Carrier Frequency Offset Estimation Scheme Research Articles

Attention Guided Multi-Task Network for Joint CFO and Channel Estimation in OFDM Systems

The existence of high carrier frequency offset (CFO) and fading channels degrades the performance of communication systems in high mobility environments significantly. To address these challenges, in this paper, we propose an attention guided multi-task network for joint CFO and channel estimation in orthogonal frequency division multiplexing (OFDM) systems. Specifically, considering the correlation between CFO estimation and channel estimation, we construct a multi-task neural network framework which consists of a shared branch (SB), a CFO estimation branch (CFOEB) and a channel estimation branch (CEB). Among them, the SB extracts common features of two tasks, and the CFOEB and CEB make more detailed CFO and channel estimation, respectively. To further suppress the influence of CFO estimation error on channel estimation, we introduce an attention guided module (AGM) which completes the re-calibration of the channel dimension on the basis of different CFO characteristics. We train the whole network by an end-to-end paradigm and construct a suitable multi-task loss function to balance the losses between different tasks. Simulation results demonstrate that the proposed attention guided multi-task learning based joint CFO and channel estimation scheme outperforms the conventional methods and has greater robustness under various conditions.

Joint DoA and CFO Estimation Scheme With Received Beam Scanned Leaky Wave Antenna for Industrial Internet of Things (IIoT) Systems

Direction-of-arrival (DoA) estimation is essential in many industrial internet of things (IIoT) applications where IoT users may require a high-resolution estimation algorithm for features limited to positioning and device localization. The conventional DoA estimation is challenging as the number of antenna arrays increases leading to an increase in design complexity and cost in RF circuitry. As a result, the conventional DoA estimation algorithms may not be suited for IIoT applications. In this work, a joint DOA and carrier frequency offset (CFO) schemes is introduced for typical uplink IIoT systems by equipping the receiver with a leaky-wave antenna (LWA). By considering that the LWA can simultaneously steer its beam direction towards the active user. As a result, each user is estimated independently. The joint algorithm is developed based on the high-resolution MUSIC scheme, which utilizes the LWA properties to compute the DoA spectrum. Furthermore, we develop an iterative CFO estimation approach as the desired solution to a multi-dimensional search. Subsequently, we further formulate the Cramer–Rao lower bounds (CRLB) to serve as a performance benchmark for the accuracy of the proposed scheme. The SDR platform is provided to validate the proposed algorithm in real-time. Finally, the simulation results and the SDR experiments justify that the proposed scheme offers more robustness than conventional counterparts with linear array antenna, which achieves a root mean square error (RMSE) approach to the derived CLRB, especially at a medium signal-to-noise ratio (SNR) range which suited for IIoT applications.

CNN-Attention-DNN Design for CFO Estimation of Non-Pilot-Assisted OFDM System

In this letter, we propose a novel attention mechanism inspired convolutional neural network (CNN)-deep neural network (DNN) called CAD model architecture for carrier frequency offset (CFO) estimation in orthogonal frequency division multiplexing (OFDM) systems. The hyperparameters used in the proposed CAD model are optimized using the Bayesian approach. The proposed CFO estimation scheme is robust to small symbol timing offset errors and does not require apriori knowledge of modulation format, channel, and transmitted OFDM signal parameters. Simulations performed over the realistic frequency-selective channel models with and without power amplifier non-linearity indicate that the proposed CAD architecture outperforms the conventional statistical-based and recurrent neural network (RNN)-based models. Lastly, the proposed CAD model is validated on the radio frequency testbed.

Maximum likelihood-based adaptive iteration algorithm design for joint CFO and channel estimation in MIMO-OFDM systems

In this paper, we present a joint time-variant carrier frequency offset (CFO) and frequency-selective channel response estimation scheme for multiple input-multiple output-orthogonal frequency-division multiplexing (MIMO-OFDM) systems for mobile users. The signal model of the MIMO-OFDM system is introduced, and the joint estimator is derived according to the maximum likelihood criterion. The proposed algorithm can be separated into three major parts. In the first part of the proposed algorithm, an initial CFO is estimated using derotation, and the result is used to apply a frequency-domain equalizer. In the second part, an iterative method is employed to locate the fine frequency peak for better CFO estimation. An adaptive process is used in the third part of the proposed algorithm to obtain the updated CFO estimation and track parameter time variations, including the time-varying CFOs and time-varying channels. The computational complexity of the proposed algorithm is considerably lower than that of the maximum likelihood-based grid search method. In a simulation, the mean squared error performance of the proposed algorithm was close to the Cramer-Rao lower bound. The simulation results indicate that the proposed novel joint estimation algorithm provides a bit error rate performance close to that in the perfect channel estimation condition. The results also suggest that the proposed method has reliable tracking performance in Jakes’ channel models.

