• Proceedings of the IEEK Conference
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• 2007.07a
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• pp.125-126
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• 2007

In this paper, we analyze the channel estimation performances associated with the interpolation order for OFDM systems. We first analyze the time varying channel and frequency selective channel, and then we derive the channel Index which indicates the ratio of time axis variation and frequency axis variation. The analyzed results show that time interpolation followed by a frequency interpolation is adequate for the channel with a channel index larger than a certain threshold value and vice versa. Computer simulation explains that the method which decides Interpolation order outperforms fixed order estimation.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.18 no.12
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• pp.1408-1418
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• 2007

In this paper, we analyze the performance of pilot symbol assisted channel estimation and equalization schemes for OFDM systems over frequency-selective time-varying channels and propose methods to improve the system performance. In the least square(LS) and linear minimum mean square error(MMSE) channel estimation, time domain windowing is introduced for banding the frequency domain channel matrix. The linear MMSE and decision feedback equalization schemes are employed with the pilot symbols for channel estimation taken into account in the equalization process. To reduce computational complexity, the band LU matrix factorization algorithm is introduced in solving the linear systems involved in the equalization, and the performances are compared with the known previous results by computer simulations. When time domain windowing is employed in the decision feedback equalization, the matrix related with the decision feedback process is shown to be unhanded and the resultant performance degradation is analyzed.

• Journal of Electrical Engineering and Technology
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• v.12 no.6
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• pp.2342-2347
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• 2017

In this study, we design a reconfigurable frequency selective surface (FSS) using fluidic channels to use the FSS in multi-frequencies. Effective permittivity can be changed using water as the fluid of fluidic channels in a dielectric slab, and the frequency characteristics of FSS can be controlled. We optimize the dimensions of the fluidic channel to design a reconfigurable FSS and measure its transmission characteristics.

• Proceedings of the IEEK Conference
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• 2002.07b
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• pp.1086-1089
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• 2002

In this paper, an adaptive packet acquisition performance of a CDMA/TDD packet radio system is analyzed and simulated in a frequency-selective Rayleigh fading channel. The frequency-selective fading process is assumed to be WSSUS model which is typical for a satellite and a mobile radio communications. The performance is evaluated in terms of packet error probability. In this adaptive packet acquisition scheme, an auxiliary training sequence is placed within data packet to estimate time-varying channel state. From the simulation results, it is confirmed that the proposed acquisition scheme significantly improves packet error performance compared to the conventional fixed acquisition scheme, especially, for the case of low Doppler frequency. The analysis in this paper can be applied to the design of a CDMA/TDD packet radio system.

• Proceedings of the KIEE Conference
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• 2003.07e
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• pp.62-69
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• 2003

In recent years, data transmission using power lines has been much highlighted. Power line is known to be cost-effective communication medium because the entire cable infrastructure has been already established and it is entirely connected with any home. Recently, the target of researches is to communicate data reliably over power lines at the speed of at least 1Mbits/s over the frequency range from 1MHz to 10MHz. OFDM communication system has been used for the high speed data transmission. Next, the conventional and adaptive OFDM systems for high speed data transmission over power line channel are investigated. The performance of AOFDM(adaptive OFDM) over the frequency selective channel with impulsive and narrow-band noise are studied to be a nice solution for high speed data transmission over power lines. The simulation results show that data the rates of the AOFDM are improved about 47% more than the ones of the conventional OFDM over the frequency response of case 4. In the results, the data rate has been much improved by the proposed adaptive algorithm in the frequency selective channel.

• Journal of Communications and Networks
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• v.10 no.1
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• pp.71-78
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• 2008

We propose a sub-band spreading technique for adaptive modulation (AM) in orthogonal frequency division multiplexing (OFDM) systems in order to reduce signaling overheads and to average frequency selective fading channels causing different signal-to-noise ratio (SNR) values for subcarriers in each subband. The conventional sub-band based AM schemes can also reduce signaling overheads and complexity for allocating a resource per sub-band at a time. However, they may suffer from the channel variation in a sub-band when the sub-band size is larger than the channel coherence bandwidth (BW). The sub-band spreading at the transmitter enables the received symbols in each sub-band to have an identical reliability even in a frequency selective fading channel. We rigorously analyze the averaged SNR value at the receiver of the sub-band spreading system and the analyzed average SNR in a sub-band is used for an adaptation criterion. The proposed AM scheme outperforms the conventional sub-band based OFDM scheme without spreading, and it can yield better throughput performance than the conventional subcarrier based AM schemes when we consider the signaling overheads.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.35 no.1C
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• pp.55-64
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• 2010

In this paper, we propose a new compensation scheme for combined channel distortions and RF impairments based on the analysis of the impacts of IQ(In-phase/Quadrature) imbalance and phase noise on the OFDM(Orthogonal Frequency Division Multiplexing) system in the direct conversion transceiver and frequency selective fading channel distortion. The proposed scheme estimates the combined distortion by the use of training symbols and the residual distortion by pilot symbols and compensates the combined distortion, including IQ imbalance, phase noise and multipath fading at the same time. The simulation results show that the proposed scheme compensates the combined distortion of IQ imbalance, phase noise and multipath fading simultaneously.

• Journal of Korea Society of Industrial Information Systems
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• v.14 no.5
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• pp.1-9
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• 2009

Relay-assisted multiple input technique has become a promising candidate for next generation broadband wireless communications. In this paper, we propose channel selective relay-based multiple input transmission system. In the proposed system, single carrier frequency division multiple access (SC-FDMA) and orthogonal frequency division multiple access (OFDMA) are adopted for uplink and downlink transmissions, respectively. The performance of relay-based system can be improved by using the subcarriers selectively based on the channel condition between relay station (RS) and mobile station, or between RS and base station. Simulation results show that the proposed relay-based system considerably outperforms the conventional relay-based system.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.15 no.10 s.89
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• pp.931-937
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• 2004

This study examines the channel capacity of a multiple antenna system based on OFDM(Othogonal Frequency Division Multiplexing) when there are correlations among received signals in the frequency selective Rayleigh fading. As for a case that two transmitting and two receiving antennas are used, the channel capacity is derived as a function of correlation in a closed-form and the effect on channel capacity is analyzed when the mean value of the of received signal power is identical/ non-identical between the two receiving antennas. Analytical results show that the channel capacity decreases with the correlation coefficient of the received signals, and the decreasing rate is accelerated when the correlation coefficient of the received signals is greater than 0.7. In addition, the channel capacity reaches its peak when the received mean signal power of each branch is identical.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.16 no.3
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• pp.248-258
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• 1991

In this paper, ISI cancellation capabilites in the frequency selective fading channels of the equalizer emplouing individual tap LMS(ITLMS) algorithm and of th equalizer using the lattice structure have been investigated through the computer simulations in terms of bit error rate and convergence speed.