Fast Image Recovery Using Variable Splitting and Constrained Optimization

We propose a new fast algorithm for solving one of the standard formulations of image restoration and reconstruction which consists of an unconstrained optimization problem where the objective includes an $\ell_2$ data-fidelity term and a non-smooth regularizer. This formulation allows both wavelet-based (with orthogonal or frame-based representations) regularization or total-variation regularization. Our approach is based on a variable splitting to obtain an equivalent constrained optimization formulation, which is then addressed with an augmented Lagrangian method. The proposed algorithm is an instance of the so-called "alternating direction method of multipliers", for which convergence has been proved. Experiments on a set of image restoration and reconstruction benchmark problems show that the proposed algorithm is faster than the current state of the art methods.Comment: Submitted; 11 pages, 7 figures, 6 table

Scene-adapted plug-and-play algorithm with convergence guarantees

Recent frameworks, such as the so-called plug-and-play, allow us to leverage the developments in image denoising to tackle other, and more involved, problems in image processing. As the name suggests, state-of-the-art denoisers are plugged into an iterative algorithm that alternates between a denoising step and the inversion of the observation operator. While these tools offer flexibility, the convergence of the resulting algorithm may be difficult to analyse. In this paper, we plug a state-of-the-art denoiser, based on a Gaussian mixture model, in the iterations of an alternating direction method of multipliers and prove the algorithm is guaranteed to converge. Moreover, we build upon the concept of scene-adapted priors where we learn a model targeted to a specific scene being imaged, and apply the proposed method to address the hyperspectral sharpening problem

An Augmented Lagrangian Approach to the Constrained Optimization Formulation of Imaging Inverse Problems

We propose a new fast algorithm for solving one of the standard approaches to ill-posed linear inverse problems (IPLIP), where a (possibly non-smooth) regularizer is minimized under the constraint that the solution explains the observations sufficiently well. Although the regularizer and constraint are usually convex, several particular features of these problems (huge dimensionality, non-smoothness) preclude the use of off-the-shelf optimization tools and have stimulated a considerable amount of research. In this paper, we propose a new efficient algorithm to handle one class of constrained problems (often known as basis pursuit denoising) tailored to image recovery applications. The proposed algorithm, which belongs to the family of augmented Lagrangian methods, can be used to deal with a variety of imaging IPLIP, including deconvolution and reconstruction from compressive observations (such as MRI), using either total-variation or wavelet-based (or, more generally, frame-based) regularization. The proposed algorithm is an instance of the so-called "alternating direction method of multipliers", for which convergence sufficient conditions are known; we show that these conditions are satisfied by the proposed algorithm. Experiments on a set of image restoration and reconstruction benchmark problems show that the proposed algorithm is a strong contender for the state-of-the-art.Comment: 13 pages, 8 figure, 8 tables. Submitted to the IEEE Transactions on Image Processin

Improving the communication reliability of body sensor networks based on the IEEE 802.15.4 protocol

Body sensor networks (BSNs) enable continuous monitoring of patients anywhere, with minimum constraints to daily life activities. Although the IEEE 802.15.4 and ZigBee® (ZigBee Alliance, San Ramon, CA) standards were mainly developed for use in wireless sensors network (WSN) applications, they are also widely used in BSN applications because of device characteristics such as low power, low cost, and small form factor. However, compared with WSNs, BSNs present some very distinctive characteristics in terms of traffic and mobility patterns, heterogeneity of the nodes, and quality of service requirements. This article evaluates the suitability of the carrier sense multiple access–collision avoidance protocol, used by the IEEE 802.15.4 and ZigBee standards, for data-intensive BSN applications, through the execution of experimental tests in different evaluation scenarios, in order to take into account the effects of contention, clock drift, and hidden nodes on the communication reliability. Results show that the delivery ratio may decrease substantially during transitory periods, which can last for several minutes, to a minimum of 90% with retransmissions and 13% without retransmissions. This article also proposes and evaluates the performance of the BSN contention avoidance mechanism, which was designed to solve the identified reliability problems. This mechanism was able to restore the delivery ratio to 100% even in the scenario without retransmissions.Fundação para a Ciência e a Tecnologia (FCT

