Designing microchannel molecular communication systems: Paper in Nano Communication Networks

I've got a new paper in Nano Communication Networks. (Of the five journal papers I've co-written on molecular communication, this is the last to be submitted and the first to be published ... quick review process FTW. But on the other hand, it's published by Elsevier.)

N. Farsad, A. W. Eckford, S. Hiyama, and Y. Moritani, “Quick system design of vesicle-based active transport molecular communication by using a simple transport model,” Nano Communication Networks, doi:10.1016/j.nancom.2011.07.003, 2011. [PDF]

In this paper, we're continuing our work on microchannel molecular communication (see also here), where molecular motors are used to transport the message-bearing molecules (MBMs).

Generating Lévy random variables from Gaussian (Updated)

Update August 17, 2011: Corrected a typo in the distribution.

Sometimes when I learn a neat mathematical trick, I write a blog post so I don't forget it.  This is one of those times.

If a random variable x has the Lévy distribution with parameters c and μ, then the pdf of x is given by

This distribution has applications in economics, finance, and physics. A famous statistical application is the first passage time of a Brownian motion: if w(t) is given by the Wiener process (with initial condition w(0) = 0), and x is the first time that w(x) = d, then x is Lévy distributed with c = d^2 and μ = 0.

Here's the trick, which is hard to find in the literature: let z represent a Gaussian-distributed random variable with mean 0 and variance v. Then 1/z^2 is distributed Lévy with c = 1/(2v^2) and μ = 0. In other words, if you have a good Gaussian random number generator, you can use it to quickly generate Lévy-distributed random variables!

This property is very briefly mentioned in the following paper (I had to do a bit more digging to verify it and get the right parameter values):

J. M. Chambers, C. L. Mallows, and B. W. Stuck, "A method for simulating stable random variables," J. Am. Stat. Soc., vol. 71, no. 354, pp. 340-344, Jun. 1976.

Talking about wireless: Elizabeth May's advice

Green Party leader and Member of Parliament Elizabeth May recently caused a stir by expressing concern over the safety of electromagnetic radiation. And here she is again this past weekend, taking an interview with her BlackBerry set to speakerphone, to avoid radiation exposure.

I'm not going to go into her concerns about safety (with which I largely disagree; if you're interested in the debate, an excellent summary is here). At the very least, May makes the valid point that there is no scientific consensus on wireless safety.  Fair enough!

But in her longer critique of wireless safety (found here on her blog), and by way of her subsequent actions, May gave some advice to those who are concerned about their exposure to EM radiation. Paraphrasing, she said this:

1. When using your phone, use speakerphone or text rather than holding it up to your ear.
2. You can use a smartphone, but don't keep it in your pocket.
3. At home, use a wireline network, not WiFi.
4. Don't get a Smart Meter.

What I will talk about is this: for people who are concerned about radio exposure (which doesn't include me), did Elizabeth May give good advice? It seems to me that #1 is good advice, #2 and #3 are questionable, and #4 is wrong to the point of embarrassing.

First Shannon Award Livetweet

Announced moments ago at the ISIT 2011 banquet in St. Petersburg, Russia: Congratulations to Abbas El Gamal on his well-deserved Shannon Award.

Livetweeted from the event by Johann Briffa:

And now a word from our sponsors (repost)

(This post was originally published in March, but was taken down as -- unknown to me at the time -- the results of the Discovery competition were under embargo. Now that the results are public, I'm republishing the post.)

NSERC Discovery results came out yesterday.  This is the program that supports almost all of the curiosity-driven, non-industrial, basic science and engineering research in the country.  It's generally not a huge amount of money -- enough to pay 1-2 graduate students is typical.  But most Canadian professors hold one, and it probably pays the salary of the majority of Canada's science and engineering graduate students.

Just now, a summary of the competition landed in my inbox.  By now, the "new regime" of Discovery funding is well known: applications are assigned a quality score, and a pot of money is assigned to each score value, divided among the applications with that value; below some score the amount is zero.  Applications are now "memoryless", meaning that the status and funding level of your last application have no bearing on your current application (I would argue this is bad for all kinds of reasons, but that's another discussion. See Ghoussoub's excellent blog for detailed summaries and a discussion of what's going on with NSERC.)

But the following details were interesting:

Let's get rid of Transactions letters (Updated)

Update August 11, 2011: According to an editorial in the August issue of Transactions on Wireless Communications (not yet available on IEEExplore), both Trans. Comm. and Trans. Wireless Comm. will stop accepting letter submissions as of September 1, 2011.  Original post follows.

Both the Transactions on Communications and Wireless Communications accept "letters". There are minor differences between the two journals, but letters are most commonly used as "enhancement of previously published work", i.e., minor results that fall short of a full paper. In both cases there are stringent length requirements: 10 double-spaced pages in the Transactions draft format excluding figures, which works out to about 4 pages in the two-column format.

Anyone who has written a journal paper -- or even a conference paper -- knows that 10 double-spaced pages is barely enough to say anything.  As both a reviewer and an author, I've seen the following dynamic happen over and over:

1. Author discovers an interesting little result. Not being quite enough for a paper, author writes a letter, leaving out details that s/he considers irrelevant to the overall point of the work, in order to meet the length requirement.
2. Reviewers are not satisfied with the level of detail in the paper. Ignoring scope and length requirements for letters, reviewers demand more detail, additional simulations, longer explanations, and so on.
3. Author adds the absolute minimum the reviewers demand, and then cuts, edits, shortens, and otherwise takes a butcher knife to the paper to meet the length requirement.
4. Go to 2.

"Letters" end up in a kind of limbo, delaying the review process and making nobody happy, even if the paper is eventually accepted. 

The problem seems to be that reviewers treat "letters" like regular papers. So why not make it official and drop the "letter" from the Transactions? Short papers could be handled just like regular papers, with the understanding that the magnitude of the paper's contribution should be proportional to its length. The Transactions on Information Theory did this years ago.

Papers at CWIT: PDFs and video

I posted earlier about my papers at CWIT.  I finally got around to uploading the PDFs and video.


J. P. K. Chu, A. W. Eckford, and R. S. Adve, “Distributed optimization of the Bhattacharyya parameter in wireless relay networks,” in Proc. 12th Canadian Workshop on Information Theory, Kelowna, BC, 2011. [PDF] (Video not available.)

L. Cui and A. W. Eckford, “The delay selector channel: Definition and capacity bounds,” in Proc. 12th Canadian Workshop on Information Theory, Kelowna, BC, 2011. [PDF]


Presentation video for the Cui/Eckford paper: