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Quadrature Amplitude Modulation (QAM) is a multi-level modulation scheme applied to achieve high data transmission rates. In the 16-QAM scheme, high Peak-to-Average Ratio (PAPR) is an issue as wireless communication devices are required to be small with high power efficiency. To resolve this issue, hexagonal constellation symbol mapping has been proposed to reduce the PAPR. In a real environment, Orthogonal Frequency Division Multiplexing (OFDM) systems are sensitive to signal distortions such as multipath channel and frequency offsets. When the pilot signals are used to compensate the signal distortion, the combination of pilot signals and mapping need to be considered. The paper investigates the PAPR of the hexagonal constellations and examines the relationship between the PAPR and the interval of pilot signal in OFDM system. Bit Error Rate (BER) performance are evaluated with computer simulations. [ABSTRACT FROM AUTHOR]

Multistatic synthetic aperture radar (SAR) is a special mode of SAR system. The radar transmitter and receiver are located on different satellites, which brings many advantages, such as flexible baseline configuration, diverse receiving modes, and more detailed ground object classification information. The multistatic SAR has been widely used in interferometry, moving target detection, three-dimensional imaging, and other fields. The frequency offset between different oscillators will cause a modulation phase error in the signal. Therefore, phase synchronization is one of the most critical problems to be addressed in distributed SAR systems. This article reviews phase synchronization techniques, which are mainly divided into two methods: synchronization by direct microwave link and synchronization by a data-based estimation algorithm. Furthermore, the future development of synchronization technology is anticipated. [ABSTRACT FROM AUTHOR]

Abstract The 5G New Radio synchronization procedure is the first step that the user must complete to access the mobile network. It mainly consists of the detection of the primary and secondary synchronization signals (PSS and SSS, respectively) and the decoding of the physical broadcast channel (PBCH). Our goal is to provide a comprehensive study of the synchronization procedure and investigate different techniques and approaches, through the implementation of a 5G New Radio-compliant simulator. Of significant interest is the investigation of impairments such as the fading channel, the frequency offset, and the delay spread. The results are provided in terms of detection probability for the PSS and SSS detection, and in terms of block error rate for the PBCH. From the data collected, there is evidence that choosing M-sequences for the PSS leads to an appreciably robust solution against frequency offset. The structure of the Gold sequences for SSS generation can be exploited to reduce the detection complexity, and different approaches can be chosen to improve reliability against delay spread. Moreover, the polar coding for 5G PBCH outperforms the former 4G coding technique, but they are still sensible to frequency offset. Finally, the simulator functionalities are validated through real captures of 5G signals.

The 5G New Radio synchronization procedure is the first step that the user must complete to access the mobile network. It mainly consists of the detection of the primary and secondary synchronization signals (PSS and SSS, respectively) and the decoding of the physical broadcast channel (PBCH). Our goal is to provide a comprehensive study of the synchronization procedure and investigate different techniques and approaches, through the implementation of a 5G New Radio-compliant simulator. Of significant interest is the investigation of impairments such as the fading channel, the frequency offset, and the delay spread. The results are provided in terms of detection probability for the PSS and SSS detection, and in terms of block error rate for the PBCH. From the data collected, there is evidence that choosing M-sequences for the PSS leads to an appreciably robust solution against frequency offset. The structure of the Gold sequences for SSS generation can be exploited to reduce the detection complexity, and different approaches can be chosen to improve reliability against delay spread. Moreover, the polar coding for 5G PBCH outperforms the former 4G coding technique, but they are still sensible to frequency offset. Finally, the simulator functionalities are validated through real captures of 5G signals. [ABSTRACT FROM AUTHOR]

Internet of Things (IoT) has widely been deployed in various industrial fields. To achieve hyperautomation in IoT-based applications, it calls for high-efficient wireless transmission technologies to improve the response speed of IoT nodes. Frequency-domain contention (FDC) is a promising contention-based channel access mechanism to meet this requirement. However, the nature of FDC's multiple-transmission-multiple-reception and the limit of IoT chip's synchronization accuracy will lead to carrier frequency offset (CFO) and hence frequent transmission collisions and null transmissions. Considering integer and fractional CFO, this paper theoretically analyzes the FDC throughput and verifies the accuracy of our model via extensive simulations. [ABSTRACT FROM AUTHOR]

Multistatic synthetic aperture radar (SAR) is a special mode of SAR system. The radar transmitter and receiver are located on different satellites, which brings many advantages, such as flexible baseline configuration, diverse receiving modes, and more detailed ground object classification information. The multistatic SAR has been widely used in interferometry, moving target detection, three-dimensional imaging, and other fields. The frequency offset between different oscillators will cause a modulation phase error in the signal. Therefore, phase synchronization is one of the most critical problems to be addressed in distributed SAR systems. This article reviews phase synchronization techniques, which are mainly divided into two methods: synchronization by direct microwave link and synchronization by a data-based estimation algorithm. Furthermore, the future development of synchronization technology is anticipated.

