Symbol Timing Offset Research Articles

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.

Interference analysis and MUI-cancellation in DCO-OFDMA-based IM/DD systems for VLC

The arresting virtue of Visible Light Communication (VLC) to expedite illumination and communication contemporaneously by exploiting sustainable and energy-efficient illuminating devices like Light Emitting Diodes (LEDs) in an indoor environment, enables it to resemble like a small scale cellular communication network providing data access to multiple users within its vicinity. This emphasizes to assure efficacious synchronization between the transmitter and receiver. Consequently, there is an urge to address the synchronization aspects in optical orthogonal frequency division multiple access (OOFDMA) reconcilable with Intensity Modulated (IM)/Direct Detection (DD) systems for VLC. Moreover, in an uplink scenario the existence of frequency offsets (FO) which emanates due to the existence of Doppler shift and symbol time offset (STO) leads to interference among the users as well induces to interfere with the desired user leading to most detrimental aspect called multiuser interference (MUI). This paper derives the analytical expressions for the deterioration of signal to interference noise ratio (SINR) in DC-Biased optical-OFDMA (DCO-OFDMA) system in view of the existence of STO. Furthermore, a modified version of single user detection (SUD) scheme and certain modified versions of synchronization algorithms to mitigate the effects of STO are illustrated for DCO-OFDMA flexible to be implemented for VLC Networks. In addition, Cramer Rao Lower Bound (CRLB) is derived for FO and STO estimators using joint parameter estimation for the uplink scenario and is verified through simulations.

Preamble Based Timing Offset Estimation and Correction in OFDM Assisted Massive MIMO Systems in the Presence of Inter-User-Interference

This paper presents a new preamble design for correct timing synchronization and symbol time offset (STO) estimation in orthogonal frequency division multiplexing (OFDM) based massive multi input multi output (M-MIMO) systems. The proposed preamble design is more robust and accurate than conventional designs at low signal to noise ratio (SNR). This paper further analyses the effect of inter user interference (IUI) on the estimation and correction. Simulations are performed for single and multi-user cases considering Rayleigh multipath channel model. Mean and standard deviation of the evaluated STO and probability of the correct timing synchronization are considered as the performance parameters. Considering the effect of fading and IUI, result shows that the proposed preamble gives better performance at low SNR.

The impact of out of band emissions: A measurement based performance comparison of UF-OFDM and CP-OFDM

Long Term Evolution (LTE) networks, along with their recent releases, rely on the cyclic prefix orthogonal frequency division multiplexing (CP-OFDM) waveform as the foundation. However, high side lobes, synchronization problems and spectral efficiency constraints of CP-OFDM cannot meet future requirements of cellular network design targets. At this point, universal filtered orthogonal frequency division multiplexing (UF-OFDM) emerges as a strong candidate. Equipped with the advantages of using filters, UF-OFDM provides additional design flexibility to address the fundamental problems of CP-OFDM. In this study, the effect of adjacent channels that causes interference between different frequency channels because of out-of-band (OoB) emissions on UF-OFDM performance is investigated by performing real-time tests on software defined radio (SDR) nodes with multiple reception paths. Frequency domain carrier frequency offset (CFO) and symbol timing offset (STO) correction techniques are used. By the application of the same correction approaches to CP-OFDM, these two waveforms are evaluated in terms of signal to noise ratio (SNR), bit/symbol error rate (BER/SER) and error vector magnitude (EVM). Test results indicate that, based on lower OoB emissions, UF-OFDM uses the spectrum more efficiently and provides better results than CP-OFDM in congested environments.

