Sampling Clock Frequency Offset Research Articles

Sampling frequency offset compensation in non-Hermitian symmetric optical OFDM

Non-Hermitian symmetric optical orthogonal frequency-division multiplexing (NHS-OFDM) can make the best use of fast Fourier transform (FFT) functions. Hence, hardware implementation of NHS-OFDM has a low complexity when maintaining the same bandwidth granularity. And the similar peak-to-average power ratio (PAPR) and bit error rate (BER) performances can be obtained in comparison with the conventional Hermitian symmetric optical orthogonal frequency-division multiplexing (HS-OFDM). However, the sampling clock frequency offset (SFO) between digital-to-analog converters (DAC) and analog-to-digital converters (ADC) always exists in asynchronous HS/NHS-OFDM systems. It will degrade BER performance seriously without compensation. In this paper, the mechanism of SFO in NHS-OFDM is analyzed and discussed. The inter-symbol differential detection (ISDD) with residual phase compensation (RPC) is applied for SFO compensation (SFOC) in the NHS-OFDM system. Moreover, enhanced residual phase estimation (ERPE) is also proposed to improve the tracking speed and accuracy of residual phase estimation. The optimal FFT size for NHS-OFDM is first identified in both additive white Gaussian noise (AWGN) channel and a 20-km standard single-mode fiber (SSMF) link. The BER performance of the ISDD with RPC/enhanced RPC (ERPC) enabled HS/NHS-OFDM is investigated by means of both numerical simulation and offline experiments. With the help of the ISDD with RPC/ERPC methods, the SFO can be compensated. Also, a slight improvement in BER performance can be achieved with the ERPC method compared to the RPC method. And the BER performance penalties can be ignored in the presence of SFO up to 200 ppm.

Compensation of DAC clock leakage in short-reach DMT system in the presence of sampling frequency offset

Digital-to-analog converter (DAC) with a high sampling-rate has been extensively applied in optical discrete multi-tone (DMT) system for high data-rate signal generation. However, there exist various imperfections during the manufacturing of DAC when its sampling rate is tens of giga-samples per second and beyond. Clock tone leakage (CTL) is a kind of interference in high-speed DACs. One or more data subcarriers of the generated DMT signal may suffer from additional clock tones, which may degrade the overall bit error rate (BER) performance of the optical DMT system. In the literature, several schemes have been proposed to compensate CTL for short-reach applications. Nevertheless, these compensation schemes may not work effectively in the asynchronous DMT system in the presence of sampling clock frequency offset (SFO) between DAC and analog-to-digital converter. In this paper, an effective CTL compensation (CLC) with the digital interpolation scheme is proposed and experimentally investigated in a short-reach 100G DMT transmission system with direct detection. The results indicate that the proposed CLC with a fourth-order interpolation (FOI) scheme can effectively compensate the CTL in the presence of an SFO up to ±1000 parts per million (ppm). What is more, we discuss the impact of SFO estimation error on the BER performance of the 64-ary quadrature amplitude modulation-encoded DMT system. It shows that the BER performance penalty can be ignored by shortening the DMT frame even though the estimation error is up to 2 ppm.

Demonstration of 2.97-Gb/s video signal transmissions in DML-based IM-DDO-OFDM systems

Abstract To further investigate the feasibility of the digital signal processing (DSP) algorithms (e.g., symbol timing synchronization, channel estimation and equalization, and sampling clock frequency offset (SCFO) estimation and compensation) for real-time optical orthogonal frequency-division multiplexing (OFDM) system, 2.97-Gb/s real-time high-definition video signal parallel transmission is experimentally demonstrated in OFDM-based short-reach intensity-modulated direct-detection (IM-DD) systems. The experimental results show that, in the presence of ∼12 ppm SCFO between transmitter and receiver, the adaptively modulated OFDM signal transmission over 20 km standard single-mode fiber with an error bit rate less than 1 × 10−9 can be achieved by using only DSP-based small SCFO estimation and compensation method without utilizing forward error correction technique. To the best of our knowledge, for the first time, we successfully demonstrate that the video signal at a bit rate in excess of 1-Gb/s transmission in a simple real-valued inverse fast Fourier transform and fast Fourier transform based IM-DD optical OFDM system employing a directly modulated laser.

