• Proceedings of the Korean Institute of Intelligent Systems Conference
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• 2008.04a
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• pp.351-354
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• 2008

유비쿼터스 네트워크의 실현을 위하여 다양한 연구가 진행되면, CR (Cognitive Radio) 구성의 연구는 주파수 자원의 효율적인 활용에 관한 연구를 부각시키고 있다. 또한, CR 적용형 소출력 무선기기의 활용에 따른 공유주파수대역의 소요대역폭에 대한 논의와 공통으로 사용하는 주파수 대역 내에서 혼용되는 통신기기들의 user 수를 만족하는 총 소요대역폭 산출은 더불어 중요한 작업이 되었다. 공통으로 사용하는 주파수 대역의 총 소요대역폭 산출은 소출력 무선기기의 간섭 회피 기술로 사용되는 Frequency Hopping (FH) 및 Listen Before Talk (LBT) 방식이 고려되어야 한다. 본 논문에서는 Cognitive Radio (CR) 기반의 LBT 방식을 사용하는 ZigBee와 FH 방식을 사용하는 DCP, RFID, Bluetooth 등의 소출력 무선기기가 공통으로 사용하는 주파수 대역에 공존할 경우를 가정하고, 대기행렬이론 (Queuing Theory)을 적용하여 소요대역폭 산출을 수행했다. 채널수 별 user의 통신시도에 따른 throughput을 분석한 결과, throughput 84% 이상의 조건에서 FH 및 LBT 방식을 사용하는 250mW 소출력 무선기기들이 공존하는 공유주파수대역의 적정 채널수는 35개를 가지며, 총 소요대역폭은 채널수와 채널대역폭의 곱으로 산출이 가능함을 보였다. 이러한 접근방법은 다른 통신시스템의 소요대역폭 산출에도 유용할 것이다.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.30 no.8A
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• pp.649-655
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• 2005

We propose a robust channel estimation method against imperfect synchronization in orthogonal frequency division multiple access (OFDMA) downlink systems. We address time and frequency synchronization, and the channel estimation at the same time, and try to minimize the error propagation from the time and frequency synchronization steps into the chailnel estimation. The simulation results show that the proposed channel estimation method outperforms the conventional algorithms by about 3dB, and circumvents the problem of mismatch among the synchronization tasks.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.1 no.1
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• pp.47-54
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• 1997

New algorithms for frequency channel assignment in small cellular mobile radio system are proposed. The algorithms are for a channel assignment method which control the transmitter power level based on the distance between the cell site and the mobile unit. These algorithms are such that any channel can be used by any cell site and mobile unit, as long as a required threshold level of carrier to interference ratio is maintained. As the cochannel reuse distance decreases, the proposed algorithms have given an increase in capacity in comparison to a fixed channel assignment.

• The Journal of The Korea Institute of Intelligent Transport Systems
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• v.4 no.1 s.6
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• pp.63-71
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• 2005

This paper proposes the channel estimation method robust to severe frequency selective fading channels in O%M system using wide bandwidth for the high data rate transmission. DDCE, which uses data between the high correlated symbols, is usually used for channel estimation in the slow fading channels. DDCE can get high gains in the non-selective channels. As the bandwidth of system gets wider, it becomes more severe frequency selective fading environments so that the reliability of data becomes lower and error flow is occurred. FE method, this paper proposed, uses the relation between sub-carriers of OFDM in frequency selective fading channels so FE method gets some gains by adapting the power value at a target frequency to the mean value of channel estimated values of adjacent sub-carriers. Because FE uses only preamble unlike DDCE using data, it is independent of data rate related to the reliability of data and the number of multipath. Consequently, FE can obtain considerable gains in the wideband systems where the errorflow of DDCE is occurred, and FE is applicable to frequency selective fading environments.

• Proceedings of the Korean Society of Broadcast Engineers Conference
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• 2011.07a
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• pp.537-538
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• 2011

기존 ATSC 시스템의 시간영역 채널 추정은 많은 탭을 사용하고 복잡도가 높지만 OFDM과 같이 보호구간을 이용하여 주파수 영역 등화기를 사용하면 복잡도를 낮추면서 정확한 채널 보상이 가능하다. 본 논문은 ATSC 시스템의 프레임에 보호구간을 삽입하여 주파수 영역 채널 등화를 제안하였다. 또한 DTV수신기의 성능을 확인하기 위해 브라질 채널을 이용하여 컴퓨터 모의실험을 통해 성능을 확인하였다.

