Toward Photon-Efficient Key Distribution over Optical Channels

This work considers the distribution of a secret key over an optical (bosonic) channel in the regime of high photon efficiency, i.e., when the number of secret key bits generated per detected photon is high. While in principle the photon efficiency is unbounded, there is an inherent tradeoff between this efficiency and the key generation rate (with respect to the channel bandwidth). We derive asymptotic expressions for the optimal generation rates in the photon-efficient limit, and propose schemes that approach these limits up to certain approximations. The schemes are practical, in the sense that they use coherent or temporally-entangled optical states and direct photodetection, all of which are reasonably easy to realize in practice, in conjunction with off-the-shelf classical codes.Comment: In IEEE Transactions on Information Theory; same version except that labels are corrected for Schemes S-1, S-2, and S-3, which appear as S-3, S-4, and S-5 in the Transaction

Finite multi-coset sampling and sparse arrays

Signals with sparse but otherwise unknown frequency content are well-represented by multi-coset samples, and efficient algorithms can be used to recover the underlying sparsity structure. While such sampling is usually analyzed over a sampling interval sufficiently large that edge effects can be ignored, in this work we develop how to take into account finite-window effects in system design. Such considerations are particularly important in the context of antenna arrays, and we analyze the associated redundancy. Additionally, we describe an efficient MIMO radar implementation of multi-coset arrays. As an example application of our results, we develop a natural two-stage architecture for direction-of-arrival estimation in sparse environments using a multi-coset array over the available aperture.Semiconductor Research Corporation. Center for Circuits and Systems SolutionsHewlett-Packard Compan

On high-efficiency optical communication and key distribution

We investigate modulation and coding techniques that approach the fundamental limits of communication and key distribution over optical channels, in the regime of simultaneously high photon and bandwidth efficiencies. First, we develop a simple and robust system design for free-space optical communication that incorporates pulse-position modulation (PPM) over multiple spatial degrees of freedom in order to achieve high photon and spectral efficiency. Further, in the context of key distribution, we determine the optimal rate using a Poisson source of entangled photon pairs and photon detectors, and show how to approach it using PPM parsing of the detected photon stream.United States. Defense Advanced Research Projects Agency. Information in a Photon Program (Contract HR0011-10-C-0159)United States. Army Research Office (Grant W911NF- 10-1-0416)United States. Air Force Office of Scientific Research (Grant FA9550-11-1-0183

On playback delay in streaming communication

We consider the problem of minimizing playback delay in streaming over a packet erasure channel with fixed bandwidth. When packets have to be played in order, the expected delay inherently grows with time. We analyze two cases, namely no feedback and instantaneous feedback. We find that in both cases the delay grows logarithmically with the time elapsed since the start of transmission, and we evaluate the growth constant, i.e. the pre-log term, as a function of the transmission bandwidth (relative to the source bandwidth). The growth constant with feedback is strictly better that the one without, but they have the same asymptotic value in the limit of infinite bandwidth.Lincoln LaboratoryUnited States. Air Force Office of Scientific Research (Grant FA9550-11-1-0183)Hewlett-Packard Compan

The effect of block-wise feedback on the throughput-delay trade-off in streaming

Unlike traditional file transfer where only total delay matters, streaming applications impose delay constraints on each packet and require them to be in order. To achieve fast in-order packet decoding, we have to compromise on the throughput. We study this trade-off between throughput and in-order decoding delay, and in particular how it is affected by the frequency of block-wise feedback, whereby the source receives full channel state feedback at periodic intervals. Our analysis shows that for the same throughput, having more frequent feedback significantly reduces the in-order decoding delay. For any given block-wise feedback delay, we present a spectrum of coding schemes that span different throughput-delay tradeoffs. One can choose an appropriate coding scheme from these, depending upon the delay-sensitivity and bandwidth limitations of the application

Multi-Coset Sparse Imaging Arrays

We develop an efficient structured sparse antenna array architecture for coherent imaging of sparse but otherwise unknown scenes. In this architecture, the array elements are configured in a periodic nonuniform pattern that can be viewed as the superposition of multiple sparse uniform subarrays. For such structure, we develop an efficient pattern design procedure using co-array analysis, and we describe robust and efficient algorithms implementing the required associated array processing, which comprise scene support recovery, followed by image reconstruction. In addition, we develop a practical method for detecting reconstruction failures when the scene density exceeds the level for which the array was designed, so that false images are not produced. As a demonstration of its viability, the architecture is used to reconstruct a simulated natural scene.National Science Foundation (U.S.) (Grant CCF-1017772)Semiconductor Research Corporation. Center for Circuits and Systems SolutionsUnited States. Air Force (Contract FA8721-05-C-0002

