Phone acting as a BTS

Dieter Spaar spaar at mirider.augusta.de
Wed Feb 24 08:30:03 UTC 2010


Hello Sylvain,

On Wed, 24 Feb 2010 00:06:54 +0100, "Sylvain Munaut" <246tnt at gmail.com> wrote:
> 
> Since the beginning, I'm wondering if it would be possible to turn one
> of those cheap phone into a BTS. I'm n ot talking a full featured BTS
> and the mod will involve hardwares changes obviously.
> The more I look at it and the more I think it might work. No, I'm not
> crazy, please bear with me until the end of this post :)

No, you are not crazy, to be true I thought about it for a while too ;-)

It all depends on what you want to do. Just transmitting a C0 so that
other phones nearby would recognize the new BTS should be rather easy.
You can synhronize on an offical network to adjust the clock, the clock
should be stable enough for a while so that other phones can detect the
new BTS. There should also be no severe limitations from any filter
when transmitting. You are probably out of the specification for the
transceiver and maybe the power amplifier but the power should still be
good enough to cover an area large enough for experiments. Transmitting
the required SBs and FBs should not be too difficult, the DSP cannot
generate them, however its rather easy to programm the ABB with the
raw bits of a burst.

Receiving is more difficult. The filters will influence the signal
quite a lot, however it should be still possible to receive strong
signals without the requirement for a hardware modification. I
already did a few experiments to find out how far the transceiver
could be tuned out of range, for GSM-900 I was able to detect the
frequency bursts of a 840 Mhz signal with about 40 dBm less strength
than receiving a similar signal inside the GSM-900 band. 840 Mhz
would be more than enough for receiving uplink signals in GSM-900.
I did not yet tried it for the transmitter, however I expect that
it can be used to transmit downlink frequencies too.

Just as an example for the influence of the filter: The Openmoko
Freerunner uses a Calypso based Tri-Band GSM modem, its available
for GSM-900 or GSM-850, the difference is mainly a different filter
in the RX path. However you can still use the GSM-850 variant with
GSM-900 if you are close enough to the BTS.

One real problem for the "phone as a BTS" idea is the detection
of the RACH bursts. You most certainly cannot use the DSP "as is"
for this task because it does not know about the special RACH burst.
It would probably require some modification (patches) to the DSP if
you want to detect and handle them.

Best regards,
  Dieter
-- 
Dieter Spaar, Germany                           spaar at mirider.augusta.de




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