Multiple Quadrature Amplitude Modulation Research Articles

Performance Analysis of MIMO-mQAM System with Pointing Errors and Beam Spreading in Underwater Málaga Turbulence Channel

Both the long-term beam spreading caused by ocean turbulence and the pointing errors induced by the jitter of transmitters and receivers degrade the performance of underwater wireless optical communication (UWOC) links. To effectively alleviate their effects, an in-depth study was carried out over the Málaga turbulence channel with pointing errors and beam spreading in multiple-input and multiple-output (MIMO) UWOC. First, we analyzed the long-term beam spreading and the received light power for the finite receiving aperture in the presence of pointing error displacements. Based on this, the relationship between beam spreading, pointing errors, and signal power was established. Second, the approximate expressions of the average bit error rate (BER) and the communication outage probability were derived theoretically for this MIMO system using maximal-ratio combining (MRC) diversity. Third, the effects of the pointing errors on the coding and the diversity gains were explored for the MIMO links. Finally, using the observed ocean data from the Global Ocean Argo gridded dataset, we numerically verified the combined effects of ocean turbulence strength, beam spreading, and pointing errors on the average BER and outage probability of this system. These results also proved that adjusting the size of the receiving aperture or the order of the multiple quadrature amplitude modulation (mQAM) could effectively mitigate their effects.

All-Optical Format Conversion of 2-Dimensional MQAM to MPSK Based on Nonlinear Mach-Zehnder Interferometer With Wavelength Preservation

An all-optical format conversion scheme of 2-dimensional multiple-quadrature amplitude modulation (MQAM) to multiple-phase shift keying (MPSK) signals is proposed and numerically simulated by using the nonlinear Mach-Zehnder interferometer (MZI) configuration with wavelength preservation. The input octal-quadrature amplitude modulation (8QAM) signal is generated through the phase modulator (PM) and amplitude modulator (AM) and injected into the nonlinear MZI configuration. The designed nonlinear MZI configuration includes two 3 dB optical couplers (OCs), a piece of high nonlinear fiber (HNLF) and a variable optical attenuator (VOA) in the upper interfering arm and a piece of standard single mode fiber (SSMF) or tunable optical delay line (TODL) and a variable phase shifter (VPS) in the lower interfering arm. The converted octal-phase shift keying (8PSK) signal can be obtained from the nonlinear MZI configuration through vector superposition between the input optical signal and itself with some nonlinear phase shift. The needed asymmetric nonlinear phase shift is mainly induced by the big nonlinear index difference between HNLF and SSMF. The error vector magnitude (EVM) and bit error rate (BER) performance of the scheme are calculated before and after format conversion with the different input and receiver optical-signal-noise-ratios (OSNRs). The scheme can be used in the intermediate node to connect the different networks which adopt 8QAM and 8PSK signals, respectively.

Symbol division multiplexing in optical fiber communication systems.

Modern emerging data services and applications have put forward an ever-increasing bandwidth requirement for fiber-optic communication channels. To this end, we propose a novel symbol division multiplexing technology (SDM) by multiplexing/de-multiplexing of multiple quadrature amplitude modulation (QAM) symbols onto one complex constellation point. In our SDM scheme, every 7-bit 128QAM symbol is multiplexed per complex valued signal sequentially according to the optimal many-to-one mapping law, forming a 32QAM in the constellation and achieving an extra 40% gain for symbol capacity in an optical discrete multi-tone transmission system. The experiments prove that the SDM-32QAM successfully mitigates the signal impairments induced by fiber chromatic dispersion and Kerr nonlinearity, thus leading to 3.91-dB superior receiver power sensitivity and 2-dB enhancement of systematic tolerance to fiber nonlinear effect. The results highly motivate a fundamental paradigm in multiplexing techniques for optical fiber communication systems.

