Estimating the Performance of Direct-Detection DPSK in Optical Networking Environments Using Eigenfunction Expansion Techniques

WOS:000281971100002 (Nº de Acesso Web of Science)“Prémio Científico ISCTE-IUL 2011”In-band crosstalk, due to multiple interferers, has been identified as one of the most severe impairments in optical transparent networks, especially in the ones with a large number of nodes and a high wavelength density. Due to its robustness to in-band crosstalk differential phase-shift keying (DPSK) emerges as an attractive modulation scheme to be used in such environments. This paper proposes a rigorous formulation to estimate the performance of direct-detection DPSK receivers using an eigenfunction expansion technique in the time domain. The method takes into account both the in-band crosstalk, due to an arbitrary number of interfering terms, and the amplified spontaneous emission noise and is able to deal with any combination of optical and electrical filter shapes. Using this method the accuracy of an approximation based on the wideband optical filtering assumption was evaluated and shown that the approximation, although not providing reliable results for the error probabilities, can be used with confidence to compute power penalties due to in-band crosstalk. Furthermore, the crosstalk tolerance of DPSK over on-off keying was quantified and shown that this tolerance is reduced when the number of interferers increases

Applying the skew-normal distribution to model coherent MPI and to evaluate its impact on PAM signals

We statistically model the coherent multipath interference (MPI) using an extended skewnormal distribution, with the results showing a good agreement with published experimental data. This distribution is very flexible and permits to model data with large skewness values, which is a typical feature in coherent MPI histograms. Furthermore, the developed model was applied to estimate the impact of coherent MPI on PAM-4 systems, with the results showing a 1-dB Q-penalty for an MPI level of -24 dB and a bit error ratio of 10^-3.info:eu-repo/semantics/acceptedVersio

© Hindawi Publishing Corp. RELIABILITY OF COMMUNICATION NETWORKS WITH DELAY CONSTRAINTS: COMPUTATIONAL COMPLEXITY AND COMPLETE TOPOLOGIES

Let G = (V,E) be a graph with a distinguished set of terminal vertices K ⊆ V. We define the K-diameter of G as the maximum distance between any pair of vertices of K. If the edges fail randomly and independently with known probabilities (vertices are always operational), the diameter-constrained K-terminal reliability of G, RK(G,D), is defined as the probability that surviving edges span a subgraph whose K-diameter does not exceed D. In general, the computational complexity of evaluating RK(G,D) is NP-hard, as this measure subsumes the classical K-terminal reliability RK(G), known to belong to this complexity class. In this note, we show that even though for two terminal vertices s and t and D = 2, R{s,t}(G,D) can be determined in polynomial time, the problem of calculating R{s,t}(G,D) forfixedvaluesofD, D ≥ 3, is NP-hard. We also generalize this result for any fixed number of terminal vertices. Although it is very unlikely that general efficient algorithms exist, we present a recursive formulation for the calculation of R{s,t}(G,D) that yields a polynomial time evaluation algorithm in the case of complete topologies where the edge set can be partitioned into at most four equi-reliable classes. 2000 Mathematics Subject Classification: 05C99, 90B25

Fast volumetric registration method for tumor follow-up in pulmonary CT exams

An oncological patient may go through several tomographic acquisitions during a period of time, needing an appropriate registration. We propose an automatic volumetric intrapatient registration method for tumor follow-up in pulmonary CT exams. The performance of our method is evaluated and compared with other registration methods based on optimization techniques. We also compared the metrics behavior to inspect which metric is more sensitive to changes due to the presence of lung tumors

Exploring the tabu search algorithm as a graph coloring technique for wavelength assignment in optical networks

The aim of this work is to study the Tabu Search algorithm as a graph coloring technique for wavelength assignment in optical networks, a crucial function in optical network planning. The performance of the Tabu Search is assessed in terms of the number of wavelengths and computation time and is compared with the one of the most common Greedy algorithm. It is concluded that for real networks with a large number of nodes and a higher variance node degree of the path graph relatively to its average node degree value, the Greedy algorithm is preferable to the Tabu Search algorithm since it returns the same number of colors of Tabu Search, but in a shorter computation time.info:eu-repo/semantics/acceptedVersio

Assessing the quality of transmission of lightpaths in multiband C+L networks through Gaussian noise models

