Phase noise influence in coherent optical DnPSK systems with DSP based dispersion compensation

We present a comparative study of the influence of dispersion induced phase noise for n-level PSK systems. From the analysis, we conclude that the phase noise influence for classical homodyne/heterodyne PSK systems is entirely determined by the modulation complexity (expressed in terms of constellation diagram) and the analogue demodulation format. On the other hand, the use of digital signal processing (DSP) in homodyne/intradyne systems renders a fiber length dependence originating from the generation of equalization enhanced phase noise. For future high capacity systems, high constellations must be used in order to lower the symbol rate to practically manageable speeds, and this fact puts severe requirements to the signal and local oscillator (LO) linewidths. Our results for the bit-error-rate (BER) floor caused by the phase noise influence in the case of QPSK, 16PSK and 64PSK systems outline tolerance limitations for the LO performance: 5 MHz linewidth (at 3-dB level) for 100 Gbit/s QPSK; 1 MHz for 400 Gbit/s QPSK; 0.1 MHz for 400 Gbit/s 16PSK and 1 Tbit/s 64PSK systems. This defines design constrains for the phase noise impact in distributed-feed-back (DFB) or distributed-Bragg-reflector (DBR) semiconductor lasers, that would allow moving the system capacity from 100 Gbit/s system capacity to 400 Gbit/s in 3 years (1 Tbit/s in 5 years). It is imperative at the same time to increase the analogue to digital conversion (ADC) speed such that the single quadrature symbol rate goes from today's 25 GS/s to 100 GS/s (using two samples per symbol)

Phase noise influence in long-range coherent optical OFDM systems with delay detection, IFFT multiplexing and FFT demodulation

We present a study of the influence of dispersion induced phase noise for CO-OFDM systems using FFT multiplexing/IFFT demultiplexing techniques (software based). The software based system provides a method for a rigorous evaluation of the phase noise variance caused by Common Phase Error (CPE) and Inter-Carrier Interference (ICI) including - for the first time to our knowledge - in explicit form the effect of equalization enhanced phase noise (EEPN). This, in turns, leads to an analytic BER specification. Numerical results focus on a CO-OFDM system with 10-25 GS/s QPSK channel modulation. A worst case constellation configuration is identified for the phase noise influence and the resulting BER is compared to the BER of a conventional single channel QPSK system with the same capacity as the CO-OFDM implementation. Results are evaluated as a function of transmission distance. For both types of systems, the phase noise variance increases significantly with increasing transmission distance. For a total capacity of 400 (1000) Gbit/s, the transmission distance to have the BER < 10-2 for the worst case CO-OFDM design is less than 800 and 460 km, respectively, whereas for a single channel QPSK system it is less than 1400 and 560 km

DNA Topoisomerase IIα contributes to the early steps of adipogenesis in 3T3-L1 cells

DNA topoisomerases (Topo) are multifunctional enzymes resolving DNA topological problems such as those arising during DNA replication, transcription and mitosis. Mammalian cells express 2 class II isoforms, Topoisomerases IIα (Topo IIα) and IIβ (Topo IIβ), which have similar enzymatic properties but are differently expressed, in dividing and pluripotent cells, and in post-mitotic and differentiated cells respectively. Pre-adipocytes re-enter the cell cycle prior to committing to their differentiation and we hypothesised that Topo II could contribute to these processes. We show that Topo IIα expression in 3T3-L1 cells is induced within 16 h after the initiation of the differentiation programme, peaks at 24 h and rapidly declines thereafter. In contrast Topo IIβ was present both in pre-adipocytes and throughout differentiation. Inhibition of PI3K with LY294002, known to prevent adipocyte differentiation, consistently reduced the expression of Topo IIα, whereas a clear effect on Topo IIβ was not apparent. In addition, inhibition of mTOR with rapamycin also reduced the protein levels of Topo IIα. Using specific class IA PI3K catalytic subunit inhibitors, we show that p110α inhibition with A66 has the greatest reduction of Topo IIα expression and of differentiation, as measured by triglyceride storage. The timing of Topo IIα expression coincides with the mitotic clonal expansion (MCE) phase of differentiation and inhibition of Topo II with ICRF-187 during this stage decreased PPARγ1 and 2 protein levels and triglyceride storage, whereas inhibition later on has little impact. Moreover, the addition of ICRF-187 had no effect on the incorporation of EdU during S-phase at day 1 but lowered the relative cell numbers on day 2. ICRF-187 also induced an increase in the centri/pericentromeric heterochromatin localisation of Topo IIα, indicating a role for Topo IIα at these locations during MCE. In summary, we present evidence that Topo IIα plays an important role in adipogenesis during MCE and in a PI3K/mTOR-dependent manner. Considering that Topoisomerases II are targets in cancer chemotherapy, our results highlight that treatment of cancer with Topo II inhibitors may alter metabolic processes in the adipose tissue.publishedVersio

