When UE Initiate Random Access Procedure In LTE:
- Transition from
RRC Idle to RRC Connected mode.
- Completing an
Intra-System Hand Over.
-
Uplink data arrives while UE is in
Non-synchronized RRC Connected State(Time alignment Timer Expiry).
- Downlink data arrives while UE is in
Non-synchronized RRC Connected State(Time alignment Timer Expiry).
- RRC Connection
Re-establishment.
PRACH : Physical Random Access Channel:
- Used to transfer RANDOM ACCESS PREAMBLES to initiate RANDOM ACCESS PROCEDURE.
- Do not transfer RRC Signaling Messages or Application Data.
RACH - Config Common Information:
 |
| RACH-ConfigCommon IE |
 |
| RACH-ConfigDedicated IE |
RACH Procedure:
-
RACH Procedure can be
-
Contention Based.
-
Non Contention Based.
-
Contention Based procedure involves the
UE selecting a Random Access Resource i.e. UE selecting a PRACH resource, a Preamble Sequence and the next available Subframe for PRACH transmission.
-
Non-Contention Based procedure involves the
eNodeB allocating the
Random Access Resource i.e. eNodeB allocating
ra-PreambleIndex and
ra-PRACH-MaskIndex.
- Contention Based RACH Procedure can be applicable for
all RACH reasons but Non Contention Based RACH Procedure can be applicable for :
- Completing an
Intra-System Hand Over.
-
Downlink data arrives while UE is in
Non-synchronized RRC Connected State.
Random Access Resource Selection:
Random Access Group Selection:
If (
ra-PreambleIndex == Allocated By eNB &&
ra-PRACH-MaskIndex == Allocated By eNB)
{
If (
ra-PreambleIndex != 000000)
{
Random Access Preamble && PRACH Mask Index =
Allocated By eNB.
}
}
Elseif
{
If (MSG3 == Not Transmitted Yet)
{
If (
Random Access Preambles group B == present)
{
If (
sizeof(MSG3) > messageSizeGroupA &&
pathloss < (P-CMAX,c– preambleInitialReceivedTargetPower – deltaPreambleMsg3 – messagePowerOffsetGroupB))
{
select the
Random Access Preamble from
Random Access Preambles group B
}
Else
{
select the
Random Access Preamble from
Random Access Preambles group A
}
}
Else
{
select the
Random Access Preamble from
Random Access Preambles group A
}
}
Elseif (MSG3 == Re-transmitted)
{
select the
Same Group of
Random Access Preambles = used for the preamble transmission attempt corresponding to the
first transmission of Msg3
}
}
Random Access Preamble Selection Within the Group:
- Randomly select a Random Access Preamble within the selected group.
- The random function shall be such that each of the allowed preamble will have equal probability.
- set PRACH Mask Index to 0.
PRACH Resource Selection:
- Subframe for PRACH transmission selected using restrictions given by the
prach-ConfigIndex and the
PRACH Mask Index.
- physical layer timing requirements i.e. UE may take into account the possible occurrence of
measurement gaps when determining the
next available PRACH subframe.
Random Access Preamble Power :
-
PREAMBLE_RECEIVED_TARGET_POWER =
preambleInitialReceivedTargetPower (RACH-ConfigCommon) +
deltaPreambleMsg3 (UplinkPowerControlCommon) + (
PREAMBLE_TRANSMISSION_COUNTER – 1) *
powerRampingStep (RACH-ConfigCommon)
- Physical layer transmit a preamble using the selected
PRACH, corresponding
RA-RNTI,
preamble index and
PREAMBLE_RECEIVED_TARGET_POWER
Random Access Response (RAR) Reception:
- Once RACH is transmitted
UE shall monitor PDCCH regardless of the possible occurrence of a measurement gap.
- RAR is identified by
RA-RNTI.
- RA Response window starts at subframe that contains the
end of the preamble transmission +
three subframes.
- RA Response window has a length
ra-ResponseWindowSize(SIB2) subframes
- The RA-RNTI is calculates as :
RA-RNTI= 1 + t_id+10*f_id
where t_id = Subframe within which start of preamble was transmitted(0<= t_id<=9).
f_id = Frequency domain index of the PRACH within the subframe.(0<= f_id<=5).
- For FDD there can be maximum one set of RBs allocated to PRACH within a subframe so f_id is always 0.
- So for
FDD RA-RNTI= 1 + t_id.
