Coherent demodulation of optical multilevel phase-shift-keying signals using homodyne detection and digital signal processing

We demonstrate coherent demodulation of optical multilevel (M-ary) phase-shift-keying (PSK) signals. Since the carrier phase is estimated accurately through digital signal processing after phase-diversity homodyne detection, the system performance is highly tolerant to the carrier phase noise. By off-line bit-error-rate measurements using distributed feedback semiconductor lasers with linewidths of 150 kHz as a transmitter and a local oscillator, it is shown that binary PSK (M=2), quadrature PSK (M=4), and eight-PSK (M=8) signals are successfully demodulated at the symbol rate of 10 Gsymbol/s.     

Linewidth-tolerant 30 Gbit/s 8-PSK self-homodyne using single modulator and phase-noise cancelling technique

Linewidth-tolerant 8-ary phase-shift keying (8-PSK) self-homodyne modulation/demodulation at symbol rates of 10 Gsymbol/s (30 Gbit/s) and 1 Gsymbol/s (3 Gbit/s) using a distributed-feedback laser diode with a linewidth of 30 MHz and a single modulator is experimentally demonstrated. Owing to the phase-noise cancelling capability of the self-homodyne scheme based on a polarisation-multiplexed pilot-carrier, a floorless bit error rate performance of <10^-9 was achieved; this value was obtained even for the symbol rate of 1 Gsymbol/s, at which the required linewidth condition is stringent. Furthermore, the tolerances against differential group delays and dispersion at the symbol rate of 10 Gsymbol/s have been evaluated.     

A fully digital noncoherent and coherent GMSK receiver architecturewith joint symbol timing error and frequency offset estimation

We propose a fully digital noncoherent and coherent Gaussian minimum shift keying (GMSK) receiver architecture with joint frequency offset compensation and symbol timing recovery. Carrier phase offset can be estimated if the coherent demodulation mode is adopted. The converted base-band complex signal is first frequency discriminated and then passed through a digital filter which performs a fast Fourier transform (FFT). The frequency offset can be estimated from the DC component of the FFT, and the symbol timing error can be estimated from the phase angle of the FFT at a specified frequency which is equal to an integral multiple of half the bit rate. These two estimated parameters are then used for frequency offset compensation and symbol timing recovery during a preamble period. Coarse carrier phase can be estimated by averaging sampled in-phase and quadrature-phase signals and finding its phase angle within the preamble period after carrier frequency offset is estimated and compensated. The bit error rate (BER) performance of this GMSK receiver architecture is assessed for an additive white Gaussian noise (AWGN) channel by computer simulation     

Nearly Shot-Noise-Limited Performance of Dual-Channel Linear Optical Sampling for Ultrafast DPSK Signals

This paper describes a newly developed dual-channel linear optical sampling technique for observing ultrafast optical differential phase-shift keying (DPSK) signals. As the proposed measurement scheme offsets two parallel interferometers by a relative delay corresponding to 1-symbol length of the DPSK signal, the measured phase distribution reflects the signal quality which is determined by the phase difference between adjacent symbols. This technique, based on interferometric optical gating by local short-pulses, also offers ultrafast measurement at symbol rates of greater than 100 Gsymbol/s. Moreover, its detection sensitivity can reach the shot noise limit. The waveform degradation caused by the coherence of the light source and the pattern effect of the phase modulator is successfully observed in continuous waves and 10-Gsymbol/s nonreturn-to-zero DPSK signals, and the constellation measurement is demonstrated for a 160-Gsymbol/s return-to-zero DPSK signal. Measurement system noise is also discussed for characterizing the detection sensitivity, and the nearly shot-noise-limited performance is experimentally verified.      

Real-Time 2.5 GS/s Coherent Optical Receiver for 53.3-Gb/s Sub-Banded OFDM

A real-time receiver for the coherent optical orthogonal frequency-division multiplexing (CO-OFDM) detection is realized in a field-programmable gate array (FPGA). Each building block of the CO-OFDM receiver, such as symbol synchronization, channel estimation, and phase estimation is described and discussed in respect of special technical requirements of real-time implementation. The real-time receiver is successfully demonstrated with a receiver sampling rate of 2.5-Gsamples/s to receive a subband of 53.3-Gb/s multiband CO-OFDM signal. The measured bit error rate (BER) is as low as $3.7times 10^{-8}$ which is a record in real-time or offline CO-OFDM demonstration.      

