• The Journal of the Acoustical Society of Korea
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• v.33 no.4
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• pp.255-261
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• 2014

This paper presents an underwater acoustic communication with nonlinear chirp modulation. The information is carried by the carrier amplitude, frequency or phase in the most common underwater acoustic communications. However, the proposed method includes the information within frequency variation of carrier wave for a symbol. Especially, as carrier wave the hyperbolic frequency modulated signal, one of the nonlinear chirp signal, is used and it is robust in the Doppler channel. The proposed method was analyzed and compared to conventional method by simulation. When the doppler shift existed, the error probability of the proposed method is reduced by 5~12 % than conventional method with linear frequency modulated signal. Sea trial was performed to analyze the performance of the proposed method.

• Korean Journal of Optics and Photonics
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• v.8 no.3
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• pp.250-253
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• 1997

The effect of dispersion in optical transmission system changes according to chirp and SPM. Chirp depends on the modulation condition of an external modulator. SPM is proportional to signal power. In this paper, we analyzed the pulse broadening due to SPM and chirp in the system with dispersion by using Split Step Fourier Method and calculated the optimum chirp parameter for 400 km transmission system. Experimental results are presented also.

• Journal of the Korean Institute of Telematics and Electronics D
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• v.35D no.6
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• pp.92-98
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• 1998

The transform limited optical short pulses are generated by compression of pulses from a gain switched distributed feedback laser at 5 GHz repetition rate. The gain-switched pulses have the minimum pulse width of 27 psec with the spectral width of 1.1 nm. Thus the output pulses have a large amount of linear chirp and nonlinear chirp. We suppress the nonlinear chirp by passing the pulses through the optical band pass filter with 3 dB band width of 0.55 nm which is narrower than spectral width of the input pulses and generate 7.1 psec pulses by compressing the output with the dispersion compensating fiber. The pulses have time-bandwidth product of 0.49 which is close to the transform limited gaussian pulse. These pulses can be utilized as optical sources in 40 Gbit/s time division multiplexed optical transmission system.

• Korean Journal of Optics and Photonics
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• v.11 no.5
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• pp.353-359
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• 2000

A harmonically mode-locked fiber ring laser cavlty was theoretically analyzed. The measured charactenstlcs of a 10 GHz erbium-doped fiber ring laser were used as a basis of the theoretical analysis. From the nonlinear Schroedinger equation of an actively mode-locked erblUm-doped fiber laser. the effects of the components inside the laser cavity have been analyzed includmg nonlinear effects from group-velocity dispersion (GVD) and self-phase modulation (SPM). Usmg the analysls. we have designed a laser cavity with minimum pulsewidth and chirp by changing the intracavity optical intensity and the bandwidth of the filter. In the new design, the chirp i, reduced by 2 times and the pulsewidth by 2A times. compared to the laser used in the experiment. iment.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.44 no.2
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• pp.131-138
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• 2016

In this paper, we studied the IRF performances of the chirp signal used in the SAR system. The most important factors that degrade IRF performances are amplitude and phase errors. Each factor can be represented to linear, quadratic, random and ripple terms. That can be extracted by a quadratic polynomial curve fitting of chirp waveform. We analyzed the IRF performances by the error terms and supposed the minimum value of RF non-linearity to meet the specification of the PSLR and ISLR.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.3 no.1
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• pp.83-95
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• 1999

Wavelet transform has localization for time or frequency. It is useful to analyze a nonstationary signal. Basic function on wavelet transform is generated dilating and translating the original wavelet(mother wavelet). In this paper, time-frequency analysis method using linear combination wavelet transform is proposed. And it is applied to diagnostic monitoring system using the proposed linear combination wavelet transform. The stationary and nonstationary signal is used linear chirp signal, fan noise signal, a sinusoid signal from revolution body, electronic signal. Transform applied to signal analysis use fast Fourier transform (FFT), Daubechies, Haar and proposed linear combination method. The result of time-frequency analysis using linear combination wavelet transform is suited for portraying nonstationary time signal as well as stationary signal. Also the diagnostic monitoring system carry out the effective the signal analysis.

• Korean Journal of Optics and Photonics
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• v.27 no.6
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• pp.203-211
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• 2016

We have studied the hybrid method for reconstructing apodized chirped fiber Bragg gratings, using both an analytic estimation of grating parameters and an optimization algorithm. The Hilbert transform of the reflection spectrum was utilized to estimate grating parameters, and then the layer-peeling algorithm was used to obtain refined parameter values by the differential-evolution optimization process. Calculations for a fiber Bragg grating with wavelength chirp rate 2 nm/cm were obtained with an accuracy of $6{\times}10^{-5}nm/cm$ for the chirp rate and $3{\times}10^{-9}nm/cm$ for the index modulation, with much improved calculation speed and high reliability.

• The Journal of the Acoustical Society of Korea
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• v.42 no.3
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• pp.169-180
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• 2023

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.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.24 no.12
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• pp.1205-1214
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• 2013

Recently spaceborne SAR systems are increasing image resolution and frequency. As a high quality image resolution, the wider bandwidth is required and a wideband signal generator with RF component is very complicated and RF impairments of device is increased. Therefore, it is very important to improve performance by reducing these errors. In this study, the transmission signal of the wideband signal generator is applied to the phase noise, IQ imbalance, ripple gain, nonlinear model of high power amplifier. And we define possible structures of wideband signal generator and measure the PSLR and ISLR for the performance assesment. Also, we extract error of the amplitude and phase from the waveform and use a quadratic polynomial curve fitting and examine the performance change due to nonlinear device. Finally, we apply a high power amplifier predistortion method for non-linear error compensation. And we confirm that distortion in the output of the amplifier by intermodulation component is decreased by 15 dB.

• Journal of the Institute of Electronics and Information Engineers
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• v.54 no.1
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• pp.111-120
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• 2017

In this paper, we propose novel feature extraction algorithm for classification of seabed sediment. In previous researches, acoustic reflection coefficient has been used to classify seabed sediments, which is constant in terms of frequency. However, attenuation of seabed sediment is a function of frequency and is highly influenced by sediment types in general. Hence, we developed a feature vector by using attenuation variation with respect to frequency. The attenuation variation is obtained by using reflected signal from the second sediment layer, which is generated by broadband chirp. The proposed feature vector has advantage in number of dimensions to classify the seabed sediment over the classical scalar feature (reflection coefficient). To compare the proposed feature with the classical scalar feature, dimension of proposed feature vector is reduced by using linear discriminant analysis (LDA). Synthesised acoustic amplitudes reflected by seabed sediments are generated by using Biot model and the performance of proposed feature is evaluated by using Fisher scoring and classification accuracy computed by maximum likelihood decision (MLD). As a result, the proposed feature shows higher discrimination performance and more robustness against measurement errors than that of classical feature.