Lieven Tytgat

Avoiding collisions between IEEE 802.11 and IEEE 802.15.4 through coexistence aware clear channel assessment

More and more devices are becoming wirelessly connected. Many of these devices are operating in crowded unlicensed bands, where different wireless technologies compete for the same spectrum. A typical example is the unlicensed ISM band at 2.4 GHz, which is used by IEEE 802.11bgn, IEEE 802.15.4, and IEEE 802.15.1, among others. Each of these technologies implements appropriate Media Access Control (MAC) mechanisms to avoid packet collisions and optimize Quality of Service. Although different technologies use similar MAC mechanisms, they are not always compatible. For example, all CSMA/CA-based technologies use Clear Channel Assessment (CCA) to detect when the channel is free; however, in each case it is specifically designed to improve detection reliability of the specific technology. Unfortunately, this approach decreases the detection probability of other technologies, increasing the amount of cross-technology collisions. In this article, we introduce the concept of coexistence aware CCA (CACCA), which enables a node operating in one technology to backoff for other coexisting technologies as well. As a proof of concept, we analyze the Packet Error Rate(PER) incurred by an IEEE 802.15.4 network in the presence of IEEE 802.11bg interference, and assess the PER reduction that is achieved by using CACCA.

Design and implementation of a generic energy-harvesting framework applied to the evaluation of a large-scale electronic shelf-labeling wireless sensor network

Most wireless sensor networks (WSNs) consist of battery-powered nodes and are limited to hundreds of nodes. Battery replacement is a very costly operation and a key factor in limiting successful large-scale deployments. The recent advances in both energy harvesters and low-power communication systems hold promise for deploying large-scale wireless green-powered sensor networks (WGSNs). This will enable new applications and will eliminate environmentally unfriendly battery disposal. This paper explores the use of energy harvesters to scavenge power for nodes in a WSN. The design and implementation of a generic energy-harvesting framework, suited for a WSN simulator as well as a real-life testbed, are proposed. These frameworks are used to evaluate whether a carrier sense multiple access with collision avoidance scheme is sufficiently reliable for use in emerging large-scale energy harvesting electronic shelf label (EHESL) systems (i.e., 12000 labels in a star topology). Both the simulator and testbed experiments yielded an average success rate up to 92%, with an arrival rate of 40 transceive cycles per second. We have demonstrated that our generic energy-harvesting framework is useful for WGSN research because the simulator allowed us to verify the achieved results on the real-life testbed and vice versa.

Analysis and experimental verification of frequency-based interference avoidance mechanisms in IEEE 802.15.4

More and more wireless networks are deployed with overlapping coverage. Especially in the unlicensed bands, we see an increasing density of heterogeneous solutions, with very diverse technologies and application requirements. As a consequence, interference from heterogeneous sources-also called cross-technology interference-is a major problem causing an increase of packet error rate (PER) and decrease of quality of service (QoS), possibly leading to application failure. This issue is apparent, for example, when an IEEE 802.15.4 wireless sensor network coexists with an IEEE 802.11 wireless LAN, which is the focus of this work. One way to alleviate cross-technology interference is to avoid it in the frequency domain by selecting different channels. Different multichannel protocols suitable for frequency-domain interference avoidance have already been proposed in the literature. However, most of these protocols have only been investigated from the perspective of intratechnology interference. Within this work, we create an objective comparison of different candidate channel selection mechanisms based on a new multichannel protocol taxonomy using measurements in a real-life testbed. We assess different metrics for the most suitable mechanism using the same set of measurements as in the comparison study. Finally, we verify the operation of the best channel selection metric in a proof-of-concept implementation running on the testbed.

Techno-economical viability of cognitive solutions for a factory scenario

Recent advances in wireless communication theory and semiconductor technology brought wireless to virtually every aspect of our life, and this trend is expected to continue to increase in the future. Unfortunately, as the number of wireless applications grows, the same scarce spectrum is reused over and over again, resulting in increased interference, which jeopardizes the prospect of wireless meeting its high expectations. Dynamic Spectrum Access proposes to mitigate this problem by adapting the operational parameters of wireless networks to varying interference conditions. However, the involved increase in cost threatens to reduce the benefit of wireless in different environments. In this paper we examine the economic balance between the added cost and the increased usability brought about by DSA. We focus on a particular real-life scenario — the production floor of an industrial installation — where there is typically extensive utilization of the ISM band. IEEE 802.15.4 wireless sensors monitor production machinery, and IEEE 802.11 WLAN is used as the data backbone. We model the benefit achieved by adding RF sensing technology in terms of reliability and battery lifetime, and qualitatively assess the cost of interference and the potential gain of introducing sensing technology. Based on this techno-economic analysis, we conclude that if implemented correctly, spectrum sensing can bring business gains in real-life applications.

Techno-economic evaluation of cognitive radio in a factory scenario

Wireless applications gradually enter every aspect of our life. Unfortunately, these applications must reuse the same scarce spectrum, resulting in increased interference and limited usability. Cognitive Radio proposes to mitigate this problem by adapting the operational parameters of wireless devices to varying interference conditions. However, it involves an increase in cost. In this paper we examine the economic balance between the added cost and the increased usability in one particular real-life scenario. We focus on the production floor of an industrial installation - where wireless sensors monitor production machinery, and a wireless LAN is used as the data backbone. We examine the effects of implementing dynamic spectrum access by means of ideal RF sensing, and model the benefit in terms of increased reliability and battery lifetime. We estimate the financial cost of interference and the potential gain, and conclude that cognitive radio can bring business gains in real-life applications.

