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Cyclic prefix (CP) is one of the most important technique to OFDM system and is reducing inter-symbol interference (ISI) effects in high speed wireless mobile communication system. At the time varying channel condition, however, fixed CP length is not only increasing power consumption but also reducing data transmission rate. So in this paper, we propose the system that has adaptive CP length for high speed data transmission system. We don’t control CP length of every symbol but adjust symbol interval depending on channel condition to CP reconstruction.

The goal of this paper is to suppress time-varying single tone jamming (STJ) in the CDMA system by using the frequency interference cancellation method. We consider the practical scenario where STJ signal experiences time-varying fading channel. In this approach, the parameters (i.e. frequency, amplitude and phase) of the time-varying STJ are estimated in frequency domain, and then it is reconstructed and subtracted from the raw signals. The theoretical BER (bit error rate) expression is analyzed to assess the performance of the method, and computer simulations are executed. Simulation results demonstrate the effectiveness of the method in mitigating time-varying STJ and approaching the interference-free performance limit over practical ranges of STJ power levels and mobility levels.

In this paper we proposed a dada-aided Doppler estimation method for underwater acoustic communication. The training sequence is non-dedicate, hence it can be designed for Doppler estimation as well as channel equalization. We assume the channel has been equalized and consider only flat-fading channel. First, based on the training symbols the theoretical received sequence is composed. Next the least square principle is applied to build the objective function, which minimizes the error between the composed and the actual received signal. Then an iterative approach is applied to solve the least square problem. The proposed approach involves an outer loop and inner loop, which resolve the channel gain and Doppler coefficient, respectively. The theoretical performance bound, i.e. the Cramer-Rao Lower Bound (CRLB) of estimation is also derived. Computer simulations results show that the proposed algorithm achieves the CRLB in medium to high SNR cases.

In high-speed multimedia satellite communication systems, it is essential to provide high-quality, economical services by using efficient transmission schemes which can overcome channel impairments appearing in the satellite link. This paper introduces techniques to compensate for rain attenuation and the Doppler shift in the satellite communication link. An adaptive transmission technique with a control algorithm to adaptively allocate transmission schemes is used as a countermeasure to rain attenuation. We introduce a new rain attenuation modeling technique for estimating system performance and propose a novel Doppler shift compensation algorithm with reduced hardware complexity. Extensive simulation results show that the proposed algorithm can provide greatly enhanced performance compared to conventional algorithms. Simulation software and hardware which incorporate the proposed techniques are also demonstrated.

In low earth orbit (LEO) satellite communication systems, more severe phase distortion due to Doppler shift is frequently detected in the received signal than in cases of geostationary earth orbit (GEO) satellite systems or terrestrial mobile systems. Therefore, an estimation of Doppler shift would be one of the most important factors to enhance performance of LEO satellite communication system. In this paper, a new adaptive Doppler compensation scheme using location information of a user terminal and satellite, as well as a weighting factor for the reduction of prediction error is proposed. The prediction performance of the proposed scheme is simulated in terms of the prediction accuracy and the cumulative density function of the prediction error, with considering the offset variation range of the initial input parameters in LEO satellite system. The simulation results showed that the proposed adaptive compensation algorithm has the better performance accuracy than Ali's method. From the simulation results, it is concluded the adaptive compensation algorithm is the most applicable method that can be applied to LEO satellite systems of a range of altitude between 1,000 km and 2,000 km for the general error tolerance level, M = 250 Hz.

수중 환경에서의 도플러 천이 주파수 추정은 채널 특성을 분석하고, 통신 성능을 향상시키기 위한 중요한 단계이다. 주로 Linear Frequency Modulated(LFM) 신호와 같은 처프 신호가 사용되었으며, 수신단에서 이의 리플리카(replica)와 상호상관도를 측정하거나 다른 형태의 처프 신호를 연접하여 도플러 천이 주파수를 추정하는 것이 가장 널리 알려진 방법이다. 본 논문에서는 하나의 LFM 신호를 이용하면서 수신단에서 이 신호의 주파수 특성 변화를 기반으로 하는 도플러 천이 주파수 추정 방법을 제안한다. 제안한 방법은 Continuous Wave(CW) 신호를 사용하여 리플리카를 구성하여 1차 도플러 천이 주파수를 추정하며, 이후 곡선 적합을 통해 보정 과정을 거친다. 제안한 방법을 이용하여 도플러 천이 주파수를 추정하는 경우 기존의 방법에 비해 약 72 %의 계산량 감소를 나타내는 것을 모의실험을 통해 확인하였다.

Doppler estimation in underwater environments is essential for analyzing channel characteristics and enhancing communication. Commonly chirp signals such as Linear Frequency Modulated (LFM) signals have been used, and the most widely known method is to measure the cross-correlation with its replica at the receiver or to estimate the Doppler shift frequency by concatenating different types of chirp signals. In this paper, we propose a Doppler shift frequency estimation method based on the change in the frequency characteristics of a single LFM signal at the receiver. The proposed method configures a replica using a Continuous Wave (CW) signal to estimate the first Doppler shift frequency, and then goes through a correction process through curve fitting. It was confirmed through simulations that the proposed method reduces the computational load by about 72% compared to the previous method when estimating the Doppler frequency.

