• Journal of Communications and Networks
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• v.13 no.4
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• pp.327-338
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• 2011

With the incitation to reduce power consumption and the aggressive reuse of spectral resources, there is an inevitable trend towards the deployment of small-cell networks by decomposing a traditional single-tier network into a multi-tier network with very high throughput per network area. However, this cell size reduction increases the complexity of network operation and the severity of cross-tier interference. In this paper, we consider a downlink two-tier network comprising of a multiple-antenna macrocell base station and a single femtocell access point, each serving multiples users with a single antenna. In this scenario, we treat the following beamforming optimization problems: i) Total transmit power minimization problem; ii) mean-square error balancing problem; and iii) interference power minimization problem. In the presence of perfect channel state information (CSI), we formulate the optimization algorithms in a centralized manner and determine the optimal beamformers using standard convex optimization techniques. In addition, we propose semi-decentralized algorithms to overcome the drawback of centralized design by introducing the signal-to-leakage plus noise ratio criteria. Taking into account imperfect CSI for both centralized and semi-decentralized approaches, we also propose robust algorithms tailored by the worst-case design to mitigate the effect of channel uncertainty. Finally, numerical results are presented to validate our proposed algorithms.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.39B no.2
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• pp.61-74
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• 2014

In recent years, development of femtocells are receiving considerable attention towards increasing the network coverage, capacity, and improvement in the quality of service for users. In 3GPP LTE-Advanced (LTE-A) system, to efficiently utilize the bandwidth, femtocell and macro cell uses the same frequency band, but this deployment poses a technical challenge of cross-tier interference to macro users. In this paper, the novel quality of service based fractional power control (QoS-FPC) scheme under the heterogeneous networks environment is proposed, which considers the users priority and QoS-requirements during the power allocation. The proposed QoS-FPC scheme has two focal points: firs, it protects the macrocell users uplink communication by limiting the cross-tier interference at eNB below a given threshold, and second, it ensures the optimization of femtocell users power allocation at each power adjustment phase. Performance gain is demonstrated with extensive system-level simulations to show that the proposed QoS-FPC scheme significantly decreases the cross-tier intereference and improves the overall users throughput.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.17 no.12
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• pp.708-715
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• 2016

Quality of experience (QoE) for multimedia services in macro-femtocell networks is one of the key issues for 5G mobile and wireless communications. A service management structure needs to guarantee the QoE for mobile users based on end-to-end negotiation to support service continuity. Resource management is necessary to maintain the QoE requirements of different multimedia applications, because service continuity may be impeded by delays. This paper proposes four types of resource management scheme to support consistent QoE for different multimedia services. For this purpose, a QoE structure is suggested, and a resource allocation scheme is proposed by utilizing a fixed amount of radio resources reserved for dedicated use to support QoE. Various multimedia services with different requirements (such as voice, image, and data) can be serviced simultaneously, because QoE can be provided under our proposed scheme. Simulation results show that our scheme provides better performance than a conventional scheme with respect to outage probability and total data throughput.

• Journal of Communications and Networks
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• v.14 no.5
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• pp.578-587
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• 2012

One of the key elements in the emerging, packet-based long term evolution (LTE) cellular systems is the deployment of multiple femtocells for the improvement of coverage and data rate. However, arbitrary overlaps in the coverage of these femtocells make the handover operation more complex and challenging. As the existing handover strategy of LTE systems considers only carrier to interference plus noise ratio (CINR), it often suffers from resource constraints in the target femtocell, thereby leading to handover failure. In this paper, we propose a new efficient, multi-objective handover solution for LTE cellular systems. The proposed solution considers multiple parameters like signal strength and available bandwidth in the selection of the optimal target cell. This results in a significant increase in the handover success rate, thereby reducing the blocking of handover and new sessions. The overall handover process is modeled and analyzed by a three-dimensional Markov chain. The analytical results for the major performance metrics closely resemble the simulation results. The simulation results show that the proposed multi-objective handover offers considerable improvement in the session blocking rates, session queuing delay, handover latency, and goodput during handover.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.36 no.3A
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• pp.259-266
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• 2011

