Telecom knowledge and experience sharing: Random Access Special Topic

After the course,you will:
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

UE power on procedure

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

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 special scenarios application

Random Access Based on Non-Competitiveness: Scenario 4