• Proceedings of the Korea Society of Information Technology Applications Conference
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• 2005.11a
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• pp.255-258
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• 2005

A blind-mode integer frequency offset estimation algorithm is proposed for an OFDM system. Imperfect integer frequency offset estimation causes ambiguity in the data sub-carrier position. Morelli's blind integer frequency offiet estimation algorithm exploits the likelihood function by comparing the power in sub-carriers. It, however, shows performance degradation when there is the fractional frequency offset. The proposed algorithm solves this by using interpolation in the frequency domain. In this algorithm, it is exploited that the effect of the frequency offset is shown as a shift of power spectrum. By calculating the covariance of over-sampled samples, most approximate samples to integer point are obtained. It enables integer frequency offset estimation less affected by fractional frequency offset.

• Proceedings of the KIEE Conference
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• 2000.07d
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• pp.3100-3102
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• 2000

In this paper, we propose to a robust frequency offset estimation method of OFDM signals. A carrier frequency offset may be decomposed into an integer multiple of the subcarrier spacing and a residual frequency offset. Fractional part of frequency offset is obtained by using the maximum likelihood estimation(MLE) method. And we use the correlation of the samples at the output of the discrete Fourier transform(DFT) to estimate integer part of frequency offset. The result shows that the estimation frequency offset is almost linear to frequency offset. We propose to an improved estimation error variance of the carrier frequency offset estimation. The proposed estimator has better performance than the conventional ones in terms of error variance and tracking range.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.31 no.6C
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• pp.623-628
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• 2006

This paper addresses the frequency offset estimation problem in the presence of the timing error for OFDM systems. When the timing error exists, the correlation value used for the frequency offset estimation could be reduced significantly due to the timing error, resulting in considerable degradation in estimation performance. In this paper, using the coherence phase bandwidth (CPB) and a threshold, a novel frequency offset estimation scheme is proposed and based on which, an efficient timing error estimation scheme is also proposed for the re-estimation of the frequency offset. The performance comparison results show that the proposed frequency offset estimation scheme is not only more robust to the timing error but also has less computational complexity, as compared with the conventional schemes. It is also demonstrated by simulation that theproposed timing error estimation scheme gives a reliable estimate of the timing error.

• 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.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.

• ETRI Journal
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• v.42 no.3
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• pp.324-332
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• 2020

In this paper, a blind symbol timing offset (STO) estimation method is proposed for offset quadrature phase-shift keying (OQPSK) modulated signals, which also works for other linearly modulated signals (LMS) such as binary-PSK, QPSK, 𝜋/4-QPSK, and minimum-shift keying. There are various methods available for blind STO estimation of LMS; however, none work in the case of OQPSK modulated signals. The popular cyclic correlation method fails to estimate STO for OQPSK signals, as the offset present between the in-phase (I) and quadrature (Q) components causes the cyclic peak to disappear at the symbol rate frequency. In the proposed method, a set of close and approximate offsets is used to compensate the offset between the I and Q components of the received OQPSK signal. The STO in the time domain is represented as a phase in the cyclic frequency domain. The STO is therefore calculated by obtaining the phase of the cyclic peak at the symbol rate frequency. The method is validated through extensive theoretical study, simulation, and testbed implementation. The proposed estimation method exhibits robust performance in the presence of unknown carrier phase offset and frequency offset.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.60 no.3
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• pp.648-653
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• 2011

We propose a new frequency offset estimation technique for multiband orthogonal frequency modulation (MB-OFDM) based ultra wideband (UWB) systems. The proposed frequency offset estimation technique is related to the scheme of Schmidl for channel model 1 (4-1Om NLOS, rms. delay =14.3ns.) using more than two symbols and with alternate symbols. Variance of frequency offset estimate obtained from the proposed frequency offset estimation technique approaches very nearing to Cramer Rao Lower Bound (CRLB) in an AWGN channel. BER performance of the proposed technique is also presented.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.47 no.10
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• pp.1-7
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• 2010

A new frequency offset estimation algorithm for chirp spread spectrum based on matched filter is proposed. Generally, the differential phase between successive symbols is used for the conventional frequency offset estimation algorithm. However, if the conventional frequency offset estimation algorithm is used for CSS, phase ambiguity arises because of long symbol duration and guard time. The phase ambiguity causes performance degradation of matched filter since the received signal is corrupted by the integer frequency offset. In this paper, we propose a new frequency offset estimation algorithm which separates integer and fractional frequency offset estimation for removing the phase ambiguity. The proposed algorithm estimates the integer frequency offset by using differential phase between matched filtering results of sub-chirps and successive symbols. Then, the fractional frequency offset is estimated by using the differential phase between successive symbols Simulation results show that the proposed algorithm well removes the phase ambiguity, and have almost same estimation performance compared with conventional one when there is not the phase ambiguity.

• 한국정보통신설비학회:학술대회논문집
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• 2008.08a
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• pp.437-440
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• 2008

In this paper, we present and analyze a robust frequency offset estimation scheme for orthogonal frequency division multiplexing (OFDM) systems over radio communication channels. When there exists frequency offset, the loss of orthogonality occurs and the interference problem among several subcarriers in OFDM signals is introduced. The guard interval of OFDM signals is employed for estimating the frequency offset. Furthermore, in order to enhance the frequency offset estimation performance, a ternary sequence is utilized. A frequency offset is estimated based on the synchronized correlator output. The proposed frequency offset estimation scheme can be applied to track the carrier frequency offset of OFDM systems.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.2
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• pp.157.2-157.2
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• 2012

Direct-conversion receivers often suffer from a DC-offset that is a by-product of the direct conversion process to baseband. In general, a basic approach to reduce the DC-offset is to do simple average of the baseband signal and remove the DC by subtracting the average. However, this gives rise to a residual DC offset which degrades the performance when the receiver adopts the coding schemes with high coding rates such as 8-PSK. Therefore, more advanced methods should be additionally required for better performance. While the training sequences are basically designed to have good auto-correlation properties to facilitate the channel estimation, they may be not good for the simultaneous estimation of the channel response and the DC-offset. Also the DC offset compensation under a bad condition does not give good results due to the estimation error. Correspondingly, the proposed scheme employs the two important points. First, the training sequence codes are divided into two groups by MSE(Mean Squared Errors) for estimating the channel taps and then SNR calculated from each group is compared to predefined threshold to do fine DC-offset estimation. Next, ON/OFF module is applied for preventing performance degradation by large estimation error under severe channel conditions. The simulation results of the proposed scheme shows good performances compared to the existing algorithm. As a result, this scheme is surely applicable to the receiver design in many communications systems.