• KSII Transactions on Internet and Information Systems (TIIS)
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• v.6 no.4
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• pp.1063-1075
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• 2012

To locate an object accurately in the wireless sensor networks, the distance measure based on time-delay plays an important role. In this paper, we propose a maximum likelihood (ML) time-delay estimation algorithm in multi-path wireless propagation channel. We get the joint probability density function after sampling the frequency domain response of the multi-path channel, which could be obtained by the vector network analyzer. Based on the ML criterion, the time-delay values of different paths are estimated. Considering the ML function is non-linear with respect to the multi-path time-delays, we first obtain the coarse values of different paths using the subspace fitting algorithm, then take them as an initial point, and finally get the ML time-delay estimation values with the pattern searching optimization method. The simulation results show that although the ML estimation variance could not reach the Cramer-Rao lower bounds (CRLB), its performance is superior to that of subspace fitting algorithm, and could be seen as a fine algorithm.

• Proceedings of the IEEK Conference
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• summer
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• pp.23-26
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• 2004

Five-port reflectometer which consists of a ring with 5 arms (two inputs, three outputs) and three RF power detectors has been used as a vector network analyser, a demodulator in the homodyne receiver as well as in Phase Looked Loop (PLL) and so on. Calibration of five-port reflectometer is an important task. In this paper, we present a calibration method of five-port for a propagation channel sounder. The method is based on measurement of the phase differences between the three voltages at the five-port's outputs in order to determine the ratio of two input incident waves. The frequency channel sounder based on five-port is calibrated for each frequency from 2.2 GHz to 2.6 GHz with 1 MHz step. This method can also determine the absolute delays of each propagation path in the propagation channel. The calibration method is validated using measurement data.

• International Journal of Computer Science & Network Security
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• v.23 no.7
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• pp.1-8
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• 2023

An Orthogonal Frequency Division Multiplexing (OFDM) based wireless communication system has drawn wide attention for its high transmission rate and high spectrum efficiency in not only radio but also Underwater Acoustic (UWA) applications. Because of the narrow sub-carrier spacing of OFDM, orthogonality between sub-carriers is easily affected by Doppler effect caused by the movement of transmitter or receiver. Previously, Doppler compensation signal processing algorithm for Desired propagation path was proposed. However, other Doppler shifts caused by delayed Undesired signal arriving from different directions cannot be perfectly compensated. Then Receiver Bit Error Rate (BER) is degraded by Inter-Carrier-Interference (ICI) caused in the case of Multi-path Doppler channel. To mitigate the ICI effect, a modified Delay and Doppler Profiler (mDDP), which estimates not only attenuation, relative delay and Doppler shift but also sampling clock shift of each multi-path component, is proposed. Based on the outputs of mDDP, an ICI canceling multi-tap equalizer is also proposed. Computer simulated performances of one-tap equalizer with the conventional Time domain linear interpolated Channel Transfer Function (CTF) estimator, multi-tap equalizer based on mDDP are compared. According to the simulation results, BER improvement has been observed. Especially, in the condition of 16QAM modulation, transmitting vessel speed of 6m/s, two-path multipath channel with direct path and ocean surface reflection path; more than one order of magnitude BER reduction has been observed at CNR=30dB.

• Journal of the Korean Institute of Telematics and Electronics A
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• v.31A no.4
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• pp.47-55
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• 1994

This paper presents the CDMA system using the RAKE receiver over a multipath fading channel and in multi-user environments. The interference between the propagation paths in a multipath fading causes a serious degradation in the mobile communication environment. In order to derive the path inteference and reduce the fading effect, the RAKE receiver is used. In a computer simulation, the path number of a fading channel and the branch number of the RAKE receiver are modified. As a result, the CDMA system using the RAKE receiver gives a better performance than the conventional CDMA system over a multipath fading channel and in multi-user environments.

• Journal of Advanced Navigation Technology
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• v.27 no.4
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• pp.410-416
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• 2023

Unlike geostationary satellite communication systems, low-earth orbit(LEO) satellite communication systems move at relatively high speeds, and the angle with the ground device is not fixed and varies over a wide range. The propagation channel condition between satellites and ground nodes cannot be assumed line of sight(LOS) anymore. This paper analyzes the low-orbit multi-path fading satellite channel model that can occur in LEO satellite communication systems and Doppler frequency transition caused by high-speed maneuvering of LEO satellites and presents effective equalization techniques for OFDM and SC-FDE transmission methods suitable for multi-path frequency selective fading satellite channel models. In addition, this paper compares and analyzes the performance of OFDM and SC-FDE transmission methods in multipath fading LEO satellite channel environment using the proposed equalization techniques through simulations. Simulation results showed that SC-FDE outpeformed OFDM.

