User`s guide

350 Chapter 15

W-CDMA Uplink Digital Modulation for Receiver Test

Understanding the PRACH

Overview

The PRACH is used by the UE (user equipment/mobile) to signal and establish

communications with a base station. It consists of two components, a preamble and message

part. However the PRACH, in an actual UE to base station application, initially transmits

only the preamble. The preamble is used to signal the base station that a UE is trying to

establish a connection, and is repeated until it receives an acknowledgment in the form of an

AICH (acquisition indication channel). Once the preamble has been identified by the base

station, the base station transmits the AICH. When the UE receives the AICH, it then sends

the message part of the PRACH transmission. The message part contains the synchronization

and request data used in establishing the base station connection.

There are two modes of PRACH operation within the ESG, single and multiple PRACH. The

single PRACH mode offers greater flexibility in configuring the PRACH signal, whereas the

multiple PRACH mode gives you the ability to transmit multiple PRACHs within an 80 ms

time period. In this mode, you can configure up to eight different UEs each having its own

signature. In addition, each UE can transmit multiple PRACHs within an 80 ms time period.

The ESG gives you the option of transmitting only the preamble or the preamble and message

part. This gives you the ability to simulate an actual PRACH transmission where multiple

preambles may be transmitted before the UE receives an acknowledgment (AICH) from a

base station. In the single PRACH mode, you can even configure the message part to transmit

when an AICH trigger is received.

In addition to controlling many other parameters of the PRACH, you can adjust the preamble

power, the message part power, the distance between the preamble and the message part, the

number of preambles prior to the message part transmission (single PRACH mode only), and

select a slot format according to the 3GPP standards.

Access Slots

The access slots provide a well defined time interval for indicating the beginning of a PRACH

transmission. An access slot is 5120 chips long (1.33 ms) and there are 15 access slots per two

10 ms radio frames (or 7.5 access slots per radio frame). A radio frame segments the

transmission into blocks of time and assists in synchronizing the mobile transmission with

the base station. So a 20 ms message part is transmitted over two consecutive radio frames.

Access slots apply to both the uplink and downlink transmissions. In an actual UE to base

station transmission, the UE transmits its access slots before it receives the base station’s