Carrier Frequency Offset Values Research Articles

Bayesian Learning-Assisted Joint Frequency Tracking and Channel Estimation for OFDM Systems

Orthogonal frequency division multiplexing (OFDM) is very sensitive to the carrier frequency offset (CFO). The CFO estimation precision heavily makes impacts on the OFDM performance. In this paper, a new Bayesian learning-assisted joint CFO tracking and channel impulse response estimation is proposed. The proposed algorithm is modified from a Bayesian learning-assisted estimation (BLAE) algorithm in the literature. The BLAE is expectation-maximization (EM)-based and displays the estimator mean square error (MSE) lower than the Cramer-Rao bound (CRB) when the CFO value is near zero. However, its MSE value may increase quickly as the CFO value goes away from zero. Hence, the CFO estimator of the BLAE is replaced to solve the problem.

A use-case efficient DFrFT-based index modulated-OFDM system in the presence of CFO

Index modulated orthogonal frequency division multiplexing (OFDM-IM) only utilizes partial resources to transmit signals according to use case requirement, it leads to higher energy efficiency and spectral efficiency than the classical OFDM. However, in the presence of inter-carrier interference (ICI) due to carrier frequency offset (CFO), the performance of index modulated OFDM system severely deteriorates. To mitigate this issue, a novel discrete fractional Fourier transform (DFrFT) based OFDM-IM scheme over Rayleigh frequency selective fading channel is proposed in this paper. The optimum DFrFT angle (αopt) for this new scheme at various CFO values has been calculated theoretically. Incorporating these estimated αopt, the proposed system achieves a gain of 8 dB over conventional FFT-based OFDM-IM at bit error rate (BER) = 10−3 in a high normalized CFO (ϵ) environment (ϵ=0.2). The system model is also verified for different user applications, i.e., high data rate and low data rate applications. Simulation and analytical results show that the proposed model has a robust and superior performance compared to the conventional FFT-based OFDM-IM, DFrFT-based OFDM, and classical OFDM. Moreover, the proposed DFrFT-OFDM-IM model achieves all these superiorities with the same complexity as the conventional FFT-based OFDM-IM system.

Impact and compensation of carrier synchronization errors in OFDM signals with very large QAM constellations

AbstractLow cost video sensors used for streaming video signals to help firefighters, require high bit rate due to uncompressed images. To increase spectral efficiency given a limited bandwidth, very high order constellations in high signal to noise ratio regimes can be used. However, noise is not the only factor effecting the high order constellations. These constellations are also sensitive to hardware impairments and system non‐linearities. Therefore, in this paper, the effect of carrier frequency offset (CFO) on the performance of an orthogonal frequency division multiplexing (OFDM) system with high order quadrature amplitude modulation (QAM) is studied. A closed form expression is derived for the maximum normalized residual CFO that an OFDM system with M‐QAM constellation can resist to have an error free symbol detection. Finally, the suitability of common previous CFO estimation techniques such as the cyclic prefix based technique and the Moose technique in these systems are investigate. The results show that the maximum residual CFO that an OFDM system with M‐QAM constellation can resist is proportional to the inverse of . The results also show that very large order QAM constellations such as 4096‐QAM are very sensitive to even small residual CFO values and their performance degrades, significantly. However, the bit error rate analysis indicate that the Moose CFO estimation technique can be used in these systems to compensate the CFO effect, accurately.

Blind symbol and carrier synchronization with carrier frequency offset estimation for space–time line coded systems

In this study, utilizing the structure of the space–time line code (STLC) signals, a blind symbol and carrier synchronization method considering the carrier frequency offset (CFO) is developed. The proposed method can achieve almost perfect synchronization performance with more than three transmit antennas, irrespective of the CFO values. Based on the results, the proposed blind symbol and carrier synchronization method is expected to accelerate the practical deployment and operation of the low-complexity STLC systems that achieve full spatial diversity gain with partial or no channel state information at the receiver.