Observability and nonlinear filtering

This paper develops a connection between the asymptotic stability of nonlinear filters and a notion of observability. We consider a general class of hidden Markov models in continuous time with compact signal state space, and call such a model observable if no two initial measures of the signal process give rise to the same law of the observation process. We demonstrate that observability implies stability of the filter, i.e., the filtered estimates become insensitive to the initial measure at large times. For the special case where the signal is a finite-state Markov process and the observations are of the white noise type, a complete (necessary and sufficient) characterization of filter stability is obtained in terms of a slightly weaker detectability condition. In addition to observability, the role of controllability in filter stability is explored. Finally, the results are partially extended to non-compact signal state spaces

Optimal Timing of Coronary Invasive Strategy in Non–ST-Segment Elevation Acute Coronary Syndromes

Background: The optimal timing of coronary intervention in patients with non–ST-segment elevation acute coronary syndromes (NSTE-ACSs) is a matter of debate. Conflicting results among published studies partly relate to different risk profiles of the studied populations. Purpose: To do the most comprehensive meta-analysis of current evidence on early versus delayed invasive treatment in NSTE-ACS. Data Sources: MEDLINE, PubMed Central, and Google Scholar databases; conference proceedings; ClinicalTrials.gov registry; and Current Controlled Trials registry through May 2012. Study Selection: Available randomized, controlled trials (RCTs) and observational studies comparing early versus delayed intervention in the NSTE-ACS population. Data Extraction: Data were extracted for populations, interventions, outcomes, and risk of bias. All-cause mortality was the prespecified primary end point. The longest follow-up available in each study was chosen. The odds ratio with 95% CI was the effect measure. Data Synthesis: Seven RCTs (5370 patients) and 4 observational studies (77 499 patients) were included. Early intervention was less than 20 hours after hospitalization or randomization for RCTs and 24 hours or less for observational studies. Meta-analysis of the RCTs was inconclusive for a survival benefit associated with the early invasive strategy (odds ratio, 0.83 [95% CI, 0.64 to 1.09]; P 0.180); a similar result emerged from the observational studies. With early versus late intervention, the odds ratios in the RCTs were 1.15 (CI, 0.65 to 2.01; P 0.63) and 0.76 (CI, 0.56 to 1.04; P 0.090) for myocardial infarction and major bleeding during follow-up, respectively. Limitation: Current evidence from RCTs is limited by the small overall sample size, low numbers of events in some trials, and heterogeneity in the timing of intervention and in patient risk profiles. Conclusion: At present, there is insufficient evidence either in favor of or against an early invasive approach in the NSTE-ACS population. A more definitive RCT is warranted to guide clinical practice. Primary Funding Source: None

Heart Rate Variability in Children and Adolescents with Cerebral Palsy-A Systematic Literature Review.

This is the final version. Available from MDPI via the DOI in this record. Cardiac autonomic dysfunction has been reported in patients with cerebral palsy (CP). The aim of this study was to assess the existing literature on heart rate variability (HRV) in pediatric patients with CP and a special attention was paid to the compliance of the studies with the current HRV assessment and interpretation guidelines. A systematic review was performed in PubMed, Web of Science, and Cumulative Index to Nursing and Allied Health Literature (CINAHL) databases searched for English language publications from 1996 to 2019 using Medical Subject Headings (MeSH) terms "heart rate variability" and "cerebral palsy" in conjunction with additional inclusion criteria: studies limited to humans in the age range of 0-18 years and empirical investigations. Out of 47 studies, 12 were included in the review. Pediatric patients with CP presented a significantly higher resting heart rate and reduced HRV, different autonomic responses to movement stimuli compared to children with normal development, but also reduced HRV parameters in the children dependent on adult assistance for mobility compared to those generally independent. None of the included studies contained the necessary details concerning RR intervals acquisition and HRV measurements as recommended by the guidelines. Authors of HRV studies should follow the methodological guidelines and recommendations on HRV measurement, because such an approach may allow a direct comparison of their results