RACH is a common transport channel
in the uplink and is always mapped one-to-one onto physical channels
(PRACHs). In one cell, several RACHs/PRACHs may be configured. If more than
one PRACH is configured in a cell, the UE performs PRACH selection randomly.
The parameters for RACH access
procedure includes: access slots, preamble scrambling code,preamble
signatures,spreading factor for data part, available signatures and
subchannels for each Access Service Class(ASC) and power control information.
The Physical channel information for PRACH is broadcasted in SIB5/6 and the
fast changing cell parameters such as uplink interference levels used for
open loop power control and dynamic persistence value are broadcasted in
SIB7.
RACH access procedure follows
slotted-ALOHA approach with fast acquisition indication combined with power
ramping in steps.
Maximum of 16 different PRACHs can
be offered in a cell, in FDD, the various PRACHs are distinguished either by
employing different preamble scrambling codes or by using common scrambling
code with different signatures and subchannels.With in a single PRACH, a
partitioning of the resources between the maximum 8 ASC is possible, thereby
providing a means of access prioritization between ASCs by allocating more
resources to high priority classes than to low priority classes.ASC 0 is
assigned highest priority and ASC 7 is assigned lowest priority.ASC 0 shall
be used to make emergency calls which has got more priority.The available 15
access slots are split between 12 RACH subchannels.
The RACH transmission consists of
two parts, namely preamble transmission and message part transmission. The preamble
part is 4096 chips, transmitted with spreading factor 256 and uses one of 16
access signatures and fits into one access slot.
ASC is defined by an identifier i
that defines a certain partition of the PRACH resources and is associated
with persistence value P(i). The persistence value for P(0) is always set to
one and is associated with ASC 0.The persistence values for others are
calculated from signaling. These persistence values controls the RACH
transmissions.
INITIATING THE RACH PROCEDURE
To start a RACH procedure,the UE
selects a random number r, between 0 and 1 and if r<= P(i), the physical
layer PRACH procedure is initiated else it is deferred by 10 ms and then the
procedure is started again. Once the UE PRACH procedure is initiated, then
the real transmission takes place.
PREAMBLE AND DATA PART
TRANSMISSION
As described above, the preamble
part transmission starts first.The UE picks one access signature of those
available for the given ASC and an initial preamble power level based on the
received primary CPICH power level and transmits by picking randomly one slot
out of the next set of access slots belonging to one of the PRACH subchannels
associated with the relevant ASC.
The UE then waits for the
appropriate access indicator sent by the network on the downlink Acquisition
Indicator Channel(AICH) access slot which is paired with the uplink access
slot on which the preamble was sent.There are 3 possible scenarios possible.
- If the Acquisition
Indication(AI) received is a positive acknowledgement, then UE sends the
data after a predefined amount of with a power level which is calculated
from the level used to send the last preamble.
- IF the AI received is a
negative acknowledgement, the UE stops with the transmission and hands
back control to the MAC layer.After a backoff period, the UE will regain
access according to the MAC procedure based on persistence
probabilities.
- If no acknowledgement is
received, then it is considered that network did not receive the
preamble.If the maximum number of preambles that can be sent during a
physical layer PRACH procedure is not exceeded,the terminal sends
another preamble by increasing the power in steps.The ability of the UE
to increase its output power,in terms of steps to a specific value is
called as open loop power control. RACH follows open loop power control
References :
1. Performance Analysis of RACH
procedure in WCDMA by Jeyaratnarajah Niththiyanathan
2. 3GPP TS 25.214, 3rd Generation
Partnership Project: Physical layer procedures (FDD), V5.1.0
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