LTE Training Course

OAI Project Team
Speaker: Gina, Ali, Nikita
Date: 2017/12/13
LTE Training course

OAI Project Team
Agenda item
• LTE Overview [Gina]
- [18:00] — [18:10]
• Channel in LTE [Ali] [Nikita]
- [18:10] — [18:25]
• Random Access Procedure [Gina]
- [18:25] — [19:00]
• Scheduling [Ali] [Nikita]
- [19:00] — [21:00]
2

OAI Project Team
• LTE Overview
What is LTE
Introduction for each layer
LTE Generic Frame Structure
• Channel in LTE
LTE MAC Layer
MAC Architecture
Why channel is important
LTE channel mapping
• Cell Search Procedure
PSS/SSS
• System information
MIB/SIB
• Random Access Procedure
Message sequence chart
Preamble
PRACH information
Various trigger events under the random access process
3
Outline

OAI Project Team
• Uplink Scheduling
Protocol Layer Architecture
- RLC
- PDCP
- RRC
BSR (Buffer Status Report)
HARQ
- HARQ vs ARQ
- Application of HARQ
- Process for Uplink
• Downlink scheduling
Downlink transmission
- Downlink transmission procedure
- Downlink control information
- Channel state information
- Downlink MAC control element
Hybrid automatic repeat request
- Hybrid automatic repeat request
- Incremental redundancy
- HARQ process
- New data indicator
- Downlink HARQ
- Downlink HARQ process
Channel quality
- Modulation coding scheme, Modulation
- Channel quality indicator
Discontinuous Reception
• Overview porting OAI UE Tutorial
4
Outline

OAI Project Team
• LTE Overview
What is LTE
• Channel in LTE
LTE MAC Layer
MAC Architecture
LTE channel mapping
PSS/SSS
MIB/SIB
Preamble
PRACH information
5
LTE Overview

OAI Project Team
What is LTE (Long Term Evolution) ?
• LTE is a wireless data communication technology standard
• LTE Enhances Data Transmission Capabilities and Data Transmission Speed of
Wireless Networks with New Technology and Modulation
6

OAI Project Team
What is LTE (Long Term Evolution) ?
7
Characteristic
High data rate
Low network latency
Save electricity
Support FDD/TDD
Support
MBSFN
*MBSFN : Multicast / Broadcast Single-
frequency Network
*FDD : Frequency Division Duplex
*TDD : Time Division Duplex

OAI Project Team
8

OAI Project Team
9
• Function: connect the radio resource of
control plane and user plane.
• Ex. Configuration for PDCP, RLC and
MAC layers

OAI Project Team
10
• Main function:
‐ Data transmit in Control and User plane
‐ PDCP serial number maintenance
‐ data encryption / decryption
‐ Make sure that the order of packets from upper
layer is correct and not repeat in reconstruction.
‐ Header compression and decompression (only
apply to DRB
‐ data integrity protection (only apply to SRB
*DRB: dedicated radio resource
*SRB: signaling radio resource

OAI Project Team
11
• Main function:
‐ Make sure that the packet order
‐ Remove duplicate packets and retransmission
‐ Packet cutting and reorganization
‐ Automatic Repeat Request (ARQ)
‐ TM/AM/UM mode handling
*SDU: Service Data Unit
*PDU: Protocol Data Unit
*TM: Transparent Mode
*AM: Acknowledged Mode
*UM: Unacknowledged Mode

OAI Project Team
12
• Main function:
‐ Managing HARQ
‐ Scheduling information reporting (DCI generation)
‐ Multiplexing
‐ Logical channel prioritization
‐ RA management and radio resource allocation to
different radio bearer
‐ Mapping between logical channels and transport
channels
*DCI : Downlink Control Information

OAI Project Team
13
• Main function:
‐ Transport channel error detection
‐ Forward Error Correction (FEC)
‐ Decode
‐ Synchronization
‐ Provide CQI to MAC layer
‐ Data transmit
‐ HARQ Ack
‐ Schedule request (SR)
*CQI: Channel Quality Indicator
*HARQ: Hybrid Automatic Repeat Request

