On Asynchronous Interference Channels

In the first part of the thesis, a decentralized wireless network of separate transmitter-receiver pairs is studied where there is no central controller to assign the resources to the users and users do not explicitly cooperate. For simplicity, we focus on a single-burst scenario where each transmitter sends a single codeword upon activation and remains silent afterwards. Users are block-asynchronous meaning there exists a mutual delay between their transmitted codewords. We show how the receivers learn about the number of active users, channel coefficients and activation times of the transmitters using piecewise linear regression. It is essential that each receiver fi nds the exact arrival time of the codeword sent by its corresponding transmitter. To achieve this goal, preamble sequences are embedded at the beginning of a transmitted codeword. As different users do not necessarily know each other's preamble sequences, there is no guarantee that a receiver can estimate the arrival times of interference bursts along its desired data with vanishingly small probability of error. Nevertheless, the estimates are reliable enough to guarantee successful decoding at each receiver. The second part of the thesis addresses a Gaussian interference channel with two transmitter-receiver (Tx-Rx) pairs under stochastic data arrival (GIC-SDA). Information bits arrive at the transmitters according to independent and asynchronous Bernoulli processes (Tx-Tx~asynchrony). Each information source turns off after generating a given total number of bits. The transmissions are asynchronous (Tx-Rx~asynchrony) in the sense that each Tx sends a codeword to its Rx immediately after there are enough bits available in its buffer. Such asynchronous style of transmission is shown to significantly reduce the transmission delay in comparison with the existing Tx-Rx synchronous transmission schemes. The receivers learn about the activity frames of both transmitters by employing sequential joint-typicality detection. As a consequence, the GIC-SDA under Tx-Rx asynchrony is represented by a standard GIC with state known at the receivers. The cardinality of the state space is $\binom{2N_1+2N_2}{2N_2}$ in which $N_1, N_2$ are the numbers of transmitted codewords by the two transmitters. Each realization of the state imposes two sets of constraints on $N_1, N_2$ referred to as the geometric and reliability constraints. In a scenario where the transmitters are only aware of the statistics of Tx-Tx~asynchrony, it is shown how one designs $N_1,N_2$ to achieve target transmission rates for both users and minimize the probability of unsuccessful decoding. An achievable region is characterized for the codebook rates in a two-user GIC-SDA under the requirements that the transmissions be immediate and the receivers treat interference as noise. This region is described as the union of uncountably many polyhedrons and is in general disconnected and non-convex due to infeasibility of time-sharing. Special attention is given to the symmetric case where closed-form expressions are developed for the achievable codebook rates