Differential Evolution Algorithm-Aided Time-Varying Carrier Frequency Offset Estimation for OFDM Underwater Acoustic Communication

Orthogonal frequency division multiplexing (OFDM) is the preferred scheme for high-speed communication in the field of underwater acoustic communication. However, it is very sensitive to the carrier frequency offset (CFO). This study used a time-varying CFO estimation method aided by the differential evolution (DE) algorithm to accurately estimate the CFO of an OFDM system. This method was based on the principle that the received OFDM signal with inter-carrier interference could be considered by a Multi Carrier-code division multiple access (MC-CDMA) system on the receiver side because MC-CDMA is a technology that combines OFDM and code division multiple access (CMDA). Because it is suitable for solving problems where there are dependencies between adjacent variables, the DE algorithm was used to capture the varying CFO values on the adjacent blocks. The spreading code of the MC-CDMA was obtained based on the estimated CFO values, which were elements in the DE solutions. Then the received signal was reconstructed. The Root-Mean-Square Error between the reconstructed and actual received signals was used as the cost function, and the CFO was estimated using the DE algorithm because of its powerful parallel search capability. The simulation results showed that the proposed method had a high estimation accuracy. Compared with other intelligent optimization algorithms such as the genetic algorithm and simulated annealing mutated-genetic algorithm, the time-varying CFO estimation performance of the DE algorithm was better because of its unique ability to solve problems with dependencies between adjacent variables. Specifically, under the condition of a high signal-to-noise ratio, the improvement of estimation accuracy reaches 36.13%, and the Bit Error Rate of demodulation is thus reduced by 75%, compared with the reference algorithms. In addition, the proposed method also has good applicability to modulation methods. For phase-shift keying and quadrature amplitude modulation, in particular, the proposed method not only achieved high-precision time-varying CFO estimation values, but also reduced the demodulation deterioration caused by noise.

MSE analysis for the finite order joint frequency tracking and channel estimation in OFDM systems

ABSTRACT The technique of orthogonal frequency division multiplexing is very sensitive to the carrier frequency offset (CFO). The requirement for high estimation accuracy when we acquire the CFO value is demanding. An efficient high-order approximation algorithm for jointly estimating the CFO and the channel impulse response (CIR) has been presented in the literature. From their simulation results, it is found that the estimator mean-square error (MSE) values are very close to the Cramer-Rao bound values, indicating that the highest estimation accuracy is achieved. However, no theoretical analysis is provided to confirm this assertion. In this article, we derive the theoretical formulas for the CFO MSE and the average CIR MSE for the estimators. It is found that the theoretical MSE for a joint CFO and CIR estimation problem approximates the Cramer-Rao bound when the signal-to-noise ratio (SNR) is high and the generalized CAZA sequence is used as the preamble. From our simulations, the theoretical results still hold true even for an SNR as low as 5 dB. Consequently, the range of SNR for which the theoretical analysis is valid is quite large.

An Optimum Signal Detection Approach to the Joint ML Estimation of Timing Offset, Carrier Frequency and Phase Offset for Coherent Optical OFDM

Coherent orthogonal frequency-division multiplexing (OFDM) is one of the prime digital modulation techniques for present and future generations of wireless and optical communications. Accurate synchronization is the main obstacle to the implementation of a reliable OFDM receiver. We propose here a joint maximum likelihood (ML) estimator for the timing offset (TO), carrier frequency offset (CFO), and carrier phase offset (CPO) for coherent optical OFDM (CO-OFDM) motivated by the theory of ML signal detection. Our approach starts conceptually with the idea of first computing L replicas of the original OFDM spectrum, where L is a power of two and is assumed sufficiently large. This is done by padding (L-1)N zeros to the end of the N received noisy OFDM samples, where N is the number of OFDM subcarriers. By computing the LN-point discrete Fourier transform (DFT) of these LN time samples, we get L replicas of the DFT of the original OFDM spectrum, where each replica corresponds to the DFT for one hypothesized value of the CFO and the set of possible CFO values is {l/L} <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l=0</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L-1</sup> . We select the most probable replica by choosing the one that is at the minimum Euclidean distance from the original OFDM spectrum, which leads to a matched-filtering (MF) operation in the frequency domain in either a blind or a data-aided manner. Building on this MF concept, we then develop a joint ML estimator of the TO and CPO for each hypothesized value of the CFO. The novelty here is that the TO and the CPO are estimated efficiently as the frequency and phase of a complex sinusoid observed in noise, via either a time-domain or a frequency-domain approach. The resulting joint CFO, CPO, and TO estimator is simpler than existing estimators, both conceptually and in implementation. A much simpler sequential approach in which we first decide on the CFO and then perform a joint TO and CPO estimation is also proposed. The performance loss of this sequential approach compared to the optimum joint approach is small at high signal-to-noise ratio (SNR). We obtain the performance of all our estimators via simulations, and show that they perform better when compared with the existing well-known estimators. Finally, we derive the Cramér-Rao lower bounds (CRLB) on the performance of our estimators, and show via simulations that our estimators for high SNR attain these performance lower bounds.

