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

Multi Carrier Settings-Cell Reselection in LTE for Intrafrequency and Interfrequency

CELL RESELECTION LTE

Brief Overview:

Cell Re selection is a procedure used to change the cell after the UE is camped on a cell and stays in the Idle mode.This procedure is used to let the UE get camped on a cell which has the best conditions among all the other cells on which the UE is allowed to camp on .
Cell reselection is done by first checking the :

1.PRIORITY
2.RADIO LINK QUALITY
3.CELL ACCESSIBILITY

PRIORITY

This is a parameter specifying the priority of EUTRA-FA(Evolved Universal Terrestrial Radio Access-Foreign Area) during idle reselection or mobility control information.
LTE cell reselection uses priority based levels (Priority 0 is Lowest and Priority 7 is highest)
This value is seen in SIB3 as cellReselectionPriority 7

Sample Priority Assigned 

(depending on the BW priorities can be interchanged)

Frequency	PRIORITY
2300(20MHz)	7
1800(5 MHz)	5
850(10MHz)	6

Let us see how priority triggers inter-frequency measurement

Example:
Serving cell priority(6)< Neighbor cell Priority(7)
Perform inter frequency measurement


CELL SELECTION

UE performs cell selection based on RSRP level according to 3GPP rel 8 or both RSRP and RSRQ level 3GPP Rel9).

The criteria for cell selection is that the Srxlev>0[Definition and details about Srxlev is available in the table below]

The role of Hysterisis can be illustrated as follows

Case 1:
Hysterisis=0

Measured value is fluctuating around the target value causing alot of unnecessary measurement report

Case 2:
Hysterisis=2dB

Even though the measured value is fluctuating around the target it does not trigger measurement report unless it fluctuates wider than hysterisis(Pink)

CELL RESELECTION FOR INTRAFREQUENCY

S_INTRA_SEARCH

This parameter is the threshold value for intra-frequency measurement i.e it is used to trigger intra frequency measurementsActual value in dB is obtained by multiplying by 2.

Corresponds to parameter s-IntraSearch specified in SIB3 

Settings:31 ,62dB

Measurement rules for intra frequency cell reselection 

Serving_Srxlev>Sintrasearch -Do not perform intra frequency measurement
Example: If measured RSRP of the serving cell is -80 dBm
Then Srxlev=-80+128=48 [Srxlev=Qrxlevmeas-Qrxlevmin(-128)]
Srxlev is 48 which is >31 (Sintrasearch Value)

Do not perform intra frequency measurement

Serving_Srxlev<=Sintrasearch -Perform RSRP measurement
Example :If measured RSRP of the serving cell is -100 dBm
Then Srxlev=-100+128=28
Srxlev is 28 which is <31 (Sintrasearch Value)

Perform intra frequency  RSRP measurement

Condition for reselection for Intrafrequency

RSRP Serving Cell+Qhyst < RSRP Neighbor cell -Qoffset during Treselection

Qmeas,n is RSRP Neighboring cell

Qmeas,s is RSRP Serving cell

UE needs to camp on the current serving cell over 1 sec(T_RESELECTION)

CELL RESELECTION FOR INTER FREQUENCY

S_NON_INTRA_SEARCH

This specifies the Srxlev threshold (in dB) for E-UTRAN inter-frequency and inter-RAT measurements.Actual value of threshold is the value * 2 (dB).

     
Example
                                        ServingCell<=Snonintrasearch(10*2=20)+Serving_qrxlevmin(SIB3,-64,-128 dBm)=-108dBm

Serving cell RSRP<=-128+20=-108dbm
UE performs target cell EUTRAN frequency measurement

THRESHX PARAMETERS

ThreshX parameters are used in LTE for idle mode cell reselection towards a      different LTE Frequency .We have 2 parameters ThreshXlow and ThreshXHigh .  The 2 parameters relate to the target cell.

THRESH_XHIGH

 The threshold used in the UE to reselect a frequency whose priority is higher       than the  current camped frequency.If the user has to move from a low             priority   LTE  frequency  band  to higher priority LTE then the   neighbor/           target LTE must have the Srxlev>ThreshXHigh.                               
   

