Deployment of Wireless Sensor Networks

1
Deployment of Wireless Sensor Networks
Supervisor Dr. Ian Wassell ijw24_at_cam.ac.uk
Ruoshui Liu rl348_at_cam.ac.uk
What are the current problems in the deployment
of Wireless Sensor Networks? A Wireless Sensor
Network (WSN) is a wireless network consisting of
spatially distributed autonomous smart devices
using sensors to cooperatively monitor physical
or environmental conditions, such as temperature,
sound, vibration, pressure, motion or pollutants,
at different locations. Currently the planning
and ease of deployment of Wireless Sensor
Networks (WSNs) is not as straightforward as
expected. This is partly as a result of
simplistic assumptions concerning wireless
propagation and its effect on the communication
links between wireless sensor motes. In
reality, radio communication links between motes
are in general notoriously variable and
unpredictable. The propagation path loss, the
channel fading, RF interference and changes in
the physical environment that block communication
links all contribute to the dramatic variation in
the received signal strength and consequently the
error rate of the communication links.
2.480 GHz (channel 11)
Radio Frequency
2.405 GHz (channel 11)
Time
Figure 2 Illustration of frequency diversity
employing the frequency hopping
Present stage of the research To investigate the
performance benefits of the proposed diversity
techniques we have developed some low cost
portable measurement equipment using
off-the-shelf MicaZ motes. Consequently, we
developed automated measurement procedures to
characterise their RF performance by establishing
accurate lookup tables for Received Signal
Strength Indication (RSSI) and transmit power as
a function of frequency channel.
What can we do to tackle the existing
problems? Therefore, we propose to investigate
some channel diversity techniques with the aim of
improving the performance of radio links between
motes. These include the use of frequency
diversity, spatial (or antenna) diversity and
novel Medium Access Control (MAC) protocols to
support the implementation of these diversity
techniques.
Preliminary results and conclusions The
validation results show that the use of our
calibrated RSSI lookup tables can provide us with
the 3 dB accuracy compared with the 6 dB
claimed in the mote datasheet. The RSSI
characteristics vary over the 16 available radio
channels. The lookup table of the transmitting
mote also reveals that the transmitted power
deviates from the manufacturers data.
Furthermore, the actual transmitted power
decreases as the radio channel number increases
with a maximum difference of about 3 dB.




The intended outcome If the performance gain
achieved by frequency diversity is judged to be
worthwhile, then it will lead to the design of a
MAC sub-layer that can support dynamic frequency
agility. In addition to this, the investigation
of space diversity will provide us with
guidelines concerning the availability of space
diversity as a function of antenna spacing.
Figure 3 Calibrated tables, where a typical RSSI
lookup table (1 out of 16) for channel 19 is on
the left and the lookup table on the right is for
the transmitting mote.

• Further work
• Carry out the investigation of the potential
  improvement in radio performance provided by
  diversity techniques using the calibrated
  measurement motes.
• Design the MAC protocol supporting dynamic
  frequency agility bearing in mind the impact of
  the MAC sub-layer on system aspects, such as
  frequency synchronisation, latency and battery
  life.

Computer Laboratory Digital Technology Group
Summer School 2008