Iterative Carrier Frequency Offset Estimation Scheme for Faster-Than-Nyquist Signaling Systems

As a nonorthogonal mode, faster-than-Nyquist (FTN) signaling transmission can effectively improve the spectrum efficiency of a system. However, FTN signaling inevitably causes inter-symbol interference (ISI). The ISI hinders traditional synchronization methods from operating accurately, and this makes the carrier synchronization of FTN systems more complex. To overcome this issue, we propose an iterative carrier frequency offset (CFO) estimation scheme for an FTN signaling system. The proposed CFO estimation scheme consists of two steps: i) a coarse estimation step based on discrete Fourier transform and ii) a fine estimation step based on a golden-section search algorithm. The simulation results show that the mean square error of the proposed scheme is lower than that of the conventional CFO estimation schemes and approaches the Cramer-Rao Bound, which is an optimal estimation performance bound.

Equalization and blind CFO estimation for performance enhancement of OFDM communication systems using discrete cosine transform

SummaryIn wireless communication systems, equalization is one of the most important schemes to improve the system performance. This paper consists of two main parts. The first part presents a blind carrier frequency offset (CFO) estimation scheme based on discrete cosine transform (DCT). The second part presents a joint low‐complexity equalization, and CFO compensation in orthogonal frequency division multiplexing (OFDM) systems. Moreover, in the second part, we present a joint low‐complexity regularized zero‐forcing (JLRZF) equalizer based on the proposed CFO estimation scheme. Simulation results show that the proposed configuration has the ability to perform blind CFO estimation and enhance the system performance in the presence of estimation errors.

Data-Aided Frequency Offset Estimation for CE-OFDM Broadband Satellite Systems

In recent years, the constant-envelope orthogonal frequency-division multiplexing (CE-OFDM) has been considered as a candidate waveform in broadband satellite systems as it has a 0-dB peak-to-average power ratio (PAPR). However, the carrier frequency offset (CFO) estimation scheme for CE-OFDM broadband satellite systems directly affects system performance. In this paper, we analyze the network architecture and the propagation environment of CE-OFDM broadband satellite systems, and we propose a data-aided CFO estimation strategy based on the frequency domain pilot symbols. The Cramer–Rao bound (CRB) of our CFO estimator is given by mathematical analysis, and the effect of the number of pilot symbols on the estimation performance is analyzed. The pilot symbol-based CFO estimator is composed of a phase demodulator and a discrete Fourier transform (DFT) module, and it can obtain a large estimation range under a small pilot overhead. The simulation results show that the CE-OFDM broadband satellite systems can achieve a good bit error rate (BER) performance by using the proposed strategy to estimate and compensate the CFO.

Sparse Repeated Preamble-Based Direct Current Offset and Frequency Offset Estimator in OFDM Direct Conversion Receivers

Direct conversion receiver (DCR) architecture has drawn considerable attention in orthogonal frequency-division multiplexing (OFDM) systems. However, OFDM DCRs are sensitive to non-ideal front-end effects and suffer from direct current offset (DCO) and carrier frequency offset (CFO). In this paper, a novel method to estimate DCO and CFO is presented in OFDM DCRs by exploring the sparse symbols embedded in the preamble. In the proposed method, CFO and DCO estimations can be easily decoupled. Next, a robust low-complexity DCO estimation method is proposed by using the sparse properties of the preamble in the frequency domain. Moreover, a novel CFO estimation scheme independent of the DCO is presented based on the repeated preamble structure in which the DCO component focuses on the DC subcarrier when the received preamble signal was not CFO compensated in the frequency domain. The simulation results show that the proposed CFO and DCO estimators can achieve better performance than any available methods.

Blind Carrier Frequency Offset Estimation for MIMO-OFDM Systems Based on the Banded Structure of Covariance Matrices for Constant Modulus Signals

This paper addresses the problem of blind carrier frequency offset (CFO) estimation for multi-input-multi-output orthogonal frequency division multiplexing systems. Benefiting from the banded structure of the circulant channel matrix from each transmitting antenna to each receiving antenna, the covariance matrices formulated by circular shifts of received signals also possess the banded structure in the absence of CFO for constant modulus signals. Thus, the cost function can be constructed by minimizing the elements outside the band and a closed-form CFO estimation algorithm is proposed. Since the channel length has an effect on the proposed estimator and channels with high delay spreads may deteriorate the estimation performance, an improved version of the proposed algorithm is investigated under the assumption that the channel remains constant within two consecutive OFDM symbols which is often the case. Experimental results demonstrate that the proposed algorithm shows better performance than the conventional CFO estimation schemes in frequency-selective fading channels and the improved version of the proposed algorithm can further optimize the estimation performance under long channel length conditions.

