1. Supervisor: Dr. Ian Wassell
Deployment of Wireless Sensor Networks
Supervisor: Dr. Ian Wassell
Ruoshui Liu
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)
B <-> C
A <-> B
C <-> D
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 manufacturer’s data. Furthermore, the actual transmitted power decreases as the radio channel number increases with a maximum difference of about 3 dB. A C B
D (BS)
Server
Figure 1: Illustration of a WSN employing the space diversity
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