• ETRI Journal
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• v.38 no.1
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• pp.30-40
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• 2016

Co-channel interference between macro-femtocell networks is an unresolved problem, due to the frequency reuse phenomenon. To mitigate such interference, a secondary femtocell must acquire channel-state knowledge about a co-channel macrocell user and accordingly condition the maximum transmit power of femtocell user. This paper proposes a pilot-based spectrum sensing (PSS) algorithm for overlaid femtocell networks to sense the presence of a macrocell user over a channel of interest. The PSS algorithm senses the pilot tones in the received signal through the power level and the correlation metric comparisons between the received signal and the local reference pilots. On ensuring the existence of a co-channel macrocell user, the maximum transmit power of the corresponding femtocell user is optimized so as to avoid interference. Time and frequency offsets are carefully handled in our proposal. Simulation results show that the PSS algorithm outperforms existing sensing techniques, even at poor received signal quality. It requires less sensing time and provides better detection probability over existing techniques.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.41 no.9
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• pp.1020-1025
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• 2016

In the recent, the communication market increased every year. Because of the user's demands and exhaustion of wireless resource, we needs a new technology of the communication system. The femtocell is one of the new technology of the communication system. 'Femto' means very small. By using the femtocell that made up the very small cell, the frequency efficiency increase. The femtocell has different cell environments. The Power Control is important for femtocell to avoid interference and unnecessary handover. In this paper, we propose a new femtocell power control that is improvement of handover probability and throughput. And we simulate and check the result.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.35 no.8A
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• pp.743-751
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• 2010

OFDMA femtocell becomes an effective solution to support indoor high data rate services instead of the macrocell systems. Although the advantage of the femtocell, the co-channel interference between the femocell and the macrocell is the most significant problem that reduces the system performance. Macrocell users who have no permission to access the femtocell suffer from interference of the downlink transmission of femtocell. Therefore, the femtocell should use transmission power as small as possible to reduce interference to macrocell users. In this paper, we define the margin adaptive power allocation problem for the femtocell and propose a heuristic power allocation algorithm to solve the problem. Simulation results show the performance of the proposed algorithm.

• IEIE Transactions on Smart Processing and Computing
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• v.3 no.5
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• pp.275-284
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• 2014

This paper considers two-tier networks consisting of macrocells and femtocells operating in the same spectrum. This paper proposes a femtocell spectrum reuse scheme that determines the shared spectrum and transmit power for the femtocells to mitigate the effects of cross-tier interference between the macrocells and femtocells. The proposed scheme provides macrocell throughput that is unaffected by the increasing number of femtocells per cell site and improves the femtocell signal quality at the same time by limiting the cross-tier interference. This study analyzed the per-tier signal-to-interference ratio (SIR) and outage probability of the proposed scheme to investigate the macrocell and femtocell performance. The total throughput of the proposed scheme was analyzed based on the outage probabilities. The analysis and numerical results proved that high femtocell throughput can be achieved using only a small fraction of the spectrum while protecting the macrocell throughput. As a result, an improved total throughput was achieved enforcing higher spatial reuse.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.38B no.1
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• pp.18-25
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• 2013

In this paper, we propose a self-optimization scheme for indoor femtocell coverage in heterogeneous networks. If the femtocell coverage is larger than indoor area, neighbor cell users passing by the outer area of the femtocell coverage may request an unnecessary handover which incurs wasteful signaling overhead. On the other hand, if the coverage is smaller than the indoor area, some of indoor users might not be connected to the indoor femtocell. Therefore, we propose the method by which the femtocell coverage attains the exact indoor area employing self-organized scheme. Autonomous self TX power adjustment of the femtocell is possible because the proposed method utilizes handover request events and membership information of users that can be obtained by the femtocell itself. We show that the TX power obtained by the proposed algorithm converges to the optimal TX power that can be obtained analytically to attain the indoor coverage area.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.8 no.4
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• pp.1424-1441
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• 2014

