Application of iterative techniques for electromagnetic scattering from dielectric random and reentrant rough surfaces

Cataloged from PDF version of article.Stationary [e.g., forward–backward method (FBM)] and nonstationary [e.g., conjugate gradient squared, quasi-minimal residual, and biconjugate gradient stabilized (Bi-CGSTAB)] iterative techniques are applied to the solution of electromagnetic wave scattering from dielectric random rough surfaces with arbitrary complex dielectric constants. The convergence issues as well as the efficiency and accuracy of all the approaches considered in this paper are investigated by comparing obtained scattering (in the form of normalized radar cross section) and surface field values with the numerically exact solution, computed by employing the conventional method of moments. It has been observed that similar to perfectly and imperfectly conducting rough surface cases, the stationary iterative FBM converges faster when applied to geometries yielding best conditioned systems but exhibits convergence difficulties for general geometries due to its inherit limitations. However, nonstationary techniques are, in general, more robust when applied to arbitrarily general dielectric random rough surfaces, which yield more ill-conditioned systems. Therefore, they might prove to be more suitable for general scattering problems. Besides, as opposed to the perfectly and imperfectly conducting rough surface cases, the Bi-CGSTAB method and FBM show two interesting behaviors for dielectric rough surface pro- files: 1) FBM generally converges for reentrant surfaces when the vertical polarization is considered and 2) the Bi-CGSTAB method has a peculiar convergence problem for horizontal polarization. Unlike the other nonstationary iterative techniques used in this paper, where a Jacobi preconditioner is used, convergent results are obtained by using a block-diagonal preconditioner

Cross-Sender Bit-Mixing Coding

Scheduling to avoid packet collisions is a long-standing challenge in networking, and has become even trickier in wireless networks with multiple senders and multiple receivers. In fact, researchers have proved that even {\em perfect} scheduling can only achieve $\mathbf{R} = O(\frac{1}{\ln N})$. Here $N$ is the number of nodes in the network, and $\mathbf{R}$ is the {\em medium utilization rate}. Ideally, one would hope to achieve $\mathbf{R} = \Theta(1)$, while avoiding all the complexities in scheduling. To this end, this paper proposes {\em cross-sender bit-mixing coding} ({\em BMC}), which does not rely on scheduling. Instead, users transmit simultaneously on suitably-chosen slots, and the amount of overlap in different user's slots is controlled via coding. We prove that in all possible network topologies, using BMC enables us to achieve $\mathbf{R}=\Theta(1)$. We also prove that the space and time complexities of BMC encoding/decoding are all low-order polynomials.Comment: Published in the International Conference on Information Processing in Sensor Networks (IPSN), 201

Bilateral Pneumothoraces Following Central Venous Cannulation

We report the occurrence of a bilateral pneumothoraces after unilateral central venous catheterization of the right subclavian vein in a 70-year-old patient. The patient had no history of pulmonary or pleural disease and no history of cardiothoracic surgery. Two days earlier, she had a median laparotomy under general and epidural anaesthesia. Prior to the procedure, the patient was hemodynamically stable and her transcutaneous oxygen saturation was 97% in room air. We punctured the right pleural space before cannulation of the right subclavian vein. After the procedure, the patient slowly became hemodynamically instable with respiratory distress. A chest radiograph revealed a complete left-side pneumothorax and a mild right-side pneumothorax. The right-side pneumothorax became under tension after left chest tube insertion. The symptoms finally resolved after insertion of a right chest tube. After a diagnostic work-up, we suspect a congenital “Buffalo chests” explaining bilateral pneumothoraces and a secondary tension pneumothorax

Application of iterative techniques for electromagnetic scattering from dielectric random and reentrant rough surfaces

Stationary [e.g., forward-backward method (FBM)] and nonstationary [e.g., conjugate gradient squared, quasi-minimal residual, and biconjugate gradient stabilized (Bi-CGSTAB)] iterative techniques are applied to the solution of electromagnetic wave scattering from dielectric random rough surfaces with arbitrary complex dielectric constants. The convergence issues as well as the efficiency and accuracy of all the approaches considered in this paper are investigated by comparing obtained scattering (in the form of normalized radar cross section) and surface field values with the numerically exact solution, computed by employing the conventional method of moments. It has been observed that similar to perfectly and imperfectly conducting rough surface cases, the stationary iterative FBM converges faster when applied to geometries yielding best conditioned systems but exhibits convergence difficulties for general geometries due to its inherit limitations. However, nonstationary techniques are, in general, more robust when applied to arbitrarily general dielectric random rough surfaces, which yield more ill-conditioned systems. Therefore, they might prove to be more suitable for general scattering problems. Besides, as opposed to the perfectly and imperfectly conducting rough surface cases, the Bi-CGSTAB method and FBM show two interesting behaviors for dielectric rough surface profiles: 1) FBM generally converges for reentrant surfaces when the vertical polarization is considered and 2) the Bi-CGSTAB method has a peculiar convergence problem for horizontal polarization. Unlike the other nonstationary iterative techniques used in this paper, where a Jacobi preconditioner is used, convergent results are obtained by using a block-diagonal preconditioner. © 2006 IEEE

Risk factors for mortality-morbidity after emergency-urgent colorectal surgery

Background: The aim of this study was to assess the risk factors associated with mortality and morbidity following emergency or urgent colorectal surgery. Materials and methods: All data regarding the 462 patients who underwent emergency colonic resection in our institution between November 2002 and December 2007 were prospectively entered into a computerized database. Results: The median age of patients was 73 (range 17-98)years. The most common indications for surgery were: 171 adenocarcinomas (37%), 129 complicated diverticulitis (28%), and 35 colonic ischemia (7.5%). Overall mortality and morbidity rates were 14% and 36%, respectively. In multivariate analysis, the only parameter significantly associated with postoperative mortality was blood loss >500cm3 (odds ratio (OR) = 3.33, 95% confidence interval (CI) 1.63-6.82, p = 0.001). There were three parameters which correlated with postoperative morbidity: ASA score ≥3 (OR = 2.9, 95% CI 1.9-4.5, p < 0.001), colonic ischemia (OR = 3.4, 95% CI 1.4-7.7, p = 0.006), and stoma creation (OR = 2.2, 95% CI 1.4-3.4, p = 0.0003). Conclusions: The main risk factors for postoperative morbidity and mortality following emergency colorectal surgery are related to: (1) patients' ASA score, (2) colonic ischemia, and (3) perioperative bleeding. These variables should be considered in the elaboration of future scoring systems to predict outcome of emergency colorectal surger