With the prevalence of renewable energy sources such as wind power in the power system, analyzing the fault characteristics of systems composed of DFIGs is becoming increasingly important. Therefore, this article analyzes, at first theoretically, the fault characteristics of a doubly fed induction generator (DFIG) during fault periods. It was found that the fault current of the DFIG exhibited the frequency offset phenomenon, which is affected by the depth of voltage dips and can negatively impact traditional distance protection. Furthermore, a method using a dynamic voltage restorer (DVR) based on superconducting magnetic energy storage (SMES) was adopted to compensate for the fault voltage of DFIG, which can mitigate the voltage dips of the DFIG. This method can not only achieve the fault ride through for DFIG but also significantly improve the frequency offset of the fault current during fault periods. Finally, a model composed of a 2.5 MW DFIG-based wind turbine and a 2.5 MW DVR-based SMES was built using a real-time digital simulator (RTDS) platform, and the simulation results showed that the fault stator voltage of DFIG can be compensated at a rated value of 0.69 kV, and the frequency of fault current can be maintained at 50 Hz These results validate the excellent performance of the method in achieving the fault ride through of DFIG and improving the frequency offset of the fault current by comparing multiple type faults while employing different protection methods. [ABSTRACT FROM AUTHOR]

Summary: In magnetically resonant wireless power transfer (MR‐WPT) systems, transmitter, and receiver usually operate at the same resonant frequency. However, parameters of inductor, capacitor, or other components are susceptible to changes due to environmental factors and cause resonant frequency deviation. This would lead to decrease of transmission performance. To solve the problem, a wireless power transfer system based on a nonlinear compensation network is proposed to suppress frequency offset. The modeling of the nonlinear MR‐WPT system is first established. On this basis, the mathematical equation of the equivalent nonlinear circuit is deduced and the amplitude‐frequency response characteristics are analyzed. The influence of inductance parameter on the nonlinear characteristic is studied. Furthermore, the effects of excitation amplitude, compensation capacitance, load, and magnetic ring turns on the operating characteristics of the nonlinear resonant circuit are analyzed. The proposed nonlinear resonant wireless power transfer system is constructed and is experimentally studied. Compared with the traditional linear MR‐WPT system, the results show that frequency offset can be effectively suppressed, which ensures efficient transmission performance with the proposed method. [ABSTRACT FROM AUTHOR]

In recent years, there have been increased demands for aggregating sensor information from several sensors owing to the spread of the Internet of Things (IoT). However, packet communication, which is a conventional multiple-access technology, is hindered by packet collisions owing to simultaneous access by sensors and waiting time to avoid packet collisions; this increases the aggregation time. The physical wireless parameter conversion sensor network (PhyC-SN) method, which transmits sensor information corresponding to the carrier wave frequency, facilitates the bulk collection of sensor information, thereby reducing the communication time and achieving a high aggregation success rate. However, when more than one sensor transmits the same frequency simultaneously, the estimation accuracy of the number of accessed sensors deteriorates significantly because of multipath fading. Thus, this study focuses on the phase fluctuation of the received signal caused by the frequency offset inherent to the sensor terminals. Consequently, a new feature for detecting collisions is proposed, which is a case in which two or more sensors transmit simultaneously. Furthermore, a method to identify the existence of 0, 1, 2, or more sensors is established. In addition, we demonstrate the effectiveness of PhyC-SNs in estimating the location of radio transmission sources by utilizing three patterns of 0, 1, and 2 or more transmitting sensors. [ABSTRACT FROM AUTHOR]

Due to the modulation of tiny frequency offset on the array elements, a frequency diverse array (FDA) jammer can generate multiple range-dimension point false targets, and many deception jamming methods against SAR using an FDA jammer have been studied. However, the potential of the FDA jammer to generate barrage jamming has rarely been reported. In this paper, a barrage jamming method against SAR using an FDA jammer is proposed. To achieve two-dimensional (2-D) barrage effect, the stepped frequency offset of FDA is introduced to generate range-dimensional barrage patches, and the micro-motion modulation is employed to increase the extent of barrage patches along the azimuth direction. Mathematical derivations and simulation results demonstrate the validity of the proposed method in generating flexible and controllable barrage jamming. [ABSTRACT FROM AUTHOR]

One of the orthogonal frequency division multiplexing (OFDM) system disadvantages is its sensitivity to frequency offset and phase noise, which lead to losing the orthogonality between the subcarriers and thereby degrade the system performance. In this paper a joint scheme for frequency offset and pilot-based channel estimation is introduced in which the frequency offset is first estimated using an autocorrelation method, and then is fined further by applying an iterative phase correction by means of pilot-based Wiener filtering method. In order to verify the capability of the estimation algorithm, the scheme has been implemented and tested using a real measurement system in a multipath indoor environment. The results show the algorithm capability of compensating for the frequency offset with different transmission and channel conditions.

Automatic modulation recognition is a necessary part of cooperative and noncooperative communication systems and plays an important role in military and civilian fields. Although the constellation diagram (CD) is an essential feature for different digital modulations, it is hard to be extracted under noncooperative complex communication environment. Frequency offset, especially the nonlinear frequency offset is a vital problem of complex communication environment, which greatly affects the extraction of traditional CD and the performance of modulation recognition methods. In the current paper, we propose an antifrequency offset constellation diagram (AFO-CD) extraction method, which combines the constellation diagram with a convolutional neural network (CNN). The proposed method indicates the change of the CD with time and enables us to suppress the influence of frequency offset efficiently. Additionally, a residual units-based classifier is designed for multiscale feature extraction and modulation classification. The experimental results demonstrate that the proposed method can effectively improve the recognition accuracy and has a good application prospect in the complex electromagnetic environment. [ABSTRACT FROM AUTHOR]

Accuracy of range measurements using navigation satellites primarily depends on on-board clock correction parameters which are broadcasted to users periodically. The on-board clock offset, a critical correction parameter which accumulates due to fractional frequency offset as well as frequency drift, has to be maintained within the allocated broadcast limit throughout the satellite's service. In IRNSS satellites, the clock offset is maintained within the broadcast limit by applying frequency offset corrections to the on-board atomic clock monitoring unit. In this paper, we discuss the methodology implemented for computing the clock offset using one-way range measurement data. Further, we report the time keeping methodology employed in IRNSS satellites that enables maintaining the satellite clock offset within the broadcast limit with minimum number of intervening frequency offset corrections throughout their operational life time. [ABSTRACT FROM AUTHOR]

