• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.13 no.3
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• pp.316-322
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• 2002

The Orthogonal Frequency Division Multiplexing(OFDM) is a special case of multicarrier transmission, where a single data stream is divided into many subcarriers and transferred in a parallel way. It reduces the necessary bandwidth using the orthogonality between the subcarriers. Therefore it requires the transmission channel which has stable characteristic. When the delay spread of the channel exceed the guard interval, then the orthogonality of the subcarriers cannot maintain and as a result the system performance degrade. In this paper, the XCP-OFDM(OFDM using cross-handed Circular Polarization) system is newly proposed. This system divides the channel in order to eliminate the overlapping of subcarrier's spectrum by using cross-handed circular polarization. Therefore, the proposed XCP-OFDM system can improve the performance without increasing the guard interval. Both theoretical analysis and simulation results are described.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.59 no.11
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• pp.2108-2113
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• 2010

The Long Term Evolution (LTE) system is based on the Orthogonal Frequency Division Multiplexing (OFDM) and relies its channel estimation on the lattice-type pilot samples in the multipath fading channel environment. The estimation of the channel frequency response (CFR) makes use of the least squares estimate (LSE) for each pilot samples, followed by an interpolation both in time- and in frequency-domain to fill up the channel estimates for subcarriers corresponding to data samples. Any interpolation scheme could be adopted for this purpose. Depending on the requirements of the target system, we may choose a simple linear interpolation or a sophisticated one. For any choice of an interpolation scheme, these is a trade-off between estimation accuracy and numerical cost. For those wireless communication systems based on the OFDM and the preamble-type pilot structure, the DFT-based channel estimation and its variants have been successfully. Yet, it may not be suitable for the lattice-type pilot structure, since the pilot samples are not sufficient to provide an accurate estimate and it is known to be sensitive to the location as well as the length of the time-domain window. In this paper, we propose a simple interpolated based on the upsampling mechanism in the multirate signal processing. The proposed method provides an excellent alternative to the DFT-based methods in terms of numerical cost and accuracy. The performance of the proposed technique is verified on a multipath environment suggested on a 3GPP LTE specification.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.35 no.5C
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• pp.487-494
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• 2010

This paper presents an OFDM-based dual-hop multi-relay system with best relay selection maximizing the mutual information. For the system either with decode-and-forward (DF) relays or with amplify-and-forward (AF) relays, we derive a lower-bound on the outage probability and the diversity order achievable in frequency selective fading channels and provide the outage capacity from simulation. Performance evaluation shows that both DF and AF provide the same diversity order as in the lower-bound but DF of which the outage probability is much closer to the lower-bound provides a better outage capacity than AF. It is also observed that the SNR gain of DF over AF gets larger as either the number of resolvable multipaths or the number of relay candidates increases, which makes DF relaying more favorable to the OFDM-based multi-relay system.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.15 no.11
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• pp.1026-1033
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• 2004

Clipping is a simple and convenient PAPR reduction method to prevent the high-speed OFDM communication system from the nonlinear distortion. However, it inherently produces clip noise so that BER performance can be degraded. Clipping noise can be divided into in-band and out-of-band clipping noise. In this paper, we study the clipping effect on the forward link of the OFDM-FDMA system. BER performance of OFDM-FDMA system using clipping method is theoretically analyzed. We also investigate the BER performance of the clipped OFDM-FDMA signal through the HPA. From the simulation results it can be quantified how much the BER performance of OFDM-FDMA system is degraded by the clipping method. From the study results, we can choose the appropriate IBO value and CR(clip ratio) to get the desired BER performance.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.6 no.4
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• pp.567-573
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• 2002

In this paper, we propose a new symbol time synchronization scheme suitable for the OFDM system with an interpolator. The proposed scheme performs the following three steps. In the first step, the coarse symbol time synchronization is achieved by continuously measuring the average power of the received envelope signal. Based on this average power, the detection possibility for the symbol time synchronization is determined. It the signal is sufficient for synchronization, we next perform a relatively accurate symbol time synchronization by measuring the correlation between a short training signal and the received envelope signal. Finally, an additional frequency synchronization is performed with a long training signal to correct symbol synchronization errors caused by the phase rotation. From the simulation results, one can see that the proposed synchronization scheme provides a good synchronization performance over frequency selective channels.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.30 no.5A
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• pp.382-388
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• 2005

