Multiple Frequency Signals Research Articles

Centralized dynamics multi‐frequency GNSS carrier synchronization

In this article, we propose a new centralized multi-frequency carrier tracking architecture using an adaptive Kalman filter to enhance the loop sensitivity and reliability of individual signal tracking in challenging signal environments. The main task of the centralized dynamics-tracking filter is to effectively blend multiple frequency carrier phase observations in order to estimate the common geometric Doppler frequency of multiple-frequency received signals. Conventionally, multi-frequency signals are tracked independently with a fixed-loop noise bandwidth tracking approach, which is suboptimal in time-varying signal environments. A suitable collaboration in multiple-frequency signal tracking using a centralized dynamics-tracking loop enables robust carrier tracking even if some of the frequency channels are affected by ionospheric scintillation, carrier-phase multipath, or interference. Additionally, computational efficiency of the multiple-frequency tracking improves by using the proposed tracking loop architecture. Performance of the proposed multi-frequency tracking-loop architecture is verified with experiments using live multi-frequency satellite signals collected from GPS Block-IIF satellites under the influence of frequency-selective interference signals.

A Novel Distributed Antenna Access Architecture for 5G Indoor Service Provisioning

In order to overcome the non-line-of-sight nature of the indoor environment so as to achieve a cost-effective solution of 5G indoor service provisioning, distribution of antenna units in indoor chambers is the most straightforward solution. This paper investigates a novel distributed antenna access architecture that allows the antenna units to be distributed over a wide geographical area via multi-pair LAN cables. The proposed architecture supports simultaneous transmission of multiple intermediate frequency signals between the remote radio unit and each distributed antenna unit. To explore the capacity of the LAN cables, we introduce a real-time multi-pair air-to-cable (MP-A2C) scheduler that allows for a graceful mapping between the radio signal spectrum and sub-channels of the cable twisted pairs, as well as power shaping of each sub-channel signal. We will first provide the problem formulation of the optimal MP-A2C process, which is nonetheless non-convex and computationally intractable. To achieve real-time solution of the problem, we reformulate the problem into two sub-problems that can be solved in a divide-and-conquer manner. Extensive numerical results show that the formulated MP-A2C problem can easily lead to quasi-optimal schedules.

Adaptive millimeter-wave radio generation and transmission of phase shift-keying signal based on radio over fiber system

A scheme to realize the flexible photonic frequency multiplication at millimeter-wave (mm-wave) and radio over fiber (RoF) transmission has been proposed and demonstrated. To enable the generated multiple-frequency signal to have a better performance of phase preserving, we theoretically investigated the phase change during the generation and transmission and proposed an adaptive coding algorithm to reduce the impact of phase fluctuation. The coding algorithm brings the frequency multiplication and fiber transmission distance together into consideration and adjusts the phase of the signal consequently. Due to the proposed coding algorithm, the system not only realizes the flexible frequency multiplication but also eliminates the phase deviation induced by the modulation and fiber dispersion, which effectively increases the transmission distance of fiber link. In addition, it also reduces the complexity of phase retrieval at receiver side. A 50-GHz to 100-GHz RoF system with 1 Gbit / s radio signal has been implemented by numerical simulation, and the result shows that the adaptive mm-wave RoF system could realize arbitrary multiple-frequency of quadrature phase shift-keying signal from double to decuple and reliable transmission of the tenfold signal at least 50 km.

A fast-estimation approach to multiple frequency excitation for online electromagnetic rail detection

This paper proposes a fast-estimation approach to demodulating multiple frequency signal for online electromagnetic rail detection. In order to speed up the demodulation, the approach uses the sine function’s expression to build Kalman filtering model, and by only one measured point, it can demodulate each component’s amplitude in multi-frequency signal induced in sensors. Instead of using all measured points in a period, the demodulating speed is faster than other demodulation methods, like FFT and fast correlation. Finally, the experimental result shows that this approach can follow the amplitude variation caused by the defect for online electromagnetic rail detection.

A novel method of identifying motor primitives using wavelet decomposition.

This study reports a new technique for extracting muscle synergies using continuous wavelet transform. The method allows to quantify coincident activation of muscle groups caused by the physiological processes of fixed duration, thus enabling the extraction of wavelet modules of arbitrary groups of muscles. Hierarchical clustering and identification of the repeating wavelet modules across subjects and across movements, was used to identify consistent muscle synergies. Results indicate that the most frequently repeated wavelet modules comprised combinations of two muscles that are not traditional agonists and span different joints. We have also found that these wavelet modules were flexibly combined across different movement directions in a pattern resembling directional tuning. This method is extendable to multiple frequency domains and signal modalities.

