We discuss error floor asympotics and present a method for improving the performance of low-density parity check (LDPC) codes in the high SNR (error floor) region. The method is based on Tanner graph covers that do not have trapping sets from the original code. The advantages of the method are that it is universal, as it can be applied to any LDPC code/channel/decoding algorithm and it improves performance at the expense of increasing the code length, without losing the code regularity, without changing the decoding algorithm, and, under certain conditions, without lowering the code rate. The proposed method can be modified to construct convolutional LDPC codes also. The method is illustrated by modifying Tanner, MacKay and Margulis codes to improve performance on the binary symmetric channel (BSC) under the Gallager B decoding algorithm. Decoding results on AWGN channel are also presented to illustrate that optimizing codes for one channel/decoding algorithm can lead to performance improvement on other channels.

This letter introduces a combinatorial construction of girth-eight high-rate low-density parity-check codes based on integer lattices. The parity-check matrix of a code is defined as a point-line incidence matrix of a 1-configuration based on a rectangular integer lattice, and the girth-eight property is achieved by a judicious selection of sets of parallel lines included in a configuration. A class of codes with a wide range of lengths and column weights is obtained. The resulting matrix of parity checks is an array of circulant matrices.

Objective: To assess the effects of low-dose acetazolamide treatment on volumetric MRI markers and clinical outcome in idiopathic normal-pressure hydrocephalus (iNPH). Methods:We analyzed MRI and gait measures from 8 patients with iNPH with serial MRIs from an institutional review board-approved imaging protocol who had been treated off-label with lowdose acetazolamide (125-375 mg/day). MRI studies included fluid-attenuated inversion recovery and 3D T1-weighted high-resolution imaging. Automated analyses were employed to quantify each patient's ventricular, global white matter hyperintensities (WMH), and periventricular WMH (PVH) volumes prior to and throughout treatment. Clinical outcome was based on gait changes assessed quantitatively using the Boon scale.Results: Five of 8 patients responded positively to treatment, with median gait improvement of 4 points on the Boon scale. A significant decrease in PVH volume (26.1 6 1.9 mL, p 5 0.002) was seen in these patients following treatment. One patient's gait was unchanged and 2 patients demonstrated worsened gait and were referred for shunt surgery. No reduction in PVH volume was detected in the latter 2 patients. Nonperiventricular WMH and lateral ventricle volumes remained largely unchanged in all patients.Conclusions: These preliminary findings provide new evidence that low-dose acetazolamide can reduce PVH and may improve gait in iNPH. PVH volume, reflecting transependymal CSF, is shown to be a potential MRI indicator of pharmacologic intervention effectiveness. Further studies of pharmacologic treatment of iNPH are needed and may be enhanced by incorporating quantitative MRI outcomes. Classification of evidence:This study provides Class IV evidence that low-dose acetazolamide reverses PVH volume and, in some cases, improves gait in iNPH. Neurology ® 2014;82:1347-1351 GLOSSARY ACZ 5 acetazolamide; FLAIR 5 fluid-attenuated inversion recovery; iNPH 5 idiopathic normal-pressure hydrocephalus; IRB 5 institutional review board; NPVH 5 nonperiventricular white matter hyperintensities; PVH 5 periventricular white matter hyperintensities; TE 5 echo time; TI 5 inversion time; TR 5 repetition time; WMH 5 white matter hyperintensities.Idiopathic normal-pressure hydrocephalus (iNPH) manifests as a progressive gait disorder accompanied by disturbance in urination and cognition.1 The neuroimaging hallmark of iNPH is nonobstructive enlargement of the cerebral ventricles disproportionate to brain atrophy. In many cases, periventricular white matter hyperintensities (PVH) are also noted.1 In this context, origin of PVH has been hypothesized to be transependymal movement of ventricular CSF. Ventricular CSF diversion by shunting is currently the standard of care and the only treatment known to reduce symptom severity.3 Shunting is associated with risk of morbidity and moderate response rate (50%-80%). 4 It has been noted that CSF diversion decreases PVH in iNPH and that PVH width reduction is associated with postshunting symptom improvement, with gait demonstrating the...

Achievable information rates for high-speed optical transmission (40 Gb/s and above) are calculated using finite-state machine approach with combined effect of ASE noise, Kerr nonlinearity (SPM, intrachannel FWM, intrachannel XPM), SRS, chromatic dispersion, and (optical/electrical) filtering.