GPRS vs. voice with openbsc

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.