Fwd: NMT-450 - partial success

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Sipos Csaba dchardware at gmail.com
Mon Oct 26 14:16:21 UTC 2020


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
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