Sampling Frequency Offset Estimation Research Articles

Sampling Frequency Offset Estimation and Compensation for Asynchronous Optical IMDD FBMC Systems

Sampling Frequency Offset Estimation and Compensation for Asynchronous Optical IMDD FBMC Systems

Hybrid Cramer-Rao Bound on Carrier and Sampling Frequency Offset Estimation for OFDM Systems in Rayleigh Fading Channels

For the carrier frequency offset (CFO) and sampling (clock) frequency offset (SFO) estimation, the hybrid Cramer-Rao bound (HCRB) is developed when the CFO, SFO, information-bearing symbols are deterministic and channel coefficients are random. Both noise and channel coefficients are complex Gaussian. The HCRB is a lower bound on the mean squared estimation error for any unbiased estimator of a parameter. For the HCRB to be applicable, it is necessary for deterministic parameters to be identifiable (uniquely determined). Some necessary identifiability conditions of some deterministic parameters are found and presented. The HCRB is dependent on the initial time instant. The HCRB is used to assess the performances of some existing methods via simulation. Our results demonstrate that even the best performance is still around 10 dB higher than the HCRB. Further effort is needed to develop more accurate methods.

Low-Complexity Sampling Frequency Offset Estimation and Compensation Scheme for OFDM-Based UWOC System

In this paper, a simple sampling frequency offset (SFO) estimation and compensation scheme based on two phase-conjugated pilots is proposed and experimentally demonstrated in an OFDM-based underwater wireless optical communication (UWOC) system. The phase shift is obtained by simple multiplication for phase-conjugated pilots, and the results are averaged to perform more accurate phase estimation. The experimental results show that the estimation offset is limited within ±3 ppm when the SFO ranges from −1000 ppm to +1000 ppm over a 1 m tap water channel. Moreover, with the help of the proposed scheme, up to ±300 ppm SFO can be well-compensated with error vector magnitude (EVM) penalties below 1 dB after 1 m underwater transmission. In addition, the results demonstrate that, compared with the ideal case without SFO, our proposed SFO compensation scheme can provide nearly negligible bit error rate (BER) penalties in saltwater with the highly scattering property.

Comparison of digital interpolation and phase compensation in asynchronous optical DMT transmission systems

The sampling frequency offset (SFO) will induce phase rotation, inter-symbol interference and inter-carrier interference, which may seriously deteriorate the receiver’s performance in asynchronous optical discrete multi-tone (DMT) systems. In this work, a high-accuracy training symbols-based SFO estimation (SFOE) method is studied in both the additive white Gaussian noise channel and short-reach directly-modulated laser-based DMT transmission system. Two general SFO compensation (SFOC) methods, i.e., digital interpolation (DI) and phase compensation (PC), are comparatively investigated through both simulations and experiments. The results indicate that the accuracy of the SFOE method can be within ±0.2 ppm under the SFO range of ±500 ppm and different received optical powers. With the high-accuracy SFOE method, the 4th order DI has the best compensation performance compared to PC, 2nd and 3rd order DI methods. Moreover, the negligible EVM performance penalty compared with the case of SFO-free can also be achieved in an optical DMT transmission system. Besides, the PC-based SFOC method has similar EVM performance to the 4th order DI in the case of small SFO within ±300 ppm.

Simple blind estimation of sampling frequency offset for real-time asynchronous DDO-OFDM system

Sampling frequency offset (SFO) will result in phase rotation, inter-channel interference, and inter-symbol interference, which can seriously degrade the transmission performance of the asynchronous direct-detection optical OFDM (DDO-OFDM). In this paper, we study a simple and overhead-free SFO blind estimation technique in a DDO-OFDM system. The key parameters in this technique are optimized by theoretical analysis, numerical simulation, and experiments. A simplified method of the SFO blind estimation is also proposed to further reduce the hardware implementation complexity. The accuracy and stability of the SFO estimation are experimentally investigated in the real-time asynchronous DDO-OFDM transmission system. The results indicate that the SFO estimation accuracy can be within ±1.5 ppm and shows high stability even at an SFO of up to 200 ppm after 20-km standard single-mode fiber transmission.

