Multi-Channel Wireless Sensor Networks: Protocols, Design and Evaluation

Pervasive systems, which are described as networked embedded systems integrated with everyday environments, are considered to have the potential to change our daily lives by creating smart surroundings and by their ubiquity, just as the Internet. In the last decade, "Wireless Sensor Networks" have appeared as one of the real-world examples of pervasive systems by combining automated sensing, embedded computing and wireless networking into tiny embedded devices.A wireless sensor network typically comprises a large number of spatially distributed, tiny, battery-operated, embedded sensor devices that are networked to cooperatively collect, process, and deliver data about a phenomenon that is of interest to the users. Traditionally, wireless sensor networks have been used for monitoring applications based on low-rate data collection with low periods of operation. Current wireless sensor networks are considered to support more complex operations ranging from target tracking to health care which require efficient and timely collection of large amounts of data. Considering the low-bandwidth, low-power operation of the radios on the sensor devices, interference and contention over the wireless medium and the energy-efficiency requirements due to the battery-operated devices, fulfilling the mentioned data-collection requirements in complex applications becomes a challenging task.This thesis focuses on the efficient delivery of large amounts of data in bandwidth-limited wireless sensor networks by making use of the multi-channel capability of the sensor radios and by using optimal routing topologies. We start with experimenting the operation of the sensor radios to characterize the behavior of multi-channel communication. We propose a set of algorithms to increase the throughput and timely delivery of the data and analyze the bounds on the data collection capacity of the wireless sensor networks.
The main contributions of the thesis are listed as follows:
Contribution 1 - Characteristics, challenges and the use of multi-channel communication in wireless ad hoc networks and wireless sensor networks: We review the state of the art channel assignment protocols in wireless multi-hop networks, particularly in wireless ad hoc networks and wireless sensor networks. We classify the existing solutions according to the number of transceivers required per node and according to the dynamics of the channel assignment. Since the channel assignment methods designed for general wireless ad hoc networks may not be directly applicable to wireless sensor networks, we give brief comparisons of them and discuss the additional challenges and requirements for wireless sensor networks.
Contribution 2 - Characterization of multi-channel interference: The assumption of perfectly orthogonal, interference-free channels, which is adopted in most of the multi-channel communication studies, may fail in practice. Radio signals are not limited to their allocated frequency band, but cause interference in adjacent bands as well —how much depends on the filtering characteristics of the transceivers. We conduct an extensive set of experiments, using NrF905 radio, to investigate the properties of multi-channel communication in wireless sensor networks. Based on these experiments, we explore an analytical model on the interference characteristics and by using the analytical model we discuss the impact of channel orthogonality on the network performance with extensive simulations.
Contribution 3 - Design and implementation of a multi-channel MAC protocol for wireless sensor networks: We design a multi-channel MAC protocol, namely MCLMAC (Multi-Channel Lightweight Medium Access Control), which is a schedule based multi-channel MAC protocol that takes advantage of interference and collision free parallel transmissions over different channels. MC-LMAC is designed to provide high throughput and high delivery ratio during high-rate traffic whereas it also meets the traditional requirements of wireless sensor networks such as energy efficiency and scalability.
Contribution 4 - Enhancing the rate of aggregated data collection: We consider enhancing the data collection rate of aggregated convergecast, which is one of the fundamental communication patterns in wireless sensor networks. We focus on the problem of finding the fastest rate of aggregated data collection with TDMA scheduling which is equivalent to minimizing the TDMA schedule length. We explore different techniques to address this question, such as transmission power control and multi-channel communication. We show that, once multiple frequencies are employed along with spatial-reuse TDMA, the aggregated data collection rate often becomes no longer interference-limited, but rather topology-limited. Accordingly, we show that the final step to enhance the rate of periodic aggregated data collection is to use an appropriate degree-constrained tree topology.
Contribution 5 - Fast convergecast scheduling in wireless sensor networks: We focus on data delivery models where data cannot be aggregated and raw sensor readings need to be relayed towards the sink node. We study the minimum time to complete the delivery of the messages in a convergecast operation. Similar to the aggregated convergecast problem, we investigate the benefits of transmission power control and multiple channels to eliminate the effects of interference. Once the interference is completely eliminated, we show that with half-duplex single-transceiver radios, the achievable schedule length is lower-bounded by max(2nk −1, N), where nk is the maximum number of nodes on any subtree and N is the number of nodes in a network organized as a tree. We study a distributed time slot assignment algorithm to achieve this bound when a suitable routing scheme over a capacitated minimal spanning tree is employed.