Summary of GSM RF Power control

historical

On Fri, Jun 19, 2009 at 12:05:47AM +0200, Harald Welte wrote:
> On Thu, Jun 18, 2009 at 11:58:48PM +0200, Harald Welte wrote:
> > On Thu, Jun 18, 2009 at 11:32:55PM +0200, Harald Welte wrote:
> > > If we use the knowledge of the behavior as described above, we can also deduct:
> > > 
> > > * the BS-11 is configured to a NM_ATT_RF_MAXPOWR_R of 0, i.e. it will transmit
> > >   with the power level that is configured by LMT / ipaccess_config.  By default
> > >   this is set to 30mW 
> > > 
> > > * the nanoBTS 900 has 20dBm (1800 is 23dBm) TRX output power.  bsc_hack is 
> > >   configured to a NM_ATT_RF_MAXPOWR_R of 12, i.e. 24dB.  This means we are
> > >   transmitting with a mere -4dBm (398uW) or -1dBm (794uW) which would be _really_
> > >   low.  So either the nanoBTS are not following specs, or we're really
> > >   transmitting something that would barely be possible to receive.  Or my
> > >   calculations are wrong ;)
> > 
> > I've now also realised that every CHANNEL ACTIVATE message contains a BS power
> > and MS power IE with 0x0f as value, i.e. another 30dB decrease in initial power
> > levels.  This really makes me suspicious...  at levels that low, everything
> > should never be working.  Probably the MS and BTS cannot operate below a
> > certain level, and thus they choose whatever is the minimum level they support.
> 
> For the BTS I'm still puzzled.  For the MS, a power level of 0xf (15) means:
> 13dBm (19mW) output power in GSM900 and 0dBm (1mW) in GSM1800 - which would
> probably work well unless there's actual uplink interference on the respective
> channel.

However, what contradicts this is:

The BCCH never gets 'activated', i.e. we just set the 12.21 CHANNEL ATTRIBUTEs
and don't call a 08.58 'ACTIVATE CHANNEL'.    12.21 CHANNEL ATTRIBUTEs don't
allow us to set any power level.  Thus the TS0 should currently be instructed
to transmit at the maximum TRX power.

Since the TRX that carries TS0 cannot employ any power control but has to
operate at constant power, the BTS will have to use the highest power of any
TS on that TRX.

What we could try is to send a 'BS POWER CONTROL' 08.58 message containing a
'BS POWER' IE for TS0 and see if that further reduces the TRX power.  I think
especially on the nanoBTS this is desirable, since it's nominal output can not
be reduced to 30mW like we can do for the BS-11 with lmt or bs11_config.

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