OAI Project Team
14
=10 subframes
=2 slots =TTI (Transmission Time Interval)
Vs.
• 6 ODFM symbols (short cyclic prefix)
=6 or 7 ODFM symbols
• 7 ODFM symbols (short cyclic prefix)
* CP : cyclic prefixExtended CP Normal CP

OAI Project Team
• LTE Overview
What is LTE
• Channel in LTE
LTE MAC Layer
MAC Architecture
LTE channel mapping
PSS/SSS
MIB/SIB
Preamble
PRACH information
15
Channel in LTE

OAI Project Team
LTE MAC Layer
• MAC is a radio network protocol which resides both in the UE and in the E-UTRAN.
• MAC protocol is available for both User plane and also for the control plane.
User Plane Control Plane
UE Protocol Stack eNB Protocol Stack
NAS
RRC
PDCP
RLC
MAC
PHY
RRC
PDCP
RLC
MAC
PHY
UE Protocol Stack
RRC
PDCP
RLC
MAC
PHY
NAS
RRC
PDCP
RLC
MAC
PHY
eNB Protocol Stack
EPC
P-GW
S-GW
MME
NAS IP Address
Packet
Filtering
Mobility
Anchoring
Inter
-net

OAI Project Team
MAC Architecture
E-UTRA defines two MAC entities
• one in the UE and one in the E-UTRAN.
The MAC layer is composed of
• a HARQ entity,
• a multiplexing/de-multiplexing entity,
• a logical channel prioritization entity,
• and a control entity.

OAI Project Team
Why Channel is needed in LTE
• In order that data can be transported across the LTE radio interface
• Variety type of data will be transported across the radio access
network
• In the LTE Protocol Structure, different channel provide interface to
the higher layers

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Mapping Channel
19 http://www.artizanetworks.com/resources/tutorials/lay_2_log.htmlCC
Uplink Mapping Channel
Downlink Mapping Channel
• Physical channels: These are transmission channels that carry user data and control messages.
• Transport channels: The physical layer transport channels offer information transfer to Medium Access
Control (MAC) and higher layers.
• Logical channels: Provide services for the Medium Access Control (MAC) layer within the LTE protocol
structure.

OAI Project Team
• LTE Overview
What is LTE
• Channel in LTE
LTE MAC Layer
MAC Architecture
LTE channel mapping
PSS/SSS
MIB/SIB
Preamble
PRACH information
20
Cell Search Procedure

OAI Project Team
UE connect to network after booting
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Cell selection Random Access

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Cell search procedure
• Mainly related to PSS / SSS and UE through PSS / SSS can get useful information.
• Main purpose:
• Scenario:
22
*PSS: Primary Synchronization Signal
*SSS: Secondary Synchronization Signal
*PCI: Physical-layer cell identity
Get
synchronization
Get
system frame timing
Determine the PCI
UE
• Handover • Cell re-selection
UE

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PSS / SSS time domain location
• In order to support community search, LTE defines two downlink sync signals
- PSS: Primary Synchronization Signal
- SSS: Secondary Synchronization Signal
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1 slot
1 symbols = 1/7 slot

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PSS
• PSS structure
• Zadoff-chu sequence
The sequence corresponding to the PSS of a certain cell is determined
by the PCI of this cell .(𝑁𝐼𝐷
𝑐𝑒𝑙𝑙
%3)
24
5 subcarriers is for protecting the frequency band

OAI Project Team
How the UE decodes the SSS
• After knowing PSS, UE will know
the possible location of SSS
• The exact location of SSS is also
related to the length of the Cyclic
Prefix
25
Structure of PSS / SSS frame and slot in time domain in FDD mode

OAI Project Team
The information obtained at each stage
26
Check the reference signal Decode PBCH
Check PSS
Check SSS
• 𝑁𝐼𝐷
2
• 5 ms timing
• 𝑁𝐼𝐷
1
• 10 ms timing
• TDD/FDD
• Time-frequency position
• The length of Cyclic Prefix
Initial synchronization
• Check PBCH timing
• Get the system message
Identify neighborhood cells
• RSRP/RSRQ
• Indicate to RRC
*RSRP: Reference Signal Receiving Power
*RSRQ: Reference Signal Receiving Quality
Check PSS
Check SSS

