I understand what GRES is saying - mostly due to my [bad] experience with Bryanston-BS, in a nutshell the receiver unit on the BS is angled in such a way that when your UT sends to the BS, your UT's signal is not received by the BS receiver unit...crumbs that sounds more complicated that GRES' explanation...
It does not help to be too close to a WBS iBurst BS - due to the small number of overlapping base-stations, WBS try to increase the coverage of existing base-stations by adjusting the antenna array stuff upwards - the effect is you still get a signal from the BS, but your UT cannot get its sgnal to the BS...
I assume the bit GRES mentioned about the modem dropping its power/transmit level to be accurate, in which case it is presumably a firmware adjustable thing which WBS could fix/tweak if they really wanted to...
Presumably when the modem transmits at a higher power level, your UT to BS signal presumably bounces off stuff & eventually finds its way to the BS...?