• The Journal of Korean Institute of Communications and Information Sciences
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• v.41 no.5
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• pp.512-519
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• 2016

This paper presents spectrum utilization strategies in cognitive radio (CR) networks powered by multi-antenna based wireless energy transfer. Secondary access point (AP) with multiple antennas should transfer the energy to the secondary sensor nodes with energy beamforming and simultaneously induces no interference to PUs. In addition, sensor nodes can transmit information to the secondary AP using the harvested energy, only when the spectrum is not utilized by PUs. We analyze the achievable rate of the CR sensor networks and propose an interference nulling energy beamforming method to maximize the achievable rate. Finally, we also propose a frame scheduling algorithm in which the durations of wireless energy transfer/information transfer frames (phases) are optimized.

• The KIPS Transactions:PartC
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• v.12C no.7 s.103
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• pp.1025-1030
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• 2005

Cooperative communications among users in multiple access wireless environments is an efficient way to obtain the powerful benefits of multi-antenna systems without the demand for physical arrays. This paper proposes a solution to optimize the weights of partnering users' signals for the minimum error probability at the output of maximum likelihood (ML) detector under the transmit power constraints by taking advantage of channel state information (CSI) feedback from the receiver to the transmitter. Simulation programs are also established to evaluate the performance of the system under flat Rayleigh fading channel plus AWGN (Additive White Gaussian Noise).

• The Journal of Korean Institute of Communications and Information Sciences
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• v.35 no.2A
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• pp.134-141
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• 2010

In this paper, we propose a scheduling algorithm to improve the performance of IEEE 802.16 based wireless mesh networks using directional antenna. The performance is presented in terms of throughput of system and delay between each node by varying number of users. The result show that proposed scheduling algorithm improving the performance by reducing the delay of mesh network system. Our work may be useful as a guideline to control the fairness between SSs for multi-hop systems such as multi-hop relay and mesh networks.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2018.10a
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• pp.464-466
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• 2018

In this paper, a dense cellular network is considered in which each base station equipped with multiple antennas simultaneously communicates with multiple single-antenna users. Based on limited feedback, each user feeds back its quantized channel state information (CSI) to its associated transmitter, and the transmitter broadcasts multiple data streams prepared for the scheduled users using a space-division multiple access scheme. As the amount of CSI is limited at the transmitter, the downlink throughput increases with the number feedback bits. However, the increased number of feedback bits requires the correspondingly increased amount of uplink resources. Thus, an appropriate balance between the downlink throughput and the uplink resource usage should be considered in realistic systems. A net spectral efficiency defined in this context is used in this paper, and the optimal number of feedback bits that maximizes the net spectral efficiency is analyzed. This paper particularly focuses on the case when the received signal power is much smaller than the noise power.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.16 no.12
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• pp.2614-2618
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• 2012

Recently, multimedia broadcasting traffic is explosively increasing since the performance of smartphones is significantly enhanced and capacity of mobile access network such as LTE is steeply increasing. We can accommodate the multimedia broadcasting traffic via general unicast scheme or eMBMS which is a broadcast-specific scheme. First, the unicast scheme provides higher spectral efficiency compared to the eMBMS. However, unfortunately, the efficiency of the unicast scheme dramatically decreases as the number of users increases. On the other hand, although the spectral efficiency of the eMBMS is lower than that of the unicast, it can serve many users with a channel. Thus, this paper defines and investigates the capacity efficiencies of the both schemes for multimedia broadcasting traffic. In addition, we derive the optimal number of users where the eMBMS can provide the higher capacity efficiency than that of the unicast scheme.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.37A no.12
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• pp.1093-1105
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• 2012

We propose an interference space reuse (ISR) algorithm for the MU-MIMO design in 3-cell downlink interference channels. Also, we provide a strategy for the adoption of the ISR scheme in the cellular network. In the multicell interference channels, the cell edge users may undergo severe interferences and their signals should be protected from the interferers for reliable transmissions. However, the intra cell users do not only experience small interferences but also they require small transmission power for stable communication. We provide a vector design algorithm based on ISR, where intra cell users are served through reusing the cell edge users' interference space. The performance enhancement reaches 20% compared to the fractional frequency reuse (FFR) scheme combined with IA through the scheduling between the cell edge users and the intra cell users. Also, it can be used to enhance the cell edge throughput when the quality of service (QoS) requirements of the intra cell users are fixed.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.39B no.11
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• pp.803-809
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• 2014

In this paper, we investigate physical layer security in a cooperative cognitive radio networks (CRN) with a relay selection in the presence of a primary user and an eavesdropper. To protect the CRN from wiretapping by the eavesdropper, we propose employing an opportunistic relay selection scheme and multiple antennas at the destination that work based on the availability of channel state information at the receivers. Under these configurations, we derive an exact closed-form expression for the secrecy outage probability of the CRN, and also derive an asymptotic probability. Numerical results will be presented to verify the analysis.

• The KIPS Transactions:PartC
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• v.13C no.6 s.109
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• pp.749-756
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• 2006

Broadcast nature of wireless medium and path-loss reduction create a favourable condition for collaborative communications (CC) among single-antenna users to gain the powerful benefits of multi-antenna system without the demand for physical arrays. This paper proposes a CC strategy adapting to the propagation environment changes by optimizing the transmit signal amplification factors to simplify the structure of maximum likelihood (ML) detector and to obtain the minimum error probability as well. The closed-form BER expression was also derived and compared to the simulation results to evaluate the performance of the suggested solution. A variety of numerical results revealed the cooperation significantly outperforms non-cooperative counterpart under flat Rayleigh fading channel plus AWGN (Additive White Gaussian Noise).

• The Journal of Korean Institute of Communications and Information Sciences
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• v.40 no.2
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• pp.273-284
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• 2015

The feasibility conditions of interference alignment (IA) are analyzed for reverse duplex systems, in which one cell operates as downlink (DL) while the other cell operates as uplink (UL) assuming one-sid base station (BS) cooperation. Under general multiple-input and multiple-output (MIMO) antenna configurations, a necessary condition and a sufficient condition for one-shot linear IA are established, i.e., linear IA without symbol or time extension. In several example networks, optimal sum degrees of freedom (DoF) is characterized by the derived necessary condition and sufficient condition. For some special class of networks, a sufficient condition is established in a more compact expression, which also yields the necessary and sufficient condition. Simulation results demonstrate that the proposed IA does not only achieve larger DoF but also significantly improves the sum rate in the practical signal-to-noise ratio (SNR) regime.

• Journal of Satellite, Information and Communications
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• v.12 no.2
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• pp.51-55
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• 2017

This paper aims to maximize the energy harvesting rate and channel capacity in RF-energy harvesting networks (RF-EHNs) under the constraints of maximum transmit power and minimum quality of service (QoS) in terms of rate capacity for each user. We study a multi-user RF-EHN with frequency division multiple access (FDMA) in a Rayleigh channel. An access point (AP) simultaneously transmitting wireless information and power in the RF-EHN serves a subset of active users which have a power-splitting antenna. To gauge the network performance, we define energy efficiency (EE) and propose an optimization solution for maximizing EE with Lagrangian dual decomposition theory. In simulation results, we confirm that the EE is effectively maximized by the proposed solution with satisfying the given constraints.