basic call flow in lte
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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