Quadrature Phase Shift Keying Signals Research Articles

Optical likelihood calculation for quadrature phase‐shift keying signal based on silicon integrated waveguide

All-optical likelihood calculation has the potential to improve the performance of operating speed and power consumption in future communication systems. The author previously proposed a novel scheme of likelihood calculation, which is capable of applying multi-value modulation. However, these studies have a limitation that requires higher integration into an actual system. In this study, an optical device of likelihood calculation for a 4-bit quadrature phase-shift keying (QPSK) modulated signal (i.e. two sequential QPSK symbols) is demonstrated. The integrated device consists of two delayed interferometers with silicon waveguide. The device is designed according to the types of 4-bit code string as follows (00 00), (00 11), (11 00), and (11 11), and the output light waveform from the device is observed by an oscilloscope for a plurality of 4-bit QPSK signals. The light intensity obtained from the device accurately corresponds to the Hamming distance between the code string and the input signal. The results indicate that the proposed scheme correctly calculates a likelihood for a 4-bit QPSK signal at 10 Gbaud.

Independent dual-single-sideband QPSK signal detection based on a single photodetector.

The two sidebands of the independent dual-single-sideband (dual-SSB) signal can carry different information to achieve higher spectral efficiency. However, the two sidebands of the independent dual-SSB vector signal are received independently. Generally, the receiver divides the signal into two channels. For each channel, we use an optical bandpass filter (OBPF) to select the left sideband (LSB) or right sideband (RSB), respectively. Then a photodetector (PD) is used for photoelectric conversion, followed by subsequent digital signal processing (DSP). To reduce the complexity and cost of the receiver, we propose a new independent dual-SSB vector signal detection scheme based on a single PD combined with conventional DSP. An electric bandpass filter (EBPF) filters out high-frequency components after photoelectric conversion, and then the signal is quadrature demodulated and processed by the DSP algorithm. The LSB and RSB are quadrature phase-shift keying (QPSK) modulated with an initial phase difference of π/4. Simulation results show that the proposed scheme performs better bit error rate (BER). For back-to-back (BTB) transmission, the BER of 2-Gbaud independent dual-SSB vector signal (1-Gbuad RSB and 1-Gbaud LSB) can reach the hard-decision forward error correction (HD-FEC) threshold of 3.8 × 10-3 when the input optical power into PD is -20 dBm. For 1-km and 2-km weak turbulence free-space optical (FSO) channel transmission, the BER of 2-Gbaud independent dual-SSB vector signal can reach the HD-FEC threshold when the input optical power into PD is -18.8 and -17 dBm, respectively. For 1-km weak turbulence FSO channel transmission, the BER of 4-, 8-, and 16-Gbuad independent dual-SSB vector signal can reach the HD-FEC threshold when the input optical power into PD is -17.8, -16, and -15 dBm, respectively.

Chaotic optical communications of 12.5-Gbaud OOK and 10-Gbaud QPSK signals based on mutual injection of semiconductor lasers.

Chaotic optical communications can provide a high level of security in data transmission. High-speed chaotic optical communications have hardly been implemented so far limited by the bandwidth of chaotic signals and the difficulties of wideband chaos synchronization. Here, we experimentally demonstrate all-optical wideband chaos synchronization and communications based on mutual injection of semiconductor lasers. Both 12.5-Gbaud on-off keying (OOK) signals and 10-Gbaud quadrature phase shift keying (QPSK) signals are successfully encrypted and transmitted over a 10-km and 2-km single-mode fiber (SMF), respectively.

Neural network-based surrogate model for inverse design of metasurfaces

Metasurfaces composed of spatially arranged ultrathin subwavelength elements are promising photonic devices for manipulating optical wavefronts, with potential applications in holography, metalens, and multiplexing communications. Finding microstructures that meet light modulation requirements is always a challenge in designing metasurfaces, where parameter sweep, gradient-based inverse design, and topology optimization are the most commonly used design methods in which the massive electromagnetic iterations require the design computational cost and are sometimes prohibitive. Herein, we propose a fast inverse design method that combines a physics-based neural network surrogate model (NNSM) with an optimization algorithm. The NNSM, which can generate an accurate electromagnetic response from the geometric topologies of the meta-atoms, is constructed for electromagnetic iterations, and the optimization algorithm is used to search for the on-demand meta-atoms from the phase library established by the NNSM to realize an inverse design. This method addresses two important problems in metasurface design: fast and accurate electromagnetic wave phase prediction and inverse design through a single phase-shift value. As a proof-of-concept, we designed an orbital angular momentum (de)multiplexer based on a phase-type metasurface, and 200 Gbit/s quadrature-phase shift-keying signals were successfully transmitted with a bit error rate approaching 1.67 × 10 − 6 . Because the design is mainly based on an optimization algorithm, it can address the “one-to-many” inverse problem in other micro/nano devices such as integrated photonic circuits, waveguides, and nano-antennas.

