Sampling Frequency Offset Research Articles

Symbol synchronization and sampling frequency synchronization techniques in real-time DDO-OFDM systems

In this paper, we propose and experimentally demonstrate a symbol synchronization and sampling frequency synchronization techniques in real-time direct-detection optical orthogonal frequency division multiplexing (DDO-OFDM) system, over 100-km standard single mode fiber (SSMF) using a cost-effective directly modulated distributed feedback (DFB) laser. The experiment results show that the proposed symbol synchronization based on training sequence (TS) has a low complexity and high accuracy even at a sampling frequency offset (SFO) of 5000-ppm. Meanwhile, the proposed pilot-assisted sampling frequency synchronization between digital-to-analog converter (DAC) and analog-to-digital converter (ADC) is capable of estimating SFOs with an accuracy of <±5-ppm for initial SFOs as large as 1000-ppm. Moreover, the sampling frequency synchronization technique can also compensate SFO effects within a small residual SFO caused by deviation of SFO estimation and low-precision or unstable clock source. The two synchronization techniques are suitable for high-speed DDO-OFDM transmission systems.

Pilot-aided sampling frequency offset estimation and compensation using DSP technique in DD-OOFDM systems

To improve the outage performance of an optical orthogonal frequency-division multiplexing (OFDM) system under the frequency offset between the sampling clocks in the transmitter and receiver, a pilot-aided sampling frequency offset (SFO) estimation and compensation scheme for the optical OFDM system with intensity-modulation and direct-detection (DD-OOFDM) is experimentally demonstrated. The experimental and simulated results show that the scheme can work effectively even with large sampling frequency offsets. In addition, it can achieve a good bit error rate (BER) performance without the sampling clock frequency synchronization in the receiver.

A Low-Complexity ML Estimator for Carrier and Sampling Frequency Offsets in OFDM Systems

This letter considers the joint acquisition of the carrier frequency offset (CFO) and sampling frequency offset (SFO) in OFDM systems using two long training symbols in the preamble. Conventional maximum-likelihood (ML) methods require a two-dimensional exhaustive search. To overcome this problem, a low-complexity closed-form ML estimator is proposed. It is shown that the CFO can be solved in closed-form. Then we develop an approximate ML estimation algorithm for the SFO by taking the second-order Taylor series expansion. Simulation results show that the proposed algorithm achieves almost the same performance as existing ML methods, but no exhaustive search is needed.

An Improved Joint Detection of Frame, Integer Frequency Offset, and Spectral Inversion for Digital Radio Mondiale Plus

In digital radio broadcasting systems, long delays are incurred in service start time when tuning to a particular frequency because several synchronization steps, such as symbol timing synchronization, frame synchronization, and carrier frequency offset and sampling frequency offset compensation are necessary. Therefore, the operation of the synchronization blocks causes delays ranging from several hundred milliseconds to a few seconds until the start of the radio service after frequency tuning. Furthermore, if spectrum inversed signals are transmitted in digital radio broadcasting systems, the receivers are unable to decode them, even though most receivers can demodulate the spectral inversed signals in analog radio broadcasting systems. Accordingly, fast synchronization techniques and a method for spectral inversion detection are required in digital radio broadcasting systems that are to replace the analog radio systems. This paper presents a joint detection method of frame, integer carrier frequency offset, and spectrum inversion for DRM Plus digital broadcasting systems. The proposed scheme can detect the frame and determine whether the signal is normal or spectral inversed without any carrier frequency offset and sampling frequency offset compensation, enabling fast frame synchronization. The proposed method shows outstanding performance in environments where symbol timing offsets and sampling frequency offsets exist.

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.

Performance Evaluation of Synchronization Method for Reducing the Overall Synchronization Time in Digital Radio Mondiale Receivers

In this paper, we present a comparative performance evaluation of the sampling frequency synchronization method that eliminates the initial sampling frequency offset (SFO) to reduce the overall synchronization time in Digital Radio Mondiale (DRM) receivers. To satisfy the advanced synchronization performance requirements of DRM receivers, we introduce the conventional DRM synchronization method (Method 1) and another method (Method 2), which does not perform the initial sampling frequency synchronization in the conventional synchronization method (both methods are mentioned later in this paper). To demonstrate the effectiveness of the performance evaluation, analytical expressions for frame detection are derived and simulations are provided. Based on the simulations and numerical analysis, our result shows that Method 2, with a negligible performance loss, is robust to the effects of the initial sampling frequency synchronization even if SFO is present in the DRM signal. In addition, we verify that the inter-cell differential correlation used between reference cells is robust to the effect of the initial SFO.