Randomized Resource Allocaion in Decentralized Wireless Networks

Ad hoc networks and bluetooth systems operating over the unlicensed ISM band are in-stances of decentralized wireless networks. By definition, a decentralized network is com-posed of separate transmitter-receiver pairs where there is no central controller to assign the resources to the users. As such, resource allocation must be performed locally at each node. Users are anonymous to each other, i.e., they are not aware of each other's code-books. This implies that multiuser detection is not possible and users treat each other as noise. Multiuser interference is known to be the main factor that limits the achievable rates in such networks particularly in the high Signal-to-Noise Ratio (SNR) regime. Therefore, all users must follow a distributed signaling scheme such that the destructive effect of interference on each user is minimized, while the resources are fairly shared. In chapter 2 we consider a decentralized wireless communication network with a fixed number of frequency sub-bands to be shared among several transmitter-receiver pairs. It is assumed that the number of active users is a realization of a random variable with a given probability mass function. Moreover, users are unaware of each other's codebooks and hence, no multiuser detection is possible. We propose a randomized Frequency Hopping (FH) scheme in which each transmitter randomly hops over a subset of sub-bands from transmission slot to transmission slot. Assuming all users transmit Gaussian signals, the distribution of the noise plus interference is mixed Gaussian, which makes calculation of the mutual information between the transmitted and received signals of each user intractable. We derive lower and upper bounds on the mutual information of each user and demonstrate that, for large SNR values, the two bounds coincide. This observation enables us to compute the sum multiplexing gain of the system and obtain the optimum hopping strategy for maximizing this quantity. We compare the performance of the FH system with that of the Frequency Division (FD) system in terms of the following performance measures: average sum multiplexing gain and average minimum multiplexing gain per user. We show that (depending on the probability mass function of the number of active users) the FH system can offer a significant improvement in terms of the aforementioned measures. In the sequel, we consider a scenario where the transmitters are unaware of the number of active users in the network as well as the channel gains. Developing a new upper bound on the differential entropy of a mixed Gaussian random vector and using entropy power inequality, we obtain lower bounds on the maximum transmission rate per user to ensure a specified outage probability at a given SNR level. We demonstrate that the so-called outage capacity can be considerably higher in the FH scheme than in the FD scenario for reasonable distributions on the number of active users. This guarantees a higher spectral efficiency in FH compared to FD. Chapter 3 addresses spectral efficiency in decentralized wireless networks of separate transmitter-receiver pairs by generalizing the ideas developed in chapter 2. Motivated by random spreading in Code Division Multiple Access (CDMA), a signaling scheme is introduced where each user's code-book consists of two groups of codewords, referred to as signal codewords and signature codewords. Each signal codeword is a sequence of independent Gaussian random variables and each signature codeword is a sequence of independent random vectors constructed over a globally known alphabet. Using a conditional entropy power inequality and a key upper bound on the differential entropy of a mixed Gaussian random vector, we develop an inner bound on the capacity region of the decentralized network. To guarantee consistency and fairness, each user designs its signature codewords based on maximizing the average (with respect to a globally known distribution on the channel gains) of the achievable rate per user. It is demonstrated how the Sum Multiplexing Gain (SMG) in the network (regardless of the number of users) can be made arbitrarily close to the SMG of a centralized network with an orthogonal scheme such as Time Division (TD). An interesting observation is that in general the elements of the vectors in a signature codeword must not be equiprobable over the underlying alphabet in contrast to the use of binary Pseudo-random Noise (PN) signatures in randomly spread CDMA where the chip elements are +1 or -1 with equal probability. The main reason for this phenomenon is the interplay between two factors appearing in the expression of the achievable rate, i.e., multiplexing gain and the so-called interference entropy factor. In the sequel, invoking an information theoretic extremal inequality, we present an optimality result by showing that in randomized frequency hopping which is the main idea in the prevailing bluetooth devices in decentralized networks, transmission of independent signals in consecutive transmission slots is in general suboptimal regardless of the distribution of the signals. Finally, chapter 4 addresses a decentralized Gaussian interference channel consisting of two block-asynchronous transmitter-receiver pairs. We consider a scenario where the rate of data arrival at the encoders is considerably low and codewords of each user are transmitted at random instants depending on the availability of enough data for transmission. This makes the transmitted signals by each user look like scattered bursts along the time axis. Users are block-asynchronous meaning there exists a delay between their transmitted signal bursts. The proposed model for asynchrony assumes the starting point of an interference burst is uniformly distributed along the transmitted codeword of any user. There is also the possibility that each user does not experience interference on a transmitted codeword at all. Due to the randomness of delay, the channels are non-ergodic in the sense that the transmitters are unaware of the location of interference bursts along their transmitted codewords. In the proposed scheme, upon availability of enough data in its queue, each user follows a locally Randomized Masking (RM) strategy where the transmitter quits transmitting the Gaussian symbols in its codeword independently from symbol interval to symbol interval. An upper bound on the probability of outage per user is developed using entropy power inequality and a key upper bound on the differential entropy of a mixed Gaussian random variable. It is shown that by adopting the RM scheme, the probability of outage is considerably less than the case where both users transmit the Gaussian symbols in their codewords in consecutive symbol intervals, referred to as Continuous Transmission (CT)

