research activities in 2007 - CSEM

Exploiting Directive Antennas for Wireless Sensor Networks
L. von Allmen, P. Dallemagne, Q. Xu, J. R. Farserotu
CSEM identified and characterized a number of applications using wireless sensor networks that would benefit from directive antennas. The goal is
to reduce the power consumption of the communication system, while preserving or even improving the quality of service.
CSEM is investigating energy saving techniques using
directive antennas for wireless communication devices [1] in
specific applications. The use of directive antennas offers the
potential for improved overall system performance. For
example, one measure of performance is the environmental
impact of the radio-communication system, which may be
reduced by lowering the emissions, which in turn translates
into power savings, as well as, improved coexistence. Further,
the use of directive antennas offers the potential for reducing
the overall power consumption of the communication system;
especially, in high density networks, where the number of
transmitting devices can be large. Reducing the radiated
power also improves spatial re-use, global reduction of the
electro-smog and implicit improvement of the end user privacy
by focusing and segmentation. Ultimately, this relates to the
broadening of the concept of quality of service by involving
factors like environmental-friendliness, compliance to more
stringent standards – related to communication as well as
safety – and operational conditions, or even transparency to
the user.
In radio communication systems, energy may be wasted in the
following situations, some of which directional antennas can
help mitigate:
Idle listening − A node is turning on its radio to listen, but
there is no transmitter in the receiving range. Directional
antennas do not bring a solution to idle listening. In fact this
could even make the situation worse if there is no way to
change the beam orientation when the antenna is incorrectly
oriented (pointing to an area without nodes).
“Crying in the wilderness” − A node is sending when there
is no destination present. This cannot be solved by smart
antennas.
Overhearing − A node receives a radio signal but the content
is not addressed to it. In this case smart antennas will reduce
overhearing.
Collisions − Smart antennas could offer an improvement in
the case where collisions occur between traffic from pair-wise
communications; i.e. the traffic between A and B is in collision
with the traffic between C and D. With directive antennas, the
benefitt is due to improved spatial usage.
Range − To reach long range requires more energy than
short distance to transmit information with the same quality. In
this situation, adaptable directive antennas, in combination
with a suitable MAC protocol, can enhance performance by
concentrating the energy in the direction of the destination. As
such, for the same radio energy consumption, radio directional
antennas yield a longer range.
Potential applications that can benefit from using directional
antennas are (names are taken from the terminology defined
in the IST project e-SENSE):
• Wireless hospital
• Sub-network interconnection
• Store of the future
• Entertainment
In order to identify applications that would benefit from
directive antennas, the study considered the scenarios (see
Figure°1) elaborated within the e-SENSE project.
Figure 1: Application spaces, Use Cases, Applications and Scenarios
within e-SENSE
Each scenario has been analyzed with regards to the
applicability and potential benefits of using directional
antennas. The analysis highlighted for instance that scenarios
involving mobility during communication are difficult if not
impossible to benefit from directional antennas. However, the
use of directional and automatically reconfigurable (smart)
antennas would help in this regard.
The analysis has also shown that in applications where
directional antennas add benefits, the main benefits expected
are:
• Energy use improvement on both terminal and base
• Spatial reuse and system capacity improvement
• Extension of range
Another point linked to the use of directional antennas is the
need for dedicated or modified MAC (Medium Access Control)
protocols (i.e., for adaptation and control of directivity). The
study added a review of the state of the art and a classification
of MAC protocols using directional antennas. This work
classifies the typical MAC protocol algorithms with respect to
the constraints and specifics brought by directional antennas,
so that the most adequate algorithm can be chosen given the
application properties.
This work was partly funded by IST project e-SENSE. CSEM
thanks them for their support.
[1] Q. Xu, et al., “UWB Antenna with Improved Bandwidth and
Spatial Diversity using RF-MEMS Switches”, in this report,
page 80
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