Robust Multilevel Coherent Optical Systems With Linear Processing at the Receiver (original) (raw)
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A novel multilevel coherent optical system: 4-quadrature signaling
Journal of Lightwave Technology, 1991
A novel multilevel coherent optical system is proposed. It is based on the exploitation of the property that the electromagnetic field propagating in a single-mode optical fiber can be represented by a four-dimensional vector whose components are the phase and quadrature terms of the two polarization components of the electrical field. This allows a wider use of the resources of the electromagnetic field for information transmission so to obtain a spectrally efficient modulation format with a limited penalty. The different states of the transmitted field can be viewed as points in a four-dimension Euclidean space whose positions are optimized so to minimize the bit-error probability. The polarization state fluctuations due to conventional single-mode fiber propagation are compensated by means of a pure electronic algorithm allowing the state of polarization of the transmitted field to be recovered without additional optical processing. The net performance gain with respect to N-APK and N-PSK increases with increasing the number of levels N. For instance, for N = 32 the gain is of 1.6 and 7.7 dB with respect to N-APK and N-PSK systems. Finally, the effect of lasers phase noise on the system performance is evaluated.
Multilevel Optical Systems With MLSD Receivers Insensitive to GVD and PMD
IEEE/OSA Journal of Lightwave Technology, 2008
This paper analyzes optical transmission systems based on high-order modulations such as phase-shift keying signals and quadrature amplitude modulations. When the channel is affected by group velocity dispersion (GVD), polarization mode dispersion (PMD), and phase uncertainties due to the laser phase noise, the optimal receiver processing based on maximum-likelihood sequence detection and its practical implementation through a Viterbi processor is described without a specific constraint on the receiver front end. The implementation issues are then faced, showing that at least a couple of widely known front ends, with proper modifications, can be used to extract the required sufficient statistics from the received signal. The aspects related to the receiver adaptivity, the complexity reduction of the Viterbi processor, and the possibility of employing polarization diversity at the transmitter end are also discussed. It is demonstrated that, as long as a sufficient number of Viterbi processor trellis states is employed, GVD and PMD entail no performance degradation with respect to the case of no channel distortions (the back-to-back case).
Homodyne optical coherent systems based on polarization modulation
Journal of Lightwave Technology, 1991
In this paper the first homodyne receiver for polarization modulated optical signals is proposed and analyzed. It is based on the detection at the receiver of the four quadratures that completely characterize the received optical field and on the estimation of the Stokes parameters starting from them. This allows homodyne detection to be carried out without phase locking the local oscillator (LO) to the received signal. Even if in this paper a binary antipodal polarization modulation homodyne system is analyzed in detail, such a receiver scheme can be implemented for baseband detection of all the kinds of polarization modulated systems. Insensitiveness to polarization fluctuations can be achieved by using electronic algorithms similar to those used in the heterodyne receivers and immunity to phase noise as high as that of all the polarization modulated systems is maintained. For this properties the proposed receiver scheme seems to be attractive for application in high-speed FDM optical networks in which an efficient exploitation of the receiver front end bandwidth and a good immunity to phase noise and polarization fluctuations are important issues.
Coherent quadrature phase shift keying optical communication systems
Coherent optical fiber communications for data rates of 100 Gbit/s and beyond have recently been studied extensively because high sensitivity of coherent receivers could extend the transmission distance. Spectrally efficient modulation techniques such as M-ary phase shift keying (PSK) can be employed for coherent optical links. The integration of mul-ti-level modulation formats based on coherent technologies with wavelength-division multiplexed (WDM) systems is vital to meet the aggregate bandwidth demand. This paper reviews coherent quadrature PSK (QPSK) systems to scale the network capacity and maximum reach of coherent optical communication systems to accommodate traffic growth.
Symmetric signal and local oscillator polarization diverse coherent optical receiver
Optics express, 2016
An improved coherent optical receiver architecture that compensates for a random drift in the state of polarization (SOP) of both the signal and the local oscillator (LO) is presented for the first time. The proposed architecture comprises two conventional coherent optical receiver front-ends in tandem, where the SOP of the LO is first divided into its two orthogonal components and then distributed to each coherent optical receiver front-end module. Two distinct methods of polarization diversity recovery of the modulation based on the MRC technique and an eigenvalue-eigenvector decomposition of the covariance matrix have been used to effectively recover the transmitted signal. The concept is validated by numerical simulations, where a differential quadrature phase-shift keyed (DQPSK) modulated signal with a random time-varying SOP is first generated. After its mixing with a LO also possessing a random time-varying SOP, the algorithms that have been developed are provided with eight ...
