Dave Cavalcanti

Issues in integrating cellular networks WLANs, AND MANETs: a futuristic heterogeneous wireless network

The popularity of wireless communication systems can be seen almost everywhere in the form of cellular networks, WLANs, and WPANs. In addition, small portable devices have been increasingly equipped with multiple communication interfaces building a heterogeneous environment in terms of access technologies. The desired ubiquitous computing environment of the future has to exploit this multitude of connectivity alternatives resulting from diverse wireless communication systems and different access technologies to provide useful services with guaranteed quality to users. Many new applications require a ubiquitous computing environment capable of accessing information from different portable devices at any time and everywhere. This has motivated researchers to integrate various wireless platforms such as cellular networks, WLANs, and MANETs. Integration of different technologies with different capabilities and functionalities is an extremely complex task and involves issues at all layers of the protocol stack. This article envisions an architecture for state-of-the-art heterogeneous multihop networks, and identifies research issues that need to be addressed for successful integration of heterogeneous technologies for the next generation of wireless and mobile networks.

Spectrum Sensing for Dynamic Spectrum Access of TV Bands

In this paper we address the issue of spectrum sensing in cognitive radio based wireless networks. Spectrum sensing is the key enabler for dynamic spectrum access as it can allow secondary networks to reuse spectrum without causing harmful interference to primary users. Here we propose a set of integrated medium access control (MAC) and physical layer (PHY) spectrum sensing techniques that provide reliable access to television (TV) bands. At the MAC level, we propose a two-stage spectrum sensing that guarantees timely detection of incumbents while meeting the quality of service (QoS) requirements of secondary users. At the PHY level, we introduce FFT-based pilot energy and location detection schemes that can detect a TV signal on a TV channel at levels as low as -116 dBm. We have evaluated these schemes through simulation and prototyping and show their effectiveness, reliability, and efficiency. These mechanisms are also part of the current IEEE 802.22 draft standard which is based on cognitive radio technology.

Coexistence challenges for heterogeneous cognitive wireless networks in TV white spaces

In order to improve utilization of TV spectrum, regulatory bodies around the world have been developing rules to allow operation by unlicensed users in these bands provided that interference to incumbent broadcasters is avoided. Thus, new services may opportunistically use temporarily unoccupied TV channels, known as television white space. This has motivated several standardization efforts such as IEEE 802.22, 802.11af, 802.19 TG1, and ECMA 392 to further cognitive networking. Specifically, multiple collocated secondary networks are expected to use TVWS, each with distinct requirements (bandwidth, transmission power, different system architectures, and device types) that must all comply with regulatory requirements to protect incumbents. Heterogeneous coexistence in the TVWS is thus expected to be an important research challenge. This article introduces the current regulatory scenario, emerging standards for cognitive wireless networks targeting the TVWS, and discusses possible coexistence scenarios and associated challenges. Furthermore, the article casts an eye on future considerations for these upcoming standards in support of spectrum sharing opportunities as a function of network architecture evolution.

Cognitive Radio Based Wireless Sensor Networks

In recent years, we have seen tremendous growth in the applications of wireless sensor networks (WSNs) operating in unlicensed spectrum bands. However, there is evidence that existing unlicensed spectrum is becoming overcrowded. On the other hand, with recent advances in cognitive radio (CR) technology, it is possible to apply the dynamic spectrum access (DSA) model in WSNs to get access to less congested spectrum, possibly with better propagation characteristics. In this paper we present a conceptual design of CR-based WSNs, identify the main advantages and challenges of using CR technology, and suggest possible remedies to overcome the challenges. As an illustration, we study the performance of CR-based WSN used for the automation and control applications in residential and commercial premises. Our simulation results compare the performance of a CR-based WSN with a standard ZigBee/802.15.4 WSN.

