Multilevel Modulation Formats Research Articles

Optical phase erasure based on FWM in HNLF enabling format conversion from 320-Gb/s RZDQPSK to 160-Gb/s RZ-DPSK

Through four-wave mixing (FWM) process in highly-nonlinear fiber (HNLF) or semiconductor device, the phase-modulation depth of one converted FWM component could be doubled compared with that of the original input signal. Therefore, with a multilevel phase-modulated signal and a CW light as inputs, after FWM process in a nonlinear media, phase pattern (0, pi) carried in the input multilevel phase-modulated signal will not be transferred to one converted FWM component, which could be referred to as an optical phase erasure process. We experimentally demonstrated format conversion from 320-Gb/s return-to-zero differential quadrature phase-shift keying (RZ-DQPSK) to 160-Gb/s return-to-zero differential phase-shift keying (RZ-DPSK) through the proposed all-optical phase erasure scheme. The phase information carried in the converted binary RZ-DPSK is logically equal to the input Q-component, or a logical XOR operation result between I and Q components of the input RZ-DQPSK, which correspond to a serial or parallel DQPSK transmitter for the input RZ-DQPSK signal. It can be applied to erase a binary tributary from a multilevel modulation format.

INFLUENCE OF EXTINCTION RATIO ON THE PERFORMANCE OF RZ-ODPASK MODULATION FORMAT

The influence of extinction ratio (ER) on the performance of 120 Gbps return-to-zero octal differential phase-amplitude-shift keying (RZ-ODPASK) modulation format in single-channel transmission is investigated. Based on the modulation and demodulation schemes, the structures of RZ-ODPASK transmitter and receiver are presented; and expressions for the output optical fields and electrical currents of amplitude and phase tributaries are analytically acquired. As ER is a tradeoff for good performance of the two tributaries, the optimal ER for back-to-back configuration is theoretically derived as 2.32 dB for the first time, which is verified by simulation. Then the tolerances to residual chromatic dispersion (CD) and fiber nonlinearity of RZ-ODPASK transmission are numerically studied, respectively. The results indicate that phase tributary is more robust against both dispersion and nonlinearity impairment than the amplitude tributary. To balance the performance of the two tributaries, curves of the optimal ER values with the change of residual CD and fiber nonlinearity are also obtained, respectively. The results can be useful for the design of RZ-ODPASK systems; in the meantime, the analytical methodology described in this paper is applicable to other multilevel RZ-DPASK modulation formats.

Generation and transmission of multilevel quadrature amplitude modulation formats using only one optical modulator: MATLAB Simulink simulation models

A geometrical and phasor representation technique is presented to illustrate the modulation of the lightwave carrier to generate quadrature amplitude modulated (QAM) signals. The modulation of the amplitude and phase of the lightwave carrier is implemented using only one dual-drive Mach-Zehnder interferometric modulator (MZIM) with the assistance of phasor techniques. Any multilevel modulation scheme can be generated, but we illustrate specifically, the multilevel amplitude and differential phase shift keying (MADPSK) signals. The driving voltage levels are estimated for driving the traveling wave electrodes of the modulator. Phasor diagrams are extensively used to demonstrate the effectiveness of modulation schemes. MATLAB Simulink models are formed to generate the multilevel modulation formats, transmission, and detection in optically amplified fiber communication systems. Transmission performance is obtained for the multilevel optical signals and proven to be equivalent or better than those of binary level with equivalent bit rate. Further, the resilience to nonlinear effects is much higher for MADPSK of 50% and 33% pulse width as compared to non-return-to-zero (NRZ) pulse shaping.

Experimental Demonstration of RZ-8-APSK Generation Through Optical Amplitude and Phase Multiplexing

In this letter, we propose and experimentally demonstrate an optical amplitude and phase multiplexing scheme to generate a multilevel modulation format signal, return-to-zero eight-amplitude and phase-shifted keying (RZ-8-APSK). The proposed optical amplitude and phase multiplexing is mainly based on four-wave mixing (FWM) in highly nonlinear fiber. Cross-absorption modulation in an electroabsorption modulator is employed to intentionally adjust the extinction ratio of the generated RZ-8-APSK signal. Amplitude and phase information carried in the inputs is multiplexed and transparently transferred to the generated FWM components. In the experiment, 30-Gb/s RZ-8-APSK signals are generated from input 10-Gb/s on-off keying and 20-Gb/s RZ differential quadrature phase-shifted keying in an all-optical manner.