Dynamic Range of Single-Ended Detection Receivers for 100GE Coherent PM-QPSK

Recently, coherent polarization-multiplexed quadrature-phase-shift keying has attracted considerable interest. Enabled by progress in ultrafast digital signal processing electronics, it can carry 4 bits per symbol and has been proposed for 100-Gb/s transmission at a reduced 25-GBaud line rate. Several different receiver structures are possible, whose performance, complexity, and design issues are varied. In this letter, we concentrate on a receiver employing single-ended, rather than balanced, detection. We analyzed its performance and found that, by properly tailoring its design parameters, the single-ended receiver may closely approach the performance of the more expensive balanced receiver, over a practically relevant received-signal dynamic range.     

Linear Fiber Impairments Mitigation of 40-Gbit/s Polarization-Multiplexed QPSK by Digital Processing in a Coherent Receiver

To provide higher capacity networks, 40-Gb/s transmission systems are under active development and their cost is on the way to be competitive with the one of 410 Gb/s. However, their lower tolerance to linear and nonlinear fiber impairments remains a major drawback for field deployment. To address the issue of linear impairments, coherent detection of multilevel formats with polarization division multiplexing appears as a promising solution by reducing the symbol rate to 10 Gbaud. Indeed, such coherent based systems have already demonstrated an improved tolerance to optical noise and an interesting capability to compensate for large amount of chromatic dispersion. In this paper, the tolerances to narrow optical filtering, chromatic dispersion, and polarization mode dispersion are investigated with coherent detection of 10-Gbaud quadrature phase shift keying (QPSK) with and without polarization division multiplexing. Moreover, the efficient mitigation of these linear impairments by digital processing in a coherent receiver is demonstrated in an ultralong haul transmission (4080 km) of 40-Gb/s QPSK polarization multiplexed data.     

Performance analysis of both hybrid and frequency-hoppedphase-coherent spread-spectrum systems. I. A hybrid DS/FH system

The authors present and analyze a model for both hybrid and frequency-hopped spread-spectrum systems in which a fully digital coherent receiver is used to demodulate the data. A receiver for a hybrid DS/FH (direct-sequence/frequency-hopped) system using a digital delay-lock loop is considered. In the absence of frequency uncertainty, it is shown that the tracking error can be modeled as an ergodic Markov chain with a finite-state set, and the probability density function of the steady-state tracking error is evaluated. When there is a frequency uncertainty, the dynamics of the resulting nonstationary phase error can be obtained, and an expression to evaluate the probability distribution of the first time at which the phase error hits predetermined boundary values is derived. Bit error rate performance is determined in the presence of both additive white Gaussian noise and various types of interference, and the performance is compared to that of noncoherent FSK systems     

Coherent homodyne optical communication receivers withphotorefractive optical beam combiners

The principles of operation and the results of performance measurements are reported of a new type of coherent optical receiver that used a dynamic volume index of refraction grating formed inside a photorefractive material to coherently combine signal and local oscillator light prior to photodetection. Because the refractive index grating is formed by the interference pattern generated where mutually coherent optical beams overlap, the receiver can automatically adjust to changes in angle of arrival or optical wavefront profiles which occur on time scales longer than the grating formation time. The grating appears stationary to high-speed phase modulation imposed on the signal beam and coherently diffracts local oscillator light into the signal beam direction. Performance measurements are reported for a prototype system that used two independent Nd:YAG lasers at 1.064 μm, an iron-doped indium phosphide photorefractive crystal, and a four-slot phase modulation signal format. A receiver BER of 10^-6 was obtained at received signal powers that corresponded to an average of 70 detected signal photons per bit at a source data rate of 50 Mb/s, 130 detected signal photons/bit at 220 Mb/s, and about 400 detected signal photons/bit at a 325 Mb/s source data rate. Quantum-limited operation corresponds to an average of six detected signal photons per transmitted information bit for this signal format     

