uncalibrated general-purpose SDRs (was Re: Balancing BTS'es for handover)

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Gullik Webjorn gullik.webjorn at corevalue.se
Sun Jan 13 12:32:25 UTC 2019


Thanks Harald,

On 2019-01-13 10:24, Harald Welte wrote:
> Hi Gullik,
>
> 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??

     RXL-FULL-dl: -110 dBm, RXL-SUB-dl: -110 dBm RXQ-FULL-dl: 5, 
RXQ-SUB-dl: 5
     RXL-FULL-ul:  -74 dBm, RXL-SUB-ul:  -73 dBm RXQ-FULL-ul: 3, 
RXQ-SUB-ul: 1
>
>> below uplink, which of course is natural when the MS can produce 30 dBm, and
>> the BTS which
>>
>> in this case uses a bareback Limesdr min, can only produce 10 dBm.

Given that the mobile *can* produce up to 30 dBm, and the limesdr *can* 
produce 10 dBm,

the asymmetry makes sense. But 36 dB = 4000 !!

> If you are using a general-purpose SDR hardware you cannot expect that
> any of the signal levels written anywhere actually meany anything at
> all.  There is no absolute levels reported by SDR hardware anywhere, and
> there is no calibration of either transmit nor receiver.  This means:
> * there's no general calibration curve for the chip/board
> * there's no per-unit individual calibration curve for the unit you have
>
> This is *very* different from a real GSM base station.  Without
> designing a calibration procedure for the above, as well as some
> mechanism to apply it in production, I don't think one can expect any of
> the readings to state anything realistic, nor expect any power control loops
> to operate.
>
> Please don't get me wrong, general-purpose SDRs such as USRPs or LimeSDR
> are great tools for experiments in the lab.  But that's what they are -
> at least for the time being.  There is a *big* difference between a
> real-world base station and a GP-SDR board.

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.

For a particular SDR, it should be possible to have a rough calibration 
based on

the similarity of devices. Also, manual attenuation to bring levels 
within reasonable

range can be done with sdr gain adjust or attenuation pads.


Some years ago when I was playing with OpenBTS, i spent a lot of time 
trying to get

the correct TX level and RX gain, so that up and downlink were within 
some reasonable

levels, and the BER was OK.

Being a novice with osmobts, I am aiming for the same thing, eliminating 
small issues one at the time.

>> the bsc parameter ms max power is set to 15 for both bts 0 and bts 1.
>> However, it seems MS output power is *much* stronger.
> The 16dBm is probably the closest one can get to the 15 dBm you
> requested:

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?

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 )?

>>      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....

Again, thanks for your reply, this is really fun.....and educational...

Best Regards,

Gullik





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