Analysis on the Compensation for the Demodulated Phase Errors Caused by Sampling Frequency Offset in OFDM Systems

Due to the excellent spectral efficiency and inherent resistance to multipath fading effect, OFDM has been widely adopted in Digital video broadcasting-terrestrial (DVB-T). However, as a multi-carrier transmission technique, there is sampling frequency offset (SFO) in the OFDM systems, which will seriously degrade performance of demodulation by resulting in interference between adjacent sub-carriers (ICI) and demodulated phase deviation. In this paper, to achieve correction of phase errors and solve the problem that OFDM signals cannot be correctly demodulated for complex constellation when SFO exists, we propose an adaptive sampling frequency tracking and demodulated phase error compensation scheme based on synchronization symbol information of OFDM signals. Numerical results supported by simulations are provided to show that the proposed scheme in an ADSL environment indicate great performance improvement of demodulation.

Reduction of Intercarrier Interference Based on Window Shaping in OFDM RoF Systems

In this letter, we theoretically investigate the intercarrier interference (ICI) that is induced by carrier frequency offset (CFO) and sampling frequency offset (SFO) for orthogonal frequency division multiplexing (OFDM) radio-over-fiber (RoF) system. A window shaping technique in the time domain at the OFDM receiver is presented, which can improve the tolerance to the frequency offset and reduce ICI. Compared with the conventional demodulation scheme, the simulation shows that the utilization of the window shaping method can increase the resistance to residual CFO to about eight times at power penalty of 2 dB. It can also improve the tolerance to SFO. A 4.65 Gb/s 4QAM OFDM RoF system over 40 km fiber link and 60 GHz wireless link is demonstrated experimentally. The bit error rate (BER) performance of the scheme is measured, with and without the proposed window shaping method, respectively. About 0.9 dB improvement of the required receiver power at a BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> shows the feasibility of the window shaping method for ICI reduction in OFDM RoF system.

MSE Analysis of Sampling Frequency Offset Estimation Scheme for OFDM-Based Digital Radio Mondiale (DRM) Systems

A precise estimation and compensation of sampling frequency offset (SFO) is an important issue in an orthogonal frequency division multiplexing (OFDM) system because of its time-variant behavior. T...

Iterative Sampling Frequency Offset Estimation for MB-OFDM UWB Systems With Long Transmission Packet

A multiband orthogonal frequency-division multiplexing (MB-OFDM) system, which is one of the effective modulation techniques that have been adopted for high-speed ultrawideband (UWB) systems, is very sensitive to sampling frequency offset (SFO) due to the mismatch between local oscillators at the transmitter and the receiver. In this paper, we propose an iterative SFO estimation method for a high-data-rate MB-OFDM UWB system to improve its SFO estimation accuracy in the case of a long transmission packet. The proposed method is an iterative process of 2-D SFO estimation across pilot subcarriers and consecutive OFDM symbols, together with joint channel estimation. Furthermore, we derive the Cramer-Rao lower bound (CRLB) for the proposed SFO estimation method. This CRLB can be used as a guide for algorithm design and to explore the theoretical limit. Our performance analysis and simulation results show that the proposed iterative SFO estimation method is both practical and effective.

A Subspace-Based Two-Way Ranging System Using a Chirp Spread Spectrum Modem, Robust to Frequency Offset

Herein, we propose and implement a subspace-based two-way ranging system for high resolution indoor ranging that is robust to frequency offset. Due to the frequency offset between wireless nodes, issues about sampling frequency offset (SFO) and carrier frequency offset (CFO) arise in range estimation. Although the problem of SFO is resolved by adopting the symmetric double-sided two-way ranging (SDS-TWR) protocol, the CFO of the received signals impacts the time-of-arrival (TOA) estimation, obtained by conventional subspace-based algorithms such as ESPRIT and MUSIC. Nevertheless, the CFO issue has not been considered with subspace-based TOA estimation algorithms.Our proposed subspace-based algorithm, developed for the robust TOA estimation to CFO, is based on the chirp spread spectrum (CSS) signals. Our subspace-based ranging system is implemented in FPGA with CSS modem using a hardware/software co-design methodology. Simulations and experimental results show that the proposed method can achieve robust ranging between the CSS nodes in an indoor environment with frequency offset.