全海深高速水声通信机设计与试验

为了适应深海科学考察领域的快速发展,实现对全海域水声通信能力的覆盖,本文设计了一种全海深水声通信机。该水声通信机采用大功率D类功率放大器进行信号发射,利用三基元接收通道进行空间分集接收,最大通信距离12 km。水声通信机采用单载波相干水声通信技术,相干解调算法采用了时频域双向turbo均衡器结构,均衡器系数调整采用了改进比例归一化最小误符号率准则算法。该通信机于2018年12月12日在海洋最深处马里亚纳海沟挑战者深渊进行了实时水声通信试验,实现了水下10500 m到水面的高速水声通信,通信速率6000 bps,通信误比特率为10-5量级。      

Evaluation of Sensitivity of the Digital Coherent Receiver

In this paper, we investigate the sensitivity of the digital coherent receiver both theoretically and experimentally. The receiver sensitivity close to the shot-noise limit is demonstrated in the 10-Gbit/s binary phase-shift keying system with the help of a low-noise optical preamplifier. We also introduce polarization diversity into our receiver. Maximal-ratio polarization combining in the digital domain makes the receiver sensitivity independent of the state of polarization of the incoming signal without power penalty.      

Pilot tone aided coherent GMSK systems for sound/data mobilebroadcasting

A Gaussian minimum-shift keying (GMSK) modem with built-in fade canceler is proposed narrowband mobile digital broadcasting systems. The proposed scheme combines DC suppressing line code and high-pass filters to create a spectral null at the carrier frequency of the transmit GMSK signal spectrum. This enables an unmodulated carrier pilot tone to be transmitted with the digital broadcast signal without mutual interference. In the receiver, the recovered pilot is used to coherently demodulate the received signal without phase ambiguity. The pilot can be also be used to track and cancel out the random phase noise induced by fast fading. The scheme is applied to a 16 kb/s BT=0.25 GMSK signal with B_v=800 Hz, and its performance over a fast Rayleigh fading channel is investigated using computer simulation. The results demonstrates that the proposed coherent modem is capable of significantly outperforming conventional differential detection in fast fading environments such as a broadcasting channel for mobile reception     

2.5 Gbit/s码位重叠快跳频光码分多址实验系统

设计并实现了单用户、数据速率2.5 Gbit/s的码位重叠快跳频光码分多址(SO-FFH OCDMA)实验系统。采用波长数为7和码长为4的单重合码,在数据速率2.5 Gbit/s时设计和制作了基于光纤布拉格光栅(FBG)的光编码/解码器,并测试了光编码器和光解码器的频谱图。脉冲发生器产生2.5 Gbit/s的非归零(NRZ)脉冲信号,外调制放大自发辐射(ASE)宽带光源后,通过光环行器进入光编码器进行光信号的扩频编码。编码后的光信号经掺铒光纤放大器(EDFA)放大后,输入到光解码器进行扩频解码,并通过2.5 Gbit/s接收模块转换为电信号。从解码信号的波形看,在用户数据速率为2.5 Gbit/s时,该系统能够正确解码用户的数据信息。实验结果表明,相对于传统的快跳频光码分多址系统,码位重叠快跳频光码分多址可大大提高用户的数据速率。     

A DOP feedback controlling multi-stage electrical PMD compensator in digital coherent receiver

We proposed a degree of polarization (DOP) controlling multi-stage electrical polarization mode dispersion (PMD) compensator in digital coherent receiver. The compensator is modulation format independent and can mitigate both first order and higher order PMD. We evaluated this PMD compensator in both 100-Gb/s 16-QAM and QPSK signal transmission systems with 15 ps and 20 ps average differential group delay (DGD) respectively. The results show that, for both two cases, less than 0.2 dB optical signal to noise ratio (OSNR) penalty at 1e^?3 symbol error rate (SER) can be achieved after 4-stage PMD compensation.     

Ultrafast Measurement of Optical DPSK Signals Using 1-Symbol Delayed Dual-Channel Linear Optical Sampling

We propose and demonstrate a 1-symbol delayed dual-channel linear optical sampling for observing optical differential phase-shift keying (DPSK) signals. As this technique is based on optical gate processing by means of interference with local short pulses, its performance allows ultrafast measurement for a symbol rate of greater than 100 Gsymbol/s. Moreover, as the new measurement apparatus employs a two-series interferometer system with a 1-symbol delay, the measured phase distribution reflects the signal quality between adjacent symbols of the optical DPSK signals. In our experiment, we successfully observe the waveform degradation caused by the coherence of the light source and the pattern effect of the phase modulator. The measurement system noise is also discussed.     

