Friday, October 22, 2010

The people's phone network

Today I came across a very interesting article on the Internet.

It describes a system called "cognitive radio" which, in this case, allows the provision of cellular mobile services on an uncontrolled part of the radio spectrum known as an ISM (Industrial, Scientific and Medicine) band.

There are a number of ISM bands, some of which are recognised around the world as areas where unlicensed transmitters can be used for almost any purpose, so long as they conform to some basic rules.

Perhaps the most well-known ISM band is the one which resides between 2.4GHz and about 2.483GHz (depending a little on the country you're in). This is the band used by Bluetooth devices, wireless video cameras, radio-control systems for models and even microwave ovens.

The system in the article linked to above works on the 900MHz ISM band which is a little more suited for long-range communications but which is not as universally recognised or implemented as the 2.4GHz one.

The big bonus of ISM-band systems is that they don't require specific spectrum allocations for services and therefore allow for much lower costs.

The downside of course, is that each user of these ISM bands isn't guaranteed sole use and therefore some clever techniques have to be implemented to cope with the inevitable prospect of interfering signals created by other users.

In most cases, the interference problem is pretty readily handled by the use of spread spectrum technologies. These allow many different systems to effectively share the same piece of spectrum without significantly affecting each other.

It wasn't too long ago that spread-spectrum technology was pretty much restricted to the likes of military and scientific uses -- the hardware required to create and decode spread-spectrum (SS) signals being complex and expensive. However, as is always the case, advances have meant that SS is now very widely used.

WiFi systems are inevitably SS-based, using a technique known as direct-sequence spread spectrum (DSSS) transmission.

Another widely used SS technique is frequency-hopping spread spectrum (FHSS) which offers its own set of benefits.

Here is an article that explains spread spectrum and the various flavours thereof in a little more detail.

But back to this "cognitive radio" concept...

Some time ago (in another blog) I suggested that the 2.4GHz ISM band would be a great place to create a P2P cellular network whereby the concept of having fixed towers would become redundant. It appears as if the cognitive radio system is part-way there.

Although ISM-based P2P is unsuited to voice, its potential as a method for handling non-realtime information, such as text messages is huge.

My own experiments indicate that as little as 60mW transmitter power when used with an SS system and a small antenna suitable for hand-held devices, can deliver a range of up to 6Kms, depending on terrain and surrounds. It's clear therefore that such a system has a huge potential as a fee-free P2P SMS network, should suitable hardware become available.

Whether or not we actually see a "people's phone network" remains to be seen -- rest assured however, that the technology is here and people are getting interested already.

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