Periodic Fading Research Articles

Frequency-octupled millimeter-wave ROF system without bit walk-off effect by polarization modulators

A novel, to the best of our knowledge, frequency-octupled millimeter-wave (MMW) radio over fiber (ROF) system without bit walk-off effect, based on two polarization modulators (PoIM) connected in parallel, is proposed. At the central station, the downlink data signal is split into two beams; one beam modulates the radio frequency (RF) carrier through an electrical phase modulator (PM), the other beam is amplified by an electrical amplifier (EA), and the two beams are combined into one, which is used as the RF driving signal of the PoIM. By adjusting the modulation index of the PoIMs, the phase shift of the phase shifter, and the polarization angle of the polarizer (Pol), the main components of the output signal from the PoIMs are ±4th-order sidebands. By adjusting the phase modulation index of the PM and the amplification of the EA, the downlink data signal is modulated only on the +4th-order sideband. At the base station, a part of the −4th-order sidebands are reflected by a fiber Bragg grating (FBG) and used as the optical carrier for uplink for carrier reuse; the ±4th-order sidebands passing the FBG are injected into the photodetector (PD) for photoelectrical conversion, and the frequency-octupled MMW with the downlink data signal is generated. The main innovation of the scheme proposed in this paper is that the data of the uplink and the downlink is modulated on ±4th-order sidebands separately, which eliminates the periodic fading effect and bit walk-off effect due to fiber dispersion and greatly improves the fiber transmission distance of the system, and also avoids the problems of DC bias drift and extinction ratio limitation of the Mach–Zehnder modulator.

Chiral standing spin waves in skyrmion lattice

This work studies the resonance excitations of the three-dimensional skyrmions lattice in the finite thickness plate of an isotropic chiral magnet using spin dynamics simulations. We found that the absorption spectra and resonance modes differ from those predicted by the two-dimensional model and the model of the unconfined bulk crystal. The features observed on the spectra can be explained by the formation of chiral standing spin waves, which, contrary to conventional standing spin waves, are characterized by the helical profile of dynamic magnetization of fixed chirality that is defined by the Dzyaloshinskii–Moriya interaction. In this case, the dynamic susceptibility becomes a function of the plate thickness, which gives rise to an interesting effect that manifests itself in periodical fading of the intensity of corresponding modes and makes excitation of these modes impossible at specific thicknesses.

Robot Localization in Tunnels: Combining Discrete Features in a Pose Graph Framework.

Robot localization inside tunnels is a challenging task due to the special conditions of these environments. The GPS-denied nature of these scenarios, coupled with the low visibility, slippery and irregular surfaces, and lack of distinguishable visual and structural features, make traditional robotics methods based on cameras, lasers, or wheel encoders unreliable. Fortunately, tunnels provide other types of valuable information that can be used for localization purposes. On the one hand, radio frequency signal propagation in these types of scenarios shows a predictable periodic structure (periodic fadings) under certain settings, and on the other hand, tunnels present structural characteristics (e.g., galleries, emergency shelters) that must comply with safety regulations. The solution presented in this paper consists of detecting both types of features to be introduced as discrete sources of information in an alternative graph-based localization approach. The results obtained from experiments conducted in a real tunnel demonstrate the validity and suitability of the proposed system for inspection applications.

3-D Fadings Structure Analysis in Straight Tunnels Toward Communication, Localization, and Navigation

Tunnels represent strong multipath environments for communications, where wireless propagation differs from regular indoor or outdoor scenarios. Moreover, with an adequate operating frequency choice, a tunnel behaves as an oversized waveguide, extending the communication range but affecting the signal with strong fadings. These fadings have been closely studied in the literature in the longitudinal dimension (tunnel length). Nevertheless, they also occur in the cross section, where they have only been analyzed statistically and on a small scale. In this paper, we present the results from an extensive measuring campaign carried out at the Somport tunnel, in Spain. We have adopted the modal theory approach to analyze the large-scale fadings (also known as slow fadings ) in the longitudinal, transversal, and vertical dimensions. More specifically, the modes’ geometry allowed for the adequate placement of transmitters to create different periodic fading structures and to virtually divide the tunnel cross section into sectors, where the fadings presented a phase shift of 180°. This study enabled the development of large-scale spatial diversity schemes and navigation techniques to improve communication and localization inside these scenarios.

