Stochastic Subgradient Algorithms for Strongly Convex Optimization over Distributed Networks

We study diffusion and consensus based optimization of a sum of unknown convex objective functions over distributed networks. The only access to these functions is through stochastic gradient oracles, each of which is only available at a different node, and a limited number of gradient oracle calls is allowed at each node. In this framework, we introduce a convex optimization algorithm based on the stochastic gradient descent (SGD) updates. Particularly, we use a carefully designed time-dependent weighted averaging of the SGD iterates, which yields a convergence rate of $O\left(\frac{N\sqrt{N}}{T}\right)$ after $T$ gradient updates for each node on a network of $N$ nodes. We then show that after $T$ gradient oracle calls, the average SGD iterate achieves a mean square deviation (MSD) of $O\left(\frac{\sqrt{N}}{T}\right)$. This rate of convergence is optimal as it matches the performance lower bound up to constant terms. Similar to the SGD algorithm, the computational complexity of the proposed algorithm also scales linearly with the dimensionality of the data. Furthermore, the communication load of the proposed method is the same as the communication load of the SGD algorithm. Thus, the proposed algorithm is highly efficient in terms of complexity and communication load. We illustrate the merits of the algorithm with respect to the state-of-art methods over benchmark real life data sets and widely studied network topologies

Average Consensus in the Presence of Delays and Dynamically Changing Directed Graph Topologies

Classical approaches for asymptotic convergence to the global average in a distributed fashion typically assume timely and reliable exchange of information between neighboring components of a given multi-component system. These assumptions are not necessarily valid in practical settings due to varying delays that might affect transmissions at different times, as well as possible changes in the underlying interconnection topology (e.g., due to component mobility). In this work, we propose protocols to overcome these limitations. We first consider a fixed interconnection topology (captured by a - possibly directed - graph) and propose a discrete-time protocol that can reach asymptotic average consensus in a distributed fashion, despite the presence of arbitrary (but bounded) delays in the communication links. The protocol requires that each component has knowledge of the number of its outgoing links (i.e., the number of components to which it sends information). We subsequently extend the protocol to also handle changes in the underlying interconnection topology and describe a variety of rather loose conditions under which the modified protocol allows the components to reach asymptotic average consensus. The proposed algorithms are illustrated via examples.Comment: 37 page

Waveform Design for Secure SISO Transmissions and Multicasting

Wireless physical-layer security is an emerging field of research aiming at preventing eavesdropping in an open wireless medium. In this paper, we propose a novel waveform design approach to minimize the likelihood that a message transmitted between trusted single-antenna nodes is intercepted by an eavesdropper. In particular, with knowledge first of the eavesdropper's channel state information (CSI), we find the optimum waveform and transmit energy that minimize the signal-to-interference-plus-noise ratio (SINR) at the output of the eavesdropper's maximum-SINR linear filter, while at the same time provide the intended receiver with a required pre-specified SINR at the output of its own max-SINR filter. Next, if prior knowledge of the eavesdropper's CSI is unavailable, we design a waveform that maximizes the amount of energy available for generating disturbance to eavesdroppers, termed artificial noise (AN), while the SINR of the intended receiver is maintained at the pre-specified level. The extensions of the secure waveform design problem to multiple intended receivers are also investigated and semidefinite relaxation (SDR) -an approximation technique based on convex optimization- is utilized to solve the arising NP-hard design problems. Extensive simulation studies confirm our analytical performance predictions and illustrate the benefits of the designed waveforms on securing single-input single-output (SISO) transmissions and multicasting

Decision Fusion in Space-Time Spreading aided Distributed MIMO WSNs

In this letter, we propose space-time spreading (STS) of local sensor decisions before reporting them over a wireless multiple access channel (MAC), in order to achieve flexible balance between diversity and multiplexing gain as well as eliminate any chance of intrinsic interference inherent in MAC scenarios. Spreading of the sensor decisions using dispersion vectors exploits the benefits of multi-slot decision to improve low-complexity diversity gain and opportunistic throughput. On the other hand, at the receive side of the reporting channel, we formulate and compare optimum and sub-optimum fusion rules for arriving at a reliable conclusion.Simulation results demonstrate gain in performance with STS aided transmission from a minimum of 3 times to a maximum of 6 times over performance without STS.Comment: 5 pages, 5 figure

Optimal Timer Based Selection Schemes

Timer-based mechanisms are often used to help a given (sink) node select the best helper node among many available nodes. Specifically, a node transmits a packet when its timer expires, and the timer value is a monotone non-increasing function of its local suitability metric. The best node is selected successfully if no other node's timer expires within a 'vulnerability' window after its timer expiry, and so long as the sink can hear the available nodes. In this paper, we show that the optimal metric-to-timer mapping that (i) maximizes the probability of success or (ii) minimizes the average selection time subject to a minimum constraint on the probability of success, maps the metric into a set of discrete timer values. We specify, in closed-form, the optimal scheme as a function of the maximum selection duration, the vulnerability window, and the number of nodes. An asymptotic characterization of the optimal scheme turns out to be elegant and insightful. For any probability distribution function of the metric, the optimal scheme is scalable, distributed, and performs much better than the popular inverse metric timer mapping. It even compares favorably with splitting-based selection, when the latter's feedback overhead is accounted for.Comment: 21 pages, 6 figures, 1 table, submitted to IEEE Transactions on Communications, uses stackrel.st

Assessment of hydrogen embrittlement of natural gas pipeline steels

Please click Additional Files below to see the full abstrac

Incidental finding of a double orifice mitral valve in an elderly patient: value of 3D imaging.