Abstract Electronic beam steering is an essential feature of state-of-the-art radar systems. Conventional phased array (PA) radars with fixed carrier frequencies are well-known for electronically steering their beam with high directivity. However, the resulting beampattern is angle-dependent but range-independent. Recently, a new electronic beam steering concept, referred to as frequency diverse array (FDA) radar, has attracted increasing attention due to its unique range-angle dependent beampattern. More importantly, the FDA radar employs a small frequency increment across the array elements to achieve beam steering as a function of angle, range, and time. In this paper, we review the development of the FDA radar since its inception in 2006. Since the frequency offset attaches great importance in FDAs to determine the beampattern shape, initially much of the research and development were focused on designing the optimal frequency offsets for improved beampattern synthesis. Specifically, we analyze characteristics of the FDA beampattern synthesis using various frequency offsets. In addition to analyzing the FDA beampattern characteristics, this study also focuses on the neglected propagation process of the transmitted signals in the early FDA literature, and discuss the time-variant perspective of FDA beampatterns. Furthermore, FDA can also play a significant role in wireless communications, owing to its potential advantages over the conventional PAs. Therefore, we highlight its potential applications in wireless communication systems. Numerical simulations are implemented to illustrate the FDA beampattern characteristics with various frequency offset functions.

With the prevalence of renewable energy sources such as wind power in the power system, analyzing the fault characteristics of systems composed of DFIGs is becoming increasingly important. Therefore, this article analyzes, at first theoretically, the fault characteristics of a doubly fed induction generator (DFIG) during fault periods. It was found that the fault current of the DFIG exhibited the frequency offset phenomenon, which is affected by the depth of voltage dips and can negatively impact traditional distance protection. Furthermore, a method using a dynamic voltage restorer (DVR) based on superconducting magnetic energy storage (SMES) was adopted to compensate for the fault voltage of DFIG, which can mitigate the voltage dips of the DFIG. This method can not only achieve the fault ride through for DFIG but also significantly improve the frequency offset of the fault current during fault periods. Finally, a model composed of a 2.5 MW DFIG-based wind turbine and a 2.5 MW DVR-based SMES was built using a real-time digital simulator (RTDS) platform, and the simulation results showed that the fault stator voltage of DFIG can be compensated at a rated value of 0.69 kV, and the frequency of fault current can be maintained at 50 Hz These results validate the excellent performance of the method in achieving the fault ride through of DFIG and improving the frequency offset of the fault current by comparing multiple type faults while employing different protection methods.

A number-theoretic net (NT-net)-based Gaussian particle filter (NT-GPF) is proposed for the joint estimation of frequency offset (FO) and linear phase noise (LPN) in a dynamic and real-time manner. Based on the concept of uniform design, the NT-net can uniformly generate particles on an ellipse for a given bivariate normal distribution. As a result, the NT-GPF can achieve accurate FO and LPN tracking with only a small number of particles, which greatly improves the estimation efficiency and noise tolerance compared with the conventional GPF. We also propose a recursive signal detection algorithm to further enhance the final system bit error rate (BER) performance. Simulations verify the accuracy, robustness and efficiency of the NT-GPF. [ABSTRACT FROM AUTHOR]

In this paper, a pilot-aided synchronization method based on combining multi-differential correlations (MDCS) is proposed to overcome the obstacles in the synchronization of high-speed mobile satellite communication links. The proposed approach can optimally use the original synchronization pilot, and the existing transmitter would not need any modification. Additionally, the performance of proposed method is investigated on theoretical level, including the principle of mitigating the frequency offset deterioration and the derivation of closed synchronization probability expression. Finally, a set of simulations were operated to verify the MDCS performance, and the results proved that the MDCS significantly outperformed the existing approaches for large frequency offset and strong channel noise.

The decaying DC calculation is an important task for the implementation of fault detection and transient protection in power system. Speed and precision are its fundamental requirements. Most methods take a long time (0.5 or 1 signal cycle) to obtain the first accurate calculated value. And they are sensitive to frequency offset and amplitude fluctuation. For example, if a fault occurs under the condition of out-of-step oscillation, the calculation result of decaying DC with traditional methods might have a large error. Therefore, this paper conducts targeted research. It first constructs a 90-degree phasor rotator, which owns excellent characteristics, such as superposition properties of rotation transformations, separation properties for DC component and fundamental frequency component, and etc. Next, a decaying DC instantaneous calculation method is proposed. It can obtain the first true value of decaying DC only after a few sampling points after fault. Besides, it maintains high calculation accuracy under the condition of high harmonics, frequency offset and amplitude fluctuation. Compared with the traditional methods, the new method has advantages in terms of computational speed and scenario applicability. This point is verified by experiments. • It solves the decaying DC calculation problem in the out-of-step oscillation scene. • It constructs a 90-degree phasor rotator, and proves its excellent characteristics. • It proposes an instantaneous decaying DC calculation method only with a few sampling points. [ABSTRACT FROM AUTHOR]

