Phase Dithering Research Articles

Amplify-and-Forward (AF) Relay Techniques in Wireless SC-FDE Systems to Enhance Diversity Gains

This paper introduces an amplify-and-forward (AF) relaying technique that employs phase dithering and intentional delay within single carrier-frequency domain equalizer (SC-FDE) systems. The proposed relaying technique aims to increase the gain of both frequency diversity and time diversity in slow fading channels. To achieve this, the proposed technique introduces random phase rotation and random intentional delay. The relaying scenario assumes two-hop communication with relaying between the source and destination. It is assumed that many nodes are densely distributed, allowing for many relay nodes to participate in relaying. To verify the performance through computer simulation, the number of relays is 1, 2, 3, 5, 15, and 25, and three modulation coding schemes (MCSs) are considered: QPSK with R=1/3, 16QAM with R=3/4, and 8PSK with R=7/8. The simulation results demonstrate that the proposed relaying technique improves the bit error ratio (BER) performance. The performance improves as the number of relays increases.

A Hybrid PAPR Reduction Scheme in OFDM-IM Using Phase Rotation Factors and Dither Signals on Partial Sub-Carriers

As a multi-carrier modulation technique, a high peak-to-average power ratio (PAPR) is a common issue suffered by orthogonal frequency division multiplexing with index modulation (OFDM-IM) due to its system structure. High PAPR may cause signal distortion, which affects correct symbol transmission. This paper tries to inject dither signals to the inactive (idle) sub-carriers, which is a unique transmission structure of OFDM-IM, to reduce PAPR. Unlike the previous works, which utilize all idle sub-carriers, the proposed PAPR reduction scheme utilizes selected partial sub-carriers. This method performs well in terms of bit error rate (BER) performance and energy efficiency, which are obvious drawbacks of the previous PAPR reduction works due to the introduction of dither signals. In addition, in this paper, phase rotation factors are combined with the dither signals to compensate for the PAPR reduction performance degradation due to the insufficient use of partial idle sub-carriers. Moreover, an energy detection scheme is designed and proposed in this paper in order to distinguish the index of phase rotation factor used for transmission. It is shown by extensive simulation results that the proposed hybrid PAPR reduction scheme is able to implement an impressive PAPR reduction performance among existing dither signa-based schemes as well as classical distortion-less PAPR reduction schemes. In addition, the proposed method obtains better error performance and energy efficiency than that of the previous works. At the error probability , the proposed method can achieve around 5 dB gain compared to the conventional dither signal-based schemes

Coherent combining of low-power optical signals based on optically amplified error feedback.

In free-space optical communication links, the combining of optical signals from multiple apertures is a well-known method to collect more power for improved sensitivity or mitigation of atmospheric disturbances. However, for analog optical combining no detailed analysis has been made in cases when the optical signal power is very low (<-60 dBm) as would be the case in very long-haul free-space links. We present a theoretical and experimental study of analog coherent combining of noise-limited signals from multiple independent apertures by applying low frequency optical phase dithering to actively compensate the relative phases. It is experimentally demonstrated that a 97% combining efficiency of four 10 GBaud QPSK signals is possible with a signal power per aperture exceeding -80 dBm, in fair agreement with theory. We also discuss the scaling aspects to many apertures.

Joint Phase and Timing Estimation With 1-Bit Quantization and Oversampling

Digital receivers based on 1-bit quantization and oversampling w.r.t. the transmit signal bandwidth promise lower energy consumption. However, since 1-bit quantization is a highly non-linear operation, standard off the shelf receiver algorithms cannot be applied. In this paper we consider an unknown phase rotation and timing offset and a fully digital receiver. To reduce the non-linear behavior introduced by 1-bit quantization, we assume that the receiver applies uniform phase and sample dithering, which can be implemented by sampling at an irrational normalized intermediate frequency and with an irrational oversampling factor, respectively. Based on the least squares objective function we derive a typical digital matched filter receiver with a data- and timing-aided phase estimator and square time recovery based timing estimation. Our main contribution is to show that both estimators are consistent under very general assumptions, e.g., arbitrary colored noise and stationary ergodic transmit symbols. Performance evaluations are done via simulations and are compared against a numerically computable upper bound of the Cram&#x00E9;r&#x2013;Rao lower bound. For low signal-to-noise ratio the estimators perform well but for high signal-to-noise ratio they run into an error floor. The performance loss of the phase estimator due to decision-directed operation or estimated timing information is marginal.

RF-chirp phase dither for MPI mitigation in RoF-based 5G mobile fronthaul networks.

The multipath interference (MPI) noise is one of the most important limiting factors on the performance of the mobile fronthaul network (MFN) based on the radio-over-fiber (RoF) technology. Recently, it has been proposed to suppress this MPI noise by using the Gaussian phase dither. However, it broadens the optical spectrum significantly and, as a result, increases its vulnerability to the chromatic dispersion. To overcome this problem, we propose to suppress the MPI noise by using the RF-chirp dither instead of the Gaussian dither. The results show that, due to the narrow optical spectrum achieved by the RF-chirp dither, we can increase the transmission distance of the RoF-based MFN operating in the 1.5-µm region by three times.

Bounds on Phase, Frequency, and Timing Synchronization in Fully Digital Receivers With 1-bit Quantization and Oversampling

Digital receivers based on 1-bit quantization and oversampling w.r.t. the transmit signal bandwidth promise lower energy consumption. However, since 1-bit quantization is a highly non-linear operation, standard receiver algorithms cannot be applied. Thus, we derive performance bounds for phase, timing, and frequency estimation in order to gain a deeper insight into the impact of 1-bit quantization and oversampling. We identify uniform phase and sample dithering as crucial to combat the effect of the non-linearity introduced by 1-bit quantization. Since oversampling results in noise correlation, a closed form of the likelihood function is not available. Thus, we study a system model with white noise by adapting the receive filter bandwidth to the sampling rate. Considering the aforementioned dithering, we obtain very tight closed form lower bounds on the Cramér-Rao lower bound (CRLB) in the large sample regime. We show that with uniform phase and sample dithering, all large sample properties of the CRLB of the unquantized receiver are preserved under 1-bit quantization, except for an signal-to-noise ratio (SNR) dependent performance loss that can be decreased by oversampling. Numerical computations show that the properties of the CRLB for white noise still hold for colored noise except that the performance loss due to 1-bit quantization is reduced.

Reflection-Tolerant RoF-Based Mobile Fronthaul Network for 5G Wireless Systems

We propose to utilize the filtered Gaussian phase dither technique for the effective suppression of the multipath- interference (MPI) noises caused by multiple bad fiber connectors in the mobile fronthaul network (MFN) based on the radio-over- fiber (RoF) technology. We first evaluate the viable performances of the RoF-based MFN in the 5G wireless communication systems by using this technique and deduce the required bandwidth and phase modulation depth of the filtered Gaussian dither signal for the suppression of the MPI noises by numerical simulations. The results show that, by using the proposed technique, it is possible to suppress the MPI noises by ∼10 dB regardless of the number of bad fiber connectors. These simulation results are experimentally verified in the RoF link capable of transporting the 5G wireless signals (i.e., thirty-two 100-MHz orthogonal-frequency-division- multiplexed (OFDM) signals modulated in 64QAM or 256QAM format). We also evaluate the effectiveness of the proposed dither technique in a high-capacity RoF link implemented by using four wavelength-division-multiplexed channels and demonstrate the successful transmission of 128 100-MHz OFDM signals modulated in 256QAM format in the presence of multiple bad connectors.

A 2.2-GHz 3.2-mW DTC-Free Sampling $\Delta\Sigma$ Fractional-$N$ PLL With −110-dBc/Hz In-Band Phase Noise and −246-dB FoM and −83-dBc Reference Spur

This paper presents the first sampling $\Delta \Sigma $ fractional- $N$ phase-locked loop (PLL) without a digital-to-time converter (DTC), whose design is challenging and requires complex calibration. A linear slope generator (LSG) is employed to generate a linear waveform, which has a large linear phase-to-voltage conversion range. Instead of modulating the reference signal by the DTC, phase dithering is performed at the feedback path through the combination of the LSG and a multi-modulus divider (MMDIV) with multi-phase generation. The reference signal then directly samples the linear LSG output waveform to output a sampled voltage corresponding linearly to the resulting phase error. All these enable the sampling phase detector to handle large phase dither step provided by a phase interpolator (PI), thus eliminating the need for the DTC and the associated calibration. This 2.2-GHz PLL achieves −246-dB FoM with an in-band phase noise of −110 dBc/Hz and −82-dBc reference spur while consuming only 3.2 mW in a 130-nm CMOS process.

Sequential phase locking scheme for a filled aperture intensity coherent combination of beam arrays

We present a novel phase locking scheme for the coherent combination of beam arrays in the filled aperture configuration. Employing a phase dithering mechanism for the different beams similar to LOCSET, dithering frequencies for sequential combination steps are reused. By applying an additional phase alternating scheme, this allows for the use of standard synchronized multichannel lock-in electronics for phase locking a large number of channels even when the frequency bandwidth of the employed phase actuators is limited.

