• ETRI Journal
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• v.36 no.1
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• pp.42-50
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• 2014

Orthogonal frequency division multiplexing (OFDM) is a modulation technique that allows the transmission of high data rates over wideband radio channels subject to frequency selective fading by dividing the data into several narrowband and flat fading channels. OFDM has high spectral efficiency and channel robustness. However, a major drawback of OFDM is that the peak-to-average power ratio (PAPR) of the transmitted signals is high, which causes nonlinear distortion in the received data and reduces the efficiency of the high power amplifier in the transmitter. The most straightforward method to solve this problem is to use a nonlinear mapping algorithm to transform the signal into a new signal that has a smaller PAPR. One of the latest nonlinear methods proposed to reduce the PAPR is the $L_2$-by-3 algorithm, which is based on the discrete sliding norm transform. In this paper, a new algorithm based on the $L_2$-by-3 method is proposed. The proposed method is very simple and has a low complexity analysis. Simulation results show that the proposed method performs better, has better power spectral density, and is less sensitive to the modulation type and number of subcarriers than $L_2$-by-3.

• ETRI Journal
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• v.34 no.1
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• pp.9-16
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• 2012

Precoding in the multiple-input multiple-output orthogonal frequency division multiplexing system is investigated. In conventional wideband precoding (WBP), only one precoder, obtained from the decomposition of the subcarrier independent channel matrix, is used for all subcarriers. With an investigation of the relationship between the subcarrier independent channel matrix and the temporal/frequency channels, an improved WBP scheme is proposed for practical scenarios in which a part of subcarriers are allocated to a user. The improved WBP scheme is a generalized scheme of which narrow-band precoding and conventional WBP schemes are special modes. Simulation results demonstrate that the improved WBP scheme almost achieves the optimum performance of a single precoder and outperforms the conventional WBP scheme in terms of the bit error ratio and ergodic capacity with slight complexity increase. The largest advantage of the improved WBP scheme on signal-to-noise ratio in simulation results is over 2.1 dB.

• Proceedings of the IEEK Conference
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• 2002.07b
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• pp.1066-1069
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• 2002

Orthogonal frequency division multiplexing (OFDM) is a special technique for communication systems which can support the high data rate transmission with sufficient robustness to fading channels. Tansmitter diversity with space-time block coding (STBC) is an attractive transmission scheme to improve the performance of systems. In this paper, we compare the performance of space-time block coded OFDM systems with that of conventional OFDM systems over fast fading channels. The block-interleaved (BI) STBC and frequency hopping (FH) OFDM are proposed in the study to provide the maximum achievable diversity gains. As the simulation results, the STBC OFDM, Bl-STBC OFDM and Bl-STBC FH-OFDM provide the much improved performance over the conventional OFDM. And the Bl-STBC FH-OFDM also provide the better performance than the STBC OFDM and Bl-STBC OFDM, especially, in the case of the two transmit antennas are employed while BI-STBC FH-OFDM can maintain the same data rate of 12 Mbps.

• Proceedings of the Korea Electromagnetic Engineering Society Conference
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• 2003.11a
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• pp.403-406
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• 2003

In this paper, we have proposed a carrier recovery algorithm of OFDM/QPSK-DMR(Orthogonal Frequency Division Multiplexing/Quadrature Phase Shift Keying Modulation-Digital Microwave Radio)system using BL-PSF(Band Limited-Pulse Shaping Filter) and have analyzed the carrier phase MSE(Mean Square Error) performance of OFDM/QPSK and single carrier DMR systems. The existing OFDM/QPSK-DMR system using windowing requires training sequence or CP(Cyclic prefix) to synchronize a receive. carrier frequency. Because in the OFDM/QPSK-DMR system using BL-PSF there is no training sequence or CP(Cyclic Prefix), we also propose a carrier recovery useful to the system. The simulation results confirm that the proposed carrier recovery algorithm has the same carrier phase MSE(Mean Square Error) performance for the single carrier DMR system under AWGN(Additive White Gaussian Noise) environment.

