Frequency Offset Error Research Articles

Joint DFO and channel estimation scheme for RIS assisted OFDM systems

In the reconfigurable intelligent surface (RIS) assisted mobile wireless communications, the performance gains provided by RIS cannot be achieved without the accurate channel estimation. Many time-varying channel estimation algorithms have been proposed for such systems, but none of them consider the effect of Doppler frequency offset (DFO) synchronization error, which can seriously degrade the accuracy of channel estimation, especially in the Rician fading channel for the orthogonal frequency division multiplexing (OFDM) systems. In our work, the effect of DFO synchronization error on the time-varying channel estimation is firstly analyzed, and then a deep learning based joint DFO and channel estimation scheme is proposed. Specifically, the threshold-based noise reduction algorithm and the dual convolution neural networks (CNN) are applied for improve the estimation accuracy. To enhance the practicability of the proposed method, the training target of CNN network is set as the actual estimation value with high accuracy, rather than the ideal information. The simulation results show that the proposed method exhibits superior performance compared to the existing algorithms, and it has an acceptable computational complexity.

The characterisation of a phase micro-stepper

The frequency steering of the local standard time is a common procedure for the timekeeping. The local radiofrequency reference output is modified by a phase and frequency offset generator to corresponds to the global timescales such as the International Atomic Time and Coordinated Universal Time timescales. The phase and frequency offset generator, also known as the phase micro-stepper, is used for such timescales for many years but it has not been very well characterized. It will be shown for a particular type of the phase micro-stepper that its phase delay error can exceed 10 ps and its frequency offset error can be relatively more than 10−16 of the reference frequency.

Harmonic Radar With Adaptively Phase-Coherent Auxiliary Transmitters

In harmonic radar (HR), the radio frequency transmitter illuminates a nonlinear target (the tag), causing the return signal to consist of harmonics at multiples of the transmitted carrier frequency. Of them, the second harmonic is usually the strongest and the one to which the receiver is tuned. This frequency difference distinguishes the tag reflection from environmental clutter, which remains at the uplink (transmitter to tag) frequency. However, the passive nature of HR tags severely limits the reflected power, and therefore the range of the downlink (tag to receiver) path. We propose to increase the range and/or signal to noise ratio (SNR) by novel restructuring at the physical and signal levels. For this, we accompany the original transmitter with auxiliary transmitters able to send simple tones that are synchronized to arrive at the tag in phase, and we design the receiver to detect an intermodulation component. The resulting range and SNR are much greater than those of the original, conventional HR system, and greater even than if the original system were to transmit with power equal to the aggregate power of our new system. Achieving mutually coherent, i.e., in phase, arrival of the tones at the tag is the focus of the present paper. We provide a system framework that models the tag and the uplink and downlink, then present the adaptive phase coherence algorithm and analyze the probabilistic growth of the output signal power. We also account for the effects of frequency shifts due to transmitter mobility and the frequency offset errors in the transmitter local oscillators.

OQPSK Synchronization Parameter Estimation Based on Burst Signal Detection

The fast estimation of synchronization parameters plays an extremely important role in the demodulation of burst signals. In order to solve the problem of high computational complexity in the implementation of traditional algorithms, a synchronization parameter (frequency offset, phase offset, and timing error) estimation algorithm based on Offset Quadrature Phase Shift Keying (OQPSK) burst signal detection is proposed in this article. We first use the Data-Aided (DA) method to detect where the burst signal begins by taking the segment correlation between the receiving signals and the known local Unique Word (UW). In the sequel, the above results are adopted directly to estimate the synchronization parameters, which is obviously different from the conventional algorithms. In this way, the complexity of the proposed algorithm is greatly reduced, and it is more convenient for hardware implementation. The simulation results show that the proposed algorithm has high accuracy and can track the Modified Cramer–Rao Bound (MCRB) closely.

