A survey on fiber nonlinearity compensation for 400 Gbps and beyond optical communication systems

Optical communication systems represent the backbone of modern communication networks. Since their deployment, different fiber technologies have been used to deal with optical fiber impairments such as dispersion-shifted fibers and dispersion-compensation fibers. In recent years, thanks to the introduction of coherent detection based systems, fiber impairments can be mitigated using digital signal processing (DSP) algorithms. Coherent systems are used in the current 100 Gbps wavelength-division multiplexing (WDM) standard technology. They allow the increase of spectral efficiency by using multi-level modulation formats, and are combined with DSP techniques to combat the linear fiber distortions. In addition to linear impairments, the next generation 400 Gbps/1 Tbps WDM systems are also more affected by the fiber nonlinearity due to the Kerr effect. At high input power, the fiber nonlinear effects become more important and their compensation is required to improve the transmission performance. Several approaches have been proposed to deal with the fiber nonlinearity. In this paper, after a brief description of the Kerr-induced nonlinear effects, a survey on the fiber nonlinearity compensation (NLC) techniques is provided. We focus on the well-known NLC techniques and discuss their performance, as well as their implementation and complexity. An extension of the inter-subcarrier nonlinear interference canceler approach is also proposed. A performance evaluation of the well-known NLC techniques and the proposed approach is provided in the context of Nyquist and super-Nyquist superchannel systems.Comment: Accepted in the IEEE Communications Surveys and Tutorial

Bioavailability of rifampicin following concomitant administration of ethambutol or isoniazid or pyrazinamide or a combination of the three drugs

Background & Objectives: Poor bioavailability of rifampicin (R) in combination with other anti-tuberculosis drugs such as isoniazid (H), pyrazinamide (Z), and ethambutol (E) is a subject of much concern for the last few decades. This could be due to an interaction between R and other drugs. An investigation was therefore undertaken to examine the bioavailability of R in the presence of H, Z and E or a combination of the three drugs. Methods: The study included eight healthy volunteers, each being investigated on four occasions at weekly intervals once with R alone and with three of the four combinations on the three remaining occasions. A partially balanced incomplete block design was employed and the allocation of R or the drug combinations was random. Plasma concentrations of R at intervals upto 12 h were determined by microbiological assay using Staphylococcus aureus as the test organism. The proportion (%) dose of R as R plus desacetyl R (DR) in urine excreted over the periods 0-8 and 8-12 h was also determined. Bioavailability was expressed as an index (BI) of area under time concentration curve (AUC) calculated from the plasma concentrations or proportion of dose of R excreted as R plus DR in urine with the combinations to that with R alone. Results: The bioavailability indices based on AUC were 0.96 with RE, 0.76 with RH, 1.08 with RZ and 0.65 with REHZ. The indices based on urine estimations (0-8 h) were similar, the values being 0.94, 0.84, 0.94 and 0.75, respectively. A second investigation revealed that the decrease of bioavailability of R with H was not due to the excipients present in H tablets. Interpretation & conclusion: Isoniazid alone or in combination with E and Z reduces the bioavailability of R. Urinary excretion data offer a simple and non invasive method for the assessment of bioavailability of R

Second-order perturbation theory-based digital predistortion for fiber nonlinearity compensation

The first-order (FO) perturbation theory-based nonlinearity compensation (PB-NLC) technique has been widely investigated to combat the detrimental effects of the intra-channel Kerr nonlinearity in polarization-multiplexed (Pol-Mux) optical fiber communication systems. However, the NLC performance of the FO-PB-NLC technique is significantly limited in highly nonlinear regimes of the Pol-Mux long-haul optical transmission systems. In this paper, we extend the FO theory to second-order (SO) to improve the NLC performance. This technique is referred to as the SO-PB-NLC. A detailed theoretical analysis is performed to derive the SO perturbative field for a Pol-Mux optical transmission system. Following that, we investigate a few simplifying assumptions to reduce the implementation complexity of the SO-PB-NLC technique. The numerical simulations for a single-channel system show that the SO-PB-NLC technique provides an improved bit-error-rate performance and increases the transmission reach, in comparison with the FO-PB-NLC technique. The complexity analysis demonstrates that the proposed SO-PB-NLC technique has a reduced computational complexity when compared to the digital back-propagation with one step per span

Simple spectrofluorimetric and microbiological assay methods for the estimation of ofloxacin in biological fluids

Objective: To evolve simple methods for the assay of ofloxacin in biological fluids. Methods: Simple methods for the estimation of ofloxacin in plasma, saliva and urine employing microbiological assay using plate diffusion technique and by fluorimetric method based on the measurement of native fluorescence emitted by ofloxacin, have been described. Results: The recovery of ofloxacin from all the three biologial fluids was 93-98% and the sensitivity was 0.5 μg/ml on all 5 different occasions by both the methods. Anti-TB drugs viz., rifampicin, ethambutol, isoniazid and pyrazinamide and also anti-leprosy drugs viz., dapsone and clofazimine at concentrations of 10 and 20 μg/ml did not interfere with the estimation of ofloxacin by either method. Ofloxacin is stable in biological fluids for a period of at least 8 days at -20°C. Conclusion: Both the methods described are simple, involve very few steps and do not need either costly chemicals or sophisticated equipments

New Single Asymmetric Error-Correcting Codes

New single asymmetric error-correcting codes are proposed. These codes are better than existing codes when the code length n is greater than 10, except for n = 12 and n = 15 . In many cases one can construct a code C containing at least [2^n/n] codewords. It is known that a code with |C| >= [2^n/(n + 1)] can be easily obtained. It should be noted that the proposed codes for n = 12 and n = 15 are also the best known codes that can be explicitly constructed, since the best of the existing codes for these values of n are based on combinatorial arguments. Useful partitions of binary vectors are also presented