Two-tier femtocell networks, consisting of a conventional cellular network underlaid with femtocell hotspots, play an important role in the indoor coverage and capacity of cellular networks. However, the cross- and co-tier interference will cause an unacceptable quality of service (QoS) for users with universal frequency reuse. In this paper, we propose a novel downlink interference mitigation strategy for spectrum-shared two-tier femtocell networks. The proposed solution is composed of three parts. The first is femtocells clustering, which maximizes the distance between femtocells using the same slot resource to mitigate co-tier interference. The second is to assign macrocell users (MUEs) to clusters by max-min criterion, by which each MUE can avoid using the same resource as the nearest femtocell. The third is a novel adaptive power control scheme with femtocells downlink transmit power adjusted adaptively based on the signal to interference plus noise ratio (SINR) level of neighboring users. Simulation results show that the proposed scheme can effectively increase the successful transmission ratio and ergodic capacity of femtocells, while guaranteeing QoS of the macrocell.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.37 no.8A
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• pp.648-660
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• 2012

The low-power, low-cost femtocell network has been proposed not only to alleviate traffic load to the macro base station (eNB) but also to cover the indoor coverage hole problem. However, in the dense femtocell environment where many femtocells are deployed to cover the whole large office building, performance of such femtocell environment can be deteriorated due to severe co-channel interference problem between the eNB and femtocells and among neighboring femtocells. In particular, a macro UE(mUE) located within femtocell coverage may experience severe co-channel interference from surrounding femtocells. Therefore, In this paper, we propose a novel power control schemes to mitigate interference to a mUE under such dense LTE femtocell environment. With proposed femtocell power control schemes, performance of the mUE can be greatly improved in terms of the outage probability and the SINR while maintaining satisfying femtocell performance. Simulation based performance study shows that the proposed power control scheme is able to enhance mUE performance more than 30% than the conventional dense femtocell in terms of the two performance metrics.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.9 no.7
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• pp.2530-2547
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• 2015

In the recent years, femtocell technology has received a considerable attention due to the ability to provide an efficient indoor wireless coverage as well as enhanced capacity. However, under the spectrum sharing between femtocell user equipment (FUEs) and the owner of spectrum macrocell user equipment (MUEs), both may experience higher uplink interference to each other. This paper proposes a novel distributed power control algorithm for the interference management in two-tier femtocell networks. Due to the assignment of licensed radio frequency to the outdoor macrocell users, the access priority of MUEs should be higher than FUEs. In addition, the quality of service (QoS) of MUEs that is expressed in the target signal-to-interference-plus-noise ratio (SINR) must always be achieved. On the other hand, we consider an efficient QoS provisioning cost function for the low-tier FUEs. The proposed algorithm requires only local information and converges even in cases where the frontiers of available power serve the target SINRs impossible. The advantage of the algorithm is the ability to implement in a distributed manner. Simulation results show that the proposed algorithm based on our cost function provides effective resource allocation and substantial power saving as compared to the traditional algorithms.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.38A no.10
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• pp.865-873
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• 2013

In the cellular networks adopting femtocells, the macrocell users can suffer from severe interference by the femtocells. In this paper, we propose a distributed transmission power control scheme for femtocells to protect macrocell users. With the proposed scheme, when a macrocell user experiences outage due to severe interference, it informs the interfering femtocell base station (BS) of the outage occurrence, via the macrocell BS. Then, the femtocell BS reduces the transmission power to protect the macrocell user. The proposed scheme does not require too much control information among the macrocell BS, the macrocell users, and the femtocell BS. Moreover, the computational complexity in femtocell BS is very low. By using simulation, we show that the performance of the proposed scheme is good enough to use in practice, in spite of its simplicity.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.9 no.11
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• pp.4331-4346
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• 2015

The femtocell network, which is designed for low power transmission and consists of consumer installed small base stations, coexists with macrocells to exploit spatial reuse gain. For its realization, cross-tier interference mitigation is an important issue. To solve this problem, we propose a joint access and power control scheme that requires limited information exchange between the femto and macro networks. Our objective is to maximize the network throughput while satisfying each user's quality of service (QoS) requirement. To accomplish this, we first introduce two distributed interference detection schemes, i.e., the femto base station and macro user equipment based schemes. Then, the proposed scheme dynamically adjusts the transmission power and makes a decision on the access mode of each femto base station. Through extensive simulations, we show that the proposed scheme outperforms earlier works in terms of the throughput and outage probability.