IT351 - Mobile & Wireless Computing
Tutorial_6
1. Name basic applications for satellite communication and describe the trends.
2.
Why are GEO systems for telecommunications currently being replaced by
fibre optics?
3. What characteristics do the different orbits have? What are their pros and
cons?
4.
What are the general problems of satellite signals travelling from a satellite to
a receiver?
5. Considered as an interworking unit in a communication network, what
function can a satellite have?
6. What special problems do customers of a satellite system with mobile phones
face if they are using it in big cities? Think of in-building use and skyscrapers.
Homework
1) The major application for GEO satellites is for TV broadcasting. Justify why
and explain why GEO satellites are not suitable for mobile communications
2) A 1 Mbps (microwave) satellite link transmits a digital photograph every 10
seconds from the satellite to the earth station. Assume the speed of light is 3 x
108 m/sec. The satellite is at a height of 36000 km.
a) What is the propagation delay of the link?
b) What is the minimum size of the picture file if the line is to be used 100%?
Answers
1) The traditional application for satellites is the “big cable in the sky.” i.e.,
satellites interconnect distant locations. Today, this traditional usage for
satellites is not dominant anymore. Thousands of fibres through all oceans
connect all continents offering more capacity than currently needed. However,
satellites are still required for TV/radio distribution and access to
telecommunication networks at remote places, places with destroyed
infrastructure, hostile environments etc.
2) The delay earth-GEO (Geostationary earth orbit) satellite-back to earth is
always about 250 ms. This is very high compared to delays in fibre optics.
Nothing can change this fact as the speed of light is the upper limit for the
signal propagation speed and the distance of the GEOs is almost the
circumference of the earth.
3) Characteristics, pros/cons of different orbits (see slides):
• GEO: Satellites seem to be pinned to the sky; pros: fixed antennas possible,
wide area coverage, simpler system design; cons: long delays, high
transmission power, low system capacity (difficult SDM), weak signals at high
latitudes, and crowded positions over the equator.
• LEO: low orbiting satellites; pros: low delay, lower transmission power,
inter-satellite routing; cons: high complexity, high system cost
• MEO: somewhere in-between GEO and MEO
4) Attenuation caused by the atmosphere, dust, rain, fog, snow, … Blocking of
signals due to obstacles (buildings, mountains). The lower the elevation the
longer is the way for the signals through the atmosphere, thus high output
power is needed.
5) Classical satellites were simple amplifiers that amplified the incoming
analogue signal and transmitted it again on a different frequency. The next
step came with digital signals. Satellite could then work as repeater. This
includes regeneration of the digital data and transmission of signals
representation the received data without noise (compared to analogue
amplifiers that also amplify noise). Many of today’s satellites are repeaters.
The next steps are switches/routers. Satellites can perform data forwarding
functions depending on receiver addresses and can even route data through
space from satellite to satellite.
6) Without any additional repeaters on earth, satellite phones only work outdoor
(or close to windows). Satellite signals are typically too weak to penetrate
roofs. Furthermore, satellite phones often require a line-of-sight even outdoor.
Thus, skyscrapers blocking the LOS may block communication, too.