Tuesday, January 22, 2019

RACH Configurations in LTE

OVERVIEW 


Why RACH Process?

In communication the most important step is to time synchronize between the receiver and the transmitter. 

1>In order to achieve the uplink synch between the UE and the eNodeB

    a RACH process is initiated.
2>Along with the  uplink synchronization user equipment gets the required            radio resources in the uplink direction to send L3/L2 message.

UE gets all the  random access  related parameter of the cell  by reading SIB2 .


When is the RACH initiated?

  1. Transition from RRC Idle to RRC Connected 
  2. Handover( for UL synch )
  3. DL data arrival when in non synch RRC mode
  4. UL Data arrival when in non synch RRC mode
  5. Re-establishing an RRC connection 
  6. For positioning  purpose when Timing advance is needed for UE's positioning.
The Random Access procedure has two types:

Contention based

1. UE selects a random access resource .
2. Used for initial access, the arrival of uplink data and re-establishment 

 Non-contention based:

1. eNB allocates a random access resource to UE 
2. Used for the connection to the target cell during UE's handover.
3. Used when RACH procedure is expected to be faster because there are               timing restrictions




RACH Procedure in Brief

UE triggers RACH procedure by sending RACH preamble to the eNB on the PRACH channel(Physical Random access Channel).

About the preambles-There are 64 preambles available in each cell.10 root sequences are used to generate the 64 preambles.Each root sequence can generate 7 preambles .By reading SIB2 parameter user comes to know about the preamble format and timing.User equipment randomly selects one of the preambles .


Incase of failure by the ENB in reading the Random Access Request the RACH preamble is transmitted again.The number of times user equipment should transmit the RACH preamble is determined by a parameter called as PreambletransMax which is a SIB 2 parameter otherwise UE will loose all its  battery if it continuously transmits the RACH preamble



During retransmission of RACH preamble user equipment will have to increase its power because ENB was not able to detect the RACH request previously.
When ENB is not able to detect the RACH request then SIB2 parameter power ramping step factor is used during retransmission of the RACH preamble if failed to detect at eNodeB.


eNodeB calculates  UE identifier called the RA RNTI(Random Access-Radio Network Temporary Identifier)
 by the timing of preamble transmission. ENB after receiving RACH Preamble derives RA RNTI from the time slot number in which preamble is received.  Calculates TC RNTI  for this UE .TC RNTI is the temporary cell radio network temporary identifier .It is used for further communication between the UE and ENB




Calculates the timing advance which is transmitted to the UE as part of response message .Resource block to be used for uplink transmission,MCS(Modulation and Coding Scheme)hopping flag,CSI(Channel State Information)field,power to be used by the UE for PUSCH, UL delay .




ENB
 includes all this information in the random access response and sends this to the UE.If RACH preamble is sent at time x then UE should expect RA response to be received  within the time gap where value of y lies between x+3<=y<=x+"random access response window" where RA response window is determined by RA response window size found in SIB2


After getting the RA response UE  saves the temporary C-RNTI 
from RA response applies  the received timing correction from the timing advance information received in the randoma access response , uses uplink resource information present in the RAR and  transmits RRC Connection request  to the ENB





UE is synched in the UL direction.UE does  not have an identity it picks a random number as an identity and sends it in the RRC connection request 
After sending RRC connection request UE starts the RRC Connection timer T300 which is broadcasted in SIB2  and waits for RRC connection setup message


The ENB
 accepts the transmission from the UE and sends RRC Connection setup in the downlink.This message contains the random number and is addressed by TC RNTI.




CONTENTION RESOLUTION

Since the UE randomly selects the preamble there are chances that two user equipment's may pick the same RACH Preamble in the UL at the same time 


a>There is collision and ENB Is not able to decode preamble sent by any UE .





ENB
 is not able to decode both the user equipments and  will run a back off timer with some random value and initiate the random procedure  again



B>There is collision and ENB is able to decode preamble only for UE A





UE A and B sends the same RACH preamble at the same time and ENB is able to detect preamble  from UE A only.If 2 different UE transmits preamble at the same time,ENB gives same RA-RNTI number for both UE in the Random Access Response(RAR)


RA RNTI will be same for both UE .Random access response(RAR) is intended to UE A.Both UE will decode RAR ,both UE will acquire the same TC-RNTI present in RAR UE B still does not know that eNodeB was not able to decode its preamble.

Both UE will choose some random number as initial identity and send RRC Connection request to ENB and start the timer T300.But ENB will not be able to detect message from UE B as UE B is using the timing advance value intended for UE A



Now ENB will send RRC Connection setup in the  downlink,both
 UE's will decode this message as its addressed by TC RNTI .eNodeB will include the random number that was sent by UE A in this message.Both UE will decode this message but random number sent and received by UE B will mismatch

At this stage it will understand it has lost out to some other UE in contention resolution.

 TYPICAL RACH PARAMETER CONFIGURATIONS IN AN LTE NETWORK

Preamble Sequence Group 

The set of 64 preamble sequences are divided into two groups: 


Contention based random access 

Group A: when UE has a relatively small quantity of uplink data, or is in poor coverage
Group B: when UE has a relatively large quantity of uplink data, or in good coverage

Non-contention based random access 

The UE selects a group B sequence if both of the following conditions are met: 

 Message Size > messageSizeGroupA  (56bits) AND 
Path Loss < Pmax (23) - preambleInitialReceivedTargetPower  ( - 108dB) - deltaPreambleMsg3  (4dB ) -messagePowerOffsetGroupB (0dB)  i.e measured RSRP>-112





Preamble Transmit Power 

PRACH Preamble Transmit Power=min{Pmax,PL+PreambleRXTargetPower}

Pmax :is the UE maximum transmit power according to the UE Power class ,eg 23 dBm for power class 3
PL(path Loss):Reference signal Transmit Power (15 dBm or 21 dBm )-measured RSRP
Preamble RX Target Power:    PreambleIntialReceivedTargetPower(-108)+DELTA PREAMBLE+(PREAMBLE TRANSMISSION COUNTER-1)*powerRampingstep(2dB)
DELTA PREAMBLE :Defines a power offset dependent upon the random access preamble format:0 dB for format 0-1 and -3 dB for format 2-3
PREAMBLE TRANSMISSION COUNTER: Counter by the UE .Its value starts from 1 ,incremented by 1 if no response is received.




                   Power Ramping in Random Access Procedure

    



Random Access Response Window



After transmitting the PRACH pramble ,the UE searches for a response during the time domain window defined by the Random Access Reponse Window(sf10)
The RA response window starts during the third subframe after the preamble
If the UE does not receive its random access response within the window
It increment the counter of the PREAMBLE TRANSMISSION COUNTER by 1
The UE exits the random access procedure if the maximum number of transmissions has been reached

mac-Contention Resolution Timer

The UE starts a contention resolution timer after transmitting the initial layer 3 message .
If the UE does not receive a response until the maccontentionResolutionTimer(sf64) expires then the UE returns to transmitting  PRACH Preambles 


Appendix


UE MAC RACH TRIGGER MESSAGE







UE RANDOM ACCESS MESSAGE PROCEDURE MESSAGE










ROOT_SEQUENCE_INDEX 

The first logical root sequence index used to create a random preamble. Different values should be assigned to neighboring cells. Be careful to change because it can cause inter-cell interference among the cells with same physical root sequence.


Typically  LTE RACH optimization operates in eNB's SON agent and EMS's SON manager.The enB SON Agent has RSI Collision Detection Function and the EMS SON Manager performs the RSI reallocation upon receiving the RSI collision /confusion notify information message


No comments:

Post a Comment