Angle Adjustment for Sampling Frequency Offset Estimation of OFDM-Based WLANs

Angle Adjustment for Sampling Frequency Offset Estimation of OFDM-Based WLANs

A Simple Blind Sampling Frequency Offset Estimation Scheme for Short-Reach DD-OFDM Systems

In this paper, a simple and overhead-free sampling frequency offset (SFO) blind estimation and compensation scheme is proposed and experimentally demonstrated in a short-reach direct detection orthogonal frequency division multiplexing (DD-OFDM) transmission system. After extracting subcarriers of OFDM symbols from multiple OFDM frames, the phase shift is obtained by the fourth-power algorithm, and the obtained results are averaged to perform phase estimation. It can achieve high-precision SFO estimation while ensuring low computational complexity as well as high spectral efficiency. The experimental results show that the SFO estimation accuracy using the proposed scheme can be significantly reduced to as low as ±3 ppm when SFO ranges from -1000 ppm to +1000 ppm after 20-km standard single-mode fiber (SSMF) transmission. Moreover, for the QPSK/16QAM encoded DD-OFDM systems aided by the proposed scheme, up to ±200 ppm SFO can be compensated with slight error vector magnitude (EVM) penalties after 20-km SSMF transmission. Furthermore, the results demonstrate that despite the existence of severe SFO, our proposed SFO compensation scheme can offer nearly negligible power penalties in the DD-OFDM system with optical back-to-back (OBTB) and 20-km SSMF transmission as compared to the ideal case without SFO.

Comparison of interpolation-based sampling frequency offset compensation schemes for practical OFDM-VLC systems.

Sampling frequency offset (SFO) is an important issue in the orthogonal frequency-division multiplexing (OFDM)-based visible light communication (VLC) systems with low-cost analog-to-digital or digital-to-analog converters (ADCs/DACs). A digital interpolation or resampling filter can be used to effectively compensate the SFO. In such case, oversampling at the receiver ADC is required to mitigate the aliasing effect due to imperfect DACs and nonlinearity of visible light sources that cause extra frequency components inside/outside the OFDM signal spectrum. The oversampling factor (rate) is mainly determined by the order of the digital interpolation filter and nonlinear VLC links. The design of the OFDM-VLC receiver incorporating the digital interpolation filter is vital as it affects not only the transmission performance but also the complexity of digital signal processing (DSP). To evaluate the feasibility of the digital interpolation-based SFO compensation schemes for cost-sensitive VLC applications, in this paper, a real-time OFDM-VLC receiver incorporating the 2nd/3rd/4th order interpolation filters is experimentally demonstrated. An OFDM frame structure is designed for the synchronization including SFO estimation and compensation, in which the precision and latency of DSP are considered. On the basis of the real-time OFDM-VLC receiver, the comparison in the VLC transmission performance and DSP complexity between different interpolation-based SFO compensation schemes is discussed.

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.

Joint Estimation of Symbol Timing and Sampling Frequency Offset for CDD-OFDM-Based DRM Systems

In this paper, an efficient symbol timing offset (STO) and sampling frequency offset (SFO) estimation scheme is proposed in an orthogonal frequency division multiplexing-based digital radio mondiale system using cyclic delay diversity. By using a periodic nature of channel transfer function, cyclic delay and pilot pattern with a maximum channel power are chosen, which aids to enable a robust estimation of STO and SFO against frequency selectivity of the channel. As a performance measure, the mean squared error of the proposed scheme is theoretically derived and is verified through simulations. By simulation results, the proposed estimation scheme is proved to profit from properly selected cyclic delay and pilot pattern, with a superior performance over the conventional estimation scheme.

Cost-Effective SFO Compensation Scheme Based on TSs for OFDM-PON

A cost-effective sampling frequency offset (SFO) compensation scheme based on a simple training symbol (TS) is proposed and experimentally demonstrated in an orthogonal frequency-division multiplexing passive optical network (OFDM-PON) downlink transmission system. For SFO estimation and compensation, two TSs are used, while one of them is also used for timing synchronization and channel estimation. In addition, the performance of the SFO compensation scheme is investigated using four optical network units (ONUs) and one optical line terminal in an asynchronous intensity-modulated and direct-detection OFDM system. Moreover, the error vector magnitude (EVM) performance with respect to the SFO error is evaluated. The experimental results indicate that the SFO estimation accuracy of the proposed scheme is within ±2 ppm under an SFO ranging from −200 ppm to +200 ppm. When the SFO error is 2 ppm, the EVM penalties for ONU1-ONU4 are all below 1 dB. In addition, aided by the proposed scheme, up to ±200 ppm SFO can be successfully compensated with an EVM penalty under 1 dB.

