LTE – Initial Attach Procedure
• Overview
1. Initial Cell Selection
a) Cell search
• Scanning of bands
• Downlink synchronization
b) Read Master Information Block (MIB)
c) Read System Information Block 1 (SIB1)
• Freq band
• PLMN-IdentityList
• Cell Barred
• Closed subscriber group • TrackingAreaCode
• CellIdentity
d) PLMN selection
• Manual or automatic
e) Other SIBs
f) Suitability criteria (RSRP, RSRQ)
g) Camping
2. Stored Info Cell Selection
• E-UTRA carrier frequency, PLMN, and/or cell for selection
• If no suitable cell from stored info, then perform initial cell selection
1
LTE – Downlink Synchronization
• eNodeB transmits primary synchronization signal (PSS) and secondary synchronization signal (SSS) – specific sequences
LTE Radio Frame (10msec)
0
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9
7 symbols/slot (0.5msec)
Sub-Frame (1msec)
PSS
SSS
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7
• Synch signals are transmitted twice per radio frame (first and 11th slots) of 72 subcarriers (actually 62 of 72) so UE does not need to know system BW and 64pt FFT
• PSS = Zadoff-Chu of length 63 (punctured to 62)
• PSS permits UE to acquire slot boundary timing regardless of CP length
• PSS enables with slot timing detection and physical layer ID (0,1,2)
• PSS enables propagation channel estimation via reference signal
• SSS = SSC1 and SSC2 are two shifts of the same length-31 M sequence
• The two SSS codes are alternating permitting the UE to determine 10msec UE timing • SSS enables radio frame timing, physical layer cell ID group (0-167), CP length,
FDD/TDD detection
LTE – MIB example
LTE – SIB example
LTE – SIB1 example
LTE – SIB6 specific example
LTE – Uplink Synchronization
• Uplink time alignment is affected these primary factors:
1. Distance between UE and eNB varies
2. Multipath prop changes constantly varies causing a variation in delay spread 3. Velocity of UE generates Doppler
4. Oscillator drift in UE
• Timing control is required to be established and updated! (by MAC layer)
•
eNB establishes it and then maintains it through regular updates
• Timing offset – Accounts for both DL and UL prop delays (NTA in 32.552ns
steps) and supports up to 100km cell size
• Timing advance – eNB sends command to UE for both initial and updates,
value is TA and UE derives NTA (PUCCH, PUSCH, SRS) and adjustment is applied on subframe n+6 (6 frames later)
0
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Radio Frame
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DL RF Tx @ eNB
DL RF Rx @ UE
UL RF Tx @ UE
Prop delay in DL
TO to account for UL & DL
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UL RF Rx @ eNB
Prop delay in UL
LTE – Random Access
• MAC layer performs the random access procedure and establishes L1 connectivity
• Two types
1. Contention-based
2. Noncontention-based
Random Access Procedure
Contention Free
Contention Based
• Intra-Sys Handover
• DL data transfer (synchronization
lost)
• RRC_IDLE to RRC_CONNECTED
• UL data transfer (synchronization
lost)
• RRC Connection Reestablishment
LTE – Contention-Based Random Access
• Contention-Based Random Access (5 cases)
1. RRC connection establishment: The UE performs the contention-based
random access procedure when it initiates the RRC connection
establishment procedure.
2. RRC connection reestablishment: The UE performs contention-based
random access procedure when it initiates the RRC connection
reestablishment procedures.
3. Initiation of the downlink data transfer: When the UE is in
RRC_CONNECTED state and downlink data has become available for
transmission but the UE is not UL synchronized
4. Initiation of the uplink data transfer: The UE performs the contention-based
random access procedure when it is in the RRC_CONNECTED state and it
has the uplink data available but it does not have its uplink synchronized.
5. Handover: The handover takes place while the UE is in the
RRC_CONNECTED state.
LTE – Contention-Based Random Access
• Contention-Based Random Access (steps)
1. 2.
3. 4.
