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The Challenge of Molecular Communication

Bacteria communicate with molecules, and now computer scientists want to copy them. Their first task: to derive a mathematical theory of molecular communication.

Claude Shannon, perhaps the greatest unsung hero of 20th century science, laid out the fundamental problem of communication in the 1940s. He said the basic task in communication is to reproduce at one point in space a message that has been created at another.

Shannon went on to determine, among other things, how much information can be sent from A to B through a noisy channel. Today, his work is the foundation of information science and has had a profound impact on our world. TV, radio, mobile phones, computing, the internet: none of these things would be possible without Shannon’s pioneering ideas.

Since then almost all the thinking about information transmission has focused on electromagnetic communication–0s and 1s transmitted by electric fields or electromagnetic waves.

And yet for billions of years on Earth, most communication has occurred in an entirely different way, by the transmission and reception of molecules. Bacteria, for example, exchange chemical signals to estimate their local population, a process known as quorum sensing. This kind of bacterial social networking has recently gained much attention.

But the question of how much information can be exchanged via molecular communication has only recently started to be addressed. Now Sachin Kadloor at the University of Illinois at Urbana-Champaign and a few pals have taken the bull by the horns.

They consider a transmitter that emits a series of identical molecules and in which information is encoded in the release times. The transmitter sits in a fluid in which the molecules disperse by Brownian motion and are then absorbed by a receiver capable of noting their arrival times.

The trouble, of course, is the role of Brownian motion. Although the molecules are emitted with specific intervals which encode information, Brownian motion ensures that these time intervals are scrambled by the time they reach the receiver. Indeed, the molecules may not even arrive in the same order as they were emitted.

At first glance, it’s hard to imagine that any useful information can be sent in these conditions. However, Kadloor and co show otherwise.

They point out that Brownian motion introduces an uncertainty in the time of arrival of a molecule at the receiver, which they are able to quantify. The key to their thinking is that this uncertainty plays a similar role in a molecular communication channel as noise does in a conventional channel. And as we know from Shannon, it is always possible to send a message with an arbitrarily small error provided that noise is below a certain threshold.

Kadloor go on to identify various ways in which the flow of information is sensitive to factors such as the velocity of fluid flow and the rate of molecular diffusion.

All this will be of more than passing interest to biologists studying the way that cells exchange messages. But it will also be important for a the growing number of engineers trying build systems that exploit molecular communication.

Their thinking is that it may not always be possible to arrange the building blocks of computing in highly ordered, dry suburbs carved into silicon. Instead, certain computing regimes may require the same kind of disordered wetware that nature has stumbled on. In that case, molecular communication looks like a highly efficient form of communication.

That looks exciting. And with more work like this, it may not be long before quorum sensing becomes a term as familiar to computer scientists as it is to cell biologists.

Ref: arxiv.org/abs/1006.3959: Molecular Communication Using Brownian Motion With Drift

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