Near-optimal depth-constrained codes

This note considers an n-letter alphabet in which the ith letter is accessed with probability p/sub i/. The problem is to design efficient algorithms for constructing near-optimal, depth-constrained Huffman and alphabetic codes. We recast the problem as one of determining a probability vector q/sup */=(q/sup *//sub 1/,...,q/sup *//sub n/) in an appropriate convex set, S, so as to minimize the relative entropy D(p/spl par/q) over all q/spl isin/S. Methods from convex optimization give an explicit solution for q/sup */ in terms of p. We show that the Huffman and alphabetic codes so constructed are within 1 and 2 bits of the corresponding optimal depth-constrained codes.     

First-order definability and algebraicity of the sets of annihilating and generating collections of elements for some relatively free solvable groups

We study the first-order definable, Diophantine, and algebraic subsets in the set of all ordered sets generating a group or generating a group as a normal subgroup for some relatively free solvable groups.     

Performance of YBaCuO thick film rf SQUIDs at 77 K

RF SQUID behaviour has been observed at 77 K in YBaCuO thick films prepared by screen printing technique. A hole shunted with a microbridge type of geometry is patterned manually for observing rf SQUID behaviour. Flux noise spectrum is also studied and it is found to depend on the quality of the film. The spectral density of the flux noise in the white noise region is 1.7×10⁻³ Φ₀/√Hz at 77 K.     

Computer communication networks—A study in the Indian context

Node location, topological optimization, capacity assignments, optimal cost determination using regression analysis, delay cost trade-offs and routing algorithm of a computer communication network in India are discussed.

The min-max criterion is used, which minimizes the time delay on every link. The optimum cost is calculated on the basis of a shortest-path algorithm, as length is found to be the only significant factor in determination of the cost as a result of regression analysis. As the traffic is low for the communication network under study, fixed routing strategy is found to be most appropriate.     

Energy and peak-current per-cycle estimation at RTL

We present novel macromodeling techniques for estimating the energy dissipated and peak-current drawn in a logic circuit for every input vector pair (we call this the energy-per-cycle and peak-current-per-cycle, respectively). The macromodels are based on classifying the input vector pairs on the basis of their Hamming distances and using a different equation-based macromodel for every Hamming distance. The variables of our macromodel are the zero-delay transition counts at three logic levels inside the circuit. We present an automatic characterization process by which such macromodels can be constructed. The energy-per-cycle macromodel provides a transient energy waveform, and can also be used to estimate the moving average energy over any time window, whereas peak-current-per-cycle macromodel provides peak-current which can be used for studying IR drop problems. Some key features of this technique are: 1) the models are compact (linear in the number of inputs); 2) they can be used for any input sequence; and 3) the characterization is automatic and requires no user intervention. These approaches have been implemented and models have been built and tested for many circuits. The average errors observed in estimating the energy-per-cycle and peak-current-per-cycle are under 20%. The energy-per-cycle model can also be used to measure the long-term average power, with an observed error of under 10% on average.     

Ultrafast long-wavelength photodetectors fabricated onlow-temperature InGaAs on GaAs

The authors report the first successful demonstration of a metal-semiconductor-metal photodetector (MSMPD) fabricated on low-temperature InGaAs grown on GaAs by molecular beam epitaxy (MBE) for long-wavelength fiber optic applications. Interdigitated MSMPDs with finger widths and spacings of 0.2, 0.5, 1.0, and 2.0 μm were tested using a femtosecond pulsed laser and high-speed electrooptic sampling. A FWHM pulsed response of 2 and 1.3 ps was measured for low-temperature In _0.25Ga_0.75As and In_0.35Ga_0.65As, respectively. The latter is the fastest response reported to data for a photodetector capable of detection to wavelengths as long as 1.3 μm     

An Integer-Programming Approach to the Bicriteria Multicasting Problem in Optical Networks

We consider the problem of designing quality-of-service (QoS)-constrained routes for a set of multicast groups in an all-optical network. We present an integer goal programming formulation with the objectives of minimizing the cost of the multicast solution and minimizing the maximum link load of the network. Based on the structural properties of the problem, we develop a variable-fixing heuristic which is easy to implement, requires a modest amount of CPU time, and provides high-quality solutions. An extensive computational study is presented to justify our claims     

Principles and demonstration of multi-functional adaptive electromagnetic screen

A bidirectional electromagnetic screen is proposed and demonstrated with properties that can be switched between high reflection, absorption and transmission over /spl sim/10 ns. The screen comprises frequency selective surfaces loaded with pin diodes, which under partial biasing conditions provide the loss mechanism for the absorption state of the screen.     

Kinetics and mechanism of oxidation of 1,4-butanediol by chromium(VI) in acid perchlorate medium

Summary The kinetics of oxidation of 1,4-butanediol by chromium(VI) was studied in acid perchlorate medium and the oxidation product of the diol was identified as 4-hydroxybutanal. The kinetic rate law observed accounted for the complex dependence of the hydrogen ion k=(k₂K₁[H+]+k₃K₁K₂[H+]²)/(1+K₁[H]+) where k is the observed second-order rate constant.     

SAR CHARACTERIZATION OF FOCUSED PLANAR ARRAY OF WATER-LOADED MODIFIED BOX-HORNS FOR HYPERTHERMIA

A 4×4 planar array of modified box-horns as a microwave hyperthermia applicator is theoretically studied to characterize power deposition (SAR) in heating tissue (muscle) at 2450 MHz. A modified box-horn is a novel improved version of conventional box-horn in which horn exciting the box waveguide is flared in both E-and H-planes. Modified box-horn supports TE₁₀ and TE₃₀ modes. The amplitude distribution over the H-plane of the box-horn aperture is a closer approximation to the uniform distribution. It is proposed that the interior of the box-horn be filled with water to provide a better impedance match to biological tissue. By applying Fresnel-Kirchhoff scalar diffraction field theory, the expression for electric field in heating region is derived and distribution of specific absorption rate (SAR) in that region due to planar array of modified box-horns as direct contact applicator is evaluated at 2450 MHz. The results of modified box-horn array are compared with those of a single modified box-horn operating at the same frequency. Results demonstrate that planar array of modified box-horns offers improvement in SAR distribution and penetration depth. It is shown that by changing the phase and amplitude of excitation of the modified box-horns of the array, the relative amplitude and position of the hot spot can be changed. The present analysis is validated through the results obtained by plane wave spectral technique.