A rare isolated double orifice mitral valve (DOMV) was diagnosed in a 77-year-old male patient, being assessed for surgical repair of the ascending aorta. This is a rare congenital abnormality, usually discovered as an incidental finding during investigation of other congenital heart defects. This case shows that a detailed assessment of all cardiac structures is necessary, not only in young patients, but also in the elderly population, to minimise the under-diagnosis of such rare anomalies. The use of 3D transthoracic echocardiography (TTE) has an increasingly significant role in establishing the diagnosis and extending the morphological and functional understanding of the anomaly. LEARNING POINTS: Thoroughly assessing all cardiac structures, in accordance with the minimum dataset guidelines for transthoracic echocardiography, ensures not only a comprehensive assessment of the primary indication for the scan, but also improves the detection of concomitant and otherwise unknown lesions. Despite falling under the category of congenital heart defects, several rare anomalies such as DOMV can be present in elderly patients, and the adult echocardiographer should have appropriate knowledge and awareness for detecting these conditions. 3D TTE provides a comprehensive assessment of the morphology of DOMV, over and above the information obtained by 2D imaging

Reporting radiographers in Europe survey: An overview of the role within the European Federation of Radiographer Society (EFRS) member countries

Introduction Reporting radiographers undertake an important role in healthcare and for the radiographer profession in general. First introduced in the United Kingdom, reporting radiographers are now practicing in several other European countries. Our objective was to investigate the workforce of reporting radiographers across the European Federation of Radiographer Societies (EFRS) community. Method and material A voluntary anonymous 34 item electronic survey was distributed online using social media accounts such as Twitter, Facebook and LinkedIn covering a wide range of topics relating to professional role, advanced practice, education, and seniority. The questionnaire was distributed during a 12-week period in 2022. Result A total of 345 individual responses were received from 15 countries with majorities of respondent from United Kingdom (n = 245, 71%) and Denmark (n = 66, 19%). Mean age was 41.9 (S.D 9.8), similar for females, 42.5 (S.D 9.0) and men 40.9 years (S.D 9.7). Most reporting radiographers worked in public hospitals (90%). The vast majority of the respondents (n = 270, n = 94%) authored and signed their own clinical reports while a minority (n = 18, 6%) stated that their reports were checked by radiologists. Conclusion The survey highlights the scope of practice of reporting radiographers working in Europe. Reporting is becoming a career path for an increasing number of radiographers across Europe and there is assess to academic education and clinical support. Implication for practice Reporting radiographers fulfil an important role within the current demands of healthcare. This demand is likely to increase in the future, and therefore it is vital that there is some form of standardisation in the level of education that this group of healthcare professionals receive

Reporting radiographers in Europe survey: Support, role satisfaction, and advanced clinical practice within the European federation of radiographer society (EFRS) member countries.

Increasing number of radiographers are undertaking image reporting throughout Europe. However, there are variations in practice and experience in European countries. The study aim was to investigate reporting radiographer's perceptions in relation to support for their role and workload satisfaction and elements of advanced practice that may also be undertaken. Following institutional ethical approval an online 34 item questionnaire survey was circulated via social media; Twitter, Facebook, and LinkedIn in a 12-week period in 2022 across Europe. The survey data were managed by the online secure database REDCap (Research Electronic Data Capture). Data was collected across a range of questions, of which those relating to support for, and barriers to radiographer reporting, role and job satisfaction, and other role elements are reported here. A response level of 345 individual reporting radiographers replied to the survey from 15 European countries; some questions were optional and therefore had a lower response rate. There was consensus about the need for support from radiologists and management, protected time, and funding to support the reporting role. The majority of respondents received additional pay for taking on this role and expressed satisfaction with their role and workload. In relation to elements of advanced practice, the majority of respondents were involved in educational and managerial activities, and there was interest, but limited involvement, in research. There was a consensus about the support needed, and perceived barriers to, radiographer reporting, between reporting radiographers from different countries. Whilst there is some commonality in relation to activities such as supervision and education, there was clearer variety in relation to opportunities for research between the respondents, perhaps reflecting the differences between reporting as a standalone role development and reporting as part of an advanced clinical practice role. As there is increasingly an emphasis on advanced clinical practice, reporting radiographers are likely to require support to develop their skills so that they can actively participate in the broader activities associated with this role, including education, leadership, and research. [Abstract copyright: Copyright © 2023 The Author(s). Published by Elsevier Ltd.. All rights reserved.