Wednesday, October 11, 2006

Network Coding: Unprecedented Opportunities

A new discipline called network coding uses broadcasting to save bandwidth by coding and then relaying bits for more than one receiver. Decoding involves combining several transmissions, including your own. Using this principle, we can build a telephone system in which the default is that everone can hear anyone --- a wireless party line. Stock traders, emergency workers and perhaps conference callers might find it particularly useful.

The broadcast radio spectrum, which has typically been regarded as limited and interference-plagued, will become open and accessible everywhere by anything. Of course, some broadcasters and registered networks will still rely on keeping certain airwaves empty and silent, and they will be used by legacy devices that we are loath to discard, such as cell phones and AM radios. But grander possibilities await radios that cooperatively sense one another's proximity, use one another to economize on radiated energy and battery life, and turn ever more remote regions of the spectrum into fertile territory for personal use.

Disparate demonstrations paint this new picture of wireless communications. For example, the multipath phenomenon, in which buildings and walls bounce multiple copies of a signal to a receiver, was once just the source of ghosts in TV pictures. But in essence, those reflectors are also sending additional energy that would have been lost. Thus, they can also be regarded as independent transmitters. Multiple-input, multiple-output radios built to take advantage of that effect can improve communications.

Other work has built Ad Hoc networks of mobile radios that at each moment dynamically select for intermediate relays requiring the least energetic connections. One radio might momentarily be in a dead spot, but another will be in a hot spot for passing on a communication. As radios become cheaper than their batteries, adding power to make reliable systems. Even more important, the dichotomy between broadcasting and point-to-point connectivity disappears; the two work together by design.

The broadcast nature of wireless is thus a feature rather than a bug: it can save energy, increase efficiency and nurture new ideas. And spectrum need not be regarded as a fixed and finite resource to be divided among users. Instead it can support more communication as more communicators use it. The theory has been in place for a few years, but now it is becoming ral. The pie will soon start to grow, and there will be enough slices for all.