Uplink Capacity Maximization based on Random Access Channel (RACH) Parameters in WCDMA

Abstract

Random Access Channel (RACH) of WCDNIA system is the common uplink channel that can be used to send call request message or short packet data. However, RACH could be the interference source against traffic channels. The amount of interference power generated by RACH depends on the setting of RACH parameters such as the target Signal to Interference Ratio (SIR) of preamble and the ramp step size. When the target SIR becomes higher, RACH performance, such as access success ratio, gets improved. However, the interference caused by RACH is getting severe and can degrade the quality of traffic channels. If the target SIR becomes lower, the retransmissions of preamble can occur frequently to obtain a desirable RACH performance, and then the interference level can be increased by the retransmissions. In this paper, we analyze the uplink traffic channel capacity for a level of RACH performance in WCDMA system. The retransmission probabilities of preamble on the fast fading environment are derived by a simulation developed. The simulation results are applied to the Erlang capacity formula of uplink traffic channel. Then we drive the capacity as functions of RACH parameters. Results show that the uplink traffic channel capacity can be maximized by setting the RACH parameters carefully.