In an optical network scenario, wavelength division-multiplexing (WDM) channels are constantly being added and dropped, leading to dynamic traffic variations in the lightpaths. In this work, the impact of the network traffic load and spectral occupancy on the quality of transmission, namely on the normalized nonlinear interference (NLI) power, power transfer due to stimulated Raman scattering (SRS) and optical signal-to-noise ratio (OSNR) of the lightpaths in a C+L multiband optical network is assessed using the recently proposed closed-form interchannel SRS Gaussian noise model (ISRS GN-model). We show that, due to the dynamic traffic behavior, the normalized NLI power can oscillate up to 2 dB in the highest frequency channels due to NLI variations when the tested channels have unequal spacing along the spectrum. For the optimum channel launch power and by increasing the network traffic load, the power transfer between the outer channels can increase up to 5.1 dB due to the SRS effect. With 201 WDM channels, high traffic load and for the optimum channel power, we obtained a maximum OSNR variation along the channel frequencies of only about 0.7 dB. A comparison between the OSNR predictions of the closed-form ISRS GN-model and a closed-form Gaussian noise (GN) model that does not take into account the SRS effect is also performed. In all results obtained, the maximum difference between the OSNR predictions of GN (without SRS) and ISRS GN models is below 0.7 dB at optimum OSNR and maximum C+L band occupancy. For channel launch powers higher than the optimum, the OSNR differences increase up to 3 dB.info:eu-repo/semantics/publishedVersio

Graph coloring heuristics for optical networks planning

This work focuses on the study of wavelength assignment algorithms based on Graph Coloring techniques. We analyze the performance of the Greedy heuristic, a well-known Graph Coloring heuristic, as well as the Degree of Saturation (DSATUR) and the Recursive Largest First (RLF) heuristics, for planning optical networks. These last two heuristics, to the best of our knowledge, have not yet been applied in the context of optical networks. Extensive simulations have been performed, using real network topologies under a static traffic scenario and we have concluded that the DSATUR and RLF heuristics can outperform the Greedy heuristic in network scenarios where there are several network clusters interconnected by only one or two links. In these cases, the RLF and DSATUR heuristics can provide less 9 and 5 wavelengths, respectively, than the Greedy heuristic, in networks with 34 nodes.info:eu-repo/semantics/acceptedVersio

Impact of traffic load and spectral occupancy on Gaussian noise models performance for multiband networks

In a network scenario, wavelength division multiplexing channels are added and dropped leading to fluctuations on the network traffic loads along the optical path. In this work, a comparison between the optical signal-to-noise ratio (OSNR) predictions of the recently proposed closed-form generalized Gaussian noise (GGN) model and a closed-form Gaussian noise (GN) model that does not take into account the stimulated Raman scattering (SRS) is performed, for different network traffic loads and spectral occupancy over the entire C+L band. In all results obtained, the maximum difference between the OSNR predictions of GN (without SRS) and GGN models closed forms is below 0.7 dB at optimum OSNR and maximum C+L band occupancy, indicating that the GN-model can also be used in C+L band transmission. For channel launch powers higher than the optimum, the OSNR differences increase up to 3 dB, being the GN-model (without SRS) unsuitable to assess the network performance in such situations.info:eu-repo/semantics/acceptedVersio

A framework for analyzing in-band crosstalk accumulation in ROADM-based optical networks

Reconfigurable optical add-drop multiplexers (ROADMs) are central pieces in building transparent optical transport networks. However, due to physical limitations, these devices can be a source of in-band crosstalk, which affects the quality of the lightpaths routed and the network performance. Hence, to efficiently design optical networks it is important to study how this impairment is generated and to understand its dependency on relevant network parameters, such as the number of wavelengths used and the node degree. In this paper, we propose a framework to analyze the in-band crosstalk accumulation inside of ROADM-based networks. The framework computes the number of accumulated crosstalk terms in each link of a lightpath considering different physical topologies, as well as different routing and wavelength assignment strategies. An empirical formula is also derived for computing the maximum number of accumulated crosstalk terms as a function of the network parameters. We observe that in the majority of the studied cases, there is a complete agreement between the results of the proposed framework and the empirical formula.info:eu-repo/semantics/acceptedVersio

Performance analysis of a graph coloring algorithm for wavelength assignment in dynamic optical networks

In the near future (5 to 10 years), dynamic optical networks will be crucial in global optical communications in order to respond to the fast growing of on-demand services. Routing and wavelength assignment (RWA) planning tools must be developed to effectively deal with the dynamic scenarios requirements. In this work, a simulation tool for RWA in dynamic optical networks was developed and wavelength assignment (WA) was implemented, through a recently proposed graph coloring algorithm, named Small-Buckets algorithm, that allows recolorings to occur. Several fiber based networks have been studied and it has been concluded that the Small-Buckets algorithm originates lower blocking probabilities than the ones obtained with the First fit algorithm. However, to reach this improved performance, the Small-Bucket algorithm requires a larger number of wavelengths and recolorings.info:eu-repo/semantics/acceptedVersio