Design Techniques for a 60-Gb/s 288-mW NRZ Transceiver With Adaptive Equalization and Baud-Rate Clock and Data Recovery in 65-nm CMOS Technology

Design techniques for a complete 60-Gb/s non-return-to-zero transceiver with adaptive equalization as well as baud-rate clock and data recovery (CDR) are demonstrated. A complete equalization front end with per-path adaptation and per-sampler offset calibration enables 60-Gb/s operation over realistic channels. Current integration in the front end for energy-efficient equalization is combined with integration phase dithering to realize a robust baud-rate CDR. Correlation of the adaptive error sampler output with the phase dithering sequence indicates the direction of phase offset, and the resulting baud-rate CDR saves power and complexity compared to an oversampling CDR by not requiring additional clock phases/deserializers. The proposed 65-nm CMOS transceiver operates at 60 Gb/s with an eye opening of 30% UI and consumes 288 mW while equalizing 21 dB of loss at 30 GHz over a 0.7-m Twinax cable.

A Low-Phase Noise ADPLL Based on a PRBS-Dithered DDS Enhancement Circuit

This paper aims to design an all-digital phase-locked loop (ADPLL) intended for professional digital audio data conversion applications. The method used for designing is based on an analogy between analog PLL and ADPLL. Managing a low-jitter effect, a comparative study between discrete voltage-controlled oscillator (DVCO) and direct digital synthesis (DDS) based on a pseudorandom binary sequence (PRBS) generator is performed. The features of the design in this work are high-precision and low harmonic distortion DDS which is combined with ADPLL. For model-based design validation, a rapid register transfer level (RTL) with VHSIC hardware description language (VHDL) is practiced and the dynamic performance result indicates a significant improvement in total harmonic distortion (THD) ([Formula: see text][Formula: see text]dB) and a better resolution of 18.97[Formula: see text]bits for audio applications based on the aid of phase dithering DDS enhancement circuit.

Reconfigurable microwave photonic mixer with minimized path separation and large suppression of mixing spurs

A compact reconfigurable photonic microwave mixer is proposed and demonstrated based on a dual-polarization Mach-Zehnder modulator and an optical 90-deg hybrid. By simply changing the photodetection schemes, single-ended, double-balanced, I/Q, and image-reject mixing can be implemented. Thanks to the sidebands selection by an optical filter, unwanted mixing spurs are highly suppressed. In addition, the system is insensitive to environmental vibration because the optical path separation is minimized. An experiment is carried out. Reconfigurable mixing functionalities with very small phase dithering are verified. The mixing spurs are suppressed by more than 30 dB, and the image-reject ratio for image-reject mixing is about 40 dB.

Fractional spur reduction technique using 45° phase dithering in phase interpolator based all‐digital phase‐locked loop

A spur reduction technique in fractional-N phase-locked loops based on a current-mode phase interpolator (CMPI) is presented by dithering input signals of the CMPI. CMPI shows deterministic phase error having symmetrical profile around 45° offset in each quadrant, and this non-linear property leads to fractional spurs. The proposed 45° phase rotator with digital compensation reduces the fractional spur by 18.57 dB at most, and average improvement of fractional tones is 7.89 dB in 2 MHz frequency step measurement.

Active phase locking of thirty fiber channels using multilevel phase dithering method.

An active phase locking of a large-scale fiber array with thirty channels has been demonstrated experimentally. In the experiment, the first group of thirty phase controllers is used to compensate the phase noises between the elements and the second group of thirty phase modulators is used to impose additional phase disturbances to mimic the phase noises in the high power fiber amplifiers. A multi-level phase dithering algorithm using dual-level rectangular-wave phase modulation and time division multiplexing can achieve the same phase control as single/multi-frequency dithering technique, but without coherent demodulation circuit. The phase locking efficiency of 30 fiber channels is achieved about 98.68%, 97.82%, and 96.50% with no additional phase distortion, modulated phase distortion I (±1 rad), and phase distortion II (±2 rad), corresponding to the phase error of λ/54, λ/43, and λ/34 rms. The contrast of the coherent combined beam profile is about 89%. Experimental results reveal that the multi-level phase dithering technique has great potential in scaling to a large number of laser beams.