OAI Project Team
• LTE Overview
What is LTE
• Channel in LTE
LTE MAC Layer
MAC Architecture
LTE channel mapping
PSS/SSS
MIB/SIB
Preamble
PRACH information
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System information

OAI Project Team
System information
• MIB：
- Downlink system bandwidth, PHICH configuration, System Frame Number (SFN)
- Transmission channel: PBCH -> BCH
- Period: 40 ms
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SI : System information
frame
subframe

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System information
• SIB1：
- Parameters are used to determine whether a cell is suitable for cell selection or not.
- Time-domain scheduling information of other SIBs
- Transmission channel: PDSCH -> DL-SCH
- Period: 80 ms
• Scheduling information for other SIBs
- According to schedulingInfolist specified SI messages, base station will tell UE which
sibs are in each SI message, and also how the SI messages will be transmitted.
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frame
subframe

OAI Project Team
• LTE Overview
What is LTE
• Channel in LTE
LTE MAC Layer
MAC Architecture
LTE channel mapping
PSS/SSS
MIB/SIB
Preamble
PRACH information
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Random Access Procedure

OAI Project Team
• Main purpose
- Get uplink synchronization
- Assign UE a unique identifier C-RNTI
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RNTI : Radio Network Tempory Identity
C-RNTI RA-RNTI SI-RNTI P-RNTI Temp C-RNTI
value 003D～FFF3 0001～003C FFFF FFFE 003D～FFF3
function
PDSCH
transmission for
dynamic
scheduling
For Random
Access
Response
For identify the
transmission of
SIB messages
For identify the
transmission of
paging
messages
For Msg3
transmission,
conflict
resolution

OAI Project Team
• Steps of Random access procedure
32
MSG 1
MSG 2
MSG 3
MSG 4

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When RACH Process occurs
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Contention based RA procedure Contention free RA procedure
situation
• 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
• Handover
• DL data arrival during RRC_CONNECTED
• For positioning purpose during
RRC_CONNECTED requiring random access
procedure
difference • Preamble is assigned by UE • Preamble is assigned by eNodeB
• In RRC Connected status, not in RRC idle status

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Contention based
34
• Choose preamble index
- Contention based: The preamble index is randomly selected by the UE
1. Select group (A or B)
2. the UE randomly selects a preamble from the group and sets the PRACH Mask Index to 0
- Contention free: The preamble index is specified by eNodeB
• The eNodeB assigns a dedicated preamble index to the UE and specifies a PRACH Mask Index
• Select the PRACH resource for sending preamble
• Determine the corresponding RA-RNTI
• Determine the target received power (PREAMBLE_RECEIVED_TARGET_POWER)
How to select
• there is a preamble group B
• MSG3 is larger than messageSizeGroupA
• pathloss is less than PCMAX, c -
preambleInitialReceivedTargetPower - deltaPreambleMsg3
- messagePowerOffsetGroupB
MSG1 : UE send preamble

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Contention based
• eNodeB sends "Random Access Response" to UE on DL-SCH addressed to RA-RNTI calculated
from the timeslot in which preamble was sent.
• RA response window
• RAR MAC PDU
35
• MAC RAR Used to specify the amount of time
required for UE uplink synchronization
Inform the UE of the next uplink data
transmission resources for MSG3
For the initial identity authentication of the UE,
if the RA succeeds, this temporary identity
authentication will be converted to a formal
identity identification.
MSG2 : eNodeB sends Random Access Response

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• Only MSG3 and MSG4 are required for contention-based random access
• Using UL-SCH, UE sends "RRC connection request message" to eNodeB
• MSG3 contains UE identify and the reason for launching RA
• UE is identified by temporary C-RNTI (TC-RNTI)
• Contention resolution timer – Check RA is finished or not and the connection with layer2 is done or not
• With HARQ
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UE identity (TMSI or Random Value )
• TMSI is used if UE has previously
connected to the same network.
• Random value is used if UE is connecting
for the very first time to network.
Why we need
contention
resolution??
Contention based
MSG3 : UE send MSG3

OAI Project Team
• After eNB receive MSG3, MAC layer informs the RRC layer
• If the RRC recognizes the UE and allows the connection, the MSG4 is sent to the UE
• UE and eNB start to build up control plane and user plane’s signal channel
• Uplink power control, counter setting, HARQ parameters, CQI ,uplink schedule relative
module(like scheduling request).
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In RRC_CONNECTED state,
UE has C-RNTI
Not in RRC_CONNECTED state
Contention based
MSG4 : eNodeB sends contention resolution
Vs.

