• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.35 no.8A
• /
• pp.777-784
• /
• 2010

Recently, the necessity of underwater communication and demand for transmitting and receiving various data such as voice or high resolution image data are increasing as well. The performance of underwater acoustic communication system is influenced by characteristics of the underwater communication channels. Especially, ISI(inter symbol interference) occurs because of delay spread according to multi-path and communication performance is degraded. In this paper, we study the OFDM technique to overcome the delay spread in underwater channel and by using CP, we compensate for delay spread. But PAPR which OFDM system has problem is very high. Therefore, we use DFT-spread OFDM method to avoid nonlinear distortion by high PAPR and to improve efficiency of amplifier. DFT-spread OFDM technique obtains high PAPR reduction effect because of each parallel data loads to all subcarrier by DFT spread processing before IFFT. In this paper, we show performance about delay spread through OFDM system and verify method that DFT spread OFDM is more suitable than OFDM for underwater communication. And we analyze performance according to two subcarrier mapping methods(Interleaved, Localized). Through the simulation results, performance of DFT spread OFDM is better about 5~6dB at $10^{-4}$ than OFDM. When compared to BER according to subcarrier mapping, Interleaved method is better about 3.5dB at $10^{-4}$ than Localized method.

• Journal of the Korea Institute of Military Science and Technology
• /
• v.18 no.2
• /
• pp.115-124
• /
• 2015

A methodology is proposed to estimate a trajectory of a flying target and its velocity using the time and frequency analysis of the acoustic signal. The measurement of sound emitted from a flying acoustic source with a microphone above a ground shall receive both direct and ground-reflected sound waves. For certain frequency contents, the destructive interference happens in received signal waveform reflected path lengths are in multiple integers of direct path length. This phenomenon is referred to as the acoustical mirror effect and it can be observed in a spectrogram plot. The spectrogram of acoustic measurement for a flying vehicle measurement shows several orders of destructive interference curves. The first or second order of curve is used to find the best approximate path by using nonlinear least-square method. Simulated acoustic signal is generated for the condition of known geometric of a sensor and a source in flight. The estimation based on cepstrogram analysis provides more accurate estimate than spectrogram.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
• /
• v.24 no.4
• /
• pp.417-425
• /
• 2013

Digital communication technology have been focused on the higher data efficiency and the effective coding of information. Also, it is very important to improve security and minimize the interference of the external sources, Chaos communications system is studied to improve security and minimize the interference by effective encoding of information in the nonlinear systems such as DCSK(Differential Chaos Shift Keying) and CDSK(Correlation Delay Shift Keying). In this paper, we evaluate anti-jamming performance of chaos communications system using the DCSK and CDSK system. The simulation results show that the Gauss iterated map has better BER performance than the other maps(tent map, logistic map, Henon map). Also, DCSK and CDSK system can get the better anti-jamming performance by applying different chaotic maps.

• Journal of the Institute of Convergence Signal Processing
• /
• v.3 no.2
• /
• pp.53-57
• /
• 2002

Recently, digital communication system becomes high speed, as communication demand dose not only increases sharply, but an image, voice various kinds data also comes multimedia. In transmitting data at a high speed, the main problem is fading by multipath. A linear or nonlinear distortion arise In multipath channel fading from ISI(Intersymbol Interference). For restoring this distorted signal, A lot of equalizer and adaptive algorithm is introduced. This paper compares and analysises, for improving communication quality in channel which is long delay spread, performance of decision feedback equalizer by RLS algorithm, a channel estimation by RLS-MLSE and viterbi equalizer Particularly, there Is exactly channel estimation of impluse response and excellent property of equalization about channel, which delay spread is long impluse response comparatively and is property of non-minimun phase.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.25 no.9B
• /
• pp.1542-1550
• /
• 2000

Fiber optic link systems based on SCM scheme are widely used as efficient and economic RF signal links between base station and a central station in mobile communication networks. However, its performance can be seriously limited depending on the operational conditions of not only the optical transmission system but also the wireless link in various environments. In this paper, we propose an analytic model for performance analyses of the SCM fiber optic link for CDMA RF signal transmission in various link environments. We present optimal operational conditions taking account of the nonlinear effects of the optical transmission system, and the multiple access interference produced at the wireless link. It has been shown through the BER analyses in this paper that the selection of optical modulation index of the SCM fiber optic links can be found optimally to minimize the fiber optic link noises and intermodulation distortion due to LD.

