Cooperative communication and distributed detection in
wireless sensor networks
Creators: name: Lucchi, Matteo <1979>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2008
Subjects: ING-INF/03 Telecomunicazioni

description: Recent progress in microelectronic and wireless communications
have enabled the development of low cost, low
power, multifunctional sensors, which has allowed the birth
of new type of networks named wireless sensor networks
(WSNs). The main features of such networks are: the nodes
can be positioned randomly over a given field with a high
density; each node operates both like sensor (for collection of environmental data) as well as transceiver (for transmission of information to the data retrieval); the nodes have limited energy resources.
The use of wireless communications and the small size of
nodes, make this type of networks suitable for a large number of applications. For example, sensor nodes can be used to monitor a high risk region, as near a volcano; in a hospital they could be used to monitor physical conditions of patients.
For each of these possible application scenarios, it is necessary to guarantee a trade-off between energy consumptions and communication reliability.
The thesis investigates the use of WSNs in two possible
scenarios and for each of them suggests a solution that permits to solve relating problems considering the trade-off introduced.
The first scenario considers a network with a high number of
nodes deployed in a given geographical area without detailed
planning that have to transmit data toward a coordinator
node, named sink, that we assume to be located onboard an
unmanned aerial vehicle (UAV). This is a practical example of reachback communication, characterized by the high density of nodes that have to transmit data reliably and efficiently towards a far receiver. It is considered that each node transmits a common shared message directly to the receiver onboard the
UAV whenever it receives a broadcast message (triggered for
example by the vehicle). We assume that the communication
channels between the local nodes and the receiver are subject to fading and noise. The receiver onboard the UAV must be able to fuse the weak and noisy signals in a coherent way to receive the data reliably. It is proposed a cooperative diversity concept as an effective solution to the reachback problem. In particular, it is considered a spread spectrum (SS) transmission scheme in conjunction with a fusion center that can exploit cooperative
diversity, without requiring stringent synchronization
between nodes. The idea consists of simultaneous transmission of the common message among the nodes and a Rake reception at the fusion center. The proposed solution is mainly motivated by two goals: the necessity to have simple nodes (to this aim we move the computational complexity to the receiver onboard
the UAV), and the importance to guarantee high levels of
energy efficiency of the network, thus increasing the network lifetime. The proposed scheme is analyzed in order to better
understand the effectiveness of the approach presented. The
performance metrics considered are both the theoretical limit on the maximum amount of data that can be collected by
the receiver, as well as the error probability with a given
modulation scheme. Since we deal with a WSN, both of these
performance are evaluated taking into consideration the energy efficiency of the network.
The second scenario considers the use of a chain network
for the detection of fires by using nodes that have a double
function of sensors and routers. The first one is relative to the
monitoring of a temperature parameter that allows to take a
local binary decision of target (fire) absent/present. The second one considers that each node receives a decision made by the previous node of the chain, compares this with that deriving by the observation of the phenomenon, and transmits the final result to the next node. The chain ends at the sink node that transmits the received decision to the user. In this network the goals are to limit throughput in each sensor-to-sensor link and
minimize probability of error at the last stage of the chain. This
is a typical scenario of distributed detection. To obtain good performance it is necessary to define some fusion rules for
each node to summarize local observations and decisions of
the previous nodes, to get a final decision that it is transmitted
to the next node.
WSNs have been studied also under a practical point of
view, describing both the main characteristics of IEEE802:15:4
standard and two commercial WSN platforms. By using
a commercial WSN platform it is realized an agricultural
application that has been tested in a six months on-field
experimentation.