BASIC CALL FLOW in LTE

◦ Process by which UE acquires time and frequency synchronization with a cell and detects the Physical Cell ID

◦ Cell search not only at power up but also to support mobility (i.e. terminals need to search, synchronise and estimate neighbour cells to decide HOs)

◦ Step 1: UE searches for the PSS carrying the Physical Cell ID index (3

different possibilities: 0,1,2) Coarse synchronization: frequency, slot and subframe synchronization

◦ Step 2: Detected PSS, position of SSS is known. SSS can take 168 values that are Physical Cell ID group. The set of values for the two SSS within a frame are different-> UE

gets frame synchronization (10ms)◦ In LTE, there are 504 physical layer cell identities grouped into 168

groups (SSS) each one of them contains 3 cell ID (PSS) PCI = 3*(SSS)+PSS

Cell Search Procedure (1/2)

◦ Step 3: With frame synchronisation and the Physical Cell ID the UE knows

the physical cell ID. It can identify the cell specific reference signals and begin channel estimation

Then, it can decode the BCH that carries the system information • After successful BCH decoding the UE may access the system (RACH procedure)

Cell Search Procedure (2/2)

P-SCH: coarse frequency synch and symbol, slot and subframe synchronizationS-SCH: frame synchronization (boundaries of 10ms frame)BCH: cell information

Initial Cell Search

Initial Cell Search

EPS Bearer

Types of EPS BearerDefault EPS Bearer :Be established during Attach ProcessAllocate IP address to UEDoes not have specific QoS (only Nominal QoS is applied).Similar to Primary PDP Context in UMTS Dedicated EPS Bearer :Normally be established during the call setup after idle mode. (but can be established during the attach as well).Does not allocate any additional IP address to UEIs linked to a specified default EPS bearerHave a specific (usually guaranteed) QoSSimilar to Secondary PDP Context in UMTS

◦ Used in several cases. E.g. initial access (from idle to connected), to re-establish a radio link after a failure, as part of the handover to get UL synchronization

Process: ◦ UEs selects randomly a preamble from the list of preambles

broadcasted in the BCCH◦ UE calculates Initial Tx Power based on pathloss◦ Checks contention parameters (i.e. max. number of retries)◦ UE transmits initial RACH and waits for a response before retry.

Open loop PC ensures that each retry will be at a higher power level◦ Upon receipt of successful UL RACH preamble, eNB calculates

power adjustment and timing advance parameters together with an UL capacity grant (so UE can send more info)

RACH Procedure

DL

PUSCH: UE specific data

PRACH response

ULPreamble

Not detected

Preamble

Next PRACH resource

On the resources indicated by PDCCH

More info TS 36.213

• Contention Based• Non contentions Based• RACH process happens when

• Initial access from RRC_IDLE• RRC Connection Re-establishment procedure• Handover• DL data arrival during RRC_CONNECTED requiring random

access procedure”• UL data arrival during RRC_CONNECTED requiring random

access procedure. • when UL synchronisation status is "non-synchronised" or there

are no PUCCH resources for SR* available. E.g. when timing advance is needed for UE positioning

RACH Procedure

Initial Access 8. PRACH Preamble

11. PDSCH Physical Downlink Shared Channel

12. PUSCH Physical Uplink Shared Channel

(Random Access response, ID of the received preamble, UL resources for TX,C-RNTI)

(RRC: RRC Connection Request,C-RNTI,TMSI or random number)

13. PDSCH Physical Downlink Shared Channel (Contention Resolution,C-RNTI & TMSI)

eNodeB

UE

ATTACH/DETACH Process

Network

UE

Continued.....

ATTACH/DETACH Process

Network

UE

Continued.....

ATTACH/DETACH Process

Network

UE

PAGING and SERVICE request

UEeNB

MME/

SGW

Dedicated Bearer Setup, MODIFICATION and Release

UE eNB MME/

SGW

Dedicated Bearer Setup, MODIFICATION and Release

UE eNB MME/

SGW