• Journal of Communications and Networks
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• 제9권2호
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• pp.192-197
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• 2007

We analyze the performance of code division multiple access (CDMA) system with multicarrier (MC) that employs precoding in synchronous downlink channels. Even though considerable efforts are undergoing for frequency offset estimation and correction, it is inevitable for the system to bear the remaining frequency offset. Therefore it is important to predict accurately the system performance in the presence of the residual frequency offset. We obtain the bit error rate (BER) performance in terms of the number of users, the spreading factor, the number of sub-carriers, and frequency offset. We assume that the spreading factor is equal to the number of sub-carriers, although we can generalize the case. The simulation results show that the BER of MC-CDMA with precoding shows a performance that varies with frequency offset as well as system loading.

• Journal of Communications and Networks
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• 제15권2호
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• pp.222-227
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• 2013

This paper presents an analytical framework for dynamic spectrum allocation in cognitive radio networks. We propose a distributed queuing based Markovian model each for single channel and multiple channels access for a contending user. Knowledge about spectrum mobility is one of the most challenging problems in both these setups. To solve this, we consider probabilistic channel availability in case of licensed channel detection for single channel allocation, while variable data rates are considered using channel aggregation technique in the multiple channel access model. These models are designed for a centralized architecture to enable dynamic spectrum allocation and are compared on the basis of access latency and service duration.

• Journal of Communications and Networks
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• 제15권1호
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• pp.25-30
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• 2013

Uplink multicarrier code-division multiple-access (MC-CDMA) with equal gain combining (EGC) over Nakagami fading channels is considered. An improved expression which is a feasible alternative for the bit error rate (BER) performance evaluation of MC-CDMA signals is proposed. Simulated annealing algorithm is employed to obtain the optimum value of the coefficients belonging to the proposed expression. Numerical examples show that the performance curves computed by the improved expression are in good agreement with the results obtained by the exact expression. Thus, the proposed expression can improve the accuracy of BER performance evaluation that has been realized by the approximate expression.

• Journal of Information Processing Systems
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• 제6권4호
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• pp.575-596
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• 2010

Video-on-Demand services are increasing rapidly nowadays. The load on servers can be very high, even exceeding their capacity. For popular contents, we can use a Periodic Broadcast (PB) strategy using multicast to serve all clients. Recent development of PB uses multiple channels broadcasting for segments of movies in certain patterns, so that users only need to wait for a small segment to start the service. However, users need higher download capacity to download multiple segments at a time. In order to compensate for this, a proxy server can help to reduce download bandwidth requirements by holding some segments for a certain time. This research will focus on more recent PB schemes that couldn't be covered by previous Proxy-Assisted Periodic Broadcast strategies.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제13권4호
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• pp.1904-1921
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• 2019

In this paper, we investigate the problem of secure communications in multiple-input-multiple-output interference networks from the perspective of physical layer security. Specifically, the legitimate transmitter-receiver pairs are divided into different categories of active and inactive. To enhance the security performances of active pairs, inactive pairs serve as cooperative jammers and broadcast artificial noises to interfere with the eavesdropper. Besides, active pairs improve their own security by using joint transceivers. The encoding of active pairs and inactive pairs are designed by maximizing the difference of mean-squared errors between active pairs and the eavesdropper. In detail, the transmit precoder matrices of active pairs and inactive pairs are solved according to game theory and linear programming respectively. Experimental results show that the proposed algorithm has fast convergence speed, and the security performances in different scenarios are effectively improved.

• Journal of information and communication convergence engineering
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• 제20권1호
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• pp.1-7
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• 2022

Herein, a new space-time block coding technique is proposed for a MIMO 2 × 2 multiple-input multiple output (MIMO) system to minimize the bit error rate (BER) in Rayleigh fading channels with reduced decoding complexity using ZF and MMSE linear detection techniques. The main objective is to improve the service quality of wireless communication systems and optimize the number of antennas used in base stations and terminals. The idea is to exploit the correlation product technique between both information symbols to transmit per space-time block code and their own orthogonal Walsh-Hadamard sequences to ensure orthogonality between both symbol vectors and create a full-rate orthogonal STBC code. Using 16 quadrature amplitude modulation and the quasi-static Rayleigh channel model in the MATLAB environment, the simulation results show that the proposed space-time block code performs better than the Alamouti code in terms of BER performance in the 2 × 2 MIMO system for both cases of linear decoding ZF and MMSE.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제15권12호
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• pp.4600-4616
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• 2021

In cognitive radio networks where unlicensed secondary users opportunistically access to licensed spectrum unused by licensed primary users, spectrum sensing is one of the key issues in order to effectively use the frequency resource. For enhancing the sensing performance in energy detection-based spectrum sensing, spatial diversity based on multiple antennas is utilized. However, the sensing performance can be degraded when antennas are spatially correlated, resulting in inducing the harmful interference to primary users. To overcome this problem, in this paper, an advanced energy detector is proposed. In the proposed sensing method, a weight matrix based on the eigenvalues of the spatial channels without any prior information on the primary signals is defined and utilized. In numerical simulations, it is shown that the proposed detector outperforms the conventional detector with regard to false-alarm and detection probabilities when antenna are spatially correlated.