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Preamble
• How to select preamble (0-63)?
1. Select group (A or B)
Random Access Preambles group B exists
the potential message size is greater than messageSizeGroupA
the pathloss is less than PCMAX – preambleInitialReceivedTargetPower –
deltaPreambleMsg3 – messagePowerOffsetGroupB
choose group B
else choose group A
2. Randomly select a Random Access Preamble within the selected group.
3. Set PRACH Mask Index to 0.
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OAI Project Team
Preamble Format
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*Frame structure type 2 and special subframe configurations with UpPTS lengths and only.
Longer T_SEQ :
Help decoding PRACH under
noised condition because it
provide correlation window
to detect PRACH.
Why Multiple Preamble Format ?
Ans.
Depending on the environment, the required
conditions will be differentLonger T_CP：
Have better tolerance in fading
environment and reduce ISI

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PRACH information
• PRACH Parameters and it's Physical Meaning
- prach-ConfigIndex
- prach-FreqOffset
- rootSequenceIndex
- zeroCorrelationZoneConfig and Highspeedflag
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sib2
radioResourceConfigCommon
rach-ConfigCommon
…
...
prach-Config
rootSequenceIndex: 22
prach-ConfigInfo
prach-ConfigIndex: 3
..0. .... highSpeedFlag: False
zeroCorrelationZoneConfig: 5
prach-FreqOffset: 4
...

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prach-ConfigIndex
• This parameter determines what type of
preamble format should be used and at
which system frame and subframe UE can
transmit PRACH Preamble.
41
Which "PRACH Configuration Idex" to use, the network
is most likely to detect RACH from UE? why?
- prach-ConfigIndex
- prach-FreqOffset
- rootSequenceIndex

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prach-FreqOffset
• prach-FreqOffset is the parameter that determines the location of PRACH preamble in frequency
domain. This location in frequency domain is calculated in the unit of PRB index and calculated by
following equation. As you see, the equation gets different depending on Preamble Format.
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- prach-ConfigIndex
- prach-FreqOffset
- rootSequenceIndex

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rootSequenceIndex
• There are 838 root Zadoff-Chu sequences available for preambles.
• The length of each root sequence is 839.
• This rootSequenceIndex is a logical value.
43
Ex..
prach-Config
prach-ConfigInfo
prach-FreqOffset: 4
- prach-ConfigIndex
- prach-FreqOffset
- rootSequenceIndex

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zeroCorrelationZoneConfig and Highspeedflag
• zeroCorrelationZoneConfig and Highspeedflg information elements is to specify the cyclic shift
intervals to generate 64 PRACH Sequence from a single base sequence.
- Unrestricted Set (HighSpeedFlack = False)
- Restricted Set (HighSpeedFlack = True)
44
Ex..
prach-Config
prach-ConfigInfo
prach-FreqOffset: 4
- prach-ConfigIndex
- prach-FreqOffset
- rootSequenceIndex

OAI Project Team
How to Generate 64 PRACH Preamble Sequences
1. Generate a Zaddoff Chu sequence (849 samples) using rootSequenceIndex (let's call this sequence as
'base sequence')
2. Generate 64 different sequency by doing cyclic shift of the base sequence. The cyclic shift interval is
determined by Ncs and the Ncs is determined by zeroCorrelationZoneConfig and Highspeedflag.
Ex..
Let's suppose SIB2 broadcast the parameters as follows.
a) rootSequenceindex = 22  From a, the base Zaddoff-Chu sequence with u = 1
b) Highspeedflag = false
c) zeroCorrelationZoneConfig = 5 From b and c, the Nzc (Cyclicshift interval) = 26
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64 different PRACH sequence as follows.
PRACH Sequence[0] = base sequence
PRACH Sequence[1] = do cyclic shift to base sequence by 1 * 26 samples
....
PRACH Sequence[32] = do cyclic shift to base sequence +1
PRACH Sequence[33] = do cyclic shift to base sequence +1 by 1 * 26 samples
PRACH Sequence[34] = do cyclic shift to base sequence +1 by 2 * 26 samples
……
PRACH Sequence[63] = do cyclic shift to base sequence+1 by 31 * 26 samples

