CDMA controls handset's transmit power in a real time loop, the mathematics requires to know the signal loss from the handset to the base station to figure out what the original signal coming out of the handset was. From what I just googled about GSM, it has power control, but only 8 levels, .8 watts is the lowest, then 2 watts. 99% of GSM handsets have a max transmit power of 2 watts anyways.