Frequency synchronization for FBMC-based massive MIMO system uplink

Frequency synchronization and channel equalization are of a great importance in any multicarrier communication system. Hence, in this paper, a joint carrier frequency offset (CFO) and channel estimation approach is proposed for multiuser filter bank multicarrier (FBMC) in the uplink of massive multiple input multiple output (MIMO) systems. Our proposed solution jointly estimates the CFO and channel responses based on the maximum likelihood (ML) criterion. The computational complexity of the estimator is high due to a large number of antennas at the base station (BS). To address this issue, a low-complexity CFO estimation technique is investigated after combining the received signals at the BS antennas. It is worth mentioning that our proposed method estimates both integer and fractional part of CFO values. Also, to characterize the accuracy of our estimation techniques, the asymptotic Cramér–Rao bounds (CRBs) are derived. Furthermore, the CFO correction and channel equalization techniques are performed for FBMC systems. Finally, we show the efficacy of our proposed algorithms through numerical results.

Joint Time and Frequency Synchronization in Halved Phase-Only MIMO

Halved phase-only multiple input multiple output (HPO-MIMO) completes wireless transmission based on $\pi$ -periodic phase measurements, which are directly acquired by phase detectors at the base station. However, existing synchronization algorithms for MIMO can no longer be applied into HPO-MIMO, because they work under complete measurements with both magnitude and phase information but HPO-MIMO loses magnitude information completely. This correspondence paper proposes a joint time and frequency synchronization algorithm for HPO-MIMO, which exploits the repeatability of a pilot preamble. Starting-time and carrier frequency offset (CFO) are jointly estimated under a grid-searching manner. At first, continuous starting-time and CFO estimation are uniformly sampled from their potential value ranges. On all potential starting-time points, the received phase measurements are uniformly sliced according to the repetition structure of the preamble, and then the phase rotations due to CFO are removed based on each potential CFO value. The shifted phase measurements would show strong similarities if CFO is successfully compensated. The similarities among these sliced phase sequences are characterized by a metric named phase sequence distance (PSD). They are summed up as a final PSD, whose minimum value corresponds to the estimated starting time and CFO. Simulation results validate the effectiveness of the synchronization algorithm under different signal-to-noise ratios and lengths of preamble. HPO-MIMO with our synchronization algorithm can even perform similarly as its perfect-synchronized correspondence.

Maximum‐likelihood synchronization and channel estimation with multiuser detection in GFDMA

AbstractAccurate estimation and correction of channel distortions and carrier frequency offset (CFO) are of a great importance in any multicarrier communication system. Hence, in this paper, we propose data‐aided CFO and channel estimation techniques for both multiuser uplink and downlink of the generalized frequency division multiple access (GFDMA). Our proposed solutions jointly estimate the CFO and channel responses based on the maximum‐likelihood criterion. To simplify the implementation of the proposed estimation algorithms, we suggest a preamble composed of two similar Zadoff‐Chu training sequences in a generalized frequency division multiplexing block. It is worth mentioning that our proposed technique can estimate both integer and fractional CFO values without any limitation on the acquisition range of CFO. In the uplink phase, each user aligns its carrier frequency with the base station using the estimated CFO in the downlink. However, the CFO estimates may get outdated for the uplink transmission. Thus, residual CFOs may still remain in the received signal at the base station. While being trivial in the downlink, CFO correction is a challenging task in the uplink. Thus, we also propose a joint CFO correction and channel equalization technique for the uplink of GFDMA systems. Finally, we evaluate our proposed estimation and correction algorithms in terms of estimation mean square error and bit error rate performance through simulations.

High Resolution Carrier Frequency Offset Estimation in Time-Reversal Wideband Communications

Time-reversal (TR) wideband communication systems enjoy the spatial–temporal focusing effect in a rich-scattering environment. However, the performance degrades in the presence of carrier frequency offset (CFO). The impact of CFO can be mitigated by compensating the estimated CFO values obtained using CFO estimators. Yet, CFO estimators in literature cannot work well in wideband TR systems due to the fact that the normalized CFO values are very small and thus cannot be estimated accurately using conventional schemes. To address this issue, we propose four CFO estimators which are capable of accurate CFO estimations for wideband TR systems. The theoretical performances of the proposed estimators are analyzed. Additionally, realizing that phase wrapping might introduce severe bias into CFO estimations, we present the conditions on the system parameters so that phase wrapping can be avoided. Extensive simulations and experimental results demonstrate the superiority of the proposed methods.

