LTE Layer 2 Understanding: Logical channel ID and Logical Channel group

LTE Layer 2 Understanding: Logical channel ID and Logical Channel group: Logical channel ID and Logical Channel group. ======================================================================== To understand this...

LTE Layer 2 Understanding: Scheduling Request

LTE Layer 2 Understanding: Scheduling Request: Scheduling Request - It is used to request the uplink resources. Suppose there is UL data from upper layer and UE does not have any resour...

Scheduling Request

Scheduling Request -
It is used to request the uplink resources. Suppose there is UL data from upper layer and UE does not have any resources for UL data transmission. So there should be some mechanism to inform the eNodeB like i have some data so please assign some resources.

There are two ways to request resources from eNodeB -
1. Using Rach procedure.
2. Scheduling Request.

Scheduling request is fast as compare to RACH as RACH make use of common resources whereas SR procedure make use of dedicated resources.

eNodeB configure the UE for SR resources, it not always mandatory for eNodeB to configure the SR resources, so in case there is no SR resources then there is only one option for resource request i.e. Rach procedure.

SR request can only be sent if UE is uplink time aligned means "TimeAlignmentTimer" is running.
If there is UL data but TimeAlignmentTimer has been expired then UE make use of RACH procedure.

SR parameter configuration - 

Logical channel ID and Logical Channel group

Logical channel ID and Logical Channel group.
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To understand this, we need to understand little about bearer (SRB/DRB). A Radio bearer is a logical connection between the UE & eNodeB.
Except SRB 0, all RB's make use of dedicated resources assigned by eNodeB.
SRB 0 is used when there is no RRC connection exist.

==> In LTE system, there exists three SRB i.e. SRB0, SRB1 and SRB2 and corresponding logical channel identities are 0, 1 and 2 respectively. These identities are fixed as per specification.

==> In LTE system, there exists at max 8 DRB and corresponding logical channel identities are 3 to 10 respectively.

DRB & EPS bearer has one to one mapping, so it means there can at max 8 EPS bearer. An EPS bearer is logical connection between UE & P-GW.

==>  Logical channel ID = DRB ID + 2 as 0, 1 and 2 identity is used for SRB.

                                       

Logical channel Group - There are maximum 4 logical channel group and each logical channel belongs to some logical channel group.
Logical channel ID are aggregated in a logical channel group based on some parameters like similar QoS.
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What is BSR and how it work ?

 What is BSR and how it work ?
BSR is buffer status report. As we know that DL/UL resource allocation is controlled by eNodeB, so for the planning for resource allocation it is necessary for eNodeB to get to know about the pending data from different UE.

Direction of BSR is from UE to eNodeB, there are three different type of BSR as follows-
1. Long BSR.
2. Short BSR.
3. Truncated BSR.
4. Regular BSR.
5. Periodic BSR.

To understand  the difference between Long, short and trucated BSR, it is necessary to know the concept of logical channel and logical channel group.

RACH Procedure

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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LTE Layer 2 Understanding: Transport Channel

LTE Layer 2 Understanding: Transport Channel: Transport channel define the characterstics of the trasmission. Uplink transport channel as follows - 1) RACH - used during random acce...

Transport Channel

Transport channel define the characterstics of the trasmission.

Uplink transport channel as follows -

1) RACH - used during random access procedure.

2) UL-SCH - Uplink shared channel used for higher layer data (user & control information)

Downlink transport channel as follows -

1) BCH - Used for system information broadcast (Ex- MIB)

2) DL-SCH - downlink shared channel used for higher layer data (user and control information) and system information.

3) PCH - used to page a UE.

MAC FAQ

1) What is BSR and how it work ?
2) What is PHR & how it works ?
3) Scheduling Request procedure.
4) RACH procedure.
5) Logical channel prioritization.
6) MAC SDU multiplexing.
7) HARQ procedure.
8) Resource allocation by MAC.
9) Use of MAC-CRNTI CE.
10) Contention resolution identity MAC CE and its use.
11) Impact on MAC in Carrier Aggregation (CA).
12) How HARQ procedure work in MIMO.
13) Timing adavnce procedure.
14) Uplink synchronization (Time alignment timer).
15) Logical channel ID & Logical channel group ID.

HARQ

HARQ is stop and wait mechanism. It is basically a layer 1 retransmission and controlled by MAC & PHY layer.
Each HARQ entity has some associated HARQ process ID. By considering the RTT of each HARQ transmission, maximum 8 HARQ process has been defined under the 3GPP standard.

1) Uplink HARQ procedure is synchronous and adpative/no adaptive.
2) Downlink HARQ procedure is non-synchronous and adaptive.

MIMO & HARQ procedure