Carrier Frequency Offset Estimation Research Articles

Timestamp-Free Clock Syntonization for IoT Using Carrier Frequency Offset

System-level timing fluctuations caused by unstable low-cost clocks and end-to-end communication delays are the main sources of uncertainties in existing synchronization mechanisms that rely on timestamp exchanges. This paper introduces a timestamp-free clock syntonization approach, carrier frequency offset (CFO)-assisted syntonization (CFOSynt), to estimate the clock skew between a pair of nodes by utilizing carrier frequency offset. To enable CFOSynt, we leverage the fact that RF oscillators in the radio can be used as the reference to calibrate the system clock oscillators, and the pairwise RF clock information is carried in the transmission carrier frequency. By incorporating CFO and capturing the clock frequency relationship in system clock skew estimation, CFOSynt can eliminate the need for timestamping and the impact of delay uncertainties. To validate the design, CFOSynt is implemented on two common off-the-shelf (COTS) IoT platforms with access to the CFO estimation from the radio chip. Extensive experiments are conducted to evaluate CFOSynt, and CFOSynt can estimate the clock skew of 32 kHz low-cost electronic oscillators with a mean error of -2.46 Hz. In comparison with timestamp-based approaches, CFOSynt achieves up to 70-90% improvement in skew estimation error and shows significant reliability when low-cost oscillators are used.

Novel Blind CFO Estimation Method for OFDM/OQAM System

To solve the performance degradation due to Carrier Frequency Offset (CFO) for OFDM/OQAM system, a new blind CFO estimation method is proposed in this letter. This method uses the header structure of the transmitted signal by considering that there is an OFDM/OQAM symbol of null-valued OQAM symbols before the first one and exploits the orthogonality among subcarriers as well as the orthogonality between synthesis and analysis filter banks to estimate the CFO. Therefore, without compromising the spectral efficiency of OFDM/OQAM system this method can estimate accurately the CFO. Simulation results illustrate the effectiveness of the proposed method.

Multiple CFOs Estimation and Implementation of SC-FDMA Uplink System Using Oversampling and Iterative Method

Multiple carrier frequency offsets (CFOs) estimation in high speed single carrier frequency division multiple access (SC-FDMA) uplink system is a challenging issue. Most of the existing blind CFO estimation methods work for a certain type of carrier mapping scheme (CMS) or address a single CFO estimation by utilizing long data over frequency selective fading channel. These estimation methods become irrelevant at high speed and multi-user scenarios where CFO changes with symbols and users. In this paper, we propose a blind multiple CFOs estimation method for all CMS of SC-FDMA and implement over testbed. The proposed method requires only one SC-FDMA symbol to attain reliable estimation even in a high mobility environment, i.e., in a doubly selective channel. It exploits the phase difference present between even and odd samples of the received oversampled signal to estimate CFOs. Subsequently, an iterative joint CFO estimation and compensation (IJCEC) method is introduced to remove residual interference and increase CFOs estimation accuracy. Cramer-Rao lower bound is derived to characterize the CFO estimation accuracy of the proposed method. The work is compared with the conventional methods such as MUSIC, ESPRIT, CAZAC, and some existing methods and validated through Monte Carlo simulations. Finally, we authenticate the proposed IJCEC method by performing implementation and measurement on National Instrument testbed setup in an indoor propagation environment.

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 Estimation of RF Impairments, Channel, and Low Complexity Iterative Equalization Technique for High Mobility SC-FDMA/OFDMA Uplink Systems

In this paper, we consider the problem of joint estimation and equalization of doubly selective channels and all radio frequency impairments, namely transmitter IQ imbalance (TIQI), receiver IQI (RIQI) and carrier frequency offsets (CFO) in the uplink of single carrier frequency division multiple access (SC-FDMA) and orthogonal frequency division multiple access (OFDMA) systems with direct conversion transceiver architecture and high mobility users. We combine the effect of TIQI, RIQI and DSC to a new effective channel and propose the use of a Hermite polynomial based basis expansion model for approximating it to avoid the identifiability issue associated with this complete parameter estimation problem. As the maximum likelihood solution to this joint estimation problem requires complex multi-dimensional searches, we propose a novel subspace nulling based iterative estimation technique that effectively reduces the multi-user interference and replaces the multi-dimensional search with single-dimensional searches. Further, without much compromise on the performance, we also derive a low complexity version of the proposed technique. Additionally, we derive the Cramer-Rao bound for the joint estimation of CFO and effective channel. Moreover, we propose two equalization techniques for iteratively equalizing the mirror subcarrier pairs. The proposed techniques do not put any restriction on the carrier assignment scheme and provide good performance even at high mobile speeds.