Characterization of 300-GHz Wireless Channel on a Computer Motherboard

This paper presents characterization of 300-GHz wireless channel on a computer motherboard, where several different measurement scenarios have been considered. Results indicate that the presence of the ground plane and/or vertical parallel-plate structures in the channel introduces multipath that, if constructively superimposed, may create a path loss lower than free-space propagation path loss. In addition, it has been found that a few centimeters of vertical misalignment between the transmitter and the receiver result in a path loss greater than 5 dB. Furthermore, the results indicate that vertical components such as dual in-line memory modules with reflection coefficient close to one for all incident angles can be used to create directed non-line-of-sight (LoS) links with wide coherence bandwidth. Finally, for LoS propagation obstructed by large objects, such as a heatsink, the path-loss exponent of 1.77 is found, while the rotating fan causes periodic fading in the received power, indicating that channel equalization techniques will be needed to overcome deep fades. All these results indicate that optimal communications can be achieved by carefully positioning the antennas with respect to the motherboard layout.

A reappraisal of parameters for the putative planet PTFO 8-8695b and its potentially precessing parent star

Published photometry of fading events in the PTFO 8-8695 system is modelled using improved treatments of stellar geometry, surface intensities, and, particularly, gravity darkening, with a view to testing the planetary-transit hypothesis. Variability in the morphology of fading events can be reproduced by adopting convective-envelope gravity darkening, but near-critical stellar rotation is required. This leads to inconsistencies with spectroscopic observations; the model also predicts substantial photometric variability associated with stellar precession, contrary to observations. Furthermore, the empirical ratio of orbital to rotational angular momenta is at odds with physically plausible values. An exoplanet transiting a precessing, gravity-darkened star may not be the correct explanation of periodic fading events in this system.

TESTS OF THE PLANETARY HYPOTHESIS FOR PTFO 8-8695b

The T Tauri star PTFO 8-8695 exhibits periodic fading events that have been interpreted as the transits of a giant planet on a precessing orbit. Here we present three tests of the planet hypothesis. First, we sought evidence for the secular changes in light-curve morphology that are predicted to be a consequence of orbital precession. We observed 28 fading events spread over several years, and did not see the expected changes. Instead we found that the fading events are not strictly periodic. Second, we attempted to detect the planet's radiation, based on infrared observations spanning the predicted times of occultations. We ruled out a signal of the expected amplitude. Third, we attempted to detect the Rossiter-McLaughlin effect by performing high-resolution spectroscopy throughout a fading event. No effect was seen at the expected level, ruling out most (but not all) possible orientations for the hypothetical planetary orbit. Our spectroscopy also revealed strong, time-variable, high-velocity H{\alpha} and Ca H & K emission features. All these observations cast doubt on the planetary hypothesis, and suggest instead that the fading events represent starspots, eclipses by circumstellar dust, or occultations of an accretion hotspot.

Precise measurement of fiber dispersion based on phase‐modulated signal fading

ABSTRACTWe propose a novel method for accurately measuring the chromatic dispersion based on the fading effect of the phase‐modulated signal. The fiber dispersion, acting as an optical phase filter, causes the periodical fading of the phase‐modulated signal, and the polynomial fitting of the fading curve enables the precise determination of the characteristic notch frequency and the fiber dispersion. Our method can be used for the chromatic dispersion measurement at different operating wavelengths and dispersive fibers by using a vector network analyzer. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:427–430, 2014

A Radio-Over-Fiber System With Frequency 12-Tupling Optical Millimeter-Wave Generation to Overcome Chromatic Dispersion

A radio-over-fiber system with frequency 12-tupling optical millimeter-wave (mm-wave) generation using an integrated nested Mach-Zehnder modulator (MZM) is proposed and demonstrated by simulation. Through properly adjusting the direct current bias voltages of two sub-MZMs, the RF local oscillator (LO) voltages and phases, and the gain of base-band signal, the frequency 12-tupling optical mm-wave with data only carried by one sixth-order sideband is generated. As the signal is transmitted along the fiber, there is no periodical fading and bit walk-off effect caused by chromatic dispersion. The eye diagrams stay open even when the signal is transmitted over 60 km and the power penalty is 0.67 dB at a BER of 10-10. Furthermore, it is also proved to be valid that the BER is insensitive to the deviation of modulator extinction ratio and RF LO voltage.

