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...