• Journal of Satellite, Information and Communications
• /
• v.4 no.2
• /
• pp.39-45
• /
• 2009

This paper proposes the specific code synchronization technique for DS-SS(Direct Sequence-Spread Spectrum transmission in the DVB-RCS +M standard. DS-SS is better than multi-carrier transmission method under nonlinear channel but imposes a long acquisition time. To improve the synchronization aspect, the robust correlation structure is introduced for acquisition and the nonlinear delay lock loop is done for tracking. MAT(Mean Acquisition Time) performances is shown to validate its superiority. In addition, code tracking and jitter performances are done when code tracking algorithm based on 2 oversamples which is not influenced by sampling clock timing offset and carrier freq. offset is used.

• Journal of the Institute of Electronics Engineers of Korea TC
• /
• v.40 no.3
• /
• pp.1-9
• /
• 2003

We analyze the performances of MC-CDMA and MC-DS/CDMA systems on quasi-synchronized mobile satellite return link. Quasi-synchronization is considered that chip offset between user terminals is within a few chips. Since the transmitted signals of MC-CDMA and MC-DS/CDMA systems have non-constant envelop, they are easily distorted by nonlinearity of transmit amplifier. Since the nonlinear amplifier input signal level of MC-CDMA system using Walsh code to employ frequency domain spreading is lower than that of MC-DS/CDMA system, performance of single user in MC-CDMA system is respectively 2.3dB and 1dB better than that of MC-DS/CDMA system when amplifier input backoff is 0dB and 6dB. However, since interference of MC-DS/CDMA is less than that of MC-CDMA with many subcarriers in quasi-synchronization and AWGN channel, MC-DS/CDMA system is much better than MC-CDMA system as the number of users increases.

• The Journal of the Institute of Internet, Broadcasting and Communication
• /
• v.15 no.1
• /
• pp.131-137
• /
• 2015

This paper relates with the VSS-MMA (Variable Step Size-Multiple Modulus Algorithm) adaptive equalization algorithm which is possible to improving the equalization performance by use the nonlinear fuction of error signal in the MMA adaptive equalization algorithm that are used for the minimization of the intersymbol interference due to the distortion which occurs in the time dispersive channel for the transmission of QAM signal in the system.. In the conventional MMA, we obtains the tap coefficient of adaptive equalizer using the fixed step size, but in the VSS-MMA, we obtains the tap coefficient of adaptive equalizer using the variable step size based on a nonlinear function of error signal. By adapting the variable step size, it was confirmed that the improved equalization performance were obtained by computer simulation. For this, the equalizer output signal constellation, residual isi, maximum distortion, MSE and SER were used in the performace index.

• Journal of the Korea Institute of Information and Communication Engineering
• /
• v.17 no.2
• /
• pp.294-301
• /
• 2013

OFDM technique uses multiple sub-carriers for the data transmission. Therefore, bit error rate increases because of inter-carrier interference caused by nonlinear high power amplifier and carrier frequency offset. Wireless OFDM transmission over multi path fading channels is characterized by small transmission gain in multiple sub-carrier frequency interval. Therefore bit error rate increases because of burst errors. Inter-leaver and convolution error control coding are effective for the reduction of this burst error. Pilot symbol is used for the channel estimation in OFDM systems. However, imperfect channel estimates in this systems degrade the performance. The performance of this convolution coding OFDM systems using inter-leaver, gauged by the bit error rate, is analyzed considering the nonlinear high power amplifier, carrier frequency offset and channel estimation error.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
• /
• v.13 no.3
• /
• pp.263-268
• /
• 2002

DFDM is a good candidate for beyond-3G high-speed wireless communication application because of the robustness to the intersymbol interference and multipath fading. However. an OFDM signal has a serious problem of the high PAPR, which results in the significant nonlinear distortion when it passes through a nonlinear high power amplifier. We propose a new PAPR reduction method using pre-emphasis and clipping. Via the proposed method, the OFDM output signal can have a low PAPR and BER improvement. Then. de-emphasis process is requisite in OFDM receiver. PAPR is reduced to about 5.7 ㏈ at the CCDF= 10$\^$-3/ when the subcarrier number is 16, QPSK modulation is used. pre-emphasis change point Is 3/9 of the peak amplitude of the IFFT output and clipping level is 11 in the IFFT output amplitude. The required SNR at BER=10$\^$-3/ the proposed system is improved by 2 dB than that of the original OFDM system.