Reconfigurable photonics-based millimeter wave signal aggregation for non-orthogonal multiple access.

A reconfigurable optical-to-electrical signal aggregation is proposed, for the first time, using optical signal processing and photo-mixing technology. Two optically modulated quadrature phase-shift keying (QPSK) signals are aggregated into a single 16-quadrature amplitude modulation (16-QAM) signal and, simultaneously, carried over a 28-GHz millimeter wave (MMW) carrier using an optimized heterodyne beating process through a single photodiode. To demonstrate the system reconfigurability, aggregation of two optical binary phase-shift keying signals is mapped into MMW QPSK or four-level pulse amplitude modulation signals by controlling the relative phases and amplitudes, respectively, of the input signals. In addition, the aggregation of two 16-QAM signals into a 256-QAM signal and the aggregation of three QPSK signals into a 64-QAM format are achieved. Besides, we report the effect of laser phase noise on signal aggregation performance. The de-aggregation of the aggregated MMW signals is performed electrically using a successive interference cancellation algorithm. Moreover, a proof-of-concept experiment is conducted to validate the numerical simulations. Two 1-Gbaud BPSK (1 Gbps) and QPSK (2 Gbps) optical signals are optically transmitted over a 20-km single-mode fiber as MMW over fiber signals. Then, the signals are aggregated into QPSK (2 Gbps) and 16-QAM (4 Gbps) 28-GHz MMW signals, respectively. The aggregated signal is further transmitted over a 1-m wireless channel. The performance of the proposed system is evaluated using bit error rate and error vector magnitude metrics.

Low-Cost Underwater Communication System: A Pilot Study

The aim of the paper is to present a simplified implementation of quadrature phase shift keying (QPSK) based underwater communication system. The presented solution addresses the problem of developing inexpensive, compact ultrasound modems able to be mounted on underwater robots. Simplifications introduced into the modulation and demodulation of QPSK signals do not disturb any parameter of the data link. The paper indicates that it is possible to realize modulation and demodulation on a simple microcontroller. Many hints are given on how to use hardware blocks embedded in a microcontroller, such as ADC, DMA, timers, etc. Experiments performed with the prototype modems allow to reach 4 kbps data rate on a distance of about 18 m.

Simultaneous Up-Conversion Based on a Co- & Counter-Directions SOA-MZI Sampling Mixer with Standard & Differential Modulation Modes

Simulation and experimental performance analyses of simultaneous up-converted signals, for the first time, were investigated utilizing a semiconductor optical amplifier Mach–Zehnder interferometer (SOA-MZI) sampling mixer in co- and counter-directions for standard and differential modulation modes. An optical pulse source at a sampling frequency of fs = 15.6 GHz was used as a sampling signal. The IF signal channels carrying quadrature phase shift keying (QPSK) data at frequencies fm were up-converted at different mixing frequencies up to 195.5 GHz. Using the Virtual Photonics Inc. (VPI) simulator, we realized mixed QPSK signals and studied their characteristics through a conversion gain and an error vector magnitude (EVM). Simulations of up mixing operated in a frequency range up to 158 GHz. For the standard modulation in the co-direction, the conversion gain decreased from 43.3 dB at the mixing frequency of 16.6 GHz to 21.8 dB at 157 GHz for the first channel and from 43 dB at 17.6 GHz to 21 dB at 158 GHz for the second channel. The use of the differential modulation principle improved the conversion gain by about 10 dB at 195.5 GHz compared to standard modulations in co- and counter-directions. The EVM reached, respectively, 15.5 and 17.5% for the differential modulation in both configurations, at the bit rate of 100 Gbit/s at 195.5 GHz. The benefit provided by the differential modulation was that EVM values were shifted by 20% for all channels in both configurations at 100 Gbit/s. In the real measurement, we confirmed that co-directional conversion exhibited a better performance than the counter-directional operation. In addition, the real mixed signal exhibited lower efficiency and quality in comparison with simulated signals due to the sensitivity of the receiver.

Simplified twin-single-sideband direct detection system separating left and right sideband signals using a digital signal processing algorithm instead of optical bandpass filters.