Symbol Error Probability Analysis of DFrFT-Based OFDM Systems With CFO and STO in Frequency Selective Rayleigh Fading Channels

In this paper, we derive exact symbol error probability (SEP) expressions for orthogonal frequency-division multiplexing (OFDM) based on discrete fractional Fourier transform (DFrFT) when symbol timing offset (STO) and carrier frequency offset (CFO) are present together. Closed form expressions are derived for binary phase shift keying, quadrature phase shift keying, and 16-ary quadrature amplitude modulation in the case of transmission over a frequency selective Rayleigh fading channel. As it is well known, in OFDM the presence of CFO destroys the orthogonality between the sub-carriers, generating inter-carrier interference (ICI), while the presence of STO introduces intersymbol interference, between successive symbols, and phase rotations in frequency domain, within the same symbol. It is well recognized that an OFDM system based on DFrFT is more robust to ICI introduced by CFO than that based on discrete Fourier transform (DFT). Motivated by this, the combined effect of CFO and STO on the SEP of a DFrFT-based OFDM system is analyzed. Our results show that the DFrFT-based OFDM system outperforms that based on the DFT for different values of the DFrFT “angle parameter.” Monte Carlo simulations are used to demonstrate the correctness of the derived analytical expressions.

Effects of frequency and timing offsets in DCT-based multiple access system for VLC

Abstract Visible light communication (VLC) has gained a significant momentum in the recent times due to its distinctive characteristics of imparting concurrent ‘illumination’ and ‘communication’ by exploiting sustainable and energy-efficient opto-electronic devices like light emitting diodes (LEDs). Consequently, by utilizing the already installed LED lighting fixtures a small scale cellular communication network can be created within an indoor environment imparting economic and ubiquitous data transmission services to multitude of roaming mobile stations within the proximity of LEDs. Therefore, it is vital to address the detrimental aspects like carrier frequency offset (CFO) and symbol time offset (STO) which emanates predominantly in an uplink scenario. This paper evaluates the performance of DC offset optical fast orthogonal frequency division multiple access (DCO-FOFDMA) system in the presence of both CFO and STO and accomplishes a thorough mathematical analysis emphasizing the different interferences like inter carrier (ICI) and multi user interferences (MUI) emanating due to different timing disparities and frequency discrepancies. The simulation results evidences that, with the increase in sensitivity of CFO and timing misalignments, the system performance drastically deteriorates thereby hindering the detection capability of multiple users operating at the same time. In order to estimate the STO, synchronization algorithms like Maximum Likelihood (ML) and Classen are imposed to the developed system model. Additionally, Cramer Rao Lower Bound (CRLB) is derived for the estimation of CFO and STO.

An Analysis of the Performance of ML Blind OFDM Symbol Timing Estimation

Symbol timing offset (STO) estimation constitutes an important part in orthogonal frequency-division multiplexing (OFDM) signal synchronization, and many blind methods based on the maximum likelihood (ML) principle have been proposed. A theoretical analysis of the estimation performance, however, is very difficult, due in part to the discrete nature of the STO. As a result, most studies on OFDM STO estimation have relied only on computer simulation for performance evaluation. This paper presents a theoretical performance analysis for ML-based blind OFDM STO estimation. We start with a review of the formulation of ML blind STO estimation problem and its solution. We then conduct the said performance analysis. The analysis turns out to involve a study of the eigenvalues of some matrix functions of the autocorrelation of the received signal. As the general case leads to highly complicated mathematics, we treat the case of full-band OFDM in additive white Gaussian noise in more detail. In this case, we are able to obtain mathematically tractable bounds and approximation to the estimation error probability and the corresponding mean-square error. In high signal-to-noise ratio (SNR), these bounds and approximation show inverse power-law dependence on the SNR. We also compare the bounds with simulation results and with an existing approximation in the literature.