수중음향통신의 성능은 다중경로 전달과 도플러 확산에 크게 영향을 받는다. 본 논문은 다중경로 전달 환경에서 강인한 기존의 Sweep Spread Carrier(SSC) 기법을 변형하여, 새로운 통신기법인 Triangle Spread Carrier(TSC) 기법을 제안한다. 제안한 TSC 기법은 상승-처프와 하강-처프 신호가 반복되는 반송파를 갖는 형태이며, 각각의 상관함수 특성을 활용하여 수신 신호의 도플러 천이 주파수를 추정하고 보정한다. 제안된 TSC 기법의 성능을 입증하기 위하여 수중 채널 시뮬레이터를 이용한 모의실험과 동해에서 수행된 해상실험을 결과를 제시한다. 해상실험 결과 추정된 도플러 천이 주파수만을 이용하여 복조하였을 때 비부호화 된 비트 오류율은 최대 0.194였지만, 제안한 방법을 적용하였을 때 비부화화된 비트 오류율이 0.001로 감소하였다.

The performance of underwater acoustic communication is greatly affected by multipath propagation and Doppler spread. This paper proposes a new communication technique, the Triangle Spread Carrier (TSC) technique, by modifying the existing Sweep Spread Carrier (SSC) technique that is strong in a multipath propagation environment. The proposed TSC technique is a form in which the up-chirp and down-chirp signals have repeated carriers, and each correlation function characteristic is used to estimate and correct the Doppler shift frequency of the receiving signal. To demonstrate the performance of the proposed TSC technique, we present the results of simulations using underwater channel simulators and sea trial conducted in the East Sea. When demodulating using only the estimated Doppler shift frequency as a result of the sea trial, the uncoded bit error rate was up to 0.194, but when the proposed method was applied, the uncoded bit error rate was reduced to 0.001.

본 논문에서는 이동위성통신 DS/CDMA 시스템의 상향링크 채널에 대한 이중모드 통신방식을 제안하고, 제안 시스템의 성능을 해석하고 제안 시스템의 유용성도 확인하였다. 또한 이동위성통신 DS/CDMA 시스템에 큰 도플러 옵셋의 해결방안으로 Kajiwara가 제안한 이중채널 방식을 이용하여 이동위성채널에 널리 쓰이는 라이시안 페이딩 모델을 설정하였으며 변조 방식으로는 DCPSK 신호를 이동체의 속도와 전송률 에러의 성능을 비교 분석하였다.

비정치제도 위성통신링크에서는 지표상의 단말기, 지구국 등에 대한 상대적인 위성의 빠른 이동으로 인하여 도플러천이가 발생하며, 이는 이동위성통신 시스템 수신단의 성능을 약화시킨다. 본 논문은 비정지궤도 위성을 이용하는 통신링크 상에서 발생하는 도플러천이의 특성을 분석한다. 분석된 결과는 도플러천이에 대한 시뮬레이션 모델과 보상 알고리즘의 개발에 활용활 수 있을 것이다.

LFM 신호를 이용하여 속도가 변화하는 표적의 존재 유무를 검출하는 기존의 소나시스템에서는 다수의 LFM 도플러 상관기를 통해 얻은 결과들을 시간상에서 정렬시킨 후 최대값을 테스트 셀로 선택하여 사용한다. 이때 정밀한 표적 검출을 하기 위해서는 많은 수의 LFM 도플러 상관기들이 필요하며, 이로 인해 요구되는 연산량과 메모리도 역시 증가하게 되어 정밀한 LFM 표적 검출기를 실제 구현하는 것을 어렵게 한다. 본 논문에서는 LFM 신호를 사용하는 소나시스템에서 표적 속도 변화에 강인하면서도 고속으로 표적 존재 유무를 검출하기 위한 새로운 기법을 제안하였다. 제안하는 기법에서는 많은 수의 LFM 도플러 상관기들의 정렬된 결과를 합하여 테스트 셀을 만들었으며, 이를 통해 SNR을 향상시킬 뿐만 아니라 표적 속도 변화에 강인한 SNR 값을 제공하여 검출기의 성능을 향상시킨다. 또한 제안하는 기법은 많은 수의 LFM 도플러 상관기들의 정렬된 결과를 합하는 과정을 하나의 합필터로 구현하여 연산량을 획기적으로 줄임으로써 표적을 고속으로 검출할 수 있게 하였다.

In a conventional sonar system, which uses LFM signal for detecting targets with varying speed, the results of multiple LFM Doppler correlators are aligned and the maximum alined result are selected as a test cell for detecting targets. As the number of the LFM Doppler correlators are increased for accurate target detection, as the required computational complexity and the memory are also increased. This fact makes it difficult to implement the accurate LFM target detector. In this paper, we propose a new fast target detection which is robust for the variation of target speed. Because the proposed method uses the summation of alined results of large numbers of LFM Doppler correlators, the proposed method increase SNR and provide robust SNR for the variation of target speed. And the proposed method can provide very fast target detection by implementing the process, the summation of alined results of large numbers of LFM Doppler correlators, as one summation filter.