QPSK System Research Articles

CHANCES OF WRONG INFORMATION RECEPTION DURING ELECTROMAGNETIC INFLUENCE UPON OPTICAL CABLE

Long-distance communication lines are optical fiber lines in most cases now. This moment PM QPSK system (polarization-division multiplex with quadrature phase shift keying) appear. Polarization-division multiplex uses a fact that the optical signal can propagate as two orthogonal polarized modes that allows to use the modes independently and makes it possible to double carrier capacity (together with already other used multiplex system). Two orthogonal waves using in the same fiber can cause problems during lightning discharges close by cable line. Under influence of external cross electrical field (or longitudinal magnetic) the polarizations plane of lights wave can turns on angle, which depends on field value, light path length upon field action and fiber sensitivity. During the influence of external electromagnetic field issue a part of one polarization signal can pass to other orthogonal plane and it will perceive wrongly. The Wavelength Division Multiplexing (WDM) systems development with increasing number of carriers brought to transmission in the same transparency window some tens or hundreds waves. They are shifted relative to each other for some parts of nanometers during the lightning stroke. Therefore the polarization plane rotation will be different for each wave (other conditions being equal): the short waves rotated greater than the long waves. On input of optical amplifier there will be packet of waves with different polarizations, and it can produce problems to filters, optical insulators and amplifiers, to Raman amplifiers specially, which are sensitive to polarization. It is known that short and long waves are moving with different velocity as refraction index depends upon wave length. Usually the rotation angle quantity is small if resistivity of earth near cable is not heavy and if equipment does not contain sensitive to PMD (Polarization Modes Dispersion). This phenomenon passes usually unnoticed. But sometimes with strong thunderstorm and lightning stroke at a short distance the polarization plane rotation can exceed 45º or even 90º. Hereby, it is possible the complex electrical and magnetic fields influence during thunderstorm on orthogonal polarization using, that can lead to additional component appearance in orthogonal axes and to signal reception error. Error level for year is estimated.

Kalman filter polarization demultiplexing algorithm based on diagonalized matrix treatment.

When we implement the equalizations of polarization effects using a Kalman filter (KF) in a coherent optical fiber communication system, we will require to multiply many matrices. If the state vector describing the system has a dimension of n, the state error covariance matrix P will have the dimension of n × n, and other matrices used in the Kalman filter will also have the dimension of n × l (l is the dimension of the measurement vector). If n is very large, the KF-based algorithm will suffer from significant complexity, which results in an impractical KF-based polarization demultiplexing algorithm. In this paper, we propose a new structured KF-based polarization demultiplexing algorithm in which the state error covariance matrix P is diagonalized, which we call the diagonalized Kalman filter (DKF). We theoretically analyze the rationality of the DKF, and the validity of the DKF was verified in both 64 Gbaud polarization-division multiplexed (PDM) QPSK and 16QAM Nyquist coherent optical simulation systems. Compared with the conventional KF, simulation results proved that under a rotation of state of polarization from 1 to 10 Mrad/s for QPSK and 1 to 5 Mrad/s for 16QAM, a differential group delay from 15 to 75 ps, and a residual chromatic dispersion of 100 ps/nm, the OSNR penalties for the DKF are only within 0.5 dB for QPSK at the threshold BER = 3.8 × 10-3, and within 2 dB for 16-QAM at the threshold BER = 2 × 10-2, respectively, compare to the case of no impairment. In the meantime, for the proposed DKF, a computational complexity reduction of over 30% is achieved, compared with conventional KF, at the expense of about no more than 50 symbols convergence delay.

