Notes on linear systems and frequency responses

We show that not all continuous multidimensional linear shift‐invariant systems are characterized by their frequency responses (even when they exist), but that the members of a certain important large family of multidimensional linear systems are completely characterized by their suitably defined frequency responses. Copyright © 2005 John Wiley & Sons, Ltd.     

Ferric Chloride,Thiourea and Brine Leach Recovery of Ag,Au and Pb from Complex Sulfides

The Bureau of Mines recently investigated a hydrometallurgical procedure to recover gold, silver, and byproduct lead from a complex lead-zinc sulfide ore. A ferric chloride (Fecl₃) preleach, at 40°C for six hours extracted about 50 percent of the zinc and formed insoluble silver and lead chlorides. Subsequent acid thiourea leaching, for one to three hours at 40°C, extracted about 85% of the gold and silver. Lead chloride was solubilized from the remaining residue with NaCl brine solution at 55°C in one hour. Lead was recovered from the brine solution in an electrolytic cell with an anion perm selective membrane at a current efficiency of 95 percent and current density of 100 A/m². Spent FeCl₃ leach solution was reoxidized in the anode compartment at a current efficiency between 75 and 95 percent.     

Electrical-heating-assisted recovery for heavy oil

Warming heavy oil (940–1000 kg/m³, 10°–20° API) reduces its viscosity substantially; however, conventional thermal recovery by steam injection is not applicable to a number of heavy-oil reservoirs. This paper explores localized electric resistance heating provided by mineral-insulated cable and a novel heater–well arrangement. Two-dimensional (2-D) and heterogeneous three-dimensional (3-D) reservoir simulation models employing single- and dual-lateral completion horizontal wells illustrate that an electric resistance heating element with a modest power output enhances recovery several fold. Important parameters for improved recovery are (1) solution gas, (2) formation and fluid thermal conductivity that permits conductive heating, and (3) the ability to achieve relatively low bottom-hole pressure in production wells. Economic analysis suggests that the cost of electricity is about 1.25 USD per barrel of incremental oil.     

A study of bounds on limit cycles in digital filters

We give bounds on the self-sustained limit cycles in fixed-poin implementations of state-variable-form digital filters having general stable system matrices. These bounds, which are period independent and which concern limit cycles due to quantization errors in physical realizations of digital filters, are given in closed form for second-order sections. A numerical comparison shows that the bounds are smaller than corresponding ones reported in the literature. This is due to our use of real similarity transformations.     

A note on representation theorems for linear discrete-space systems

The cornerstone of the theory of discrete-space single-input single-output linear systems is the idea that every such system has an input-output mapH that can be represented by a convolution or the familiar generalization of a convolution. This thinking involves an oversight, which, for the case of bounded inputs mapped continuously into bounded outputs, was recently corrected by adding an additional term to the representation. Here we give a more general result that addresses an important larger family of inputs.     

On steady-state errors in nonlinear control systems

In a recent paper control systems containing a memoryless nonlinear element are considered, and the steady-state response to certain inputs is characterized when the circle condition for stability is met. Here we give three results for the case in which the forward path contains an additional element, an integrator preceding the nonlinearity.     

Variable-rate hybrid ARQ for meteor-burst communications

The authors introduce a form of automatic repeat request (ARQ), referred to as variable-rate type-I hybrid ARQ, in which the code rate varies in response to the fluctuations in the power received from a meteor trail. For one implementation, the source or the destination periodically obtains estimates of the signal power at the destination, and the source uses these estimates to select the rate of the code. For an alternative approach, the code rate is determined completely by the decoding successes and failures during previous transmissions. The performance measure is the throughput per trail, which is defined as the expected number of information bits received correctly for a given meteor trail. Numerical results for Reed-Solomon codes are included to illustrate the relative performance of the various schemes. It is shown that the throughput is larger for both implementations of variable-rate type-I hybrid ARQ than for fixed-rate type-I hybrid ARQ and ARQ without forward error correction