Spectral efficiency is improved by assigning uplink channels according to expected receive signal levels at a serving base station. A first subscriber device detects a new call request indication, and then determines, as a function of its transmit power level and current location relative to the serving base station, an expected receive signal strength indication (RSSI) at its serving base station. Depending on whether it is determined that the expected RSSI is greater than or less than a RSSI threshold: one of (i) transmitting a call request to the serving base station requesting an assignment to a respective designated high-power or low-power channel for the new call and (ii) transmitting the new call on a respective pre-allocated high-power or low-power channel.