A novel carrier frequency offset algorithm based on a double Barker code in VDE-TER

The Maritime Very High Frequency (VHF) Data Exchange System (VDES) is a basic method of data exchange in the e-Navigation strategy proposed by the International Maritime Organization (IMO). Regarding the synchronization of the VDE-terrestrial (VDE-TER) communication carrier frequency, using the special structure of the training sequence in a VDE message frame, we propose a novel carrier frequency offset (CFO) estimation algorithm based on the phase difference between the Barker code and its inverse code. We derive the principle model for the Barker code and anti-Barker code algorithm and analyze the frequency offset estimation performance of the proposed algorithm with those of the Fitz, L&R and M&M algorithms. We also present a real-world experiment conducted using a software-defined radio (SDR) platform. The results show that in the VDE-TER channel, the double Barker code (DBC) carrier frequency offset estimation algorithm has good estimation performance and estimations range within the visible range.

Frequency Synchronization With Beamforming Network Optimization for Uplink Massive MIMO Systems

In this correspondence, we develop a novel frequency synchronization scheme for multiuser orthogonal frequency division multiplexing (OFDM) uplink with massive uniform linear array (ULA) at base station (BS). With proper user scheduling strategy, the directions of arrival (DOAs) of all users are considered to be disjunct. Our proposed method performs multiuser interference (MUI) cancellation through beamforming network, after which the multiuser OFDM uplink model can be decomposed into a set of parallel single user models. Then, carrier frequency offset (CFO) estimation for each user can be performed individually. The beamforming network is further optimized to minimize the mean square error (MSE) of the CFO estimation. The provided numerical results validate the proposed studies.

Efficient carrier frequency offset estimation algorithm for generalized frequency division multiplexing systems

This paper presents an efficient carrier frequency offset (CFO) estimation algorithm for generalized frequency division multiplexing (GFDM) systems. The proposed algorithm formats the preamble into multiple identical subsymbols, and then decimates the received signal in the time domain to estimate the fractional CFO through array signal processing. After compensating for the fractional CFO, the proposed algorithm uses a low-complexity searching process to estimate the integer CFO in a maximum likelihood. The proposed algorithm presents some distinctive features: (1) it provides the full range of frequency acquisition region (FAR), which was considerably larger than that provided by many existing algorithms whose FAR was limited to one subcarrier spacing; (2) the proposed algorithm can use a single subsymbol to estimate the integer CFO by using the searching process of stepsize 1, significantly reducing its computational complexity; and (3) the decimation of the received signal ensures the proposed algorithm a power gain in fractional CFO estimation, resulting in an improved estimation accuracy compared with conventional algorithms.

On the Effects of Channel Sparsity on Joint Estimators in Aeronautical Telemetry

Minimum mean-squared error (MMSE) equalizers are a viable solution to mitigate the frequency selectivity of the aeronautical telemetry channel. Because the MMSE equalizer filter coefficients are a function of the carrier frequency offset (CFO), the equivalent discrete-time channel, and the noise variance, reliable estimates of those parameters are required. The CFO is due primarily to the high velocity of the airborne transmitter. Because the equivalent discrete-time channel in aeronautical telemetry is sparse, the performance of the joint estimator based on a sparse channel estimator is superior to the performance of a traditional nonsparse maximum-likelihood-inspired joint estimator. The improvement is seen in lower estimator error variances for the parameters, particularly for the CFO and channel estimate, and in the postequalizer bit-error rate. Simulation results demonstrate that the postequalizer bit-error rate using the sparse estimator is almost as good as that using ideal estimators.