Experimental demonstration of an adaptive SFO pre-compensation scheme for OFDM-PON

An adaptive sampling frequency offset (SFO) pre-compensation scheme is experimentally demonstrated for orthogonal frequency division multiplexing-based passive optical network (OFDM-PON). Different QAM modulation formats and SFO effects for four optical network units (ONU) are investigated in OFDM-PON. In addition, the length of cyclic prefix (CP)/cyclic suffix (CS) and the number of OFDM symbols per OFDM frame are discussed for the OFDM-PON with the adaptive SFO pre-compensation scheme. And the signal-to-noise ratio (SNR) penalty with respect to SFO estimation offset is also studied. The experimental results show that, the SFO-induced ISI in each ONU can be avoided using the adaptive SFO pre-compensation scheme. After 20 km standard single-mode fiber transmission, the SFO estimation accuracy can be within ±1 ppm in the presence of SFO ranging from -200 to 200 ppm. The SNR penalty for each ONU is below 1 dB under different SFO combinations and different QAM modulation formats.

A Novel Sampling Frequency Offset Estimation Algorithm for OFDM Systems Based on Cyclostationary Properties

Sampling frequency synchronization in orthogonal frequency division multiplexing (OFDM) communications is critical for achieving the full advantages offered by this modulation scheme. In this paper, we propose a novel and efficient, blind, cyclostationarity-based sampling frequency synchronization (CB-SFS) algorithm for estimating the sampling frequency offset (SFO) in OFDM communications by exploiting the relationship between the sampling frequency and the cyclostationary properties of the sampled received signal. The proposed scheme is ignorant of the channel coefficients and does not require pilots. These two properties are rather unique in this context and are not possessed by previous schemes which achieve comparable estimation performance. These properties also make the proposed CB-SFS scheme suitable for a wide range of communications scenarios. The main novelty of the new scheme is the understanding that SFO alters the cycle frequencies at the receiver, yet these frequencies are a priori known for the discrete-time (DT) transmitted signal, as they result from the periodic operation of the DT signal generation scheme at the transmitter. We show that the mismatch between the measured and the expected cycle frequencies is directly related to the SFO. Complexity analysis and numerical simulations are carried out and demonstrate the superiority of the proposed CB-SFS algorithm compared to the existing approaches. It is illustrated that the proposed algorithm can achieve a smaller estimation error at the same complexity order of current algorithms while providing a higher spectral efficiency and robustness to additive stationary noise and to multipath.

Complexity Effective Sampling Frequency Offset Estimation Method for OFDM-Based HomePlug Green PHY Systems

The HomePlug Green PHY (HomePlug GP) specification provides an attractive solution to enable smart grid power line communication (PLC) applications by using robust orthogonal frequency division multiplexing (ROBO) mode. This paper proposes a computationally efficient sampling frequency offset (SFO) estimation technique in the HomePlug GP system without relying on pilot symbols. For this purpose, the proposed estimation scheme utilizes the redundant information contained within the repeat coding in the HomePlug GP ROBO mode, thus eliminating the need of dedicated pilots. Computer simulations are conducted to assess the performance of the proposed SFO estimation scheme and to compare it with the conventional decision-directed (DD) estimation schemes. Simulations indicate that the repeat coded ROBO signals are effectively used for the proposed estimation scheme, which provides an affordable estimation accuracy while reducing the complexity compared to the conventional DD estimation schemes.

Effective Blind Frequency Offset Estimation Scheme for BST-OFDM Based HDTV Broadcast Systems

The integrated services digital broadcasting-terrestrial (ISDB-T) system is designed in order to accommodate high-quality video/audio and multimedia services, using band segmented transmission orthogonal frequency division multiplexing (BST-OFDM) scheme. In the ISDB-T system, the pilot configuration varies depending on whether a segment uses a coherent or differential modulation. Therefore, it is necessary to perform a joint estimation of carrier frequency offset (CFO) and sampling frequency offset (SFO) independent of the segment format in the ISDB-T system. The goal is to complete those synchronization tasks while considering an information-carrying transmission and multiplexing configuration control (TMCC) signal as pilot symbols. It is demonstrated through numerical simulations that the differential BPSK-modulated TMCC information can be efficiently used for a least-squares estimation of CFO and SFO, offering an acceptable performance.

Robust Sampling Frequency Offset Estimation for OFDM over Frequency Selective Fading Channels

Digital radio mondiale (DRM) is a terrestrial radio broadcasting standard to replace existing analogue AM and FM broadcasting, which is based on an orthogonal frequency division multiplexing (OFDM) technique. This paper focuses on the issue of estimating a sampling frequency offset (SFO) in OFDM-based broadcasting systems under frequency selective fading channels. In order to design a robust SFO estimation scheme and to benchmark its performance, the performance of the various conventional SFO estimation schemes is discussed and some improvements on the conventional estimation algorithms are highlighted. The simulation results show that such a design enhances the robustness of the proposed scheme against frequency selective fading.