Coexistence and spectral efficiency in decentralized networks

Abstract—We consider a wireless communication network with a fixed number of frequency sub-bands to be shared among several transmitter-receiver pairs. In traditional frequency division (FD) systems, the available sub-bands are partitioned into disjoint clusters (frequency bands) and assigned to different users (each user transmits only in its own band). If the number of users sharing the spectrum is random, this technique may lead to inefficient spectrum utilization (a considerable fraction of the bands may remain empty most of the time). In addition, this approach inherently requires either a central network controller for frequency allocation, or cognitive radios which sense and occupy the empty bands in a dynamic fashion. These shortcomings motivate us to look for a decentralized scheme (without using cognitive radios) which allows the users to coexist, while utilizing the spectrum efficiently. We consider a frequency hopping (FH

Epidemiological and Clinical Features of Pediatric Open Globe Injuries: A Report from Southern Iran

Purpose: To evaluate the epidemiological features of open globe injury (OGI) in a tertiary ophthalmic center in the south of Iran. Methods: The medical files of pediatric patients diagnosed with OGI between March 2014 and March 2019 were reviewed retrospectively. Demographic data, laterality, time of injury, cause of trauma, location and mechanisms, complications, and the involved tissues, visual acuity, type of operation, and antibiotic therapy were all analyzed. Data were processed using the SPSS. Results: In total, 110 eyes of 108 patients were included. Ages <7 years comprised 49.1%, 7–12 years 26.4%, and 13–18 years 24.5% of cases. Of the 108 patients, 76 (70.3%) were males. No significant difference between right versus left eyes was seen. The incidence of OGI was lowest in winter and highest in spring, and it had more prevalence on the weekends. Sharp objects were the most common cause of OGI in ages <7 years, while blunt objects, accidents and falls, and guns and fireworks were more prevalent in older children. Home was the most common place of injury overall. The most common type of injury was penetrating trauma. Upon arrival, most of the children had a visual acuity <0.1 decimal. Primary wound closure was the most prevalent type of surgery done predominantly within 24 hr from admission time. Conclusion: Ages <7 years and male gender were the most common age and sex of pediatric OGI, respectively, and sharp objects were the predominant etiology. Early management and primary repair are essential for prevention of complications such as endophthalmitis and amblyopia

A comprehensive review of transvenous obliteration techniques in the management of gastric varices

Bleeding gastric varices (GVs) is a life-threatening complication of portal hypertension, with higher morbidity and mortality rates compared with bleeding esophageal varices (EVs). The endovascular techniques for the management of GVs are mainly transjugular intrahepatic portosystemic shunt (TIPS) and transvenous obliteration of the GVs. Transvenous obliteration techniques can be an alternative or an adjunct to TIPS for treatment of GVs, depending on the clinical scenario, and are less invasive than TIPS. However, these procedures are associated with increased portal pressure and related complications, mainly worsening of the EVs. In this article, the different techniques of transvenous obliteration of GVs, their indications, contraindications, and outcomes are discussed

Compliance with standard precautions, sharp injuries, and blood and body fluid exposure among healthcare workers

Background & Aim: Healthcare workers are exposed to certain infectious diseases through needlestick injuries and exposure to blood and other bodily fluids. This study aimed to determine the prevalence of needlestick injuries and blood and other body fluids exposure among healthcare workers at Namazi Hospital in Shiraz, Iran. Methods & Materials: This cross-sectional study was conducted on 656 Healthcare workers, including students and staff in medical, nursing, laboratory, service, and operating room groups, at the Namazi Hospital, in 2019. A questionnaire with four parts, including demographics, questions about using sharps containers, needle recapping, masks, and glasses, and the experience of sharps injuries or blood and body fluid exposure and reporting them, was distributed using the quota sampling method. To analyze the results, the T‑test and Chi-square test were performed. Results: Nearly 75.2% of the participants always used sharps containers, and 21.7% never recapped syringes. Sharps injuries and blood and body fluid exposure occurred in 40.7% and 39.8% of the participants. Occupation (P 0.05). Conclusion: Hospital managers should provide effective educational interventions to protect against occupational hazard exposures. Also, they should provide protective equipment for Healthcare workers

