The random access procedure of 2G and 3G mobile communication system are derived from slotted ALOHA. This paper analyzes the performance of slotted ALOHA at first, and then proves the results by simulation of the access procedure. This article proposes that the higher-power access packets would be attempted to detect after the base station detecting a collision, and analyzes the performance of this algorithm comparing to slotted-ALOHA. As results of simulation, the system throughput would increase greatly, and the access delay would decrease at the same time, if the method could be applied