RESEARCH OF THE PROCEDURE OF INITIAL ACCESS TO THE FIFTH GENERATION COMMUNICATION NETWORK

Traditional mobile networks have bandwidth and infrastructure limitations. Each base station has a limited number of users and data capacity. 5G technology uses advanced technologies, such as massive MIMO and beamforming, to increase data rates and spectrum efficiency. These features, and the large number of small cells, allow many 5G users to receive low latency and high traffic without overloading the base station. Small cells work the same way as regular cells, but use new technology and millimetre waves (5G New Radio). Low-power base stations can be easily deployed. Cell search is a procedure by which the user equipment (UE) obtains time and frequency synchronization with the base station and decodes the cell ID of the mobile device. System information is broadcast information that is periodically transmitted by the base station on the downlink. This is very important information for the mobile device (UE) needed to connect to the base station (gNB). The UE reads system information to determine the location of mobile devices when turned on, to select and re-select a cell. System information provides the following information: system frame number, system bandwidth, PLMN, mobile subscriber selection and redial thresholds, etc., required to access the network. Physical Cell ID (PCI) is used to distinguish mobile devices in the network radio communication. PLMN (Public land Mobile Network) – mobile network service area. Is a set of all mobile devices of one cellular network operator. The UE scans the bandwidth in the sync raster. The synchronization raster indicates the frequency positions of the synchronization unit that can be used by the UE to detect the system when there is no explicit indication of the position of the synchronization unit. In the next step, the UE decodes the primary and secondary synchronization signals to obtain the Physical Cell ID (PCI) and the DMRS PBCH. The base station transmits SSB packets that contain one SSB per beam. The beam direction changes for each SSB transmission. The following steps are performed to complete the synchronization procedure. The UE selects the preferred beam and decodes the PBCH MIB information, such as SFN, SSB index, raster offset, default DL numerology, RMSI configuration, DM-RS location, and cell prohibition information. If the cell is not locked, the UE attempts to decode the physical downlink control channels (PDCCH) to obtain the frequency location of the PDSCH resources. A study of the model of restoring the synchronization of symbols and frames in the QPSK system, caused by a variable synchronization error. A study of the formation of the radio channel at the physical level.

LFMTS-assisted reflection interference elimination method for a coherent same-wavelength bidirectional optical communication system.

We propose a novel reflection interference (RI) elimination method using a linear frequency modulated training sequence (LFMTS). Three LFMTSs and the signal are mixed in the electrical domain before being modulated onto an external cavity laser, and then utilized in reconstructing the RI. In the digital signal processing of the receiver, two LFMTSs identify the time mismatch between the signal and the RI, while the third is for carrier recovery of the RI. Simulations were carried out to investigate the impact of critical factors on the RI elimination, including the time mismatch, the bandwidth limitation, the laser linewidth and the frequency difference between lasers. Experiments were carried out on a 21-GBaud QPSK system to prove that the RI can be eliminated. Both simulations and experiments have proved the effectiveness of the proposed method in respect of RI elimination.

Effect of Epidemic Interference on the Performance of M-ASK, M-PSK and M-QAM Modulation Schemes

This article presents a study on the performance of different digital modulation systems in the presence of epidemic interference. This type of interference is caused by the fast increase in the number of users in a system at a given time. The epidemic interference can be modeled as a non-stationary stochastic process, which presents random power levels over time. In a previous work, the authors assessed the effect of the epidemic interference on the performance of BPSK, QPSK, and M-QAM systems. The present paper extends the previous results for MASK and M-PSK systems. Furthermore, numerical results from Monte Carlo simulations are presented for all evaluated digital modulation systems.

Machine Learning Assisted Modulation-Format Transparent and Nonlinearity Tolerant Carrier Recovery Scheme for Intelligent Receiver

A modulation-format transparent and nonlinearity tolerant carrier recovery scheme, based on machine learning and kurtosis, is proposed and demonstrated experimentally. In our scheme, kurtosis-based blind phase search (BPS) and machine learning assisted maximum likelihood (ML) estimator are deployed together to achieve precise phase noise estimation. In the first stage, kurtosis-based BPS is used to estimate phase noise coarsely in a decision-free and modulation-format transparent manner. Then, mean-shift clustering, the well-known machine learning algorithm, is employed to find the centroid points of clusters automatically. Finally, the centroid points are taken as reference in ML estimator for precise residual phase noise estimation. Compared with BPS and universal carrier phase estimation scheme (UCPE), higher nonlinearity tolerance in our scheme is demonstrated via numerical simulation. Meanwhile, the linewidth tolerance of the proposed scheme is six times than that of UCPE for 64QAM. Our scheme is also verified experimentally for both 28GS/s QPSK and 16-QAM systems in the back-to-back (BTB) and transmission case. The results indicate that our scheme obtains comparable OSNR sensitivity compared with UCPE and BPS in the BTB case. Furthermore, the proposed scheme also achieves 0.4 dB and 0.2 dB Q-factor improvement than UCPE and BPS for 16QAM after 915 km fiber link transmission, respectively.

