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 wideband code-division multiple access (W-CDMA) seems more robust when compared with 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. Also, for third-generation networks, defining and offering priorities in wireless communication systems appears to be more important for operators and system users. Indeed, after voice only 2G networks, 3G systems (UMTS) should offer more services with quality and priority. However, all priorities in the UMTS system are based on the dedicated channel, which is offered after the random-access mechanism that uses the weak access protocol, slotted ALOHA. We analyze theoretically the possibility of applying the tree random-access protocol for the W-CDMA part of the UTRA radio interface proposition. We also study a priority system applied for random access directly. This seems to be very useful to propose priority for some applications, especially emergency applications. 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.