Hybrid Wireless Networks combining WLAN, FSO
and Satellite Technology for Disaster Recovery
E. Leitgeb, S. Sheikh Muhammad, M. Gebhart, Ch. Chlestil, Institute of Broadband Communications, Graz
University of Technology, Graz, Austria
U. Birnbacher, O. Koudelka, P. Schrotter, A. Merdonig, Institute of Communication Networks and Satellite
Communications, Graz University of Technology, Graz, Austria
G. Kandus, Department of Digital Communications and Networks, Jozef Stefan Institute, Ljubljana, Slovenia
Abstract—This paper presents a combination of Free Space
Optic systems with WLAN and Satellite Communications to
provide Video conferencing and data transfer for use in Civil
Military Cooperation and disaster recovery applications. A
modular communication system developed at TU Graz is
described, which facilitates worldwide access to the Internet or
any other network (depending on the particular application) by
combining Satellite Communications, FSO and WLAN networks.
Satellite communications provide the necessary backbone for
communication between distant locations in the world, Free
Space Optics allows quick installation of broadband fixed
wireless links without the need of any cables and finally, the
WLAN offers connectivity to mobile users in the network cell,
located at the target destination.
The second part of the paper presents results of a system
demonstration at a civil-military exercise in Spring 2004 in
Austria, where mobile Satellite Earth Stations were used in
combination with FSO and WLAN to enable Videoconferencing
between military and civil organizations.
Index Terms—Disaster Recovery and Management, Hybrid
Wireless, Free Space Optics, Satellite Communications, WLAN
I. INTRODUCTION
Applications for disaster recovery require broadband access
from the disaster area to the rest of the world. In the case of
destroyed terrestrial infrastructure, connections based on
geostationary satellite technology (GEO) offer quick access to
any location on earth. For practical reasons it is convenient to
set up all applications on the Internet Protocol (IP) and to use
standard networking technologies, like Ethernet for data
transmission. This is the most flexible approach which allows
to have several applications running in parallel on standard
software, using the same network connection. Another
important aspect is the flexibility of the physical connection to
the location of the application and the time which is needed
for system installation. Considering these aspects, we have
developed a transportable ground station including Free Space
Optical (FSO) connections to the actual disaster location, as
well as an optional Wireless Local Area Network (WLAN)
distribution to mobile users. Slightly adjusted TCP/IP is
implemented over the whole link and standardized Ethernet
connectivity is offered in order to use of-the-shelf user
equipment. The paper is structured as follows: After
describing each technology in some detail, the concept of the
system combining all technologies is proposed and the
realization and demonstration described.
Finally the use of the hybrid system for civil-military
cooperation and disaster management is presented. In a
CIMIC exercise example a mobile satellite earth station
(equipped with FSO and WLAN) was used for Video-
conferencing between different organisations.
II. HYBRID WIRELESS COMMUNICATION
TECHNOLOGIES
Different wireless communication technologies exist, each of
them with specific advantages and disadvantages. By putting
together a system combining those technologies by exploiting
the specific advantage, very flexible systems can be
implemented. However considerations regarding which
protocols to use and the adaptation to the specific
characteristics of the physical transmission have to be taken
into account. Hence, the characteristics of the three used
technologies are described.
A. Satellite Communications
The main advantage of satellites in the geostationary orbit 35
785 km above the Earth equator is that seen from the Earth,
they seem to remain at a fixed location, because their rotation
movement is exactly equal to the Earth´s surface. This allows
to mount ground station antennas at fixed positions always
pointing to the selected Satellite. GEO satellites can offer
access to users in large geographical regions depending on the
provided coverage area. In principal 3 GEO satellites can
cover most of the Earth’s surface. Negative aspects are the
high free-space losses due to the large distance requiring high
antenna gain for compensation and the delay time which is in
the order of 240 ms for an earth-satellite-earth link (480 ms
round-trip time). Alternative concepts with many Low Earth
Orbit (LEO) satellites avoid large directional antennas and
reduce delay, but these systems are complex and could not
show commercial success up to now. GEO connections are
state of the art and our choice of technology.
The so called “bent pipe” satellite concept uses transparent
transponders, which relay the received signal after
amplification back to the earth again.
This concept allows different applications over the same
channel and leaves freedom for new implementations in the
ground station. It also minimizes additional delay time.
Customers can rent bandwidth in fractions of a transponder
size for time periods as needed. More efficient concepts also
offer bandwidth on demand.