In recent years, there have been increased demands for aggregating sensor information from several sensors owing to the spread of the Internet of Things (IoT). However, packet communication, which is a conventional multiple-access technology, is hindered by packet collisions owing to simultaneous access by sensors and waiting time to avoid packet collisions; this increases the aggregation time. The physical wireless parameter conversion sensor network (PhyC-SN) method, which transmits sensor information corresponding to the carrier wave frequency, facilitates the bulk collection of sensor information, thereby reducing the communication time and achieving a high aggregation success rate. However, when more than one sensor transmits the same frequency simultaneously, the estimation accuracy of the number of accessed sensors deteriorates significantly because of multipath fading. Thus, this study focuses on the phase fluctuation of the received signal caused by the frequency offset inherent to the sensor terminals. Consequently, a new feature for detecting collisions is proposed, which is a case in which two or more sensors transmit simultaneously. Furthermore, a method to identify the existence of 0, 1, 2, or more sensors is established. In addition, we demonstrate the effectiveness of PhyC-SNs in estimating the location of radio transmission sources by utilizing three patterns of 0, 1, and 2 or more transmitting sensors.

This study focused on methods estimation and compensation frequency offset in communication systems with LoRa modulation. LoRa being used in IoT applications rapidly expands its markets. The article discusses three methods for estimation and compensation frequency offset as well as details of the comparative analysis made. Aside from the well-known methods by Ghanaatian and Golden Section Search (GSS), the third method is offered by the authors of the article and represents an improved version of the method proposed by Ghanaatian. The proposed method is based on a symmetric processing signal that is divided into two parts. The advantages and drawbacks of the considered methods are described. As a result of the performed study, a software model was developed, the dependence of the root mean-square error (RMSE) estimation of the frequency offset, and cumulative density function (CDF) of the frequency estimation error was calculated. Modeling was performed under identical conditions. The frequency offset accuracy provided by the proposed method exceeded the accuracy of the considered methods. [ABSTRACT FROM AUTHOR]

Cell search is the first step to establish downlink synchronization between user equipment (UE) and base station (BS) in the fifth generation (5G) new radio (NR) system. However, frequency offset and noise will cause a massive loss of synchronization performance. To solve this problem, a novel timing synchronization algorithm of primary synchronization signal (PSS) with anti-frequency offset and anti-noise is proposed, which includes the improved coarse synchronization algorithm based on Fourier transform and the fine synchronization algorithm based on the triple auto-correlation algorithm. The improved coarse synchronization algorithm not only has a strong frequency offset resistance, especially for large frequency offset, but also obtains the estimated frequency offset. The fine synchronization algorithm is more resistant to noise than the conventional algorithm. Finally, the algorithm analysis and simulation results show that the improved algorithm can work very well even when the normalized frequency offset increases to greater than 0.8, but the conventional algorithm fails to synchronize properly. Meanwhile, when the SNR is as low as -6dB, the detection accuracy of the timing synchronization based on the proposed fine synchronization algorithm is improved by about 31.7% compared with the conventional algorithm.

PURPOSE: Combining MR phase images from multiple receive coils is a challenging problem, complicated by ambiguities introduced by phase wrapping, noise, and the unknown phase-offset between the coils. Various techniques have been proposed to mitigate the effect of these ambiguities but most of the existing methods require additional reference scans and/or use ad hoc post-processing techniques that do not guarantee any optimality. THEORY AND METHODS: Here, the phase estimation problem is formulated rigorously using a maximum-likelihood (ML) approach. The proposed framework jointly designs the acquisition-processing chain: the optimized pulse sequence is a single multiecho gradient echo scan and the corresponding postprocessing algorithm is a voxel-per-voxel ML estimator of the underlying tissue phase. RESULTS: Our proposed framework (Maximum AmbiGuity distance for Phase Imaging, MAGPI) achieves substantial improvements in the phase estimate, resulting in phase signal-to-noise ratio (SNR) gains by up to an order of magnitude compared to existing methods. CONCLUSION: The advantages of MAGPI are: (1) ML-optimal combination of phase data from multiple receive coils, without a reference scan; (2) voxel-per-voxel ML-optimal estimation of the underlying tissue phase, without the need for phase unwrapping or image smoothing; and (3) robust dynamic estimation of channel-dependent phase-offsets.

In this paper, by defining a new physical quantity called as the frequency offset in the harmonic oscillator model and combining with the established plasmon coupling model, we successfully explain the anti-crossing property caused by plasmon coupling of dimer composed of a silver nanoring and a silver nanorod. In the plasmon coupling model, we reasonably reveal that the physical mechanism of the coupling between bright mode and dark mode of the plasmons is mainly represented by the coulomb potential and the electrostatic potential. With this model, we explain also the asymmetric feature the variation of frequency offset and coupling coefficient with coupling distance. [ABSTRACT FROM AUTHOR]

A joint and robust frequency offset (FO) and phase noise tracking scheme using self-learning Kalman filter is proposed and demonstrated experimentally. In our scheme, a self-learning Kalman filter based on kurtosis error is used to track FO and phase noise simultaneously in a decision-free manner. Then, a mean-shift clustering-assisted maximum likelihood estimator is used for residual phase noise compensation. Simulation results indicate that the proposed scheme has advantages in FO estimation accuracy, OSNR sensitivity penalty, and FO jitter rate tolerance and nonlinearity tolerance, compared with the conventional FO and phase noise estimation scheme. With 7% FEC threshold, the proposed scheme can tolerate FO jitter rate up to 10 MHz for 16QAM, which is around 100 times and 20 times than that of VVPE1 +BPS and FFT+UCPE. With 1 dB OSNR penalty, the linewidth tolerance of the proposed scheme is 300 kHz for 64QAM, which is around 30 times and 5 times than that of FFT+UCPE+ML and FFT+ VVPE2. The proposed scheme also shows excellent nonlinearity tolerance over wide launched power, and it provides around 0.4 dB and 0.2 dB Q-factor improvement over VVPE1+ VVPE2, VVPE1 +BPS for 16QAM, respectively. The 28GBaud PDM 16QAM experimental results also indicate that the proposed scheme outperforms the rest of tested scheme under both back-to-back and fiber transmission cases. With optimal launched power, the proposed scheme can provide around 0.2 dB, 0.4 dB, and 0.7 dB Q-factor improvement over VVPE1 +BPS, FFT+UCPE, and VVPE1+ VVPE2 after 915 km fiber transmission, respectively. To the best of our knowledge, it is the first time that a joint and modulation transparent with robust nonlinearity and FO jitter rate tolerance is proposed and demonstrated for 64QAM. [ABSTRACT FROM AUTHOR]