• Journal of Broadcast Engineering
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• v.15 no.2
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• pp.217-223
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• 2010

In this paper, an interchannel interference (ICI) cancellation method is proposed for multiple frequency offsets in orthogonal frequency division multiplexing (OFDM) systems. When several same signals are received from different transmitters simultaneously, multiple frequency offsets may occur at the receiver because of the frequency difference of the oscillators of the two transmitters and the receiver. This causes degradation of system performance because OFDM systems are very sensitive to the frequency offsets. In this paper, we propose a method to eliminate the effect of the multiple frequency offsets for OFDM systems. The method is accomplished in two steps: compensation of the frequency offset in the time domain and subsequent cancellation of the ICI in the frequency domain. Through computer simulations, we verify the effectiveness of the proposed ICI cancellation method.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2001.05a
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• pp.197-200
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• 2001

Recently, Switching time is the principal factor in a design of frerquency synthesizer for Spread-Spectrum Communications. fast switching frequency synthesizer is important to improve the channel efficiency in a Frequency Hopping Spread Spectrum (FH-SS) tranceiver. In this paper, we design the frequency synthesizer with fast switching time as fast as 1${\mu}\textrm{s}$. In frequency synthesizer design, we use the interpolated PLL method inserted memory Look-up table of DDS to reduce switching time, and have result of improved channel efficiency about 20% by applying to FH-SS Transceiver.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.41 no.9
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• pp.47-54
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• 2004

The previous works for WLAN system based on OFDM is mainly individual study for independent frequency offset or symbol synchronization. In this paper, the performance of IEEE 802.11a WLAN(Wireless Local Area Network) system in the realistic indoor multipath channel models is analyzed with frequency offset compensation and channel estimation methods. For the performance analysis of the WLAN system indoor Rayleigh multipath channels are adopted, and the BER(Bit Error Rate) of WLAN system is calculated with y2 code-rate 16-QAM based on standard specification. From the simulation results, the difference of required Eb/No for BER of 10-3 is 1-2dB between the channel estimation and frequency offset compensation, and perfect channel estimation and no frequency offset.

• Journal of Advanced Navigation Technology
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• v.11 no.1
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• pp.37-42
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• 2007

In this paper, we analyzed the performance of OFDM systems with adaptive equalizer that considers the frequency offset, the frequency non-selective fading, and two-path microwave Rummer's model channels. First of all, it is analyzed that the performance degradation, which is caused by the offset and the non-selective fading channel, through simulation. As the results of the simulation, the performance of the OFDM system is greatly influenced by the offset and channels. The more the frequency offset is, the worse the performance of the OFDM system is. However, if the adaptive equalizer is adopted to the OFDM system, the performance is enhanced up to the limited rang.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.26 no.10B
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• pp.1367-1374
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• 2001

본 논문은 무선 통신 시스템에 사용될 수 있는 저전력의 Cascaded Integrator-Comb(CIC) 필터 뱅크 구조를 제안한다. 필터 뱅크를 만들기 위해서 CIC 필터의 적분기 섹션을 변화시켜 구성하는 방법을 제안하였으며, 원하는 채널별 중심주파수를 정확히 맞추기 위하여 Modified CIC Filter(MCF)를 사용하는 방법을 제안하였다. 이 방법으로 채널별 중심주파수가 원하는 주파수에 위치하도록 필터 뱅크를 구현할 수 있음을 보였다. 또한 각 채널별 통과대역 주파수응답특성을 개선할 수 있는 새로운 Generalized Comb Filter(GCF)를 제안하였다. 한 개의 GCF와 두 개의 GCF를 각 채널에 적용하는 방법과 그때 통과대역의 주파수응답특성이 얼마나 향상되는지를 보였다. 또한 본 논문이 제안한 CIC 필터 뱅크와 일반 FIR 필터를 사용한 필터 뱅크의 구현 소자의 수를 비교하여 구조의 우수함을 입증하였다.