Random access in LTE
Random access in LTE establishes or restores uplink
synchronization between a UE and an eNodeB. Random access is classified into
contention-based and non-contention-based random access:
-In
contention-based random access, the access may fail because a random access
channel (RACH) may not be allocated to the UE.
-In
non-contention-based random access, the eNodeB allocates a dedicated RACH to
the UE to ensure successful access. If dedicated RACHs are insufficient, the
eNodeB instructs the UE to initiate contention-based random access.
-The Random Access Channel (RACH) is an uplink channel in mobile communication system that is used to transfer control information from a mobile terminal to the network, e.g. for initial access to set up a connection or for location area updates. The RACH channel may be contention based channel where several users might access the same resource. There is usually no knowledge about the required transmit power of the mobile terminal and thus an open loop power control method is applied.
-User Equipments (UEs) can only be scheduled for uplink transmission if its uplink transmission timing is synchronized. The RACH, therefore, plays a key role as an interface between non-synchronized UEs and the orthogonal transmission scheme of the uplink radio. In UMTS, RACH is primarily used for initial network. Once uplink synchronization is achieved for a UE, the eNodeB (Enhanced Node B or simply base station) can schedule orthogonal uplink transmission resources for it.
no possibility to transmit user data in LTE RACH. This is exclusively sent on the Physical
uplink Shared CHannel (PUSCH).The main purpose of LTE RACH is to achieve uplink time
synchronization for UEs. Also, in LTE RACH, there is no power ramping as in the case of
UMTS, which can be a major course for latency and interference.
We should illustrate the difference between Contention-based and non-contention-based
Random access procedure is performed for the following five events:
• Initial access from RRC_IDLE;
• RRC Connection Re-establishment procedure;
• Handover;
• DL data arrival during RRC_CONNECTED requiring random access procedure;
• UL data arrival during RRC_CONNECTED requiring random access procedure;
E.g. when UL synchronization status is “non-synchronized” or there are no PUCCH resources for SR available.
Furthermore, the random access procedure takes two distinct forms:
• Contention based (applicable to all five events);
• Non-contention based (applicable to only handover and DL data arrival).
Normal DL/UL transmission can take place after the random access procedure.
-The Random Access Channel (RACH) is an uplink channel in mobile communication system that is used to transfer control information from a mobile terminal to the network, e.g. for initial access to set up a connection or for location area updates. The RACH channel may be contention based channel where several users might access the same resource. There is usually no knowledge about the required transmit power of the mobile terminal and thus an open loop power control method is applied.
-User Equipments (UEs) can only be scheduled for uplink transmission if its uplink transmission timing is synchronized. The RACH, therefore, plays a key role as an interface between non-synchronized UEs and the orthogonal transmission scheme of the uplink radio. In UMTS, RACH is primarily used for initial network. Once uplink synchronization is achieved for a UE, the eNodeB (Enhanced Node B or simply base station) can schedule orthogonal uplink transmission resources for it.
-RACH in UMTS versus RACH in LTE
In both schemes, the primary role of RACH is for initial network access but there existsno possibility to transmit user data in LTE RACH. This is exclusively sent on the Physical
uplink Shared CHannel (PUSCH).The main purpose of LTE RACH is to achieve uplink time
synchronization for UEs. Also, in LTE RACH, there is no power ramping as in the case of
UMTS, which can be a major course for latency and interference.
Difference between Contention based and Non-Contention based
We should illustrate the difference between Contention-based and non-contention-basedRandom access procedure is performed for the following five events:
• Initial access from RRC_IDLE;
• RRC Connection Re-establishment procedure;
• Handover;
• DL data arrival during RRC_CONNECTED requiring random access procedure;
• UL data arrival during RRC_CONNECTED requiring random access procedure;
E.g. when UL synchronization status is “non-synchronized” or there are no PUCCH resources for SR available.
Furthermore, the random access procedure takes two distinct forms:
• Contention based (applicable to all five events);
• Non-contention based (applicable to only handover and DL data arrival).
Normal DL/UL transmission can take place after the random access procedure.
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RACH procedure
RACH procedure sequence is as follows
- Preamble
selection: If Preamble index is not provided to UE, based on the msg3
size, power level and other parameters in RACH-CommonConfig, UE will
select a preamble index.
- RACH
Transmission: Based on the prach-config parameter, UE computes the
prach transmission opportunity and transmits preamble index on PRACH
channel. At the same time RA-RNTI is computed based on the RACH
transmission parameters.
RA-RNTI = t_id +10* f_id - PDCCH
RA-RNTI Montioring: 3ms/sub-frames after RACH transmission sub-frame,
UE will start monitoring the PDCCH channel for RA-RNTI for a duration of
RA-Response window irrespective of the measurement gap.
- RAR
Reception: If UE successfully receives Random Access Resonse (RAR) on
RA-RNTI and contains the same preamble index in the received RAPID.
If Preamble index was provide by eNB, RACH procedure ended and is considered as successful
else UE proceeds to contention resolution procedure.
-apply Timing Advancement Command
(TAC)
-update the Temp C-RNTI to the
value received (only in case of contention resolution process)
-use the received UL grant for
transmitting msg3 (only in case of contention resolution process)
- RAR
Reception: Else if UE successfully receives Random Access Resonse
(RAR) on RA-RNTI but contains back off indicator.
-set the back off parameter in UE
to the BI value received in RAR.
- RAR
Failure Case:If UE fails to receive RAR within RA-Response window or
if received RAR doesn't contain the transmitted preamble index in any of
the RAPID's, RACH procedure is considered as unsuccessful
-increment
PREAMBLE_TRANSMISSION_COUNT by 1
-if PREAMBLE_TRANSMISSION_COUNT is
PreambleTransMax +1, indicate RACH problem to upper layers.
-if the preamble was selected by
UE, UE will select a random back off time from 0 to back off parameter before
going to retry RACH procedure.
-retry the RACH procedure step
(1).
Contention Resolution Procedure
Contention resolution procedure is applicable only if
preamble index is selected by UE. UE starts/restarts contention resolution
timer for transmission of msg3 or non-adaptive HARQ re-transmisssion of msg3.
After transmission of msg3, UE will monitor the PDCCH channel for C-RNTI/Temp
C-RNTI till contention resolution timer is expired irrespective of the
measurement gap.
If UE receives data notification on PDCCH channel and
- If
C-RNTI MAC CE was included in the msg3 and if RACH was due to PDCCH
order and PDCCH transmission was addressed to C-RNTI or if RACH was
triggered by MAC itself and PDCCH transmission was addressed to
C-RNTI and contains UL grant for new transmission
-consider Contention resolution as
successful.
-stop contention resolution timer
-discard Temp C-RNTI
-RACH procedure is considered
successful.
- Else
if msg3 was CCCH data and UE receives PDCCH transmission addressed to Temp
C-RNTI and MAC PDU decodes successfully and contains the contention
resolution id in msg3.
-consider Contention resolution as
successful.
-stop contention resolution timer
-set C-RNTI to Temp C-RNTI.
-discard Temp C-RNTI
-RACH procedure is considered
successful.
- Else
consider contention resolution is not successful.
If contention resolution timer expires, contention
resolution is not successful.
If contention resolution is not successful then
- if
time alignment timer is running, stop the time alignment timer
- flush
HARQ buffer used for msg3
- increment
PREAMBLE_TRANSMISSION_COUNT by 1
- if
PREAMBLE_TRANSMISSION_COUNT is PreambleTransMax +1, indicate RACH
problem to upper layers.
- UE
will select a random back off time from 0 to back off parameter before
going to retry RACH procedure.
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