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• Three ways to trigger the random access procedure
1. PDCCH order trigger
2. MAC sublayer trigger
3. The upper trigger
46

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• DL Data arrival when Out-of-Sync
• UE positioning. (through a special DCI format 1A)
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Contention based Contention free

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2. MAC sublayer trigger
• UE has uplink data to send, but there is no
valid PUCCH resource available for sending
SR in any TTI
• What situation may the UE have no SR
resources?
- When the UE loses uplink synchronization, it
also releases SR resources.
• The function of the random access based
on contention may replace the function of
the SR to apply for the uplink resource.
49
*SR: scheduling request
*TTI : Transmission Time Interval

OAI Project Team
• The upper layer triggered random access process includes
- Initial access
- RRC connection re-establishment
- handover
50

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51
Initial access RRC Re-establishment

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52
Handover - Contention based Handover - Contention free

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- RLC
- PDCP
- RRC
HARQ
- HARQ vs ARQ
- HARQ process
- Downlink HARQ
Channel quality
53
Uplink Scheduling - Protocol Layer Architecture

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RLC Function
• Transfer of Upper Layer PDUs
- Signaling Radio Bearer (SRB) from RRC
- Radio Bearer (RB) from PDCP
• Error Correction through ARQ
• Concatenation, Segmentation and Reassembly of SDUs
• Re-segmentation of RLC PDUs
• In-Sequence delivery of Upper Layer PDUs
• Duplicate Detection
• Protocol Error Detection and Recovery

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RLC Modes for Uplink Logical Channel
• RLC has several modes, that is Transparent Mode (TM),
Unaknowledged Mode (UM), Aknowledged Mode (AM)

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Difference RLC Between LTE and HSPA
LTE RLC HSPA RLC
RLC located in eNB RLC located in RNC
Flexible RLC PDU sizes to match
underlying PHY layer capacity
RLC PDU sizes are semi-statically
configured by RRC layer (except
for R7 and beyond)
Re-segmentation of RLC PDUs
during retransmission to match
PHY layer capacity
Re-segmentation for
retransmissions not allowed
No ciphering at RLC layer
(performed at PDCP layer)
Ciphering supported for UM and
AM modes
Transparent mode for common
signaling channels only
Sometimes, even the traffic
channels as well as signaling
should use transparent mode

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PDCP Function
• Transfer of Data (C-Plane and U-Plane) between RLC and Higher U-Plane interface
• Maintenance of PDCP SN(Sequence Number)
• Transfer of SN Status (for use Upon Handover)
• ROHC (Robust Header Compression)
• In-Sequence delivery of Upper Layer PDUs at re-establishment of lower layer
• Elimination of duplicate of lower layer SDUs at re-establishment of lower layer for RLC
AM
• Ciphering and Deciphering of C-Plane and U-Plane data
• Integrity Protection and Integrity verification of C-Plane Data
• Timer based Discard
• Duplicate Discard
• For split and LWA bearers, routing and reordering.

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PDCP in the LTE Protocol Stack

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LTE PDCP Layer View

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RRC Function
• Broadcast of System Information related to the non-access stratum (NAS);
• Broadcast of System Information related to the access stratum (AS);
• Paging;
• Establishment, maintenance and release of an RRC connection between the UE
and E-UTRAN
• Security functions including key management;
• Establishment, configuration, maintenance and release of point to point Radio
Bearers;
• Mobility functions
• QoS management functions;
• UE measurement reporting and control of the reporting;
• NAS direct message transfer to/from NAS from/to UE.