Electromagnetic Vibration Energy Harvesting for Railway Applications

Safe localization of trains via GPS and wireless sensors is essential for railway traffic supervision. Especially for freight trains and because normally no power source is available on the wagons, special solutions for energy supply have to be developed based on energy harvesting techniques. Since vibration is available in this case, it provides an interesting source of energy. Nevertheless, in order to have an efficient design of the harvesting system, the existing vibration needs to be investigated. In this paper, we focus on the characterization of vibration parameters in railway application. We propose an electromagnetic vibration converter especially developed to this application. Vibration profiles from a train traveling between two German cities were measured using a data acquisition system installed on the train’s wagon. Results show that the measured profiles present multiple frequency signals in the range of 10 to 50 Hz and an acceleration of up to 2 g. A prototype for a vibration converter is designed taking into account the real vibration parameters, robustness and integrability requirements. It is based on a moving coil attached to a mechanical spring. For the experimental emulation of the train vibrations, a shaker is used as an external artificial vibration source controlled by a laser sensor in feedback. A maximum voltage of 1.7 V peak to peak which corresponds to a maximum of 10 mW output power where the applied excitation frequency is close to the resonant frequency of the converter which corresponds to 27 Hz.

Reconfigurable radio-over-fiber system based on optical switch and tunable filter

As the best candidate for wireless-access networks, radio-over-fiber (RoF) technology can carry a variety of business. It is necessary to provide differentiated services for different users, so the network needs to produce signals with different modulation formats and different frequencies. A reconfigurable RoF system based on a switch and tunable optical filter that can realize modulation format conversion and multiple frequency signal switching functions is designed. It has a good performance in terms of bit error rate and an eye diagram. The design can help to use radio frequency resources efficiently and make dynamic bandwidth resources controllable.

Photonics-Based Radar for Sub-mm Displacement Sensing

Photonics-based multiband radars have been demonstrated where photonics is exploited for multiple radio frequency (RF) signal generation and detection by means of a single optical local oscillator that replaces the conventional cascades of electrical local oscillators. The ultrawide band and high stability of photonics and the use of a single local oscillator assure very low system phase noise and phase coherence among the RF signals. This phase coherence among multiband signals is exploited to perform differential phase estimation in enhanced submillimeter displacement measures. The system employs stepped frequency continuous waves simultaneously in the S- and X-bands, measuring the differential phase over a frequency span up to 7.4 GHz. The high coherence among the two frequency bands, provided by the photonic architecture, enables very precise displacements measures, allowing to obtain submillimeter precision without using correction algorithms. The presented experimental results demonstrate a precision <200 μm in a range up to 3 km. Moreover, the sharing of the same hardware to handle a multiband operation enables a great reduction of size, weight, power, and footprint of the overall system.

A New GNSS Single-Epoch Ambiguity Resolution Method Based on Triple-Frequency Signals

Fast and reliable ambiguity resolution (AR) has been a continuing challenge for real-time precise positioning based on dual-frequency Global Navigation Satellite Systems (GNSS) carrier phase observation. New GNSS systems (i.e., GPS modernization, BDS (BeiDou Navigation Satellite System), GLONASS (Global Navigation Satellite System), and Galileo) will provide multiple-frequency signals. The GNSS multiple-constellation and multiple-frequency signals are expected to bring great benefits to AR. A new GNSS single-epoch AR method for a short-range baseline based on triple-frequency signals is developed in this study. Different from most GNSS multiple-constellation AR methods, this technique takes advantage of the triple-frequency signals and robust estimation as much as possible. In this technique, the double difference (DD) AR of the triple-frequency observations is achieved in the first step. Second, the triple-frequency carrier phase observations with fixed ambiguities are used with the dual-frequency carrier phase observations to estimate their ambiguity. Finally, to realize reliable GNSS single-epoch AR, robust estimation is involved. The performance of the new technique is examined using 24 hours of GPS/GLONASS/BDS observation collected from a short-range baseline. The results show that single-epoch AR of the GNSS signals can be realized using this new technique. Moreover, the AR of BDS Geostationary Earth Orbit (GEO) satellites’ observations is easier than are those of the Medium Earth Orbit (MEO) and Inclined Geosynchronous Satellite Orbit (IGSO) satellites’ observations.

A Modified MUSIC Algorithm for Direction of Arrival Estimation in the Presence of Antenna Array Manifold Mismatch

We introduce a modified multiple signal classification (MUSIC) algorithm to estimate the directions of arrival (DOAs) of multiple radio frequency signals. We specifically consider an antenna array with an imperfectly calibrated array response. The modified MUSIC algorithm estimates the DOA of each signal individually after attempting to null the other signals that are present. We compare the new approach to the conventional MUSIC algorithm in simulations, and show that the new approach leads to improved performance. The new approach converges quickly and produces DOA estimates with smaller mean error and standard deviation than MUSIC over a range of incident signal separations. We recommend our approach for systems with challenging antenna placements, a time varying host platform, and limited processing capabilities.