A Machine Learning Approach to Phase Reference Estimation With Noise

This paper concerns the problem of phase reference estimation with noise, introduced by the imperfect phase-locked loop (PLL) circuit, or the imperfect channel estimation, or both. Prior solutions for suppressing phase noise focus on improving the accuracy of phase reference estimation. The accuracy of phase reference estimation is not high enough due to the following two limitations. First, since the PLL circuit works in radio-frequency (RF), a PLL circuit with high accuracy leads to high cost and high complexity, which makes the deployment difficult. Second, as data rates increase and wireless channels become more complex, the receiver is more difficult to obtain an ideal channel estimation and the negative effect of phase noise becomes more apparent. In this paper, we propose a machine learning approach to mitigate the negative effect of phase noise by using clustering algorithms. The key intuition of our approach is that the clustering algorithm can adaptively trace the shifted constellation point due to the phase noise. Our approach is adaptive because it can adaptively find each received symbol belongs to its original constellation point if the phase noise is not too large, e.g. , no larger than $0.25 \pi $ . While the shifted distance is not too large, we can map the received symbols into the correct constellation point to mitigate the negative effect of phase noise. Instead of directly using conventional clustering algorithms into the proposed machine learning approach, we propose a new weighted ensemble clustering algorithm to further improve the performance of our approach. In comparison with prior approaches based on RF circuits, our approach has comparable reception performance but with low complexity and low cost. Our experimental results show that, for a QPSK system, our approach improves the demodulation performance and the decoding performance about 10 dB, 8 dB under BCH codes, and 3 dB under Turbo codes, respectively. Even the demodulation performance of our approach without channel coding is better than the decoding performance of the system with channel coding about 5 dB under BCH codes.

Teaching Communications with SDRs: Making It Real for Students

This article describes a class/laboratory organization that culminates in a fully synchronized QPSK system based on Analog Device's ADALM-PLUTO radio. The course is organized as a one-semester senior-level elective with minimum prerequisites and designed so that all students succeed. An emphasis is placed on what synchronization can teach about real-world impairments such as automatic gain control, frequency offset, and timing offset.

Binarized Coherent Optical Receiver Based on Opto-Electronic Neural Network

The resolution of analog to digital converter (ADC) and the power consumption of digital signal processing (DSP) are becoming the bottleneck of optical communication system. To acquire and recover signals with low resolution ADCs, we propose a binarized coherent optical receiver based on opto-electronic neural network, which consists of three layers, the broadcast layer, the optical neural layer, and the electronic neural layer. The broadcast layer that composed of Mach–Zehnder interferometers (MZIs) is to duplicate and select the input optical signal into the opto-electronic neural network. The optical neural layer is constituted by specially designed structures to map the binarized weights into optical domain, which can be integrated on silicon together with the broadcast layer. The one-bit vertical resolution ADCs act as the binarized activation after the optical neural layer. This binarized coherent optical receiver is demonstrated in simulation of 50-Gb/s single polarization (SP) QPSK system and 100-Gb/s polarization multiplexing (PDM) QPSK system. The one-bit vertical resolution ADCs can be utilized to recover the complex modulation format signal successfully. The broadcast layer and the optical neural layer can be integrated based on silicon, which could increase the calculation speed and reduce the energy consumption of the total system.

High symbol-rate polarization interference cancelation for satellite-to-ground remote sensing data transmission system

To remove the impacts of polarization interference on satellite-to-ground high-speed data transmission systems, the major factors that affect polarization interference are concluded, the deteriorations of bit error rate (BER) of different modulation systems and channel coding modes are simulated, and the parallel adaptive filtering technology is presented to resolve the high symbol-rate signal transmission polarization interference. It has been verified by experiments that the effects of this method is remarkable. In the situation of QPSK system, 500MSPS symbol rate and 10 dB polarization isolation, the BER performance can be improved 4.5 dB by using this method.

