View Full Version : Random Access Channel (RACH) Procedure

2008-10-03, 10:46 AM
<a href="http://www.3g4g.co.uk/Tutorial/SK/">Senthil Kumar</a>
<i>Last Updated: 28/07/2007</i>

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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
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
To start a RACH procedure,the UE selects a
random number r, between 0 and 1 and if r&lt;= 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.
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.
<ol><li>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.

</li><li>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.
</li><li>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<!-- End Main Code -->
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2009-11-10, 09:56 AM
Hi, How evaluate performance Randon Acces