Sensitivity of autocorrelation frequency offset estimation in a FH-OFDM system
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A new aspect of carrier frequency offset (CFO) estimation is studied in a frequency-hopping (FH) orthogonal frequency division multiplexing (OFDM) system.Unlike in a traditional nonhopped OFDM system,in FH-OFDM system it is important to verify how accurately frequency-switching must be performed before the CFO estimation can take place with adequate performance.The accuracy of frequency-switching is evaluated through frequency-hopping timing offset (FHO) and frequency-hopping phase offset (PHO) between the received signal and the signal produced by the local frequency-hopping synthesizer of the receiver.The performance of selected autocorrelation algorithms is simulated in multi-path fading channel under the FHO and PHO,and the results are compared with the performance of ideal frequency-switching and the corresponding analytical Cramer-Rao bound.It is found that the CFO estimators based on autocorrelation principle are quite robust to the FHO less than half the hopping period.Keywords:
Carrier frequency offset
Frequency-hopping spread spectrum
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This paper develops a novel and simple algorithm to perform joint carrier frequency offset and channel estimation in orthogonal frequency division multiplexing (OFDM) systems. Carrier frequency offset of upto several frequency subcarrier spacings has to be estimated and compensated in order to enhance the OFDM system performance. The time domain synchronization algorithms using the preamble structure provided for fixed broadband wireless OFDM uplink access (IEEE 802.16 LAN/MAN standards) can only measure a frequency offset with an ambiguity equal to an even number of subcarrier spacings, in case of the actual offset being more than a subcarrier spacing. Our proposed algorithm uses the received preamble to jointly estimate this remaining integer part of the frequency offset and the initial channel impulse response using a least squares criterion in an iterative manner. Computer simulations show that the proposed estimator is very robust at low signal to noise ratios (SNRs) and operate near the Cramer-Rao bound for the variance of the frequency offset estimate.
Carrier frequency offset
Preamble
UTC offset
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For the frequency offset estimation in Orthogonal Frequency Division Multiplexing( OFDM) systems,a high accuracy blind carrier frequency offset estimator is proposed. The OFDM system with inter-carrier interference( ICI) is considered as a multi-carrier CDMA system and the ICI coefficient matrix is considered as the spreading codes in CDMA system. The spreading code is designed with estimated frequency offset to reconstruct the received signal. The cost function is constructed based on minimum reconstruction error principle between the received signal and actual received signal,and the fitness function is also designed corresponding to the genetic algorithm( GA). With the strong ability of parallel search and global optimization property of the genetic algorithm,the appropriate frequency offset value is achieved by minimizing the cost function. The simulation results show the proposed algorithm performs higher estimation accuracy than conventional methods,especially in low signal-to-noise ratio( SNR) condition,and it reduces time complexity effectively,without the limitation of frequency offset estimation range and modulation type.
Carrier frequency offset
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A new carrier frequency offset estimation scheme in the orthogonal frequency division multiplexing (OFDM) system is proposed. In non-data-aided synchronization systems, maximum likelihood estimation (MLE) is often used for carrier frequency offset acquisition , but it could not correct the accumulated residue frequency offset caused by sampling clock offset and other factors. After simple comparisons at frequency domain, the proposed scheme takes correction to OFDM symbols and makes compensation for MLE algorithm. By sharing the same phase calculation block with the acquisition step, it keeps very low computational cost and reduces the implementation complexity. The system performance was investigated in additive white Gaussian noise channel.
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Frequency synchronization for OFDM is very important because the carrier frequency offset (CFO) will damage the performance. Based on the correlation between the front half-symbol and the back half-symbol in BPSK modulation, a cost function is deduced which varies with the carrier frequency offset as a sine function, and then a new carrier frequency offset estimation algorithm is presented. Then frequency offset estimation can be conducted based on this cost function. At the same time, the method that simplifies iterative calculation is given, which makes the algorithm suitable for hardware implementation and practical application. The results are also suitable for OFDM signal based on QPSK and 8PSK modulation.