AFF-SOGI-DRC Control of Renewable Energy Based Grid Interactive Charging Station for EV With Power Quality Improvement

In this article, a control of solar photovoltaic (PV) array, and wind energy conversion system (WECS) based charging station using an adaptive frequency-fixed second-order generalized integrator with dc offset rejection capability (AFF-SOGI-DRC), is presented for charging the electric vehicles (EVs) and improving the power quality of the grid. In comparison to SOGI-based algorithm, wherein a frequency feedback loop reduces the stability margin and FF-SOGI-based algorithm, where a double frequency oscillation and offset error constraint the algorithm capability, the presented algorithm precisely estimates the fundamental EV current. Moreover, the AFF-SOGI-DRC-based positive sequence extractor helps in generating the sinusoidal in-quadrature unit vectors under distorted and unbalanced grid voltages; therefore, the reference grid currents become harmonic free. The charging station is operated in the grid-connected mode (GCM) and an islanded mode (IM) to utilize the renewable energy maximally. The control of the charging station also includes the voltage synchronizing strategy that ensures the smooth transition between the GCM and IM. Moreover, a unified controller is designed to achieve four-quadrant operation vehicle to grid/grid to vehicle (V2G/G2V), vehicle to home power transfer, reactive power compensation, and active filtering, etc. The operation of the charging station complies with the IEEE 1547 standard, and the operation of the charging station is validated through the laboratory prototype.

Pulse shaping methods for inter carrier interference reduction in OFDM system

The weakness of the orthogonal freuency division multiplexing (OFDM) system is susceptible to the existence of carrier frequency offset (CFO) which causes the emergence of inter carrier interference (ICI) which causes a degradation of performance OFDM systems. This study aims to apply the suggested rectangular (REC) pulse and improved sinc-power (ISP) pulse shaping methods on OFDM system and determines ICI reduction with the effects of CFO over flat fading Rayleigh channels. The performance of each pulse shaping method is evaluated and compared based on parameter ICI power vs. normalized frequency offset, signal to interference ratio (SIR) vs. normalized frequency offset and bit error rate (BER) vs. energy bit per noise (Eb/No). The simulation result in terms of BER vs. Eb/No indicated that REC and ISP pulse shaping has better performance dealing with ICI reduction compared to OFDM system no applied pulse shaping. In addition, the ISP is able to mitigate ICI better than REC.

Evaluation and Analysis of FBMC/OQAM Systems Based on Pulse Shaping Filters

Cyclic prefix orthogonal frequency division multiplexing (CP-OFDM) is the most widespread multicarrier modulation scheme. However, the use of a rectangular filter in CP-OFDM causes poor out-of-band (OoB) radiation. Moreover, the CP causes spectral efficiency loss. To overcome these limitations, a filter bank multicarrier (FBMC) was proposed which uses prototype pulse shaping filters that can be adapted to fulfil the system requirements. The choice of the filter is crucial for FBMC/offset quadrature amplitude modulation (OQAM) due to its significant impact on the achieved performance. In this paper, new prototype pulse shaping filters in FBMC/OQAM systems are proposed, aiming to improve the system performance. A number of prototype pulse shaping filters, such as raised cosine pulse (RC), root raised cosine pulse (RRC), better than raised cosine pulse (BTRC), modified Bartlett-Hanning (MBH), improved sinc power pulse (ISP), phase modified sinc pulse (PMSP), parametric linear pulse (PLP), linear combination pulse (LCP), PHYDYAS and Hermite, are considered. The performance of each pulse shaping filter in FBMC/OQAM is evaluated and compared other in terms of the power spectral density (PSD), spectral efficiency, signal-to-interference ratio (SIR), time offset (TO), carrier frequency offset (CFO) and bit error rate (BER) performance over various channels. In addition, as multiple input multiple output (MIMO) is one of the main challenges associated with FBMC, we propose the new approach that use the frequency/time block spreading based on a Walsh-Hadamard (WH) code in MIMO FBMC/OQAM and investigate the combination of the our proposed prototype pulse shaping filters with MIMO systems. The proposed filters are shown to be suitable candidates for FBMC/OQAM systems.