Transmission of the random access preamble on the PRACH
Transmission of the Random Access Response (RAR) on the PDSCH with indication on the PDCCH
Transmission of message 3 on the PUSCH
Contention resolution
UE eNodeB UE
Random access preamble
Random access response Message 3
Contention resolution
eNodeB
Preamble sequence
Random Access Response UL Grant, Timing advance, Temp CRNTI
Connection Request
UE Random Value/TMSI, Establishment Cause
Contention Resolution CRNTI
LTE – Contention-based Random Access
• Transmission of the Random Access Preamble
• MAC layer of the UE picks a random access preamble from the available random
access preambles and sends it on the RACH PRACH
• Subset of 64 preambles used for contention-based access. Remainder are reserved
for noncontention-based random access
• Set by NumberOfRA-Preambles field of RACH-ConfigCommon IE (4 – 64)
• UE starts at low power level and then can step up by estimated path loss
• Random Access Response (RAR)
• Sent in response to the received preamble by the eNodeB on the PDSCH and an
indicator on the PDCCH
• UE waits 2msec and monitors the PDCCH instance (CRC scrambled by the Random
Access RNTI (RA-RNTI)) for a Response Window period of time. If indicator not set, UE initiates a random backoff
• UE transmits at new random backoff time with increased PRACH transmit power
• If TransMax (times) is reached (3 – 200) an error is reported to upper layers
• If successful, the MAC RAR allocates resources on PUSCH for the UE to send
Message 3 and assigns a temporary C-RNTI to the UE
• UE performs first scheduled UL transmission on the PUSCH and sends Msg3 on CCCH or
DCCH
• If contention exists due to matching random access preamble signatures, eNB picks one
and sends an RRC message to the UE on the DL-SCH. Others time out and try again.
LTE – Contention-based Random Access
Unsuccessful RAR
• Poor estimate of uplink power by UE (preamble doesn’t reach eNodeB)
• Simultaneous transmissions from multiple UEs using same resources interfere with each other, so no RAR sent
• RAR received, but Random Access Preamble Identifier (RAPID) doesn’t match
If Unsuccessful RA attempt
• UE performs backoff, picks new preamble, transmits with increased PRACH power in subframe (n+4) where n is the last subframe of the RAR window
LTE – Noncontention-Based Random Access
• Noncontention-Based Random Access (steps)
1. Assignment of random access preamble
2. Transmission of the Random Access Preamble on the PRACH 3. RAR on the PDSCH with indication on the PDCCH
UE eNodeB
Random access preamble assignment
Random access preamble Random access response
LTE – Successful RAR
Message 3 on CCCH (contention-based)
• RRC connection setup: UE sends RRCConnectionRequest to eNodeB (usually includes S- TMSI)
• RRC connection reestablishment: UE send the RRCConnectionReestablishment message as the message 3 (contains C-RNTI)
• HARQ is also part of message 3 transmission
Contention Possibilities
• Since RA preamble signatures are chosen randomly, multiple UEs may transmit same one
• Thus, CRC of the PDCCH is scrambled by the same RA-RNTI (which is dependent upon frequency and time resources utilized by UE to transmit preamble) same RAR message is sent ont eh PDSCH addressing multiple UEs
• Multiple UEs will thus be using the same PUSCH for message 3
• eNodeB receives message 3 from only one of the UEs that it considers as best time aligned and most suitable
• UE waits for contention resolution for a set period of time (MAC-ContentionResolutionTimer)
• If Contention is successful UE will receive the a PDCCH instance using the C-RNTI of the UE to scramble the CRC
• The eNodeB sends the RRCConnectionSetup or RRCConnectionSetupReestablishment, on the DL-SCH
LTE – Initial Attach Request
UE eNodeB
Random access preamble
Random access response
RRC Connection Setup
RRC ConnectionRequest
LTE – Initial Attach Procedure (Cont.)