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RRC State
The System Information required by an UE depends on the RRC state of
the UE.
• RRC Idle
• RRC Connected

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RRC_Idle
• This state indicates that there is not signaling radio bearer is
established i.e. no RRC connection is established. RRC_IDLE state can
further characterized as follows
- Transfer of broadcast/multicast data to UE.
- A UE specific DRX may be configured by upper layers.
- UE controlled mobility.
- The UE:
• Monitors control channels associated with the shared data channel to determine if data
is scheduled for it.
• Performing neighboring cell measurements and measurement reporting.
• Acquires system information

OAI Project Team
RRC_Connected
• This state indicates that there is signaling radio bearer established i.e. RRC
connection is established. RRC_CONNECTED state can be further
characterized as follows
- Transfer of unicast data to/from an UE, transfer of broadcast/multicast data to UE.
- At the lower layers, the UE may configure with a UE specific DRX/DTX
- Network Control Mobility, i.e. handover and cell change order with network
assistance (NACC) to GEREN.
- The UE:
• Monitors control channels associated with the shared data channel to determine if data is
scheduled for it
• Provides channel quality and feedback information Performing neighboring cell
measurements and measurement reporting.
• Acquires system information.

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- RLC
- PDCP
- RRC
HARQ
- HARQ vs ARQ
- HARQ process
- Downlink HARQ
Channel quality
64
Uplink Scheduling - BSR

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Uplink Scheduling
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• Uplink Data Transmission Scheduling
- Persistent Scheduling : UE can send UL data every time
- Non-Persistent Scheduling : UE will always ask NW to send UL Grant (DCI 0)
eNB UE
PUCCH
Scheduling Request
PDSCH
DCI format 0
PUSCH
Uplink Data
PHICH
Send ACK/NACK
eNB UE
PDCCH
DCI format 1, DCI format 0
PCFICH
CFI value
PUCCH
UCI (Carrying ACK/NACK)
PUSCH
Uplink Data
PHICH
Send ACK/NACK
Persistent
Non Persistent

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Buffer Status Report
BSR is a kind of MAC Control Element, which carrying an information on
how much data in UE buffer from UE to Network
• Allocate UL resources only when a particular UE has something to
transmit.
• Avoid allocating resources when UE has nothing to transmit to save
network resources.

OAI Project Team
Classification of BSR
• Data Structure
- Short BSR
- Long BSR
• BSR Timing
- Regular BSR
- Periodic BSR
- Padding BSR
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Scheduling Request
Buffer Status Report
This mechanism avoid wasting resource
allocation
1. eNB have no
idea amount of
UL Grant
needed
2. If grant
allocation is no
sufficient, UE
need to
retransmit

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Buffer Size according to Data Structure
• Buffer Size bit field size is always 6 which means it can represent only
0~63
68

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Regular BSR
• BSR will be sent by UE if :
- UE has some data to transmit in RLC or PDCP entity for a certain LCG.
- New Data Arrives in UL Buffer and no already existing data
- New Data has a higher priority than those which wait in UL Buffer
- RetxBSR-Timer expires and the UE has data available for transmission
• If there are UL Resources for new Transmission, transmitted on the
allocation resource
• If no UL resources, triggers scheduling request
69

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Periodic BSR
• Timer Controlled periodic reporting
• Trigerred if periodicBSR-Timer expires
• Tranmitted on UL resources allocated for new Tranmission
70

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Padding BSR
• Transmitted, instead of padding, on UL resources allocated for new
transmission if there are enough padding bits
• Number of padding bits after UL grant is allocated is equal to or larger
than the size of the Buffer Status Report MAC CE and its subheader.
71

OAI Project Team
When is Long or Short BSR sent?
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if( the number of LCG with allocated data > 1) --> Long BSR
else --> Short BSR
if( the number of padding bit >= the size of the Short BSR plus its subheader
&& the number of padding bit <= the size of the Long BSR plus its subheader)
{
if( the number of LCG with allocated data > 1) --> Truncated BSR
else --> Short BSR
} else {
--> Long BSR
}
Periodic & Regular BSR Padding BSR

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- RLC
- PDCP
- RRC
HARQ
- HARQ vs ARQ
- HARQ process
- Downlink HARQ
Channel quality
73
Uplink Scheduling - HARQ

