Thursday, April 26, 2012

Ceiling function considered harmful

Often in my 4th year mobile communications class, I'll ask a question like the following:
A cellular telephone system is intended to cover an area of 36 square km. Suppose FDMA is used, where the total system bandwidth is 100 MHz, and each call occupies 30 kHz. The system should support 40,000 simultaneous calls in the entire coverage area. Assuming a cluster size of 7, how many cells are needed?
This is a very easy question to solve, and I ask it to get students thinking about radio resources. (Granted that circuit-switched telephony is now obsolete with LTE.) The solution looks like this: at 30 kHz per call, 100 MHz supports 3333 calls; to get 40,000 calls, you need 40,000/3333, or about 12 times the system bandwidth (i.e. 12 cell clusters); with 7 cells per cluster, that's 12 x 7 = 84 cells.

But the problem is that 40000/3333 is slightly more than 12; it's exactly equal to 12 + 4/3333, or 12.00120012... . Multiplying this number by 7, we get 84 + 28/3333, or 84.00840084... .

Since the number of cells must be an integer, a typical student reaction is to take the ceiling of the answer: ceil(84.00840084...) = 85.

But that's bonkers. We "need" an extra 0.0084 of a cell, so the answer is to build a completely new cell tower to satisfy this tiny demand. Cell towers are expensive. And in fact, 12 clusters can cover 39,996 users, so the extra cell would serve a grand total of four calls. The revenue from that -- and even, let's say, the possible contractual penalties for not hitting exactly 40,000 calls -- would never be worth it.

Instead of blindly using the ceiling function in response to these questions, we should encourage students to think about the assumptions behind the question. Allowing a bit of flexibility (e.g., slightly relaxing the "40,000 call" assumption) would lead to a much more realistic and useful answer, and better engineering.

Tuesday, April 3, 2012

Thomas M. Cover, 1938-2012

I never met Tom Cover. But like every PhD in information theory, I came to know him through his book.  

Elements of Information Theory, better known as Cover and Thomas, is the gold standard for a graduate-level mathematics textbook: always readable, rigorous but never tedious, and comprehensive; one that captures both the elements and the spirit of the discipline. You have to imagine that, if The Book has a section on information theory, it's just Cover and Thomas, verbatim.

Its influence on the field has been so vast, a colleague once said, that "we're not information theorists, we're Coverists."

Thomas M. Cover August 7, 1938 - March 26, 2012

Monday, April 2, 2012

Hidden York: Power generation and distribution facilities

The chimneystack is York's most widely ignored major landmark. Yet at the base of that chimney is enough heavy machinery to generate and distribute 10 MW of electrical power (about half of the university's maximum power needs), plus all of York's heating and cooling. Without it, we would all literally freeze -- or swelter -- in the dark.

Because the university's facilities department wants to play a role in our new electrical engineering program (especially our planned Power specialization), I had the rare opportunity to take a tour of the generation, distribution, and steam generation facilities. Many thanks to Brad Cochrane and Gary Gazo for showing me around. Pictures after the jump.