 Settings

  12/14

 Priority of the Target Cell is higher than the serving cell

 Example 1:

 Serving cell Priority (6)<Neighbor cell priority(7)

 Measurement

 Neighbor cell priority>Serving cell priority =Always perform inter                     frequencymeasurement

 Condition for cell reselection

 RSRP Neighbor /Target cell >target_qrxlevmin(SIB5,-64,-128 dBm) +   threshXHigh(SIB5,14,28 dB)=-100dBm

THRESH_SERVING_LOW

This specifies the Srxlev threshold (dB) used by the UE on the serving cell when reselecting towards a lower priority inter-RAT frequency (SIB3)

                   

THRESH_XLOW

The threshold used to reselect low-priority frequency from high-priority frequency
This value is found in SIB 5 .If the user has to move from a high priority band to a low priority band then the target low priority band must have  the Srxlev greater than Thresh_Xlow

Settings

10 [-108 dBm]when source is 2300 to any frequency
12[-104 dBm ]when source is 1800/850 to any frequency

Priority of the Target Cell is lower than the serving cell

Current Cell priority (7)<Target Cell Priority(6)

Assumption:Serving Cell:2300MHz,Target Cell:850MHz

Measurement

Serving Cell<=Snonintrasearch(12*2=20)+Serving_qrxlevmin(SIB3,-64,-128 dBm)=-108dBm
Serving cell RSRP<=-128+20=-108dbm
UE performs target cell EUTRAN frequency measurement

Cell Re-Selection for interfrequency (when target cell is of lower priority)

Both conditions have to be met

Serving Cell's RSRP<THRESH_SERVING_LOW(10,10*2=20)+serving_qrxlevmin(SIB3,-64,-128 dBm)=-108dBm
                           
&

Neighbor/Target cell RSRP> ThreshX,Low(10,10*2=20)+Target_qrxlevmin (SIB5,-64,-128 dBm)=-108dBm

APPENDIX

1.UE always measures frequencies with higher priority.
2.UE has to measure frequencies with lower priority in the following cases
   a>perform intrafrequency measurement (same priority)only when Srxlev of         the serving cell<=Sintrasearch

   b>perform lower priority inter-frequency measurement only when Srxlev of           serving cell <=Snonintrasearch
3.LTE Cell Reselection uses priority based levels (Priority 0=Lowest,Priority     7=Highest)

Multi Carrier Settings -LTE Measurement Reports and Handover Settings

HANDOVER TYPE


Handover per carrier

Service providers can modify as per requirement

1. HoEvent   A3-Event A3 is used for both inter and intra frequency handover .Where neighboring cell has to be offset  better than the serving cell for a defined time mentioned by the parameter "time to trigger " .If a hysteresis value is defined then the neighbor cell has to be offset+hysteresis better than the serving cell. Hysteresis is a value used to avoid unnecessary triggering of measurement reports when the signal is fluctuating .Using Hysteresis is optional.

2. HoEvent   A4-Signal of serving cell is poor and UE starts to make measurement with A2 to find better neighbor cell, if the serving cell signal is getting better and stays stable during Time to Trigger then the UE stops measurement with A1. If it is not getting better during the Time to Trigger then it will make handover with A4 if neighbor cell is better than the threshold specified (A4 THRESHOLD_RSRP/A4 THRERSHOLD_RSRQ).The dotted line in the figure below marks the threshold.

3. HoEvent   A5 -Event A5 is ideally used when carrying out an inter frequency handover for FTP DL (Non Guaranteed Bit Rate)from lower *priority band to higher *priority band. UE has to switch to the target cell and perform the signal quality measurement and come back to the current cell in cases of inter frequency.

(*Note:Priority is assigned to each frequency band depending upon the Bandwidth. Higher frequencies usually have higher priority assigned considering they have higher Bandwidth)

TIME_TO_TRIGGER

This parameter is the timeToTrigger value for the Event .  The timeToTrigger is the time which should be satisfied for the UE to trigger the measurement report.Below we have 2 sample settings 

 1.Settings :            256ms ( To trigger all events.)

                                480 ms (480 ms is used typically to trigger Event A4 )
   

2.Settings :             256ms (for most events)

                                40 ms(for VOLTE services in  event A3 ) 
                                480 ms (480 ms is used typically to trigger Event A4 )

Values are subject to change according to the KPI results.

TRIGGER_QUANTITY

This parameter is used to set up the TriggerQuantity of an  Event during reporting neighboring cell's configuration. The TRIGGER_QUANTITY can be set to rsrp/rsrq. A UE transmits an event when RSRP or RSRQ meets a specific threshold according to TRIGGER_QUANTITY. 

 1.rsrp: It indicates that RSRP is used for triggerQuantity. 

 2.rsrq: It indicates that RSRQ is used for triggerQuantity. 

 3.followA2event: It indicates that the value of TriggerQuantity corresponds with the value of A2            Event triggerQuantity. 

 4.both: It indicates that RSRP and RSRQ are used for triggerQuantity

Samples of settings that could be used for a cell in an LTE network

Settings for Trigger Quantity

Trigger quantity is set to rsrp. Which means that all the handover measurement report criteria's are compared and triggered using rsrp values only.