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HARQ Vs ARQ
ARQ HARQ

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Application of HARQ

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UL HARQ
• Synchronous protocol
- Retransmission occurs at a pre-defined time after the initial transmission
- The process number can be implicitly derived
• Non-adaptive protocol, “typically”
• Retransmission must occur at the same frequency resources and with the same transmission
format as the initial transmission
• Adaptive retransmission are sometimes needed
• to avoid fragmenting the uplink frequency resource
• to avoid collisions with random-access resources

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HARQ Process for Uplink
• it have to use the specific process in a specific subframe called Synchronous
• There are two mode of operations : Adaptive and Non-Adaptive HARQ
Adaptive HARQ Non Adaptive HARQ

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HARQ Procedure Example
• Non-adaptive and adaptive HARQ procedure example in UL

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HARQ Timing
• Timing relation betweenw downlink data in subframe n and uplink HARQ
acknowledgement in subframe n+4 for FDD is as in the figure below
• Upon reception of HARQ acknowledgement, eNB can, if needed, retransmit the
downlink data in subframe n+8
- 8 HARQ processes are used
- HARQ round-trip time is 8ms

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- RLC
- PDCP
- RRC
HARQ
- HARQ vs ARQ
- HARQ process
- Downlink HARQ
Channel quality
80
Downlink Scheduling - Downlink transmission

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Downlink transmission procedure
81
UE eNB
CSI
PUCCH
DCI1
PDCCH(without DCI0)
Downlink Data:initial
PDSCH
ACK/NACK
PUCCH (If no Grant(no DCI0))
Downlink Data:new data or retransmission
PDSCH
ACK/NACK
PUCCH
http://msc-generator.sourceforge.net v6.1
UE eNB
CSI
PUSCH
DCI1
PDCCH(with DCI0)
Downlink Data:initial
PDSCH
ACK/NACK
PUSCH (If Grant(DCI0))
Downlink Data:new data or retransmission
PDSCH
ACK/NACK
PUSCH

OAI Project Team
DCI (Downlink control information)
• DCI carries those detailed information
- Which resource block carries data
- Which demodulation scheme use to decode data
- Resource allocation
- Power control command
- CSI report request
- CQI report request
• Downlink control information:
- Format 1, 1A, 1B, 1C, 1D, 2, 2A, 2B, 2C
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without DCI, UE
can’t decoding the
data from eNB

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CSI (Channel State Information)
• CSI: UE report channel status information to eNB
• CSI is a kind of collective name of several different type of indicators
- Channel Quality Indicator(CQI)
- precoding matrix indicator (PMI)
- precoding type indicator (PTI)
- rank indication (RI)
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UE
eNB
UE get CSI
Report CQI to eNB
eNB consider
channel quality
Transmit MCS by DCI

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Downlink MAC control element
• MAC control element: special MAC structure carrying the control
information.
- This special MAC structure is implemented as a special bit string in LCID field
of MAC Header.
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Values of LCID for DL-SCH
MAC PDU
MAC sub-header
R: Reserved bit
E: Extension field
LCID: Logical Channel ID field

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- RLC
- PDCP
- RRC
HARQ
- HARQ vs ARQ
- HARQ process
- Downlink HARQ
Channel quality
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Downlink Scheduling - HARQ

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HARQ (hybrid automatic repeat request)
• HARQ = FEC(forward error correction) + ARQ(automatic repeat
request)
• HARQ in MAC layer: retransmission loss or wrong data more qrickly.
• Classification:
- Time domain: synchronous and asynchronous
- Frequency domain: adaptive and non-adaptive
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HARQ Downlink Uplink
Time domain asynchronous synchronous
Frequency domain adaptive adaptive and non-adaptive

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Incremental redundancy
• HARQ with soft combining:
- Chase combining and incremental redundancy
• Use “incremental redundancy” in LTE.
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RV
RV: Redundancy Version
Value: 0 ~ 3

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HARQ process
• HARQ use “stop-and-wait protocol” to send data.
• In LTE, we use “Multiple parallel” stop-and-wait process.
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Multiple parallel HARQ process
HARQ
entity
8 HARQ
process

