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

LTE - STACKS

LTE - STACKS

Lets see the LTE complete Stack architecture with respect to MME & SGW from the EnodeB. EnodeB is nasically a protocol converter, which converts the messages received from MME with a different set of Stack layers. It Take the NAS messages & encapsulate it to the OTA stack & send it to UE's.

In Downlink, EnodeB receives 3 kind of stack messages.

1. From MME (Control Plane - S1 Interface)
2. From SGW (Data Plane - S1-U Interface)
3. From neighbour EnodeB (Control & Data Plane - X2 interface)

Lets see the Stack between UE, EnodeB & MME

Similarly you can see the Stack between UE, EnodeB & SGW:

The EnodeB to EnodeB stack comprises control & Data Planes in the X2 Interface for carrying the control information & data respectively.

Stack for X2 Interface control plane:

Stack for X2 Interface Data Plane:

I hope the you can understand the complexity in the EnodeB stack & its importance in the LTE architecture.

LTE - Terminologies & Explanations

LTE - Terminologies & Explanations

PLMN ID is a Public Land Mobile Network Identifier; serves to PLMN unique identification
PLMN ID (not more than 6 digits) = MCC + MNC

MCC is a Mobile Country Code; assigned by ITU; (3 digits)

MNC is a Mobile Network Code; assigned by National Authority; (2 or 3 digits) If the 2 digit MNC is used, then the PLMN will be like ex: 262-09 for 3 didgit MNC, PLMN is like: 262-009

MSIN (MSISDN) is a Mobile Subscriber Identification Number; assigned by operator; (10 digits)

IMSI is an International Mobile Subsciber Identity; serves to uniquely identify a mobile (LTE) subscriber; not more than 15 digits) IMSI will be 14 digits or 15 digits based on the MNC with 2 digits or 3 digits.
IMSI = PLMN ID + MSIN = MCC + MNC + MSIN

ECGI is an E-UTRAN Cell Global Identifier; serves to identify a Cell in global (globally unique); EPC knows the UE location based of ECGI
ECGI (not more than 52 bits) = PLMN ID + ECI

ECI is an E-UTRAN Cell Identifier; serves to identify a cell within PLMN; (28 bits)
ECI = eNB ID + Cell ID

Cell ID is a CELL identifier; serves to uniquely identify a cell within eNB; (8 bits)

eNB ID is an eNodeB Identifier; serves to identify an eNB within PLMN; (20 bits)

Global eNB ID is a Global eNodeB Identifier; serves to identify an eNB in global (Globally unique); (max 44 bits)
Global eNB ID = PLMN ID + eNB ID

Note: The physical Cell ID is different from the Cell ID mentioned above. The Physical Cell ID is used to decode the enodeB signalling by the UE @ phy layer. The Cell ID mentioned above is used by the NAS & RRC layers for EPC management purposes. The most important use of Phy Cell ID is:

Interference to reference signals from reference signals of other cells is eliminated by Physical Cell Identity

Reference: 3GPP 36.508 - Conformance Testing.

LTE - MFBI

MFBI - Multi Frequency Band Indicator

EnodeB will inform UE about Multiple Overlapping bands it supports in the SIB1 message provided if MFBI is enabled in the EnodeB.

Consider an example of Serving Band 4 by an EnodeB in a particular Cell. The corresponding Overlapping Band in the FDD is Band 10.

Frequency info:

Band 4:

DL Bw: 2110 to 2155 Mhz
UL Bw: 1710 to 1755Mhz

Band 10:

DL Bw: 2110 to 2170 Mhz
UL Bw: 1710 to 1770 Mhz

So from the Serving BW's its clear that both the Band 10 & 4 are overlapping each other in a range of 45Mhz

Why MFBI?

Now a UE is trying to latch on the Band 4, But the UE wont support band 10, due to the enabling of MFBI, UE will unknowingly latch on to the Band 10 as well.