A Hardware-Efficient Synchronization in L-DACS1 for Aeronautical Communications

L-band digital aeronautical communication system type-1 (L-DACS1) is an emerging standard that aims at enhancing air traffic management by transitioning the traditional analog aeronautical communication systems to the superior and highly efficient digital domain. L-DACS1 employs modern and efficient orthogonal frequency-division multiplexing (OFDM) modulation technique to achieve more efficient and higher data rate in comparison to the existing aeronautical communication systems. However, the performance of OFDM systems is very sensitive to synchronization errors such as symbol timing offset (STO) and carrier frequency offset (CFO). STO and CFO estimations are extremely important for maintaining orthogonality among the subcarriers for the retrieval of information. This paper proposes a novel efficient hardware synchronizer for L-DACS1 systems that offers robust performance at low power and low hardware resource usage. Monte Carlo simulations show that the proposed synchronization algorithm provides accurate STO estimation as well as fractional CFO estimation. Implementation of the proposed synchronizer on a widely used field-programmable gate array (FPGA) (Xilinx xc7z020clg484-1) results in a very low hardware usage which consumed 6.5%, 3.7%, and 6.4% of the total number of lookup tables, flip-flops, and digital signal processing blocks, respectively. The dynamic power of the proposed synchronizer is below 1 mW.

Synchronization in Visible Light Communication for Smart Cities

The rapid deployment of light emitting diodes and the prominence of facilitating simultaneous illumination and communication have driven Visible Light Communication (VLC) to gain enormous prevalence in the recent times. This expedited VLC to become amalgamated with many smart city applications in particular, with the lighting infrastructure to enable seamless connectivity. The underlying challenge of this technology is to assure high data rate communication while fulfilling illumination requirements in smart cities. In this regard, optical orthogonal frequency division multiplexing (OOFDM) compatible with intensity modulated/direct detection (IM/DD) system can be exploited for VLC. Where, IM and DD is one approach of envisioning VLC. DC biased Optical OFDM (DCO-OFDM), the earliest variant of OFDM for VLC is employed in this paper. It is apparent that, the advantages of OOFDM cannot be utilized if its orthogonality is destroyed by any means. The striking difference between RF-based OFDM and OOFDM is that, OOFDM necessitates the transmitted time-domain signal to be both real and unipolar. Hence, the synchronization techniques existing for RF cannot be directly applied for VLC and consequently must be revisited. This paper analyzes the effects of carrier frequency offset (CFO) and symbol time offset (STO) in DCO-OFDM over VLC channel. Here, mathematical analysis is performed showing the effects of CFO and STO in OOFDM. Synchronization algorithms, such as Moose, Classen and Training Symbol assisted are revisited. Detailed analysis of Cramer Rao lower bound is carried out considering the joint parameter estimation of both CFO and channel estimation error.

Non-data aided joint estimation of symbol timing offset and carrier frequency offset for OFDM/OQAM systems

This paper deals with the problem of blind synchronization for orthogonal frequency division multiplexing based on offset quadrature amplitude (OFDM/OQAM) systems without the prior information of the channel response and transmitting signals power. A novel non-data aided (NDA) joint estimation method of symbol timing offset (STO) and carrier frequency offset (CFO) for OFDM/OQAM is conceived in multipath fading channels condition. The multipath fading channels are equivalent to multiple parallel sub-channels with flat Rayleigh fading characteristics firstly, and then STO and CFO are estimated by the second-order cyclic-cumulant with different cycle frequency values and time delay values. Moreover, the modified Cramer-Rao Bound (MCRB) of NDA joint STO and CFO estimation is presented. The simulation results demonstrate that the proposed method is effective and feasible over frequency selective fading channels.

Pseudo-Noise Sequence Based Synchronization for Generalized Frequency Division Multiplexing in 5G Communication System

As one of the candidate waveforms for 5G, generalized frequency division multiplexing (GFDM) is a flexible non-orthogonal multicarrier technique. Similar to orthogonal frequency division multiplexing, GFDM communication system suffers from symbol timing offset (STO) and carrier frequency offset (CFO) caused by multipath effect and Doppler shift. In this paper, we design a pseudo noise (PN) based preamble and propose an approach for signal synchronization. The preamble is composed of two identical PN sequences whose autocorrelation and cross-correlation are better than cyclic prefix (CP). STO can be detected by calculating the correlation coefficient between the local PN sequence of receiver and the corresponding preamble part of sliding window. These two adjacent PN sequences can be used to calculate phase offset value by Fourier transform, and then obtain CFO value. Simulation results demonstrate that, compared with conventional CP-based synchronization, the proposed approach has the lower mean square error and symbol error rate at the cost of just a little bit more bandwidth.