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NDI(new data indicator)
• NDI : parameter of HARQ
- Same NDI  the same HARQ process  stored in HARQ buffer to soft
combining (retransmission)
- Different NDI  different HARQ process  clear the HARQ buffer (new
transmit)
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0 1 0 0 0 1 0 0 0 1 0 0 0
NDI
toggled

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Downlink HARQ
• Downlink HARQ asynchronous:
- Retransmission can happen at anytime. (ie. HARQ process can be used in any
order.)
- It is the reason why we need DCI which contain HARQ process number. (The
eNB will clearly tell the UE about the current HARQ process. )
• Downlink HARQ adaptive:
- It means that eNB can change retransmission PRB and MCS. (ie. eNB need to
reschedule downlink retransmission.)
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Downlink HARQ process
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UE Base_Station
DCI1(Downlink Assignment)
PDCCH(with/without DCI0)
New Downlink Data
PDCCH
ACK/NACK
PUCCH(No grant,no DCI0)
ACK/NACK
PUSCH(grant,with DCI0)
If NACK
DCI1(Downlink Assignment)
PDCCH(with/without DCI0)
Retransmit Downlink Data
PDSCH(with/without DCI0)
4 sub-frame delay
At least 4 sub-frame delay

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Downlink HARQ process
• PDCCH : for DCI
• PDSCH : for data
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OAI Project Team
- RLC
- PDCP
- RRC
HARQ
- HARQ vs ARQ
- HARQ process
- Downlink HARQ
Channel quality
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Downlink Scheduling - Channel quality

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Modulation
• Modulation: Located in the MAC of the eNB side, whose function is to
make good use of the radio resources of the uplink and downlink
channels.
- The two schedulers on the uplink and the downlink respectively control the
operation of the schedule.
- Allocate resources and codes on the physical layer to the uplink and downlink
transmission channels respectively.
• There are three modulation modes in LTE.
- QPSK
- 16QAM
- 64QAM
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MCS (Modulation coding scheme)
• Modulation Coding Scheme is related to Modulation Order.
- Modulation order is defined as a Parameter called Qm.
• Code rate: How many percentages of real data are in a transmission.
- Ex. 5/6: data  5
checksum  1
- Channel signal is well  64QAM
- Channel signal is bad  QPSK
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Qm Modulation Method
2 QPSK
4 16QAM
6 64QAM

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CQI (Channel quality indicator)
• CQI is the information that UE sends to the network via PUCCH
(periodic CQI) or PUSCH (aperiodic, period CQI)
- Current communication channel quality
- UE wants to get the data with transport block size
• The CQI value ranges from 0 ~ 30
- 30 indicates the best channel quality
- 0,1 indicates the poorest channel quality
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• UE reports low CQI  cause waste of radio resources
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UE Base
Station
Too big transport block
Failed to
decode it
Report low CQI
Retransmission
Response NACK

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• Base station gets “high” CQI value from UE  transmit the data with
“larger” transport block size
• Base station gets “low” CQI value from UE  transmit the data with
“smaller” transport block size
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OAI Project Team
- RLC
- PDCP
- RRC
HARQ
- HARQ vs ARQ
- HARQ process
- Downlink HARQ
Channel quality
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Downlink Scheduling - Discontinuous Reception

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DRX (Discontinuous Reception)
• DRX: It’s kind of mechanism, which can improve the battery life by
sleeping for a while there is no data transmission.
• While UE in RRC_CONNECTED state, it will deploy a “DRX cycle”.
- DRX cycle = On Duration + Opportunity for DRX
- UE receive PDCCH on Duration, otherwise UE will go to sleep
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• Case 1: Only Long DRX Cycle is configured and No PDCCH is received during the cycle
• Case 2: Only Long DRX Cycle is configured and a PDCCH is received during a cycle
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• Case 3: Only Long DRX Cycle is configured and a PDCCH and DRX Command MAC CE are received during
a cycle
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OAI Project Team
Reference
• Sharetechnote
• 深入理解LTE-A
• Spec 36.212 for DCI format
• Spec 36.213 for Downlink transmission procedure
• Spec 36.321 for MAC control element
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