The communication between the 16 terminals and the system can be divided into two stages, one is the access stage, and the other is the traffic communication stage. It can be seen that only when the terminal is correctly connected to the network can communication between the terminal and the network be possible.

The open-loop power control in UTRA FDD can be used for the initialization process of RACH (random access channel) transmission. Under normal circumstances, the open-loop power control accuracy is within the range of ±9dB. Therefore setting the transmit power based on the absolute received power will bring great uncertainty. The random access process is strictly regulated in UTRA.

The structure of a random access burst for each access slot is as shown in the figure

The access burst consists of several 1ms length preamble parts and a 10ms length message part. . There is an idle time slot of 0.25ms between the two parts. The leading part is a characteristic sequence composed of 16 complex symbols A or -A (A=1+j, where j is the mark of the imaginary part). There are 16 different such sequences. In a cell, the message about which feature sequences can be used is broadcast through the base station downlink channel.

The message part consists of a data part (corresponding to DPDCH) and a first-layer control information part (corresponding to DPCCH). These two parts are sent in parallel through two physical channels