WiFiRe: rural area broadband access using the WiFi PHY and a multisector TDD MAC

The needs of Indian rural telecom, and the economics of currently available broadband access technologies, motivate a new system for rural broadband access, which we call WiFiRe (WiFi rural extension). The system leverages the widely available, and highly cost-reduced, WiFi chipsets. We, however, retain only the PHY from these chipsets and propose a single-channel, multisector, TDD MAC using directional antennas. The proposed WiFiRe MAC is similar to the WiMAX MAC in several respects. In this article we motivate our approach, describe the system architecture and the MAC, analyse the spatial reuse, and then, using a simple scheduler, provide an assessment of the voice and data capacity of a WiFiRe system     

Simultaneous saccharification and extractive fermentation of lignocellulosic materials into lactic acid in a two-zone fermentor-extractor system

Simultaneous saccharification and extractive fermentation of lignocellulosic materials into lactic acid was investigated using a two-zone bioreactor. The system is composed of an immobilized cell reactor, a separate column reactor containing the lignocellulosic substrate and a hollow-fiber membrane. It is operated by recirculating the cell free enzyme (cellulase) solution from the immobilized cell reactor to the column reactor through the membrane. The enzyme and microbial reactions thus occur at separate locations, yet simultaneously. This design provides flexibility in reactor operation as it allows easy separation of the solid substrate from the microorganism, in situ removal of the product and, if desired, different temperatures in the two reactor sections. This reactor system was tested using pretreated switchgrass as the substrate. It was operated under a fed-batch mode with continuous removal of lactic acid by solvent extraction. The overall lactic acid yield obtainable from this bioreactor system is 77% of the theoretical.     

Designing Packet Buffers for Router Linecards

Internet routers and Ethernet switches contain packet buffers to hold packets during times of congestion. Packet buffers are at the heart of every packet switch and router, which have a combined annual market of tens of billions of dollars, and equipment vendors spend hundreds of millions of dollars on memory each year. Designing packet buffers used to be easy: DRAM was cheap, low power and widely used. But something happened at 10 Gb/s when packets started to arrive and depart faster than the access time of a DRAM. Alternative memories were needed, but SRAM is too expensive and power-hungry. A caching solution is appealing, with a hierarchy of SRAM and DRAM, as used by the computer industry. However, in switches and routers it is not acceptable to have a ldquomiss-raterdquo as it reduces throughput and breaks pipelines. In this paper we describe how to build caches with 100% hit-rate under all conditions, by exploiting the fact that switches and routers always store data in FIFO queues. We describe a number of different ways to do it, with and without pipelining, with static or dynamic allocation of memory. In each case, we prove a lower bound on how big the cache needs to be, and propose an algorithm that meets, or comes close, to the lower bound. These techniques are practical and have been implemented in fast silicon; as a result, we expect the techniques to fundamentally change the way switches and routers use external memory.     

Studies in biological excretion of inhaled plutonium in the case of a few occupational workers in a fuel reprocessing plant

A power reactor fuel reprocessing plant is in operation at Tarapur. The various processes involved in the plant are: fuel rod cutting, dissolution in nitric acid, separation of plutonium, and handling of separated plutonium. The chemical form of plutonium could be nitrate, TBP complex, or oxide depending upon the nature of the process involved. Possible internal exposure to plant personnel occurs mainly by inhalation and occasionally through a contaminated wound. Occupational workers are regularly monitored for internal contamination by urinary excretion analysis as well as by in-vivo lung counting. This paper presents a follow up study of plutonium elimination in four inhalation exposure cases.     

Light’s bending angle due to black holes: from the photon sphere to infinity

The bending angle of light is a central quantity in the theory of gravitational lensing. We develop an analytical perturbation framework for calculating the bending angle of light rays lensed by a Schwarzschild black hole. Using a perturbation parameter given in terms of the gravitational radius of the black hole and the light ray’s impact parameter, we determine an invariant series for the strong-deflection bending angle that extends beyond the standard logarithmic deflection term used in the literature. In the process, we discovered an improvement to the standard logarithmic deflection term. Our perturbation framework is also used to derive as a consistency check, the recently found weak deflection bending angle series. We also reformulate the latter series in terms of a more natural invariant perturbation parameter, one that smoothly transitions between the weak and strong deflection series. We then compare our invariant strong deflection bending-angle series with the numerically integrated exact formal bending angle expression, and find less than 1% discrepancy for light rays as far out as twice the critical impact parameter. The paper concludes by showing that the strong and weak deflection bending angle series together provide an approximation that is within 1% of the exact bending angle value for light rays traversing anywhere between the photon sphere and infinity.     

Stabilized Blue Emission from Polyfluorene‐Based Light‐Emitting Diodes: Elimination of Fluorenone Defects

Polyfluorene (PF)‐based light‐emitting diodes (LEDs) typically exhibit device degradation under operation with the emergence of a strong low‐energy emission band (at ～ 2.2–2.4 eV). This longer wavelength band converts the desired blue emission to blue–green or even yellow. We have studied both the photoluminescence (PL) and electroluminescence (EL) of PFs with different molecular structures and found that the low‐energy emission band originates from fluorenone defects which are introduced by photo‐oxidization, thermal oxidation, or during device fabrication. X‐ray photo‐emission spectroscopy (XPS) results show that the oxidation of PF is strongly catalyzed by the presence of calcium. The fluorenone defects generate a stronger contribution to the EL than to the PL. By utilization of a novel electron‐transporting material as a buffer layer between the emissive PF and the Ca/Ag (Ba/Ag) cathode, the blue EL emission from the PF was stabilized.     

Comment on “the question of an upper bound on entropy”

We show that the upper bound on the entropy of a finite system proposed by Qadir is not universal and consequently neither interesting nor relevant to the Bekenstein-Unruh-Wald debate.     

Transmission line models for negative refractive index media and associated implementations without excess resonators

Recently, three-dimensional composite periodic media comprising split-ring resonators (SRR) and thin wires have been shown to exhibit a negative refractive index in the frequency range around the SRR resonance. In this letter, we propose transmission line models for studying and interpreting the electromagnetic propagation behavior of such materials. Based on these equivalent transmission line models, we show that by periodically loading a network of transmission lines with series capacitors and shunt inductors, a negative refractive index medium can be synthesized without excess resonators, thus leading to wideband behavior. These proposed media have tailorable properties over a broad frequency range. Moreover, they are completely planar, frequency scalable, more compact, and easier to implement for RF/microwave circuit applications than their SRR/wire counterparts.     

Representation of Intermodal Dispersion in Multimode Fiber Links

This paper analyzes statistical aspects of intermodal dispersion in multimode fiber links using a linear systems model. It is shown that the dependence of the response of a fiber section on the power distribution in the input modes leads to uncertainties in the characterization of the rms time dispersion of the section when it forms a part of a fiber link. Analytical expressions for the link dispersion in terms of three section parameters are derived, and a recursive method for computing link dispersions when these parameters are known for each section of the link is outlined. Calculations indicating the applicability of the method are also presented.