Dynamic channel selection algorithms for coexistence of wireless sensor networks and wireless LANs

Due to the advances in wireless technology and spectrum scarcity, unlicensed band heterogeneous networks are growing rapidly. Increasing users of these networks should compete for the shared spectrum. Therefore, interoperability and coexistence of such networks are becoming key issues that require novel media access protocols equipped with dynamic channel selection to avoid harmful interference. In this paper we focus on dynamic channel selection for coexistence of IEEE 802.11 Wireless LAN and IEEE 802.15.4 sensor networks. Dynamic channel selection algorithm can either be implemented on top of an existing wireless sensor network or assisted with an auxiliary spectrum sensing device. In this research couple of dynamic channel selection algorithms have been developed and implemented to evaluate the added value of the auxiliary sensing device. As such, we propose a novel energy-aware metric to detect and quantify the harmfulness of dynamic interference. We also investigated the impact of interference dynamism on algorithms performance and validated the efficiency of the implemented mechanisms by three sets of experiments. Experiments results primarily validate the efficiency of both interference mitigation techniques. Besides, these measurements suggest that the auxiliary sensing device is most beneficial for highly complex interference profiles.

Building the business case for wireless sensors in a factory setting

With the advent of the low complexity wireless technology Zigbee, different cheaper and more energy efficient wireless transceivers became available on the market. These chipsets are gradually introduced in different small scale wireless applications. Especially in cases of wireless sensor networks, the low energy consumption and cost efficiency are a real gain. As such it is often used in small non-critical monitoring type battery powered sensor installations. In more time-critical applications, all wireless signals should be better orchestrated to avoid collisions, especially with Wi-Fi. So called spectral sensing engines are able to scan the wireless spectrum and severely reduce the probability of collisions, even between different technologies as in the Wi-Fi ⇔ Zigbee case. This paper investigates if both technologies - with or without spectral sensing engine - could lead to more cost efficient installations in more critical sensor network installations, where typically today a wired solution is used. As a realistic benchmarking case, we consider an installation in a factory in which many different types of sensors - critical and background - are scattered over the production facility and are used in controlling all processes. In this paper we constructed cost models for the wired and both wireless solutions. The results clearly show that wireless sensors could lead for smaller installations to large cost reductions (up to 50%) and give a more detailed view on the tradeoffs in a wired vs. wireless sensor network installations. This model can be used for identifying the most cost efficient sensor network for different factory installations, as well as beyond the realm of factories. The economic model is more generally applicable for sensor networks spread over large areas including mission critical and non-critical sensors.

Coexistence Aware Clear Channel Assessment

Wireless sensor networks are used by an ever growing number of applications which have ever increasing Quality of Service requirements. The available unlicensed industrial scientific and medical bands – where wireless sensor networks typically operate – are crowded with a number of technologies interfering with each other. Delivering a sufficiently high QoS within these frequency bands is therefore becoming more and more difficult. A theoretic concept named Coexistence Aware Clear Channel Assessment (CACCA) promises more reliable QoS when different technologies utilize the same. Within this paper we propose two methods to perform CACCA and create an SDR prototype to show that CACCA can achieve a high packet error rate reduction in an IEEE 802.15.4 network when it coexists with IEEE 802.11.

Energy awareness in self-growing sensor networks

An ever increasing variety of applications are being addressed by wireless sensor networks, resulting in a continuous proliferation of their deployments, which are in many cases co-located. This development is mostly hindered by the operational complexity involved with management and maintenance of large numbers of small, battery powered wireless sensor devices. The paradigm of energy aware self-growing networks addresses these difficulties. It focuses on power saving which reduces the major maintenance complexity of replacing batteries, and on automatic cooperation between networks which reduces the management complexity. However, cross-network cooperation requires cross-network communication, which is not straightforward as they typically operate on different frequencies. Receiver Directed Transmission is a MAC layer protocol which can bridge this gap, while also minimizing interference and thus reducing the number of transmissions. In this work we study how Receiver Directed Transmission can be combined with Low Power Listening in order to take advantage of the reduced number of transmissions to improve power consumption. We then implement the selected approach on TinyOS and verify its operation experimentally.

Spectrum Sharing in Heterogeneous Wireless Networks: An FP7 CREW Use Case

Cognitive radio (CR) techniques and cognitive networks [1] aim at optimizing the use of the wireless spectrum, by observing the wireless environment and intelligently configuring radio settings and network parameters. The aim of the FP7 CREW project [2] is to establish an open federated test platform in order to facilitate experimental research on advanced spectrum sensing, CR and cognitive networking strategies. The main goal of this demonstration is to showcase the possibilities of the Belgian branch of the CREW federation. A first aspect is the demonstration of the IBBT w-ilab.t testbed [3] which will be incorporated in the CREW federation, through an example CR set-up where Wi-Fi interference is avoided by an IEEE 802.15.4 network using distributed channel selection. Secondly, a high-performance advanced spectrum sensing design [4] by imec, based on reconfigurable analog and digital building blocks is demonstrated, showing the feasibility of spectrum sensing using low-cost low-power handheld devices. Within the CREW project, the integration of the advanced spectrum sensing component and the testbed (i) generates advanced possibilities for executing and monitoring reproducible testbed experiments, and (ii) allows the optimization of horizontal resource sharing between heterogeneous networks.