Carrier frequency offset
Modulation (music)
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Carrier frequency offset
Orthogonality
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Orthogonal Frequency Division Multiplexing (OFDM) systems are very sensitive to the frequency offset of the local oscillator at the receiver while the symbol timing offset can be absorbed in the guard interval. For the same reason, estimation of the frequency characteristics, needed for OFDM to be adapted to the frequency selective fading, can only be carried out conventionally after the frequency offset has been compensated. And accurate estimation of large frequency offset certainly requires high precision estimate of the frequency characteristics. In this paper, we propose a new joint estimation method of the frequency offset and the channel frequency response using an Expectation-Maximization (EM) algorithm for OFDM systems. The proposed algorithm overcomes the limitation of the thus far proposed algorithm. By computer simulations, we show the proposed algorithm provides estimation accuracy close to its lower bound in a wide range of the frequency offset.
Guard interval
Carrier frequency offset
Frequency deviation
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This paper proposes an improved carrier frequency offset (CFO) and sampling frequency offset (SFO) estimation scheme for orthogonal frequency division multiplexing (OFDM) based broadcasting system with cyclic delay diversity (CDD) antenna.By exploiting a periodic nature of channel transfer function, cyclic delay and pilot pattern with a maximum channel power are carefully chosen, which helps to enable a robust estimation of CFO and SFO against the frequency selectivity of the channel.As a performance measure, a closed-form expression for the achievable mean square error of the proposed scheme is derived and is verified through simulations using the parameters of the digital radio mondiale standard.The comparison results show that the proposed frequency estimator is shown to benefit from properly selected delay parameter and pilot pattern, with a performance better than the existing estimator.
Carrier frequency offset
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Based on cyclostationary characteristics, a blind algorithm for estimating carrier frequency and symbol offset of multi-carrier Orthogonal Frequency Division Multiplexing (OFDM) system in Rayleigh multipath channels is proposed in this paper. Through selecting appropriate cycle frequencies, we first isolate the timing offset from frequency offset, and then propose independently time and frequency offset estimation method respectively. Computer simulation results show that the proposed algorithm can resist Rayleigh multipath channel; the performance of the estimator is robust to Signal-to-Noise (SNR) in wide-sense stationary noise. Furthermore, the symbol timing estimation errors do not affect the frequency offset results, and similarly, the frequency offset estimation result doesn't affect the symbol timing estimation results.
Carrier frequency offset
Cyclostationary process
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For orthogonal frequency division multiplexing (OFDM) systems, the frequency synchronization and channel estimation are always tightly coupled with timing synchronization. In this paper, we investigate the effects of timing offset on the performance of a recently proposed subspace based joint carrier frequency offset (CFO) and channel estimation scheme. It is found that this scheme is highly sensitive to timing offsets. An improved timing robust scheme is then proposed. In the proposed CFO estimation scheme, the orthogonality loss between two subspaces due to timing offset is restored by exploiting a reduced null-subspace. The missing taps in the original channel estimate are recovered by using an upper bound of the channel length instead of the actual value. The effectiveness of the proposed scheme is verified by simulations.
Carrier frequency offset
Orthogonality
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OFDM is sensitive to carrier frequency offset, so the frequency offset estimation must be accurate. A new joint estimation algorithm of OFDM frequency offset and phase offset was proposed. The new algorithm combines with a coherent demodulation decision feedback algorithm, estimates the frequency offset and phase offset simultaneously. An OFDM system with 128 sub carriers, QPSK modulation, a multi path fading channel and SNR=15dB was simulated. The result indicates that the frequency offset and phase offset can be estimated accurately, when the frequency offset is less than 12% and the phase offset is less than 0.05 π.The joint estimation algorithm can be used to keep the track of frequency offset and phase offset effectively for OFDM systems.
Carrier frequency offset
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