Develop of CFO in High Mobility Environments for OFDM-DMWT Based Mobile Wireless Applications

An “Orthogonal Frequency division Multiplexing (OFDM)” systems are sensitive to “service Frequency Offset (CFO)” mistakes. OFDM- “Discrete Multi-wavelet remodel (DMWT)” based totally cellular wireless with a sender, a provider Frequency Offset errors turns into similarly tough for cellular applications in view that power postpone outline of the channel may trade quick because of the sporadic start and dying of the channel paths. We discover analytical expressions for channel estimation mistakes inside the existence of CFO mistakes and excessive mobility. We display that the sensitivity of the channel estimator be capable of still be exploited to increase CFO in high mobility environments. A comparative have a look at amongst extraordinary techniques of service frequency offset estimation in OFDM systems is offered. We keep in mind strategies in frequency area are considered, namely; training image approach and Pilot tone technique. Mean square blunders (MSE) is the evaluation criteria used within the look at. Simulation consequences display that the CFO estimation strategies the schooling image technique outperforms the pilot tone approach with the aid of about 2dB. The contrast results are considered at MSE of 10-4 and normalized CFO of 0.2. Keywords— CFO, ICI, BER, SNR, OFDM, DMWT, MSE, Frequency domain. DOI : 10.7176/NCS/10-04 Publication date :July 31 st 2019

Experimental demonstration of a 16.9 Gb/s link for coherent OFDM PON robust to frequency offset and timing error

Due to its merits of flexible bandwidth allocation and robustness towards fiber transmission impairments, coherent optical orthogonal frequency division multiplexing (CO-OFDM) technology draws a lot of attention for passive optical networks (PON). However, a CO-OFDM system is vulnerable to frequency offsets between modulated optical signals and optical local oscillators (OLO). This is particularly serious for low cost PONs where low cost lasers are used. Thus, it is of great interest to develop efficient algorithms for frequency synchronization in CO-OFDM systems. Usually frequency synchronization proposed in CO-OFDM systems are done by detecting the phase shift in time domain. In such a way, there is a trade-off between estimation accuracy and range. Considering that the integer frequency offset (IFO) contributes to the major frequency offset, a more efficient method to estimate IFO is of demand. By detecting IFO induced circular channel rotation (CCR), the frequency offset can be directly estimated after fast Fourier transforming (FFT). In this paper, circular acquisition offset frequency and timing synchronization (CAO-FTS) scheme is proposed. A specially-designed frequency domain pseudo noise (PN) sequence is used for CCR detection and timing synchronization. Full-range frequency offset compensation and non-plateau timing synchronization are experimentally demonstrated in presence of fiber dispersion. Based on CAO-FTS, 16.9 Gb/s CO-OFDM signal is successfully delivered over a span of 80-km single mode fiber.

A novel energy-efficient ICI cancellation technique for bandwidth improvements through cyclic prefix reuse in an OFDM system

Orthogonal frequency division multiplexing (OFDM), a very promising technique that is leading the evolution in wireless mobile communication to sideline the bandwidth scarcity issue in spectrum allocation, is severely affected by the undesirable effects of the frequency offset error, which generates inter carrier interference (ICI) due to the Doppler shift and local oscillator frequency synchronization errors. There are many ICI cancellation techniques available in the literature, such as self-cancellation (SC), maximum likelihood estimation (MLE), and windowing, but they present a tradeoff between bandwidth redundancy and system complexity. In this study, a new energy-efficient, bandwidth-effective technique is proposed to mitigate ICI through cyclic prefix (CP) reuse at the receiver end. Unlike SC and MLE where the whole OFDM symbol data is transmitted in duplicate to create redundancy at the transmitter end, the proposed technique uses the CP data (which is only 20% of the total symbol bandwidth) to estimate the channel, and it produces similar results with a huge bandwidth saving. The simulation results show that the proposed technique has a significant improvement in error performance, and a comparative analysis demonstrates the substantial improvement in energy efficiency with high bandwidth gain. Therefore, it outperforms the legacy ICI cancellation schemes under consideration.

Zero-Padded Conjugate Transmission to Cancel ICI in OFDM System

In OFDM (Orthogonal Frequency Division Multiplexing) to minimize effect of Inter-carrier interference the proposed technique Conjugate transmission with Zero padding and adaptive receiver is proposed in this research article. To transmit the signal according to proposed scheme, first regular OFDM signal is transmitted as a signal in the first path and conjugate signal of regular OFDM is transmitted using proposed algorithm in second path. Two consecutive symbols of the proposed OFDM signal is transmitter after padding Zeros between them to minimize the effect of inter-carrier interference forbetter time and frequency synchronization between transmitter and receiver signal. To adaptively update the frequency offset error receiver is designed with block least mean-squared algorithm (BLMS). BPSK, QPSK and 16-QAM modulation techniques are used and simulation results are carried out using MATLab software. Simulation results for proposed scheme are compared with regular OFDM system, Conjugate Cancellation (CC) and Phase Rotation Conjugate technique(PRCC) with reference to BER and simulation graphs shows that proposed scheme shows better BER rate performance as comparison to conventional techniques for AWGN wireless channel.

