Visvesvaraya Technological University (VTU) 2007 B.E Computer Science on mobile communications - Question Paper

1. Introduction
1.1 Discover the current numbers of subscribers for the various systems. As mobile
communications boom, no printed number is valid for too long!
1.2 Check out the strategies of various network operators while migrating towards third
generation systems. Which are reasons why a single common system is not in sight?
2. Wireless Transmission
2.1 Frequency regulations may differ ranging from countries. Check out the regulations valid
for your country (within Europe the European Radio Office may be able to help you,
www.ero.dk, for the US try the FCC, www.fcc.gov, for Japan ARIB, www.arib.or.jp).
2.2 Why can waves with a very low frequency follow the earth’s surface? Why are they
not used for data transmission in computer networks?
2.3 Why does the ITU-R only regulate ‘lower’ frequencies (up to a few hundred GHz) and
not higher frequencies (in the THz range)?
2.4 elaborate the 2 various approaches in regulation regarding mobile phone systems
in Europe and the US? elaborate the consequences?
2.5 Why is the international availability of the identical ISM bands important?
2.6 Is it possible to transmit a digital signal, e.g., coded as square wave as used inside a
computer, using radio transmission without any loss? Why?
2.7 Is a directional antenna useful for mobile phones? Why? How can the gain of an
antenna be improved?
2.8 elaborate the main issues of signal propagation? Why do radio waves not always
follow a straight line? Why is reflection both useful and harmful?
2.9 Name several methods for ISI mitigation. How does ISI depend on the carrier
frequency, symbol rate, and movement of sender/receiver? elaborate the influences
of ISI on TDM schemes?
2.10 elaborate the means to mitigate narrowband interference? What is the complexity of
the various solutions?
2.11 Why, typically, is digital modulation not enough for radio transmission? elaborate
general goals for digital modulation? elaborate typical schemes?
2.12 Think of a phase diagram and the points representing bit trends for a PSK scheme
(see Figure 2.29). How can a receiver decide which bit trend was originally sent
when a received ‘point’ lies somewhere in ranging from other points in the diagram? Why
is it, thus, difficult to code more and more bits per phase shift?
2.13 elaborate the main benefits of a spread spectrum system? How can spreading be
achieved? What replaces the guard space in Figure 2.31 when compared to Figure
2.32? How can DSSS systems benefit from multipath propagation?
2.14 elaborate the main reasons for using cellular systems? How is SDM typically realized
and combined with FDM? How does DCA influence the frequencies available in other
cells?
2.15 What limits the number of simultaneous users in a TDM/FDM system compared to a
CDM system? What happens to the transmission quality of connections if the load
gets higher in a cell, i.e., how does an additional user influence the other users in the

Earning:   Approval pending.