Most of the existing deception jamming techniques focus on designing effective jamming signals for static targets or scenes. This paper proposes a multi-scene deception jamming against synthetic aperture radar (SAR) imaging with frequency diverse array (FDA) antenna. Different from traditional single-element and phased-array antennas, FDA antenna employs a small frequency offset across the array elements to produce angle-, range- and even time-dependent transmit beampattern, which brings bright prospect for the application of new jamming techniques against SAR imaging. Such a new method will generate many false targets or multiple deception scenes for the jammed SAR, which helps jam better on the static scene and moving target. Moreover, the location and number of false targets can be manipulated. This paper will illustrate the FDA-based deception jamming against SAR imaging, specially designed waveform and the relationship between the number of false targets or scenes and FDA parameters. The relationship between the location of false targets and the position of jammer will also be described in detail. All proposed methods are validated by extensive simulation results.

The DFT-based demodulator for BFSK has been introduced for applications where the received signal experiences a carrier frequency offset (CFO) much larger than the symbol rate. The Ultra-Narrowband (UNB) communication techniques have been introduced for implementing the emerging Low Power Wide Area Networks (LPWAN). Since UNB communication is prone to CFO, a DFT-based BFSK demodulator is an interesting option for this type of communication. However, for proper operation in a large frequency offset, the DFT-based demodulator requires a complex window synchronization which is not desirable for low power nodes. The main source of complexity, is calculating the DFT of a window which slides over the preamble. In this work, the complexity is alleviated by considering the window synchronization algorithm and its implementation together. First, a new window synchronization algorithm is proposed which is designed such that an efficient class of implementations of the sliding DFT (SDFT), called Single Bin SDFT (SB-SDFT) in this work, can be used. Moreover, a new stable implementation of SB-SDFT is designed to enable zero-padding which is required by the demodulator. The complexity of the proposed algorithm implemented using the SB-SDFT, scales more efficiently compared to the conventional algorithm when the range of tolerable CFO increases. Using the proposed method, for a CFO tolerance in the order of 14.5 times the symbol rate (±14.5 kHz for a symbol rate equal to 100 Hz), the number of complex operations is reduced by more than 85% (and memory by 90%) compared to the conventional method.

In order to ensure the performance of burst signal demodulation, forward symbol timing offset estimation algorithm is usually used for symbol timing recovery. Aiming at the shortcomings of common forward symbol timing estimation algorithms, a new algorithm based on maximum likelihood estimation joint trigonometric polynomial interpolation is proposed in this article, which is suitable for the data of four samples per symbol and can resist large frequency offset. Hereafter, in order to make it more suitable for engineering practice, optimize it into an improved data-aided (DA) forward symbol timing offset estimation algorithm with multiple characteristics, e.g., moderate frequency offset capture range, insensitive to shaping coefficient, relatively low complexity, excellent estimation performance, flexible algorithm structure and less sample data required for calculation. The simulation results show that the performance of the improved algorithm in this article can approach the modified cramer-rao bound (MCRB) within the frequency offset capture range, under the conditions of low signal to noise ratio (SNR) and small shaping coefficient.

Due to the modulation of tiny frequency offset on the array elements, a frequency diverse array (FDA) jammer can generate multiple range-dimension point false targets, and many deception jamming methods against SAR using an FDA jammer have been studied. However, the potential of the FDA jammer to generate barrage jamming has rarely been reported. In this paper, a barrage jamming method against SAR using an FDA jammer is proposed. To achieve two-dimensional (2-D) barrage effect, the stepped frequency offset of FDA is introduced to generate range-dimensional barrage patches, and the micro-motion modulation is employed to increase the extent of barrage patches along the azimuth direction. Mathematical derivations and simulation results demonstrate the validity of the proposed method in generating flexible and controllable barrage jamming.

Electronic beam steering is an essential feature of state-of-the-art radar systems. Conventional phased array (PA) radars with fixed carrier frequencies are well-known for electronically steering their beam with high directivity. However, the resulting beampattern is angle-dependent but range-independent. Recently, a new electronic beam steering concept, referred to as frequency diverse array (FDA) radar, has attracted increasing attention due to its unique range-angle dependent beampattern. More importantly, the FDA radar employs a small frequency increment across the array elements to achieve beam steering as a function of angle, range, and time. In this paper, we review the development of the FDA radar since its inception in 2006. Since the frequency offset attaches great importance in FDAs to determine the beampattern shape, initially much of the research and development were focused on designing the optimal frequency offsets for improved beampattern synthesis. Specifically, we analyze characteristics of the FDA beampattern synthesis using various frequency offsets. In addition to analyzing the FDA beampattern characteristics, this study also focuses on the neglected propagation process of the transmitted signals in the early FDA literature, and discuss the time-variant perspective of FDA beampatterns. Furthermore, FDA can also play a significant role in wireless communications, owing to its potential advantages over the conventional PAs. Therefore, we highlight its potential applications in wireless communication systems. Numerical simulations are implemented to illustrate the FDA beampattern characteristics with various frequency offset functions. [ABSTRACT FROM AUTHOR]