Non-Access Stratum (NAS) Layer
• NAS signaling connection is created to facilitate communication between UE and the MME – specifically the NAS layer and MME
• UE moves from the ECM-IDLE state to the ECM-CONNECTED state
• NAS signaling connection = RRC connection + S1 logical connection
1. RRC connection establishment
2. MME selection
3. UE-associated logical S1-connection established
• Initial NAS message NAS signaling connection
RRC connection
ECM-CONNECTED
S1 logical connection
LTE-Uu
S1-MME
1
LTE – NAS Signaling Connection
Initial NAS messages
1. ATTACH REQUEST
2. TRACKING AREA UPDATE REQUEST
3. SERVICE REQUEST
4. EXTENDED SERVICE REQUEST
5. DETACH REQUEST
LTE – NAS Signaling Connection
RRC Connection Establishment
• Initiated by NAS request
• Used to establish SRB1
• Establishes NAS signaling connection
UE
eNodeB
Message 3 using SRB0 on CCCH logical channel
NAS
RRC
Random access preamble
Random access response
RRCConnectionRequest
RRCConnectionSetup
RRCConnectionSetup
Complete
Request to est. RRC connection
Radio Resource Manager in eNB decides if connection can be est.
eNodeB
LTE – RRC Connection Establishment
LTE – RRC Connection Establishment
Message 3 using SRB0 on CCCH logical channel
• Emergency calls
• Mobile originating signaling
• Mobile originating data transfer • Mobile terminating access
RRC Connection Establishment
RRC Connection Establishment
LTE – RRC Connection Establishment
LTE – MME Selection
• After the RRC connection is established, the eNodeB selects an MME in order to establish an S1-MME connection for the particular UE unless the eNodeB is only connected to one MME
• In an MME pool area the eNodeB is connected to multiple MMEs. MME uses the NAS Node Selector Function (NNSF) to determine which MME is best to associate with the UE based on:
1. Reduction of the possibility of changing MMEs (mobility)
2. UE indicated MME if current, or utilize to reachback
3. MME with higher weight factor for load balancing
LTE – MME Connection
• The eNodeB and the selected MME communicate over the S1-MME interface
• Signaling services between eNodeB and MME uses S1AP protocol over S1-MME
interface:
1. Non-UE-Associated services: related to the whole S1 interface instance (general)
2. UE-Associated services: Related to a particular UE using UE-associated logical S1-connection. Identified by:
• Pair: eNB UE S1AP ID and MME UE S1AP ID (unique pair)
• When the MME sends signaling message to the UE, it includes the assigned eN UE S1AP ID allowing the eNodeB to forward the message to the right UE.
• When eNodeB forwards signaling message from UE to MME, it inserts the assigned MME UE S1AP ID so the MME can identify the correct UE
LTE: S1 Setup
• The S1 setup procedure occurs during initial provisioning of S1-MME interface setup
• Procedure involves exchange of configuration information needed for interoperability
• S1 interface uses Stream Control Transmission Protocol (SCTP/IP) in the Transport Network Layer (TNL)
• Pair of SCTPs are set up to exchange signaling
• Procedure
1. eNodeB requests the MME to set up S1 by sending S1 SETUP REQUEST message including:
• eNodeB Name (human readable)
• Global eNB ID
• Supported tracking areas
2. S1 connection is then established and the MME sends S1 SETUP RESPONSE containing:
• MME Name (human readable)
• PLMNs – a list of served PLMNs
• Relative MME capacity – processing capacity relative to other MMEs in pool area used for load balancing
LTE – Logical S1-Connection Setup (UE-Associated)
• The UE-associated logical S1-connection supports dedicated information signaling between for a UE between the eNodeB and MME. It is established by:
1. Transmission of INITIAL UE MESSAGE message – Initial NAS message from the UE it must forward it to the MME:
• eNB UE S1AP ID: the eNB assigns a unique identifier for the UE
• NAS-PDU: containing the initial NAS message from the UE
• TAI: tracking are of UE
• E-UTRAN CGI: identifier of cell serving UE
• S-TMSI: if UE has provided it
• GUMMEI: if NodeB does not support NNSF
2. Transmission of the DL NAS TRANSPORT message: so the MME can send a NAS message to the UE (may include MME UE S1AP ID if link has not been established)
3. INITIAL CONTEXT SETUP REQUEST message: this establishes the UE- associated logical S1-connection if it does not exist
4. Path switch: used during the switching of a DL GTP tunnel towards a new GTP tunnel endpoint
5. Trace start: message sent by MME
LTE – GUTI and GUMMEI
• Globally Unique Mobility Management Entity Identifier – part of Globally Unique Temporary Identifier (GUTI)
http://blog.eventhelix.com/2012/07/14/lte-identifiers-guti-gummei-imsi-rnti-tai-and-more/