Opportunities and challenges in using WPAN and WLAN technologies in medical environments [Accepted from Open Call]

The present view of medical environments, where isolated networks are used for IT and medical applications, is changing toward an integrated heterogeneous network scenario that can support a wide range of applications. WPAN and WLAN technologies play a fundamental role in enabling such integrated environment that is expected to support both medical and nonmedical applications. The ultimate goal is to exploit WPAN and WLAN technologies, as well as other wireless networks, such as 3G cellular systems and satellite networks, to support highly efficient medical care delivery, anytime and anywhere. However, the life-critical nature of some medical applications imposes additional challenges that have not been considered in nonmedical scenarios. This article discusses some future scenarios where WLAN and WPAN technologies can be used to provide an integrated and ubiquitous network in medical environments, and identifies the main issues to be addressed in order to meet the QoS requirements of different medical applications when operating in integrated environment

The deafness problems and solutions in wireless ad hoc networks using directional antennas

This paper addresses the issue of deafness in directional antennas for wireless ad hoc networks. Deafness is caused when a node X repeatedly attempts to communicate with node Y but is unsuccessful, because Y is presently tuned to some other antenna beam. In this paper, we first outline the different factors which contribute to such deafness in directional antennas and its significant impact on network performance. We then propose two schemes to overcome deafness scenarios which are transparent to the underlying directional MAC protocol in use. In addition, we also claim that IEEE 802.11 short retry limit (SRL) needs a special handling in directional environment because of the presence of deafness. We have done a detailed performance evaluation of our schemes with different directional MAC protocols running over switched beam antennas and the initial results are found to be very promising.

Exploiting the Small-World Effect to Increase Connectivity in Wireless Ad Hoc Networks

This paper investigates how the small world concept can be applied in the context of wireless ad hoc networks. Different from wireless ad hoc networks, small world networks have small characteristic path lengths and are highly clustered. This path length reduction is caused by long-range edges between randomly selected nodes. However, in a wireless ad hoc network there are no such long-range connections. Then, we propose to use a fraction of nodes in the network equipped with two radios with different transmission ranges in order to introduce the long-range shortcuts. We analyze the system from a percolation perspective and show that a small fraction of these “special nodes” can improve connectivity in a significant way. We also study the effects of the special nodes on the process of information diffusion and on network robustness.

Performance Analysis of 802.15.4 and 802.11e for Body Sensor Network Applications

This paper studies the energy efficiency and QoS performance of 802.15.4 and 802.11e MAC protocols for body sensor network applications. We simulated a stand-alone body sensor network, as well as co-existence scenarios where the body sensors operate in the presence of voice, video and IT traffic. Our results indicate that although 802.15.4 and 802.11e can provide an acceptable compromise between power consumption and QoS in some scenarios, there are situations (e.g. co-existence with video and heavy data traffic) in which both performance criteria can not be met simultaneously. This highlights the need for improving existing MAC protocols or designing new solutions that can provide both extremely low power and QoS for body sensor networks (BSNs).

A novel queue management mechanism for improving performance of multihop flows in IEEE 802.11s based mesh networks

Wireless mesh networks exploit multi-hop wireless communications between access points to replace wired infrastructure. However, in multi-hop networks, effective bandwidth decreases with increasing number of hops, mainly due to increased spatial contention. Longer hop length flows suffer from extremely low throughputs which is highly undesirable in the envisioned scenarios for mesh networks. In this paper, we show that queue/buffer management, at intermediate relay mesh nodes, plays an important role in limiting the performance of longer hop length flows. We propose a novel queue management algorithm for IEEE 802.11s based mesh networks that improves the performance of multihop flows by fairly sharing the available buffer at each mesh point among all the active source nodes whose flows are being forwarded. Extensive simulations reveal that our proposed scheme substantially improves the performance of multihop flows. We also identify some important design issues that should be considered for the practical deployment of such mesh networks

CMAC - A multi-channel energy efficient MAC for wireless sensor networks

Tins paper presents CMAC, a fully desynchronized MAC protocol that is designed to exploit the existing multi-channel support in sensor nodes. The hardware requirements of our protocol are minimal, requiring a single half-duplex transceiver and a low-power wake-up radio. CMAC takes into account the fundamental energy constraint in sensor nodes by placing them in a default sleep mode and waking them up only when necessary. As a contrast to other dual radio wake-up schemes, our protocol focuses on how communication and its preceding control message exchange mechanism can be undertaken in a multi-channel scenario without assuming a separate control channel. CMAC enables spatial channel re-use, nearly collision free communication, and addresses the deafness problem without incurring a tradeoff in fairness or latency. When compared with a recent MAC protocol SMAC, results show that CMAC obtains nearly 200% reduction in energy consumption, significantly improved throughput, and end-to-end delay values that are 50-150% better than SMAC for our simulated topologies