Stefano Savazzi

Spectrum Leasing to Cooperating Secondary Ad Hoc Networks

The concept of cognitive radio (or secondary spectrum access) is currently under investigation as a promising paradigm to achieve efficient use of the frequency resource by allowing the coexistence of licensed (primary) and unlicensed (secondary) users in the same bandwidth. According to the property-rights model of cognitive radio, the primary terminals own a given bandwidth and may decide to lease it for a fraction of time to secondary nodes in exchange for appropriate remuneration. In this paper, we propose and analyze an implementation of this framework, whereby a primary link has the possibility to lease the owned spectrum to an ad hoc network of secondary nodes in exchange for cooperation in the form of distributed space-time coding. On one hand, the primary link attempts to maximize its quality of service in terms of either rate or probability of outage, accounting for the possible contribution from cooperation. On the other hand, nodes in the secondary ad hoc network compete among themselves for transmission within the leased time-slot following a distributed power control mechanism. The investigated model is conveniently cast in the framework of Stackelberg games. We consider both a baseline scenario with full channel state information and information-theoretic transmission strategies, and a more practical model with long-term channel state information and randomized distributed space-time coding. Analysis and numerical results show that spectrum leasing based on trading secondary spectrum access for cooperation is a promising framework for cognitive radio.

Energy aware power allocation strategies for multihop-cooperative transmission schemes

This paper deals with a cooperative decoded relaying scheme in multihop wireless network and the corresponding transmitters power allocation strategies for nodes belonging to a single primary route towards a destination. The proposed transmission strategy is referred as multihop cooperative transmission chain (MCTC). The MCTC is based on the relays of the same message by multiple previous terminals along the route and on their linear combination at the receiver to maximize the multihop diversity. Power allocations among transmitting nodes in the route can be obtained according to the average (not instantaneous) node-to-node attenuation using a recursive power assignment that can be employed locally with minimal signalling exchange among nodes. In this paper the MCTC with selection combining strategy at receivers and power assignment that maximize the minimum spread of received power (min-max strategy) is able to better exploit the multihop diversity. In addition, for ad hoc network where the energy of each node is an issue, the MCTC considerably increases the network lifetime when compared to non-cooperative multihop schemes.

A Bayesian Approach to Device-Free Localization: Modeling and Experimental Assessment

Device-free positioning allows to localize and track passive targets (i.e., not carrying any electronic device) moving in an area monitored by a dense network of low-power and battery-operated wireless sensors. The technology is promising for a wide number of applications, ranging from ambient intelligence in smart spaces, intrusion detection, emergency and rescue operations in critical areas. In this paper, a new approach is proposed where both the average path-loss and the fluctuations of the received signal strength induced by the moving target are jointly modeled based on the theory of diffraction. A novel stochastic model is derived and used for the evaluation of fundamental performance limits. The model is proved to be tight enough to be adopted for real-time estimation of the target location. The proposed localization system is validated by extensive experimental studies in both indoor and outdoor environments. The model calibration is addressed in practical scenarios to compare the performance of different Bayesian online localization methods. The test-bed system supports efficient and flexible target tracking, without requiring any action from the end-users. In addition, the technology is proven to be readily applicable over the existing IEEE 802.15.4 compliant PHY layer standard, by adapting the low-level MAC firmware.

Virtual MIMO Channels in Cooperative Multi-hop Wireless Sensor Networks

In this paper we analyze the performance of virtual multiple-input-multiple-output (MIMO) channels for multi-hop transmission in wireless sensor networks. First, we propose a clustered network topology with sensor nodes grouped in collaborative sets attending to proximity. Then, we propose that all nodes belonging to any given cluster cooperatively transmit and receive data from other clusters, exploiting the diversity advantages of cluster-to-cluster virtual MIMO channels. In order to construct the cooperative transmission, we arrange each hop into two consecutive time slots: the intracluster slot, that accounts for data sharing within the cluster, and the intercluster slot for transmission between clusters; and we devise a cooperative reception protocol within the clusters based upon a simplified selection diversity algorithm. Optimum time assignment and power allocation for both slots are derived taking the cluster-to-cluster probability of outage as the metric. Results for one hop networks and for multi-hop networks are obtained, showing substantial diversity gains and energy savings. Furthermore, results show that the performance of the proposed virtual MIMO channels is equal to that of real MIMO channels but for a small SNR loss.

Wireless Cloud Networks for the Factory of Things: Connectivity Modeling and Layout Design

Large-scale adoption of dense cloud-based wireless network technologies in industrial plants is mandatorily paired with the development of methods and tools for connectivity prediction and deployment validation. Layout design procedures must be able to certify the quality (or reliability) of network information flow in industrial scenarios characterized by harsh propagation environments. In addition, these procedures must account for possibly coexisting heterogeneous radio access technologies as part of the Internet of Things (IoT) paradigm, easily allow post-layout validation steps, and be integrated by industry-standard CAD-based planning systems. The goal of the paper is to set the fundamentals for comprehensive industry-standard methods and procedures supporting plant designer during wireless coverage prediction, virtual network deployment, and post-layout verification. The proposed methods carry out the prediction of radio signal coverage considering typical industrial environments characterized by highly dense building blockage. They also provide a design framework to properly deploy the wireless infrastructure in interference-limited radio access scenarios. In addition, the model can be effectively used to certify the quality of machine-type communication by considering also imperfect descriptions of the network layout. The design procedures are corroborated by experimental measurements in an oil refinery site [modeled by three-dimensional (3-D) CAD] using industry-standard ISA IEC 62734 devices operating at 2.4 GHz. A graph-theoretic approach to node deployment is discussed by focusing on practical case studies, and also by looking at fundamental connectivity properties for random deployments.