相干光OFDM系统中基于导频辅助的采样时钟频率偏差估计和补偿算法

相干光OFDM系统中基于导频辅助的采样时钟频率偏差估计和补偿算法

A novel sampling synchronization scheme for OFDM-based system with unified reference clock

Due to the constraint of cost and size for mobile wireless communication terminals, many orthogonal frequency division multiplexing (OFDM)-based systems required the same crystal driving the sampling and the channel frequencies, which leads to the challenge of a more comprehensive sampling clock synchronization scheme needed. In this article, the effect of sampling clock error on the system performance was analyzed by dividing it into sampling clock frequency offset (SFO) and sampling timing error (STE) firstly. After that, we proposed a two-stage scheme of sampling clock synchronization based on theoretical derivation: the preliminary SFO was jointly acquired with the carrier frequency offset by using the improved preamble-aided algorithm firstly, the timing drift resulted from residual SFO and STE was tracked based on a phase looped lock in the second stage. The deviation properties of the estimation were achieved theoretically, which reveals that both the estimating variances of SFO and timing drift are in inverse proportion to signal-to-noise ratio and grow linearly by the number of total subcarrier. The results of the simulation show that the proposed synchronization scheme can introduce preferable tracking and robust synchronizing performance for this kind of OFDM-based system.

Implementation and Co-Simulation of Hybrid Pilot-Aided Channel Estimation With Decision Feedback Equalizer for OFDM Systems

This paper introduces novel hybrid pilot-aided channel estimation with decision feedback equalizer(DFE) for OFDM systems and its corresponding hardware co-simulation platform. This pilot-aided channel estimation algorithm consists of two parts: coarse estimation and fine estimation. In the coarse estimation, the combined classical channel estimation methods including carrier frequency offset (CFO) and channel impulse response (CIR) estimation are used. Based on the received training sequence and pilot tones in the frequency domain, the major CFO, sampling clock frequency offset (SFO) and CIR effect coefficients are derived. In the fine estimation, the pilot-aided polynomial interpolation estimation combined with a new decision feedback equalizer scheme based on minimum mean squared error (MMSE) criteria is proposed to reduce the residual effect caused by imperfect CIR equalizer, SFO and CFO. At the same time, for the purpose of speeding up the whole development and verification process, a new architecture of co-simulation platform which combines software and hardware is introduced. The simulation results on the co-simulation platform indicate that the proposed hybrid channel estimation scheme can enhance the receiver performance by 6 dB in terms of error vector magnitude (EVM) over large ranges of CFO and SFO and BER performance by 7 dB for SNR range over 15 dB.

Spectrum Sensing of OFDM Waveforms Using Embedded Pilots in the Presence of Impairments

Given the widespread deployment of orthogonal frequency-division multiplexing (OFDM)-based wireless systems, reliable spectrum sensing of OFDM waveforms is of great interest for future cognitive radio systems. Use of the embedded pilots of OFDM signals has been extensively studied in the literature. Such techniques rely on having correlated pilots to improve detection sensitivity. However, the impact of synchronization in accuracy and implementation impairments has not been thoroughly studied in the literature. This paper discusses the pilot-aided cyclostationary detection (PACSD) method as a practical structure to detect OFDM signals under asynchronous conditions. The presented analysis quantifies the performance of a detection method without exhaustive searching, compared with an ideal system. This paper goes on to investigate the impact of radio-frequency (RF) front-end nonidealities on the overall performance of the PACSD. We develop closed-form expressions for performance loss in the presence of in-phase/quadrature (I/Q) imbalance, carrier frequency offset, phase noise, and sampling clock frequency offset (SCFO). These results show that the most detrimental impairment is that of SCFO, where a 20-ppm offset can result in a 9-dB performance loss. We then present a modified detector structure to compensate for this huge loss. Analysis and simulation show that, using the modified technique, we can get within 2 dB of the ideal no-offset case.

Low-overhead sampling clock frequency synchronization for OFDM-based DRM systems

In this paper, an improved and low-complexity sampling clock frequency offset (SFO) estimation scheme is proposed for orthogonal frequency division multiplexing (OFDM) based digital radio mondiale(DRM) systems. In order to devise the low-complexity SFO estimator, some modifications on the conventional estimator are highlighted with an emphasis on the selection of pilot subcarrier.

An Algorithm Combining Synchronization and Channel Estimation for OFDM Systems

OFDM (Orthogonal Frequency Division Multiplexing) is more and more popular in applications of digital communications because of the effective spectrum and less impacts of multipath fading. However, beside these advantages, OFDM signals are destroyed easily by errors such as CFO (Carrier Frequency Offset), SFO (Sampling Clock Frequency Offset). Thus, it’s necessaryto have robustly offset algorithms to overcome these disadvantages. Studies about OFDM we just examined channel estimation with assumptions that synchronization is perfect, and vice versa. However, they have a close relationship, channel estimation can be restricted if synchronization is bad, and vice versa. This paper presents an algorithm combining synchronization and channel estimation in OFDM systems. The algorithm is compared with other proposed algorithms by simulation. The simulation result of the algorithm combining synchronization and channel estimation is close to that of ideal conditions: perfect channel estimation and synchronization.