Coherent detection associated with digital signal processing for fiber optics communication

Abstract Optical transmission systems have seen their performance (system capacity and system reach) drastically improved in the last 15 years by using new technologies such as wide band optical amplification, wavelength division multiplexing (WDM), dispersion management, introduction of 10 Gbit/s channel rate and now 40 Gbit/s, Differential Phase Shift Keying (DPSK) modulation… . In order to further increase system capacity while relaxing the constraints associated with higher channel rate (i.e. 100 Gbit/s), the association of multilevel modulation format with coherent detection and digital signal processing appears as a key enabler. This article describes the actual modulation and detection methods in order to explain how a coherent receiver including digital signal processing works. The efficiency of coherent receiver to compensate for linear distortions induced by fiber optic propagation will also be detailed. The promising association of coherent receiver with multilevel modulation format, especially for 100 Gbit/s channel rate will be described. Eventually, the limitations of optical communication systems relying on coherent detection will be explained. To cite this article: G. Charlet, C. R. Physique 9 (2008).

Multi-amplitude minimum shift keying modulation format for optical communications

We present the multi-amplitude level minimum shift keying (M-ary MSK) modulation scheme for long haul optically amplified transmission systems. New configurations of optical M-ary MSK transmitters using two cascaded electro-optic phase modulators are proposed and its detailed operation principles are expressed. The optical receiver for optical multi-amplitude MSK modulation format requires both amplitude direct-detection and differential phase balanced-detection. Numerical results on spectral characteristics, dispersion tolerance and the relationship between amplitude and phase with launched average power for long-haul transmission of multi-level MSK modulation format are presented. The dispersion tolerance shows that the 2-amplitude MSK at 80Gb/s is well compared with 40Gb/s binary MSK modulation format.

1.6-b/s/Hz Spectrally Efficient Transmission Over 1700 km of SSMF Using 40 $\times$ 85.6-Gb/s POLMUX-RZ-DQPSK

We discuss high spectral efficient long-haul transmission using multilevel modulation formats. In particular, we investigate long-haul transmission with a 1.6-b/s/Hz spectral efficiency using polarization-multiplexed return-to-zero differential quadrature phase-shift keying. In an experimental demonstration, we show 40 times 85.6-Gb/s transmission with a 50-GHz channel spacing over 1700 km (18 times 94.5 km) of standard-single-mode fiber

Bit pattern dependence in optical DQPSK modulation

A strong bit pattern dependence in optical transmission links employing multilevel modulation formats when pseudorandom binary sequences are used for modulation is demonstrated. The use of pseudorandom quaternary sequences for multilevel modulation is proposed and an efficient method to generate such sequences is presented.

Effect of Linear Crosstalk of WDM Filters on Multilevel Modulation

The effect of the crosstalk of wavelength-division-multiplexing (WDM) optical filters on the performance of WDM transmission systems that use multilevel modulation formats are examined and it is found that the currently common specification of -30 dBm is not adequate when more than eight signal levels are used. Also, the systems using coherent detectors and those using direct detection receivers are compared

Polarization Stabilization in Optical Communications Systems

The control of the state of polarization (SOP) of light remains one of the open issues in optical communications. In particular, the achievement of a stabilization of the SOP can find many applications in advanced optical communication systems: from the mitigation of polarization-mode dispersion to the development of novel multilevel modulation formats. In this paper, theoretical and experimental aspects of polarization stabilization are dealt with, and a novel algorithm to overcome the issues related to the practical availability of finite-range birefringent components and to solve the requirement for endless stabilization is also presented. A complete analysis of the control algorithm, based on the Jones matrix formalism, is also presented. The practical implementation of the polarization stabilizer is discussed, and experimental demonstrations based on liquid crystal and magnetooptical retarders are shown

