Hi out there,
I am using OpenBTS with an USRP1, so not absolutely on-topic here, but I have here some effects with GPRS that are a bit annoying, and I would like to ask if something similar is known with the openbsc familiy when using GPRS.
GPRS works, but with a reduced operating distance. While voice and SMS work over 200m (USRP1, WBX, small omni antennae) GPRS stops after 20-25 m, when downlink level is around -70dBm.
Digging a bit deeper showed me that GPRS uses a somehow strange power control model, not a closed loop but a kind of dedicated guess, derived from the downlink RSSI at the ME. Setting the corresponding gamma value has a strong influence, but even with its lowest value I do not reach more than those 25m, and I can reduce the operating range to 2m with it. Playing with rxgain and other parameters did not really improve things.
Is similar behavior known with the openbsc familiy GSM systems? I even know similar effects from commercial operators (O2 Germany with EDGE), not knowing if it is the same issue or something completely different. A first glance at some documents showed me that this power level calculation seems to be normal - is there really no closed loop for GPRS available?
With best regards
Ralph.
Ralph A. Schmid, dk5ras wrote:
Digging a bit deeper showed me that GPRS uses a somehow strange power control model, not a closed loop but a kind of dedicated guess, derived from the downlink RSSI at the ME. Setting the corresponding gamma value has a strong influence, but even with its lowest value I do not reach more than those 25m, and I can reduce the operating range to 2m with it. Playing with rxgain and other parameters did not really improve things.
hi ralph,
maybe try this: use a gamma of 0. (this means that the maximum power of your phone is not reduced.) use an alpha of 0. (this means no reduction depending on the downlink RSSI.) also be sure to set MS_TXPWR_MAX_CCH to 33dbm, so the maximum allowed output power of the mobile is 2w, if supported. (at openbsc the parameter is "ms max power 33")
best regards,
andreas
maybe try this: use a gamma of 0. (this means that the maximum power of your phone is not
This I already am using, with only partial success, reaching about 10% coverage, compared to voice.
reduced.) use an alpha of 0. (this means no reduction depending on the
With the alpha I did not yet play very much, has to be tested this evening :)
downlink RSSI.) also be sure to set MS_TXPWR_MAX_CCH to 33dbm, so the maximum allowed output power of the mobile is 2w, if supported. (at openbsc the parameter is "ms max power 33")
My license allows only 100mW :) Anyway, this leads to jamming the receiver when nearby subscribers drive the AGC (?) into limiting the signal, and not recovering fast enough to decode the next (weaker) timeslot.
best regards,
andreas
Ralph.
Ralph,
On Mon, Feb 3, 2014 at 4:06 PM, Ralph A. Schmid, dk5ras ralph@schmid.xxx wrote:
Digging a bit deeper showed me that GPRS uses a somehow strange power control model, not a closed loop but a kind of dedicated guess, derived from the downlink RSSI at the ME.
GPRS has two modes of operation - closed loop and open loop. What you're describing here and what is implemented in both OpenBTS and OsmoPCU. I'm pretty sure that commercial networks use closed loop control nowadays, and it's a TBD item in OsmoPCU.
Is similar behavior known with the openbsc familiy GSM systems? I even know similar effects from commercial operators (O2 Germany with EDGE), not knowing if it is the same issue or something completely different.
It's a different issue with EDGE. EDGE uses 8-PSK modulation scheme, which is a bit more more sensitive to noise than GMSK and thus may not work at the edge of coverage.
Hi,
GPRS has two modes of operation - closed loop and open loop. What you're describing here and what is implemented in both OpenBTS and OsmoPCU. I'm pretty sure that commercial networks use closed loop control nowadays, and it's a TBD item in OsmoPCU.
OK, I understand.
It's a different issue with EDGE. EDGE uses 8-PSK modulation scheme, which is a bit more more sensitive to noise than GMSK and thus may not work at the edge of coverage.
Well, it is only with one operator, and the problem came up when they implemented EDGE, it only worked when you could see the BTS. Others have EDGE running until the end of coverage without any problems :)
-- Regards, Alexander Chemeris.
Ralph.
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Alexander Chemeris -
Andreas Eversberg -
Ralph A. Schmid, dk5ras