GPRS vs. voice with openbsc
by Ralph A. Schmid, dk5ras
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.