Methods, apparatus, and systems for measuring, on a per-QoS-class basis, the average number of user devices active on the uplink of a wireless communication system are disclosed. In an exemplary method, a number of mobile terminals with buffered data for transmission to a base station is estimated based on received buffer status reports. An estimated quantity of active mobile terminals is calculated, based on the received buffer status reports and a number of mobile terminals for which a semi-persistent scheduling grant has been granted. In some embodiments, the estimated quantity of active mobile terminals is estimated on a per-traffic-class basis, in which case the technique includes calculating first and second estimated quantities, corresponding to mobile terminals having active data bearers for first and second traffic classes, respectively, based on the received buffer status reports and the number of mobile terminals for which a semi-persistent scheduling grant has been granted.