DRM 시스템에서 초기 샘플링 주파수 옵셋의 영향에 강인한 고속 동기화 방식

본 논문에서는 DRM(Digital Radio Mondiale) 수신기를 위한 고속 동기화 방식을 제안한다. 제안하는 방식에서는 DRM 수신기에서 요구하는 고속 수신 성능을 만족시키기 위해 기존 동기화 방식에서 수행하는 초기 샘플링 주파수 동기과정을 생략하고 정수배 주파수(Integer Frequency) 동기 및 프레임(Frame) 검출을 수행한 후, 추적(Tracking) 과정에서 샘플링 주파수 동기를 수행한다. 샘플링 주파수 옵셋(Sampling Frequency Offset)이 포함된 OFDM(Orthogonal Frequency Division Multiplexing) 복조 심볼로부터 프레임 옵셋을 추정하기 위해 참조셀(Reference cell)에 상관 기반 추정 알고리즘을 적용하고 그 성능을 분석하여 샘플링 주파수 옵셋에 강한 알고리즘을 선정한다. 제안된 구조는 DRM 수신 신호에 샘플링 주파수 옵셋이 존재하는 경우에도 초기 샘플링 주파수 동기화 과정에 소요되는 시간을 단축할 수 있으며, 참조셀간 차동 상관(Inter-cell Differential Correlation) 기법을 사용할 경우 초기 샘플링 주파수 옵셋의 영향에 강인함을 프레임 검출 모의실험을 통해 확인한다. In this paper, we propose a high-speed synchronization method for Digital Radio Mondiale (DRM) receivers. In order to satisfy the high-speed synchronization requirement of DRM receivers, the proposed method eliminate the initial sampling frequency synchronization process in conventional synchronization methods. In the proposed method, sampling frequency tracking is performed after integer frequency synchronization and frame synchronization. Different correlation algorithms are applied to detect the first frame of the Orthogonal Frequency Division Multiplexing (OFDM) demodulation symbol with sampling frequency offset (SFO). A frame detection algorithm that is robust to SFO is selected based on the performance analysis and simulation. Simulation results show that the proposed method reduces the time spent for initial sampling frequency synchronization even if SFO is present in the DRM signal. In addition, it is verify that inter-cell differential correlation used between reference cells is roubst to the effect of initial SFO.

A Frequency Offset Estimation and Compensation Scheme for MB-OFDM UWB Modem

We present a carrier and sampling frequency offset estimation and compensation scheme for a multi-band orthogonal frequency division multiplexing (MB-OFDM) ultra-wideband (UWB) modem. We first perform initial carrier frequency offset (CFO) estimation and compensation during the preamble period, and then conduct the estimation and compensation of the residual CFO and sampling frequency offset (SFO) during the payload period. The proposed design scheme reduces the logic gate count of the frequency offset compensation block by about 10%, while it gives almost the same performance at the packet error rate (PER) of 10-4 in the CM1 channel. The frequency offset estimation and compensation block is implemented using 90nm CMOS technology and tested.

Comparison of Passband and Baseband Transmission Schemes for Power-Line Communication OFDM Systems

Passband and baseband transmissions are two competing transmission schemes for power-line communication (PLC) orthogonal frequency-division multiplexing (OFDM) systems. The advantage of the former is that it is simpler to implement for digital circuits as it needs the half-size inverse fast Fourier transform/fast Fourier transform. As will be shown, there are synchronization errors, such as carrier frequency offset (CFO) and sampling frequency offset (SFO) that degrade the performance of passband transmission. On the other hand, baseband transmission is expected to be less complex in an analog circuit implementation and to be affected by only SFO, since it does not need up-down conversion. Both CFO and SFO are caused by the frequency errors of the reference clocks in the transmitter and receiver. In this paper, signal-to-interference-plus-noise ratios and capacities are considered in comparing the effects of synchronization errors in passband and baseband transmissions. The results indicate that the performance degradation of passband transmission is almost the same as that of baseband transmission in the presence of synchronization errors.