Dispersing the Mists: An Experimental History of Medicine Study into the Quality of Volatile Inhalations

This document is the Accepted Manuscript version. The final publication is available from Mary Ann Liebert, Inc. Publishers at https://doi.org/10.1089/jamp.2016.1357.Background: Dr. Nelson's Improved Inhaler was first marketed with an advertisement in The Lancet in 1865. Revolutionary at the time for its ease of use and patient-friendliness, the inhaler is still in use for self-treatment by many all over the world. On the occasion of its 150th anniversary, this study reports an experimental historical medicine approach to identify evidence for the quality of vapor inhalers. Methods: Through accessing reviews of the device's use by the contemporary medical establishment, it was established that Dr. Nelson's Inhaler enjoyed a reputation of quality and efficacy among reputable physicians generating empirical evidence of clinical performance. There was a general absence of product performance tests during this period. Therefore, modern inhalation performance testing was applied to test the aerosol delivery performance for Friars' Balsam, and its key chemical constituent, benzoic acid (BA). Results: A respirable dose of 59.9 ± 9.0 μg of BA was aerosolized in a 10 minutes period from a dose of 3.3 mL Friars' Balsam (equivalent to 35.1 ± 0.2 mg of BA) in 375 mL of steaming water using the glass twin stage impinger at a flow rate of 60 L·min−1. The respirable dose from a standardized aqueous BA inhalation formulation increased from 115.9 ± 10.6 to 200.2 ± 19.9 μg by increasing the simulated inhalation period from 5 to 10 minutes. When tested with a simulated inhalation maneuver (500 mL tidal volume, 13 minutes−1 respiration rate, 1:2 inspiratory:expiratory ratio) a respirable dose of 112.8 ± 40.3 μg was produced. Conclusions: This work has highlighted the potential for aerosol drug delivery using steam inhalers that are popular with patients. Physicians should therefore be aware of the potential for lung dosing with irritants when patients self-medicate using the Nelson Inhaler with vaporizing formulations such as Friars' Balsam.Peer reviewedFinal Accepted Versio

Formability analysis of pre-strained AA5754-O sheet metal using Yld96 plasticity theory: Role of amount and direction of uni-axial pre-strain

Automotive industries are very much interested in formability of different pre-strained aluminum alloy sheets in the context of multistage stamping to fabricate complex components. In the present work, different uni-axial pre-strains of 6.4% and 12.2% were induced in AA5754-O aluminum alloy both along rolling direction (RD) and transverse direction (TD). The true stress-strain response, limiting dome height (LDH) and strain based forming limit diagram (ε-FLD) of as received and all pre-strained materials were evaluated experimentally. The anisotropy constitutive material model was developed using the Yld96 plasticity theory in-conjunction with the Hollomon isotropic hardening law to predict the yield strength evolution of the pre-strained materials. Also, it was found that the limiting strains in ε-FLD shifted significantly depending on the amount and direction of uni-axial pre-strain. Hence, the limiting strains of the as-received materials were transposed into stress space to estimate the stress based forming limit diagram (σ-FLD) using the anisotropy constitutive material model. Further, the dynamic shifts of ε-FLDs of four different pre-strained materials were predicted by successfully decoupling the σ-FLD of as-received materials within root mean square error of 0.008. Finite element models of both uni-axial pre-straining and subsequent LDH tests were developed, and the forming behavior of the pre-strained materials were predicted implementing the Yld96 plasticity model and estimated σ-FLD. It was found that LDH was significantly influenced by the amount of pre-strain, and the maximum thinning location shifted close to pole in the case of 12.2% pre-strained materials. However, the effect of uni-axial pre-strain direction on both LDH and maximum thinning location in AA5754-O material was very negligible