency and seamless connectivity in heterogeneous networks, 3GPP and IEEE have recently incorporated several architectural and functional enhancements to the baseline operation of their standards for cellular and wireless local area network access, respectively. Based on the 3GPP Access Network Discovery and Selection Function (ANDSF) and the advanced measurement capabilities provided by the IEEE 802.11-2012 and the 3GPP Long Term Evolution - Advanced (LTE-A) Standards, we propose an ANDSF-assisted energy-efficient vertical handover decision algorithm for the heterogeneous IEEE 802.11-2012 / LTE-A network. The proposed algorithm enables a multi-mode mobile terminal to select and associate with the network point of attachment that minimizes its average overall power consumption and guarantees a minimum supported quality of service for its ongoing connections. System-level simulation is used to evaluate the performance of the proposed algorithm and compare it to that of other competing solutions.

An SDN-based virtual cell framework for enhancing the QoE in TD-LTE pico cells

In this paper, we propose a Software Defined Network (SDN)-based framework for enhanced Quality of Experience (QoE) in the presence of Time Division-Long Term Evolution (TD-LTE) pico-cell hotspots. The proposed framework enables users to utilize radio-resources from multiple base stations and enhance their QoE, by utilizing the concept of virtual cells and by customizing the TD-LTE frame at the picocell stations. In this way, the proposed framework resolves pseudo-congestion and enables elastic radio-resource management while meeting the individual traffic requirements at the end users. The formation of virtual cells is driven by a QoE-centric approach that monitors whether the Mean-Opinion Score (MOS) of specific applications drops below a prescribed user-specific threshold. Accordingly, the TD-LTE frame is centrally optimized by an SDN Controller so as to meet the individual QoE requirements at the end users. System-level simulations demonstrate that the employment of the proposed framework leads to notable improvements in terms of MOS, end-to-end delay and packet loss rate, as compared to existing approaches with static and adaptive frame reconfiguration in TD-LTE picocells.

Dynamic resource allocation in adaptive wireless OFDMA systems

Modern wireless orthogonal frequency division multiple access (OFDMA) systems incorporate dynamic resource allocation (DRA), adaptive modulation and coding (AMC), and power control (PC) to exploit multiuser diversity and achieve higher system throughput. In the literature, only a few proposed algorithms deal with the contiguous DRA problem according to which a contiguous collection of resources can be allocated to each user. This paper formulates this high complexity problem, provides a suitable decision metric and a simple yet efficient solution. The proposed algorithm allocates in each step a contiguous collection of resources to the pending user that leads to the highest estimated correctly received number of bits. Simulation results show that, in this way, considerably improved performance can be achieved in terms of overall system throughput, spectral efficiency, and served traffic. Copyright