In this paper, we apply the advanced waveform universal filtered multi-carrier (UFMC) to a heterogeneous small-cell network, where both the inner-cell interference and the inter-cell interference are considered. By employing UFMC, a dynamic spectrum allocation scheme and a precoding scheme are proposed to mitigate the interference. The proposed UFMC based interference management schemes are analyzed in detail, and the achievable data rate of the proposed scheme is derived. Simulation results show that the proposed UFMC based schemes can efficiently mitigate the interference, and reduce the effect of frequency offsets.

Similar to all mobile communication networks, synchronization in the time-frequency domain is a fundamental step that allows a fifth-generation (5G) new radio (NR) user equipment (UE) to properly receive and transmit its data. Due to the wide range of frequencies that are defined for the 5G NR systems, the corresponding synchronization procedure becomes critical and presents many challenges, especially for the applications that would need accurate oscillators to reduce the large values of the frequency offset. In this paper, we present and detail the 5G NR physical layer. Then, we describe the required synchronization procedure for 5G NR. And finally, we present the main challenges and issues within the 5G NR synchronization.

In this paper, a pilot-aided synchronization method based on combining multi-differential correlations (MDCS) is proposed to overcome the obstacles in the synchronization of high-speed mobile satellite communication links. The proposed approach can optimally use the original synchronization pilot, and the existing transmitter would not need any modification. Additionally, the performance of proposed method is investigated on theoretical level, including the principle of mitigating the frequency offset deterioration and the derivation of closed synchronization probability expression. Finally, a set of simulations were operated to verify the MDCS performance, and the results proved that the MDCS significantly outperformed the existing approaches for large frequency offset and strong channel noise. [ABSTRACT FROM AUTHOR]

This study focused on methods estimation and compensation frequency offset in communication systems with LoRa modulation. LoRa being used in IoT applications rapidly expands its markets. The article discusses three methods for estimation and compensation frequency offset as well as details of the comparative analysis made. Aside from the well-known methods by Ghanaatian and Golden Section Search (GSS), the third method is offered by the authors of the article and represents an improved version of the method proposed by Ghanaatian. The proposed method is based on a symmetric processing signal that is divided into two parts. The advantages and drawbacks of the considered methods are described. As a result of the performed study, a software model was developed, the dependence of the root mean-square error (RMSE) estimation of the frequency offset, and cumulative density function (CDF) of the frequency estimation error was calculated. Modeling was performed under identical conditions. The frequency offset accuracy provided by the proposed method exceeded the accuracy of the considered methods.

Different from the conventional phased array radar, a frequency diverse array (FDA) applies additional frequency shift across the array elements, yielding a range–angle-dependent transmit beampattern that provides a huge potential opportunity for target estimation and range-dependent clutter and interference suppression. Generally, the conventional FDA employs a linear small frequency increment, forming a time-varying “S”-shape range–angle beampattern, which is range–angle coupled. In this article, a range–angle transmit beampattern synthesis method for FDA based on frequency offset optimization is proposed. Particle swarm optimization (PSO) is adopted in the FDA element frequency increment design to achieve a dot-shaped transmit beampattern. Meanwhile, matched weights are designed for time-modulation compensation. Numerical simulation results demonstrate that the proposed approach achieves better time-invariant performance over other FDA frameworks. [ABSTRACT FROM AUTHOR]

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A newfangled family of Nyquist-I pulses is proposed and named improved dual sinc pulse (IDSP). The IDSP is designed to improve performance in orthogonal frequency division multiplexing (OFDM)-based systems. The IDSP is a generalization of the dual sinc pulse (DSPP). This is because the DSP was formulated for α = 1 whereas the IDSPP is valid for 0 ≤ α ≤ 1. The behavior of the IDSP is promising in terms of its frequency and time domain responses. Theoretical and numerical outcomes indicate that the IDSP outperformed other existing pulses applied in OFDM-based systems for various key evaluation metrics. [ABSTRACT FROM AUTHOR]

We consider a wireless wideband downlink transmission for high-speed railway. In this scenario, the wireless channel is double selective, i.e., multipath and highly time-varying, and involves non-Gaussian noise at the receiver side. Conventional receiver algorithms designed for Gaussian noise and time-invariant channels are inefficient especially for impulsive noise and time-varying multipath channels, which are the main challenges in high-speed railway. They may lead to communication outage and involve large carrier offsets, respectively. Regarding the challenges, a synthesis scheme is proposed to resolve these issues in this paper. First, in order to mitigate the impulsive noise, three impulsive noise detection algorithms are adopted to discriminate impulsive noise from the received signal. The blanking algorithm is used to suppress the discriminated noise. Then, the refined signal goes through a well-designed filtering, which transforms the received signal with varying frequency offsets into an angle domain. The advantage in the angle domain is that the frequency offset in each angle becomes nearly constant. Thus, the frequency offsets can be estimated within each angle to realize accurate estimation. In the simulation, three different impulsive noise models are considered to evaluate the communication reliability in this circumstance. The simulation results show that the proposed scheme significantly improves the reliable performance of the high-speed railway communication systems.

