Performance analysis of the random access channel (RACH) in WCDMA

In addition to call origination and registration messages, the random access channel (RACH) in 3GPP will carry traffic including short messaging service (SMS) packets and possibly short data bursts in the absence of a dedicated traffic channel. Because of this increase in traffic volume it is important to accurately predict performance of this channel and size the resources appropriately. It has commonly been assumed that RACH throughput is comparable to that of slotted ALOHA. Using the infinite user assumption we show that the resource utilization at node B is significantly higher than e^-1. This analysis provides a close estimate of the RACH capacity when the actual protocol is in place. We present physical layer results that provide preamble detection probability at the receiver and use these results in a detailed MAC layer simulation with finite number of mobiles. We analyze the capacity per resource in node B for two message sizes and we investigate performance impacts of power capture and the maximum number of preamble cycles