Due: September 15, 2003 M. Shamos

For general homework policies, see Homework 1.

All homework must be submitted as a Microsoft Word .doc file, by email to jieh@cs.cmu.edu, with a copy to shamos@cs.cmu.edu. Homework is due by 11:59 p.m. on the announced date.

Mobile Systems

[100 points total] Third-generation (3G) cell phone technology is extremely expensive because of the new infrastructure required and the cost of frequency licenses. Two years ago, a group of companies paid 50 billion Euros for the 3G spectrum in Germany, which worked out to about $600 for every person in the country. It seems much more likely that cellphones of the future will use less expensive unlicensed frequencies and high-bandwidth protocols like 802.11g at 54 megabits/second (WiFi), or even UWB at even higher data rates. The question is whether this could ever be feasible and how much it would cost.

A) [20 points] The problem with high-bandwidth unlicensed frequencies is that transmission distances are quite short because of limitations on radiated power. Assume that a receiver must be within 200 feet of a transmitter for reliable communication. Assume that every land phone in the world (that is, telephones that are in fixed positions, such as in homes, offices and phone booths) is going to be replaced with a digital wireless 802.11g phone. How many access points will be needed so there is at least one within 200 feet of each fixed telephone? How much will it cost to buy and install the access points?

B) [20 points] A major problem is that mobile phones are not necessarily always within 200 feet of one of these access points. For example, when you are out in the woods away from any home or office you might not be able to receive signals and will have to use some other technology, like a satellite phone. However, most of us spend most of our time within 200 feet of a home, office, building or some form of roadway or transportation line, like a subway or railroad. Suppose we install enough access points so that everyone who is within 200 feet of one of these facilities will be able to communicate. (Note that this handles people who use cellphones in cars.) How many access points will be required and how much will this cost?

C) [60 points] Read about the Iridium low Earth-orbit (LEO) satellite system at www.iridium.com. Iridium is available everywhere on Earth but has high cost because of the large number of satellites required. However, it may provide a possible response to the fact that WiFi access points are not available everywhere. The idea is to use Iridium when no WiFi signal is detected at a particular place. However, when the user comes into range of a WiFi access point, the Iridium signal should be switched over to WiFi automatically to save money. Draw a block diagram of the components of a combined WiFi and Iridium system you would design to allow inexpensive worldwide communication. Explain in words how you would deal with the problem of handover when a call needs to be switched from Iridium to WiFi and vice versa. (As an example to show you the proper level of detail needed in your solution, take a look at the GSM diagram from the Wireless lecture). We are interested here in what databases will be required, what information they contain, and how the information necessary to switch a call can be obtained quickly as the user passes between transmitters. You do not need to deal with the details of the hardware involved, either the computers or the radio transmission equipment. The purpose of the question is to clarify your understanding of cell system components.