• Proceedings of the Korea Electromagnetic Engineering Society Conference
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• 2003.11a
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• pp.394-397
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• 2003

In this paper, we have proposed a clock recovery algorithm of OFDM/QPSK-DMR(Orthogonal Frequency Division Multiplexing/Quadrature Phase Shift Keying Modulation-Digital Microwave Radio)system using BL-PSF(Band Limited-Pulse Shaping Filter) and have analyzed the clock phase error variance performance of OFDM/QPSK and single carrier DMR systems. The existing OFDM/QPSK-DMR system using the windowing requires training sequence or CP(Cyclic Prefix) to synchronize a receiver clock frequency Because there is no training sequence or CP(Cyclic prefix) in our proposed DMR system, the proposed clock recovery algorithm is useful to the OFDM/QPSK-DMR system using BL-PSF, The simulation results confirm that the proposed clock recovery algorithm has the same clock phase error variance performance in a single carrier DMR system under AWGN(Additive White Gaussian Noise) environment.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.10 no.4
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• pp.752-757
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• 2006

In the case of the requirement of high speed transmission, OFDM is a powerful technique employed in communications systems suffering from frequency selective fading. In this paper, we apply an optimal adaptive bitloading algorithm technique. The BER performance of the fixed-rate SISO and adaptive SISO is simulated. Specially, we can decompose the MIMO channel into the SISO channel by making use of the singular value decomposition(SVD) assuming channel knowledge in a multipath environment. As a results of simulation, we confirmed that the BER enhancement of MIMO-OFDM system with the bitloadins algorithm was superior to the SISO-OFDM system.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.12 no.8
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• pp.3704-3724
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• 2018

This paper proposes an advanced turbo receiver with joint inter-carrier interference (ICI) self cancellation and channel equalization for multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems over rapidly time-varying channel environment. The ICI caused by impairment of local oscillators and carrier frequency offset (CFO) is the major problem for MIMO-OFDM communication systems. The existing schemes (conjugate cancellation (CC) and phase rotated conjugate cancellation (PRCC)) that deal with the ICI cancellation and channel equalization can't provide satisfactory performance over time-varying channels. In term of error rate performance and low computational complexity, ICI self cancellation is the best choice. So, this paper proposes a turbo receiver to deal with the problem of joint ICI self cancellation and channel equalization. We employ the adaptive phase rotations in the receiver to effectively track the CFO variations without feeding back the CFO estimate to the transmitter as required in traditional existing scheme. We also give some simulations to verify the proposed scheme. The proposed schene outperforms the existing schemes.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.35 no.12B
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• pp.1251-1258
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• 2010

Orthogonal Frequency Division Multiplexing (OFDM) has been investigated as promising technology for future broadband communication. In this paper, we propose a power allocation algorithm for a frequency selective fading channel. However, many investigated algorithms that perform high computation overhead are not appropriate for sensor network systems due to hardware limitations. The proposed algorithm significantly reduces computational overhead and satisfies the power budget. Throughput of the algorithm is comparable to the optimum solution. Simulation results support the claim stated.

• ETRI Journal
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• v.33 no.3
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• pp.326-334
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• 2011

This paper outlines the time jitter effect of a sampling clock on a software-defined radio technology-based digital intermediate frequency (IF) transceiver for a mobile communication base station. The implemented digital IF transceiver is reconfigurable to high-speed data packet access (HSDPA) and three bandwidth profiles: 1.75 MHz, 3.5 MHz, and 7 MHz, each incorporating the IEEE 802.16d worldwide interoperability for microwave access (WiMAX) standard. This paper examines the relationship between the signal-to-noise ratio (SNR) characteristics of a digital IF transceiver with an under-sampling scheme and the sampling jitter effect on a multichannel orthogonal frequency-division multiplexing (OFDM) signal. The simulation and experimental results show that the SNR of the OFDM system with narrower band profiles is more susceptible to sampling clock jitter than systems with relatively wider band profiles. Further, for systems with a comparable bandwidth, HSDPA outperforms WiMAX, for example, a 5 dB error vector magnitude improvement at 15 picoseconds time jitter for a bandwidth of WiMAX 3.5 MHz profile.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.15 no.5
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• pp.1031-1037
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• 2011

Pilot symbol assisted modulation (PSAM) can be used for the channel estimation in orthogonal frequency division multiplexing (OFDM) transmission systems. However, imperfect channel estimates in PSAM systems degrade the bit error rate (BER) performance. I derive the BER of PSAM OFDM systems over time dispersive fading channels. The BER performance of OFDM systems with SFBC antenna diversity are analyzed, where data is transmitted over time and frequency selective Rayleigh fading channel. The performance of this PSAM SFBC OFDM systems, gauged by the average bit error rate, is analyzed considering the channel estimation error.