Albert Bel

Opportunistic relay selection with outdated CSI: outage probability and diversity analysis

In this paper, we analyze the outage probability and diversity order of opportunistic relay selection in a scenario based on decode and forward and where the available channel state information (CSI) is outdated. The study is conducted analytically by obtaining a closed-form expression for the outage probability, which is defined as the probability that the instantaneous capacity is below a target value. We derive high-SNR approximations for the outage probability. By doing so, we demonstrate that the diversity order of the system is reduced to 1 when CSI is outdated, being this behavior independent of the level of CSI accuracy. A physical explanation for this extreme loss of diversity is provided along with numerical results to support the analytical study.

IEEE 802.11AH: the WiFi approach for M2M communications

M2M communications are positioned to be one of the fastest growing technology segments in the next decade. Sensor and actuator networks connect communication machines and devices so that they automatically transmit information, serving the growing demand for environmental data acquisition. The IEEE 802.11ah Task Group is working on a new standard to address the particular requirements of M2M networks: a large number of power-constrained stations; a long transmission range; small and infrequent data messages; low data rates; and a non-critical delay. This article explores the key features of this new standard, especially those related to the reduction of energy consumption in the medium access control layer. Given these requirements, a performance assessment of IEEE 802.11ah in four common M2M scenarios, i.e. agriculture monitoring, smart metering, industrial automation, and animal monitoring, is presented.

Outage probability versus fairness trade-off in opportunistic relay selection with outdated CSI

We analyze the existing trade-offs in terms of system performance versus fairness of a cooperative system based on opportunistic relay selection (ORS) and with outdated channel state information (CSI). In particular, system performance is analytically evaluated in terms of outage probability, and the fairness behavior is assessed based on the power consumption at the different relays. In order to improve the fairness behavior of ORS while keeping the selection diversity gain, we propose a relay selection mechanism where the relay with the highest normalized signal-to-noise ratio (SNR) is selected for relaying the sources information. The proposed strategy is compared with existing relay selection strategies by adopting a novel graphical representation inspired by expected profit versus risk plots used in modern portfolio theory. As shown in the paper, this strategy allows operating the system in more favorable points of the outage versus fairness region.

Capacity Analysis of IEEE 802.11ah WLANs for M2M Communications

Focusing on the increasing market of the sensors and actuators networks, the IEEE 802.11ah Task Group is currently working on the standardization of a new amendment. This new amendment will operate at the sub-1GHz band, ensure transmission ranges up to 1 Km, data rates above 100 kbps and very low power operation. With IEEE 802.11ah, the WLANs will offer a solution for applications such as smart metering, plan automation, eHealth or surveillance. Moreover, thanks to a hierarchical signalling, the IEEE 802.11ah will be able to manage a higher number of stations (STAs) and improve the 802.11 Power Saving Mechanisms. In order to support a high number of STAs, two different signalling modes are proposed, TIM and Non-TIM Offset. In this paper we present a theoretical model to predict the maximum number of STAs supported by both modes depending on the traffic load and the data rate used. Moreover, the IEEE 802.11ah performance and energy consumption for both signalling modes and for different traffic patterns and data rates is evaluated. Results show that both modes achieve similar Packet Delivery Ratio values but the energy consumed with the TIM Offset is, in average, 11.7% lower.

Localization Algorithm with On-line Path Loss Estimation and Node Selection

RSS-based localization is considered a low-complexity algorithm with respect to other range techniques such as TOA or AOA. The accuracy of RSS methods depends on the suitability of the propagation models used for the actual propagation conditions. In indoor environments, in particular, it is very difficult to obtain a good propagation model. For that reason, we present a cooperative localization algorithm that dynamically estimates the path loss exponent by using RSS measurements. Since the energy consumption is a key point in sensor networks, we propose a node selection mechanism to limit the number of neighbours of a given node that are used for positioning purposes. Moreover, the selection mechanism is also useful to discard bad links that could negatively affect the performance accuracy. As a result, we derive a practical solution tailored to the strict requirements of sensor networks in terms of complexity, size and cost. We present results based on both computer simulations and real experiments with the Crossbow MICA2 motes showing that the proposed scheme offers a good trade-off in terms of position accuracy and energy efficiency.