A 4 Gb/s 2-level to 2 Gsymbol/s 4-level converter GaAs IC forsemiconductor optical amplifier QPSK modulators

The design of a 50 Ω impedance matched two-to-four level converter GaAs IC for two-electrode semiconductor optical amplifier (SOA) modulators is presented. The designed IC exhibits eye diagrams with eye openings of better than 0.30 V and a spacing between adjacent output signal levels of 0.33 V for output symbol rates of up to 2 Gsymbol/s corresponding to input bit rates of up to 4 Gb/s. A novel differential super buffer output driver is applied, for which output reflection coefficients |S₂₂| of less than -12 dB for frequencies up to 10 GHz are obtained. A 1 Gb/s optical QPSK microwave link transmission experiment using a packaged sample of the designed IC and a two-electrode semiconductor optical amplifier phase modulator has been conducted     

Optimum receiver design for wireless broad-band systems using OFDM.I

Orthogonal frequency-division multiplexing (OFDM) is the technique of choice in digital broad-band applications that must cope with highly dispersive transmission media at low receiver implementation cost. In this paper, we focus on the inner OFDM receiver and its functions necessary to demodulate the received signal and deliver soft information to the outer receiver for decoding. The effects of relevant nonideal transmission conditions are thoroughly analyzed: imperfect channel estimation, symbol frame offset, carrier and sampling clock frequency offset, time-selective fading, and critical analog components. Through an appropriate optimization criterion (signal-to-noise ratio loss), minimum requirements on each receiver synchronization function are systematically derived. An equivalent signal model encompassing the effects of all relevant imperfections is then formulated in a generalized framework. The paper concludes with an outline of synchronization strategies     

The effect of interpolation on the BER performance of narrowbandBPSK and (O)QPSK on Rician-fading channels

This paper deals with the bit error rate (BER) performance of a fully digitally implemented receiver performing coherent detection on a narrowband BPSK or (O)QPSK signal, transmitted over a slow non-frequency selective Rician fading channel. The considered digital receiver operates on samples of the received signal, taken by a fixed clock which is not synchronized to the transmitter clock. Signal samples needed for detection of the information sequence are obtained from interpolation between the available samples. In the case of nonideal interpolation a BER degradation occurs. By means of theoretical analysis, we demonstrate that even simple interpolators, operating at only a few samples per symbol, yield BER degradations that are limited to a small fraction of a dB. This paper is an extension of the work of Erup, Gardner and Harris (see ibid., vol.41, no.6, p.998-1008, 1993) where only the AWGN channel was considered and results were obtained by computer simulations     

偏振复用QPSK相干光数字通信系统实验研究

建立了偏振复用正交相移键控(QPSK)传输及相干检测的数字通信实验系统,并分析系统的性能。40Gbit/s的偏振复用QPSK信号在单模光纤(SMF)中传输了100km;信号光与本振光在接收端进行混频并下变频,最后用数字信号处理(DSP)算法对接收信号离线处理。比较了光纤传输前后不同的光信噪比(OSNR)下的误码率(BER),实验结果表明,所使用的DSP算法能克服光纤信道对信号的主要损伤,对高速的相干检测信号进行有效的恢复。     

光接入网络中的数字信号处理技术

采用先进的数字信号处理(DSP)技术,在发射机和接收机分别引入预处理和后处理,以提高光接入网络的频谱效率并延长传输距离。研究了一种基于光超奈奎斯特(Super-Nyquist)滤波的类9状态正交振幅调制(9QAM)信号多模均衡(MMEQ)后端DSP算法,使用这种方案,能够有效提高频谱效率,实现了频谱效率高达4 bps/Hz的正交移相键控(QPSK)信号传输;还研究了一种基于数字SuperNyquist信号前端预处理的方案,此方案的优点是不需要光预滤波即可达到相同的频谱效率。使用一个采用直接调制激光器(DML)、直接探测和数字均衡技术的高速无载波幅度相位-64状态正交振幅调制(CAP-64QAM)系统,在20 km标准单模光纤(SSMF)上实现了创纪录的60 Gbit/s CAP-64QAM信号传输;使用相干探测,实现了速率高达100 Gbit/s的64状态正交振幅调制-正交频分复用(16QAM-OFDM)实时传输系统,解决了实时OFDM信号处理中的关键问题。