• The Journal of Korean Institute of Communications and Information Sciences
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• v.31 no.1A
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• pp.38-47
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• 2006

In this paper, we propose a velocity-adaptive channel estimation scheme using the simple zero-forcing technique in the frequency domain. Channel estimation is performed by removing frequency components that are higher than the maximum Doppler frequency in the received signal. The proposed scheme can be extended to the combined estimation scheme for channel coefficients and mobile velocity using one FFT/IFFT module. Simulation results show that the proposed scheme outperforms conventional schemes for a wide range of mobile velocities($3{\sim}300\;Km/h$). Finally, the MSE for the proposed channel estimation scheme is analyzed.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.19 no.9
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• pp.1028-1036
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• 2008

In the OFDM system, IQ imbalance problem happens at the RF front-end of transceiver, which degrades the BER(bit error rate) performance because it affects the constellation in the received signal. Also, phase noise is generated in the local oscillator of transceivers and it destroys the orthogonality between the subcarriers. Conventional PNS algorithm is effective for phase noise suppression, but it is not useful anymore when there are jointly IQ(In-phase and Quadrature) imbalance and phase noise. Therefore, in this paper, we analyze the effect of IQ imbalance and phase noise generated in the down-conversion of the receiver. Then, we estimate and compensate the IQ imbalance and phase noise at the same time. Compared with the conventional method that IQ imbalance after IFFT is estimated and compensated in front of FFT via the feedback, this proposed method extracts and compensates effect of IQ imbalance after FFT stage. In case IQ imbalance and phase noise exist at the same time, we can decrease complexity because it is needless to use elimination of IQ imbalance in time domain and training sequences and preambles. Also, this method shows that it reduces the ICI and CPE component using adaptive forgetting factor of MMSE after FFT.

• Proceedings of the IEEK Conference
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• 2000.06a
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• pp.236-239
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• 2000

OFDM Transmission system can effectively reduce ISI(Inter Symbol Interference) caused by multipath fading, especially in the case of broad band data transmission. Nevertheless, the delayed waves existing in a symbol degrade the transmission Performance at the receiver. Therefore when OFDM transmission system applies to mobile radio communication system, we have to remove the influence without intricacy. This paper proposes and design a new scheme in order to estimate propagation characteristics using the characteristics of distinctive OFDM signal which is inserted pilot signal periodically in frequency axis before IFFT at the transmitter Moreover, we introduce a cancellation scheme of delayed signal on the basis of the proposed estimation technique. Using these schemes, it is possible to estimate and cancel the Interference of the delayed signal before FFT at the receiver.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.59 no.12
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• pp.2324-2328
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• 2010

In this paper we proposed a new algorithm that can reduce the open-loop pitch estimation time of G.723.1. The time domain cross-correlation method is simple but has long processing time by recursive multiplication. For reduction of processing time, we use the method that compute the cross-correlation by multiplying the Fourier value of speech by it's complex conjugate. Also, we can reduce the processing time by omitting the bit-reversing of FFT and IFFT for time-frequency domain transform. As a result, the processing time of improved hybrid domain cross-correlation algorithm is reduced by 67.37% of conventional time domain cross-correlation.

• Proceedings of the Acoustical Society of Korea Conference
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• autumn
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• pp.107-110
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• 2004

본 논문에서는 시간-주파수 혼성 영역 피치 검출법을 제안하였다. 음성신호를 주파수 영역으로 변환하고 주파수 영역에서 위상 성분을 조절하여 시간영역으로 역변환 함으로써 피치 피크가 최대가 되도록 하여 용이한 피치검출이 가능하였다. 또한 처리시간을 단축하기 위하여 FFT와 IFFT의 비트 재정렬을 생략하여 처리할 수 있는 방법을 제안하였다. 성능 측정 결과 기존의 켑스트럼 검출법에 비하여 검출성능은 우수하면서도 처리시간은 $84.8\%$로 단축됨을 알 수 있었다.

• JSTS:Journal of Semiconductor Technology and Science
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• v.7 no.4
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• pp.229-234
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• 2007

In this paper, we present an efficient instruction set architecture using vector register file hardware to accelerate operation of general matrix-vector operations in DSP microprocessor. The technique enables in-situ row-access as well as column access to the register files. It can reduce the number of memory access significantly. The technique is especially useful for block-based video signal processing kernels such as FFT/IFFT, DCT/IDCT, and two-dimensional filtering. We have applied the new instruction set architecture to in-loop deblocking filter processing in H.264 decoder. Performance comparisons show that the required load/store operations for the in-loop deblocking filter can be reduced about 42%. The architecture would improve the processing speed, and code density in DSP microprocessor especially for video signal processing substantially.

