lte radio interface - afralti • overview of ofdma & sc-fdma • lte frame structure • lte...

LTE Radio Interface

Agenda

• Overview of OFDMA & SC-FDMA

• LTE Frame Structure

• LTE Resource Grid

• LTE Bandwidth/Resource Configuration

• LTE Channels

• LTE Protocol Architecture

User 1 User 2 User 3 User ..

Overview of Radio Technologies

f

t

TDMA

Time Division

t

CDMA

Code Division

f

OFDMA

Frequency Division Orthogonal subcarriers

f

t

f

t

FDMA

Frequency Division

Each user has a unique frequency

All users transmit at the same time

AMPS, NMT, TACS

Each user has a unique timeslot

Several users share the same frequency

IS-136, GSM, PDC

Each channel has a unique code

Several users share the same frequency & time

IS-95, CDMA2000, WCDMA

Each user and channel has a unique time and frequency allocation

Users are separated in frequency and/or time

LTE, Wimax, 802.11

f0 f1 f2 f3 f4

OFDM carriers

n = Κ −1,0,1,2.Κ f = f + nf = f + n 1 n 0 s 0 T s

OFDM carriers have no Adjacent Carrier Interference (ACI)

FDM vs OFDM Po

wer

Den

sity

Pow

er D

ensi

ty

Frequency (f/fs) Frequency (f/fs)

Saved Bandwidth

Compared to conventional FDM, OFDM allows orthogonal subcarriers to overlap tightly

OFDM vs OFDMA

OFDM allocates users in the time domain only

OFDMA allocates users in the time and frequency domains

UL Allocation (SC-FDMA)

OFDMA vs SC-FDMA

User 1

User 2

User 3

User 1

User 2

User 3

Sub-carriers

TTI: 1ms

Frequency

System Bandwidth

Sub-band：12Sub-carriers Time User 1

User 2

User 3

TTI: 1ms

Frequency

Time

With OFDMA, the subcarriers are shared among multiple users. This results in: •Different spectrum bandwidths can be utilized without impacts on system design. •Transmission resources of variable bandwidth can be allocated to different users and scheduled freely in the frequency domain. •Fractional frequency re-use and interference coordination between cells are facilitated.

System Bandwidth

• With SC-FDMA, each user is assigned part of the system bandwidth

• SC-FDMA has a significantly lower PAPR, providing the advantages of multicarrier technology without excessive cost for the mobile terminal transmitter, while retaining a reasonable degree of commonality between uplink and downlink technologies.

Sub-carriers

Sub-band：12Sub-carriers

User 1

User 2

User 3

User 1 User 2 User 3

User 1

User 2

User 3

Advantages • Support flexible bandwidth operation • Support FDD (Frequency Division Duplex), TDD (Time Division Duplex), and

half duplex FDD mode (HD-FDD) • Frequency Diversity • Support MIMO Disadvantages • Sensitivity to Doppler • Overhead • High PAPR; in UL, SCFDMA has much lower PAPR (Peak to Average Power

Ratio) than OFDM

OFDMA & SC-FDMA

Agenda





• LTE Channels


LTE Frame Structure • Two radio frame structures

defined. – Frame structure type 1 (FS1): FDD. – Frame structure type 2 (FS2): TDD.

• A radio frame has duration of 10 ms.

• A resource block (RB) spans 12 subcarriers over a slot duration of 0.5 ms.

• One subcarrier has bandwidth of 15 kHz, thus 180 kHz per RB.

