• 한국자동차공학회논문집
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• 제9권2호
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• pp.210-215
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• 2001

The pole antenna of vehicles for AM/FM radio broadcasting receptions has many probems, such as damageable mechanical system and noise etc. It is considered that noise is composed of the structural vibration noise and the air flow induced aerodynamic noise. Also we find out that the setting process of a printed on-glass automobile antenna has many difficulties. Recently, the above mentioned problems can be reduced by employing enhanced wiper antenna which utilizes the windshied wiper arm. The new system is a passive antenna. In this study, experiments for the characteristics of a wiper antenna by measuring the SWR, radiation pattern and received powe have been carried out. The experimental results show that the efficiency of the wiper antenna has better performance than any other antenna.

• Journal of electromagnetic engineering and science
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• 제12권3호
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• pp.203-209
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• 2012

In order to improve sensing performance when the noise variance is not known, this paper considers a so-called blind spectrum sensing technique that is based on eigenvalue models. In this paper, we employed the spiked population models in order to identify the miss detection probability. At first, we try to estimate the unknown noise variance based on the blind measurements at a secondary location. We then investigate the performance of detection, in terms of both theoretical and empirical aspects, after applying this estimated noise variance result. In addition, we study the effects of the number of SUs and the number of samples on the spectrum sensing performance.

• Journal of Communications and Networks
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• 제11권2호
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• pp.148-156
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• 2009

The fixed spectrum assignment policy in today's wireless networks leads to inefficient spectrum usage. Cognitive radio network is a new communication paradigm that enables the unlicensed users to opportunistically use the spatio-temporally unoccupied portions of the spectrum, and hence realizing a dynamic spectrum access (DSA) methodology. Interference temperature model proposed by Federal Communications Commission (FCC) permits the unlicensed users to utilize the licensed frequencies simultaneously with the primary users provided that they adhere to the interference temperature constraints. In this paper, we formulate two NP-hard optimal scheduling methods that meet the interference temperature constraints for cognitive radio networks. The first one maximizes the network throughput, whereas the second one minimizes the scheduling delay. Furthermore, we also propose suboptimal schedulers with linear complexity, referred to as maximum frequency selection (MFS) and probabilistic frequency selection (PFS). We simulate the throughput and delay performance of the optimal as well as the suboptimal schedulers for varying number of cognitive nodes, number of primary neighbors for each cognitive node, and interference temperature limits for the frequencies. We also evaluate the performance of our proposed schedulers under both additive white gaussian noise (AWGN) channels and Gilbert-Elliot fading channels.

• Journal of information and communication convergence engineering
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• 제13권4호
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• pp.241-247
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• 2015

Cognitive radio has been attracting increased attention as an effective approach to improving spectrum efficiency. One component of cognitive radio, spectrum sensing, has an important relationship with the performance of cognitive radio. In this paper, after a summary and analysis of the existing spectrum-sensing algorithms, we report that the existing eigenvalue-based semi-blind detection algorithm and blind detection algorithm have not made full use of the eigenvalues of the received signals. Applying multi-antenna systems to cognitive users, we design a variety of spectrum-sensing algorithms based on the joint distribution of the eigenvalues of the received signal. Simulation results validate that the proposed algorithms in this paper are able to detect whether the signal of the primary user exists or not with high probability of detection in an environment with a low signal-to-noise ratio. Compared with traditional algorithms, the new algorithms have the advantages of high detection performance and strong robustness

• 한국전자파학회논문지
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• 제18권5호
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• pp.511-521
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• 2007

본 논문에서는 복합 소형화된 Ka-대역 주파수 합성기 모듈을 제작하였다. 본 논문을 통하여 소형화 구성시 배치 방법과 체계적인 검증 방법을 제시하였다. 제작된 주파수 합성기는 X-대역 전압 제어 발진기(VCO: Voltage Controlled Oscillator)의 주파수를 3체배하여 구성하였으며, 제작된 모듈은 500 MHz 주파수 가변 범위와 약 14 dBm의 출력 전력, 그리고 100 kHz 오프셋 주파수에서 -96.17 dBc/Hz의 위상 잡음 특성을 보여주고 있다.

• 한국위성정보통신학회논문지
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• 제1권1호
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• pp.5-11
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• 2006

무선인지기술은 환경에 능동적으로 적응하여 스펙트럼 이용을 효율적으로 할 수 있도록 하는 기술이다. 무선인지 기술의 가장 중요한 첫 단계는 센싱 부분으로서 본 논문에서는 주파수 상관관계를 이용한 방법을 제안하였다. 통신신호의 주기적인 특성을 기반으로 잡음과 간섭의 효과를 최소화하기 위해 주파수 상관함수를 사용하였다. 이미 알려진 에너지 측정 방법의 동작특성을 보이고 시뮬레이션을 통하여 제안된 측정 방법과 기존의 방법을 비교하여 무선인지환경을 이용한 스펙트럼 센싱의 효율성을 보인다.