Server selection on the internet using passive probing

This paper describes a server selection mechanism for connection oriented services based on passive probing. The criterion of selection is the quality of service expected from each server, expressed as a function of availability and response time. Measures from previous connections to servers made by local clients are used to continuously update a QoS database which the prediction algorithm uses to compute the response time expected in subsequent connections. The forecasting approach is mainly based on prior measurements of TCP connection establishment time. The maximum segment size in a connection is also considered. The proposed metric is compared with other ones normally used to measure network proximity. Results show that the proposed server selection mechanism achieves a reduction of response time of over 50 percent compared with a random selection mechanism

Simulation analysis of IEEE 802.15.4 for wireless networked control systems

The deployment of wireless networks in industrial environments can bring several advantages over their wired counterparts; however, the characteristics of the wireless channels pose challenges to the provisioning of quality of service (QoS) that are not seen in wired networks. This paper provides an analysis of how important QoS parameters, such as message delivery ratio, delay and energy consumption, vary as a function of the number of sensor nodes injecting traffic in IEEE 802.15.4 networks operating in unbeaconed mode. Results show that the peer-to-peer topology enables better performance than the star topology, under the same conditions, indicating that the former is a better option if centralized control is not required. The use of retransmissions increases significantly the reliability of the network; however, even a high number of retransmissions are not enough to provide full reliability when the channel is highly affected by errors. Results also show that presence of hidden nodes can severely degrade the performance of the network, while the reduction of the turnaround time can improve the performance.Fundação para a Ciência e a Tecnologia (FCT

Fast retransmission of real-time traffic in HIPERLAN/2 systems

Automatic repeat request (ARQ) schemes are effective to recover non-real-time data corrupted by channel errors, but their use with real-time traffic is seldom considered because packet retransmissions tend to increase the delay beyond the deadline. However, the high-speed of modern wireless local area networks, together with their inherent low propagation times, favors the use of ARQ schemes with real-time traffic. This paper proposes a fast retransmission scheduling mechanism for reliable transport of real-time traffic in HIPERLAN/2 systems, as well as an adaptive rate scheme designed to reduce the delay introduced by retransmissions while keeping the same level of network efficiency. The achieved performance is evaluated in the scenario of a distributed data acquisition and control system

Performance simulation of HIPERLAN/2 with low debit traffic for real time data acquisition and control applications

Wireless local area networks can be suitable for remote interconnection of different data acquisition and control systems over a standardized telecommunications network, using several communication technologies, such as ISDN, ATM or IP. This paper presents an overview of the HIPERLAN/2 specifications as well as simulation results of this system, with channel errors and mixed traffic generated by control applications

Wireless body area network for cycling posture monitoring

This work presents the design and implementation of a wireless body area network based on Bluetooth Low Energy (BLE) which enables the integration of multiple sensor nodes into a smartphone-based system in order to monitor the posture of cyclists. The wireless posture monitoring system presented in this chapter obtains the orientation in space of each body segment in which the sensor nodes are placed and calculates the trunk angle, the knee angle and the angle of inclination of the road. This system collects raw sensor data from accelerometers, magnetometers and gyroscopes and sends the data via BLE to an Android smartphone, which plays the role of central station and performs the data processing concerning the posture calculation. This chapter describes the development of the hardware and software of the sensor nodes, which are based on the CC2540 BLE system-on-chip, as well as the development of the Android application. Experimental results concerning the measurement of the posture of a cyclist are provided in order to validate the implementation.This work has been supported by FCT (Fundação para a Ciência e Tecnologia) in the scope of the project: UID/EEA/04436/2013.info:eu-repo/semantics/publishedVersio