Frequency diverse array (FDA) is capable of generating a range-angle-dependent beampattern via adjusting the frequency offset across array elements. The classical linear FDA generates an S-shaped beampattern due to the range-angle-dependent coupling. In order to remove the coupling, and obtain a well-focused dot-shaped beampattern, several FDA frameworks are proposed via adjusting frequency offsets. It seems that the window based tapering frequency offsets are able to form a single maximum with a satisfactory mainbeam sharpness. However, most window-based FDA schemes are focused on classical fixed windows. In this letter, an FDA framework with Taylor windowed frequency offsets is first proposed. Considering that adjustable tapering windows allow adjustments by parameter manipulation, the optimized parameters of the window function are determined. The FDA beamforming results with all kinds of frequency offsets are measured, and compared. Numerical results validate the superiority of proposed FDA in terms of mainlobe width, and sidelobe level. [ABSTRACT FROM AUTHOR]

In orthogonal frequency-division multiple access (OFDMA), carrier frequency offset (CFO) destroys the orthogonality between subcarriers due to Doppler effect of the channel and also causes inter-carrier interference (ICI) as well as multiple access interference (MAI). In this paper, CFO tracking algorithm for OFDMA system is presented to overcome the effect of ICI as well as MAI. The proposed algorithm employs normalized least-mean-square (NLMS) algorithm which tracks the residual carrier frequency offset in OFDMA system. The proposed NLMS-based tracking algorithm uses average of all residual carrier frequency offset (RCFO) for improving the mean square error (MSE) performance. The proposed NLMS-based frequency tracking along with frequency offset compensator improves the error performance by reducing the effect of ICI and MAI. Simulation results indicate that the proposed algorithm achieves better tracking performance in the presence of frequency offset. From the results, it is observed that the MSE of the proposed estimator is improved about 3.8 dB at SNR = 20 dB with the frequency offset. [ABSTRACT FROM AUTHOR]

Summary: In this paper, we consider a rateless coded Orthogonal Frequency Division Multiplexing (OFDM) system under a quasistatic fading channel. During each transmission round, transmitter keeps transmitting to the receiver using Raptor code until the receiver feeds back an acknowledgement (ACK). On the other hand, frequency offset between the transmitter and receiver ruins the orthogonality of OFDM subcarriers and cause intercarrier interference (ICI). We resort to ICI self‐cancelation precoding to combat ICI, wherein the data symbol vectors are multiplied with some precoding matrix before transmission. To improve the system robustness, we jointly optimize the precoding matrix and the degree profile of Raptor code, with only statistical channel state information (CSI) being assumed at the transmitter. The optimization problem is formulated based on the extrinsic information transfer (EXIT) analysis of the decoding process at the receiver. The advantage of the proposed design is that the instant CSI is not required at the transmitter, which reduces the system overhead. Simulation results verify that the proposed scheme with the optimized precoding matrix and degree profile can effectively combat ICI and achieve good performance both in bit error ratio (BER) and average transmission rate. [ABSTRACT FROM AUTHOR]

Precoding is proved to be highly efficient to improve the bit error rate (BER) of orthogonal frequency division multiplexing (OFDM) in frequency-selective fading channels. Nevertheless, most of the work reported in the literature ignores the radio frequency impairments such as carrier frequency offset (CFO), which is known to be of significant impact on OFDM. Consequently, this paper considers the performance evaluation of precoded OFDM (P-OFDM) in the presence of CFO. The performance of the considered P-OFDM is evaluated in terms of the signal-to-interference plus-noise-ratio (SINR) and BER. Various channel models are considered and closed-form analytical expressions are derived for the exact SINR. The obtained analytical results, corroborated by simulation, show that P-OFDM is substantially more sensitive to CFO compared with conventional OFDM. Generally speaking, if the normalized CFO is about 18%, then OFDM will outperform P-OFDM for most practical channel models. It is also interesting to note that the subcarriers in P-OFDM may experience different SINRs, which is not the case for conventional OFDM. The paper also considers the SINR of OFDM after the equalization process. The obtained results show that ignoring the impact of the equalization process results in inflated SINR degradation. [ABSTRACT FROM AUTHOR]

Improving the stability of national time scale (TS) is a main target for all time keepers. The stability of TS can be improved by either increasing the number of atomic clocks in the ensemble or by adding clocks of higher stability. In both cases, a large financial budget is needed which may be unaffordable by some developing economics. In this paper, we propose a new method for improving the stability of national TS by a factor approaching 50% with zero expenses by replacing the real clocks in the ensemble by virtual clocks (VCs). The VCs are obtained by downscaling (DS) the frequency offset of the real clocks. The DS process improves the frequency stability of the clocks and hence that of the resultant average TS. Since the downscaled clocks are correlated to the original ones, they replace the real clocks and not added to the ensemble. The performance of the proposed method is verified by applying it on the daily clocks comparison data of the Observatory of Paris (OP) published on the time department server of the Bureau International des Poids et Measures. The performance of the proposed method is compared with that of Kalman filter (KF) method presented in the literature. It is found that the proposed method is simpler and acting as a wider bandwidth de-noising filter than KF method for achieving the same performance. [ABSTRACT FROM AUTHOR]