Performance Enhancement of OFDM-Based Systems Using Improved Parametric Linear Combination Pulses

As the demand for better performance increases in next-generation networks, such as 5G technologies, more efficient modulation techniques are required. In this work, we evaluate the performance of the improved parametric linear combination pulses (IPLCP) in orthogonal frequency division multiplexing (OFDM) based systems. OFDM-based systems are very sensitive to frequency offset errors, and are characterized by producing high peak-to-average power ratio (PAPR) values. The IPLCP is evaluated in terms of average inter-carrier interference power, average signal to interference power, PAPR, and bit error rate. Theoretical and numerical simulations show that the IPLCP pulse performs well in OFDM-based systems in comparison to other existing pulses.

A Methodology for Estimating Frequency Responses of Electric Power Grids

Currently, there is a lack of standard methodology to characterize frequency responses of electric power grids for power line communication purpose. As a result, fair comparisons among measurement campaigns carried out in different parts of the world are missing. Aiming at to deal with this issue, this contribution discusses a complete sounding-based methodology to estimate frequency responses of electric power grids combining sampling frequency offset error estimation and correction, timing synchronization, channel estimation, and channel estimation enhancement techniques. The effectiveness of this methodology is validated by using well-known power line channel models, as well as measured ones, covering the frequency band from \(1.7\) up to \(50\) MHz. The attained results show that the methodology provides estimates in shorten period of time in comparison with the network analyzer based methodology and because of that it is capable of characterizing the periodically and time-varying behavior of electric power grids. Additionally, it is shown that the methodology can be successfully applied to characterize frequency responses of electric equipment and, as a consequence, it is very useful for both power line communication and power system applications.

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.

A Novel Approach for Reduction in CFO at Channel Estimator in WiMax OFDM System

During Multicarrier transmission in OFDM(Orthogonal Frequency Division Multiplexing), the signal suffers due to fading environment and results in timing and frequency errors at the receiver end.An efficient timing and frequency offset estimation algorithm has been designed in VHDL using ISE XILINX 10.1. The maximum value at which the timing offset is achieved and input signal with a delay are compared to get the frequency offset. This operation ensures that the frequency offset calculation is done at the best time, i.e., when the correlation over the actual received symbol cyclic prefix is complete. The values for frequency offset has been obtained after simulations as .00034232 and timing offset has been observed as1.864ns.The design is achieved at 75.5 Mhz and uses 2% of the total memory of the Virtex-6 Device. The design when implemented was found to reduce 95% of carrier frequency offset(CFO) error, than the performance of a simple frequency estimator.

Cooperative OFDM Channel Estimation in the Presence of Frequency Offsets

Channel estimation in the presence of frequency offsets is developed for cooperative orthogonal frequency-division multiplexing (OFDM) systems. A two-time-slot cooperative channel estimation protocol is proposed. The source broadcasts the training sequence to the relays and the destination (first time slot), and the relays retransmit the training sequence (second time slot). Pilot designs for amplify-and-forward (AF) and decode-and-forward (DF) relays are derived. These designs eliminate interrelay interference (IRI), which occurs due to the simultaneous relay retransmissions, and minimize the mean square error (MSE). Consequently, the number of AF and DF relays is constrained to be less than lfloorN/(2L - 1)rfloor and lfloorN/Lrfloor, respectively, where N is the total number of subcarriers, L is the channel order, and lfloorarfloor is the maximum integer part of alpha. The pairwise error probability (PEP) of orthogonal space-time coding in cooperative OFDM due to both frequency offset and channel-estimation errors is also evaluated. The optimal power allocation ratio between the source and the relays to minimize the PEP is derived for AF and DF relays. When L < 16, DF relays outperform AF relays in terms of PEP. With L = 4 and 16 active relays, the gap is 9 dB for a frequency offset error variance of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , and this gap increases to about 11.3 dB when the variance increases to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> .

