Questions for Mobile Communications 2 nd ed. Jochen H. Schiller, Freie Universität Berlin, Germany schiller@computer.org, www.jochenschiller.de 1. Introduction 1.1 Discover the current numbers of subscribers for the different systems. As mobile communications boom, no printed number is valid for too long! 1.2 Check out the strategies of different 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 between 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 some hundred GHz) and not higher frequencies (in the THz range)? 2.4 What are the two different approaches in regulation regarding mobile phone systems in Europe and the US? What are the consequences? 2.5 Why is the international availability of the same 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 What are the main problems 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? What are the influences of ISI on TDM schemes? 2.10 What are the means to mitigate narrowband interference? What is the complexity of the different solutions? 2.11 Why, typically, is digital modulation not enough for radio transmission? What are general goals for digital modulation? What are typical schemes? 2.12 Think of a phase diagram and the points representing bit patterns for a PSK scheme (see Figure 2.29). How can a receiver decide which bit pattern was originally sent when a received ‘point’ lies somewhere in between other points in the diagram? Why is it, thus, difficult to code more and more bits per phase shift? 2.13 What are 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 What are 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 cell?