Home Brew StarTrek Communicator
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?
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
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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