TDD Frame

FDD Frame

TYPE 1 – FDD FRAME STRUCTURE

Symbol Time = 66.7 µsec

TYPE 2 - TDD Frame Structure

TDD Frame Configurations

D = Downlink Subframe U = Uplink Subframe S = Special Subframe

Uplink- downlink

configuration

Downlink-to-Uplink Switch-point periodicity

Subframe number

0 1 2 3 4 5 6 7 8 9

0 5 ms D S U U U D S U U U

1 5 ms D S U U D D S U U D

2 5 ms D S U D D D S U D D

3 10 ms D S U U U D D D D D

4 10 ms D S U U D D D D D D

5 10 ms D S U D D D D D D D

6 5 ms D S U U U D S U U D

CYCLIC PREFIX INSERTION

FDD/TDD Summary

Agenda





• LTE Channels


LTE Resource Grid

SUB-CARRIER TYPES

• DC Subcarriers

• Guard Subcarriers

• Data Subcarriers

• Reference Signals

Agenda





• LTE Channels


CHANNEL BANDWITH vs BANDS

Source: 3GPP TS 36.101 version 10.7.0 Release 10

Bandwidth vs RBs • LTE physical layer supports any bandwidth

from 1.4 MHz to 20 MHz in steps of 180 kHz (resource block)

• Current LTE specification supports a subset of 6 different system

• All UEs must support the maximum bandwidth of 20 MHz

Source: 3GPP TS 36.101 version 10.7.0 Release 10

BANDWIDTH vs SUBCARRIERS

LTE Frame and Bandwith

Adaptive Modulation Coding

Maximum Data Rate

2X2 and 4X4 MIMO would provide higher data rate

Channel Bandwith (MHz)

1.4 3 5 10 15 20

Data Sub-carriers 72 180 300 600 900 1,200

Symbols per Slot (0.5 msec)

6 6 6 6 6 6

Symbols per Second (per Sub-carrier)

12,000 12,000 12,000 12,000 12,000 12,000

QPSK (max = 2 bits per Symbol) Mbps

1.73 4.32 7.20 14.40 21.60 28.80

16QAM (max = 4 bits per Symbol) Mbps

3.46 8.64 14.40 28.80 43.20 57.60

64QAM (max = 6 bits per Symbol) Mbps

5.18 12.96 21.60 43.20 64.80 86.40

Agenda





• LTE Channels


LTE Physical Channels • DL

– Physical Broadcast Channel (PBCH): Carries system information for cell search, such as cell ID. – Physical Downlink Control Channel (PDCCH) : Carries the resource allocation of PCH and DL-SCH, and

Hybrid ARQ information. – Physical Downlink Shared Channel (PDSCH) : Carries the downlink user data. – Physical Control Format Indicator Channel (PCFICH) : Carriers information of the OFDM symbols

number used for the PDCCH. – Physical Hybrid ARQ Indicator Channel (PHICH) : Carries Hybrid ARQ ACK/NACK in response to uplink

transmissions. – Physical Multicast Channel (PMCH) : Carries the multicast information. – Reference Signal (RS) – Synchronization Signal (P-SS and S-SS)

• UL – Physical Random Access Channel (PRACH) : Carries the random access preamble. – –

Physical Uplink Shared Channel (PUSCH) : Carries the uplink user data. Physical Uplink Control Channel (PUCCH) : Carries the HARQ ACK/NACK, Scheduling Request (SR) and Channel Quality Indicator (CQI), ... Uplink Reference Signal

Physical channels determine how data is processed and then mapped –

via dynamical scheduling onto resource blocks.

DL Reference Signals (RS) DL Reference Signals

Used for downlink physical channel demodulation and channel quality measurement (CQI)

Cell-Specific Reference Signals are generated from cell-specific RS sequence and frequency shift mapping.

The frequency interval of RS is 6 subcarriers. RS distributes discretely in the time-frequency

domain, sampling the channel situation which is the reference of DL demodulation.

RS distribution leads to accurate channel estimation, at the cost of high overhead and reduced system capacity.

Used for – coherent demodulation in the UE – channel-quality measurements for scheduling – measurements for mobility

Synchronization Signals: used for time-frequency synchronization between UE and E-UTRAN during cell search procedure. synchronization signal comprise two parts:

Primary Synchronization Signal (P-SS), used for symbol timing, frequency synchronization and part of the cell ID detection.

Secondary Synchronization Signal (S-SS), used for detection of radio frame timing, CP length and cell group ID.

Characteristics: Occupies 63 subcarriers (62+DC), located at

the center of the system bandwidth Synchronization signals are transmitted in

the 1st and 11rd slots of every 10ms frame. The primary synchronization signal is

located in the last symbol of the transmit slot. The secondary synchronization signal is located in the 2nd last symbol of the transmit slot.