Optimizing Training Lengths and Training Intervals in Time-Varying Fading Channels

In time-varying faded channels the transmissions are organized into frames where the channel estimation is mainly training-based. The optimal design of the training structure is formulated here by finding the training length (the optimal number of contiguous pilots) and the training interval (the interval among two successive training phases) to maximize system throughput. The optimal balance of training and payload depends on the combination of Doppler frequency and frame length. The level of the signal to noise ratio and the fading dynamics constrain the quality of the estimate from training. It is shown that the length of the training can be conveniently traded for lower training intervals to reduce the estimate out-dating. For fast-varying fading and for high enough signal to noise ratio, there is a definite advantage in fragmenting the frame with dispersed segments of training symbols of smaller length rather than having a highly reliable channel estimate by concentrating all the training symbols at the beginning of the frame. Extensive simulations corroborate the design criteria. System throughput is maximized either for noisy binary transmission and for Gaussian input symbol distribution (i.e., by using information theoretic analysis).

Device-Free Radio Vision for Assisted Living: Leveraging wireless channel quality information for human sensing

Wireless propagation is conventionally considered as the enabling tool for transporting information in digital communications. However, recent research has shown that the perturbations of the same electromagnetic (EM) fields that are adopted for data transmission can be used as a powerful sensing tool for device-free radio vision. Applications range from human body motion detection and localization to passive gesture recognition. In line with the current evolution of mobile phone sensing [1], radio terminals are not only ubiquitous communication interfaces, but they also incorporate novel or augmented sensing potential, capable of acquiring an accurate human-scale understanding of space and motion. This article shows how radio-frequency (RF) signals can be employed to provide a device-free environmental vision and investigates the detection and tracking capabilities for potential benefits in daily life.

Cooperative Fading Regions for Decode and Forward Relaying

Cooperative transmission protocols over fading channels are based on a number of relaying nodes to form virtual multi-antenna transmissions. Diversity provided by these techniques has been widely analyzed for the Rayleigh fading case. However, short range or fixed wireless communications often experience propagation environments where the fading envelope distribution is meaningfully different from Rayleigh. The main focus in this paper is to investigate the impact of fading distribution on performances of collaborative communication. Cooperative protocols are compared to co-located multi-antenna systems by introducing the concept of cooperative fading region. This is the collection of fading distributions for which relayed transmission can be regarded as a competitive option (in terms of performances) compared to multi-antenna direct (noncooperative) transmission. The analysis is dealt with by adopting the information theoretic outage probability as the performance metric. Cooperative link performances at high SNR are conveniently expressed here in terms of diversity and coding gain as outage parameters that are provided by the fading statistics of the channels involved in collaborative transmission. Advantages of cooperative transmission compared to multi-antenna are related to the propagation environment so that the analysis can be used in network design.

Wireless Sensor Network Modeling and Deployment Challenges in Oil and Gas Refinery Plants

Wireless sensor networks for critical industrial applications are becoming a remarkable technological paradigm. Large-scale adoption of the wireless connectivity in the field of industrial monitoring and process control is mandatorily paired with the development of tools for the prediction of the wireless link quality to mimic network planning procedures similar to conventional wired systems. In industrial sites, the radio signals are prone to blockage due to dense metallic structures. The layout of scattering objects from the existing infrastructure influences the received signal strength observed over the link and thus the quality of service (QoS). This paper surveys the most promising wireless technologies for industrial monitoring and control and proposes a novel channel model specifically tailored to predict the quality of the radio signals in environments affected by highly dense metallic building blockage. The propagation model is based on the diffraction theory, and it makes use of the 3D model of the plant to classify the links based on the number and density of the obstructions surrounding each individual radio device. Accurate link classification opens the way to the optimization of the network deployment to guarantee full end-to-end connectivity with minimal on-site redesign. The link-quality prediction method based on the classification of propagation conditions is validated by experimental measurements in two oil refinery sites using industry standard ISA SP100.11a compliant devices operating at 2.4 GHz.

Energy Aware Power Allocation strategies for Multihop-Cooperative transmission schemes

This paper is focused on the optimization of transmitted power in a cooperative decoded relaying scheme for nodes belonging to the single primary route towards. a destination. The proposed transmission protocol, referred to as Multihop Cooperative Transmission Chain (MCTC), is based on the linear combination of copies of the same message by multiple previous terminals along the route in order to maximize the multihop diversity. Power allocations among transmitting nodes in the route can be obtained according to the average (not instantaneous) node-to-node path attenuation using a recursive power assignment. The latter can be employed locally on each node with limited signalling exchange (for fixed or nomadic terminals) among nodes. In this paper the power assignments for the MCTC strategy employing conventional linear combining schemes at receivers (i.e., selection combining, maximal ratio combining and equal gain combining) have been derived analytically when the power optimization is constrained to guarantee the end-to-end outage probability. In particular, we show that the power assignment that minimize the maximum spread of received power (min-max strategy) can efficiently exploit the multihop diversity. In addition, for ad hoc networks where the energy of each node is an issue, the MCTC protocol with the min-max power assignment increases considerably the network lifetime when compared to non-cooperative multihop schemes