Quadrature Phase Shift Keying Signals Research Articles

Demonstration of using two aperture pairs combined with multiple-mode receivers and MIMO signal processing for enhanced tolerance to turbulence and misalignment in a 10 Gbit/s QPSK FSO link

We utilize aperture diversity combined with multiple-mode receivers and multiple-input-multiple-output (MIMO) digital signal processing (DSP) to demonstrate enhanced tolerance to atmospheric turbulence and spatial misalignment in a 10 Gbit/s quadrature-phase-shift-keyed (QPSK) free-space optical (FSO) link. Turbulence and misalignment could cause power coupling from the fundamental Gaussian mode into higher-order modes. Therefore, we detect power from multiple modes and use MIMO DSP to enhance the recovery of the original data. In our approach, (a) each of multiple transmitter apertures transmits a single fundamental Gaussian beam carrying the same data stream, (b) each of multiple receiver apertures detects the signals that are coupled from the fundamental Gaussian beams to multiple orbital angular momentum (OAM) modes, and (c) MIMO DSP is used to recover the data over multiple modes and receivers. Our simulation shows that the outage probability could be reduced from >0.1 to <0.01. Moreover, we experimentally demonstrate the scheme by transmitting two fundamental Gaussian beams carrying the same data stream and recovering the signals on OAM modes 0 and +1 at each receiver aperture. We measure an up to ∼10dB power-penalty reduction for a bit error rate (BER) at the 7% forward error correction limit for a 10 Gbit/s QPSK signal.

Low complexity QPSK/8QAM millimeter-wave signal generation at D-band without phase pre-coding

In this paper, we propose a D-band quadrature-phase-shift-keying (QPSK) and 8-quadrature-amplitude-modulation (8QAM) millimeter-wave (mm-wave) signal generation based on frequency quintupling by only using a phase modulator (PM). The radio frequency (RF) at 25 GHz is used to drive PM. -3rd sideband and 2nd sideband of PM output is selected to generate 125 GHz mm-wave. We apply star-QPSK constellation distribution which is suitable for frequency quintupling mm-wave generation without pre-coding. For 8QAM, the distribution of constellation can be divided into QPSK set and star-QPSK set. Two sets of constellations will be rotated after photo detector (PD) according to the positive or negative of Bessel function, respectively, so as to realize 8QAM frequency quintupling photonic mm-wave generation without phase pre-coding. Moreover, there is no redundant operation in the generation, transmitting and receiving of mm-wave. The simulation results show that the bit-error-rate (BER) of 2Gbaud QPSK and 8QAM signals are lower than 3.8 ×10−3 in 15-km and 10-km optical fiber transmission, respectively. Compared with previous mm-wave generation schemes without phase pre-coding, the complexity of the proposed scheme is the lowest.

Phase-sensitive amplification of a QPSK signal using a dispersion engineered silicon-graphene oxide hybrid waveguide.

We numerically investigate phase-sensitive amplification of a quadrature phase shift keying (QPSK) signal in a 35 µm dispersion engineered silicon-graphene oxide hybrid waveguide. The four-wave mixing efficiency is effectively enhanced by exploiting the ultrahigh Kerr nonlinearity and low loss of graphene oxide in the ultrawide wavelength range. A new structure of dispersion flat silicon-graphene oxide hybrid waveguide is proposed and used to achieve the phase regeneration of a QPSK signal using a dual-conjugated-pump degenerate scheme. The phase-dependent gain and phase-to-phase transfer functions are calculated to analyze the properties of a phase-sensitive amplifier (PSA). The constellation diagrams of the QPSK signal and the error vector magnitude are used to assess the regeneration capacity. The simulation results show that the proposed PSA with a good phase noise squeezing capability has potential applications in all-optical signal processing.

