• Journal of Digital Contents Society
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• v.10 no.2
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• pp.189-198
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• 2009

In this paper, we propose a method to improve the performance of orthogonal frequency division multiplexing (OFDM) receivers by using oversampling the received signals. Demodulation of the received OFDM signals is to detect the amplitude and phase components of the subcarriers. From the oversampled OFDM signals, we can get redundant informations in frequency domain for the data, which are different in phase but the same in amplitude. By using these properties, we can obtain signal to noise ratio (SNR) gain by the oversampling ratio compared to the receivers which sampled with symbol rate. In this paper, we propose oversampled receivers whose oversampling ratio is expanded from integer to general rational number. Through computer simulations, we show the validity of the proposed methods by comparing the performance of the receivers with nonideal band-limiting filters.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.66 no.6
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• pp.949-954
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• 2017

Multiple input, multiple output orthogonal frequency division multiplexing (MIMO OFDM) systems are the candidate for the future wireless communications. However, the main drawback of MIMO OFDM systems is their sensitivity to carrier frequency offset (CFO) similar to the single input, single output OFDM (SISO OFDM) systems. The demodulation of a signal with CFO causes large bit error rate and degrade the performance of a symbol synchronizer. It is important to estimate the frequency offset and minimize or eliminate its impact. In this paper, we propose a technique based on observation training symbols for estimating CFO by employing block-by-block estimation for SISO OFDM systems. The technique of SISO OFDM is extended to the MIMO OFDM systems. Simulation results show that the proposed techniques have a superior performance and better accuracy compared to the conventional techniques in the sense of mean square error.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.40 no.5
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• pp.806-808
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• 2015

In this paper, we suggest the method of pilot tone design for a compressed channel sensing in order to decrease the peak to average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.41 no.9
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• pp.1038-1040
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• 2016

This paper introduces a fine timing synchronization based on reference signals. The proposed method first estimates a channel impulse response (CIR), and acquires the timing offset based on the change point of statistics of the estimated CIR. It is confirmed that the proposed estimator can significantly improve the fine timing synchronization performance compared to conventional schemes over diverse channel environments.

• 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.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.5 no.4
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• pp.621-629
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• 2001

In this thesis a wireless data transmission system has been proposed and analysed that uses the multi-carrier technique with the adaptive modulation scheme. In general, the OFDM(Orthogonal Frequency Division Multiplexing) system is assigning a same amount of information to all sub-carriers in a wireless data transmission. In the proposed system, the different amount of information is assigned to each sub-carrier depending on the state of channel and the target probability of error of system. With the proposed scheme, the transmission rate can be maximized with the fixed power and the required power to transmit the information can be minimized with the target probability of error of system.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.2C
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• pp.226-233
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• 2009

A novel frequency offset estimation scheme using a periodogram is proposed for orthogonal frequency division multiplexing (OfDM) systems. A conventional scheme proposed by Ren has a problem that the fractional and residual frequency offset estimation steps do not compensate for the difference between the real frequency offset and estimated integer frequency offset in the noise channel. So, the frequency offset estimation performance of Ren's scheme is degraded. In this paper, a frequency offset estimation scheme is proposed, which has a larger estimation range than that of the factional and residual frequency offset estimation steps in Ren's scheme. The proposed scheme can compensate for the difference between the real frequency offset and estimated integer frequency offset, unlike Ren's scheme. From the simulation results, we can observe that the performance of the proposed scheme is better than that of Ren's scheme.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.5C
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• pp.548-554
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• 2009

OFDM systems are very sensitive to the frequency offset. Therefore, frequency offset estimation is one of the most important procedures in OFDM systems. Ren's periodogram-based frequency offset estimation scheme consists of three steps which estimates the frequency offset accurately. However, there are the drawbacks of the limited frequency offset estimation ranges in the second and third estimation steps of the Ren's scheme. This ranges are insufficient compared with the overall signal bandwidth, making the high complexity of the first estimation step. In this paper, we propose a novel lower complexity frequency offset estimation scheme for OFDM systems which extends the frequency offset estimation range. The simulation results show that the proposed scheme has a wider frequency offset estimation range and lower complexity than those of the Ren's scheme.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.6A
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• pp.643-651
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• 2008

In this paper, we analyze the performance of normalized SNR based proportional fair scheduling with partial feedback information for multiuser MIMO-OFDMA systems. The closed form expression on the downlink capacity of the selective partial CQI feedback scheme is derived and its asymptotic behavior is investigated. From the performance analysis and numerical results, it is found that the optimal growth rate of downlink capacity can be achieved with bounded average feedback overhead irrespective of the number of users.

• Korean Journal of Optics and Photonics
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• v.14 no.6
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• pp.600-605
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• 2003

We have experimentally demonstrated a Direct Optical Switching (DOS) CDMA for future wide-band mobile communication systems at the 40 ㎓ band by using orthogonal (crosscorrelation $\leq$2) unipolar type codes with code length of 16 and chip rate of 2.5 Gcps for radio-over-fiber (RoF) systems. Pulse-amplitude-equalized 40 ㎓ laser pulses were provided by rational-harmonically mode-locking a 10 ㎓ fiber ring laser.