Improved frequency offset estimation scheme for UWB systems with cyclic delay diversity

This paper proposes an improved pilot-based algorithm for joint estimation of carrier frequency offset (CFO) and sampling frequency offset (SFO) in ultrawideband orthogonal frequency division multiplexing (UWB-OFDM) systems with cyclic delay diversity. By proper selection of cyclic delay time and a frequency band with maximum channel power, a joint estimation of CFO and SFO is derived. Via computer simulation and analysis, the proposed frequency estimator is shown to benefit from the properly selected delay parameter and frequency band.

Joint Maximum Likelihood Estimation of Carrier and Sampling Frequency Offsets for OFDM Systems

In orthogonal frequency-division multiplexing (OFDM) systems, carrier and sampling frequency offsets (CFO and SFO, respectively) can destroy the orthogonality of the subcarriers and degrade the system performance. In this paper, we propose a new joint CFO and SFO maximum-likelihood (ML) estimation scheme extending Moose's CFO estimation scheme using two long training symbols in the frequency-domain (FD). In particular, we derive FD Cramér-Rao bounds (CRBs) for the mean square errors (MSEs) of the CFO and SFO estimation. Simulation results show that the proposed ML scheme exhibits better performance than the other existing methods.

Accurate Pilot-Aided Sampling Frequency Offset Estimation Scheme for DRM Broadcasting Systems

In this paper, we propose an improved sampling frequency offset (SFO) estimation scheme for orthogonal frequency division multiplexing (OFDM) based digital radio mondiale (DRM) broadcasting systems. To demonstrate the performance of the proposed frequency estimator, analytical expression of the mean square error (MSE) is derived and performances are compared with conventional pilot-assisted estimators. Based on the simulation and theoretical analysis, our proposed estimator shows an improved estimation performance compared to the conventional estimators.

Improved and low-complexity sampling frequency offset estimation scheme for DRM systems

In this letter, an improved sampling frequency offset (SFO) estimation scheme is proposed for an orthogonal frequency division multiplexing based digital radio mondiale (DRM) system. An analytical closed-form expression of the mean square error is derived for the proposed SFO synchronization scheme. The new scheme is demonstrated and verified under simulations carried out within the framework of DRM systems.

Sampling Frequency Offset Estimation for Pilot-Aided OFDM Systems in Mobile Environment

In orthogonal frequency division multiplexing (OFDM) systems, most of the conventional sampling frequency offset (SFO) estimation methods work under the assumption of time-invariant or slow time-variant channels. In mobile environment, the time-variant channel significantly degrades the accuracy of SFO estimation. To solve the problem, we first analyze the properties of time-variant channels. If terminal moves within some tens of the wavelength of radio frequency (RF) signal, channel path delay almost remains unchanged. For most practical OFDM systems, our analysis indicates that channel path delay can be regarded as unchanged during the interval of some tens of OFDM symbols in time-variant channels. Based on the analysis, we propose a novel SFO estimation method for pilot-aided OFDM systems. Different from the conventional methods, the proposed method estimates SFO by detecting the variation of the symbol timing error caused by SFO. The detection is finished by implementing correlation between the channel impulse responses (CIRs) estimated by different OFDM symbols. Performance of the proposed method is simulated and compared with that of two conventional post-FFT methods. Numerical results show that, the proposed SFO estimation method performs better than the conventional methods not only in time-variant channels, but also at low SNRs and large residual carrier frequency offsets (CFOs).

Fine carrier and sampling frequency synchronization in OFDM systems

This paper investigates the joint pilot-assisted estimation of the residual carrier frequency offset (RCFO) and sampling frequency offset (SFO) in an orthogonal frequency division multiplexing (OFDM) system. As it is known, the exact maximum-likelihood (ML) solution to this problem involves a bidimensional grid-search that cannot be pursued in practice. After introducing an enlarged set of auxiliary unknown parameters, however, the RCFO and SFO recovery tasks can be decoupled and the bidimensional search is thus replaced with a simpler mono-dimensional search. This results into an estimation algorithm of reasonable complexity which is suitable for practical implementation. To further reduce the processing load, we also present an alternative scheme yielding frequency estimates in closed-form. Numerical simulations indicate that the proposed methods outperform existing estimators available in the literature in terms of both estimation accuracy and error-rate performance.