In this paper, a coupled oscillator model is employed to describe the eigen-frequencies of the coupled plasmons, which are dominated by the coupled coefficient and the frequency offset defined by us. From this, we present a novel physical model to describe the influence of the Coulomb potential and the electrostatic potential (CPEP) on the coupled coefficient and the frequency offset. Based on the finite element method, we simulate numerically the evolution of the absorption spectra of the microstructure unit composed of a simply metal nanostructure. The simulated results show that two resonant absorption peaks are asymmetrical with the change of the coupling distance between the nano-elements. With the CPEP model, we not only explain successfully the asymmetrical behaviors of the peaks values of the absorption spectra with the change of the coupling distance but also obtain the resonant frequencies for different plasmons. The absorption spectra are simulated by the harmonic oscillator model, in which their calculated parameters originate from the CPEP model, and the results are very well association with those of the numerical simulation. [ABSTRACT FROM AUTHOR]

Ｉｎ ｔｈｅ ｃａｓｅ ｏｆ ｇｒｉｄ ｆａｕｌｔ ｃｏｎｄｉｔｉｏｎｓ, ｔｈｅ ｔｒａｄｉｔｉｏｎａｌ ＳＲＦ-ＰＬＬ ｈａｓ ｌａｒｇｅ ｆｒｅｑｕｅｎｃｙ ａｎｄ ｐｈａｓｅ ｓｉｇｎａｌ ｄｅｔｅｃｔｉｏｎ ｅｒｒｏｒｓ ｗｈｅｎ ｐｅｒｆｏｒｍｉｎｇ ｇｒｉｄ ｓｉｇｎａｌ ｓｙｎｃｈｒｏｎｉｚａｔｉｏｎ, ａｎｄ ａ ｇｒｉｄ ｓｉｇｎａｌ ｓｙｎｃｈｒｏｎｉｚａｔｉｏｎ ｍｅｔｈｏｄ ｂａｓｅｄ ｏｎ ｃａｓｃａｄｅｄ ｆｉｌｔｅｒ ｐｈａｓｅ-ｌｏｃｋｅｄ ｌｏｏｐ ｗａｓ ｐｒｏｐｏｓｅｄ. Ｉｎ ＱＴ１-ＰＬＬ, ｔｈｅ ｃａｓｃａｄｅｄ ｆｉｌｔｅｒｉｎｇ ｓｔｒｕｃｔｕｒｅ ｗａｓ ｕｓｅｄ ｔｏ ｉｍｐｒｏｖｅ ｔｈｅ ｆｉｌｔｅｒｉｎｇ ｐｅｒｆｏｒｍａｎｃｅ ａｎｄ ｔｈｅ ｒｅｓｐｏｎｓｅ ｓｐｅｅｄ ｏｆ ｔｈｅ ｓｙｓｔｅｍ. Ｔｈｅ ｅｆｆｅｃｔ ｏｆ ｇｒｉｄ ｆｒｅｑｕｅｎｃｙ ｏｆｆｓｅｔ ｏｎ ｐｈａｓｅ-ｌｏｃｋｅｄ ｒｅｓｕｌｔｓ ｃａｎ ｂｅ ｅｌｉｍｉｎａｔｅｄ ｂｙ ａｄｄｉｎｇ ｐｈａｓｅ ａｎｄ ａｍｐｌｉｔｕｄｅ ｃｏｍｐｅｎｓａｔｉｏｎ ｌｉｎｋｓ ｉｎ ｔｈｅ ｐｈａｓｅ-ｌｏｃｋｅｄ ｌｏｏｐ. Ｓｉｍｕｌａｔｉｏｎ ａｎｄ ｅｘｐｅｒｉｍｅｎｔａｌ ｒｅｓｕｌｔｓ ｓｈｏｗｅｄ ｔｈａｔ ａｃｃｕｒａｔｅ ｆｒｅｑｕｅｎｃｙ ａｎｄ ｐｈａｓｅ ｄｅｔｅｃｔｉｏｎ ｃａｎ ｂｅ ａｃｈｉｅｖｅｄ ｉｎ １􀆰 ５ ｇｒｉｄ ｐｅｒｉｏｄｓ ｗｈｅｎ ｔｈｅ ｖｏｌｔａｇｅ ｓｉｇｎａｌ ｈａｓ ａ ２０° ｐｈａｓｅ ｓｔｅｐ ａｎｄ ４ Ｈｚ ｆｒｅｑｕｅｎｃｙ ｓｔｅｐ. Ｗｈｅｎ ｔｈｅ ｆｒｅｑｕｅｎｃｙ ａｎｄ ｐｈａｓｅ ｏｖｅｒｓｈｏｏｔ ａｒｅ ０􀆰 ５ Ｈｚ ａｎｄ ４°, ｒｅｓｐｅｃｔｉｖｅｌｙ, ｉｔ ｃａｎ ｍｅｅｔ ｔｈｅ ｆａｓｔ ａｎｄ ａｃｃｕｒａｔｅ ｒｅｑｕｉｒｅｍｅｎｔｓ ｏｆ ｔｈｅ ｇｒｉｄ-ｃｏｎｎｅｃｔｅｄ ｓｙｓｔｅｍ, ａｎｄ ｖｅｒｉｆｙ ｔｈｅ ｃｏｒｒｅｃｔｎｅｓｓ ａｎｄ ｅｆｆｅｃｔｉｖｅｎｅｓｓ ｏｆ ｔｈｅ ｐｒｏｐｏｓｅｄ ｔｈｅｏｒｙ. [ABSTRACT FROM AUTHOR]

MIMO-OFDM models of RPAS communication channels based on LTE Standard were built. Dependencies of the BER on the SNR for Extended Pedestrian A and Extended Vehicular A models using 2-by-2 multiple antennas were obtained. Dependencies of the BER on the SNR for different levels of Frequency Offset at satellite transponder were studied. [ABSTRACT FROM AUTHOR]