A Robust Relay Selection Strategy for Cooperative Systems with Outdated CSI

In this paper, we consider a cooperative system based on relay selection in a scenario where the available channel state information (CSI) is subject to delays. In order to exploit the selection diversity gains of the system while providing robustness against CSI inaccuracy, we propose a robust relay selection strategy based on a minimum mean square error (MMSE) Bayesian estimator. As shown in the paper, the proposed robust strategy provides significant gains in scenarios with different levels of CSI inaccuracy.

Real-time path loss and node selection for cooperative localization in wireless sensor networks

RSS-based localization is considered a low-complexity algorithm with respect to other range techniques such as TOA or AOA. The accuracy of RSS methods depends on the suitability of the propagation models used for the actual propagation conditions. In indoor environments, in particular, it is very difficult to obtain a good propagation model. For that reason, we present a cooperative location algorithm that dynamically estimates the path loss exponent by using RSS measurements. Since the energy consumption is a key point in sensor networks, we propose a node selection in order to avoid the use of all the nodes in the network for positioning purposes. Hence, we derive a practical solution tailored to the strict requirements of sensor networks in terms of complexity, size and cost. We present results based on both computer simulations and experimentation with the Crossbow IRIS motes showing that the proposed scheme offers a good performance in terms of position accuracy.

CUIDATS: An RFID–WSN hybrid monitoring system for smart health care environments

Abstract The continuous development and improvement of low-power wireless communication technologies is enabling the emergence of many new applications in the field of the Internet of Things (IoT). One of the main areas of research within the Smart City context is smart health, which engages novel IoT initiatives to improve both quality and access to health care and smart services in general. In this paper we present CUIDATS, an IoT hybrid monitoring system for health care environments which integrates RFID and WSN technologies in a single platform providing location, status, and tracking of patients and assets. After its performance validation in a suitable testbed, CUIDATS has been deployed and evaluated with a high degree of success in a real hospital.

Multi-hop communication in the uplink for LPWANs

Abstract Low-Power Wide Area Networks (LPWANs) have arisen as a promising communication technology for supporting Internet of Things (IoT) services due to their low power operation, wide coverage range, low cost and scalability. However, most LPWAN solutions like SIGFOX™or LoRaWAN™rely on star topology networks, where stations (STAs) transmit directly to the gateway (GW), which often leads to rapid battery depletion in STAs located far from it. In this work, we analyze the impact on LPWANs energy consumption of multi-hop communication in the uplink, allowing STAs to transmit data packets in lower power levels and higher data rates to closer parent STAs, reducing their energy consumption consequently. To that aim, we introduce the Distance-Ring Exponential Stations Generator (DRESG) framework, 1 designed to evaluate the performance of the so-called optimal-hop routing model, which establishes optimal routing connections in terms of energy efficiency, aiming to balance the consumption among all the STAs in the network. Results show that enabling such multi-hop connections entails higher network lifetimes, reducing significantly the bottleneck consumption in LPWANs with up to thousands of STAs. These results lead to foresee multi-hop communication in the uplink as a promising routing alternative for extending the lifetime of LPWAN deployments.

Node selection for cooperative localization: Efficient energy vs. accuracy trade-off

In this paper, we present a cooperative localization algorithm for wireless sensor networks. We adopt an RSS-based localization algorithm due to its lower complexity with respect to other ranging techniques such as TOA or AOA. Since the energy consumption is a key aspect in sensor networks, we propose a node selection algorithm in order to avoid the use of all the nodes in the network for positioning purposes. Thus, we derive a practical solution tailored to the strict requirements of sensor networks in terms of complexity, size and cost. In particular, nodes in the network select the cooperating nodes by simply comparing their received RSS with a threshold. We present results based on both computer simulations and experimentation with the Crossbow IRIS motes showing that the proposed scheme offers a good trade-off in terms of position accuracy vs. energy consumption.