Phase Coherent Signals Research Articles

An Anti-Islanding Protection Method Based on Voltage-Synchronous Impedance Angle Measurements

Grid-tied distributed generators (DGs) need to be equipped with anti-islanding protection to avoid the impact of unplanned islanding, which would affect system stability, auto-reclosing, and personal safety. Among the active anti-islanding protections, impedance measurements based on signal injection have the advantages of a low non-detection zone (NDZ) and are less prone to maloperation during grid disturbances; however, there are problems with signal interference in multi-DG systems. Hence, the impedance angle measurement method with signals injected synchronously is proposed. In this method, each DG injects phase-coherent signals to detect islanding using zero-cross points of the voltage (ZCPV) to avoid the protection failures caused by signal interference. An islanding identification criterion based on the measured impedance angle is proposed by analyzing the impedance characteristics of grid connection and islanding, which avoids the influence of variation of the DG operation state on islanding detection. Finally, we present a signal injection strategy and performance analysis in combination with an existing DG control platform, avoiding additional hardware investment. RTDS-based simulation verification shows that the proposed method can 100% avoid DG maloperation due to voltage and frequency disturbances during grid-connected operation and exit operation within 2 s when islanding occurs.

Transmission of precise phase coherent optical signals using a VCSEL stabilization system

To improve the time and frequency reference standards used for radio astronomy and high precision atomic physics applications, we describe a novel stable frequency transmission technique. The approach uses a vertical-cavity surface- emitting laser (VCSEL) based phase correction actuator. Using a frequency mixing process for feedback control, the phase noise induced along the 26 km G.655 optical fibre link was actively reduced. This was achieved by using a 1 310 nm dither controlled VCSEL phase error correction actuator together with the inherent chromatic dispersion properties of the fibre. The technique corrects phase noise and optical signal drift along the fibre at the transmission end. Fractional frequency instabilities across the G.655 fibre link of 4.44×10−12 at 1 s and 4.86×10−14 at 104 s are successfully reported.

Performance analysis of direct-sequence spread-spectrum underwater acoustic communications based on at-sea data

Direct sequence spread spectrum (DSSS) signal uses phase coherent signals where the information symbols are multiplied with a code sequence, commonly known as chips. The signals are processed at the receiver using the code sequence as a matched filter to extract the information symbols. Taking advantage of the spread processing gain derived from the matched filter, communications can be carried out at low signal levels, which is the preferred method for high quality underwater acoustic (UWA) communication and remote UWA communication. However, in practical applications, DSSS UWA communication will face lots of interferences including multipath interference, Doppler compression interference and phase fluctuation interference, which will seriously affect the performance of DSSS. This paper first analyzes the interferences that DSSS faces in practical applications based on multiple actual received data collected in different water areas. By calculating the gain of the matched filter loss, one can find that influence of phase fluctuation interference has the most serious impact on DSSS. Secondly, the corresponding receiver processing algorithms are given for these interferences, including the differential energy detector and Doppler estimation method. The performance of differential energy detector under the above interferences is analyzed, which shows that the differential energy detector is insensitive to slow phase fluctuation interference and multipath interference and the differential energy detector combined with Doppler compensation can work well based on moving at-sea data. Finally, the parameter design of DSSS system in practical application is discussed and parameter design of DSSS UWA communication system should comprehensively consider the Doppler compression interference and phase fluctuation interference.

Widely Tunable Photonic-Assisted Microwave Phase Shifter Based on Single Polarization Multiplexing Electro–Optic Modulator

We demonstrate an approach to achieve a widely tunable photonic-assisted microwave phase shifter (MPS) based on single polarization multiplexing electro-optic modulator (PM-EOM). The PM-EOM can generate orthogonally polarized and phase coherent optical signals. A phase shifted microwave signal is generated by combining the orthogonally polarized signals and generating the signal in a photodetector. A polarization controller (PC) continuously tunes phase of the recovered microwave signals and ensure its magnitude is constant. Through joint adjusting of the wavelength plates inside the PC before a polarizer, phase of the recovered microwave signal in a frequency range of 1–18 GHz can be fully tuned over 360°. Phase deviation is less than 1.5°. The corresponding magnitude almost remains unchanged. Magnitude deviation is less than 2 dB. Phase drift is less than 4° within 120 s. The motivation of this work lies in the fact that the previously reported MPSs using EOMs are hard to be realized with single modulator without special devices or an optical bandpass filter. The photonic-assisted MPS can be expanded into a phase shifter array with a help of a 1 × N optical coupler, which can be applied in optically controlled beamforming.