Frequency offset compensation for satellite communication system with CE-OFDM

Constant envelope orthogonal frequency division multiplexing (CE-OFDM) is a waveform that can achieve 0dB peak-to-average power ratio and avoid the signal distortion caused by the nonlinear power amplifier. However, the carrier frequency offset (CFO) in CE-OFDM systems can cause errors at phase unwrapper module. In this paper, a CFO estimation scheme is proposed for CE-OFDM in satellite communication system. As the null subcarrier is inherent in the conjugate symmetric symbol structure at the transmitter, the proposed scheme uses the null subcarrier as prior information to estimate the CFO at the receiver. The ideal estimation range of normalized CFO is obtained by mathematical analysis. Simulation results show that the proposed scheme can estimate the CFO accurately under additive white Gaussian noise (AWGN) channel and multipath fading channel, especially for moderate and high signal-to-noise ratio (SNR).

A Novel Hybrid CFO Estimation Scheme for UFMC-Based Systems

In this letter, a robust hybrid carrier frequency offset (CFO) estimation scheme based on training symbols is devised for universal-filtered multi-carrier (UFMC) based systems. In this scheme, the CFO estimator based on a least square (LS) criterion is first derived to acquire a coarse frequency offset estimation with the use of a large frequency searching step. And then, the residual frequency offset is accurately estimated through the auto-correlation operation of received signals, which have been compensated. This algorithm has the features of larger estimation range as well as higher estimation precision. Through numerical simulations, it is validated that the performance of the proposed algorithm is better than that of existing algorithms.

Blind carrier frequency offset estimation for single carrier and orthogonal frequency division multiplexing signals using least‐order cyclic moments

A definition of least cyclostationary order (LCO) is proposed based on the inner cyclic period of single carrier and orthogonal frequency division multiplexing (OFDM) signals. The LCOs of single carrier and OFDM signals are derived. Since the LCO has a relationship with the carrier frequency offset (CFO), a blind CFO estimation scheme for single carrier and OFDM signals can be proposed. The novel scheme is entirely blind without any priori information and is proved to be the same as cumulants estimation when the LCO is known. However, the new scheme is superior to the estimation based on cumulant when the LCO is unknown. The mean-square error of M-phase shift keying signals approaches the Cramer–Rao bound in additive white Gaussian noise (AWGN) channel. Simulation results are given to verify the effectiveness of the proposed blind estimation scheme in Rician fading channel and in AWGN channel.

A Robust Estimation of Residual Carrier Frequency Offset With I/Q Imbalance in OFDM Systems

In this paper, we focus on improving the accuracy of frequency offset estimation for an orthogonal frequency-division multiplexing (OFDM) system under the conditions of the joint impairments in both carrier frequency offset (CFO) and in-phase/quadrature (I/Q) imbalance. To propose a robust CFO estimation scheme and to benchmark its performance, the performance of the conventional frequency estimation algorithm in the presence of I/Q imbalance is analyzed, and some modifications to the conventional estimation scheme are highlighted. We show via simulations that such a design achieves a remarkable robustness against I/Q imbalance; thereby, the proposed scheme can efficiently estimate the CFO, irrespective of the I/Q mismatch.

PARAFAC-based frequency synchronization for SC-FDMA uplink transmissions

Frequency synchronization is a critical requirement for single-carrier frequency division multiple-access (SC-FDMA) uplink transmissions. Based on the key observation that the received signal at the base station can be formulated as the parallel factor analysis model, we present a joint carrier frequency offset (CFO) estimation and compensation scheme for SC-FDMA uplink transmissions with interleaved sub-carrier allocation. The proposed scheme, employing identical training symbols, guarantees the identifiability of CFO estimation, allows the system to operate on full load, and it is readily extendible to multi-antenna base station (BS) reception. Comparisons with existing approaches in the literature reveal that the proposed algorithms provide lower complexity and superior performance.

An Efficient Blind Estimation of Carrier Frequency Offset in OFDM Systems

In this paper, we propose a low-complexity blind carrier frequency offset (CFO) estimation scheme for constant modulus (CM)-signaling-based orthogonal frequency-division multiplexing (OFDM) systems. Provided that the channel can be assumed to be slowly time-varying, subcarriers having the same indexes in two consecutive OFDM symbols will experience nearly the same channel effect. This assumption enables us to derive a cost function that is determined by the sum of the products of the signal amplitudes on each pair of equivalent subcarriers from two successive OFDM symbols. The maximization process of this cost function makes it possible to find an appropriate estimate of the CFO. Over frequency-selective Rayleigh fading channels, the proposed CFO estimation method provides improved performance over existing techniques. Moreover, in the context of narrow-band noise and signal gain variations, the simulations demonstrate the robustness and immunity of our scheme.