Pilot-Assisted Fine Frequency Synchronization for OFDM-Based DVB Receivers

After coarse synchronization in orthogonal frequency division multiplexing (OFDM) systems, there might still be a residual frequency offset (RFO) and a sampling clock frequency offset (SFO), which seriously degrade the performance of the systems. This paper proposes a simple way of jointly estimating the RFO and SFO by using a continual pilot (CP) defined in an OFDM-based digital video broadcasting (DVB) system. For deriving an unbiased joint RFO and SFO estimator in the current DVB system, a CP subset is selected to simultaneously remove the effect of RFO and SFO. We can observe by simulation that the proposed frequency estimator endowed with a properly selected CP subset is unbiased and performs robustly.

RLS-Based Joint Estimation and Tracking of Channel Response, Sampling, and Carrier Frequency Offsets for OFDM

This paper proposes a pilot-aided joint channel estimation and synchronization scheme for burst-mode orthogonal frequency division multiplexing (OFDM) systems. Based on the received signal samples containing pilot tones in the frequency domain, a cost function that includes the carrier frequency offset (CFO), sampling clock frequency offset (SFO) and channel impulse response (CIR) coefficients is formulated and used to develop an accompanying recursive least-squares (RLS) estimation and tracking algorithm. By estimating the channel <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CIR</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">coefficients</i> instead of the channel frequency response in the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">frequency</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">domain</i> , the proposed scheme eliminates the need for an IFFT block while reducing the number of parameters to be estimated, leading to lower complexity without sacrificing performance and convergence speed. Furthermore, a simple maximum-likelihood (ML) scheme based on the use of two long training symbols (in the preamble) is developed for the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">coarse</i> estimation of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">initial</i> CFO and SFO to suppress dominant ICI effects introduced by CFO and SFO and to enhance the performance and convergence of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">fine</i> RLS estimation and tracking. Simulation results demonstrate that, over large ranges of CFO and SFO values, the proposed pilot-aided joint channel estimation and synchronization scheme provides a receiver performance that is remarkably close to the ideal case of perfect channel estimation and synchronization in both AWGN and Rayleigh multipath fading channels.

Joint Effects of Synchronization Errors of OFDM Systems in Doubly-Selective Fading Channels

The majority of existing analyses on synchronization errors consider only partial synchronization error factors. In contrast, this work simultaneously analyzes joint effects of major synchronization errors, including the symbol time offset (STO), carrier frequency offset (CFO), and sampling clock frequency offset (SCFO) of orthogonal frequency-division multiplexing (OFDM) systems in doubly-selective fading channels. Those errors are generally coexisting so that the combined error will seriously degrade the performance of an OFDM receiver by introducing intercarrier interference (ICI) and intersymbol interference (ISI). To assist the design of OFDM receivers, we formulate the theoretical signal-to-interference-and-noise ratio (SINR) due to the combined error effect. As such, by knowing the required SINR of a specific application, all combinations of allowable errors can be derived, and cost-effective algorithms can be easily characterized. By doing so, it is unnecessary to run the time-consuming Monte Carlo simulations, commonly adopted by many conventional designs of synchronization algorithms, in order to know those combined error effects.

An Improved Sampling Frequency Offset Estimator for OFDM-Based Digital Radio Mondiale Systems

After coarse synchronization in orthogonal frequency division multiplexing (OFDM) systems, there might still be a residual frequency offset (RFO) and a sampling clock frequency offset (SFO), which seriously degrade the performance of the systems. This paper suggests a simple way of estimating the SFO in OFDM-based digital radio mondiale (DRM) system. To demonstrate the efficiency of the proposed SFO estimator, comparisons are made with other existing estimators in terms of mean square error (MSE) performance, estimation range, and complexity.

Decision-Directed Fine Synchronization in OFDM Systems

A new decision-directed (DD) synchronization scheme is proposed for joint estimation of carrier frequency offset (CFO) and sampling clock frequency offset (SFO) in orthogonal frequency-division multiplexing (OFDM) systems. By exploiting the hard decisions, we report accurate estimators of residual CFO and small SFO. The performance analysis and simulation results indicate that the proposed novel DD scheme achieves much better performance than the conventional pilot-based schemes in both additive white Gaussian noise and frequency-selective channels.