Investigation of Multilevel Phase and Amplitude Modulation Formats in Combination With Polarization Multiplexing up to 240 Gb/s

We present experimental investigations of the receiver sensitivity and dispersion tolerance of multilevel optical communication systems with a symbol rate of 40 Gbaud. Four- and eight-level communication is obtained by combining binary amplitude modulation with either binary or quadrature differential phase modulation. We experimentally compare the dispersion tolerance, and show that multilevel modulation formats offer much better dispersion tolerance compared to binary formats. By combining multilevel modulation with polarization multiplexing, bit rates up to 240 Gb/s were obtained. We demonstrate transmission over 50-km fiber span with no power penalty for a pseudorandom binary sequence length of 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> -1 bits

Optical DPASK and DQPSK: a comparative analysis for linear and nonlinear transmission

The performance of two multilevel modulation formats, differential-phase-amplitude shift-keying (DPASK) and differential quadrature-phase-shift keying (DQPSK), is compared for linear and nonlinear channels. For DPASK, the optimal extinction ratio is derived for both linear and nonlinear transmission. It is shown that for single-channel transmission, DQPSK outperforms DPASK in linear channels, whereas DPASK achieves better performance when nonlinear impairments are present

Theoretical sensitivity of direct-detection multilevel modulation formats for high spectral efficiency optical communications

Sensitivities of several direct-detection multilevel modulation formats, including M-ary differential phase-shift keying (M-DPSK), M-ary differential amplitude-phase-shift keying (M-DAPSK), and M-ary differential polarization-phase-shift keying (M-DPolPSK) are systematically calculated. The theory is based on a unified mathematical framework for direct-detection receivers using optical delay interferometers and balanced detectors. Results quantitatively elucidate the tradeoffs between these direct-detection multilevel modulation formats as well as the classic multilevel modulation format, M-ary amplitude-shift keying (M-ASK). Although the conventional bit error rate estimation method based on the eye Q-factor with Gaussian approximation is not accurate enough for DPSK and DQPSK, it is found to be reasonably accurate for M-DPSK with M>8 and in M-DPolPSK.

Phase-diversity homodyne detection of multilevel optical modulation with digital carrier phase estimation

This paper describes a phase-diversity homodyne receiver that which can cope with multilevel modulation formats. The carrier phase drift is estimated with digital signal processing (DSP) on the homodyne-detected signal, entirely restoring the complex amplitude of the incoming signal. Our DSP-based phase-estimation scheme consists of a simple and demultiplexable architecture that allows the system to reach significantly higher performance than conventional optical delay detection. Since the whole optical signal information is preserved with our receiver, various kinds of postprocessing of the received signal become possible. For example, we can demultiplex wavelength-division/optical time-division multiplexed channels and compensate for group velocity dispersion of fibers as well as the nonlinear phase noise in the electrical domain. We also experimentally evaluate the performance of our receiver. Our offline bit-error rate experiments show the feasibility of transmitting polarization-multiplexed 40-Gb/s quadrature phase-shift keying signals over 200 km with channel spacing of 16 GHz, leading to spectral efficiency of 2.5 b/s/Hz

Fiber communications using convolutional coding and bandwidth-efficient modulation

In this paper we evaluate numerically the advantage of combining convolutional coding and bandwidth-efficient modulation. We compare different multilevel modulation formats, line codes, and hard/soft decision decoding. Compared with DPSK modulation (with the same bandwidth and information transmission rate), an improvement of almost 5 dB is observed for bit error rates around 10-8. We also study the robustness to intersymbol interference in the form of chromatic dispersion, and find that the improvement of the coded transmission lines improves over the uncoded even in presence of chromatic dispersion.

