Know the details of channel and channel mapping
Understand UE power on procedure
Master UE random access procedure
Master UE initial attach procedure
Content:
LTE channel introduction and channel mapping
UE power on procedure
UE random access procedure
Random access special scenarios application
UE attach signalling flow
LTE physical
channel introduction
- LTE downlink physical channel
- Broadcast channel：PBCH
- Control channel：PCFICH，PHICH，PDCCH
- Service channel：PDSCH
- MBMS channel：PMCH
- LTE uplink physical channel
- Control channel ：PUCCH
- Service channel ：PUSCH
- Random access channel：PRACH
Downlink physical
channel
- PBCH ( Physical Broadcast Channel )
- BCCH includes MIB and SIB，MIB is mapping on PBCH,SIB is mapping on PDSCH.
- PBCH is in the first 4 OFDM symbols of subframe 0 1 time slot on time domain, and 6 central RBs on frequecydomain,modulation is QPSK.
- MIB includes downlink system bandwidth, system frame number (SFN), PHICH duration.
- Control Channel：PCFICH
- PCFICH : Physical Control Format Indicator Channel
- PCFICH are transmitted in each sub-frame, informing UE of the OFDM symbols that PDCCH occupies in a sub-frame. The OFDM symbols are indicated by CFI which can be valued as 1, 2, 3, 4 (4 is reserved).
- Modulation is QPSK。
- PCFICH is mapped to the four resource-element groups in the first OFDM symbol in a downlink subframe.
- Control channel: PHICH
- PHICH: Physical Hybrid-ARQ Indicator Channel
- Send NAK/ACK responding information for the PHICH，modulation is BPSK.
- Multiple PHICH can be sent in a group at the same time.
- 1 PHICH group=8 PHICHs (normal cp)
- 1 PHICH group=4 PHICHs (extend cp)
- Control channel: PDCCH
- PDCCH: Physical Downlink Control Channel
- PDCCH sends scheduling information, transmission format, resource allocation, uplink scheduling permission, power control and uplink-transmission-related ACK/NACK.
- PDCCH is mapped to the first n (n<=4) OFDM symbols in each sub-frame. The value of n is indicated by CFI in the PCFICH channel.
- PDCCH supports 4 types of physical-layer formats which occupy one, two, four, and eight CCEs respectively.
- Service Channel: PDSCH
- PDSCH: Physical Downlink Shared Channel
- PDSCH Bears service data, L3 signalling message, paging and SIB information.
- Modulation: QPSK, 16QAM, 64QAM.
- MBMS Channel: PMCH
- Physical Multicast Channel (PMCH)
- If MBMS is support, MBMS sends cell MBMS broadcast information.
- Modulation:QPSK, 16QAM, 64QAM。
Downlink channel mapping
Uplink physical channel
- Control Channel：PUCCH
- PUCCH: Physical Uplink Control Channel
- PUCCH periodically feed back different CQI, PMI, RI, HARQ-ACK、SR.
- For the same UE, PUCCH does not transmit with PUSCH.
- PUCCH format:
- Service Channel: PUSCH
- PUSCH : Physical Uplink Shared Channel
- PUSCH Bears service data, L3 signalling message
- Modulation: QPSK, 16QAM, 64QAM.
- Localized resource allocation for PUSCH, consecutive RBs are allocated to user’s PUSCH.
- PRACH: Physical Random Access Channel
- During the random access procedure, UE send preamble on PRACH.
- PRACH occupies 6 RBs on frequency domain.
- PRACH time domain structure：
- Preamble: CP + Sequence
- There is a guarding interval behind Preamble.
- PRACH
- Format0~3 frequency domain location
- Subcarrier interval is 1.25KHz, 1/12 of normal subcarrier bandwidth.
- 1 PRACH include 864 subcarrier (6×12×12=864).
- The length of preamble sequences is 839, mapped on the central 839 subcarrirs.
- PRACH
- Format4 frequency domain location
- Subcarrier interval 7.5KHz , half of normal subcarrier bandwidth.
- 1 PRACH include 144 subcarrier (6×12×2=144) .
- The length of preamble sequences is 139, mapped on the central 139 subcarrirs.
Uplink channel mapping
Physical layer signals
- Downlink Physical Signal
- Reference Signal
- Cell-Specific Reference Signal (CRS)
- MBSFN Reference Signal
- \UE-Specific RS/Dedicate RS(DRS)
- Synchronization Signal
- PSS : Primary Synchronization signal
- SSS : Secondary Synchronization signal
- Uplink Physical Signal
- Demodulation RS(DMRS)
- Sounding RS(SRS)
Downlink physical layer
signals
- Function of downlink reference signal
- Downlink channel quality measurement
- Downlink channel estimate, for UE to demodulate data
- Downlink synchronization
- Function of cell-specific reference signal
- Cell-specific reference signals shall be transmitted in all downlink subframes in a cell supporting PDSCH transmission.