Pseudo-Noise Sequence Based Synchronization for Generalized Frequency Division Multiplexing in 5G Communication System

As one of the candidate waveforms for 5G, generalized frequency division multiplexing (GFDM) is a flexible non-orthogonal multicarrier technique. Similar to orthogonal frequency division multiplexing, GFDM communication system suffers from symbol timing offset (STO) and carrier frequency offset (CFO) caused by multipath effect and Doppler shift. In this paper, we design a pseudo noise (PN) based preamble and propose an approach for signal synchronization. The preamble is composed of two identical PN sequences whose autocorrelation and cross-correlation are better than cyclic prefix (CP). STO can be detected by calculating the correlation coefficient between the local PN sequence of receiver and the corresponding preamble part of sliding window. These two adjacent PN sequences can be used to calculate phase offset value by Fourier transform, and then obtain CFO value. Simulation results demonstrate that, compared with conventional CP-based synchronization, the proposed approach has the lower mean square error and symbol error rate at the cost of just a little bit more bandwidth.

Generalised error analysis of FRFT‐OFDM over Nakagami‐ m fading channel with arbitrary m

In this study, generalised expression of probability of error for binary phase shift keying (BPSK) and quadrature PSK (QPSK) modulated fractional Fourier transform (FRFT)-based orthogonal frequency division multiplexing (OFDM) system has been derived over frequency selective Nakagami-m fading channel for any arbitrary Nakagami parameter m in the presence of carrier frequency offset (CFO). The symbol error rate (SER) performance of FRFT-based OFDM system has been found to be better than fast FT-based OFDM system at different values of FRFT angle for BPSK and QPSK. For different values of CFO, the optimum at which the error rate is minimum has been found and used for analysis and simulation. It has been observed that over Nakagami-m frequency selective channel, SER expression of BPSK and QPSK for FRFT-OFDM system exactly follows the Monte Carlo simulation in MATLAB.

UFMC 시스템에서 CFO의 영향

The UFMC(Universal Filtered Multi-Carrier) is one of multi-carrier transmission technique as a candidate of transmission method for the recent 5G communication system. The UFMC can be considered as the system which has both the advantages of OFDM(Orthogonal Frequency Division Multiplex) and the simplicity of FBMC(Filter Bank Multi-Carrier). CFO(Carrier Frequency Offset) causes the problem to lower the BER performance due to a mismatch between the sub-carriers in multi-carrier transmission scheme. The effect of CFO on UFMC is not greater than the OFDM. But it still degrades the system performance caused by interference between the sub-band. In this paper, we analyze the SNR degradation due to changes in the value of EsNo and CFO on UFMC system.

Performance Assessment of OFDM Utilizing FFT/DWT over Rician Channel Effected by CFO

Orthogonal Frequency Division Multiplexing (OFDM) is a popular kind of multi-carrier based transmission technique that has found its applications in some wireless and wireline systems. But this system suffers from several facts that degrade its performance resulting in the interference of sub-carriers. Carrier Frequency Offset (CFO) is considered as one of the factor that causes interference in the system, leading to an Inter-Carrier Interference (ICI) between the carriers that results in a degradation in the performance factor of the system. In this research work, a comparative estimation is done on the Bit Error Rate (BER) performance of Discrete Wavelet Transform (DWT) OFDM and Fourier Transform (FFT) OFDM in the presence of CFO in both of the given systems. The research focuses mainly on the improvement of the BER in the system. The complete analysis is done by considering the channel to be Rician and the systems performance is estimated for some practical values of CFO. The complete simulation is done in the MATLAB. As the CFO increases the error in the system also increases, but still in that situation, DWT shows better improvement than the conventional system. The BER performance of OFDM systems shows improvement for all the diverse PSK modulation.

Compressive sensing based joint frequency offset and channel estimation for OFDM

We consider joint estimation of carrier frequency offset (CFO) and channel impulse response (CIR) for orthogonal frequency division multiplexing (OFDM) with pilot symbols. A new method based on compressed sensing is proposed. It has been shown that the CIR can be represented as a 1-block sparse signal by using a dictionary constructed by concatenating subspaces of CFO values taken from a search space. Recovery of both CFO and CIR is accomplished by the block orthogonal matching pursuit algorithm. The proposed method uses only one OFDM training block and does not require any initialization. The performance of the proposed method is compared against the well-established pilot based estimators: Moose, Classen, the maximum likelihood estimator, and the p-algorithm. Numerical results show that the performance of the proposed method does not depend on the value of the CFO. We also give worst-case upper bounds for the mean squared error of the CIR estimate for a sparse multipath channel.