• Proceedings of the Korea Information Processing Society Conference
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• 2023.05a
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• pp.567-569
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• 2023

GAN은 이미지 생성모델로서 이미지 공간에서 좋은 결과를 보여왔다. 우리는 이러한 GAN의 능력을 더욱 향상하기 위하여 본 연구에서 주파수 영역에서 이미지를 학습하고 생성하는 새로운 방법을 제안한다. 이를 위하여 먼저 학습데이터를 2D FFT로 주파수 영역으로 변환한 후 변환된 학습데이터를 GAN이 학습하게 한다. 학습 후에 GAN은 새로운 이미지를 생성하며 생성된 이미지를 2D IFFT하여 이미지 공간으로 변환한다. 이렇게 주파수 영역에서 이미지를 생성하는 방법은 이미지 공간에서 생성하는 방법보다 다양한 장점이 있다. 생성된 이미지의 품질을 평가하기 위하여 4개 데이터 셋에 4개의 평가지표를 사용하여 평가한 결과 주파수 영역에서 생성한 이미지가 IS, P&R, D&C 측면에서 더 좋은 것으로 평가되었다.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.31 no.5A
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• pp.493-500
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• 2006

This paper presents the efficient structure and parameter optimization of downlink beamforming transmitter in OFDMA/TDD system. To design downlink beamforming transmitter for multiple transmit antennas, an efficient beamforming structure for multiple users and the choice of word-length of each block are critical in the aspect of its performance and hardware complexity. We propose an efficient beamforming scheme, which stores the weights of subcarriers into memory without user identification at the receiver of base station and calculates the weights for corresponding user in a subcarrier unit of IFFT input at high speed. Also, we obtain the word-length of main data path and other design parameters by fixed-point simulation analysis. The proposed architecture could reduce the memory size proportional to the maximum number of users per frame, and the processing time of an OFDM symbol at the receiver of base station without the need of additional processing time for calculating the weights at the transmitter.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.47 no.2
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• pp.9-17
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• 2010

Recently, there has been much interest in orthogonal frequency division multiplexing (OFDM) for next generation wireless wideband communication systems. OFDM is a special case of multicarrier transmission, where a single data stream is transmitted over a number of lower-rate subcarriers. One of the main reasons to use OFDM is to increase robustness against frequency-selective fading or narrowband interference. However, in the radio systems it is also important to distortion introduced by high power amplifiers (HPA's) such as solid state power amplifier (SSPA) considered in this paper. Since the signal amplitude of the OFDM system is Rayleigh-distributed, the performance of the OFDM system is significantly degraded by the nonlinearity of the HPA in the OFDM transmitter. In this paper, we propose a canonical piecewise-linear (PWL) model based digital predistorter to prevent signal distortion and spectral re-growth due to the high peak-to-average power ratio (PAPR) of OFDM signal and the nonlinearity of HPA's. Computer simulation on an OFDM system under additive white Gaussian noise (AWGN) channels with QPSK, 16-QAM and 64-QAM modulation schemes and modulator/demodulator implemented with 1024-point FFT/IFFT, demonstrate that the proposed predistorter achieves significant performance improvement by effectively compensating for the nonlinearity introduced by the SSPA.

• Proceedings of the Korea Electromagnetic Engineering Society Conference
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• 2003.11a
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• pp.384-388
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• 2003

In this paper, we have proposed a OFDM/QPSK-DMR(Orthogonal frequency Division Multiplexing/Quadrature Phase Shift Keying Modulation-Digital Microwave Radio)system using BL-PSF(Band-limited pulse shaping filter) over the Microwave channel. In the proposed DMR system, STS-1(51.84 Mbps) of SONET(Synchronous Optical NETwork) is first modulated by OFDM/QPSK symbol and used Band-limited pulse shaping filter. The advantage of the proposed DMR system is to simplify system complexity and increase IFFT/FFT block use-efficiency. The system performance of single carrier and OFDM systems is already proved that those of DMR systems have the same performance over AWGN(Additive White Gaussian Noise) channel environment. Therefore, the system is analyzed between proposed OFDM/QPSK-DMR and single carrier DMR systems and simulated by BER performance and Signature curve over Microwave channel environment. Simulation result is that the proposed system performances are approaching to the performance of single carrier DMR system as the number of Sub-carriers increasing.