• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.14 no.10
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• pp.1030-1035
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• 2003

In this paper, we compare the performance of IS-OFDM(interference suppressing orthogonal frequency division multiplexing) system with that of the conventional OFDM system. Each parallel branch symbol of IS-OFDM is multiplied by the orthogonal sequence and distributed into all sub-carriers. Then, each sub-carrier transmits information composed of the symbol components of all parallel branches in the given frame. The structure of IS-OFDM shows the frequency diversity characteristic. Therefore, IS-OFDM can be robust to the narrowband interference. BER performances are found in the narrow-band interference and a harmonic noise channels. When JSR(Jamming to signal ratio) increases from -10 dB to +10 dB in the constant interference bandwidth, IS-OFDM outperforms the conventional OFDM and the BER difference between IS-OFDM and OFDM is reduced as JSR increases. Also, the similar properly can be shown when interference bandwidth increases at the constant JSR. In the harmonic noise channel, we can find the good performance of IS-OFDM when the h-rate(harmonic noise bandwidth to signal bandwidth ratio) increases from 0.01 to 0.8. So, simulation results show that the IS-OFDM is more robust to the interference than the conventional OFDM.

• Journal of the Institute of Electronics and Information Engineers
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• v.49 no.9
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• pp.138-144
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• 2012

In this paper, we introduce the time-domain equalizer in orthogonal frequency division multiplexing (OFDM) systems using orthogonal matching pursuit (OMP) algorithm. Since OFDM system inserts guard intervals, it shows robust performance against multi-path fading. However, in Doppler channel, inter-carrier interference (ICI) occurs because an orthogonality of sub-carriers does not maintain. A least squares (LS) algorithm is common method of time-domain equalizer, but if a channel length is longer, the performance deteriorates by noise. The multi-path fading is a summation of the different delay signal. And that has sparse properties in time-domain. Because the OMP algorithm of the compressive sensing (CS) algorithm restores the channel by choosing the important elements of sparse channel, it can reduce the influence of noise. We simulate the performance of time-domain equalizer in OFDM system with various channel environments using OMP algorithm compared with other equalization method.

• Proceedings of the IEEK Conference
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• 2002.07a
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• pp.381-384
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• 2002

Orthogonal Frequency Division Multiplexing (OFDM), which employs orthogonal overlapping subcarriers to modulate the signals, has been attracted much attention in recent time as a favorable option for future generation wireless networks due to its ability to overcome many impairments in wireless channels. Since OFDM is a multi-carrier system adaptive modulation techniques can be effectively and efficiently used to enhance the system performance in terms of both BER and overall system capacity. This paper discusses a COFDM-CDMA system with adaptive modulation schemes for future generation wireless networks.

• Proceedings of the Korean Society of Broadcast Engineers Conference
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• 2011.07a
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• pp.470-471
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• 2011

본 논문에서는 MIMO(multi-input multi-output) 시스템에서의 시간영역 채널 추정방법을 제안하였다. 제안된 알고리듬은 각 안테나에서 전송되는 시간영역의 OFDM(orthogonal frequency division multiplexing) 심볼 구간에 직교코드를 곱하여 기존의 채널 추정보다 긴 채널 추정이 가능하다. 제안된 알고리듬을 바탕으로 다양한 길이의 직교코드를 이용하여 채널추정이 가능함을 컴퓨터 시뮬레이션을 통해 검증하였다.

• Proceedings of the IEEK Conference
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• 2008.06a
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• pp.291-292
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• 2008

In orthogonal frequency division multiplexing based wireless system, a pilot signal is often employed to estimate channel state information (CSI). However, the received pilot signal is interfered by other cell interference in multi-cell environments. We consider the estimation of CSI by utilizing orthogonal preambles and channel correlation.

• ETRI Journal
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• v.28 no.1
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• pp.87-90
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• 2006

In this letter, we propose a quasi-orthogonal space-time-frequency (QOSTF) block coded orthogonal frequency division multiplexing (OFDM) that can achieve full symbol rate with four transmit antennas. Since the proposed QOSTF-OFDM cannot achieve full diversity, we use a diversity advantage collection with zero forcing (DAC-ZF) decoder to compensate the diversity loss at the receiving side. Due to modulation advantage and collected diversity advantage, the proposed scheme exhibits a better bit-error rate performance than other orthogonal schemes.

• IEIE Transactions on Smart Processing and Computing
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• v.6 no.2
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• pp.117-123
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• 2017

N-continuous orthogonal frequency division multiplexing (OFDM) is a precoding method for sidelobe suppression of OFDM signals and seamlessly connects OFDM symbols up to the high-order derivative for sidelobe suppression, which is suitable for suppressing out-of-band radiation. However, it severely degrades the error rate as it increases the continuous derivative order. Two schemes for orthogonal precoding of N-continuous OFDM have been proposed to achieve an ideal error rate while maintaining sidelobe suppression performance; however, the large size of the precoder matrices in both schemes causes very high computational complexity for precoding and decoding. This paper proposes matrix decomposition of precoder matrices with a large size in the orthogonal precoding schemes in order to reduce computational complexity. Numerical experiments show that the proposed method can drastically reduce computational complexity without any performance degradation.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.15 no.10
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• pp.6296-6302
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• 2014

A new technique, which can increase the channel bandwidth in an optical wireless orthogonal frequency division multiplexing (OFDM) link based on a light emitting diode (LED), is proposed. The technique uses adaptive sampling to convert an OFDM signal to a sparse waveform. In compressive sensing (CS), a sparse signal that is sampled below the Nyquist/Shannon limit can be reconstructed successively with sufficient measurements. The data rate of the proposed CS-based visible light communication (VLC)-OFDM link increases from 30.72 Mb/s to 51.2 Mb/s showing an error vector magnitude (EVM) of 31 % at the quadrature phase shift keying (QPSK) symbol.

• Proceedings of the KIEE Conference
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• 2004.11c
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• pp.277-279
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• 2004

In this paper, we propose an channel estimation method for Multi-Input Multi-Output-Orthogonal frequency Division Multiplexing (MIMO-OFDM). The proposed method estimates uniquely all channel frequency responses needed in space-frequency block coded OFDM systems using "comb-type" pilot symbols. To reduce the computational complexity of the proposed method, least square(LS) and linear minimum mean square error(LMMSE) are used in the frequency-domain. The performance of the proposed approach is evaluated by computer simulation for rayleigh fading channel.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.8C
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• pp.613-616
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• 2008

In this paper, a fine residual frequency offset estimation scheme is proposed for multiband orthogonal frequency division multiplexing ultra-wideband (MB-OFDM UWB) systems. The basic idea of our approach is based on the fact that two adjacent OFDM symbols carry the identical information in the MB-OFDM UWB system, thus removing the need of pilot symbols. The mean square error of the synchronization scheme is evaluated and simulation results are used to verify the effectiveness of the proposed estimator. When compared to the pilot-aided conventional estimator, the proposed estimator has a lower estimation error.