Data-Driven Bandpass Filter Design for Estimating Symbol Rate of Sporadic Signal at Low SNR

Symbol rate is one of the most important parameters in signal demodulation process. In real-time signal processing, traditional symbol rate estimation algorithms for the Multiple Phase Shift Keying (M-PSK) and the Multiple Quadrature Amplitude Modulation (M-QAM) are based on the Fourier transform of signal’s complex envelope. At the low signal-to-noise ratio (SNR), the accuracy of symbol rate estimation can be improved by increasing the number of symbols as much as possible. However, this improvement is infeasible in many applications such as the energy-limited Internet of Things devices and sporadic noncooperative transmissions. In this paper, we propose a data-driven bandpass filter (BPF) design scheme for accurate estimation of symbol rate under low SNR with only a small number of symbols available. The proposed scheme considerably improves the estimation performance by optimizing the BPF design using the equivalent dynamic linearization model with time-varying pseudo-partial derivatives. Specifically, the proposed scheme iteratively optimizes the upper and lower cut-off frequencies of the BPF based on the measured complex envelope spectrum until achieving the optimal BPF. Therefore, the peaks of the complex envelope spectrum are extracted as the estimate of the symbol rate by applying the optimal BPF. Experimental results indicate the promise of the proposed scheme as an efficient symbol rate estimator for sporadic signal at low SNR and with a small number of symbols.

Robust Blind Equalization Algorithm Using Convolutional Neural Network

In this paper, we introduce a novel blind equalization algorithm based on the convolutional neural network (CNN), to improve the bit error rate (BER) performance of equalizers against the multipath fading effect and nonlinear distortion. In contrast to existing neural network (NN)-based blind equalization algorithms, the proposed algorithm performs equalization and soft demapping jointly, which allows it to obtain soft bits directly from received data. In addition, the input preprocessing module is used to reorganize the received data to fully exploit the feature information of the received signals. Afterwards, the two-dimensional (2D) convolutions can be operated on a three-dimensional (3D) input array obtained by input preprocessing to recover the transmitted bits. Simulation results show that the proposed algorithm outperforms other CNN-based blind equalization algorithms, in terms of BER performance, under a wide range of channel models, signal-to-noise (SNR) levels and modulation schemes. Strikingly, the BER obtained by the proposed algorithm approaches the theoretical BER of multiple quadrature amplitude modulation (M-QAM) in linear channels.

Flexible and fast multi‐level modulation format identification using optical orthogonal frequency division multiplexing redundancy subcarriers

AbstractCompared with the orthogonal frequency division multiplexing (OFDM) communication system, the subcarrier‐index modulation OFDM communication system can alleviate the problem of peak‐to‐average power ratio and the frequency offset sensitivity. Therefore, the problem of inter carrier interference (ICI) and low power of the radio frequency amplifier in OFDM communication system can be solved to some extent. A fast and flexible modulation format recognition (MFR) scheme utilizing OFDM redundancy subcarriers is proposed in order to solve the MFR problem in coherent optical OFDM transmission. The scheme uses the silent subcarrier position of OFDM to carry modulation format information (MFI) and the receiving end decodes the MFI index by comparing the modulus values of the redundant symbols. It can recognize the phase shift keying (PSK) and multiple quadrature amplitude modulation (M‐QAM) modulation formats block by block. The number of symbols required by the proposed MFR is equal to the number of OFDM subcarriers, and the average time to get the recognition result in a block is O(1). We numerically investigate the proposed MFR among 20 Gbaud 4/16/32/64 QAM and 8/16 PSK. The results show that the correct recognition rate of the proposed scheme is not affected by the modulation formats. In particular, the proposed MFR is a potential solution for future flexible optical networks.

