• JSTS:Journal of Semiconductor Technology and Science
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• 제15권2호
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• pp.202-207
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• 2015

A CMOS relaxation oscillator, with high robustness over process, voltage and temperature (PVT) variations, is designed in $0.18{\mu}m$ CMOS. The proposed oscillator, consisting of full-differential charge-discharge timing circuit and switched-capacitor based voltage-to-current conversion, could be expanded to a simple open-loop frequency synthesizer (FS) with output frequency digitally tuned. Experimental results show that the proposed oscillator conducts subcarrier generation for frequency-modulated ultra-wideband (FM-UWB) transmitters with triangular amplitude distortion less than 1%, and achieves frequency deviation less than 8% under PVT and phase noise of -112 dBc/Hz at 1 MHz offset frequency. Under oscillation frequency of 10.5 MHz, the presented design has the relative FS error less than 2% for subcarrier generation and the power dissipation of 0.6 mW from a 1.8 V supply.

• 한국멀티미디어학회논문지
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• 제16권5호
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• pp.601-609
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• 2013

One of the most serious factors constraining the next generation cellular mobile consumer communication systems will be the severe co-channel interference experienced at the cell edge. Such a capacity-degrading impairment combined with the limited available spectrum resource makes it essential to develop more spectrally efficient solutions to enhance the system performance and enrich the mobile user's application services. This paper proposes a unique hybrid method of frequency hopping (FH) and subcarrier-reuse-partitioning that can maximize the system capacity by efficiently utilizing the available spectrum while at the same time reduce the co-channel interference effect. The main feature of the proposed method is that it applies an optimal combination of different frequency reuse factors (FRF) and FH-subcarrier allocation patterns into the partitioned cell regions. From the simulation results, it is shown that the proposed method can achieve the optimum number of subcarrier subsets according to the frequency-reuse distance and results in better performance than the fixed FRF methods, for a given partitioning arrangement. The results are presented in the context of both blocking probability and BER performances. It will also be shown how the proposed scheme is well suited to FH-OFDMA based cellular systems aiming at low co-channel interference performance and optimized number of subcarriers.

• 한국통신학회논문지
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• 제34권3A호
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• pp.281-289
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• 2009

무선 중계기 기술은 기지국과 음영 지역 사용자와의 통신을 가능케 함으로써 안정적인 신호 전송을 가능케 하고 셀 영역을 넓힐 수 있어 차세대 무선통신 핵심 기술로 주목받고 있다. 본 논문에서는 OFDM 시스템에서 복수개의 복조 후 전달 (DF) 중계기를 사용할 때, 고정된 비트당 오율 (BER) 및 전력 합 조건을 만족하면서 데이터전송률을 높일 수 있는 송신 전력 및 부반송파 할당 기법들을 제안한다. 컴퓨터 모의 실험을 통해 제안하는 기법들의 평균 전송률을 비교 평가하였다. 최적의 성능을 갖는 exhaustive 기법에 비교하였을 때, 제안하는 기법은 달성 가능한 평균 전송률 값의 차이가 거의 없으면서도 계산 복잡도는 크게 줄일 수 있음을 확인하였다.

• 한국위성정보통신학회논문지
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• 제1권1호
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• pp.40-47
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• 2006

차세대 이동 통신 기술은 무선 환경에서 고속 및 고품질의 데이터 전송을 요구하고 있다. OFDM 기술은 대역폭당 전송 속도의 향상과 멀티패스 간섭 등의 방지를 위한 디지털 변조 방식으로 무선 환경에서 다양하게 응용되고 있다. 본 논문에서는 MBMS(Multimedia Broadcast and Multicast Service)와 같이 무선 환경에서 여러 개의 서비스를 동시에 제공하는 다중 서비스 환경에 OFDM을 적용할 때 필요한 부채널 배정에 관한 알고리즘을 제안하였다. 제안한 방법은 각 서비스의 채널 환경을 고려하여 부채널을 분배할 수 있을 뿐만 아니라 서비스의 우선순위를 고려해서 우선순위가 높은 서비스에 더 많은 부채널 분배가 가능하고 기존의 부채널 배정 알고리즘과 비교할 때 수렴 속도의 향상과 계산량 감소의 장점이 있다.

• 한국통신학회논문지
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• 제38A권7호
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• pp.580-591
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• 2013

Orthogonal Frequency Division Multiple Access(OFDMA)는 4세대 무선 통신을 위해 고려되는 변조와 다중 접속 기술이다. 본 논문에서는, OFDMA 시스템에서 더 나은 proportional rate의 자원할당 기법을 위해 전체 전력, 비트 오류율, 유저 간의 rate proportionality에 대한 제한 조건을 만족시키면서, 유저의 전송률을 최대화 시키는 전략 할당 알고리즘에 대해 기술하였다. 그리고 subcarrier 할당의 비율과 유저의 정규화된 proportionality 조건에 기반을 둔 전력 할당 방식을 제안하였다. 유저에게 subcarrier 할당을 위한 방법으로는 greedy 알고리즘과 waterfilling 기술이 적용되었다. 제안된 알고리즘에 대한 평가는 시뮬레이션을 통해서 이루어졌으며, 시뮬레이션을 통해서 제안된 알고리즘이 유저 간의 rate proportionality를 유지하면서 더 높은 시스템 채널 용량과 적은 소요 시간을 보인다는 것을 확인할 수 있었다.