Characterization and calibration techniques for multi-channel phase-coherent systems

Electronic Warfare (EW) and Radio Detection and Ranging (RADAR) are two of the many applications that rely on multi-channel and phase-coherent configurations for signal processing. We provide an overview of the complexities and requirements of a multi-channel phase-coherent measurement system. Multiple Input Multiple Output [MIMO] systems have to overcome key technical challenges related to phase, time, and frequency synchronization to coherently receive and process the data acquired/ generated from each input/output. In practical MIMO systems, the radio hardware should be capable of acquiring and/or generating such phase coherent signals across the multiple channels. Further, the systems need to be able to sustain the phase coherence over considerable duration of time, depending on the sensitivity of the system. However, drifts will occur, owing to the effects of, e.g., temperature, thermal expansion, mismatched cable lengths, uncorrelated phase noise, ADC sample, clock phase noise, and quantization noise. Thus, a calibration process is required to compensate for the drift, whenever it crosses a particular threshold value that defines the accuracy of the phase-coherent system. In this paper, an FPGA based software-defined calibration method is presented for synchronizing the phase and magnitude across multiple channels of a system. This method allows the phase/magnitude drift over time to be periodically monitored and calibrated when there is a need. With the FPGA built into the system, the calibration can take place remotely without the need of connecting the system to an external calibration kit. Also, measurement results are provided for a state of the art super heterodyne receiver system to show that the phase drift is lesser than ± 1 degree across a 500 MHz to 26.5 GHz frequency range at 23 °C ± 5 °C for two and four channel configurations. Such systems can be useful in a variety of real-world MIMO implementations such as Direction Finding, Beam Steering, Passive Radars, MIMO and Phased-Array Radar Systems, where phase coherence, alignment, and/or synchronization has added advantages to multi-channel systems.

Coherent cavity-enhanced dual-comb spectroscopy.

Dual-comb spectroscopy allows for the rapid, multiplexed acquisition of high-resolution spectra without the need for moving parts or low-resolution dispersive optics. This method of broadband spectroscopy is most often accomplished via tight phase locking of two mode-locked lasers or via sophisticated signal processing algorithms, and therefore, long integration times of phase coherent signals are difficult to achieve. Here we demonstrate an alternative approach to dual-comb spectroscopy using two phase modulator combs originating from a single continuous-wave laser capable of > 2 hours of coherent real-time averaging. The dual combs were generated by driving the phase modulators with step-recovery diodes where each comb consisted of > 250 teeth with 203 MHz spacing and spanned > 50 GHz region in the near-infrared. The step-recovery diodes are passive devices that provide low-phase-noise harmonics for efficient coupling into an enhancement cavity at picowatt optical powers. With this approach, we demonstrate the sensitivity to simultaneously monitor ambient levels of CO2, CO, HDO, and H2O in a single spectral region at a maximum acquisition rate of 150 kHz. Robust, compact, low-cost and widely tunable dual-comb systems could enable a network of distributed multiplexed optical sensors.

Noisy quantum phase communication channels

We address quantum phase channels, i.e communication schemes where information is encoded in the phase-shift imposed to a given signal, and analyze their performances in the presence of phase diffusion. We evaluate mutual information for coherent and phase-coherent signals, and for both ideal and realistic phase receivers. We show that coherent signals offer better performances than phase-coherent ones, and that realistic phase channels are effective ones in the relevant regime of low energy and large alphabets.