Analytical estimation in differential optical transmission systems influenced by equalization enhanced phase noise

An analytical model is presented for assessing the performance of the bit-error-rate (BER) in the differential m-level phase shift keying (m-PSK) transmission systems, where the influence of equalization enhanced phase noise (EEPN) has been considered. Theoretical analysis has been carried out in differential quadrature phase shift keying (DQPSK), differential 8-PSK (D8PSK), and differential 16-PSK (D16PSK) optical transmission systems. The influence of EEPN on the BER performance, in term of signal-to-noise ratio (SNR), are investigated for different fiber dispersion, LO laser linewidths, symbol rates, and modulation formats. Our analytical model achieves a good agreement with previously reported EEPN induced BER floors, and can give an accurate prediction for the DQPSK system, and a leading-order approximation for the D8PSK and the D16PSK systems

Study of EEPN mitigation using modified RF pilot and Viterbi-Viterbi based phase noise compensation

We propose - as a modification of the optical (RF) pilot scheme -a balanced phase modulation between two polarizations of the optical signal in order to generate correlated equalization enhanced phase noise (EEPN) contributions in the two polarizations. The method is applicable for n-level PSK system. The EEPN can be compensated, the carrier phase extracted and the nPSK signal regenerated by complex conjugation and multiplication in the receiver. The method is tested by system simulations in a single channel QPSK system at 56 Gb/s system rate. It is found that the conjugation and multiplication scheme in the Rx can mitigate the EEPN to within 1/2 orders of magnitude. Results are compared to using the Viterbi-Viterbi algorithm to mitigate the EEPN. The latter method improves the sensitivity more than two orders of magnitude. Important novel insight into the statistical properties of EEPN is identified and discussed in the paper

Digital Adaptive Carrier Phase Estimation in Multi-Level Phase Shift Keying Coherent Optical Communication Systems

The analysis of adaptive carrier phase estimation is investigated in long-haul high speed n-level phase shift keying (n-PSK) optical fiber communication systems based on the one-tap normalized least-mean-square (LMS) algorithm. The close-form expressions for the estimated carrier phase and the bit-error-rate floor have been derived in the n-PSK coherent optical transmission systems. The results show that the one-tap normalized LMS algorithm performs pretty well in the carrier phase estimation, but will be less effective with the increment of modulation levels, in the compensation of both intrinsic laser phase noise and equalization enhanced phase noise.Comment: 5 pages in [IEEE] International Conference on Information Science and Control Engineering (ICISCE) 2016. arXiv admin note: text overlap with arXiv:1602.0685

Variable-step-size LMS adaptive filter for digital chromatic dispersion compensation in PDM-QPSK coherent transmission system

High bit rates optical communication systems pose the challenge of their tolerance to linear and nonlinear fiber impairments. Digital filters in coherent optical receivers can be used to mitigate the chromatic dispersion entirely in the optical transmission system. In this paper, the least mean square adaptive filter has been developed for chromatic equalization in a 112-Gbit/s polarization division multiplexed quadrature phase shift keying coherent optical transmission system established on the VPIphotonics simulation platform. It is found that the chromatic dispersion equalization shows a better performance when a smaller step size is used. However, the smaller step size in least mean square filter will lead to a slower iterative operation to achieve the guaranteed convergence. In order to solve this contradiction, an adaptive filter employing variable-step-size least mean square algorithm is proposed to compensate the chromatic dispersion in the 112-Gbit/s coherent communication system. The variable-step-size least mean square filter could make a compromise and optimization between the chromatic dispersion equalization performance and the algorithm converging speed. Meanwhile, the required tap number and the converged tap weights distribution of the variable-step-size least mean square filter for a certain fiber chromatic dispersion are analyzed and discussed in the investigation of the filter feature