Twin-single-sideband (twin-SSB) signals can be generated based on an in-phase- quadrature (I/Q) modulator, and two independent left sideband (LSB) and right sideband (RSB) signals carry individual data to effectively harvest the advantage of twin-SSB modulation, which achieves higher spectral efficiency. However, the conventional twin-SSB scheme employs two optical bandpass filters (OBPFs) and two photodetectors (PDs) for complete separation and detection at the receiver side. To mitigate the crosstalk between RSB and LSB signals and reduce the complexity and cost of the twin-SSB system, we propose a new scheme to realize twin-SSB without OBPFs separating LSB and RSB signals by a single-ended PD to improve system performance. According to the beating characteristics of the LSB and RSB, we can separate two independent sideband signals using a digital signal processing (DSP) algorithm added to the receiver end. Our simulation results demonstrate that our proposed scheme can obtain good bit error ratio (BER) performance of LSB and RSB signals. We designed a twin-SSB system with different modulation formats in the two sidebands, adopting geometric shaping 3PSK (GS-3PSK) modulation for the LSB and quadrature phase shift keying (QPSK) modulation for the RSB. The BER of the LSB GS-3PSK and RSB QPSK signal can reach hard-decision forward error correction (HD-FEC) when the received optical power (ROP) was > -17.5 and > -16 dBm, respectively, at different baud rates of 1-, 2-, and 4-Gbaud with a carrier frequency of 12-GHz over 10-km standard single-mode fiber (SSMF) transmission. For an 8-Gbaud baud rate with a carrier frequency of 12-GHz over 5-km SSMF transmission, the BER of the two sideband signals can still be below the HD-FEC threshold of 3.8 × 10-3 when the ROP was > -17 and > -16 dBm, respectively.

Investigation of modulation format conversion for optical channel deaggregation in elastic optical network

Our research investigates an optical channel deaggregation technique based on modulation format conversion that may be used in elastic optical networks (EONs) to achieve high spectral efficiency, flexibility, and high data rate. The modulation format conversion technique from 8-ary phase shift keying (8-PSK) to two simultaneous quadrature phase shift keying (QPSK) at 50 Gb / s has been simulated and investigated. 8-PSK and the converted QPSK signals are investigated and compared in terms of bit error rate (BER) and quality factor (Q-factor), and it is demonstrated that they substantially reduce the complexity of transmission and reception process, at high data rates in EON. Additionally, this approach is further exploited to tailor a protection scheme for network survivability. Channel deaggregation, which divides the channel into working and protection paths, is used to implement a dedicated path protection scheme in EON. To evaluate the effect of various network conditions such as traffic load, modulation format, and BER on EON survivability, network simulations are performed on the ARPANET network.

Exact SER and BER Analysis on Jointly Optimal Maximum Likelihood Detection of Two QPSK Signals With Phase Offset

Analysis on the exact error rate of jointly optimal (JO) maximum likelihood (ML) multiuser detection is limited in the existing literature owing to its high computational complexity. In this paper, the JO ML detection of two quadrature phase-shift keying (QPSK) signals with an arbitrary relative phase offset is investigated. Exact, analytical symbol error rate (SER) and bit error rate (BER) formulas are derived for two users with no less than 3 dB power difference between them. Closed-form union bounds on the user SER and BER with an arbitrary phase offset and an arbitrary user power ratio are also given as a simpler approach to acquire the asymptotic error rate performance. Furthermore, insights on the potentials and properties of the JO ML detector regarding its error rate behaviors for each user are revealed through comparisons with the successive interference cancellation receiver. The accuracies of the obtained formulas are demonstrated through simulation. The results show the error rates for the two-user QPSK case no longer follow the classic knowledge on the error rate behaviors in the single signal detection. In addition, the performance and dB penalty employing different modulation types are also compared and discussed.

Mode-division multiplexing phase-sensitive amplification for mode selective and mode equalized phase regenerative amplifications

In this work, mode-division multiplexing (MDM) phase-sensitive amplification (PSA) for all-optical mode selective and mode equalized phase regeneration is presented and investigated. MDM PSA relies on reasonable phase-matching conditions, which makes intramode four-wave mixing in the fiber much stronger than that of cross-mode. It enables multimode signals to interact mainly with the same spatial mode pumps and idlers. Thus, MDM signals can obtain synchronous phase regeneration and avoid cross talk with different mode signals. To achieve this, we designed an appropriate few-mode fiber by an inverse design method based on a neural network for obtaining the desired phase mismatch and low modal dispersion. The results show that effective phase regeneration can be achieved for the degraded 40 Gbit/s binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK) signals. For BPSK signals, error vector magnitudes (EVMs) of the regenerated signal on two modes are reduced from − 8.013 d B and − 8.068 d B to − 24.867 d B , and − 26.090 d B , respectively. For QPSK signals, the EVMs are reduced from − 16.767 d B and − 16.583 d B to − 24.867 d B and − 24.822 d B , respectively.

