User`s guide

Chapter 15 351
W-CDMA Uplink Digital Modulation for Receiver Test
Understanding the PRACH
transmission of access slots, thus creating an offset between the two. This offset is used to
time the reception of the AICH by the UE. This timing/offset of the AICH is called Tp-a (the
distance from the beginning of the preamble to the beginning of the AICH transmission) and
is defined in the 3GPP standards. The PRACH also uses the access slots to set the
timing/distance between preamble transmissions (Tp-p) and the preamble and the message
part (Tp-m). Figure 15-3 shows these timing relationships. You will notice that all
measurements are from the beginning of one component to the next.
Figure 15-3 Access Slot Timing
The preamble is 4096 chips in length (1.067 ms), so it does not occupy the entire length of an
access slot. When a 10 ms message part (7.5 access slots) is used, it only partially occupies the
last access slot.
In multiple PRACH mode, the ESG provides you access slot resolution. Because of this, you
can select the access slot for the beginning of your PRACH transmission. However when
transmitting multiple PRACHs for the same UE with the ESG, they cannot occupy any of the
same access slots within an 80 ms time period. If they do, the subsequent (second PRACH,
etc.) PRACH transmission that uses any of the same access slots, will not occur. See
“Understanding the 80 ms Transmission/Time Period” on page 375 for more information.
Preamble
The preamble is used to signal a base station that a UE is trying to establish communications.
It is composed of a complex signature and scrambling code that is used by the base station to
uniquely identify the UE. This is a 1.067 ms bursted signal, and is the first part of the
Tp-a
Preamble
AICH
Tp-m
Message Part
AICH Access
Slots RX at UE
PRACH Access
Slots TX at UE
Tp-p
One Access Slot