Sampling Frequency Offset Research Articles

Sampling frequency offset compensation scheme based on machine learning for MB-OFDM UWBoF system

Digital to analogue converter (DAC) at the transmitter side and analogue to digital converter (ADC) at the receiver side with not exactly same clock period leads to sampling frequency offset (SFO) that affects the performance of multi-band ultra-wideband orthogonal frequency division multiplexing over fiber (MB-OFDM UWBoF) system, four machine-learning algorithms are proposed for compensating the SFO for the system and are compared in this paper. Simulation results show that the best compensation among the three conventional machine learning algorithms after 100 km standard single mode fiber (SSMF) transmission is the self-organising mapping (SOM) algorithm, which is able to compensate the SFO in the range of ±150 ppm, with a high complexity. The next best is the conventional kmeans algorithm which can compensate ±40 ppm and is slightly better compared to the density-based spatial clustering of applications with noise (DBSCAN) algorithm. The DBSCAN algorithm only compensates around ±25 ppm of SFO, when the constellation points are slightly chaotic, the wrong cluster results in failure of demodulation, which makes it less applicable for problems of SFO. Moreover, the improved kmeans algorithm proposed in this paper can compensate the SFO of ±225 ppm, better than three conventional machine learning algorithms. And it can still make the BER lower than 3.8e-3 at received optical power (ROP) of −15 dBm when the SFO is 225 ppm.

Frequency Offset Estimation for UWB-OFDM System using Weight and Exhaustiveness based Monarch Butterfly Optimization Approach

The performance of an OFDM system has limitation because the carrier frequency offset (CFO) and sampling frequency offset (SFO) is not estimated correctly. Current methods propose the way of obtaining frequency estimation by using the evolutionary algorithm, but the evolution algorithm suffers from the premature convergence problem. In order to provide efficient frequency offset estimation, this paper proposed weight and exhaustiveness based monarch butterfly optimization approach. First, on the transmitter block, for modulating an input symbol, the Hybrid-Quadrature Phase-Shift Keying modulation scheme is used. Brunn’s IFFT is done at each transmitter for symbol mapping and then in each transmitter block cyclic prefix is inserted. After that, WE-MBO algorithm estimates the frequency offset. On receiver side, the inverse operation of the transmitter is performed. After comparison of proposed system with the existing system, the proposed system gave better performance.

Non pilot data-aided carrier and sampling frequency offsets estimation in fast time-varying channel

This paper considers the non pilot data-aided estimation of the carrier frequency offset (CFO) and sample frequency offset (SFO) of orthogonal frequency division multiplexing (OFDM) signals in fast time-varying channel. The main obstacle is the time-variant channel response, which deteriorates the estimation validity. A practical approach to mitigate this impact is to reduce the time consumption of one-shot estimation. In this way, we propose a method to reduce the time consumption to within one OFDM symbol duration. The maximum likelihood (ML) estimator is derived based on the observations of frequency domain constellations output of two FFTs on one symbol; its closed-form approximation is then derived to reduce the calculation burden. Remarkably, our method does not require any training symbol or pilot tone embedded in the signal spectrum, therefore achieves the highest spectral efficiency. Theoretical analysis and simulation results are employed to assess the performance of proposed method in comparison with existing alternatives.

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

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

Fast Acquisition Method for DSSS in the Presence of Sampling Frequency Offset

Fast Acquisition Method for DSSS in the Presence of Sampling Frequency Offset

Estimating and Tracking Wireless Channels Under Carrier and Sampling Frequency Offsets

This article addresses the challenge of estimating and tracking wireless channels under carrier and sampling frequency offsets, which also incorporate phase noise and sampling time jitter. We propose a novel adaptive filter that explicitly estimates the channel impulse response, carrier frequency offset, and sampling frequency offset by minimizing the mean-square error (MSE) and, when the estimated parameters are time-varying, inherently performs tracking. The proposed filter does not have any requirements for the structure of the waveform, but the digital transmitted waveform must be known to the receiver in advance. To aid practical implementation, we derive upper bounds for the filter's step sizes. We also derive expressions for the filter's steady-state MSE performance, by extending the well-known energy conservation relation method to account for the self-induced nonstationarity and coupling of update equations that are inherent in the proposed filter. Theoretical findings are verified by comparison to simulated results. Proof-of-concept measurement results are also provided, which demonstrate that the proposed filter is able to estimate and track a practical wireless channel under carrier and sampling frequency offsets.