Coherent detection of optical quadrature phase-shift keying signals with carrier phase estimation

This paper describes a coherent optical receiver for demodulating optical quadrature phase-shift keying (QPSK) signals. At the receiver, a phase-diversity homodyne detection scheme is employed without locking the phase of the local oscillator (LO). To handle the carrier phase drift, the carrier phase is estimated with digital signal processing (DSP) on the homodyne-detected signal. Such a scheme presents the following major advantages over the conventional optical differential detection. First, its bit error rate (BER) performance is better than that of differential detection. This higher sensitivity can extend the reach of unrepeated transmission systems and reduce crosstalk between multiwavelength channels. Second, the optoelectronic conversion process is linear, so that the whole optical signal information can be postprocessed in the electrical domain. Third, this scheme is applicable to multilevel modulation formats such as M-array PSK and quadrature amplitude modulation (QAM). The performance of the receiver is evaluated through various simulations and experiments. As a result, an unrepeated transmission over 210 km with a 20-Gb/s optical QPSK signal is achieved. Moreover, in wavelength-division multiplexing (WDM) environment, coherent detection allows the filtering of a desired wavelength channel to reside entirely in the electrical domain, taking advantage of the sharp cutoff characteristics of electrical filters. The experiments show the feasibility to transmit polarization-multiplexed 40-Gb/s QPSK signals over 200 km with channel spacing of 16 GHz, leading to a spectral efficiency as high as 2.5 b/s/Hz.

Quaternary optical transmission system combining phase and polarization-shift keying

A new optical transmission scheme which combines polarization-shift keying and differential phase-shift keying on the same optical carrier is proposed. Simulations carried out show a considerable tolerance toward different physical impairments, such as polarization-mode dispersion, nonlinear effects, and chromatic dispersion. When compared to other multilevel modulation formats, it shows the advantage of reusing the carrier power, thus, sensitivity is improved.

Constellation Rearrangement: Enhancement for Multilevel Modulation Formats and Transmit Diversity

Radio link adaptivity will be a key feature of the air interfaces of future mobile communication systems. This adaptivity includes techniques such as fast scheduling, hybrid ARQ, transmit diversity and adaptive modulation and coding. The performance of adaptive modulation generally suffers from the power inefficiencies of multilevel modulation formats. This is due to the variations in bit reliabilities caused by the bit-mapping onto the signal constellation. This article presents a concept, called constellation rearrangement (CoRe), which improves the multilevel modulation power efficiency by the joint application with transmit diversity schemes. It is shown by a theoretical analysis for 16-QAM that the presented concept can equalize variations in bit reliabilities by employing different mapping rules for the transmission over the diversity branches. This significantly improves the receiver block error rate performance and, hence, the performance of the adaptive modulation and coding. This is proven by simulations at link-level in a multicarrier CDMA system for AWGN and fading channels employing turbo coded transmission.

InP DHBT technology and design methodology for high-bit-rate optical communications circuits

High-bit-rate optical communication links require high performance circuits. Electrical time division multiplex (ETDM) single channel bit-rate of 40 Gb/s is at hand, due to recent progress in both technology and design methodology. Multilevel modulation format can be envisaged for ETDM transmission. An InP double heterojunction bipolar transistor technology is presented in this paper. The methodology used and tools developed with optical communications in mind are also discussed. Fabricated circuits are reported: 40 Gb/s multiplexer and demultiplexer, a 20 Gb/s driver, a 30 Gb/s selector-driver, a 22 Gb/s decision circuit, and a decision-decoding circuit for multilevel transmissions.

Toward an optimum use of the optical channel capacity

Abstract In recent decades, in addition to long-haul, high-capacity links, research activity in optical fiber communication systems has addressed possible advantageous applications in distribution networks, local and metropolitan area networks, CATV backbones and distribution, and high-speed computer networks. As a consequence, new possibilities in optical transmission systems have been explored to face the new requirements in terms of capacity, spectral efficiency, and receiver sensitivity. Among the possible solutions capable to exploit effectively the potentials offered by the optical channel, novel multilevel modulation formats have been proposed for future communication environment. This paper reviews multilevel modulation techniques for optical transmission: a comparative analysis is performed between conventional modulations, such as N-PSK and N-QAM, and new modulation/demodulation techniques, such as N-4QSK, N-SPSK, and PM-DD, taking into account both system performance and technological constraints.