Improved Artificial Bee Colony Optimization Approach in UWBOFDM for Frequency Offset Estimation

Abstract Orthogonal frequency division multiplexing (OFDM) is considered a crucial and fundamental potential approach because of its significance and role with regards to broadband wireless communication systems. The swarm intelligence optimization approach has been deployed to facilitate frequency offset optimization. An improved artificial bee colony (IABC) based optimization approach is proposed in this work that will enable ultra wideband orthogonal frequency division multiplexing (UWBOFDM) system, which will then be deployed in the carrier frequency offset (CFO) joint estimation and also for sampling frequency offset (SFO). By proper selection of cyclic delay times, correlation existing between adjacent sub carriers, a joint estimation of both CFO and SFO has been derived by employing IABC. The performance of the proposed algorithm is compared with the existing estimators, and this comparison is carried out by MATLAB7.2 simulation, which clearly depicts that the performance of the proposed algorithm is superior in terms of estimation accuracy. The performances are measured in terms of the mean square error (MSE), signal-to-noise ratio (SNR) and normalized mean square error (NMSE). The proposed approach considered for transmitter (t = 4) and receiver (r = 4) with SNR is 30 dB, which achieves MSE of about 0.35 %. Similarly, when (t = 2) and (r = 2), the proposed IABC approach achieves MSE of about 0.4 %, whereas particle swam optimization (PSO) and maximum likelihood estimator (MLE) methods achieve 0.42 and 0.44 % MSE values, respectively. On the other hand, considering (t = 4) and (r = 4) with SNR of 8 dB, the proposed approach achieves NMSE of about 0.0008 %. However, considering (t = 2) and (r = 2), the NMSE values of IABC approach, PSO, and MLE are observed to be 0.002, 0.008, and 0.08 %, respectively, for Channel Model 4 (CM4).

An Efficient Frequency Offset Estimation Scheme for Band Segment Transmission OFDM Systems

The integrated services digital broadcasting-terrestrial (ISDB-T) system is designed to individually configure transmission parameters of each orthogonal frequency division multiplexing (OFDM) segment using band segmented transmission OFDM (BST-OFDM). Since the pilot configuration relies on whether a segment is a differential or coherent modulation, it is important to develop the frequency-offset estimation scheme that conforms to the segment-dependent pilot structure in the BST-OFDM system. This paper proposes an efficient sampling frequency offset (SFO) in the ISDB-T system using BST-OFDM. Extensive simulations are conducted not only to verify the effectiveness of the proposed SFO estimation scheme but also to compare the new scheme with the conventional scheme. It has confirmed that the performance of the proposed SFO estimation algorithm is better than that of the existing algorithm at a reduced computational cost.

Joint statistical framework for the estimation of channel and SFO in OFDM systems

Joint estimation of channel and sampling frequency offset (SFO) in orthogonal frequency division multiplexing (OFDM) systems, using Bayesian framework, is shown in this study. Hybrid Cramér–Rao lower bounds (HCRLBs) for the estimation of SFO together with channel are obtained. The significance of Bayesian approach in the formulation of joint estimator is shown by comparing HCRLB with the corresponding standard CRLB. The authors propose a joint maximum a posteriori (JMAP) algorithm for the estimation of channel and SFO in OFDM, utilising the prior statistical knowledge of channel. To reduce the complexity of JMAP estimator, a modified MAP algorithm, which has no grid searches, is also proposed. Also, they analyse the effect of inaccurate knowledge of channel statistics and signal-to-noise ratio on the estimation accuracy. The estimation methods are analysed by numerical simulations and resultant conclusions validate the better performance of the proposed algorithms when compared with previous algorithms.

Joint Estimation of Carrier and Sampling Frequency Offsets in the Uplink of Multiuser OFDM Systems

This paper considers the joint estimation of the carrier frequency offset (CFO) and sampling frequency offset (SFO) in the uplink of multiuser orthogonal frequency division multiplexing systems, where the received signal at the base station is a mixture of all users’ signals and each user is affected by different frequency offsets. Usually, users’ separation is needed to cope with this challenging multi-parameter estimation problem. Conventional methods adopt the zero-forcing criterion, which will lead to noise enhancement. To overcome this problem, a novel joint CFO and SFO estimation algorithm is proposed. We use the QR decomposition to achieve users’ separation, and the successive interference cancellation technique is employed to estimate the frequency offsets of different users. To further improve the estimation accuracy, the minimum variance unbiased estimator is exploited. Numerical results show that the proposed algorithm offers better performance than existing methods in terms of mean squared error and bit error rate .