Synchronization Signals Structure

DL Synchronization Signals (P-SS & S-SS)

UL Reference Signals Used for synchronization between E-UTRAN and UE, as well as uplink channel estimation. Two types of UL reference signals:

DM RS (Demodulation Reference Signal), associated with PUSCH and PUCCH transmission. SRS (Sounding Reference Signal), without associated PUSCH and PUCCH transmission.

Each UE occupies parts of the system

bandwidth since SC-FDMA is applied in uplink. DM RS only transmits in the bandwidth allocated to PUSCH and PUCCH.

The slot location of DM RS differs with associated PUSCH and PUCCH format.

Sounding RS’s bandwidth is larger than that allocated to UE, in order to provide the reference to e-NodeB for channel estimation in the whole bandwidth.

Sounding RS is mapped to the last symbol of sub-frame. The transmitted bandwidth and period can be configured. SRS transmission scheduling of multi UE can achieve time/frequency/code diversity.

LTE DL Channels

FDD Downlink – 20 MHz – 4 Antennas

LTE UL Physical Channels

kWave

LTE Transport Channels • DL

– Broadcast Channel (BCH): System Information broadcasted in the entire coverage area of the cell.

– Downlink Shared Channel (DL-SCH): User data, control signaling and System Info. HARQ and link adaptation. Broadcast in the entire cell or beamforming.

– Paging Channel (PCH): Paging Info broadcasted in the entire cell. – Multicast Channel (MCH): MBMS traffic broadcasted in entire cell. MBSFN is

supported.

• UL

– Uplink Shared Channel (UL-SCH): User data and control signaling. HARQ and link adaptation

– Random Access Channel (RACH): Random Access transmissions (asynchronous and synchronous). Transmission typically contention based.

Define how is something transmitted over the air, e.g. what are encoding, interleaving options used to transmit data, bit rates (transport block sizes, number of blocks), a transmission time

interval, delay, support for HARQ, support for beam-forming, , and so on.

LTE Logical Channels • Control Channels: Control-plane information

– Broadcast Control Channel (BCCH): DL broadcast of system control information – Paging Control Channel (PCCH): DL paging information. UE position not known

on cell level – Common Control Channel (CCCH): UL/DL when no RRC connection exists – Multicast Control Channel (MCCH): DL point-to-multipoint for MBMS

scheduling and control – Dedicated Control Channel (DCCH): UL/DL dedicated control information.

Used by UEs having an RRC connection

• Traffic Channels: User-plane information – Dedicated Traffic Channel (DTCH): UL/DL Dedicated Traffic to one UE, user

information – Multicast Traffic Channel (MTCH): DL point-to-multipoint. MBMS user data

Define what type of information is transmitted over the air, e.g. traffic channels, control channels, system broadcast, etc.

LTE Channel Mapping

Physical Channels (DL)

eNodeB User Equipment

(UE)

Synchronization Channel (SCH): timing & frequency sync Physical Broadcast Channel (PBCH): basic system broadcast info

Physical Control Format Indicator Channel (PCFICH): Time span of PDCCH

Physical Downlink Control Channel (PDCCH): DL Scheduling Grant

Physical Downlink Shared Channel (PDSCH): DL Traffic

Physical Hybrid ARQ Indicator Channel (PHICH): HARQ Feedback for UL

Physical Uplink Control Channel (PUCCH) HARQ Feedback for DL (CQI)

Physical Channels (UL)

eNodeB User Equipment

(UE)

Physical Random Access Channel (PRACH): initial UL access & timing Physical Uplink Shared Channel (PUSCH): traffic & channel reference signal Physical Uplink Control Channel (PUCCH): UL scheduling request Physical Downlink Control Channel (PDCCH): UL Scheduling Grant

Physical Hybrid ARQ Indicator Channel (PHICH): HARQ Feedback for UL

Agenda





• LTE Channels


Radio Layer Protocol

Source: 3GPP TS 36.300

Layer 3 Protocols

FDD | TDD - Layer 1 ( DL: OFDMA, UL: SC-FDMA )