A novel digital phase interpolation control for clock and data recovery circuit

In this express, we present a new architecture of digital phase interpolation (PI) controller with clock and data loops, which can greatly reduce the jitter of recovery clock by reducing the probability of the coarse phase jumping and interpolating among several fine phases. A demo design was implemented using 0.13 µm CMOS technology for verification, and the simulation results demonstrate that the recovered clock of the presented architecture has a peak to peak jitter no more than 29 ps under 2.5 Gbps received data, which shows no coarse phase dithering happening. The area of this proposed PI controller is only 0.1 mm2.

An active phase locking of multiple fiber channels via square wave dithering algorithm

A novel active phase locking technique base on single detector is proposed and demonstrated experimentally. Using dual-level rectangular-wave phase modulation and time division multiplexing, the proposed multi-level phase dithering technique can achieve the same phase error signal as single/multi-frequency dithering technique, but without coherent demodulation circuit. Experimental investigation on coherent beam combination of 10 fiber laser beams is successfully demonstrated, with imposing an additional phase disturbances to mimic the phase noises in the high power fiber amplifiers. The combining efficiency with modulated phase noise is achieved above 97.55% and the accuracy of phase control is improved to λ/42rms. One of the application fields, i.e., beam steering based on coherent beam combining using square wave dithering algorithm are proposed.

Phase measurement of fast light pulse in electromagnetically induced absorption

We report the phase measurement of a fast light pulse in electromagnetically induced absorption (EIA) of the 5S₁/₂ (F = 2)-5P₃/₂ (F' = 3) transition of ⁸⁷Rb atoms. Using a beat-note interferometer method, a stable measurement without phase dithering of the phase of the probe pulse before and after it has passed through the EIA medium was achieved. Comparing the phases of the light pulse in air and that of the fast light pulse though the EIA medium, the phase of the fast light pulse at EIA resonance was not shifted and maintained to be the same as that of the free-space light pulse. The classical fidelity of the fast light pulse according to the advancement of the group velocity by adjusting the atomic density was estimated to be more than 97%.

Fiber optical parametric oscillator based on highly nonlinear dispersion-shifted fiber

The development of fiber optical parametric oscillators (FOPO) based on highly nonlinear dispersion-shifted fiber is reviewed in this paper. Firstly, the background and motivation are introduced, and it is pointed out that the FOPO is promising to act as optical source in non-conventional wavelength bands. Subsequently, the context focuses principally on the problem of inherent multiple-longitudinal-mode characteristic of FOPO and the corresponding solutions to it. The primary technique is by locking the phase of multiple longitudinal modes. The first reported actively mode locked FOPO is also presented in this article. However, it is not probable to realize passively mode locked FOPO because of the random phase dithering of the pump required for suppressing stimulated Brillouin scattering. Furthermore, a regeneratively mode locked FOPO is demonstrated, which can generate wide band tunable radiation in nonconventional wavelengths. Besides mode locked FOPO, the single-longitudinal-mode FOPO is also introduced. Finally, potential future directions are discussed.

Four-State Modulation Continuous Variable Quantum Key Distribution over a 30-km Fiber and Analysis of Excess Noise

We report a fiber-based four-state discrete modulation continuous variable quantum key distribution system based on homodyne detection. A secret key rate of 1kbit/s is achieved at a transmission distance of 30.2 km. Two factors that result in the excess noises of the quantum key distribution system are analyzed. The first is the relative phase dithering between the signal and local fields, and the second is the local field leakage into the signal field due to the scattering process that depolarizes the local field. It is found that the latter has a significant impact on the excess noise, which is the main limiting factor to the long-distance secure quantum transmission. Some protocols are also given to decrease the excess noise effectively.

Continuous-wave, short-wavelength infrared mixer using dispersion-stabilized highly-nonlinear fiber

A new type of highly nonlinear fiber (HNLF) was designed and fabricated. The new HNLF was engineered to reduce dispersion shift due to transverse fluctuations while maintaining the modal confinement superior to that of the conventional fibers. The new design strategy was validated by the measurements of the global and local dispersive characteristics under considerable core and index profile deformation induced by tensile stress, which indicated that the dispersive and phase matching characteristics of the fiber did not change even under the highest tensile stress. The characteristics effectively decoupled tension-based Brillouin suppression from phase-matching impairments in parametric mixers for the first time. The new HNLF was used to demonstrate the first coherence-preserving mixer operating in the short-wavelength infrared (SWIR) band. The SWIR mixer was driven by continuous-wave near-infrared (NIR) pump and did not require pump phase dithering to suppress Brillouin scattering.