A novel demodulation structure for quadrate modulation signals using the segmentary neural network modelling

In digital communication, the baseband information signal is modulated by the high-frequency carrier to produce a passband signal and applied to the transmission line. QASK (quadrature amplitude-shift keying), QFSK (quadrature frequency-shift keying), and QPSK (quadrature phase-shift keying) modulations from Quadrate type digital modulations are used for high-speed communication in passband digital modulations. They are a widespread modulation for fast and easy data transmission, especially in wireless communication and modem devices. In these modulations, four separate carriers are used, and since each carrier represents 2 bits, the transmission rate doubles compared to conventional digital modulation. This study aims to obtain the baseband signal from quadrate type modulation signals. For this purpose, all 8-bit data between Decimal 0–255 were obtained in QASK, QFSK, and QPSK modulations and signal matrices were formed. The various SNR (signal to noise ratio) values of 5 dB-10 dB-15 dB-20 dB noise were added to these signals to examine the system performance during the modulated signal transmission. The generated signals were given to the demodulation system developed using different methods and tested. The best results were obtained in developed a segmentary NN (Neural Network). In this study, it was observed that modulation signal matrices were given directly to an ANN (Artificial Neural Network) and that the results could not be predicted with the application of noise matrices for testing. Then the modulation signals with the proposed method are divided into four parts. Each segment represents a two-bit piece of data. In the case of a column matrix, four parts were applied as input to the neural network model. In the output of ANN, the result matrix to be predicted is created. Each modulation signal applied to the network input was classified between 0 and 3 at the output. Modulation data-carrying 8 bits are applied to the network in 4 steps and classified. 4 separate classification data from the ANN output is converted back to 2-bit logic. Therefore, signals carrying 8 bits of data are obtained in 4 steps. After the formation of the ANN network, baseband digital signal estimation was performed quickly in 4 steps across each byte modulation signals under different noises coming into the network, and demodulation data was successfully achieved.

Continuous delay tunability using a combination of three types of fiber Bragg gratings, wavelength conversion, and wavelength multicasting with a frequency comb

We experimentally demonstrate a delay subsystem that utilizes a combination of three types of fiber-Bragg gratings (FBGs), wavelength conversion, and wavelength multicasting using a frequency comb source. While the linearly chirped FBG together with a channelized dispersion compensation one, provide the continuous delay, an array of standard FBGs utilizes the multicast copies to extend the delay in steps determined by the array geometry. Following the characterization of the group-delay dispersion performance of the FBGs, a 10 Gbaud quadrature phase-shift keying (QPSK) signal is continuously delayed by 2.2 ns and discretely delayed in 3 steps of ∼2 ns, to provide a total gap-less delay of 8.1 ns. The delayed signal is finally recovered to its original wavelength with an optical signal-to-noise ratio (OSNR) penalty of ∼2.3 dB. Furthermore, we show that the penalty can be reduced to 1.2 dB after equalization and report delaying a 20 Gbaud QPSK signal. The demonstrated high-quality delay performance is quite scalable. The continuous delay can be extended to tens of nanoseconds, limited only by the performance of the used components, while the discrete delay is determined by the number of multicast copies and propagation distance between the reflections in the array of FBGs.

Mitigation of crosstalk between RSB and LSB signals generated by one I/Q modulator.

Optical independent sideband (ISB) signals can be generated by exploiting one external In-phase/Quadrature (I/Q) modulator. Our theoretical analysis shows crosstalk between the two ISB (right and left side) signals can attribute to two main imperfections: amplitude difference and phase unmatched in I/Q data. To reduce the impact of crosstalk between the two ISB signals, we propose three schemes. The first is precise phase match of the I and Q data. The second has been made possible by setting different frequencies for the left sideband (LSB) and the right sideband (RSB) signals, and the last is achieved by adding Multiple-Input Multiple-Output (MIMO) equalization digital signal processing (DSP) at the receiver side. Our experimental results have shown that these schemes can improve the performance of ISB signals. In our experimental system we designed dual ISB system with different modulation formats in two sidebands. Precise phase match can bring a ∼2.2dB improvement at BER of 1×10-2 and a ∼4.3dB improvement at BER of 1×10-3 for 16-ary quadrature-amplitude-modulation (16QAM) and quadrature-phase-shift-keying (QPSK) signals, respectively, in 4Gbaud with carrier frequency of 36GHz system. The BER of 4Gbaud 16QAM ISB signal at 30GHz and 4Gbaud QPSK ISB signal at 38GHz can reach hard-decision forward-error-correction (HD-FEC) when the input power is larger than -5.5 and -7.4dBm respectively in different frequencies system. For 4Gbaud with carrier frequency of 36GHz system, the BER of 16QAM signal and QPSK signal reduce ∼2.1 and ∼2.2dB at HD-FEC after using MIMO. In addition, MIMO can further improve the performance of the matched phase system or the system with different frequencies.