Simple Algorithms for Estimating the Symbol Timing Offset in DCT‐Based Multicarrier Systems

Multicarrier modulation based on discrete cosine transform (DCT‐MCM) is a candidate technique for future wireless communication. DCT‐MCM provides a better usage of the spectrum, allowing for much lower out‐of‐band radiation than conventional OFDM. In this paper, we address the problem of correcting the symbol timing offset in DCT‐MCM. Two sliding window‐based correlation algorithms are proposed. The new approaches consider a prefix and a suffix that are inserted into each data symbol, which are the replica of part of the useful data. The performance of the proposed approaches is verified and tested, and their usefulness and effectiveness are illustrated by computer simulations.

Combined Subband-Subcarrier Spectral Shaping in Multi-Carrier Modulation Under the Excess Frame Length Constraint

This paper investigates spectral shaping of multi-carrier-modulation waveforms based on combination of Nyquist windowing and subband filtering. The combined windowing/filtering allows simultaneous control on both subcarrier and subband spectra. When compared with the existing Nyquist windowing or subband filtering techniques under a fixed excess frame length constraint, the proposed scheme offers reduced sensitivity to carrier frequency and symbol timing offsets. Establishing an analytical tool based on the error spectrum stack consisting of the error signals evaluated at different signal delay positions, we explore the window-filter trade-off and provide the minimum interference power solution for given ranges of carrier frequency and symbol timing offsets. Our design targets low-latency applications having no provisions for high-precision synchronization and having potential need for spectrum aggregation.

Performance Evaluation of Timing Estimation Technique Using Extended Cyclic Prefix for Correlation Sequence for OFDM Systems

This paper presents an analysis of symbol timing offset (STO) estimation method for orthogonal frequency division multiplexing systems. The proposed method does not require any preamble and with the use of available cyclic prefix (CP) samples we have proposed an extended CP. The extended CP is used to exploit more number of correlation products in correlation sequence so that a more accurate starting point of any symbol can be estimated. A simplified maximum likelihood estimation process for proposed timing estimation technique is given which enables us much more accurate timing estimate due to decreased error variance. The performance of the proposed method is evaluated in terms of mean square error of timing estimator against different signal to noise ratio values and in terms of error distribution of the estimators with and without the effect of carrier frequency offset in various channel scenarios such as AWGN, HIPERLAN/2A and HIPERLAN/2E. Simulation results shows that proposed method has significantly better performance compared to the CP based method for STO estimation.

Synchronisation algorithm based on zero correlation code pair for OFDM‐based VLC systems

Visible light communication (VLC) system based on orthogonal frequency division multiplexing (OFDM) is promising for its ability to link a huge number of devices with high reliability and low latency. However, OFDM-based VLC systems are very sensitive to the symbol timing offsets. Thus, synchronisation algorithm is very important for OFDM-based VLC system. In this study, the authors first propose zero correlation code pair (ZCC pair) and investigate the periodical impulse-shaped correlation property of ZCC pair. Then the authors propose a novel timing offset estimation algorithm based on ZCC pair. The preamble is processed via peak-to-average power ratio (PAPR) reduction algorithms to overcome the PAPR problem in VLC systems. This algorithm achieves high accuracy and has low synchronisation error probability. The effectiveness of the authors' proposed algorithm has been demonstrated using computer simulation under non-light-of-sight VLC and Gaussian channels. The results show that the authors' algorithm outperforms the Schmidl Cox's and Park's algorithms in terms of synchronisation error probability and mean square error of timing error.