Signal Estimation in Cognitive Satellite Networks for Satellite-Based Industrial Internet of Things

Satellite industrial Internet of Things (IIoT) plays an important role in industrial manufactures without requiring the support of terrestrial infrastructures. However, due to the scarcity of spectrum resources, existing satellite frequency bands cannot satisfy the demand of IIoT, which have to explore other available spectrum resources. Cognitive satellite networks are promising technologies and have the potential to alleviate the shortage of spectrum resources and enhance spectrum efficiency by sharing both spectral and spatial degrees of freedom. For effective signal estimations, multiple features of wireless signals are needed at receivers, the transmissions of which may cause considerable overhead. To mitigate the overhead, part of parameters, such as modulation order, constellation type, and signal to noise ratio (SNR), could be obtained at receivers through signal estimation rather than transmissions from transmitters to receivers. In this article, a grid method is utilized to process the constellation map to obtain its equivalent probability density function. Then, binary feature matrix of the probability density function is employed to construct a cost function to estimate the modulation order and constellation type for multiple quadrature amplitude modulation (MQAM) signal. Finally, an improved ${M_2}{M_\infty }$ method is adopted to realize the SNR estimation of MQAM. Simulation results show that the proposed method is able to accurately estimate the modulation order, constellation type, and SNR of MQAM signal, and these features are extremely useful in satellite-based IIoT.

Blind modulation format identification using decision tree twin support vector machine in optical communication system

This paper proposes a method of blind modulation format identification using decision tree twin support vector machine classifier trained with the features extracted from the high-order cumulant and cyclic spectrum after compressed sensing in order to solve the problem of identification efficiency and computing speed and reduce the performance requirements of the sampling system in low optical signal to noise ratio. By reconstructing the feature parameters of fourth-order, eighth-order cumulants and cyclic spectrum under the theory of compressed sensing, and introducing the decision tree twin support vector machine classifier to achieve high-precision classification, the different modulation formats of amplitude shift keying, multiple phase-shift keying and multiple quadrature amplitude modulation are effectively identified. Simulation analysis of the influence of identification accuracy and identification time improves the identification performance and achieves the purpose of identifying more signals with fewer feature parameters. The results indicate that the average identification accuracy of the optical modulation format signals can be achieved over 94% when the OSNR is -5dB, and the identification time is 4.3 times higher than the standard SVM. Owing to its excellent performance, this method can be employed in the next generation optical transport network for auto-adaption real-time modulation format identification.

Linewidth-tolerant and multi-format carrier phase estimation schemes for coherent optical m-QAM flexible transmission systems.

Multi-format and linewidth-tolerant carrier phase estimation (CPE) is a vital part of digital signal processing (DSP) units for future elastic optical transmissions to relax the laser linewidth limitation. In this paper, an innovative CPE scheme outperforming existing CPEs in both universality and performance is presented and verified for multiple quadrature amplitude modulation (QAM) formats. Based on the technique of extended QPSK partitioning and quasi-linear approximation, accurate phase estimation is determined by calculating the intersection of two symmetric straight lines with very low complexity. Comprehensive simulation results of square 4/16/32/64-QAM not only demonstrate that the scheme can be applied to different modulation formats with a universal structure, i.e., indicate its flexibility in the format-adaptive elastic optical networks (EONs), but also show that the linewidth tolerance is greatly enhanced even in comparison with traditional BPS schemes. In addition, taking 64-QAM as an example, the computational efforts can be significantly reduced by a factor of 15.7 (or 10.3) in the form of multipliers (or adders). The slightly better OSNR performance is experimentally validated in polarization multiplexing 16GBaud 4/16-QAM systems respectively, which shows the potential application for flexible receiver-side DSP unit in EONs.

High Temperature Quadrature Amplitude Modulation over Orthogonal Frequency Division Multiplexing