Subharmonic All-Optical Clock Recovery of up to 320 Gb/s Signal Using a Quantum Dash Fabry–Pérot Mode-Locked Laser

In this study, we provide an experimental assessment of a quantum dash Fabry-Pérot mode-locked laser for all-optical clock recovery using data streams at 40, 80, 160, and 320 Gb/s. The data streams at 80, 160, and 320 Gb/s are phase coherent signals featuring no spectral component at 40 GHz. The 40 GHz recovered optical clock signal is characterized in terms of phase noise, timing jitter, dynamic power range, and wavelength dependence for the different data rates. Our experiments demonstrate a recovered optical clock signal from a 320 Gb/s data stream with a timing jitter of 94 fs and wavelength detuned by 23 nm. In addition, the performance of the optical time division multiplexing receiver at 80 and 160 Gb/s data signals employing the recovered clock as a demultiplexing control signal and as a clock for the bit-error-rate tester (BERT) is evaluated by bit error rate measurements on the demultiplexed data signals.

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The article reviewed the results of theoretical studies of the effect of correlated interference caused by multipath channel distribution of radio, on the accuracy of estimates of temporary positions (AFP) on the envelope and phase coherent carrier signals received

The dependence of the intensity of surface scattered signals on surface wave characteristics.

The compensation for the fluctuations in surface scattered signals remains one of the challenges for reliable and high rate underwater acoustic communications systems. Communication using phase coherent signals relies on the ability of the receiver to track the fluctuations in the impulse response of the channel between the transmitter and receiver. Techniques that can exploit a priori statistical characterizations of the fluctuations of the arrivals in the channel impulse response (fading statistics) will generally outperform systems that do not have access to such information. While fading statistics in radio communications is generally assumed to be Rayleigh or Rician, prior work [J. C. Preisig and G. B. Deane, J. Acoust. Soc. Am. 116, 2067–2080 (2004)] has shown that surface wave focusing can significantly change channel fading statistics. The physical basis for the intensity of surface scattered signals for both isolated specular scattering points and caustics developed in [C. T. Tindle and G. B. Deane, J. Acoust. Soc. Am. 117, 2783–2794 (2005)] relates intensity to ray geometry parameters. In this work, surface wave characteristics are related to both the source to receiver ranges at which normal focused specular scattering and caustics will occur and to the intensities of the surface scattered arrivals in each case. Experimental data are presented comparing theoretically predicted and measured intensities.

Estimation of Rapidly Time-Varying Sparse Channels

The estimation of sparse shallow-water acoustic communication channels and the impact of estimation performance on the equalization of phase coherent communication signals are investigated. Given sufficiently wide transmission bandwidth, the impulse response of the shallow-water acoustic channel is often sparse as the multipath arrivals become resolvable. In the presence of significant surface waves, the multipath arrivals associated with surface scattering fluctuate rapidly over time, in the sense that the complex gain, the arrival time, and the Dopplers of each arrival all change dynamically. A sparse channel estimation technique is developed based on the delay-Doppler-spread function representation of the channel. The delay-Doppler-spread function may be considered as a first-order approximation to the rapidly time-varying channel in which each channel component is associated with Doppler shifts that are assumed constant over an averaging interval. The sparse structure of the delay-Doppler-spread function is then exploited by sequentially choosing the dominant components that minimize a least squares error. The advantage of this approach is that it captures both the channel structure as well as its dynamics without the need of explicit dynamic channel modeling. As the symbols are populated with the sample Dopplers, the increase in complexity depends on the channel Doppler spread and can be significant for a severely Doppler-spread channel. Comparison is made between nonsparse recursive least squares (RLS) channel estimation, sparse channel impulse response estimation, and estimation using the proposed approach. The results are demonstrated using experimental data. In training mode, the proposed approach shows a 3-dB reduction in signal prediction error. In decision-directed mode, it improves significantly the robustness of the performance of the channel-estimate-based equalizer against rapid channel fluctuations.

Investigation of a VHF transequatorial path between Europe and southern Africa

Long range VHF transequatorial propagation experiments between Athens (Greece) and Roma (Lesotho) on 34, 40 and 45.1 MHz, in progress since 1967, have an equinoctial character regarding occurrence and strength. Studies of fading characteristics and topside electron density profiles lead to the conclusions that phase coherent signals are most likely at least 2-hop great circle F-transmissions, while other afternoon and evening types, including flutter signal, which is strongly related to equatorial spread- F, are off great circle supermodes involving two reflections from the F-layer without intermediate ground reflection.