Design of Multiple Modulated Frequency Lock-In Amplifier for Tapping-Mode Atomic Force Microscopy Systems

Nonlinear contact between tip and sample in tapping mode atomic force microscopy (AFM) systems induces higher harmonics, which may be useful for extraction of some characteristics of the sample. In this paper, a new efficient method for implementing multichannel digital lock-in technique is presented, which is able to measure the amplitude and phase of multiple modulated frequency signals. In order to solve the two most important problems in AFM systems, which are resolution and cost, previous multichannel lock-in amplifier methods are investigated and an improved algorithm for amplitude and phase extraction using fast Fourier transform is developed. In addition, we propose a novel multichannel digital lock-in algorithm including an $N$ -slow digital filter and a multichannel direct digital synthesis block for generating reference signals. We achieve optimal noise reduction using the appropriate decimation filters prior to harmonic extraction. This modification results in a significant hardware reduction in implementation compared with classical designs. Since the precision of the design is sensitive to the sampling frequency, we perform the suitable sampling rate conversion using fractional delay and decimation filters, which enhance the accuracy and noise resiliency of the designed architectures. Experimental results prove the efficiency of the proposed method in multiple modulated frequencies extraction. Also, FPGA implementation results show that the proposed architecture is superior to previous structures, especially in the aspect of area.

A Take on Arbitrary Transient Electric Field Reconstruction Using Wavelet Decomposition Theory Coupled With Particle Swarm Optimization

Destructive and constructive interference of multiple time-shifted and amplitude-adjusted higher frequency signals (wavelet signals) is exploited in order to reproduce a desired signal at a given point in the far-field regime of radiating antennas. The number of individual wavelets is intentionally kept small in keeping with a realistic antenna array size, where each antenna would emit wavelets at conceivably very high power levels. Wavelet decomposition theory is coupled with particle swarm optimization to determine the necessary time shifts and amplitude adjustments of the wavelet signals. In this application, the reconstructed signal can be specified by a desired frequency or arbitrary shape. A pyramidal horn antenna array is used in the analysis of the far-field propagation of the wavelet signals due to its relatively large bandwidth and known analytical electric field solutions. It is found that when the wavelet signals are appropriately superpositioned and added in the far field, the desired signal may be reconstructed with the quality of reconstruction mostly governed by the intentionally low number of wavelets. The reconstructed signal is solely found on the centerline while the signal drastically changes off the centerline or at distances too close or too far from the antenna array.

Digital linearization for broadband multicarrier analog photonic link incorporating downconversion

The multicarrier analog photonic links suffer from both the traditional third-order intermodulation distortions (IMD3) and the cross-modulation distortions (XMDs), severely limiting the dynamic range of the links. This paper proposes and demonstrates an effective technique based on a single modulator and photodetector to simultaneously realize the downconversion and receiving of multiple radio frequency signals, as well as suppress the nonlinearities, including the IMD3 and XMD. In the scheme, the nonlinear compensation information is directly obtained from hardware then the distortion compensation is carried out in the digital domain. Experimental results show that the XMD and IMD3 distortions are suppressed with 36.6 and 25.8 dB, respectively, and the link dynamic range is improved by 25 dB, preventing the degradation of the dynamic range of the link. Moreover, the structure of our scheme can eliminate the stringent requirement for hardware.

New OTA-C Current-mode Comb Filter

A new circuit realization for OTA-C current-mode comb filter is proposed to remove the undesirable multiple frequency signals. The proposed filter uses only OTAs and capacitors, hence suitable for monolithic integrated circuit implementation. The proposed OTA-C comb filter is simulated in PSPICE using a behavioral macro-model of the OTA as well with a practical CMOS OTA circuit. The workability of the comb filter is verified for hum signal frequencies of 60, 180 and 300 Hz that are found in bio-medical signals. The results obtained are in good agreement with theor

A new ionosphere-free ambiguity resolution method for long-range baseline with GNSS triple-frequency signals

New GNSS systems (i.e. GPS modernization, BeiDou, and Galileo) will provide multiple navigation signals for reliable navigation services. The triple or even multiple frequency signals are expected to bring great benefits to the ambiguity resolution (AR) over long-range baselines, which is always regarded as a huge challenge. Another issue in the long baseline AR is the unmodeled ionospheric delay, which is one of the major errors in ranging signals. A new triple-frequency, ionosphere-free technique for ambiguity resolution of long-range baseline is developed in this study. In this technique, the optimal observation combinations are chosen considering the effect of ionospheric delay. At the same time, using this technique, the double difference (DD) ionospheric delay is nullified in the ambiguity search process. The performance of the new technique is examined using the simulated GPS triple frequency data as well as the real BDS observation. Results show that the ambiguity can be fixed within 10min for GPS and BDS long-range baselines with this new technique.