Settings for Trigger Quantity(the new Release-sample)

1.Network providers could use trigger quantity "both"since the new release for 2300 MHz intra frequency and inter frequency handover(where source is 2300MHz to any other band )in FTP DL.
(2300Mhz band has the maximum bandwidth among other bands we want users to be in this band )

2.Handover for  VOLTE (Guaranteed Bit rate service)inter and intra frequency could be done using Event  A3 and it has trigger quantity "rsrp"

3.Handover for FTP DL (Non Guaranteed Bit Rate Service)inter frequency handover (Event A3)could use trigger quantity "rsrp" 

4.Handover for FTP DL (Non GBR Service)intra frequency handover (Event A3) could use
trigger quantity "rsrq"

5.Handover for FTP DL(Non GBR) i.e handover from lower band to higher band could be
 done  using Event A5 and it  has its trigger quantity "rsrq".

REPORT_QUANTITY

This information is for setting whether to report only values equal to the triggerQuantity (RSRP or RSRQ), or all values (RSRP and RSRQ), when a device is reporting measurement results.

sameAsTriggerQuantity: If it is set to sameAsTriggerQuantity, the UE transmits only the result corresponding to the TRIGGER_QUANTITY

both: A UE transmits both RSRP/RSRQ if the REPORT_QUANTITY is set to both for the measurement result

This  value can be set to both since the new release.
old releases  could have sameAsTriggerQuantity

                                               Figure 4.Sample Multicarrier Settings

            

A1_THRESHOLD_RSRP

Event A1 occurs when serving becomes better than threshold.

This parameter is set to a value between 0-97

The actual RSRP measurement in dBm can be calculated as follows
 A1_THRESHOLD_RSRP-140 (dBm).


A1_THRESHOLD_RSRQ

This parameter is the RSRQ threshold for Event A1 which is used to perform measurement gap deactivation which means user equipment will not search for measurements if its serving RSRQ is better than the value specified in A1
This parameter is set to a value between integer values 0-34
The actual RSRQ measurement in dB can be calculated as follows A1_THRESHOLD_RSRQ-40/2 (dB).

A2_THRESHOLD_RSRP

Measurement is carried out by an event A2 that means the quality of the serving cell's DL reference signal becomes worse than the absolute threshold.

The higher the parameter is,  measurement gap is activated for a stronger signal strength and too frequent measurement gap activation can impact service experience.

The lower the parameter is, the weaker signal strength measurement gap is activated and the later measurement gap activation can impact HO success rate.

The values in A2 is usually A1+5db .

A3_OFFSET

RSRP threshold used for triggering the measurement report for Event A3 to the eNB 

Event A3  occurs when Neighbor becomes offset  better than serving.In the network if the neighboring cell is offset better than the serving cell the event A3 occurs
The actual offset value in dB can be calculated as follows Integer Value*0.5 =Value in dB

When eNB receives a measurement report including Event A3 from UE, eNB triggers LTE handover to the best cell indicated in the measurement report. Because handover target cell is decided by UE's measurement results for neighboring cells.

Event A3 is used for LTE Intra-Frequency handover. This event type can also be used for Inter-Frequency handover and it is configurable in eNodeB side to choose the event type for Inter-Frequency

The higher the value of A3 offset the more we drag the calls to neighboring cells.This is very useful where we have coverage holes.The smaller the value of A3 offset the faster we release the calls to neighboring cells .This is very useful in those scenarios where a large number of LTE cells exist in a given geographical area.

A3_OFFSET_FOR_RSRQ

RSRQ threshold used for triggering the LTE neighboring cell's measurement report for Event A3.
The actual offset value in dB can be calculated as follows Integer Value*0.5 =Value in dB

Example from the Multicarrier settings shared in figure 4
For triggering handover  neighboring intra frequency cell has to have an RSRQ value 3dB better than the serving RSRQ during FTP DL services 


A4 THRESHOLD_RSRP/A4 THRESHOLD_RSRQ

Triggers when a neighboring cell becomes better than threshold as specified for event A4
This event type could be used for inter frequency handover and it comes after event A2.

Sample Settings:
The A4 THRESHOLD_RSRP could be set to -105 dBm
and the A4 THRESHOLD_RSRQ could be set to -4dB

However Handover_type has to be set to A4 for using this type of handover and the parameter called
Trigger_quantity rsrp or rsrq will determine if A4 THRESHOLD_RSRP or A4 THRESHOLD_RSRQ will be used as threshold

A5_THRESHOLD1_RSRP & A5_THRESHOLD2_RSRP

Event A5 is triggered when the serving becomes worse than the threshold1 and neighbor becomes better than threshold 2

The A5_Threshold1 and A5_Threshold2 value of Event A5 is used during reporting measurement configurations of the neighboring LTE cells.

Its range is 0-97.(Calculation is as follows :Integer Value-140)

Typically handover for FTP DL in event A5(which is used for  handover from lower band to higher band) using trigger quantity "rsrp"  

A5_THRESHOLD1_RSRQ & A5_THRESHOLD2_RSRQ

It is set to 0-34. (Calculation is as follows :Integer Value-40/2)

Typically handover for FTP DL in event A5(which is used for  handover from lower band to higher band) using trigger quantity "rsrq"