26 Oct
2020
26 Oct
'20
7:54 p.m.
Hello Csaba,
I am not sure if you know this, wanted to mention it: you could easily measure your SDR’s
oscillator accuracy using the kalibrate tool. It uses nearby GSM BTSs if I am correct to
give you a good estimation about how much off your oscillator is.
I used to use it with rtl-sdr a lot, because those USB dongles are crazy off in terms off
accuracy.
Once you have an offset figured out for the current temperature I guess you could try to
offset the frequency set by osmocom by that amount. Maybe even try to expose it as a
variable if needed.
Other solution would be using some kind of a GPSDO or similar 10 MHz input if the LineSDR
supports it.
Sorry again if this is known info, but wanted to chime in with some hopefully helpful
bits.
Cheers,
Domi
2020. okt. 26. dátummal, 16:08 időpontban Sipos Csaba
<dchardware(a)gmail.com> írta:
Hi Ralph,
I attached the patch for the country specific settings for Hungary, you can apply it
against "master" . TAs are not yet verified, but the phone goes
"green" with all TAs from 1 to 15. The RF part and country code are verified.
One thing, maybe your MCR has drifted, due to
ageing?!
Well, that was my first impression too, as if you look at my picture (
http://www.imagebam.com/image/d17e881357285965 ) you will see that the center of the
downlink and uplink transmissions are not exactly 10MHz apart, they have a couple kHz
drift. On the picture the two markers are indicating the nominal frequency of the channel.
As you can see the uplink burst is actually closer to the nominal frequency, compared to
the downlink. It is not in the standard (or I was not able to find it, in general the RF
spec is not very detailed...), so I am not sure if the SDRs downlink signal is off, and
the phone is correct or the other way around. Although it sure looks like a low layer
fault, as the NMT network is not even detecting is as bad or incorrect frames.
I did a loopback test with a jumper between the TX and RX of the SDR and that seemed to
work (frame level and frame quality were 90+%). On the other hand if the local oscillator
would be off, the phone would not be able to lock on the DL signal as well. I have no high
precision counter at home, but with a relatively primitive multimeter I was able to
measure 12.7999MHz on the 12.8MHz local oscillator. And both identical phones behave the
same.
The question is if I can somehow offset the uplink frequency on the SDR side or via
config/source modification to remove this couple kHz drift?
If someone has seen a lot of these signals and can take a look at the image above, would
be lovely to hear if any of the signals are off.
I will also get another more "modern" phone hopefully soon, so I can try with
that.
Regards,
Csaba
Ralph A. Schmid, dk5ras <ralph(a)schmid.xxx> ezt írta (időpont: 2020. okt. 26., H,
15:33):
Hi Chaba,
I have a Nokia handheld phone from Hungary, I can try with your settings and let you
know.
Andreas seems to be difficult to reach these days, I found the same :/
With best regards
Ralph.
From: OpenBSC [mailto:openbsc-bounces@lists.osmocom.org] On Behalf Of Sipos Csaba
Sent: Monday, October 26, 2020 3:16 PM
To: openbsc-request(a)lists.osmocom.org <openbsc(a)lists.osmocom.org>
Subject: Fwd: NMT-450 - partial success
Hi,
I know it is a bit off topic, but as osmocom-analog has no dedicated mail list and my
every attempt to contact Andreas lead to silence, I thought this is the closest one to
discuss it.
I try to create an NMT-450 network with Motorola MCR4800XL phones, and a LimeSDR-mini. As
the phones are locked to "Hungary" using a specific raster and a large gap in
the middle, first I needed to dig out the details, find out the country code and create a
patch so at least the phone is willing to lock onto the DS signal. I managed to do all
that, so now the phone is actually able to decode the network and lock onto the signal.
My issue is with the uplink: when the phone tries Traffic Area update (the phone's
uplink transmission burts is clearly seen with a spectrum analyzer), the network side is
not able to detect the uplink burst at all. Not even as bad, or incorrectly formatted
frame. Andreas has a site which describes how to set up the uplink side and do some
tests:
http://osmocom-analog.eversberg.eu/docs/sdr.html
I followed that guide and when the uplink burst from the phone arrives, the RX IQ
constellation monitor indicates a correct burst with proper power (the burst is nicely
round and in the green area). If I try to set up a call to the phone using the correct
country code and phone number, the phone clearly responds to the paging request, as the 3
paging attempt generates 3 uplink bursts. Again, with no reception/decoding on the network
side. Tried with two phones of the same type, the effect is the same.
I have two questions:
1. Where to send patches for the osmocom-analog project?
2. Does anyone have an idea what can be wrong with my setup?
One more thing I noticed: compared to the channel frequency used to set the NMT network
up, the uplink is a couple kHz shifted:
http://www.imagebam.com/image/d17e881357285965
As it can be seen, the uplink burst appears 3-4kHz left relative to the downlink signal.
The network is started with the following command:
nmt -k 239 -k 235 -Y HU,1 --limesdr-mini --sdr-rx-gain 20
Any and all help is appreciated.
Regards,
Csaba
<0001-Add-country-specific-settings-for-Hungary.patch>