On the capacity of a cellular CDMA system

It is shown that, particularly for terrestrial cellular telephony, the interference-suppression feature of CDMA (code division multiple access) can result in a many-fold increase in capacity over analog and even over competing digital techniques. A single-cell system, such as a hubbed satellite network, is addressed, and the basic expression for capacity is developed. The corresponding expressions for a multiple-cell system are derived. and the distribution on the number of users supportable per cell is determined. It is concluded that properly augmented and power-controlled multiple-cell CDMA promises a quantum increase in current cellular capacity     

Transcranial Doppler ultrasound signals associated with prosthetic heart valves: an in vitro study

The application of transcranial Doppler (TCD) ultrasonography to asymptomatic prosthetic heart valve patients can result in detection of localized bursts of high intensity signals, similar to those caused by the passage of emboli. The origin of these signals is not known. In order to investigate this phenomenon in a simplified, more controllable environment, a TCD machine was used to record flow downstream from mechanical prosthetic heart valves in a mock circulatory loop. The model, which uses a saline solution seeded with silk particles (< 15 micrometers) as the circulatory fluid, recreates the principal hydrodynamic characteristics of the left heart and systemic circulation. Reproducibility of the system was established through repeated testing of a Monostrut valve. Three different mechanical valve types, (Monostrut, Medtronic Hall, St. Jude Medical) were tested over a range of simulated cardiac outputs, and the effect of valve size was investigated with four Omniscience tilting disc valves (21, 23, 25 and 29 mm). Average energy of the reflected Doppler signal was used to quantify the amount of high intensity Doppler signal, QTCD. TCD signals recorded in vitro were visually and aurally similar to those found in prosthetic heart valve patients. All valve types generated exponentially more QTCD with increasing simulated cardiac output. Differences amongst valve types were only significant at higher flow outputs, with the Monostrut valve producing the greatest QTCD. Larger valves consistently generated greater QTCD than smaller valves. In conclusion, TCD signals found in prosthetic heart valve patients can be reproduced, at least qualitatively, using a mock circulatory loop which does not incorporate the formed elements of blood.(ABSTRACT TRUNCATED AT 250 WORDS)     

Time-dependent thermal convection in dilute solutions of 3He in superfluid 4He

Time-dependent thermal convection can occur in a unity-aspect-ratio Rayleigh-Beard convection cell containing a dilute solution of ³He in superfluid ⁴He when the fluid is heated from above. Results are presented primarily for a 0.24 mole % He solution at 0.925 K. Means is provided for introducing heat at the top and separately for a central plug and an outer ring such that both are at a constant temperature gDT above the bottom. A critical temperature difference T _cfor convection can be defined above which both steady and time-dependent convection occur. The time-dependent effects include a region of T. near T _cand characterized only by excessive noise, a region of somewhat higher T where there are intermittent major changes in the plug heating rate with a time distribution like that for random events, and a region at still higher T where periodic but nonsinusoidal variation of the heat flow is observed. When a long enough time, several months, has elapsed after cooling down the apparatus, time-dependent states no longer occur, and the heat flow above T _cis limited to steady convection. Briefly raising the temperature of the apparatus to 77 K is sufficient to restore the possibility of time-dependent states.     

Evaluation of four blood pump geometries: the optical tracer technique

Artificial blood pump assistance of the failing human heart can allow it to recover. Analysis of blood pump fluid flow is a useful tool for design development and thrombosis minimization. The aim of this study was to investigate fluid flow, particularly ventricular clearance rate and stagnation areas, in four different blood pump geometries and to determine the best design. The blood pumps consisted of a polyurethane ventricle, and combinations of inlet/outlet pipe angles and compression plate shapes. A video camera recorded the motion of fluid labelled with an optical tracer (Methyl Blue histological dye). A novel processing method was developed to produce colour maps of tracer concentration, experimentally calibrated. An overall picture of fluid flow in each pump geometry was generated by considering clearance curves, tracer concentration maps and inflow jet animations. Overall and local mixing coefficients are calculated for each pump. The best geometry featured straight inlet/outlet pipes and a domed compression plate. This optical tracer technique has proven convenient, economical, sensitive to low concentrations of tracer and provides instantaneous pictures of tracer distribution in a ventricle.     

Ice‐Templated Scaffolds with Microridged Pores Direct DRG Neurite Growth

Successful spinal cord repair is thought to be promoted with hierarchically structured scaffolds. These should combine aligned porosity with additional linear features on the micrometer scale to guide axons across multiple length scales. Such scaffolds are generated through the carefully controlled directional solidification of an aqueous biopolymer solution, followed by lyophilization. Under specific freezing conditions this yields a highly regular and aligned lamellar architecture. This architecture exhibits uniform ridges of controlled height and width on the lamellar surface. These ridges run parallel to the pore axis, serving as secondary guidance features. The ridges are capable of linearly aligning 62.4% of chick dorsal root ganglia neurites to within ±10° of the ridge direction. Notably, neurites sprouting perpendicular to the ridge are guided into alignment with these microridged features.     

Microvascular Experimentation in the Chick Chorioallantoic Membrane as a Model for Screening Angiogenic Agents including from Gene-Modified Cells

The chick chorioallantoic membrane (CAM) assay model of angiogenesis has been highlighted as a relatively quick, low cost and effective model for the study of pro-angiogenic and anti-angiogenic factors. The chick CAM is a highly vascularised extraembryonic membrane which functions for gas exchange, nutrient exchange and waste removal for the growing chick embryo. It is beneficial as it can function as a treatment screening tool, which bridges the gap between cell based in vitro studies and in vivo animal experimentation. In this review, we explore the benefits and drawbacks of the CAM assay to study microcirculation, by the investigation of each distinct stage of the CAM assay procedure, including cultivation techniques, treatment applications and methods of determining an angiogenic response using this assay. We detail the angiogenic effect of treatments, including drugs, metabolites, genes and cells used in conjunction with the CAM assay, while also highlighting the testing of genetically modified cells. We also present a detailed exploration of the advantages and limitations of different CAM analysis techniques, including visual assessment, histological and molecular analysis along with vascular casting methods and live blood flow observations.     

Zero-sound studies in superfluid 3He

The propagation of 5-, 15-, and 25-MHz sound in superfluid ³He has been studied in zero magnetic field over a wide pressure range. Measurements of the structure and location of the attenuation peaks, velocity changes, and the low-temperature attenuation are described. The degree of strong coupling in the B phase is discussed. Observations of superheating and supercooling near the polycritical point have been extended and comparison is made with theory.Work supported by the U.S. Energy Research and Development Administration under Contract No. E(04-3)-34, P.A. 143.     

Principles and methods of dilution refrigeration. II

This paper is an extension of, and complementary to, an earlier paper by Wheatley, Vilches, and Abel. Some important fundamental questions which lead to degradation in the performance of a dilution refrigerator are considered: namely, convection on the dilute side and superfluid on the concentrated side. Experimental results are presented for a film suppressing still used in conjunction with a continuous-heat exchanger (several designs are considered) and a variable number (0–6) of step-heat exchangers of a copper-foil type with a high surface-area-to-viscous-impedance ratio. The first quantitative measurements of viscous heating on the dilute side are presented as well as measurements of thermal resistance on the dilute and concentrated side. Finally, the properties of the refrigerator under external heat load are considered.Supported by the U.S. Atomic Energy Commission under Contract AT(04-3)-34, P.A. 143.