Overlapping Bands in FDD & TDD:

E-Utra Operating Bands	Overlapping E-Utra Bands	Duplex Mode
2	25	FDD
3	9	FDD
4	10	FDD
5	18, 19, 26	FDD
9	3	FDD
10	4	FDD
12	17	FDD
17	12	FDD
18	5, 26, 27	FDD
19	5, 26	FDD
25	2	FDD
26	5, 18, 19, 27	FDD
27	18, 26	FDD
33	39	TDD
38	41	TDD
39	33	TDD
41	38	TDD

PHY - PUCCH

PUCCH - Physical Uplink Control Channel

PUCCH carries uplink control information. PUCCH will never be transmitted along with PUSCH in the same subframe.

There are multiple PUCCH formats: (Source 3GPP spec 36.211 Rel 9)

We will see how PUCCH RB allocation happens in the upcoming blog updates.

RRC STATES

RRC STATES

RRC states in LTE are very simple. Its just 2 states as explained in the 3GPP spec 36.331

1. RRC_IDLE
2. RRC_CONNECTED

RRC_IDLE:

1. A UE specific DRX may be configured by upper layers.
2. UE controlled mobility;
    The UE:
     i. Monitors a Paging channel to detect incoming calls, system information change, for ETWS capable UEs, ETWS notification, and for CMAS capable UEs, CMAS notification;
     ii. Performs neighbouring cell measurements and cell (re-)selection;
     iii. Acquires system information
     iv. Performs logging of available measurements together with location and time for logged measurement configured UEs.

RRC_CONNECTED:

Transfer of unicast data to/from UE.

At lower layers, the UE may be configured with a UE specific DRX.
1. For UEs supporting CA, use of one or more SCells, aggregated with the PCell, for increased bandwidth;
2. Network controlled mobility, i.e. handover and cell change order with optional network assistance (NACC) to GERAN;

The UE:
1. Monitors a Paging channel and/ or System Information Block Type 1 contents to detect system
information change, for ETWS capable UEs, ETWS notification, and for CMAS capable UEs, CMAS notification;
2.  Monitors control channels associated with the shared data channel to determine if data is scheduled for it;
3. Provides channel quality and feedback information;
4. Performs neighbouring cell measurements and measurement reporting;
5. Acquires system information.

You can find the RRC state change Diagram across LTE/UMTS/GPRS due to inter RAT handovers here:

Source: 3GPP Spec 36.331

DCI

DCI - Downlink Control Information

DCI is transmitted in the PDCCH. It tells the UE how to decode the PDSCH for getting the DL data transmitted to it in the same subframe.

There are many DCI formats which tells specific predefined downlink data formats to be used by UE.

So without decoding the DCI, UE will never come to know about the DL data sent to it by the eNodeB.

What DCI informs to UE?

UE will get the following informations from DCI after successful decoding of the DCI in PDCCH.

1. Number of RB's allocated (This will differ with channel Bandwidth)
2. Modulation & Coding scheme (MCS) used in PDSCH for the DL data.(QPSK or 16QAM or 64QAM)
3. Redundancy version - RV
4. New Data Indicator
5. HARQ process number
6. TPC command for PUCCH

Note: 
Incase of MIMO DCI formats like DCI 2 & 2A, 3 Transport blocks informations will be present in DCI for TB1 & 2. Each contails MCS, RV & NDI informations in it.

CRC added to diff CDI formats is of 16 bits & its scrambled with UE-RNTI.
After that 1/3 Rate matching will happen.

So DCI carries all those above data in it. One quick question which comes to the mind now is? How UE's will come to know whether the DCI is intended for a particular UE?

While the process of encoding, DCI will be padded with the CRC bits scrambled with the UE-RNTI. So, once the UE, descrambles the CRC of DCI, it will come to know whether the intended UE is that or not. If the UE-RNTI passed in DCI CRC not matches with the decoded UE, then UE will simply ignore it.

Different DCI formats:

There are as many as 9 DCI formats as of now in Rel8 3GPP. They are:

DCI - 0
DCI - 1
DCI - 1A
DCI - 1B
DCI - 1C
DCI - 1D
DCI - 2
DCI - 2A
DCI - 3
DCI - 3A

In those formats, DCI 0 is always used to indicate Uplink Data Transmission.

DCI formats 1 to 2A are used to indicate DL data transmission format in PDSCH

DCI formats 3 & 3A are used for Transmit Power Control information for PUCCH & PUSCH.