Joint low‐complexity equalization and CFO estimation and compensation for UWA‐OFDM communication systems

SummaryFrequency synchronization has a great importance in preserving the performance of the underwater acoustic (UWA) orthogonal frequency division multiplexing (OFDM) systems. The carrier frequency offset (CFO) estimation can be blind or data‐aided. In this paper, the Zadoff‐Chu (ZC) sequences are used for OFDM synchronization in UWA communications, and they are compared with different data‐aided algorithms. We propose a low‐complexity algorithm for CFO estimation based on ZC sequences. Also, a joint equalization and CFO compensation scheme for UWA‐OFDM communication systems is presented. Simulation results demonstrate that the proposed CFO estimation algorithm allows estimation of the CFO accurately with a simple implementation in comparison with the traditional schemes. Also, the performance of the UWA‐OFDM system can be preserved in the presence of frequency offsets.

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.

Self-improved grey wolf optimization for estimating carrier frequency offset in SCM-OFDM systems

PurposeThe purpose of this paper is to demonstrate a proficiency for accomplishing optimal CFO and keep down the error among the received and transmitted signal. Orthogonal frequency-division multiplexing (OFDM) is considered as an attractive modulation scheme that could be adopted in wireless communication systems owing to its reliability in opposition to multipath interruptions under different subchannels. Carrier frequency offset (CFO) establishes inter-carrier interference that devastates the orthogonality between the subcarriers and fluctuates the preferred signal and minimizes the effectual signal-to-noise ratio (SNR). This results in corrupted system performance. For sustaining the subcarriers’ orthogonality, timing errors and CFOs have to be approximated and sufficiently compensated for. Single carrier modulation (SCM) is a major feature for efficient OFDM system.Design/methodology/approachThis paper introduces a novel superposition coded modulation-orthogonal frequency-division multiplexing (SCM-OFDM) system with optimal CFO estimation using advanced optimization algorithm. The effectiveness of SCM-OFDM is validated by correlating the transmitted and received signal. Hence, the primary objective of the current research work is to reduce the error among the transmitted and received signal. The received signal involves CFO, which has to be tuned properly to get the signal as closest as possible with transmitted signal. The optimization or tuning of CFO is done by improved grey wolf optimization (GWO) called GWO with self-adaptiveness (GWO-SA). Further, it carries the performance comparison of proposed model with state-of-the-art models with the analysis on bit error rate (BER) and mean square error (MSE), thus validating the system’s performance.FindingsFrom the analysis, BER of the proposed and conventional schemes for CFO at 0.25 was determined, where the adopted scheme at 10th SNR was 99.6 per cent better than maximum likelihood, 99.6 per cent better than least mean square (LMS), 99.3 per cent better than particle swarm optimization (PSO), 75 per cent better than genetic algorithm (GA) and 25 per cent better than GWO algorithms. Moreover, MSE at 1st SNR, the proposed GWO-SA scheme, is 4.62 per cent better than LMS, 60.1 per cent better than PSO, 37.82 better than GA and 67.85 per cent better than GWO algorithms. Hence, it is confirmed that the performance of SCM-OFDM system with GWO-SA-based CFO estimation outperformed the state-of-the-art techniques.Originality/valueThis paper presents a technique for attaining optimal CFO and to minimize the error among the received and transmitted signal. This is the first work that uses GWO-SA for attaining optimal CFO.

Successive Interference Cancellation-Based Weighted Least-Squares Estimation of Carrier and Sampling Frequency Offsets for WLANs

A successive interference cancellation (SIC)-based weighted least-squares (WLS) estimation for the carrier frequency offset (CFO) and the sampling frequency offset (SFO) is presented for wireless local area networks (WLANs) based on orthogonal frequency division multiplexing (OFDM). The proposed SIC-based WLS performs the estimation by exploiting the phase rotation in the frequency domain caused by the CFO and the SFO. SIC-based WLS estimates the CFO and the SFO successively instead of by traditional simultaneous estimation. The estimation of CFO based on the Taylor series is performed first, and then the WLS estimation of SFO based on successive cancellation of the CFO is carried out. The simulation results show that the SIC-based WLS can estimate the CFO and the SFO effectively. Compared to the WLS scheme, a performance improvement of more than 0.6 dB is achieved by SIC-based WLS, and nearly 10 percent of the complexity is reduced.