Training Sequence-Based Chromatic Dispersion Estimation With Ultra-Low Sampling Rate for Optical Fiber Communication Systems

We present a novel method to estimate chromatic dispersion with ultra-low sampling rate based on training sequence. The under-sampling can be equivalent to over-sampling by splicing points with periodic sequence. The functional relationship between equivalent over-sampling rate and symbol rate, actual sampling rate and sequence period is deduced. To demonstrate the feasibility of the method, the simulation of a 28 GBaud QPSK optical fiber communication system is carried, in which 76 MSa/s sampling rate is equivalent to 66.5 GSa/s sampling rate. The results show that maximum estimation error is less than 160 ps/nm after 100 km to 2300 km SSMF transmission. We also demonstrate the robustness of the proposed method to amplified spontaneous emission and nonlinear noise in a three-channel optical fiber communication system. Moreover, the proposed method has been experimentally verified with a 20.5 GBaud QPSK system, 1.25 GSa/s sampling rate is equivalent to 51.25 GSa/s sampling rate, and estimation error is less than 100 ps/nm and 150 ps/nm after 100 km and 200 km SSMF transmission, respectively. The proposed method eliminates the requirement of high speed ADC and is cost effective, which can be used for optical performance monitoring.

Joint monitoring of chromatic dispersion, OSNR and inter-channel nonlinearity by LFM pilot

A novel joint monitoring method is proposed for dynamic and reconfigurable optical transmission systems to estimate chromatic dispersion (CD), optical signal-to-noise ratio (OSNR) and inter-channel nonlinearity (NL) simultaneously. This method utilizes two linear-frequency modulation (LFM) pilots with different center frequencies at two polarizations. The CD is estimated by the peak position difference of two pilots in fractional domain. Compare to previous work of CD estimation, the monitoring precision of CD is improved by zero padding at the receiver. The OSNR and the inter-channel NL are obtained in frequency and fractional domain. By verification of 9-channel 28 GBaud wavelength division multiplexing QPSK and 16QAM systems after 1000 km transmission, it proves that this method achieves stable CD (<200 ps/nm error), nonlinearity tolerant OSNR and accurate inter-channel NL (<1 dB error) monitoring.

Monitoring inter-channel nonlinearity based on differential pilot

We modify and simplify the inter-channel nonlinearity (NL) estimation method by using differential pilot. Compared to previous works, the inter-channel NL estimation method we propose has much lower complexity and does not need modification of the transmitter. The performance of inter-channel NL monitoring with different launch power is tested. For both QPSK and 16QAM systems with 9 channels, the estimation error of inter-channel NL is lower than 1 dB when the total launch power is bigger than 12 dBm after 1000 km optical transmission. At last, we compare our inter-channel NL estimation method with other methods.

Near-Coherent QPSK Performance With Coarse Phase Quantization: A Feedback-Based Architecture for Joint Phase/Frequency Synchronization and Demodulation

As communication systems scale up in bandwidth, the limited resolution in high-speed analog-to-digital converters (ADCs) is a key challenge in realizing low-cost “mostly digital” transceiver architectures. This motivates a systematic effort to understand the limits of such architectures under the severe quantization constraints imposed by the use of low-precision ADCs. In particular, we investigate a canonical problem of blind carrier phase and frequency synchronization with coarse phase quantization in this paper. We develop a Bayesian approach to blind phase estimation, jointly modeling the unknown data, unknown phase and the quantization nonlinearity. We highlight the crucial role of dither, implemented via a mixed signal architecture with a digitally controlled phase shift prior to the ADC. We show the efficacy of random dither, and then improve upon its performance with a simple feedback control policy that is close to optimal in terms of rapidly reducing the mean squared error of phase estimation. This initial blind phase acquisition stage is followed by feedback-based phase/frequency tracking using an Extended Kalman Filter. Performance evaluations for a QPSK system show that excellent bit error rate (BER) performance, close to that of an unquantized system, is achieved by the use of 8 phase bins (implementable using 4 one-bit ADCs operating on linear combinations of in-phase and quadrature components).