• 한국위성정보통신학회논문지
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• 제5권1호
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• pp.27-31
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• 2010

급증하는 주파수 수요를 해결하기 위해 최근 FCC(Federal Communications Commission)에서는 스펙트럼 사용 효율을 높이기 위해 주파수를 공유하는 CR(Cognitive radio)를 적용하기로 하였다. 그리고 이러한 CR 시스템을 LTE 시스템에 적용하고자 CR 기술 중 스펙트럼 센싱 이론을 LTE 환경에 적용을 시켜 실험을 하여 성능분석을 하였다. 본 실험에 있어서 LTE 하향링크 OFDMA (Orthogonal frequency division multiple access) 신호를 센싱했고, 채널은 가우시안 채널로 가정하여 실험을 하였다. 또한 실험에 필요로 한 임계값(threshold)은 5%, 10%로 달리 적용하여 성능분석을 하였다. 그리고 추후 LTE 시스템에 CR 시스템 적용을 위한 향후 연구 방향에 대하여 논의를 하였다.

• 전자공학회논문지D
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• 제36D권9호
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• pp.13-20
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• 1999

위성통신용 30GHz대 저잡음 증폭기 MMIC 회로가 $0.15 {\mu}m$ 게이트를 구현하는 pHEMT 기술로 제작되었다. 하나의 저잡음 증폭용 pHEMT 소자는 길이 $20{\mu}m$인 게이트 핑거 4개로 구성되어 있다. HEMT 소자의 소오스에 연결한 직렬 궤환 회로로 인하여 30GHz대 주파수에서 고이득과 저잡음, 그리고 우수한 입력측 반사계수를 동시에 얻을 수 있었다. 바이어스 회로 및 증폭기 안정화 회로를 이용하여 제 1단, 제 2단 및 증폭기 전체가 전대역에서 모두 인정한 동작을 하도록 하였다. 제작된 2단 MMIC 저잡음증폭기는 사용대역인 29 ~ 33GHz에서 15.7dB 이상의 이득과 .2.09dB 이하의 잡음지수를 가진 것으로 측정되어 설계시의 예측된 성능과 잘 일치한다.

• Journal of Communications and Networks
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• 제16권4호
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• pp.397-406
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• 2014

Radio frequency (RF) jamming is a denial of service attack targeted at wireless networks. In resource-hungry scenarios with constant traffic demand, jamming can create connectivity problems and seriously affect communication. Therefore, the vulnerabilities of wireless networks must be studied. In this study, we investigate a particular type of RF jamming that exploits the semantics of physical (PHY) and medium access control (MAC) layer protocols. This can be extended to any wireless communication network whose protocol characteristics and operating frequencies are known to the attacker. We propose two efficient jamming techniques: A low-data-rate random jamming and a shot-noise based protocol-aware RF jamming. Both techniques use shot-noise pulses to disrupt ongoing transmission ensuring they are energy efficient, and they significantly reduce the detection probability of the jammer. Further, we derived the tight upper bound on the duration and the number of shot-noise pulses for Wi-Fi, GSM, and WiMax networks. The proposed model takes consider the channel access mechanism employed at the MAC layer, data transmission rate, PHY/MAC layer modulation and channel coding schemes. Moreover, we analyze the effect of different packet sizes on the proposed jamming methodologies. The proposed jamming attack models have been experimentally evaluated for 802.11b networks on an actual testbed environment by transmitting data packets of varying sizes. The achieved results clearly demonstrate a considerable increase in the overall jamming efficiency of the proposed protocol-aware jammer in terms of packet delivery ratio, energy expenditure and detection probabilities over contemporary jamming methods provided in the literature.

• International Journal of Railway
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• 제3권2호
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• pp.39-45
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• 2010

The RF-CBTC (Radio Frequency-Communication Based Train Control) System is a communication system in railroad systems. The communication method of RF-CBTC system is the wireless between the wayside device and on-board device. The wayside device collects its location and speed from each train and transmits the distance from the forwarding train to the speed-limit position to it. The on-board device controlling device controls the speed optimum for the train. In the case of the RF-CBTC system used in Korea, transmission frequency is 2.4 [GHz]. It is the range of ISM(Industrial Scientific and Medical equipment) band and transmission of voice and data is performed by CDMA (Code Division Multiple Access) method. So noises are made in the AWGN (Additive White Gaussian Noise) and fading environment. Currently, the SNR (Signal to Noise Ratio) is about 20 [dB], so due to bit errors made by noises, transmission of reliable information to the train is not easy. Also, in the case that two tracks are put to a single direction, it is needed that two trains transmit reliable voice and data to a wayside device. But, by noises, it is not easy that just a train transmits reliable information. In this paper, we estimated the BER (Bit Error Rate) related to the SNR of voice and data transmission in the environment such as AWGN and fading from the RF-CBTC system using the CDMA method. Also, we supposed the SNR which is required to meet the BER standard for voice and data transmission. By increasing the processing gain that is a ratio of chip transmission to voice and data transmission, we made possible voice and data transmission from maximally two trains to a wayside device, and demonstrated it by using Matlab program.