Blind symbol timing synchronization scheme for real-time industrial wireless control network using high-speed preambleless OFDM systems over multipath fading channels

An accurate estimation of symbol timing is crucial for orthogonal frequency division multiplexing (OFDM) demodulation, since synchronization errors might destroy the orthogonality among all subcarriers and have greater impact on the system performance than single carrier systems. The symbol timing offset (STO) that leads to inter-symbol interference (ISI) and inter-channel interference (ICI) must be avoided as much as possible. In this paper, a novel symbol timing synchronization algorithm is proposed for OFDM systems on the basis of cyclic prefix (CP). A metric function and a positioning function have been designed by taking advantage of the correlation properties of OFDM signals. Computer simulations demonstrate that the schemes are superior because they achieve accurate timing offset estimation with smaller standard deviation (SD) and relatively lower computational complexity compared with the existing methods based on CP under high signal–noise ratio (SNR) assumption over frequency-selective channels, which can meet the real-time requirements in wireless local area network (WLAN) environments for factory automation (FA).

A Joint Timing Offset and Channel Estimation Using Fractional Fourier Transform and Cazac Sequences

This paper deals with symbol timing offset and channel estimation in OFDM (orthogonal frequency division multiplexing) system in fast varying channel. Symbol timing offset (STO) estimation is a major task in OFDM. Most of existing methods for estimating STO used cyclic prefix or training sequences. In this paper, we consider a new system for STO estimation using constant amplitude zero autocorrelation (CAZAC) sequences as pilot sequences in conjunction with fractional Fourier transform (FRFT). After STO estimation is done, timing compensation is made. Thereafter, channel is estimated to well recover the original transmitted signal. This method gives good results in terms of MSE in comparison with other known techniques, it estimated well the channel and it is important for fast varying channel. MATLAB Monte-Carlo simulations are used to evaluate the performance of the proposed estimator.

Analyse Performance of Fractional Fourier Transform on Timing and Carrier Frequency Offsets Estimation

This paper deals with the performance of the use of fractional Fourier transform (FRFT) instead of conventional Fourier transform (FFT) in either symbol timing offset (STO) and carrier frequency offset (CFO) estimation. Orthogonal frequency division multiplexing is widely used in many systems due to advantages of theses technique compared with mono-carrier systems. In spite of his advantages, OFDM presents drawbacks such as sensitivity to timing and frequency offsets. Many techniques are used in the literature to estimate these two parameters in order to compensate them (synchronization task). These techniques used conventional Fourier transform. In this paper, we are interested in estimating STO and CFO using fractional Fourier transform. Monte Carlo simulation demonstrates the performance of the use of FRFT instead of FFT.

QoS enhancement in OFDM based systems under transceiver impairments

Broadband wireless systems generally use orthogonal frequency division multiplexing (OFDM) with link adaptation (LA) to achieve high throughput while meeting bit error rate (BER) constraint. OFDM systems are known to be affected by non-linearity of high power amplifier (HPA) at transmitter, carrier frequency offset (CFO), symbol timing offset (STO) and channel estimation error at the receiver. The delay in feedback of channel state information (CSI) further affects the performance of LA procedures. The focus of this work is on performance analysis in presence of simultaneous affect of all these impairments on LA based OFDM systems. The results are found to be useful for threshold readjustment which is essential for successful implementation of LA scheme to counter the effects of change in operating conditions from ideal to as listed above.

Preamble-based frequency-domain joint CFO and STO estimation for OQAM-based filter bank multicarrier

Abstract Filter bank multicarrier systems, similarly to orthogonal frequency division multiplexing (OFDM), are very sensitive to carrier frequency offset (CFO) and symbol timing offset (STO). In this paper, a low-complexity preamble-based joint CFO and STO technique is presented. It is based on a relatively long preamble in order to improve the CFO estimation performance as well as avoid interference coming from the data following this preamble. After CFO and STO correction, the preamble can be reused to estimate the channel. Unlike most current techniques, the CFO and STO estimation occurs in the frequency domain. This allows for a low-complexity estimation with respect to time-domain techniques and, as will be shown by simulations, provide even better performance in a reasonable range. The drawback however is that the estimation range is shorter. Specifically, for large STOs (and to a smaller extent large CFOs), the performance decreases below time-domain estimations. Two versions of the STO estimation technique will be presented, the second one being an approximation of the first one, making it less complex yet also less precise. The performance is assessed by means of computer simulations, testing for both large and small STOs, and compared with existing techniques.