Medium Access Control (MAC)

Physical Channels

Transport Channels

RLC (Radio Link

Control) …

PDCP’ (Packet Data Convergence

Protocol) …

RLC (Radio Link

Control)


Protocol)

RLC (Radio Link

Control)

PDCP (Packet Data Convergence

Protocol)

RLC (Radio Link

Control)


Protocol)

RLC (Radio Link

Control)


Protocol)

Logical Channel

(E-)RRC (Radio Resource Control)

IP / TCP | UDP | …

Application Layer

Radio Bearer

NAS Protocol(s) (Attach/TA Update/…) NAS System Information (BCCH)

E-UTRAN System Info. (BCCH)

Paging (PCCH)

RRC Connection Management

Temporary Identifiers UE-EUTRAN

Allocation of Sign. Radio Bearers

Mgmt. of ptp radio bearers

Mobility Functions (LTE_ACTIVE)

UE measurement reporting/control

Inter-cell handover

Control of cell (re-)selection

UE context transfer between eNB

MBMS

Notification for MBMS services

Mgmt. of MBMS radio bearers

QoS control

Transfer of NAS messages

E-UTRAN Security

Integrity protection for RRC msg.

Layer 1 & 2 Protocols



Physical Channels

Transport Channels

RLC (Radio Link

Control) …


Protocol) …

RLC (Radio Link

Control)


Protocol)

RLC (Radio Link

Control)


Protocol)

RLC (Radio Link

Control)


Protocol)

RLC (Radio Link

Control)


Protocol)

Logical Channel


NAS Protocol(s) Application Layer (Attach/TA Update/…)


Radio Bearer

Header Compression

De-Ciphering

Segment./Reassembly

ARQ

HARQ

Interleaving Modulation

Resource Mapping/MIMO

CRC

Coding/Rate Matching

Scheduling / Priority Handling

De-Multiplexing

PDCP Layer



Physical Channels

Transport Channels

RLC (Radio Link

Control) …


Protocol) …

RLC (Radio Link

Control)


Protocol)

RLC (Radio Link

Control)


Protocol)

RLC (Radio Link

Control)


Protocol)

RLC (Radio Link

Control)


Protocol)

Logical Channel



Application Layer

Radio Bearer

NAS Protocol(s) (Attach/TA Update/…)

• User Plane: – Header compression and decompression:

ROHC – Transfer of user data: PDCP receives PDCP

SDU from the NAS and forwards it to the RLC layer and vice versa

– In-sequence delivery of upper layer PDUs at handover for RLC AM

– Duplicate detection of lower layer SDUs at handover for RLC AM

– Retransmission of PDCP SDUs at handover for RLC AM

– Ciphering – Timer-based SDU discard in uplink

• Control Plane: – Ciphering and Integrity Protection – Transfer of control plane data: PDCP

receives PDCP SDUs from RRC and forwards it to the RLC layer and vice versa

RLC Layer

FDD | TDD - Layer 1


Transport Channels

RLC (Radio Link

Control) …


Protocol) …

RLC (Radio Link

Control)


Protocol)

RLC (Radio Link

Control)


Protocol)

RLC (Radio Link

Control)


Protocol)

RLC (Radio Link

Control)


Protocol)

Logical Channel



Application Layer

Radio Bearer


• Transfer of upper layer PDUs • Error Correction through ARQ • Segmentation of SDUs according to

the size of the TB • Re-segmentation of PDUs that need

to be retransmitted • Concatenation of SDUs for the same

radio bearer • In-sequence delivery of upper layer

PDUs except at HO • Protocol error detection and recovery • Duplicate Detection • SDU discard

( DL: OFDMA, UL: SC-FDMA )

Physical Channels

MAC Layer



Transport Channels

RLC (Radio Link

Control) …


Protocol) …

RLC (Radio Link

Control)


Protocol)

RLC (Radio Link

Control)


Protocol)

RLC (Radio Link

Control)


Protocol)

RLC (Radio Link

Control)


Protocol)

Logical Channel



Application Layer

Radio Bearer


• Mapping between logical channels and transport channels

• Multiplexing/demultiplexing of RLC PDUs (Protocol Data Unit) belonging to one or different radio bearers into/from TB (transport blocks ) delivered to/from the physical layer on transport channels

• Traffic volume measurement reporting

• Error correction through HARQ • Priority handling between logical

channels of one UE • Priority handling between UEs

(dynamic scheduling) • Transport format selection

Physical Channels • Padding

HARQ

Packet

L1 NACK

eNodeB UE

ReTX

• The lower part of the MAC entity is the HARQ (Hybrid Automatic Retransmission on reQuest) entity.