Single-Channel Blind Separation of Unsynchronized Multiuser PSK Signals With Non-Identical Sampling Frequency Offsets

Single-channel blind source separation (SCBSS) of uncoordinated, non-spread, co-frequency interfering multi-user signals is a challenging task. When the multi-user signals have sampling frequency offset (SFO) due to the imperfection of the local oscillator (LO), this problem becomes even more difficult to be solved. In this letter, a select-stitch data-aided channel estimation (CE) initialized per-survivor processing (SS-DCE-PSP) algorithm is proposed for SCBSS of multi-user signals with SFOs caused by imperfect LOs. By dividing a long sequence into a number of segments and selecting the segments with highest detection reliability to stitch together, the time slip and symbol slip due to the non-identical SFOs can be resolved. The data-aided CE in SS-DCE-PSP further enhances the performance of PSP, especially for the initial symbols. Simulation results show that the proposed SS-DCE-PSP algorithm is robust to the normalized SFO up to 10−4, and the BERs obtained by the SS-DCE-PSP algorithm are close (within 0.5dB) to the BERs obtained for multi-user signals without SFOs.

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.

Performance of a Method for the Estimation of Carrier and Sampling Frequency Offsets in OFDM WLAN Systems

A method was published in 2020 for the estimation of the residual carrier frequency offset (RCFO) and sampling frequency offset (SFO) in WLAN OFDM systems. The purpose of this paper is to compare this method with one existing method for more examples to make a fair comparison in terms of estimation accuracy and computational load.

Position-Dependent MIMO Demultiplexing Strategy for High-Speed Visible Light Communication in Internet of Vehicles

The context of the Internet of Vehicles (IoV) and the growing vehicular deployment of sensors and smart devices have demanded alternative wireless communication technologies. Despite the prevalent works on radio-frequency (RF)-based vehicular communication technologies, visible-light communication (VLC) is envisaged to be a promising candidate due to its unlicensed spectrum and high transmission rates. However, the long-distance and high-speed transmission performance of vehicular VLC needs more investigation considering the massive data transmission needs for automotive applications. In this article, a vehicular multiple-input–multiple-output (MIMO) VLC system based on two commercial headlights and a self-designed PIN array is experimentally demonstrated as a proof of concept for the IoV. Considering the relative movement of vehicles, different communication areas are divided based on the position-dependent receiver receiving states. The boundary condition of each area is derived. For the first time, the selection of the strategy that best suits the MIMO demultiplexing scheme is discussed by analyzing the rank and type of the channel matrix. A modified pilot-aided phase recovery method based on polynomial curve fitting (PCF) is proposed to compensate for the phase noise caused by the sampling frequency offset (SFO). The proposed method maintains good robustness even when the VLC system suffers from the strong nonlinearity and bandwidth limitation of the headlights. Based on the proposed MIMO VLC system, we achieve a record-breaking data rate of 3.08 Gb/s at a 2-m indoor transmission link. We further extend the transmission distance to 100 m, and successfully achieve overall data rates of 336 and 362 Mb/s during the daytime and nighttime, respectively. To the best of our knowledge, these are the highest transmission data rates ever reported for a 100-m vehicular VLC system. Our experimental demonstration clearly verifies the feasibility of VLC in the application of IoV.

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.

A novel sampling frequency offset mitigation scheme based on rotated K-means clustering for OFDM-VLC system

Sampling frequency offset (SFO) between digital-to-analog converter (DAC) and analog-to-digital converter (ADC) will severely degrade the transmission performance of orthogonal frequency division multiplexing (OFDM) based visible light communication (VLC) system. In this paper, a novel SFO mitigation scheme based on rotated K-means clustering is proposed, which differs from traditional SFO compensation based on phase estimation and feedback. When SFO causes the constellation points to deviate from their original positions, this scheme can adaptively update the clustering centroids to achieve the correct symbol demapping without phase estimation and compensation. Experimental results indicate that after 3.5 m free-space transmission, the SFO tolerance of quadrature phase shift keying (QPSK) and 16-ary quadrature amplitude modulation (16-QAM) can reach ±130 and ±65 ppm with a bit error rate (BER) less than the hard-decision forward error correction (HD-FEC) threshold of 3.8e−3, respectively.

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.