Post-FEC performance evaluation of coherent QPSK system with an enhanced pilot-aided CPE scheme

In this paper, a simple and robust pilot-aided carrier phase estimation scheme with a modified phase unwrapping is proposed. The principle of the proposed phase unwrapping is shown with a step-by-step procedure. Further, we test the post-FEC BER performance for the pilot-aided carrier phase estimation scheme with the traditional and proposed phase unwrapping. For the proposed phase unwrapping, a cycle slip rate reduction of about 10 times is achieved than the case using the traditional phase unwrapping through simulations in the presence of Wiener phase noise. Finally, we present a 2.2 Eb/N0 gain and post-FEC error-free performance with only 0.78 % pilot overhead in a 32-Gbaud QPSK system using soft-decision forward error correction.

带宽受限QPSK相干光侦听接收机最佳性能分析

带宽受限QPSK相干光侦听接收机最佳性能分析

FPGA-Based Emulation of a Synchronous Phase-Coded Quantum Cryptography System

We present FPGA-based emulation of a synchronous phase-coded quantum cryptography system. Several of the emulated subsystems are used for implementation in a free space demonstrative QPSK scheme for quantum key distribution with continuous variables (CV-QKD) using a base and optical phase synchronization. The CV-QKD systems are commonly implemented using QPSK modulation with switched or simultaneous detection. In this paper we only make use of one base of the QPSK system in order to get a simpler modulation (BPSK) scheme, used for demonstrative purposes. The reported results from the emulation and the experiment in terms of Quantum Bit Error Rate (QBER) and mutual information for different values of the number of photons per bit are in good agreement.

Biased SNR estimation using pilot and data symbols in BPSK and QPSK systems

In wireless communications, knowledge of the signal-to-noise ratio is required in diverse communication applications. In this paper, we derive the variance of the maximum likelihood estimator in the data-aided and non-data-aided schemes for determining the optimal shrinkage factor. The shrinkage factor is usually the constant that is multiplied by the unbiased estimate and it increases the bias slightly while considerably decreasing the variance so that the overall mean squared error decreases. The closed-form biased estimators for binary-phase-shift-keying and quadrature-phase-shift-keying systems are then obtained. Simulation results show that the mean squared error of the proposed method is lower than that of the maximum likelihood method for low and moderate signal-to-noise ratio conditions.

Comparison of Split-Step Fourier Schemes for Simulating Fiber Optic Communication Systems

This paper mainly focuses on efficient schemes for simulating propagation in optical fibers. Various schemes based on split-step Fourier techniques to solve the nonlinear Schrodinger equation (NLSE), which describes the propagation in optical fibers, are compared. In general, the schemes in which the loss operator is combined with nonlinearity operator are found to be more computationally efficient than the schemes in which the loss is combined with dispersion. When the global error is large, the schemes with variable step size outperform the ones with uniform step size. The schemes based on local error and/or minimum area mismatch (MAM) further improve the computational efficiency. In this scheme, by minimizing the area mismatch between the exponential profile and its stepwise approximation, an optimal step size distribution is found. The optimization problem is solved by the steepest descent algorithm. The number of steps to get the desired accuracy is determined by the local error method. The proposed scheme is found to have higher computational efficiency than the other schemes studied in this paper. For QPSK systems, when the global error is 10 -8 , the number of fast Fourier transforms (FFTs) needed for the conventional scheme (loss combined with dispersion and uniform step size) is 5.8 times that of the proposed scheme. When the global error is 10 -6 , the number of FFTs needed for the conventional scheme is 3.7 times that of the proposed scheme.

Automatic bias control of Mach−Zehnder modulators for QPSK and QAM systems

A close-loop control technique for automatic bias control of inner and outer Mach–Zehnder modulators in a dual parallel configuration is presented. The Mach–Zehnders are configured as dual parallel optical modulators. The technique is based on applying low amplitude pilot tones at three different direct current biasing ports of the modulators. By carefully monitoring the amplitude of the beating between these pilot tones, a close-loop control technique can be developed that will automatically monitor and follow the drift of the direct current biasing voltage from its optimum. Hence the technique will enable the modulators to exhibit long term stability for the quadrature phase shift keying or M-quadrature amplitude modulation systems.