• HARQ has already been used for HSDPA and HSUPA. • L1-based signaling to indicate need for retransmission =

fast round trip time facilitated between UE and eNB • HARQ especially increases the performance (delay and

throughput) for cell edge users. • HARQ simply implements a retransmission protocol on

layer 1/2 that allows to send retransmitted blocks with different coding than the 1st one.

• Only certain transport channel types (UL-SCH) can have this unit.

• The assembled transport block from the multiplexer will be stored in one of the HARQ’s buffers and simultaneously sent to the physical layer:

entity will retransmit the transport block.

– If the eNB/UE receives the transport block correctly, it will send an ACK indication via a special physical channel. This would delete the transport channel from the buffer.

– If no indication or a NACK indication is received, the HARQ

Data Transfer – Layer 1 & 2

User Datarate

Protocol Overhead

Channel Bitrate

L1 Procedures (1/2) • Cyclic Redundancy Check (CRC) : used to detect if there are any uncorrected errors left after

error correction. Blocks of data are passed through a CRC generator, which will perform a mathematical division on the data producing a remainder or checksum. LTE uses a 24 bit CRC for the user data channels.

• Channel Coding/Forward Error Correction (FEC) : a type of digital signal processing which improves data reliability by introducing parity information (redundancy) into a data sequence prior to transmission. This enables a receiver to detect and correct transmission errors. LTE utilizes turbo coding for the user data quite similar to the coding in WCDMA, but improved with a QPP (Quadrature Permutation Polynomial) interleaver between two rate ½ encoders. The overall coding rate is 1/3.

• Interleaver : used to randomize the bursty errors. Interleaving is determined by the delay requirements of the service.

• Rate Matching : performed on the data to change the data rate to one that can be accommodated by the system. Used to reduce the data rate (by puncturing bits) but also to increase the data rate (by padding it with extra bits).

L1 Procedures (2/2) • Scrambling : Cell specific bit-level scrambling used in LTE for all datastreams in UL and DL;

used in order to achieve interference randomization between cells • Link adaptation (AMC: adaptive modulation and coding) with various modulation schemes

and channel coding rates is applied to the shared data channel. The same coding and modulation is applied to all groups of resource blocks belonging to the same L2 PDU scheduled to one user within one TTI and within a single stream.

• Power Control The power spectral density of the uplink transmissions can be influenced by the eNB

• Cell search : procedure by which a UE acquires time and frequency synchronization with a cell and detects the Cell ID of that cell. It is based on the primary and secondary synchronization signals.

• Uplink timing control: sent by the eNB to the UE which the UE uses to advance/delay its timings of transmissions to the eNB so as to compensate for propagation delay and thus time align the transmissions from different UEs with the receiver window of the eNB.

Modulation • The sub-carriers are modulated with a certain modulation scheme: the data bits are mapped

into a carrier phase and amplitude (symbols) • E-UTRAN user data channels supports QPSK, 16QAM and 64QAM • 16QAM allows for twice the peak data rate compared to QPSK • 64QAM allows for three times the data rate compared to QPSK • Higher order modulation more sensitive to interference; i.e. useful mainly in good radio

channel conditions (high C/I, Little or no dispersion, Low speed) e.g. close to cell site & Micro/Indoor cells

• BPSK is used for some signaling (PHICH)

Link Adaptation

Available modulation schemes per physical channels

Physical channel Modulation

PDSCH QPSK, 16QAM, 64QAM

PBCH QPSK

PDCCH (and PCFICH) QPSK

PHICH BPSK

PUSCH QPSK, 16QAM

PUCCH BPSK and QPSK (for ACK/NACK and CQI messages) On/off keying (for scheduling request)