Low-complexity CFO compensation technique for interleaved OFDMA system uplink

Carrier frequency offset (CFO) is a challenging problem in the uplink of orthogonal frequency division multiple access (OFDMA). In this paper, we present a new CFO compensation technique for interleaved OFMA uplink. The received signal for each user is corrected by the CFO composed of CFO values from two adjacent users and then the residual interference is cancelled by the parallel interference canceller (PIC) algorithm. In the following we analyze the performance of the proposed method and derive a signal-to-interference (SIR) expression. Finally, simulation results show acceptable performance of the proposed method in comparison with some conventional techniques with lower complexity.

Performance Analysis of FFT-OFDM and DWT-OFDMover AWGN Channel under the Effect of CFO

Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier based system that is selected to be used in the broadband wireless communication system. However, the main drawback considered for OFDM system is the Carrier Frequency Offset (CFO) which causes the Inter-Carrier Interference (ICI) that degrades the performance of the OFDM system. This paper discusses the performance of the system based on Bit Error Rate (BER) in the presence of CFO. For this, the estimation of CFO is carried over in both Discrete Wavelet Transform (DWT) OFDM and FFT OFDM for five different values of CFO (CFO = 0, 0.05, 0.1, 0.15, 0.2) in AWGN (Additive White Gaussian Noise) channel using the same procedure and their BER performance are simulated using MATLAB software. A comparative analysis on FFT-OFDM and DWT-OFDM systems is offered that shows that the performance given by DWT-based OFDM system is better in comparison to the FFT based OFDM system. BER results are simulated for BPSK, QPSK, 8-PSK, 16-PSK, 32-PSK modulations for both DWT and FFT based OFDM models. The analysis of MATLAB simulation clearly shows that the DWT-OFDM offers an improvement of 5-10 dB for the diverse order of modulations over AWGN channel.

Performance Evaluation for Wavelet Based Ofdm System Effected by Cfo over Rayleigh Channel

OFDM systems due to its orthogonality factor provides significant spectrum efficiency, robustness to fading and simple cost effective implementation. One of the factors that effects the OFDM system performance is the Carrier Frequency Offset (CFO) that results in the Inter-Carrier Interference (ICI) which causes a degradation of the OFDM system performance. In this paper, a discussion on the performance of Discrete Wavelet Transform (DWT) OFDM and Fourier Transform (FFT) OFDM is done by considering the presence of CFO in both the systems. A comparative study of both the systems is done on the basis of Bit Error Rate (BER) performance these systems show. The system is considered on the basis of improvement in the BER. Simulations conclude that the system that uses wavelet implementation performs better than the conventional OFDM system. The DWT performance even with CFO factors shows considerable improvement in the bit error performance than the FFT based traditional system. Further, the whole simulation is done under the Rayleigh channel, the performance calculated for five practical values of CFO from 0 to 0.2 with step difference of 0.05. The BER performance is simulated using MATLAB software. Further, the whole simulation is done under the Rayleigh channel, the performance calculated for five practical values of CFO from 0 to 0.2 with step difference of 0.05. The DWT based OFDM outperforms the FFT based OFDM for all the values of CFO.

ICI Reduction in Multicarrier Systems

Inter-Carrier Interference (ICI) is a major contribution to performance degradation in multicarrier systems. The main reason of the ICI is the Carrier Frequency Offset (CFO). Some researchers reduced the effect of this ICI by estimating the CFO value then compensating for it. The CFO estimation accuracy affects in turn the multicarrier system performance. The Repeated Prefix (RP) is a new technique that is recently proposed to reduce the ICI effect by estimating the CFO value without producing mathematical derivations. The drawback of this technique is its low performance at low guard interval values. This paper enhances the performance of this method at low guard interval values, giving the Maximum Likelihood Estimation (MLE) derivation for this process in addition to a derivation of the relative Cramer-Rao Lower Bound (CRLB). The proposed technique (RP-combined) reduces the effect of the residual ICI and can be used once in the transmitted frame with less data rate losses.Simulations show comparable results between both theoretical and simulated cases. Simulations also show the system performance with/without using channel estimation stage to show that, the channel estimator increases the immunity of the system performance to ICI for small offset values.