Transmission of 128 Gb/s Optical QPSK Signal over FSO Channel under Different Weather Conditions and Pointing Errors

Free space optics (FSO) is a promising technology for high data rate transmission where data is transmitted wirelessly from one place to another. It has a variety of applications nowadays including indoor, outdoor, underwater, and deep space communications. However, the availability of link under various atmospheric conditions is a major concern. In this study, we evaluate the transmission of 128 Gb/s optical quadrature phase shift keying (QPSK) signal under different weather conditions and pointing errors. Simulation parameters such as FSO link range, atmospheric attenuation, pointing loss, and wavelength are taken into consideration. The system performance evaluation parameters including eye diagram, constellation, and received signal to noise ratio (SNR) are compared to find the best performance under different weather conditions. Numerical simulations show that the pointing errors up to 10 µrad and bad weather conditions up to 15 dB attenuation has sever effects on the system performance and we should use digital signal processing and equalization to enhance it.

All-Optical OOK -to-QPSK Modulation Format Conversion With Wavelength Multicasting Based on Cascaded SOA Configuration

An on–off keying (OOK) to quadrature phase shift keying (QPSK) format conversion scheme has been demonstrated in this paper. Two-channel OOK signals are converted into one-channel QPSK signal, and accompanied with a multicast function. The proposed scheme is based on a cascaded semiconductor optical amplifiers (SOA) configuration where the processes of cross phase modulation (XPM) and nonlinear polarization rotation (NPR) are properly controlled by OOK signals in each SOA. The experimental results show that after conversion, four-channel QPSK signals are successfully obtained with open eye-diagrams, and all the converted signals experience a small power penalty of less than 3.4 dB at a bit error rate (BER) of $10^{-9}$ compared to the original signal. In addition, the optical spectra and the constellation diagrams of the converted signal also indicate the high quality performance exhibited by the proposed format conversion scheme.

In-Band OSNR Measurement Method for All-Optical Regenerators in Optical Domain

We propose an in-band measurement method of optical signal-to-noise ratio (OSNR) output from an all-optical regeneration system with a nonlinear power transfer function (PTF) according to the fact that there are different average gains of signal and noise. For the all-optical quadrature phase-shift keying (QPSK) regenerator as an example, the output OSNR is derived from the input OSNR and the total gain of the degraded QPSK signal. Our simulation shows that the OSNR results obtained by this method are in agreement with those calculated from the error vector magnitude (EVM) formula. The method presented here has good applicability for different data rates but is also useful for analyzing the OSNR degradation of other nonlinear devices in optical communication links.

4 × 32-Gbps Block-Wise QPSK 1200-km SSMF Direct Detection Transmission Based on Delta-Sigma Virtual Carrier Generation

Direct detection (DD) is one of the most attractive solutions for short and medium reach optical fiber transmission in comprehensive consideration of digital signal processing (DSP), power consumption, and cost. Various single side band (SSB) DD techniques have been widely investigated to detect and process the complex signal. In this paper, we propose a novel approach which uses only high-speed digital input/outputs (IO) to generate both quadrature phase shift keying (QPSK) signal and virtual carrier signal. A transmission of 4 × 32 Gb/s QPSK signal over 1200-km standard single mode fiber (SSMF) in block-wise manner has been successfully demonstrated. Only a single polarization in-phase-and-quadrature (IQ) modulator and one single photodiode (PD) are used for each channel. Compared with conventional DSP of coherent detection, the DSP in the proposed scheme is also simplified that only fixed frequency compensation is required without additional phase noise elimination. The overall system complexity has been greatly reduced.

Demonstration of wavelength tunable optical modulation format conversion from 20 and 30 Gbit/s QPSK to PAM4 using nonlinear wave mixing

We experimentally demonstrate tunable modulation format conversion of quadrature phase shift keying (QPSK) to four-level pulse amplitude modulation (PAM4) using nonlinear wave mixing in periodically-poled Lithium-Niobate (PPLN) waveguides. The conversion operation is accomplished by: (a) rotating the data constellation by applying a phase offset, and (b) offsetting the data constellation in the radial direction by adding a constant bias optical power. In this manner, the constellation data points can be tailored to have different amplitude levels, thus realizing the PAM4 format. In a tunable fashion, we convert 20- and 30-Gbit/s QPSK channels to PAM4, and open eye diagrams and low bit-error-rates (BER) are obtained at the receiver. We investigate the impact of phase rotation on the eye diagram of the received signal, and we determine that a phase rotation of ∼71°provides substantially good performance. Wavelength tunability of the output PAM4 is demonstrated by tuning the wavelength of the pump. In addition, open eye is also obtained when a high phase noise laser is used for modulating QPSK signal.

