Home Brew StarTrek Communicator

Ian Stirling OpenMoko at mauve.plus.com
Tue Jun 12 14:55:20 CEST 2007

Dean Collins wrote:
> With the FCC 700 mhz spectrum coming up soon I thought some of you might 
> like to join us for the Yi-Tan call next week?
> http://deancollinsblog.blogspot.com/2007/06/home-brew-startrek-communicator.html

Part of the reason mobile phones work reasonably well - 2W will get you 
some 15Km or so in good conditions - is that they have relatively low 
and controlled noise levels.
The towers can have beam shaping and are closely planned, so they don't 
interfere overly with each other.
The transmissions are closely scheduled, so that interference is greatly 

With no regulation, people will want applications like - for example - 
streaming music from their home server.
Collision avoidance technologies help a little.
But the problem is that you can't do collision avoidance of signals you 
can't recieve. But those signals still interfere.

Consider an infinite plane of trancievers, evenly distributed.
As you go away from the source, the contribution of an individual 
transmitter to your noise level falls off as the distance squared.
But, the number of sources increases as the distance squared too.
The noise sums to infinity.

In practice, it won't be quite that bad - 700Mhz doesn't have infinite 
range, but it can reach hundreds of kilometers especially in some 
atmospheric conditions.

I don't know how wide the band that is proposed is - say 20Mhz.

Using really good coding, in good channel conditions, you may get 20 
megabytes a second.
Neglecting distant interference, and assuming collision avoidance works 
- you get possibly 10 megabytes/sec shared amongst the users.
With a 2Km range (say) - if you've got phones scattered every 100m, then 
there are 300 or so phones in range.
It doesn't take more than a few percent of these users to be doing stuff 
like streaming MP3 radio before it basically stops working.

And then you get people who when their connection stops working, they 
ramp up the power, or ignore collision avoidance.
It also gets worse as as the noise level rises, the bitrate drops rapidly.

Mobile phones don't really have these problems, as they are closely 
controlled by the operator.

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