A Novel Low-Complexity Estimation of Sampling and Carrier Frequency Offsets in OFDM Communications

This paper presents a novel low-complexity sequential, blind, pilot-assisted estimator for the sampling frequency offset (SFO) and the carrier frequency offset (CFO), for orthogonal frequency-division multiplexing (OFDM) communications. The proposed algorithm processes the received subcarriers to obtain a cost function which depends only on a single unknown parameter at a time, either the SFO or the CFO, as well as on a specifically designed auxiliary parameter, while ignoring the noise. Then, by computing the cost function at a few selected values of the auxiliary parameter, an explicit estimator for each unknown parameter is derived, thereby avoiding the need for a search. To the best of our knowledge, this is the first time such a deterministic approach is applied to the joint estimation of the SFO and the CFO. Moreover, the proposed estimator does not require knowledge of the channel coefficients at the pilot subcarriers, and achieves good performance with a relatively small number of pilot symbols, which results in a low computational complexity. Simulation results show that at low computational complexity, there are many scenarios in which the new estimator achieves smaller estimation errors compared to other existing methods.

Carrier frequency offset estimation for FM and symbiotic FM radio data system hybrid signal

The symbiotic FM radio data system (SRDS) is a radio data system that a specially designed OFDM signal co-lives with FM signal, which enables a significantly higher data rate than existing radio data systems. The cyclic prefix of the OFDM symbol has the same length as the OFDM body, which enables the analytic separation of the co-channel OFDM and FM signal at receiver side, utilizing the fact that the OFDM body and prefix is equal. In this work, we show that the OFDM body and prefix cannot be viewed as equal when there is sufficient carrier frequency offset (CFO). Thus, we propose a two-step CFO estimation algorithm for FM and SRDS hybrid signal. The first step estimates the coarse CFO by exploring the characteristics of the FM signal. Once the coarse CFO is removed, the residual CFO is small enough for FM and OFDM separation. The second step fine estimates CFO from the OFDM-only signal using its repeated PN structure after the separation. Detailed mathematical equations are formulated and simulation results are given. The results show that the proposed algorithm works fine with the simulation setup and has a final residual CFO less than 3.9Hz.

Implementation-Friendly and Energy-Efficient Symbol-by-Symbol Detection Scheme for IEEE 802.15.4 O-QPSK Receivers

In this article, the noncoherent detection scheme for the receiver in wireless sensor nodes is discussed. That is, an implementation-friendly and energy-efficient symbol-by-symbol detection scheme for IEEE 802.15.4 offset-quadrature phase shift keying (O-QPSK) receivers is investigated under both pure additive white Gaussian noise (AWGN) channel and fading channel. Specifically, the residual carrier frequency offset (CFO) of the chip sample is estimated and compensated with the aid of the preamble; then, the standard noncoherent detection scheme with perfectly known CFO is directly configured. The corresponding simulation results show that only 4 preamble symbols is sufficient for accurate CFO estimation. Compared with the conventional noncoherent detector, the average running time per data packet of our enhanced detector is only 0.17 times of the former; meanwhile, at the packet error rate of 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , our enhanced detector can obtain 2.2 dB gains in the (32, 4) direct sequence spread spectrum system. A more reasonable trade-off between complexity and reliability is thus achieved for energy-saving and maximum service life in wireless sensor networks (WSNs).

Double-Sequence Frequency Synchronization for Wideband Millimeter-Wave Systems With Few-Bit ADCs

In this paper, we propose and evaluate a novel double-sequence low-resolution frequency synchronization method in millimeter-wave (mmWave) systems. In our system model, the base station uses analog beams to send the synchronization signal with infinite-resolution digital-to-analog converters. The user equipment employs a fully digital front end to detect the synchronization signal with low-resolution analog-to-digital converters (ADCs). The key ingredient of the proposed method is the custom designed synchronization sequence pairs, from which there exists an invertible function (a ratio metric) of the carrier frequency offset (CFO) to be estimated. We use numerical examples to show that the ratio metric is robust to the quantization distortion. To implement our proposed method in practice, we propose to optimize the double-sequence design parameters such that: (i) for each individual user, the impact of the quantization distortion on the CFO estimation accuracy is minimized, and (ii) the resulting frequency range of estimation can capture as many users' CFOs as possible. Numerical results reveal that our proposed algorithm can provide a flexible means to estimate CFO in a variety of low-resolution settings.