- Cell-specific reference signals are transmitted on one or several of antenna ports 0 to 3.
- Cell-specific reference signals are defined for subcarrier interval is 15kHz only.
- Cell-Specific Reference Signal
- The RS location has the relation with PCI, antenna port number, OFDM symbol sequence, slot number, CP mode,etc.
- MBSFN Reference Signal
- Transmitted only when the PMCH is transmitted. MBSFN reference signals are transmitted on antenna port 4.
- Defined for extended cyclic prefix only.
- UE-Specific Reference Signal
- Send to a dedicated user, the eNodeB will indicate sending this signal or not, and indicate UE to demodulate use the signal or not.
- Only transmitted on the RBs bearing PDSCH, using the antenna port 5.
Downlink synchronization signals
- LTE Synchronization Signal
- PSS : Primary Synchronization Signal
- SSS : Secondary Synchronization Signal
- Function of Synchronization Signal
- Get Physical Cell ID
- Dectect SSS to get PCI group number(0-167)
- Dectect PSS to get ID in group(0-2)
- PCI=3*group number+ID in group
- Downlink synchronization
- PSS: 5ms synchronization
- SSS: 10ms synchronization
- Identify it is a FDD or TDD system and CP mode.
- SCH includes P_SCH and S_SCH. The frequency-domain is located in the 72 subcarriers near direct current. Only 62 subcarriers are actually occupied. Other 10 subcarriers do not hold synchronization sequences.
- There are two same P-SCHs in a wireless frame. Their time-domain is located in the last symbol of the slot no.0 and the last symbol of the slot no. 10.
- There are also two S-SCHs in a wireless frame. But their synchronization symbols are different. The time-domain is located in the last symbol but one of the slot no.0, and the last symbol but one of the slot no. 10.
Uplink physical layer
signal
- DMRS( for PUSCH)
- Generated by Zad-off Chu seqence,maping on RE without any proceess.
- Transmitted on 4th OFDM symbol of each slot, the bandwidth of DMRS is same with PRB for PUSCH.
- Differerent users’ DMRS sequence will have different cyclic shift.
- Sounding RS: SRS
- Used for uplink channel quality estimate and selection, uplink scheduling.
- Location: last SC-FDMA symbol of uplink subframe which configured to send SRS, for UpPTS all the symbols can be used to transmit SRS.
- SRS sub-frame configuration: UE read the broadcast information to get which subframe will send SRS.
- Duration: eNodeB will notice UE to send SRS once or all the time.
- Period: eNodeB will notice SRS transmiting period, it can support 2、5、10、20、40、80、160ms
FDD Downlink Channel
Location
TDD Downlink Channel
Location
LTE Physical Layer
Procedure: Synchronization
UE Power on Procedure
- PLMN selection and cell selection P-SCH、S-SCH and PBCH has the same width and position for all system bandwidth options. So UE can do the cell search without kowning the system bandwith
- PLMN selection and cell selection
- PLMN selection
- UE scan the whole channel in the band according to its frequecy capability to find a available PLMN. UE will search the cell with the strongest signal, and then read the cell broadcast information to get the PLMN. If UE read one or more than PLMN in one cell, UE will report all the PLMN as the high quality PLMN whose signal strength satisfy a dedicated threshold to the NAS layer. If UE can read PLMN ID, but the signal strength doesn’t satisfy a dedicated threshold, UE also will report PLMN to NAS layer with the measurment value.
- PLMN selection result is given by NAS layer.
- After selecting PLMN, UE will select serving cell.
- Cell search purpose
- Detect the Physical Cell-ID
- Get the downlink time and frequency synchronization
- Detect CP format：normal or extended format
- Detect eNodeB antenna port number
- Read PBCH (Master Information Block)
- Get system bandwidth, system frame number (SFN), PHICH duration
- Cell search type: Initial and Stored Information Cell Selection
- Initial Cell Selection: According to its capability, UE scan the whole channel in the band and find the best serving cell on each carrier, if UE find, it wil select the cell as the serving cell.
- Stored Information Cell Selection: UE select the cell according to the pre-stored cell frequency information.If UE finds a proper cell, it will select this one as the serving cell, or UE will initiate Initial Cell Selection.
UE read the broadcast
information
UE Random
Access Procedure
- Random access characteristic
- Exists in both TD-LTE and FDD-LTE.
- No relation with the scale of cell.
- Divided into competitive and non- competitive random access.
- purpose of random access
- During attach procedure
- UE status changed from idle to connected.
- During handover procedure
- Get/ recover uplink synchronization
- Request UE ID from eNodeB
- Request uplink resoure from eNodeB
- Random access process can be used in the following situations:
- Access at RRC_IDLE status
- Access when the wireless link fault occurs
- Access in changeover
- Access at RRC_Connected status
- When there are downlink data (eg. The uplink is at non-synchronization status.)
- When there are uplink data (eg. The uplink is at non-synchronization status or no PUCCH resource can be used for scheduling request.)
