Phone acting as a BTS

Harald Welte laforge at gnumonks.org
Wed Feb 24 06:25:42 UTC 2010


Hi Sylvain,


On Wed, Feb 24, 2010 at 12:06:54AM +0100, Sylvain Munaut wrote:
 
> * RX filters :
> 
> Obviously you would need to remove one and replace it with one for the
> good band. I would only replace one of them so that you can use the
> 900 band for BTS and the 1800 band for MS for example (so that you can
> still listen to a official bts and calibrate the clock).

One problem is physical:  All the BTS type SAW filters I've been able to
find (and there are very few on the market) are a number of times larger
than the MS type filters, i.e. you won't be able to mechanically fit them.

> * RX/TX switch
> 
> Those have ports that have frequency bands ... but .. do they really
> filter that much ?

no, they're really just switches.  The only difference is Rx/Tx outputs.

> * MS can't TX/RX simultaneously.
> 
> I think you can take advantage of the fact. Imagine that you only ever
> allocate channels on TS0,1 & 2. ( BCCH+SDCCH/4 + 2*TCH/F ), you could
> divide your time like this :
> 
> TS0 - Capture FCCH of a nearby station to calibrate local vcxo
> TS1 - Our BTS TS0 TX
> TS2 - Our BTS TS1 TX
> TS3 - Our BTS TS2 TX
> TS4 - Our BTS TS0 RX
> TS5 - Our BTS TS1 RX
> TS6 - Our BTS TS2 RX
> TS7 - nothing ...

Interesting idea, but I doubt it would work all that well.  A C0 of a BTS is
required to transmit continuously on all timeslots.  The first step when
scanning for BTS's is a power scan.  So if a MS does a power scan, it might
do that at a time when your poor-mans-BTS is not transmitting and thus not find
it.

There might be other reasons why a MS is having problems with a
'discontinuously trasmitting' C0 of a BTS, as it is required by the spec.

David Burgess might know more about it.  Also, this behavior could be
simulated with OpenBTS, just to see how phones react to it.

But I'd say definitely worth a try...

-- 
- Harald Welte <laforge at gnumonks.org>           http://laforge.gnumonks.org/
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