Spectral analysis comparisons of Fourier-theory-based methods and minimum variance (Capon) methods

In recent years, adaptive (data dependent) methods have been introduced into many areas where Fourier spectral analysis has traditionally been used. Although the data-dependent methods are often advanced as being superior to Fourier methods, they do require some finesse in choosing the order of the relevant filters. In performing comparisons, we have found some concerns about the mappings, particularly when related to cases involving many spectral lines or even continuous spectral signals. Using numerical simulations, several comparisons between Fourier transform procedures and minimum variance method (MVM) have been performed. For multiple frequency signals, the MVM resolves most of the frequency content only for filters that have more degrees of freedom than the number of distinct spectral lines in the signal. In the case of Gaussian spectral approximation, MVM will always underestimate the width, and can misappropriate the location of spectral line in some circumstances. Large filters can be used to improve results with multiple frequency signals, but are computationally inefficient. Significant biases can occur when using MVM to study spectral information or echo power from the atmosphere. Artifacts and artificial narrowing of turbulent layers is one such impact.

Simultaneous beam steering of multiple signals based on optical wavelength-selective switch

A novel, photonics-based scheme for the independent and simultaneous beam steering of multiple radio frequency signals at a wideband phased-array antenna is presented. As a proof of concept, a wavelength-selective switch (WSS) is employed both as a wavelength router to feed multiple antenna elements and as a tunable phase shifter to independently control the phase of each signal at any antenna element. In the experiment, two signals at 12.5 and 37.5 GHz are simultaneously fed to the four output ports of the WSS with independent and tunable phase shifts, emulating the independent steering of two signals in a four-element phased-array antenna. The results confirm the precision and flexibility of the proposed scheme, which can be realized both with bulk components or resorting to photonic integrated circuits, especially for wide-band applications. The architecture for a possible integrated implementation of the proposed solution is presented, employing a structure based on micro-ring resonator. Starting from these results, the feasibility of an integrated version of the presented architecture is also considered. The proposed photonic integrated circuit realizing the beam-forming network might be based on tunable true-time delay, as well as on phase shift through micro-ring resonators, and could be conveniently implemented with CMOS-compatible silicon technology.

Instantaneous high-resolution multiple-frequency measurement system based on frequency-to-time mapping technique

A new microwave photonic instantaneous frequency measurement system that can simultaneously measure multiple-frequency signals while achieving very high resolution and wide frequency measurement range is presented. It is based on the frequency-to-time mapping technique implemented using a frequency shifting recirculating delay line loop and a narrowband optical filter realized by the in-fiber stimulated Brillouin scattering effect. Experimental results demonstrate the realization of a multiple-frequency measurement capability over a frequency range of 0.1-20GHz that can be extended to 90GHz, and with a measurement resolution of 250MHz.

Multiple microwave frequencies measurement based on stimulated Brillouin scattering with improved measurement range

We numerically investigate the operational principle and performance of stimulated Brillouin scattering based multiple microwave frequency signals measurement. The unknown signals are processed specially to generate a gain region which is measured by phase modulation to amplitude modulation converting. By sweeping the vector network analyzer, both single and multiple frequencies measurement can be achieved. The loss spectrum generated by one of the pumps is fully compensated by the gain spectrum of the other pump, which increases the measurement range from 2νB to 4νB.

Sparse Frequency Diverse MIMO Radar Imaging for Off-Grid Target Based on Adaptive Iterative MAP

The frequency diverse multiple-input-multiple-output (FD-MIMO) radar synthesizes a wideband waveform by transmitting and receiving multiple frequency signals simultaneously. For FD-MIMO radar imaging, conventional imaging methods based on Matched Filter (MF) cannot enjoy good imaging performance owing to the few and incomplete wavenumber-domain coverage. Higher resolution and better imaging performance can be obtained by exploiting the sparsity of the target. However, good sparse recovery performance is based on the assumption that the scatterers of the target are positioned at the pre-discretized grid locations; otherwise, the performance would significantly degrade. Here, we propose a novel approach of sparse adaptive calibration recovery via iterative maximum a posteriori (SACR-iMAP) for the general off-grid FD-MIMO radar imaging. SACR-iMAP contains three loop stages: sparse recovery, off-grid errors calibration and parameter update. The convergence and the initialization of the method are also discussed. Numerical simulations are carried out to verify the effectiveness of the proposed method.