An efficient multilevel coherent optical system: M-4Q-QAM
… Technology, Journal of, 1992
In this paper a new coherent multilevel transmission system is presented based on the modulation of all the optical field degrees of freedom (the four quadratures of the optical field). The transmitted symbols are represented in the space of the field quadratures by a square lattice of points, so that both the transmitter and the decision devices can be easily implemented. The proposed modulation formats allow high spectral efficient systems to be designed without paying a great sensitivity penalty with respect to binary systems. For this reason the proposed modulation format seems promising for application in densely spaced very high capacity FDM networks and in high-speed data transmission in parallel computers.
Decision-Aided Carrier Phase Estimation for Coherent Optical Communications
Journal of Lightwave Technology, 2000
We analytically studied the block length effect (BLE) of decision-aided maximum likelihood (DA ML) carrier phase estimation in coherent optical phase-modulated systems. The results agree well with the trends found using extensive Monte Carlo simulations. In order to eliminate the BLE and accurately recover the carrier phase, an adaptive decision-aided (DA) receiver is proposed that does not require knowledge of the statistical characteristics of the carrier phase, or any parameter to be preset. The simulation results show that using the adaptive DA receiver, the maximum tolerance ratio of the linewidth per laser to symbol rate (1) at a bit error rate (BER) = 10 4 has been increased to 2 5 10 4 4 1 10 5 , and 9 5 10 6 , respectively, for quadrature-, 8-and 16-phase-shift keying formats. The ratio (1) of the adaptive DA receiver in 16 quadrature amplitude modulation (QAM) is decreased to 2 10 5 due to the constellation penalty from 2 5 10 5 by using DA ML with optimum memory length, though it consistently performs well without optimizing any parameters as in DA ML. The phase error variance of the adaptive DA receiver is also analytically investigated. In addition, an analog-to-digital converter with bit resolution higher than 4 bits is shown to be sufficient to implement our adaptive DA receiver. Index Terms-Adaptive receiver, block length effect (BLE), coherent optical fiber communication, decision-aided maximum likelihood (DA ML) carrier phase estimation, phase noise, PSK/QAM. I. INTRODUCTION A DVANCED modulation formats with coherent optical detection receivers, such as-ary phase-shift keying (PSK) and even quadrature amplitude modulation (QAM), have received much interest due to the potentially high spectral efficiency, and tolerances to fiber dispersion effects [1]-[3]. One of the challenges in coherent optical systems is to recover the carrier phase, which is perturbed by the laser phase noise.
Optimal Polarization Demultiplexing for Coherent Optical Communications Systems
Journal of Lightwave Technology, 2000
Spectrally-efficient optical communications systems employ polarization division multiplexing (PDM) as a practical solution, in order to double the capacity of a fiber link. Polarization demultiplexing can be performed electronically, using polarization-diversity coherent optical receivers. The primary goal of this paper is the optimal design, using the maximum-likelihood criterion, of polarization-diversity coherent optical receivers for polarization-multiplexed optical signals, in the absence of polarization mode dispersion (PMD). It is shown that simultaneous joint estimation of the symbols, over the two received states of polarization, yields optimal performance, in the absence of phase noise and intermediate frequency offset. In contrast, the commonly used zero-forcing polarization demultiplexer, followed by individual demodulation of the polarization-multiplexed tributaries, exhibits inferior performance, and becomes optimal only if the channel transfer matrix is unitary, e.g., in the absence of polarization dependent loss (PDL), and if the noise components at the polarization diversity branches have equal variances. In this special case, the zero-forcing polarization demultiplexer can be implemented by a 2 2 lattice adaptive filter, which is controlled by only two independent real parameters. These parameters can be computed recursively using the constant modulus algorithm (CMA). We evaluate, by simulation, the performance of the aforementioned zero-forcing polarization demultiplexer in coherent optical communication systems using PDM quadrature phase shift keying (QPSK) signals. We show that it is, by far, superior, in terms of convergence accuracy and speed, compared to conventional CMA-based polarization demultiplexers. Finally, we experimentally test the robustness of the proposed constrained CMA polarization demultiplexer to realistic imperfections of polarization-diversity coherent optical receivers. The PMD and PDL tolerance of the proposed demultiplexer can be used as a benchmark in order to compare the performance of more sophisticated adaptive electronic PMD/PDL equalizers.
Optical Fiber Technology, 2015
The joint iterative detection and decoding (JIDD) technique has been proposed by with the objective of compensating the time-varying phase noise and constant frequency offset experienced in satellite communication systems. The application of JIDD to optical coherent receivers in the presence of laser frequency fluctuations has not been reported in prior literature. Laser frequency fluctuations are caused by mechanical vibrations, power supply noise, and other mechanisms. They significantly degrade the performance of the carrier phase estimator in high-speed intradyne coherent optical receivers.