Femtocells are expected as the surest way to increase the system capacity with higher-quality links and more spatial reuse in future networks. In spite of their great potential, the system capacity is highly susceptible to network density because a large portion of users are exposed to inter-cell interference (ICI). In this work, we proposed a dynamic interference avoidance scheme in densely deployed cell environments. Our proposed DDIA (Distributed Dynamic ICI Avoidance) scheme not only works in a fully distributed manner, but also controls interference link connectivity of users with high agility so that it is suited for self-organizing networks (SONs). We introduced the concept of ICI-link and two-tier scheduling in designing the DDIA scheme. To avoid ICI without any central entity, our scheme tries to harmonize all base stations (BSs) with users adaptively. Through extensive simulations, it was shown that our proposed scheme improves the throughput of users by more than twice on average compared to the frequency reuse factor 1 scheme, who are exposed to ICI while maintaining or even improving overall network performance. Our scheme operates well regardless of network density and topology.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.38C no.4
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• pp.387-393
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• 2013

In shipyard site, a ship is constructed by integrating lots of modules that are made in several sites with different capabilities and specifications. Recently, it needs to make a shipyard to be smart with wired and wireless broadband networks between hierarchical shipyards. In this paper, we propose an installation of LTE femtocell local gateway at a remote shipyard, an information exchange method between hierarchical shipyards, and an offload method to separate the general traffic. We define the mode change in a femtocell gateway for supporting the offload of the general traffic between the headquarter and a remote shipyard, the offload data management and trigger message, and cache entry fields. To show the operation of our proposed off-loading function, we consider the transcipient message flow at the femtocell gateway with its state transition diagram. Hence, it is expected to increase the productivity of shipyard industry with mobile communications and broadband Internet technologies.

• ETRI Journal
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• v.42 no.2
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• pp.175-185
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• 2020

Inter-cell interference (ICI) is a major problem in heterogeneous networks, such as two-tier femtocell (FC) networks, because it leads to poor cell-edge throughput and system capacity. Dynamic ICI coordination (ICIC) schemes, which do not require prior frequency planning, must be employed for interference avoidance in such networks. In contrast to existing dynamic ICIC schemes that focus on homogeneous network scenarios, we propose a novel semi-distributed dynamic ICIC scheme to mitigate interference in heterogeneous network scenarios. With the goal of maximizing the utility of individual users, two separate algorithms, namely the FC base station (FBS)-level algorithm and FC management system (FMS)-level algorithm, are employed to restrict resource usage by dominant interference-creating cells. The distributed functionality of the FBS-level algorithm and low computational complexity of the FMS-level algorithm are the main advantages of the proposed scheme. Simulation results demonstrate improvement in cell-edge performance with no impact on system capacity or user fairness, which confirms the effectiveness of the proposed scheme compared to static and semi-static ICIC schemes.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.27 no.9
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• pp.783-790
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• 2016

Since scarcity of spectrum in future mobile networks, millimeter wave frequencies from 30 GHz to 300 GHz have been proposed to be used in an important part of 5G mobile communication backhaul links to provide several giga bits services. In ITU-R has been invited to conduct and complete in time for WRC-19 the appropriate studies to determine the spectrum needs for the terrestrial component of IMT in the frequency range between 24.25 GHz and 86 GHz. Also, small cells such as a femtocell, and heterogeneous networks have been deployed through world in order to enhance the communication capacity. At this stage, it is important to develop millimeter wave frequencies to provide 5G mobile broadband services, and thus this paper proposes the effective usage of these frequencies by summarizing the FCC allocation of millimeter waves, their propagation characteristics, the required minimum path length, and the interference effect.

• Journal of the Korea Society of Computer and Information
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• v.22 no.8
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• pp.25-32
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• 2017

In this paper we propose a resource management method which enables to guarantee the quality of experience (QoE) for handover in the overlaid macro-femtocell networks. How to cope with the resource demand of handover calls is necessary to efficiently support the movement of mobile terminals, the QoE degradation or the load control. We attempt to satisfy the QoE requirements of users and maximize the capacity of the system at the same time. In order to achieve this goal, this scheme divides the shared resources into two part for the movement of MT and QoE degradation, and allocates those resources with the competition between four types of handovers. Simulation results show that our scheme provides better performances than the conventional one with respect to the outage probability, data transmission throughput.

• Journal of the Korea Society of Computer and Information
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• v.21 no.12
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• pp.73-79
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• 2016

This paper proposes a novel resource allocation scheme which enables to guarantee the quality of experience (QoE) for handovers from femtocell to macrocell and vice versa, and between femtocells in the overlaid macro-femto networks. The proposed scheme guarantees QoE to a certain extent during handover by utilizing a fixed amount of radio resource reserved in advance for the dedicated use to support QoE. We take both the type of the handover and the temporal sensitiveness characteristics of user services into consideration in order to support QoE. Performance of our scheme is analyzed by simulation with respect to the outage probability and total throughput.