OFDMA uplink frequency offset estimation via cooperative relaying

Frequency offset estimation for an orthogonal frequency-division multiple access (OFDMA) uplink for amplify-and-forward (AF) relays and a new type of relay (R) called decode-and-compensate-and-forward (DcF) relays are studied. Multiple relays are considered, and the relay with the best S rarr R channel is chosen to perform re-transmission, where S and R represent the source and relay nodes, respectively. Frequency offsets due to the mismatches between the transmitter and receiver oscillators are considered, and without considering the effect of Doppler shift, both S rarr D and S rarr R rarr D links have the same frequency offset, where D represents the destination. Thus, by using these two transmissions, D generates two frequency offset estimates, which are combined to minimize the mean square error (MSE). Power allocation between S and R can be adaptively adjusted to optimize the cooperative scheme in terms of frequency offset error variance. When channel state information (CSI) is available at each mobile node, a scheme where the relays adaptively switch between the cooperative and conventional (no relaying) transmissions is proposed to optimize the frequency offset estimation. Although the frequency offset estimation accuracy in the DcF mode is somewhat worse than the AF mode, both modes outperform the conventional transmission. However, DcF (or decode-and-forward (DF)) relays outperform AF relays in terms of channel capacity and bit error rate (BER).

Joint ML time–frequency synchronisation and channel estimation algorithm for MIMO-OFDM systems

The maximum-likelihood (ML) time–frequency synchronisation algorithm combined with channel estimation for multiple input multiple output-orthogonal frequency division multiplexing (MIMO-OFDM) systems in frequency selective fading channels is addressed. In the proposed algorithm, the authors use two steps to maximise an ML metric to obtain first the frequency offset and then timing. A fast Fourier transform algorithm is used to estimate the frequency offset. Using these two estimates, the channel is identified. A simple iterative algorithm is proposed to improve the frequency offset estimation. The performance of the proposed synchronisation approach, in terms of timing failure probability and mean square error of the estimated frequency offset and bit error rate, is compared with others in the literature. Comparison of simulation results with the Cramer–Rao lower bound clearly illustrates the accuracy of the proposed algorithm, which outperforms the state-of-the-art synchroniser devices in the open literature.

Mathematical Formulations of Signal Propagation in Ultra-Wideband Transceiver Systems under a UWB Channel Environment with an Extension of Frequency Offset Correction

This paper analyzes mathematically the crucial aspects of signal processing in a Multi-Band (MB) Orthogonal Frequency Division Multiplexing (OFDM) based system considering Ultra-Wideband (UWB) channel environment. In the process of analysis, it emphasizes the significant features of UWB receiver design in comparison with ‘conventional’ narrow-band system. The analysis shows that the high dispersive nature of a frequency selective UWB channel effects the design of different signal processing blocks like pre-select filter, low noise amplifier (LNA) and analog-to-digital (A/D) converter in the receiver front end. The characteristic functions of each of these stages are now dominated by the channel characteristics and it needs to be modified accordingly. This analysis is extended further with the study of frequency offset error and its correction. The unbiased Cramer Rao Lower Bound (CRLB) of estimation error is calculated and supported by computer simulation. The performance of an MB-OFDM system with frequency offset correction in terms of Bit-Error-Rate (BER) is also reported.

Modified back-propagation algorithm applied to decision-feedback equalisation

This study presents a modified back-propagation (BP) algorithm for a multilayer perceptron (MLP) to perfect its ability to cope with the problem of binary phase shift keying channel equalisation. For a typical BP algorithm, the error signal is obtained from the comparison between the target and estimated signal. The error signal is propagated layer by layer from the output layer to the input layer to adaptively adjust all weights in the MLP. Therefore all parameters of the MLP are obtained by a single BP algorithm. However, the structure of the MLP with a hidden layer provides the feasibility to modify the BP algorithm to improve its performance. The MLP can be divided from the hidden layer into two sub-MLPs, and each sub-MLP is optimised by its own BP algorithm. Accordingly, the whole MLP is adjusted by two BP algorithms independently. In this study, the modified BP algorithm is utilised to cope with the problem of channel equalisation. The simulation results show that the modified BP algorithm indeed improves the typical BP algorithm especially for an environment with nonlinear distortion, frequency offset, and phase and timing errors. Moreover, the computation complexity of the proposed algorithm almost equals that of the conventional BP algorithm.