Optical digital‐to‐analogue conversion for 2‐bit BPSK‐modulated signal based on delay line interferometer with balanced photodetector

Optical digital-to-analogue (DA) conversion has the potential to realise fully-transparent optical communication systems, as it is considered to be fundamental in the most of operations, including packet switching and label processor. However, most DA converters have been confined to on–off keying modulation as opposed to phase-shift keying (PSK) modulation. In this Letter, the author provides an optical DA converter for binary PSK (BPSK) signal. The proposed scheme converts a digitised information into an optical intensity by using delay line interferometer and balanced photodetector. The scheme generates a quadrature PSK (QPSK) signal from two BPSK signals, then, the generated QPSK signal is coupled with a -shifted signal. The four constellations of the QPSK signal are converted into four levels at equal intervals when the angle is set at and . It is concluded that the optical 2-bit DA conversion is experimentally demonstrated for BPSK signal at 10 Gbps.

Efficient QPSK signal demodulation in channels with unfavorable conditions of radio reception

The main task of the receiver in digital communication systems is to provide a minimum bit error when transmitted data is received. Therefore, the quality of digital communication largely depends on the quality of demodulation of signals in general. In addition, in channels with unfavorable conditions of radio receiving, which are characterized by rapid change in signal parameters due to time and frequency Doppler spread, as well as unknown characteristics of the main transmission channel, there are situations where the most modems don’t allow for high-quality receiving of the signal. Thus, increasing the efficiency of demodulation was and remains a relevant task for modern technology of radio receiving and signal processing. The article is devoted to the problem of signal reception with phase manipulation in channels with unfavorable transmission conditions, since in most modern digital communication systems, phase manipulation is the basis of the physical level of transmission. The existing solutions are analyzed and the method of hybrid signal demodulation is considered, which combines the algorithms of feedforward estimation of signal parameters with subsequent synchronization of parameters in feedback schemes. The initial estimates of the carrier frequency offset, phase, and time delay of the signal are calculated according to the known unique insertion and the obtained estimates are used when initializing the feedback circuits for instant entering into tracking mode. A comparative modeling of known traditional methods of demodulation and method of hybrid demodulation is done on an example of a signal with quadrature phase-shift keying (QPSK). Consequently, the obtained results confirm that in the conditions of rapid change of signal parameters the hybrid algorithm provides the minimum value of bit error, depending on SNR, in comparison with traditional methods. The considered synchronization approach can also be used for signals with other types of phase manipulation. Moreover, the proposed hybrid combination of feedback and feedforward algorithms allowed the use of feedback algorithms for signals with packet data transmission.

60 Gbit/s coherent wavelength-division multiplexing free-space optical modulating retro-reflector in a turbulence-tunable atmospheric cell

A novel high-speed coherent modulating retro-reflector (MRR) is presented and experimentally demonstrated for free-space optical (FSO) communications by using an in-phase quadrature (IQ) modulator with quadrature phase-shift keying (QPSK) signals to break the conventional speed limitation. The transmission performance is tested in a turbulence-tunable atmospheric cell. The received sensitivity is measured to be <14.7 dB PPB, and the power penalty is <2.0 dB for 20 Gbit/s single-channel QPSK signal at the BER of 0.01 when the temperature difference is <250 °C. Moreover, this MRR scheme is compatible to wavelength-division multiplexing (WDM) technology, and a 60 Gbit/s WDM MRR is demonstrated using three channels with only about 0.6 dB degradation.

Design of All-Optical Modulation Format Converter From One 8PSK to Two QPSK Signals Based on Phase Sensitive Amplification in Elastic Optical Network

Flexible conversion between modulation formats enables intelligent grooming techniques for an elastic optical network (EON) aimed at dynamical spectral efficiency allocation in optical fiber transmission. In this paper, we propose and demonstrate an all-optical modulation format conversion scheme between an 8-ary phase shift keying (8PSK) signal and two quadrature phase shift keying (QPSK) signals based on phase-sensitive amplification in highly nonlinear fiber. Benefiting from two simultaneous but different phase-squeezing processes, the two QPSK signals can be obtained simultaneously to avoid the loss of original information. The system transfer functions show the feasibility of the proposed scheme, and the system performance is evaluated by input-output constellations, calculated error vector magnitude (EVM), and bit error rate (BER) in numerical simulation. At BER of 3.8 × 10 -3 , about 5-dB optical signal-to-noise-ratio (OSNR) improvement between the input 8PSK signal and output QPSK signals can be observed. Furthermore, several potential applications in optical fiber transmission can also be found based on the converter.

