RACH is used to get the initial uplink synchronization with the eNodeB. Before the starting of RACH procedure, UE is already downlink synchronized. During the RACH procedure, eNodeB allocate the UL-SCH resources, timing advance value. RACH is MAC layer procedure, please refer TS36.321 for the MAC layer specification.
There are two types of RACH procedure -
1) Contention base RACH procedure.
2) Contention free RACH procedure.
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Why we need the RACH procedure ?
1. To get the UE uplink sync.
2. To get the uplink resources.
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How we identify, UE is uplink sync or not.
There is a timer running at MAC layer, timeAlignmentTimer. If this timer is running that means UE is uplink sync.
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Who can initiate the RACH procedure ?
1. PDCCH order.
2. MAC layer
3. RRC layer.
==> Suppose MAC layer receive the UL data transfer request and UE is not uplink time aligned (uplink time aligned is decide based on the timeAlignmentTimer) then MAC layer start the RACH procedure. UL data transfer can be control plane/user plane data.
==> Suppose there UL data transfer request and UE is uplink time aligned then UE check the possibility of SR (Scheduling Request) request. Please refer the SR section for the detail.
If SR is possible then UE request the UL-SCH resources via SR procedure.
Failure case when UE is uplink time aligned - Either SR is not possible or SR procedure failed after max number of re-try then again UE will use the RACH procedure to get the UL-SCH resources.
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1. Contention free RACH - In case of contention free RACH, eNodeB provides a dedicated preamble ID using the PDCCH order message.
2. Contention base RACH - Multiple UE compete for the resources and the one who win, use the allocated resources. All rest UE again compete.
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UE requirement to start the RACH procedure.
All necessary parameters are broadcast-ed in SIB2
1. RACH Configurations
2. PRACH Configurations
PRACH Configuration parameter.
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There are two types of RACH procedure -
1) Contention base RACH procedure.
2) Contention free RACH procedure.
========================================================================
Why we need the RACH procedure ?
1. To get the UE uplink sync.
2. To get the uplink resources.
========================================================================
========================================================================
How we identify, UE is uplink sync or not.
There is a timer running at MAC layer, timeAlignmentTimer. If this timer is running that means UE is uplink sync.
========================================================================
========================================================================
Who can initiate the RACH procedure ?
1. PDCCH order.
2. MAC layer
3. RRC layer.
==> Suppose MAC layer receive the UL data transfer request and UE is not uplink time aligned (uplink time aligned is decide based on the timeAlignmentTimer) then MAC layer start the RACH procedure. UL data transfer can be control plane/user plane data.
==> Suppose there UL data transfer request and UE is uplink time aligned then UE check the possibility of SR (Scheduling Request) request. Please refer the SR section for the detail.
If SR is possible then UE request the UL-SCH resources via SR procedure.
Failure case when UE is uplink time aligned - Either SR is not possible or SR procedure failed after max number of re-try then again UE will use the RACH procedure to get the UL-SCH resources.
========================================================================
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1. Contention free RACH - In case of contention free RACH, eNodeB provides a dedicated preamble ID using the PDCCH order message.
2. Contention base RACH - Multiple UE compete for the resources and the one who win, use the allocated resources. All rest UE again compete.
========================================================================
========================================================================
UE requirement to start the RACH procedure.
All necessary parameters are broadcast-ed in SIB2
1. RACH Configurations
2. PRACH Configurations
PRACH Configuration parameter.
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