• The Journal of the Korea Contents Association
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• v.8 no.4
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• pp.79-85
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• 2008

These days, orthogonal frequency division multiplexing (OFDM) transmission method is widely used for broadband communication systems. The OFDM, which uses sine waves as orthogonal basis functions, is one of the orthogonal waveform modulation techniques. In this paper, we investigate a wavelet packet modulation method which uses wavelet packets instead of sine waves as the basis functions. The wavelet packets may have different patterns in two dimensional time-frequency domain, and we can design the packets appropriate for the channel environments with much flexibility. In this paper, we investigate the characteristics of the wavelet packet modulation as one of the multicarrier modulation methods, And we illustrate by simulations that narrowband interference can be reduced effectively by control the bandwidth of the wavelet packets.

• Journal of the Korean Institute of Telematics and Electronics D
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• v.35D no.2
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• pp.101-109
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• 1998

In this paper, we implemented holographic optical memory systm which can store and reconstruct many images using new input and angular multiplexing method. In the new input method, phase infomation of input image is inputed in the recording material instead of brightness information. To do so, we represented the images, which captured with CCD camera or displayed on the computer monitor, on the liquid crystal television(LCTV) which removed polarizer/analyzer. Therefore, we can generate uniform input beam intensity regardless of the total brightness of input image, and apply the scheduled recording method. Also we can increase the intensity of input beam so reduce the recording time of input image. And reconstructedimage is acquired by transforming phase information into brightness information of image with analyzer. The incident angle of reference beam is acquired by Fourier transform of the binary phase hologram(BPH) which designed with SA algorithm on the LCTV. The proposed optical memory system is stable because the incident angle of the reference beam is controlled easy and electronically. We demonstreated optical experiment which store and reconstruct various type images in BaTiO$_{3}$ using proposed holographic memory system.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.19 no.10
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• pp.2275-2284
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• 2015

In this paper, the Walsh coded orthogonal training signals for 4 × 4 multiple input multiple output-orthogonal frequency division multiplexing (MIMO-OFDM) systems are designed and the channel estimation equations are derived as a closed form, taking account of the inter training signal interference problems caused by the multi-path delayed signals. The performances of the proposed channel estimation method are analyzed and compared with the conventional methods[9,14] by using computer simulation. The simulation results show that the proposed methods has better performances, compared with the conventional methods[9,14]. As a result, the proposed method can be used for MIMO-OFDM systems with null sub-carriers.

• 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.

• Journal of Broadcast Engineering
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• v.15 no.2
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• pp.217-223
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• 2010

In this paper, an interchannel interference (ICI) cancellation method is proposed for multiple frequency offsets in orthogonal frequency division multiplexing (OFDM) systems. When several same signals are received from different transmitters simultaneously, multiple frequency offsets may occur at the receiver because of the frequency difference of the oscillators of the two transmitters and the receiver. This causes degradation of system performance because OFDM systems are very sensitive to the frequency offsets. In this paper, we propose a method to eliminate the effect of the multiple frequency offsets for OFDM systems. The method is accomplished in two steps: compensation of the frequency offset in the time domain and subsequent cancellation of the ICI in the frequency domain. Through computer simulations, we verify the effectiveness of the proposed ICI cancellation method.

• Journal of Communications and Networks
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• v.9 no.4
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• pp.419-427
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• 2007

In wavelength-division-multiplexing(WDM) networks a link failure may cause the failure of several high-bit-rate optical channels, thereby leading to large data loss. Recently, various protection and restoration mechanisms have been proposed to efficiently deal with this problem in mesh networks. Among them, dedicated path protection(DPP) is a promising candidate because of its ultra-fast restoration time and robustness. In this work we investigate the issue of planning and optimization of WDM networks with DPP. Integer linear programming(ILP), in particular, is one of the most common exact method to solve the design optimization problem for protected WDM networks. Traditional ILP formalizations to solve this problem rely on the classical flow or route formulation approaches, but both these approaches suffer from a excessively high computational burden. In this paper, we present a variable-aggregation method that has the ability of significantly reducing the complexity of the traditional flow formulation. We compare also the computational burden of flow formulation with variable aggregation both with the classical flow and route formulations. The comparison is carried out by applying the three alternative methods to the optimization of two case-study networks.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.48 no.6
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• pp.6-13
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• 2011

Multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) with space-time coding (STC) is a promising technology for future wireless communication systems. However, MIMO-OFDM systems are greatly impaired by large cochannel interference (CCI) from the multiple transmitters. In this paper, we propose pre-fast Fourier transform (FFT) multibeamforming based on MMSE(minimizing the mean squared error) for a MIMO-OFDM system to preserves the STC diversity and to remove the CCI. The improvement in bit error rate is investigated through computer simulation of a MIMO-OFDM system in a multipath channel with CCI.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.30 no.7C
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• pp.624-632
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• 2005

We consider an adaptive bit-loading technique for bit interleaved coded modulation-orthogonal frequency division multiplexing(BICM-OFDM) systems. By adjusting transmission parameter of each subcarrier adaptively depending on the subchannel state, the performance of OFDM system can be improved dramatically. In this paper, the number of bits for each subcarrier is allocated to minimize bit error rate keeping the constant throughput for the adaptive transmission technique of BICM-OFDM system which can be applied to real time transmission. Also, We use the discrete Lagrange multiplier method to get the optimum solution under the integer bit allocation constraint. Simulation results show that computational amount of the proposed bit allocation technique is not high and BICM-OfDM system using the proposed technique can get the SNR gain by 2$\~$3 dB over nonadaptive one.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.30 no.1A
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• pp.55-61
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• 2005

Mobile communication systems are required not only to support high-data-rate transmissions in favorable channel conditions but also to be able to tolerate hostile environments possibly encountered by cellular communications. This paper compares CDMA(Code Division Multiple Access), OFDM(Orthogonal Frequency Division Multiplexing), and MC-CDMA(Multi-Carrier CDMA)with inaccurate channel estimates and low SNR environments. The equations and simulation results show that the performance losses in CDMA systems due to imprecise channel estimates are not considerable while frequency-hopping of OFDM systems can result in more than l0dB SNR losses. Also, this paper show that frequency-spreading of MC-CDMA can be very helpful for channel compensation performances than frequency-hopping or time-spreading of OFDM.

• ETRI Journal
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• v.35 no.4
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• pp.706-709
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• 2013

As an alternative to the traditional pilot-aided orthogonal frequency division multiplexing (OFDM), the time-domain pseudonoise (PN)-padded OFDM provides a higher spectral efficiency. However, the carrier frequency offset (CFO) attenuates peaks of the conventional PN correlation output, which limits the CFO estimation range of the OFDM synchronizer. An improved correlation is proposed in this letter to remove the CFO-induced amplitude attenuation of correlation peaks. For a synchronizer adopting the designed correlator, a larger range of CFO acquisition is obtained through using wider correlation windows with a smaller interval between them. The proposed method of CFO acquisition is verified in a digital terrestrial multimedia broadcast receiver, in which the synchronizer is able to acquire CFOs up to ${\pm}320$ kHz in the DVB-T F1 channel. Furthermore, the acquisition range can be expanded in more favorable channels.