Channel Feedback Reporting

• CQI – Channel Quality Indicator

• RI – Rank Indicator : 1, 2, or 4 (*)

• PMI – Pre-coding Matrix Indicator : TX mode 4, 5, and 6 (*)

(*) MIMO only

CQI Modulation Coding rate * 1024

Bits/res element

0 Out-of-range

1 QPSK 78 0.1523

2 QPSK 120 0.2344

3 QPSK 193 0.3770

4 QPSK 308 0.6016

5 QPSK 449 0.8770

6 QPSK 602 1.1758

7 16QAM 378 1.4766

8 16QAM 490 1.9141

9 16QAM 616 2.4063

10 64QAM 466 2.7305

11 64QAM 567 3.3223

12 64QAM 666 3.9023

13 64QAM 772 4.5234

14 64QAM 873 5.1152

15 64QAM 948 5.5547

MAC Scheduler



Physical Channels

Transport Channels

RLC (Radio Link

Control) …


Protocol) …

RLC (Radio Link

Control)


Protocol)

RLC (Radio Link

Control)


Protocol)

RLC (Radio Link

Control)


Protocol)

RLC (Radio Link

Control)


Protocol)

Logical Channel



Application Layer

Radio Bearer

Header Compression

De-Ciphering

Segment./Reassembly

ARQ

Scheduling / Priority Handling

HARQ

CRC Coding/Rate Matching

Interleaving Modulation


Resource Mapping/MIMO

De-Multiplexing

MAC Scheduler

Payload Selection

Priority Handling

Retransmission Control

Modulation Scheme

Antenna & Resource Mgt

MAC Scheduler • eNB scheduler controls the time/frequency resources for a given time for uplink and

downlink – dynamically controls the terminal(s) to transmit to and, for each of these terminals, the set of resource blocks upon which the

terminal’s DL-SCH should be transmitted

• Scheduler dynamically allocates resources to UEs at each TTI • The scheduling strategy is implementation specific and not specified by 3GPP

– scheduler selects best multiplexing for UE based on channel conditions – preferably schedule transmissions to a UE on resources with advantageous channel condition

• Most scheduling strategies need information about: – channel conditions at the terminal – buffer status and priorities of the different data flows – interference situation in neighboring cells (if some form of interference coordination is implemented)

• UE transmits – channel-status reports reflecting the instantaneous channel quality in the time and frequency domains – information necessary to determine the appropriate antenna processing in case of spatial multiplexing

• Downlink LTE considers the following schemes as a scheduler algorithm: – Frequency Selective Scheduling (FSS) – Frequency Diverse Scheduling (FDS) – Proportional Fair Scheduling (PFS)

• Interference coordination, which tries to control the inter-cell interference on a slow basis, is

Frequency Domain Scheduling

• Exploits frequency selective power variations on the desired signal

• Uses Resource Blocks that are not faded

• Effective with system bandwidths equal to or larger than 5 MHz

• Specific to OFDMA

• Not possible with CDMA, W-CDMA

Applicable Standards • 3GPP TS 36.101: User Equipment (UE) radio transmission and reception • 3GPP TS 36.104: Base Station (BS) radio transmission and reception • 3GPP TS 36.201: Physical layer - general description • 3GPP TS 36.211: Physical channels and modulation • 3GPP TS 36.212: Multiplexing and channel coding • 3GPP TS 36.213: Physical layer procedures • 3GPP TS 36.214: Physical layer – measurements • 3GPP TS 36.300: Overall description • 3GPP TS 36.304: UE procedures in idle mode • 3GPP TS 36.321: MAC protocol specification • 3GPP TS 36.322: RLC protocol specification • 3GPP TS 36.323: PDCP specification • 3GPP TS 36.331: RRC protocol specification • 3GPP TR 36.803: User Equipment (UE) radio transmission and reception

THANK YOU

lte radio interface - afralti • overview of ofdma & sc-fdma • lte frame structure • lte...

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