Athanasios Vgenis

Optimal Polarization Demultiplexing for Coherent Optical Communications Systems

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.

Nonsingular Constant Modulus Equalizer for PDM-QPSK Coherent Optical Receivers

Adaptive electronic equalizers using the constant modulus algorithm (CMA) algorithm often converge to a singular coefficient matrix that produces the same signal at multiple outputs. We address this issue in the context of optical communications systems with polarization-division multiplexing and coherent receivers. We study, by computer simulation, the performance of multiuser CMA equalizer, an enhanced CMA equalizer initially proposed for use in wireless multiuser and later multiple-input/multiple-output communications systems. We show that the proposed adaptive electronic equalizer does not exhibit singularities and, therefore, is superior to the commonly used CMA equalizer.

Quadrature Imbalance Compensation for PDM QPSK Coherent Optical Systems

In this letter, we study the impact of quadrature imbalance (QI) on the performance of optical communications systems using polarization-division-multiplexed (PDM) quadrature phase-shift keying (QPSK), with coherent detection and digital signal processing. We compare, via simulation, the performance of three QI compensation algorithms, suitable for PDM QPSK coherent optical receivers, including a novel, blind, adaptive, constrained equalizer, based on the constant modulus algorithm. We show that dedicated QI compensation is mandatory and cannot be substituted by conventional adaptive electronic equalizers designed for intersymbol interference mitigation.

Impact of transmitter and receiver imperfections on the performance of coherent optical QPSK communication systems

We investigate transmitter and receiver imperfections in coherent QPSK systems, and how their impact can be mitigated using DSP algorithms. Quadrature imbalance was found to be a particularly significant effect that will require compensation.

Performance Comparison of Electronic PMD Equalizers for Coherent PDM QPSK Systems

Polarization division multiplexed (PDM) quadrature phase-shift keying (QPSK) coherent optical systems employ blind adaptive linear electronic equalizers for polarization-mode dispersion (PMD) compensation. In this paper, we compute the performance of various adaptive, fractionally spaced, feed-forward electronic equalizers, using the outage probability as a criterion. A parallel programming implementation of the multicanonical Monte Carlo method is developed, which automatically performs concurrent loop computation on multicore processors, for the estimation of the tails of the outage probability distribution. The constant modulus algorithm (CMA), the decision-directed least mean squares (DD-LMS), and their combination are applied for the adaptation of electronic equalizer filter coefficients. In the exclusive presence of PMD, we demonstrate that half-symbol-period-spaced CMA-based adaptive electronic equalizers perform slightly better than their DD-LMS counterparts, at links with strong PMD, whereas the opposite holds true at the weak PMD regime. It is shown that the successive application of CMA and DD-LMS with 20 complex, half-symbol-period-spaced taps per finite impulse response filter is adequate to reduce the outage probability of coherent PDM QPSK systems to less than 10-5, for a mean differential group delay of more than twice the symbol period.

Wavelength-space permutation switch with coherent PDM QPSK transmission for supercomputer optical interconnects

We experimentally study the performance of an economically-viable, high-capacity supercomputer optical interconnect employing wavelength-space optical packet switching, polarization division multiplexed (PDM) quadrature phase shift keying (QPSK) modulation and coherent detection.

Outage Probability Due to PMD in Coherent PDM QPSK Systems With Electronic Equalization

Polarization-division-multiplexed (PDM) quadrature phase-shift-keying (QPSK) coherent optical systems employ blind adaptive electronic equalizers for polarization-mode dispersion compensation. In this letter, we compare the performance of fractionally spaced, linear electronic equalizers, composed of four parallel finite impulse response (FIR) filters of various lengths, using the outage probability as a performance criterion. The constant modulus algorithm is applied for the adaptation of FIR filter coefficients. A parallel programming implementation of the multicanonical Monte Carlo method is adopted for the estimation of the tails of the outage probability distribution. It is shown that less than 20 complex, half-symbol-period-spaced taps per FIR filter suffice, in order to reduce the outage probability of PDM QPSK coherent optical systems to less than 10-5 , for a mean differential group delay up to twice the symbol period.

Quadrature imbalance compensation algorithms for coherent PDM QPSK systems

We compare three quadrature imbalance compensation algorithms for coherent PDM-QPSK systems, including a novel, blind, adaptive equalizer. We show that dedicated quadrature imbalance compensation is mandatory and cannot be performed by regular distortion mitigating equalizers.

Design guidelines for electronic PMD equalizers used in coherent PDM QPSK systems

We theoretically study the performance of various fractionally-spaced, electronic PMD equalizers in coherent optical PDM QPSK systems using the multicanonical Monte Carlo method. We calculate the required number of equalizer taps for a specific outage probability and different system margins.

Pilot-less time synchronization for OFDM systems: application to power line receivers

Power line networks provide high-speed broadband communications without the need for new wirings. However, these networks present a hostile environment for high-speed data communications. The most common modulation method used in such systems is OFDM, since it copes effectively with noise, multipath, fading selectivity, and attenuation. A potential drawback of OFDM is its sensitivity to receiver synchronization imperfections, such as timing and sampling frequency offsets. Although several approaches have been proposed for estimating the time and frequency offset, they are based on the use of pilot sequences that are not available in power line communication standards. More importantly, they focus on isolated algorithms for compensating either time or frequency offsets without providing a complete, low complexity, OFDM receiver architecture that mitigates jointly time and frequency errors. This paper focuses on providing an OFDM receiver architecture that can be compatible with many power line standards. Extensive simulation studies show under realistic channel and noise conditions that the proposed receiver provides enhanced robustness to synchronization imperfections as compared to conventional approaches.