• ETRI Journal
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• 제42권6호
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• pp.846-858
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• 2020

The non-orthogonal multiple access (NOMA) technique offers throughput improvement to meet the demands of the future generation of wireless communication networks. The objective of this work is to further improve the throughput by including an underlay cognitive radio network with an existing multi-carrier NOMA network, using cooperative communication. The throughput is maximized by optimal resource allocation, namely, power allocation, subcarrier assignment, relay selection, user pairing, and subcarrier pairing. Optimal power allocation to the primary and secondary users is accomplished in a way that target rate constraints of the primary users are not affected. The throughput maximization is a combinatorial optimization problem, and the computational complexity increases as the number of users and/or subcarriers in the network increases. To this end, to reduce the computational complexity, a dynamic network resource allocation algorithm is proposed for combinatorial optimization. The simulation results show that the proposed network improves the throughput.

• IEIE Transactions on Smart Processing and Computing
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• 제1권1호
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• pp.50-64
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• 2012

Quality of Service (QoS) and fairness considerations are undoubtedly essential parameters that need to be considered in the design of next generation scheduling algorithms. This work presents a novel game theoretic cross-layer design that offers optimal allocation of wireless resources to heterogeneous services in Orthogonal Frequency Division Multiple Access (OFDMA) networks. The method is based on the Axioms of the Symmetric Nash Bargaining Solution (S-NBS) concept used in cooperative game theory that provides Pareto optimality and symmetrically fair resource distribution. The proposed strategies are determined via convex optimization based on a new solution methodology and by the transformation of the subcarrier indexes by means of time-sharing. Simulation comparisons to relevant schemes in the literature show that the proposed design can be successfully employed to typify ideal resource allocation for next-generation broadband wireless systems by providing enhanced performance in terms of queuing delay, fairness provisions, QoS support, and power consumption, as well as a comparable total throughput.

• 융합신호처리학회논문지
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• 제6권2호
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• pp.100-106
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• 2005

OFDM은 심볼간 간섭에 대한 강인성으로 인하여 4세대 광대역 이동통신에 적합한 변조방식으로 활발히 연구되고 있다. OFDM 전송 기술은 신호 대 잡음비에 따라서 각 부반송파의 변조기법을 적응적으로 적용할 수 있다. 본 논문에서는 광대역 이동 채널에서 OFDM 시스템을 위한 적응적 변조기법을 제안하였다. 제안한 알고리즘은 목표 BER의 SNR 임계값을 이용하여 각 부반송파에 초기 비트를 할당하고, Chow 알고리즘을 적용하여 전체 비트수를 목표 비트수와 같게 제어한다. 또한, 제안한 알고리즘은 부반송파들을 블록으로 구성함으로써 부반송파의 비트할당을 위한 시그날링 오버헤드를 줄였다. 제안한 알고리즘은 광대역 이동채널에서 블록내의 부반송파의 수, 목표 BER, 그리고 도플러 주파수에 대한 적응형 OFDM 시스템의 성능을 평가하였다.

• International Journal of Computer Science & Network Security
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• 제23권9호
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• pp.8-16
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• 2023

In Japan, high-speed ground transportation service using linear motors at speeds of 500 km/h is scheduled to begin in 2027. To accommodate 5G services in trains, a subcarrier spacing frequency of 30 kHz will be used instead of the typical 15 kHz subcarrier spacing to mitigate Doppler effects in such high-speed transport. Furthermore, to increase the cell size of the 5G mobile system, multiple base station antennas will transmit identical downlink (DL) signals to form an expanded cell size along the train rails. In this situation, the forward and backward antenna signals are Doppler-shifted in opposite directions, respectively, so the receiver in the train may suffer from estimating the exact Channel Transfer Function (CTF) for demodulation. In a previously published paper, we proposed a channel estimator based on Delay and Doppler Profiler (DDP) in a 5G SISO (Single Input Single Output) environment and successfully implemented it in a signal processing simulation system. In this paper, we extend it to 2×2 MIMO (Multiple Input Multiple Output) with spatial multiplexing environment and confirm that the delay and DDP based channel estimator is also effective in 2×2 MIMO environment. Its simulation performance is compared with that of a conventional time-domain linear interpolation estimator. The simulation results show that in a 2×2 MIMO environment, the conventional channel estimator can barely achieve QPSK modulation at speeds below 100 km/h and has poor CNR performance versus SISO. The performance degradation of CNR against DDP SISO is only 6dB to 7dB. And even under severe channel conditions such as 500km/h and 8-path inverse Doppler shift environment, the error rate can be reduced by combining the error with LDPC to reduce the error rate and improve the performance in 2×2 MIMO. QPSK modulation scheme in 2×2 MIMO can be used under severe channel conditions such as 500 km/h and 8-path inverse Doppler shift environment.

• Journal of Communications and Networks
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• 제17권1호
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• pp.27-33
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• 2015

In this paper, we consider a joint frequency-domain scheduling and user-pairing problem for virtual MIMO in the single carrier frequency division multiple access (SC-FDMA) system, e.g., the uplink transmission for third generation partnership project-long term evolution (3GPP-LTE) standard. Due to the subcarrier adjacency constraint inherent to SC-FDMA, its complexity becomes unmanageable. We propose a greedy heuristic algorithm for PF scheduling so as to deal with the complexity issue in this joint problem. It has been shown that its performance can reach up to 90% of its upper bound.