Throughput comparison of random access schemes in 3GPP

Abstract

Random access channel (RACH) and common packet channel (CPCH) are the two common link transport channels in 3GPP based on random access. Although their design goals were different both access schemes could be used for short message transport. To assist the system engineering of data services in UMTS it is of interest to compare performance of the two schemes. It has commonly been assumed that the normalized RACH throughput is comparable to that of slotted ALOHA and thus bounded by 36%. Additionally, it has been assumed that the collision probability is RACH is significantly higher than in CPCH due to collision detection phase introduced in the latter. In this paper we challenge these two commonly used assumptions. Using the same method we derive the probability of successful transmission for both RACH and CPCH. We then show analytically for a single demodulation resource at node B and under certain set of assumptions, that the maximum normalized throughput is greater than 80% in both cases. We make two main observations: the throughput advantage for CPCH is mainly due to the longer message size and the effect on throughput of the CD phase is not significant. For multiple resources we use simulations to obtain normalized throughput per channel resource.