Analysis of the priority stack random-access protocol in W-CDMA systems

The use of the stack protocol (also called tree protocol) to introduce a priority mechanism on the random access stage in wide-band code-division multiple access (W-CDMA) seems more robust when compared with the slotted Aloha. In fact, the protocol stability in infinite population and the guaranteed low-contention delay appear very attractive for a random-access system proposal. In addition, from the third-generation network standpoint, defining and offering priorities in wireless communication systems appears to be the more important point for operators and system users. Indeed, after second-generation networks supporting voice service only, the third-generation systems (UNITS) should offer more services with quality and priority. However, all priorities in the UNITS system are based on the dedicated channel, which is offered after the random-access mechanism that uses the weak access protocol: slotted Aloha. In this paper, we analyze theoretically the possibility of applying the tree random-access protocol for the W-CDMA part in the UTRA radio interface proposition. We also study a priority system applied for the random access directly. This seems to be very useful to propose priority for some applications, especially the emergency application. The analytical model uses generating functions and an algebraic method in order to show the stack protocol performance. Also, numerical and simulation results are presented and show the predominance of this protocol compared with the slotted-Aloha mechanism.