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Harald Welte laforge at gnumonks.orgHi Gullik,
On Sun, Jan 13, 2019 at 01:32:25PM +0100, Gullik Webjorn wrote:
> > On Sat, Jan 12, 2019 at 08:22:05PM +0100, Gullik Webjorn wrote:
> > > Below is a measurement on my "most distant" BTS, BTS 1. From what I see,
> > > downlink is 35 dB
> > What do you mean by "downlink is 35dB"?
>
> This is what osmocom thinks, if I understand correctly.
>
> 110 -74 = 36 (ok, i was careless in subtracting)
>
> i.e. osmocom thinks that the level it sees is -74, and the mobile reports a
> signal of -110 , Right??
Yes. However, the big difference is that the mobile has been calibrated to
(if I remember) 2dB accuracy, while the GP-SDR based BTS has not been at all.
> Given that the mobile *can* produce up to 30 dBm, and the limesdr *can*
> produce 10 dBm,
FYI: I've never seen a LimeSDR generate anywhre near 10dBm of a GSM carrier.
> the asymmetry makes sense. But 36 dB = 4000 !!
Yes, if such readings were correct, you'd have an unbalanced link. I'm still
not really clear what you're trying to say here. I somehow have the feeling
that between the lines you're saying the weak downlink should have an
influence on the uplink? That's now how GSM power control is specified.
> Yes, I do understand this, but thought that there *was* at least a power
> loop, where osmobsc tells the mobile to increase or decrease power based on rx level.
yes, there is. There's an uplink target receive level to which the power control
loop will try to adjust. This is -75 dBm by default, and can be configured
using 'uplink-power-target' at the BTS node of the configuration.
> For a particular SDR, it should be possible to have a rough calibration
> based on the similarity of devices.
yes, it should be possible. Yet, nobody of the many users or proponents
of related devices have been working on anything like that, to the best
of my knowledge. I would love to see work in that area, and I'm
honestly surprised that nobody with an interest in this has been doing
so during the past almost 10 years of OpenBTS and OsmoBTS being out
there. At least not in the public.
> This is fine by me, and should I interpret this as the power loop in
> osmocom *works*, and that it has actually downregulater mobile power
> output to a "correct" value?
If the "L1 MS Power" as seen in "show lchan" changes depending on your path loss,
then the power control loop is working. You can also enable "logging level loop debug"
to see it in action.
> Or, has the BSC just set the power to 15 db ( as in the config file) without
> regard of the uplink level measured ( -74 dBm with uncertain accuracy )?
Simply adjust your path loss by increasing attenuation between BTS and
MS and see if the L1 MS Power increments.
Please note that "-74 dBm" are by no means -74dBm at all due to the lack
of any calibration. However, as long as OsmoBTS thinks it's -74 dBm,
it thinks it's almost exactly at the target receive level of the uplink
power control loop.
> > > L1 MS Power: 16 dBm, Timing Advance: 0
> The 16 dBm, is what the mobile reports, right?
> > How do you establish this fact? Did you attach a RF power meter to the
> > MS output and masure the output power?
> >
> I have not got to this point yet, but I have a suitable power meter, and a
> spectrum analyzer,
>
> so there are tests to be done once I have the setup stable.
>
> I am trying to get this to work the way you intended, so that enabling
> handover has a chance
> of succeeding. Some figures *you* have been aiming for would be nice....
Seriously: We are aiming for "figures" that reflect absolute power levels,
everything else feels like stumbling in the dark.
I think that the best way to go ahead is to ensure that OsmoBTS is able to
deal with loosely calibated (let's say compensated? absolute?) values.
All the algoriths deal with that as input data, and trying to get them
working without getting that first stepping stone right feels like
trying to build the first floor without the foundation first.
Regards,
Harald
--
- Harald Welte <laforge at gnumonks.org> http://laforge.gnumonks.org/
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