• International Journal of Computer Science & Network Security
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• v.23 no.6
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• pp.27-34
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• 2023

This paper proposed an Inter-Carrier-Interference (ICI) Canceling Orthogonal Frequency Division Multiplexing (OFDM) receiver for 5G mobile system to support 500 km/h linear motor high speed terrestrial transportation service. A receiver in such high-speed train sees the transmission channel which is composed of multiple Doppler-shifted propagation paths. Then, a loss of sub-carrier orthogonality due to Doppler-spread channels causes ICI. The ICI Canceler is realized by the following three steps. First, using the Demodulation Reference Symbol (DMRS) pilot signals, it analyzes three parameters such as attenuation, relative delay, and Doppler-shift of each multi-path component. Secondly, based on the sets of three parameters, Channel Transfer Function (CTF) of sender sub-carrier number 𝒏 to receiver sub-carrier number 𝒍 is generated. In case of 𝒏≠𝒍, the CTF corresponds to ICI factor. Thirdly, since ICI factor is obtained, by applying ICI reverse operation by Multi-Tap Equalizer, ICI canceling can be realized. ICI canceling performance has been simulated assuming severe channel condition such as 500 km/h, 2 path reverse Doppler Shift for QPSK, 16QAM, 64QAM and 256QAM modulations. In particular, for modulation schemes below 16QAM, we confirmed that the difference between BER in a 2 path reverse Doppler shift environment and stationary environment at a moving speed of 500 km/h was very small when the number of taps in the multi-tap equalizer was set to 31 taps or more. We also confirmed that the BER performance in high-speed mobile communications for multi-level modulation schemes above 64QAM is dramatically improved by the use of a multi-tap equalizer.

• The Journal of Information Technology
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• v.6 no.4
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• pp.1-9
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• 2003

This paper is about power line communication(PLC) over the low power voltage grid. The main advantage with power line communication is the use of an existing infrastructure. The PLC channel can be modeled as having multi-path propagation with frequency-selective fading, typical power lines exhibit signal attenuation increasing with length and frequency. OFDM(Orthogonal Frequency Division Multiplexing) is a modulation technique where multiple low data rate carriers are combined by a transmitter to form a composite high data rate transmission. The performance in consideration of the multi-path(echoes) powerline scheme is analyzed and verified by computer simulation.

• Journal of Institute of Control, Robotics and Systems
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• v.15 no.3
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• pp.346-352
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• 2009

UTC (Coordinated Universal Time) has been made by the comparison results and the statistical analysis of primary clocks maintained by national standard institutes. Some kinds of technique have been used for international time transfer; since 1980s the study on methods and development of time transfer has conducted with activation of GPS application. And the more accurate and easier method made it use the official time transfer method for the generation of UTC. But recently TWSTFT (Two-Way Satellite Time and Frequency Transfers) as well as GPS time transfer are increasing in number because the TWSTFT is able to improve the accuracy and precision of time comparison owing to the elimination of the ionospheric and tropospheric delay errors thanks to the reciprocal propagation path. In this paper, we introduce the TWSTFT results by a multi-channel modem comparing with GPS P3-code.

• The Journal of the Institute of Internet, Broadcasting and Communication
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• v.8 no.5
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• pp.29-34
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• 2008

In this paper, we propose the channel sounding scheme which improves the reliability and performance of communication link and mitigate the fading effect of channel in MTMO-OFDM system having correlation between Tx and Rx antennas. And we estimate the channel propagation characteristics of channels between the transmitter and receiver with CS scheme by measuring the channel propagation characteristics using sounding signal. Multi agent system models can be used to analyze the path of the system within any time frame. Further, using this proposed method, we can increase the channel capacity by simple calculation in receiver and transmitter and allocate the optimal power by channel information in transmitter. Also, the increment of channel capacity using the prorposed method is induced as the number of antenna is increased. So, we can anticipate the performance improvement with the presented scheme in more complicated Tx and Rx antenna system.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.29 no.5A
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• pp.492-501
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• 2004

The performance of mobile communication systems depends on the state of the time-varying multi-path fading channel. To effectively prevent the corruption of video stream and its propagation in spatial and temporal domain, proactive error controls are widely being deployed. Among possible candidates, the rate compatible punctured convolutional (RCPC) code has been widely used for multimedia data, since its rate can be determined flexibly. In this paper, the adaptive channel estimation and the adaptive error correction techniques over the time-varying mobile channel have been proposed. Extensive computer simulations show that the proposed techniques yield the superior performance than the fixed rate system.