As downhole tools advance, they require increasingly higher data rates in communication with the surface. Current data rates over HT single-conductor wireline are limited to approximately 200 kbps. Long cables and harsh environments create a limiting challenge to standard serial bit rates due to high cable impedance and component availability. Keying techniques are often used to improve the robustness and speed of data links over long cables. Frequency shift keying (FSK) is a common implementation where data is encoded by specific frequencies on the data line. Amplitude shift keying (ASK) and phase shift keying (PSK) work on similar principles. In this project we apply quadrature amplitude modulation (QAM), a combination of PSK and ASK, and carry it over multiple frequency channels simultaneously using orthogonal frequency division multiplexing (OFDM). QAM over OFDM is a digital communications technique similar to those used in the telecommunications industry for digital subscriber line (DSL) connections and cellular networks. Using our data link, laboratory tests have demonstrated data rates of up to 3.8 Mbps over a 5000 ft (1524 m) single conductor wireline cable with less than 1e-8 bit error rate. Higher data rates are possible if error mitigation and correction coding is used. Gray encoding is an error mitigation technique where nearby points on the QAM constellation are similar in value so the errors are off only by least significant bits. A prototype point to point link protocol was developed in which the cable bandwidth is assessed empirically to optimize the OFDM by allowing for correction of cable distortion at each frequency independently. The protocol can enable use of the developed data link for a variety of tools with little to no changes by hiding all the complexity from the tool. Future versions of the protocol will automatically detect and correct transmission errors, allow for data buffering/retransmission and provide feedback as to the quality of the link. However, this new approach to the problem requires more complex computation to encode the data and signal generation electronics to produce the required waveform. One of the key challenges to full implementation of this concept on high temperature electronics is that a downhole fast Fourier transform (FFT) is required to decode the QAM information for downlink and an inverse FFT (IFFT) is required to efficiently encode data for uplink. The IFFT allows combination of multiple QAM data streams into a time series which can be converted to analog waveforms. High temperature testing of FFT and IFFT performance on the RelChip RC10001 microcontroller is included here. Also included here are the details of development of a high temperature digital to analog converter (DAC) to create composite signals and a high temperature line driver to boost analog signal transmission. This paper describes the communication architecture, the systems developed for high temperature implementation, and evaluations/implementations on high temperature computation devices.

High order modulation format identification based on compressed sensing in optical fiber communication system

We propose a method of modulation format identification based on compressed sensing using a high-order cyclic cumulant combined with a binary tree classifier. Through computing the fourth-order cyclic cumulant of the pretreated band signal, which is obtained by compressed sensing with the sampling rate much less than the Nyquist sampling value, the feature vector for classification is extracted. Simulations are carried out in the optical coherent fiber communication system with different modulation formats of multiple phase-shift keying and multiple quadrature amplitude modulation. The results indicate that this method can identify these modulation formats correctly and efficiently. Meanwhile, the proposed method is insensitive to laser phase noise and signal noise.

Energy-Efficient Communications in MIMO Systems Based on Adaptive Packets and Congestion Control With Delay Constraints

In this paper, we propose adaptive techniques for multiple input and multiple output (MIMO) systems, to solve the problem of energy efficient communications with delay constraint, where the energy efficiency is defined as the number of bits per second correctly received per power consumed. We first investigate the optimal multiple quadrature amplitude modulation (MQAM) constellation size for each transmission stream and the optimal packet size. By exploiting the intrinsic relationship among the constellation size, the packet size, the symbol error rate (SER) and delay, we propose an adaptive transmission mode for different delay demands. For the case of user's buffer overflow, we use the congestion control algorithm to schedule the average queue length, and maintain the optimal delay performance for energy efficiency. Simulations show that to maximize the energy efficiency and offer different Quality of Service (QoS) of delay simultaneously, the transmitter should adaptively choose the constellation size and the packet size as well as the transmission mode. In this framework, the tradeoff between energy efficiency and delay demand are well demonstrated.

Simply decoded efficient full‐rate space–time block codes over correlated Rician fading channels

A novel technique is presented to design efficient full-rate space–time block codes (STBCs) over correlated Rician fading channels. The authors first derive a formula for the achievable information rate of a linear dispersion coded multi-input single-output system as a function of correlation at the transmitter in a Rician fading channel. In addition, the authors obtain the bit-error-rate (BER) equations for this system, when multiple phase shift keying and multiple quadrature amplitude modulation schemes are used at the transmitter. Therefore with regard to derived formulae and using a genetic algorithm, a method is presented to construct STBCs over correlated Rician fading channels. The efficient designed STBCs for two and four transmit antennas enjoy full-rate, simple structure, simple processing at the receiver, better BER performance than associated orthogonal STBCs at practical signal-to-noise ratios (SNRs) and higher achievable information rate than them at all SNRs.

