LTE - RACH Procedure

LTE RACH Procedure

RACH Procedure is the 1st procedure through which UE will send its first Uplink message to the Network element EnodeB. Since its the 1st message sent by UE in UL, its MSG1

1. UE sends Rach Request in MSG1 to EnodeB
2. EnodeB sends Rach Response in MSG2 to UE with Temp_C_RNTI
3. UE sends UE Identification Message in MSG3 to EnodeB
4. EnodeB sends Contention Resolution Message in MSG4 to UE


 UE                                                                          EnodeB


------------------- RACH REQUEST in MSG1 ----------------->


<------------------- RACH RESPONSE in MSG2 ---------------


--------- UE IDENTIFICATION MESSAGE in MSG3 ------->


<--- CONTENTION RESOLUTION MESSAGE in MSG4 --


Note:

Important points to remember about the RACH procedure:

1. RRC connection request will be sent in MSG3 to EnodeB by UE with UE- Identity filled with a Random Value (Random value will be used when the RRC connection Request will be sent for the 1st time by the UE after Switch ON, i.e., UE is not having any identifiers with it.)

2. Once the RRC connection request is received by EnodeB, MSG4 will be sent by the EnodeB MAC  to UE, since MSG4 is a MAC level Message. (All MSG1, MSG2, MSG3, MSG4 are MAC level messages)

3. After RRC connection req is processed, RRC Connection Setup will be sent by EnodeB to UE on Signalling Radio Bearer 0 - SRB0 with a C- RNTI assigned & details to setup the SRB1.

MSG 1:

LTE Random Access Request (MSG1) Report

Version                  = 5

Preamble Sequence        = 25

Physical Root Index      = 129

Cyclic Shift             = 325

PRACH Tx Power           = -44 dBm

Beta PRACH               = 242

PRACH Frequency Offset   = 0

Preamble Format          = 0

Duplex Mode              = FDD

Density Per 10 ms        = 1

PRACH Timing SFN         = 907

PRACH Timing Sub-fn      = 1

PRACH Window Start SFN   = 907

RACH Window Start Sub-fn = 4

PRACH Window End SFN     = 908

PRACH Window End Sub-fn  = 4

RA RNTI                  = 2

PRACH Actual Tx Power    = -44

MSG 2:

LTE Random Access Response (MSG2) Report

Version                 = 1

SFN                     = 907

Sub-fn                  = 6

Timing Advance          = 0

Timing Advance Included = Included

RACH Procedure Type     = Contention Based

RACH Procedure Mode     = Initial Access

RNTI Type               = TEMP_C_RNTI

RNTI Value              = 53

MSG 3:

LTE UE Identification Message (MSG3) Report

Version                   = 1

TPC                       = 3

MCS                       = 1

RIV                       = 302

CQI                       = Disabled

UL Delay                  = Don't Delay

Hopping Flag              = Disabled

SFN                       = 908

Sub-fn                    = 2

Starting Resource Block   = 2

Num Resource Blocks       = 4

Transport Block Size Index = 1

Modulation Type           = QPSK

Redundancy Version Index  = 0

HARQ ID                   = 2

MSG 4:

LTE Contention Resolution Message (MSG4) Report
Version              = 1
SFN                  = 908
Sub-fn               = 7
Contention Result    = Pass
UL ACK Timing SFN    = 909
UL ACK Timing Sub-fn = 1