Random access, realized in WCDMA through the Random Access Channel (RACH), is the mechanism through which the mobile terminal performs the initial access to the system. As the terminal is not yet connected to the system, no uplink codes have been allocated, nor is there any control of the uplink power. Thus, mechanisms for RACH code allocation and power setting were needed. To minimize signaling and required power, a solution was adopted in WCDMA in which no explicit signaling of codes is needed. All RACH-related codes are derived according to given rules using only the downlink, that is the transmission from the network, scrambling code and a signature that is randomly selected by the mobile device.
As illustrated in Figure 10, in WCDMA preambles with randomized signatures are sent (uplink) with increasing power in a “ramping” procedure. When the power is sufficient, the network transmits an acquisition indicator to request transmission of the actual message from the terminal. This ramping procedure is very fast and together with the signature randomization leads to an efficient, high-capacity RACH in WCDMA [17]. In contrast, the IS-95/Cdma2000 standard uses random access parameters derived from the serial number of the mobile device and is based on time randomization instead of signature randomization. In IS-95/Cdma2000 the entire random access message is transmitted and retransmitted until received by the network,which is less efficient.
References : http://www.ericsson.com/
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