• 통합자연과학논문집
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• 제16권1호
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• pp.39-45
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• 2023

P2X receptors are ATP-activated ion channels in the plasma membrane. P2X receptors have a role in a diverse range of disorders, making them a valuable therapeutic target. Hence, the present investigation employed homology modelling of the P2X receptor based on the crystal structure of 5SVJ, 6AH4, 5YVE and 5SVL. Twenty models, using both single- and multiple template-based methods, were developed, and the best model was chosen based on the validation result. We observed that a strategy based on multiple templates provided greater accuracy. Future studies involving binding site and docking analysis can make use of the produced structures.

• 한국통신학회논문지
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• 제29권9A호
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• pp.1022-1030
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• 2004

차세대 이동 및 우선통신 시스댐은 현재보나 얘우 큰 고속의 데이터 전송과 시스템 용량을 요구하고 였으며 이러한 요구를 충족시키기 위해서 복수의 안테나를 사용하여 송수신을 하는 MIMO(m 비 tiple input, multiple output) 기술이 널리 연구되고 있다 이 논문은 다수의 안테나를 가지고 있는 기지국, 소수의 안테나를 가지고 있는 단말기의 하향 링크에서 스케터령과 변동이 적으며 평탄한 페이딩 채널을 가정한다. 단영 사용가 MIMO 시스템에서 가지국이 채널상태정보를 가지고 있다연 SVD(singular value decomposition) 와 water filling 읍 사용 한 MIMO 기숭이 최대의 채널용량을 이룬다. 그러나 복수 사용자 시스템의 경우에는 공간분할 다중접속 기술을 이용하여, 보다 큰 전체 채널용량을 얻는 것이 가능하다 이 논문은 복수 사용가 시스템에서 각 사용자에게 복수 의 데이터 스트림을 전송하는 MIMO 공간분할 다중접속 기술을 제안한다. 제안하는 기술은 SVD 기반의 MIMO 기술이나 스마트 안테나를 사용한 공간분항 다중정속 기술보다 더 큰 전체 채널용량을 얻을 수 있다.

• 대한전자공학회논문지TC
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• 제46권6호
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• pp.78-85
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• 2009

시변 채널에서 상향 직교 주파수 분할 다중 접속 (OFDMA uplink : Orthogonal Frequency Division Multiple Access uplink) 시스템은 도플러 현상 (Doppler Effect)과 시간 비동기에 의한 부반송파 간의 간섭 (ICI : Inter-Channel Interference)으로 인해 치명적인 성능 열화가 발생한다. 본 논문에서는 OFDM uplink 시스템에서 부반송파 간의 간섭으로 인한 열화를 줄이기 위해 혼합 간섭 제거 기법 (HIC : Hybrid Interference Cancellation)을 제안한다. 제안된 혼합 간섭 제거 기법은 직렬 간섭 제거 기법 (SIC : Successive Interference Cancellation)의 강인한 간섭 제거 효과와 병렬 간섭 제거 기법 (PIC : Parallel Interference Cancellation)의 복잡도 감소 효과를 동시에 얻을 수 있도록 설계되었다. 이것은 신호 대 간섭 및 잡음 비 (SINR : Signal-to-Interference and Noise Ratio)에 따라 정렬된 수신 신호의 효과적인 군(Group) 선택과 위너 여파기 (WF : Wiener Filter), 그리고 시변 채널 특성을 고려한 신호 선택 기준을 통해 구현된다. 모의 실험 결과에서는 기존 OFDMA 기법의 등화기가 시변 페이딩 채널의 영향이 커짐에 따라 성능 열화가 심해지는 것에 반해 제안된 간섭 제거 기법은 간섭 제거를 위한 신호 선택 기준에 의해 저복잡도에서도 비트 오류 확률 성능이 더욱 향상되며, 또한 성능과 복잡도의 교환이 가능한 유연한 시스템인 것을 보여 준다.