Efficient CFO Estimation and Compensation Approach in OFDMA for Uplink Mobile WiMax

Synchronization is a complex task in uplink Orthogonal Frequency Division Multiple Access (OFDMA) for Mobile WiMax as each user presents different Carrier Frequency Offsets (CFO). Synchronization begins with the CFO estimation and followed by the compensation of residual CFO present in the received signal. This paper deals with various techniques in time and frequency domains to compensate the effect of CFO on the received signal for different estimation techniques. SImple time domain MultiUser Interference Cancellation (SIMUIC) performs in time domain. It employs delays in the compensation of CFO while synchronizing the last user. Decorrelation Successive Interference Cancellation (DC-SC) and Integrated Estimation and Compensation (IEC) perform in frequency domain for the compensation of CFO. These approaches are more complex for Inter-Carrier Interference (ICI) cancellation. In this paper, we propose a new efficient CFO compensation technique in frequency domain for different estimation methods. This technique is a modified version of IEC for reducing the CFO effect on the received signal with lesser computations. Simulation results show that the modified IEC performs better than SIMUIC, DC-SC and IEC for different estimation methods.

Simplified ML-Based Carrier Frequency Offset and Phase Noise Estimation for CO-OFDM Systems

The coherent optical orthogonal frequency division multiplexing (CO-OFDM) has become prominent among emerging telecommunication techniques and applications. However, carrier frequency offset (CFO) and laser phase noise adversely impact and degrade the performance of the CO-OFDM systems. In this paper, a simplified maximum-likelihood (ML) approach, which eliminates the need for the exhaustive search associated with traditional ML methods, is derived and utilized for the estimation of CFO and laser phase noise in CO-OFDM systems. Furthermore, to obtain an improved performance, the proposed simplified low-complexity ML estimator is uniquely combined with an efficient data-dependent pilot-aided (DD-PA) technique, for the acquisition of both the CFO and the laser phase noise. The performance of the simplified ML-based estimators is compared with existing methods and verified in a 16-ary quadrature amplitude modulation (16-QAM) CO-OFDM system with polarization mode dispersion (PMD), chromatic dispersion (CD) and other polarization dependent losses (PDLs) along the fiber link.

Performance Analysis of Hardware Impairment Aware MMSE Receiver With Channel and CFO Estimation Error Statistics for a Large MIMO-OFDM System

In this treatise, we conceive a hardware impairment aware receiver design of a large multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system, taking into account the error statistics of parameter estimations. The hardware impairment includes the quantization noise, phase noise etc. There are two key contributions in this work. Firstly, consideration of error statistics of residual parameters in the receiver design improves receiver performance. In this work, we propose a linear equalizer based data detector at the receiver for a large MIMO-OFDM system and the parameters of interest are the channel impulse response (CIR) and carrier frequency offset (CFO) along with the hardware impairments awareness both at transmitter and receiver. The design criterion chosen for designing the detector equalizer is based on the linear minimum mean square error (LMMSE) one and it will consider the statistics of estimation error and hardware impairments. However, in any practical communication system, the availability of statistics for these parameter errors is inconceivable. Hence, we propose further to consider the Bayesian Crammer Rao lower bound (BCRLB) as the variance of the error with the key assumption of zero mean. Numerical simulation demonstrates that, consideration of such statistics attains an SNR gain of $2$ dB with respect to a fixed symbol error ratio (SER). It also shows that the SER performance with BCRLB matches very closely with the case, when exact error variance is used. For the second contribution, an extensive theoretical analysis has been performed to evaluate the MSE improvement of data detection with the LMMSE criterion with parameter estimation error statistics consideration in the large MIMO-OFDM system. This theoretical analysis of improved MSE has been conceived with the usage of large dimension random matrix theory and a closed-form expression has been obtained. Numerical simulation demonstrates that the theoretical analysis for MSE improvement matches closely with the simulated one.