Optical Mitigation of Interchannel Crosstalk for Multiple Spectrally Overlapped 20-GBd QPSK/16-QAM WDM Channels Using Nonlinear Wave Mixing

Optical mitigation of interchannel interference (ICI) for multiple spectrally overlapped data channels is experimentally demonstrated without multichannel detection and channel spacing estimation. The ICI mitigation takes place in three stages of periodically poled lithium niobate (PPLN) waveguides. In the first PPLN stage, the conjugate copies of channels are generated. In the second stage, the overlapped signals are coherently multiplexed with different complex taps. In the third stage, the concurrent nonlinear processes and the coherent multiplexing of the signals with their delayed copies compensate the ICI. The bit error rate and the constellation diagrams of wavelength division multiplexed (WDM) channels carrying quadrature phase-shift keying (QPSK) or 16 quadrature amplitude modulation (16-QAM) formats demonstrate the potential capability of the proposed method to reduce the ICI and its possible modulation format transparency. The effect of channel spacing on the performance of the method is also demonstrated. After optical ICI mitigation, a reduction of almost 4 dB is achieved for the value of optical signal-to-noise ratio at BER of 10–3 for 20-GBd QPSK signals with a channel spacing of 17.5 GHz. The overlapped WDM system of 20-GBd 16-QAM signals with channel spacing of 17.5 GHz is also ICI mitigated and error vector magnitudes are reduced by almost 28%.

Scalable and reconfigurable optical tapped-delay-line for multichannel equalization and correlation using nonlinear wave mixing and a Kerr frequency comb

We experimentally demonstrate a scalable and reconfigurable optical tapped-delay-line (TDL) for multichannel equalization and correlation of 20-Gbaud quadrature-phase-shift-keyed (QPSK) signals using nonlinear wave mixing and a microresonator Kerr frequency comb. The optical TDL mainly consists of two stages: one being a multicasting of the original signals in a periodically poled lithium niobate (PPLN) waveguide with Kerr comb lines functioning as mutually coherent pumps, while the other is a coherent multiplexing of the delayed and weighted signal replicas in a second PPLN. A two- or three-tap optical TDL is demonstrated to simultaneously equalize a distorted QPSK data signal, reducing the error vector magnitude (EVM) from 22.5% to either 19.9% or 18.2%, and search two- or three-symbol patterns on another QPSK signal.

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.

Optical Comparison Operation for 8-Bit QPSK-Modulated Signal by Using Serially-Cascaded Delay Line Interferometer

An all-optical comparator is desirable to realize large-capacity, fully-transparent, and energy-efficient communication systems, as it is considered to be a fundamental component to perform most of the operations, including packet switching, label recognition, error detection and correction, and so on. However, most of the previous studies have been confined to the on–off keying (OOK) modulation format, not phase-shift keying (PSK) modulation. In this paper, the author provides a novel optical comparator designed for quadrature PSK (QPSK)-modulated signal, which comprises a code word with 8-bit length, using a serially-cascaded delay line interferometer. The proposed comparator yields constellations having the information of a Hamming distance based on the designed code, when several patterns of QPSK signal are injected into the comparator. The paper experimentally demonstrates the feasibility of the optical comparison operation for 8-bit QPSK-modulated return-to-zero (RZ) signal at 10 Gbaud.

A novel modulation format identification based on amplitude histogram space

In this paper, we proposed a novel modulation format identification method for square M-quadrature amplitude modulation (M-QAM) signals which is based on amplitude histogram space of the incoming data after analog-to-digital conversion, chromatic dispersion compensation at the receiver. We demonstrated the identification of quadrature phase-shift keying (QPSK), 16-QAM, 64-QAM formats with an amplitude histogram space. Simulation results show that it achieve 100% identification accuracy when the incoming signal OSNR is 14 dB to identify the modulation format of QPSK, 16-QAM, and 64-QAM signals in digital coherent systems. The method has low complexity and small delay.