Robust Pilot-aided Frequency Offset Estimation Scheme for OFDM-based Broadcasting System with Cyclic Delay Diversity

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.

Simultaneous Multiple Carrier Frequency Offsets Estimation for Coordinated Multi-Point Transmission in OFDM Systems

Orthogonal frequency division multiplexing (OFDM) combined with the coordinated multi-point (CoMP) transmission technique has been proposed to improve performance of the receivers located at the cell border. However, the inevitable carrier frequency offset (CFO) will destroy the orthogonality between subcarriers and induce strong inter-carrier interference (ICI) in OFDM systems. In a CoMP-OFDM system, the impact of CFO is more severe because of the mismatch in carrier frequencies among multiple transmitters. To reduce performance degradation, CFO estimation and compensation is essential. For simultaneous estimation of multiple CFOs, the performance of conventional CFO estimation schemes is significantly degraded by the mutual interference among the signals from different transmitters. In this work, our goal is to propose an effective approach that can simultaneously estimate multiple CFOs in the downlink by using the composite signal coming from multiple base stations corresponding to CoMP transmission. Based on the Zadoff-Chu sequences, we design an optimal set of training sequences, which minimizes the mutual interference and is robust to the variations in multiple CFOs. Then, we propose a maximum likelihood (ML)-based estimator, the robust multi-CFO estimation (RMCE) scheme, for simultaneous estimation of multiple CFOs. In addition, by incorporating iterative interference cancellation into the RMCE scheme, we propose an iterative scheme to further improve the estimation performance. According to the simulations, our scheme can eliminate the mutual interference effectively, approaching the Cramer-Rao bound performance.

Carrier frequency offset estimation method for 2 × 1 MISO TDS-OFDM systems

Abstract In time-domain synchronous (TDS)-orthogonal frequency division multiplexing (OFDM) systems, a pseudo noise (PN) sequence is inserted instead of the cyclic prefix. The PN sequence is used not only as a guard interval but also as a training sequence for channel estimation and synchronization in the time domain. Recently, research studies on 2 × 1 multi input-single output (MISO) TDS-OFDM systems have been conducted, and different PN sequences (which are orthogonal to one another or cyclically shifted) are transmitted at each transmit antenna for channel estimation, which are modulated by binary phase shift keying in the same phase angle. However, when the absolute phase difference among the transmitted PN sequences is π, a PN sequence cancellation problem occurs, making the estimation of an accurate carrier frequency offset (CFO) difficult. In this paper, a CFO estimation method with the aid of PN sequences for 2 × 1 MISO TDS-OFDM systems is proposed. In the proposed method, the phase of the PN sequences at each antenna is rotated differently and transmitted to prevent a PN sequence-canceling problem. In addition, a CFO estimation scheme using channel state information is proposed to estimate an accurate CFO in time-varying channels. We show by computer simulations that the mean square error performance of the proposed method over an additive white Gaussian noise environment and time-varying Rayleigh channel is higher than that of the conventional method.

Performance Analysis of Carrier Frequency Offset Estimation Techniques for OFDM Systems

In this study we will study the effects of Carrier Frequency Offset (CFO) upon Signal to Noise Ratio (SNR) for an OFDM system. We will also present performance analysis of two CFO estimation schemes: time domain Cyclic Prefix (CP) based estimation scheme and frequency domain based Moose scheme, Simulation results show that frequency domain based scheme can achieve better performance in term of Mean Square Error (MSE) than the time domain CP based scheme. Both schemes are analyzed under Doppler fading.

An OFDM Carrier Frequency Offset Estimation Scheme with Wide Fractional Offset Estimation Range

In this paper, we propose a carrier frequency offset (CFO) estimation scheme which is robust to the fractional CFO variation for orthogonal frequency division multiplexing (OFDM) systems. The proposed scheme first performs the envelope equalization process to convert the offset estimation problem to a carrier estimation problem, and then, estimates the integer and fractional parts of CFO by using periodogram of the received signal. Especially, in the estimation stage for fraction CFO, the ratio of the square-roots of periodograms is employed enlarging the estimation range of the stage than that of the conventional scheme. Numerical results demonstrate that the proposed scheme has better estimation performance than the conventional scheme for wider fractional CFO range in various channel conditions.