- Random access categories:
- Random access based on competitiveness
- Used in the five mentioned situations
- UE selects a preamble sequence randomly in the available preamble set in a competitive way.
- Possible collision: two UEs use the same preamble sequence.
- Perform the synchronization process through four steps. The fourth step is used to solve the collision.
- Random access based on non-competitiveness
- In handover or when the downlink data arrives
- The eNB allocates a preamble sequence.
- Perform the synchronization through three steps without solving the collision.
- Random Access Based on Competitiveness
- MSG1：send preamble sequence on PRACH
- According to root sequence number, cyclic shift and preamble formats , UE generates the preamble sequence.
- Select preamble in group A/B randomly according to the MSG3 size and pathloss information, then send the preamble on PRACH.
- According to the target PRACH received power, preamble formats in BCCH and transmitting counter, UE caculates the PRACH initial emission power.
- eNB：according to the received preamble measurement, estimates the distance between UE and eNB,generates the timing ajdustment.
- MSG2：Random Access Response
- eNB Send RAR on the PDSCH, the location is indicated by PDCCH and No HARQ.
- MSG2 content:
- Preamble indication
- Timing adjustment information
- Temporary C-RNTI
- Msg3 resource allocation information
- UE detects RA response in a time window after sending MSG1. If UE fails to receive the RA respondence in a time window, this RA process is terminated, otherwise it goes to step3.
- MSG3
- According to MSG2 TPC indication, estimate pathloss and RB number forPUSCH, UE cacaulates the MSG3 transmitting power. It’s open loop power control.
- According to random access response, UE send MSG3 on dedicated resource. MSG3 needs HARQ.
- MSG3 content:
- In attach：send RRC Connection Request on CCCH, include NAS UE ID and establishment cause, but no NAS signalling message.
- In RRC Connection Re-establishment: Send RRCConnection ReestablishmentRequest on CCCH no NAS message
- In handover：send Handover Confirm on DCCH with UE C-RNTI.
- Other (uplink or downlink arrives):send UE C-RNTI.
- MSG4： contention resolution
- Collision detect: eNB sends contention resolution ID ( UE NAS ID ) on PDSCH or send C-RNTI on PDCCH to UE.
- With HARQ
- MSG4 content:
- UE NAS ID
- Resource allocation information
- UE：
- If UE finds its own NAS-layer ID is sending, UE sends ACK and temporary C-RNTI becomes C-RNTI.Uplink synchronization finish.UE waits for being schedulled and send data on uplink.
- If UE fails to detect NAS ID, there is a collision, UE waits a random time and do random access again.
- Random Access Based on Non-Competitiveness
- Random Access Based on Non-Competitiveness
- 1) eNB send non-contention Random Access Preamble to UE,and this preamble is not broadcasted in the Broadcast information channel.
- 2)UE send dedicated preamble on RACH. 3) eNB generates the RAR on MAC layer, and send the RAR in DL-SCH,random access success.
Random Access Based on Non-Competitiveness: Scenario 4
- eNB èMSG0
- Send Msg0 on PDCCH；
- Include Dedicated Preamble and Mask Index；
- With HARQ；
- Initiated reason：
- UE is out of synchronization on uplink,
- Downlink data coming
- No need to allocateC-Rnti in Msg2
- UE
- UE is Out of synchronization and monitoring to receive PDCCH order;
- Send Msg1 with dedicated preamble
- Msg3 is not useful，if there is data ,sending data , or sendingPadding。
Random
Access Based on Non-Competitiveness: Scenario 4（UE leave）
- eNB è send MSG0 on PDCCH
- Msg0 is send on PDCCH；
- Include Dedicated Preamble and Mask Index；
- With HARQ；
- Initiated reason ：
- UE is out of synchronization on uplink,
- Start timer and timer is time out.
- purpose：suspecting UE leaving, so release connection to avoid UE process hanging
- No need to allocateC-Rnti in Msg2
- UE UE hardware problem or power off, not camping in the network
Random
Access Based on Non-Competitiveness : Scenario 3
- eNB è send MSG0 on PDCCH
- Msg0 is send on PDCCH
- Include Dedicated Preamble and Mask Index
- With HARQ
- Initiated reason ：
- Source eNB apply dedicated preamble from target eNB and then send preamble to UE.
- UE
- Monitor to receive PDCCH order.
- Send Msg1 with dedicated preamble
Random
Access Based on Competitiveness: Scenario 5
- eNB -- No need MSG4
- Send Temporary C-Rnti in RAR.
- MAC identifying Msg3 is for UE re-synchronization；
- MAC notice L3 that UE re-synchronization success.
- L3 send RRC Connection Reconfiguration to re-setup connection.
- UE
- Uplink data coming but uplink is at non synchronization.
- Send RA request
- Include C-Rnti in Msg3,and scrambling MSG3 by Temporary C-Rnti.
UE attach Flow
Attach/dettach signalling flow
End of Course