Design of efficient full‐rate linear dispersion space‐time block codes over correlated fading channels

In this study, the authors present a method to design efficient full-rate linear dispersion space-time block codes (LD-STBCs) in correlated Rayleigh fading channels with simple detection at the receiver. The authors first derive a formula for the achievable capacity of an LD-STBC coded multi-input multiple-output (MIMO) system as a function of correlation at the transmitter and the receiver. Moreover, the authors compute closed-form bit-error rate equations for multiple phase shift keying (M-PSK) and multiple quadrature amplitude modulation (M-QAM) schemes. Then, the authors present a method to construct LD-STBCs over correlated Rayleigh fading MIMO channels. Finally, the authors design LD-STBCs for two and four transmit antennas. The designed LD-STBCs enjoy full-rate, minimum power fluctuation, simple structure, simple processing at the receiver. Moreover, the designed LD-STBCs for two transmit antennas provide better performance than Alamouti and LYC codes and the designed LD-STBCs for four transmit antennas provide better performance than TV codes at practical signal-to-noise ratios. Simulation results show that when the proposed LD-STBC for two transmit antennas is concatenated with an efficient outer channel encoder, it provides better performance than LYC code over correlated fading channels.

Low complexity soft demapping for non-binary LDPC codes

This article focuses on non-binary wireless transmission, where "non-binary" refers to the use of non-binary Low Density Parity Check (LDPC) codes for Forward Error Correction. The complexity of the non-binary soft demapper is addressed in particular when one non-binary Galois Field (GF) symbol spreads across multiple Quadrature Amplitude Modulation (QAM) symbols and Space-Time Block Code (STBC) codewords. A strategy is devised to guarantee an efficient mapping at the transmitter, together with an algorithm at the receiver for low complexity soft Maximum Likelihood demapping. The proposed solution targets a trade-off between performance and complexity, and removes any restriction on the setting of the GF order, QAM constellation order, and STBC scheme. This makes the non-binary LDPC codes even more appealing for potential use in practical wireless communication systems.

Two-Transmitter Two-Receiver Cooperative MAC Protocol: Cross-Layer Design and Performance Analysis

Cooperative diversity is a promising and practical technique for realizing spatial diversity through a virtual antenna array formed by multiple distributed antennas of different nodes. In this paper, we propose and evaluate a novel cooperative cross-layer medium access control (CC-MAC) protocol, which combines space-time coding and adaptive modulation at the physical layer and employs truncated automatic repeat request (ARQ) at the data-link layer. The transmitter's partner is selected in the previous hop, and the receiver invites a partner to participate in data reception in the current hop, and in this situation, a two-transmitter two-receiver model is constructed. Because distributed space-time coding is applied at the transmitting set (transmitter and its partner), cooperative diversity and coding gain can be obtained. Based on the instantaneous channel state measured at the receiver, an adaptive modulation scheme is adopted to improve the link throughput at the physical layer. In addition, the truncated ARQ scheme is employed to further enhance the throughput performance at the data-link layer. Furthermore, under different system models with or without the truncated ARQ scheme, performances of the average packet error rate (PER) and symbol error rate (SER) with multiple phase shift keying (M-PSK) modulation and multiple quadrature amplitude modulation (M-QAM) over Rayleigh fading channel are analyzed. Simulation results verify the correctness of the performance analysis and show that CC-MAC can significantly improve system throughput and energy efficiency compared with other transmission schemes.

Clock multiplier using digital CMOSstandard cells for high-speed digitalcommunication systems

The authors propose and evaluate the performance of a 2N times clock multiplier that controls memory components for high-speed data communications. To improve the reliability of the circuit, a symmetric circuit structure is used, while to verify circuit operation by means of a simple method, an MVU estimator is found from simulation data. The proposed circuit can provide clock rates, which are usually required in the multiple phase shift keying (MPSK) or multiple quadrature amplitude modulation (MQAM) modulation schemes, of 2 to 2N times that of the input clock.