A wireless communication system as described here employs control signaling for contention-based uplink access from user equipment devices to a base station. Contention-based access configuration is performed via physical downlink control channel signaling. Configuration data sent to the user equipment devices identifies multiple contention-based access zones, along with minimum power headroom values for each contention-based access zone. A probability factor may also be used to lower collision possibility by influencing whether the user equipment devices perform contention-based uplink access. An uplink grant message can be used to acknowledge contention-based transmission; contention resolution is achieved implicitly via the uplink grant.