Demonstration of QPSK data correlation and equalization using a tunable optical tapped delay line based on orbital angular momentum mode delays

We demonstrate a tunable optical tapped delay line (OTDL) using delays based on orbital angular momentum (OAM) modes to perform data correlation and equalization for a quadrature phase-shift-keyed (QPSK) signal. The system: (i) creates N signal replicas as N taps on different OAM modes through mode multicasting, (ii) tunes the complex tap weights (i.e., amplitude and phase of the modes), (iii) applies a tunable mode-dependent delay to each replica, and (iv) coherently combines the taps in order to realize the OTDL functions of correlation and equalization. The experiment results: (i) identify the correlation occurrence of a sequence of 2 or 3 symbols in an incoming 10 or 20 Gbaud QPSK signal, and (ii) equalize the distortion caused by chromatic dispersion for a 20-Gbaud QPSK data signal and reduce error vector magnitude (EVM) from ∼25.4 to ∼12.9%.

All-Optical Aggregation and De-Aggregation Between 8QAM and BPSK Signal Based on Nonlinear Effects in HNLF

An all-optical aggregation and de-aggregation scheme between one 8-ary quadrature amplitude modulation (8QAM) signal and three binary phase shift keying (BPSK) signals is proposed and theory simulated based on nonlinear effects in high nonlinear fiber (HNLF). In this scheme, the input 8QAM signal is de-aggregated into three BPSK signals by deploying self-phase modulation (SPM), cross-phase modulation (XPM), four wave mixing (FWM) and parameter amplification (PA) effect. Firstly, an improved amplitude compression loop (ACL) is designed based on SPM effect to convert 8QAM signal into quadrature phase shift keying (QPSK) signal. A degenerate phase sensitive amplifier (PSA) based on FWM effect is used to decompose QPSK signal into the first two BPSK signals (BPSK-1 and BPSK-2). The third BPSK signal (BPSK-3) can be extracted from the probe light modulated in its phase by the input 8QAM signal thanks to XPM and PA effect. Secondly, BPSK-3 is converted into an atypical on-off keying (OOK) signal through a 1-bit delay interference (DI). BPSK-1 and BPSK-2 are recombined into one QPSK signal through a coupler. Finally, the recombined QPSK signal and the atypical OOK signal are aggregated into one 8QAM signal by using XPM and PA effect. The error vector magnitude (EVM) and bit error ratio (BER) of the 8QAM and BPSK signal before and after de-aggregation and aggregation are calculated to analyse the performance of the scheme. The scheme can be applied in the network node to connect different networks which explore to use 8QAM and BPSK signal respectively.

High-speed Stokes vector receiver enabled by a spin-dependent optical grating

Stokes vector direct detection is a promising, cost-effective technology for short-distance communication applications. Here, we design and fabricate a spin-dependent liquid crystal grating to detect light polarization states. By separating the circular and linear components of incident light, the polarization states can be resolved with accuracy of up to 0.25°. We achieved Stokes vector direct detection of quadrature phase-shift keying (QPSK), 8PSK, and 16-ary quadrature amplitude modulation signals with 32, 16, and 16 GBd rates, respectively. We integrated the system, including the grating, photodetectors, and optical elements, on a miniaturized printed circuit board and demonstrated high-speed optical communications with 16 GBd rate QPSK signals.

0.36 THz photonic terahertz-wave generation by simple optical frequency comb

A novel 0.36 THz photonic-assisted terahertz-wave (THz-wave) signal generation scheme is demonstrated via simulation. We employ only one single phase modulator (PM) as optical frequency comb generator to get the desired optical carries with high-frequency stability. We get two target optical carriers of symmetrical orders (−6th, +6th) chosen from the generated optical frequency comb via wavelength selector switch (WSS). One optical carrier from one port is acted as optical local oscillator (LO) for heterodyne beating. And one optical carrier from the other port is sent into the Mach Zendell modulator (MZM), which is modulated with a 2-Gband 16 quadrature amplitude modulation (16QAM) baseband signal or 4-Gbaud quadrature phase shift keying (QPSK) baseband signal generated in the MATLAB. Employing our proposed 0.36 THz photonic-assisted THz-wave signal generation scheme, we demonstrate the performance of 4-Gband vector QPSK signal or 2-Gbaud vector 16 QAM signal at back to back (BTB) transmission case or 10 km single-mode fiber transmission case. The bit error ratio (BER) under aforementioned conditions in every case is under the hard decision forward error correction (HD-FEC) threshold of 3.8e−3. To our knowledge, it’s the first time to utilize only one PM as optical frequency comb generator in the photonic-assisted THz-wave signal generation.