Generation of frequency-doubling mm-wave signal using a Mach–Zehnder modulator with three arms to overcome fiber chromatic dispersion

We propose a novel method to generate optical frequency-doubling mm-wave signal by using a three-arm Mach–Zehnder modulator (MZM) for radio-over-fiber (RoF) systems. Since the optical carrier and its two 2nd-order sidebands are dominant, the 2nd-order harmonic in the photocurrent gets maximal. When the optical mm-wave signal is transmitted in standard single-mode fiber (SSMF), there is no time shift of code edges because the data signal is only carried by the optical carrier. The 2nd-order harmonic in the photocurrent consists of two beating products, so it experiences a periodical fading effect when the optical mm-wave signal is transmitted in SSMF. However, if a proper direct-current (DC) bias is applied to the data modulation arm in the MZM architecture to adjust the position of the fading nodes, the degradation on the photocurrent can be avoided. A RoF link is built based on the novel scheme by simulation, and the transmission performance of the proposed scheme is presented. The eye diagram still keeps open and clear even after 40 km transmission. Simulation results prove our theoretical analysis.

Generation of Carrier and Odd Sidebands Suppressed Optical MM-Wave with Signal Only on One Sideband Using an External Integrated Mach–Zehnder Modulator

A novel scheme to generate millimeter (mm)-wave is proposed where the quadrupling of local radio frequency is formed by using an external integrated Mach–Zehnder modulator through intensity modulation or phase modulation. Generated optical mm-wave signal suffers from neither power periodical fading nor time shift of the sidebands as it is transmitted along the fiber. Receiver sensitivity of our 10 Gbit/s radio-over-fiber system based on the proposed scheme is −28.3 dBm under intensity modulation while −24 dBm under phase modulation after 65km transmission, and bit error rate is at 10 level after 100 km transmission. Optical carrier in uplink is provided by the central station to simplify the base station, which also reduces the cost of the base station.

Universal serially concatenated trellis coded modulation for space-time channels

This paper presents serially concatenated trellis coded modulations (SCTCMs) that perform consistently close to the available mutual information for periodic erasure channel (PEC), periodic fading channel (PFC) and the 2 times 2 compound matrix channel. We use both the maximum-likelihood decoding criteria and iterative decoding criteria to design universal SCTCMs for the PEC and the PFC. For the space-time channel, by demultiplexing the symbols across the antennas, the proposed universal SCTCMs for the period-2 PFC deliver consistent performance over the eigenvalue skew of the matrix channel. Within the family of channels having the same eigenvalue skew, a time-varying linear transformation (TVLT) is used to mitigate the performance variation over different eigenvectors. The proposed space-time SCTCMs of 1.0, 2.0 and 3.0 bits per transmission require excess mutual information in the ranges 0.11-0.15, 0.23- 0.26 and 0.35-0.53 bits per antenna, respectively. Because of their consistent performance over all channels, the proposed codes will have good frame-error-rate (FER) performance over any quasi-static fading distribution. In particular, the codes provide competitive FER performance in quasi-static Rayleigh fading.

DSB optical mm-wave with signal only on optical carrier generated by embedded LiNbO 3 Mach–Zehnder modulator

We have investigated the generation of the 40-GHz double-sideband optical millimeter (mm)-wave with signal carried only by its optical carrier via an embedded LiNbO 3 Mach–Zehnder modulator (LN-MZM). Since the optical carrier and its two first-order sidebands are dominant and their powers are well balanced, the first-order harmonic in the photocurrent gets maximal. As the optical mm-wave signal is transmitted along the fiber, there is no code outline distortion because the signal is only modulated on the optical carrier. Although the first-order harmonic shows the periodical fading effect when the optical mm-wave signal is transmitted along the fiber, its degradation on the radio-over-fiber link can be avoided by adjusting the position of the fading nodes via varying the main MZM bias voltage, and the signal still keeps much good eye diagram even after 50-km fiber transmission. The experimental results prove our theory.

The Universal Operation of LDPC Codes Over Scalar Fading Channels

Root and Varaiya proved the existence of a code that can communicate reliably over any linear Gaussian channel for which the channel mutual information level exceeds the transmitted rate. This paper provides several examples of scalar (single-input single-output) fading channels and shows that on these channels the performance of low-density parity-check (LDPC) codes lies in close proximity to the performance limits identified by Root and Varaiya. Specifically, we consider periodic fading channels and partial-band jamming (PBJ) channels. A special case of periodic fading is the variation of signal-to-noise ratio across orthogonal frequency division modulation subchannels. The robustness of LDPC codes to periodic fading and PBJ across parameterizations of these different channels is demonstrated through the consistency of the required mutual information to provide a specified bit error rate. For the periodic fading case, the Gaussian approximation to density evolution has been adapted such that asymptotic threshold measures can be compared to simulated code performance in various periodic fading scenarios

An Alternative Approach for the Design of Millimetric Wave Drivers for SSB Optical Modulator

For the millimetric wave transport over fiber, one of the most simple ways to eliminate the periodic fading due to chromatic dispersion is to use an optical Single Side Band signal. In accordance with this principle, this paper presents highly linear millimetric amplifiers driving a SSB modulator. For the first time, the real-frequency technique has been successfully applied to the design of the matching networks of 26.5–29.5 GHz and 40.9–41.8 GHz amplifiers.

