Performance analysis of slotted random access channels for W-CDMA systems in Nakagami fading channels

We perform a new analysis of the performance of the wide-band code-division multiple access (W-CDMA) randomized slotted-offset random access channel with Nakagami-m fading and compare it with the performance of a comparable randomized slotted-ALOHA-based random access channel. In a previous paper, it was shown through simulations that the W-CDMA random access channel is fast, robust, and simple to implement. However, the paper assumed that any random access packet arrival events involving multiple packets from cochannel mobile stations would result in lost packets. In this paper, closed-form equations are developed for the collision process of the arriving random access packets in slotted-offset and slotted-ALOHA random access channels. An analysis is then developed of the signal-to-interference and noise ratio of a packet involved in a multipacket direct-sequence code-division multiple-access collision with a generalized Nakagami fading channel and diversity reception at the base station. Finally, a closed-form equation is developed for the bit error rate and packet throughput of the slotted-offset- and slotted-ALOHA-based random-access channels. The results of the analysis show that considerable improvement in the random-access channel performance can be achieved when multiple arrival collisions, on the same timeslot and with the same signature code, are partially resolved. The results also show the random-access channel's sensitivity to fading severity, packet E-b/N-o, spreading gain, and number of base station receive selection diversity antennas. Finally, it is shown that the W-CDMA random-access channel performs as well as comparable randomized Slotted-ALOHA designs in light fading conditions and slightly worse than Slotted-ALOHA in severe fading conditions.