Integrated dual-polarization coherent receiver without a polarization splitter-rotator.

We propose and demonstrate a simple integrated dual-polarization (DP) coherent receiver that does not require a polarization splitter-rotator (PSR). Based on a novel concept, a DP coherent signal is mixed with the local-oscillator (LO) waves inside a single interferometer and detected by five single-ended photodetectors. The signal-signal and LO-LO beat noises are eliminated through differential detection. We design and fabricate a proof-of-concept device on InP and experimentally demonstrate complete retrieval of DP quadrature phase-shift keyed signals. Requiring minimal number of optical components without a PSR, the demonstrated scheme would be attractive particularly for the InP and thick-silicon photonic platforms due to its significantly reduced footprint and ease of fabrication.

Simultaneous Generation of Wired and Wireless Signals Using a DP-MZM in a RoF System

We experimentally demonstrate a simultaneous generation of independent 10-Gbit/s wired on/off keying (OOK) signal and 20-Gbit/s wireless quadrature phase shift keying (QPSK) signal by using a dual-polarization Mach-Zehnder modulator (DP-MZM) in a millimeter-wave-band radio-over-fiber system at 30 GHz. The polarization directions of two signals are orthogonal, so there is not any interference between two polarizations in the photodiode. By using a 100/200-GHz interleaver, the power penalty after 15-km fiber transmission for the wireless QPSK signal can be reduced from 3.9 to 1.9 dB. As we know, this is the first time to use QPSK modulation format for the wireless signal in the simultaneous generation and delivery of wireless and wired signals by using a dual-polarization modulator.

Integrated optical digital‐to‐analogue converter for a 2‐bit BPSK‐modulated signal based on a silicon photonics waveguide

An integrated optical digital-to-analogue (DA) converter, which is intended for a phase-shift keying (PSK) modulation signal, is presented to realise a fully transparent optical communication system. The converter is provided with a silicon-based optical waveguide structure in a compact size equal to 1/3000 that of the previous study. The device optically performs a conversion of digital data to their equivalent analogue values by generating a quadrature PSK (QPSK) signal from two binary PSK (BPSK) modulated pulses. In the author's experiment, the resulting QPSK signal is converted into four different intensities depending on the pattern of the 2-bit BPSK signal at 10 GHz. This fact indicates the experimental demonstration of optical DA conversion for PSK modulation using the integrated device.

Coherent optical communications enhanced by machine intelligence

Accuracy in discriminating between different received coherent signals is integral to the operation of many free-space communications protocols, and is often difficult when the receiver measures a weak signal. Here we design an optical communication scheme that uses balanced homodyne detection in combination with an unsupervised generative machine learning and convolutional neural network (CNN) system, and demonstrate its efficacy in a realistic simulated coherent quadrature phase shift keyed (QPSK) communications system. Additionally, we design the neural network system such that it autonomously learns to correct for the noise associated with a weak QPSK signal, which is distributed to the receiver prior to the implementation of the communications. We find that the scheme significantly reduces the overall error probability of the communications system, achieving the classical optimal limit. We anticipate that these results will allow for a significant enhancement of current classical and quantum coherent optical communications technologies.

Photonic-assisted image rejection mixer based on Hilbert transform in the optical domain using a phase-shifted fiber Bragg grating

A photonic-assisted image rejection mixer is proposed based on Hilbert transform in the optical domain using a phase-shifted fiber Bragg grating (PS-FBG). The image signal and the desired RF signal are converted to the optical domain by carrier-suppressed single-sideband modulation, which is then split into two parts. One part is Hilbert transformed by a specially designed PS-FBG and a 90-deg optical phase shifter and then combined with the other part to directly reject the image signal in the optical domain. The image-free optical signal is downconverted to an intermediate frequency signal by combining it with an optical local oscillator signal and then detecting them in a photodetector. The image rejection capability of the system is analyzed, and an image rejection ratio of 68.0 or 58.6 dB is achieved when the bandwidth of the image signal is 20 or 60 MHz, respectively, while having an acceptable influence on the desired RF signal. Quadrature phase-shift keying (QPSK) signals, 8 phase-shift keying (8PSK) signals, and 16 quadrature amplitude modulation (16-QAM) signals are used to evaluate the image rejection performance. When the bandwidth of the image signal is 18.75 MHz, the error vector magnitudes of the downconverted QPSK, 8PSK, and 16-QAM signals with the same power, modulation format, and bandwidth are 2.5%, 2.5%, and 2.6%, respectively.