Digital interpolation-based SFO compensation in OCT precoding-enabled NHS-OFDM transmission systems

The non-Hermitian symmetric optical orthogonal frequency division multiplexing (NHS-OFDM) system exhibits lower hardware implementation complexity and power consumption as maintaining the same bandwidth granularity, compared to the conventional Hermitian symmetric optical OFDM (HS-OFDM) system. Also, NHS-OFDM can obtain a similar peak-to-average power ratio as well as bit error rate (BER) performance. It is expected to be a new candidate for the next-generation passive optical network. In the asynchronous NHS-OFDM system, the sampling clocks for the analog to digital converter in the receiver and the digital to analog converter in the transmitter generated from different sources and their frequencies always have a certain degree of deviation, i.e., sampling frequency offset (SFO). It may significantly deteriorate the receiver performance. Besides, the imperfect frequency response of the optical/electrical devices is another factor for the degraded BER performance. In this work, we experimentally investigate the digital interpolation (DI)-based SFO compensation in a short-reach NHS-OFDM transmission system. The orthogonal circulant matrix transform (OCT) precoding technique is employed to equalize the signal-to-noise ratio over data-carrying subcarriers and then improve the BER performance. The experimental results indicate that, with the help of the 4-th order DI, the SFO up to 200 ppm can be well compensated without power penalties in comparison with the SFO-free case. Furthermore, the receiver sensitivity of the NHS-OFDM enabled by OCT precoding can be improved by more than 2 dB at the BER of 1e-3 after 20 km standard single-mode fiber transmission.

Analysis of synchronous interpolated signals in Tactical Sensor Network-Visible light communication (TSN-VLC) link

Sampling frequency offset (SFO) is an important issue in the tactical sensor network (TSN) based visible light communication (VLC) systems in Tactical Sensor Network. A digital interpolation 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 signal spectrum. The oversampling factor is mainly determined by the order of the digital interpolation filter and TSN links. The design of the TSN-VLC receiver incorporating the digital interpolation filter is vital as it affects not only the transmission performance but also the complexity of signal processing. To evaluate the feasibility of the digital interpolation-based SFO compensation schemes for cost-sensitive TSN applications. Based on the real-time VLC tactical sensor network, the comparison in the VLC-TSN transmission performance and DSP complexity between different interpolation-based SFO compensation schemes is analyzed.

Human opinion dynamics optimization for ICI mitigation in MC-CDMA systems

This paper presents a novel social impact based approach named as Human Opinion Dynamics Optimization (HODO) to detect signals of Multi-Carrier Code Division Multiple Access (MC-CDMA) systems in the presence of Inter Subcarrier Interference (ICI). The ICI occurs because of various factors, such as Carrier Frequency Offset (CFO), Sampling Frequency Offset (SFO) and channel mobility. The ICI degrades orthogonality between different subcarriers in MC-CDMA system due to which Multiple Access Interference (MAI) occurs. The HODO is based on the socio-psychological behavior of humans in the society. Opinion dynamics leads to human consensus and consensus builds up due to human interactions in a society. The proposed approach could be used to introduce human opinion based intelligent communication networks. Our method is novel in the sense that with this algorithm, social sciences could be made compatible with wireless communication technology, which will go a long way to bridge the gap between humans and the communication technology. This new method works with very few number of control parameters and reduces the bit error rate (BER) better as compared with other conventional stochastic methods like Genetic Algorithm (GA) and Particle Swarm Optimization (PSO). Numerical simulation results show that the proposed detector also reduces the computational complexity manifold.

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.

WS-WiFi: Wired Synchronization for CSI Extraction on COTS-WiFi-Transceivers

WiFi channel state information (CSI) is a key enabler of precise radio sensing applications for commercially available off-the-shelf hardware. By utilizing colocated WiFi devices, this article aims to perform Angle-of-Arrival (AoA) estimation as one instance of radio sensing. However, unsynchronized local oscillators (LOs) in a WiFi network can cause carrier frequency offsets and sampling frequency offsets, which can lead to imperfect CSI. To this end, we propose a wired LO clocking scheme to eliminate residual impairments on regular WiFi transceivers. By utilizing a common oscillator, we transform a standard WiFi setup to a frequency-aligned transceiver architecture across several colocated devices. Building upon the proposed scheme, we present a detailed investigation of hardware-related impairments and environmental conditions. In particular, we are the first to investigate the temperature-dependent behavior of the CSI phase across multiple devices. We validate the performance in a lab setup as well as in a real-world application assembly, namely, a nine-antenna uniform circular array for AoA estimation. Furthermore, we show that with only a single received packet and minor hardware adjustments, we are able to obtain the AoA of a WiFi transmitter with a mean absolute error of 1.5° in line-of-sight conditions and 19.9° in strong multipath living room conditions.

Sub‐Nyquist receiver using sampling frequency and instant offsets

With rapid advance in analogue-to-digital converters, more analogue-to-digital converters in the market can cover multiple Nyquist zones. As a result, the implementation of a sub-Nyquist receiver designed to cover a bandwidth wider than what is dictated by Nyquist theory has become more feasible than ever. This article presents a sub-Nyquist receiver using both different sampling rates and an offset at sampling instant. Compared with other sub-Nyquist receivers, the proposed design is straightforward to implement and can avoid issues conventional sub-Nyquist receivers encounter at the presence of multiple signals.

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

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