An OFDM system may be a good candidate for a next-generation wireless communication system which requires high-speed and high-rate data transmission. In OFDM systems, guard intervals are inserted to mitigate the effects of ISI (Inter-Symbol Interference). But guard interval insertion degrades the system performance from the standpoint of data rate and bandwidth efficiency. In this paper, a new method to improve the performance of an OFDM system without guard interval. First, PIC (Parallel Interference Cancellation) scheme which can reduce ISI by subtracting the estimated multi-path components from the received signal will be considered. And the proposed system with Pseudo-Decorrelator estimates transmitted signals by using the inverse matrix of cross-correlation matrix relating to interference components. The performance of the proposed system is evaluated through computer simulations and compared with that of the PIC system and an OFDM system without guard interval.

• Proceedings of the Korean Society of Broadcast Engineers Conference
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• 2014.06a
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• pp.153-154
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• 2014

DMB 시스템은 직교주파수분할 (OFDM : Orthogonal Frequency Division Multiplexing) 기반의 전송방식을 사용하기 때문에, 단일 주파수 네트워크 (SFN : Single Frequency Network) 기반 방송망 구축이 가능하다. 이러한 SFN 방송망은 인접 송신기의 시간 및 주파수 동기, 송신소 사이의 거리, 송신 전력의 크기를 고려하여야 하며, 이를 위해서는 먼저 특정 수신점에서 채널 임펄스 응답 (CIR : Channel Impulse Response) 을 분석하여 간섭을 최소화되도록 설계하는 것이 중요하다. 본 논문에서는 DMB 시스템에서 다중경로에 의한 간섭을 효율적으로 측정하는 기법을 제안하고, 모의실험을 통해 제안된 기법의 성능을 검증하고자 한다.

• Proceedings of the IEEK Conference
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• 2007.07a
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• pp.153-154
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• 2007

In this paper, we propose context information communication system using RF module to prevent railway cross accident. Since the communication module transmits to the train with high bit rate, OFDM(Orthogonal frequency division multiplexing) modulation method that distributes high speed data and transmits multiple times is applied. And image information is transmitted to the train's transceiver device by using ISM band (2.4GHz frequency band) that is proper to mobile communication. By using this system, we can deal with urgent situations at the railway cross and prevent railway cross accidents in advance.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.8A
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• pp.583-591
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• 2009

OFDM(orthogonal frequency division multiplexing) signal with high PAPR(peak to average power ratio) produces the nonlinear distortion and/or decreases down the power efficiency of HPA(high power amplifier). So, the IMD(inter-modulation distortion) reduction method was proposed to reduce the nonlinear distortion, which shows better BER(bit error rate) performance than the PAPR reduction methods. However, IMD reduction method has inherent problem which system complexity and processing time increases because the FFT(fast Fourier transform) processor is added in transmitter and decision criterion of IMD reduction method is computed in frequency domain,. In this paper, therefore, we propose a new IMD reduction method to reduce the computational complexity and structure of IMD computation. And we apply this proposed method into OFDM system using PTS(partial transmit sequence) scheme and compare the computational complexity between conventional and proposed IMD reduction method. This method can reduce the system size and computational complexity. Also, the proposed has almost same BER performance with the conventional IMD reduction method.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.32 no.7A
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• pp.755-761
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• 2007

In this paper the uplink single carrier scheme is proposed for wireless Internet application and its performance is evaluated in terms of PAPR (peak to average power ratio) and ABR (achievable bit rate) compared with the uplink OFDM scheme. There exist constraints regarding the signal amplification because the non-linear amplifier should be exploited instead of the linear amplifier due to its high cost of linear amplifier and power inefficiency at the WiBro terminal. Therefore characteristics in the uplink scheme design are discussed in order to maximize ABR and relieve the requirements for the amplifier specifications.