Degree of polarization technique used in PMD compensation of optical microwave transmission systems

Polarization-mode dispersion (PMD) can severely degrade the performance of optical microwave transmission systems by inducing a periodic power fading of the received RF signal that depends on the subcarrier frequency and accumulated differential group delay (DGD) along fiber. We derive a compact analytical expression of the degree of polarization (DOP) of optical signal using Jones and Stokes representations based on first-order assumption. Using this expression, we quantify the signal DOP fading induced by PMD by means of numerical simulations for BPSK and ASK modulations. The dependences of signal DOP on subcarrier frequency, accumulated DGD, and modulation formats have been demonstrated. It is found that signal DOP has similar periodic fading with the power of received RF signal, which is caused by DGD. Moreover, if the DOP technique is used in the PMD compensation of the optical microwave transmission systems, the DOP degradation is more sensitive to the DGD in the system modulated by BPSK than by ASK. The performance of this technique is immune to residual chromatic dispersion of the fiber.

Tunable compensation of channel degrading effects using nonlinearly chirped passive fiber Bragg gratings

Several channel-degrading effects are present in nonstatic and dynamically reconfigurable wavelength-division-multiplexed systems and networks due to various types of dispersion in the optical transmission fiber. These effects must be addressed by tunable methods so that data signals do not fade with time. The relevant effects for which we demonstrate tunable compensation include: chromatic dispersion accumulated in a single channel and in multiple channels, polarization mode dispersion, and periodic RF power fading. We utilize a nonlinearly chirped fiber Bragg grating that provides a dispersive function that can be varied continuously by tuning a single mechanical stretching element.

Changes in the state of water, induced by radiofrequency electromagnetic fields

Microwave irradiation ( f = 36 GHz) changes the properties of distilled water within the first 1–10 min. The new state is retained for at least tens of minutes and manifests itself as changes in power density spectrum of periodic fading voltage fluctuations that are generated during discharge of a capacitor in which water is used as a dielectric. It is assumed that long-term changes in water properties mediate the effect of electromagnetic fields on biological systems.

Developments in computer simulation of X-ray diffraction contrast images of stacking faults

Computer-simulated X-ray section topograph images of a specimen containing a single large-area stacking fault are compared with the experimental image to illustrate useful improvements in simulation realism with regard to both image geometry and stacking-fault fringe contrast. The computer-program developments include exact treatment of the X-ray polarization factor so that anomalous transmission and the periodic fadings of fringe visibility occurring when unpolarized or partially polarized sources are used can be correctly computed. A procedure is described for photographing a cathode-ray-tube display to obtain records with acceptable resolution, contrast and freedom from distortion.

Spectral study of the irregular variables

Nineteen spectrograms for three variables with periodic fadings of the, namely, SV Cep, UX Ori, and DD Ser, obtained on the two meter telescope of the Shemakha Astrophysical Observatory with a reciprocal dispersion of 94 A/mm in H..gamma.., were used to compile a two dimensional quantitative spectral classification of those variables. On the basis of certain classification criteria based on the lines of the metals, the mean spectral classes were determined for each star: A0 for SV Cep, A3 for UX Ori, and A5 for DD Ser. It was discovered that the spectral class of SV Cep varied from A0 to A3 from night to night; this may possibly be connected with the presence of cold regions on the surface of the star. The mean values of the absolute magnitudes found from the spectra of SV Cep, UX Ori, and DD Ser are, respectively, M /sub V/ = -0 /sup m/ .5, M /sub V/ = 0 /sup m/ .6, and M /sub V/ = 0 /sup m/ .2, which corresponds to giants of luminosity class III. The depths of the hydrogen lines and of K Ca II markedly exceed the depths in the spectra of standard stars of themore » same spectral class and luminosity class, and the explanation for this is the contribution of the additional absorption in the envelope. A consideration of the photographic observations available in the literature for DD Ser, which cover a period of 25 years, suggests a cyclicity of the fadings of about 4 yr.« less