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PM104

LTE: Downlink Physical-Layer Overview and Throughput Simulation Results

Nov 5, 2008

Ian C. Wong, Ph.D. Jason WongSystems Engineer, Algorithms and Standards System & Application EngineerCellular Products Group R&D GSM

TMFreescale Semiconductor Proprietary Information. Freescale™ and the Freescale logo are trademarksof Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners. © Freescale Semiconductor, Inc. 2008. 2

Agenda

►Overview of LTE• LTE philosophy• LTE performance requirements

►LTE Physical Layer • General description• Physical channels• Channel coding• Modulation

►Throughput simulation results• Freescale’s LTE downlink simulator• Downlink shared channel• Control channel


3GPP – LTE Philosophy and Criteria

►Philosophy• LTE focus is on:

Enhancement of the Universal Terrestrial Radio Access (UTRA)Optimization of the UTRAN architecture

• With HSPA (downlink and uplink), UTRA will remain highly competitive for several years

• LTE project aims to ensure the continued competitiveness of the 3GPP technologies for the future

►Criteria• Demand for higher data rates• Expectations of additional 3G spectrum allocations• Greater flexibility in frequency allocations• Continued cost reduction• Keeping up with other (unlicensed) technologies (eg WiMAX)


Performance Requirements for LTE Category Requirement

Peak data rate DL: 100 Mbps in 20 MHz BW (5 bps/Hz)UL: 50 Mbps in 20 MHz BW (2.5 bps/Hz)

Control-plane latency < 100 ms for Idle to Active mode transition

Control-plane capacity > 200 users per cell in Active state within 5 MHz

User-plane latency < 5 ms for 1 user with 1 data stream and small IP packet

Average user throughput DL: 3-4 times of HSDPA per MHzUL: 2-3 of HSUPA per MHz

MobilityOptimized for 0-15 km/h

Support with high performance for 15-120 km/hSupport for 120-350 km/h or even 500 km/h

CoverageAll targets met for 5 km cells

Slight degradation for 5-30 km cellsSupport for 30-100 km cells

Spectrum flexibility Support for 1.25 – 20 MHz BandwidthsPaired or unpaired spectrum allocations


LTE Protocol Architecture Around the Physical Layer

►PHY interfaces MAC sub-layer of Layer 2 and RRC of Layer 3

►PHY offers a transport channel to MAC

• Transport channel characterized by how information is transferred

►MAC offers logical channels to the RLC

• Logical channel characterized by the type of information transferred.

Radio Resource Control (RRC)

Medium Access Control (MAC)

Physical Layer(PHY)

Logical channels

Transport channels

Con

trol/M

easu

rem

ents Layer 2

Layer 1

Layer 3

Radio Link Control (RLC)

…

Physical channels


Layer 1 General Description

►Multiple access• OFDMA in the downlink• SC-FDMA in the uplink

►Duplexing• Frequency-division duplexing (FDD) for paired spectrum• Time-division duplexing (TDD) for unpaired spectrum• Half-duplex FDD for paired spectrum (optional)


Frame Structure

►Frame structure type 1 applicable to FDD and half duplex FDD►Each 10 ms radio frame is divided into ten equally sized sub-frames►Each sub-frame consists of two equally sized slots

Slot 0 Slot 1 Slot 2 Slot 3 … Slot 18 Slot 19

One radio frame = 10 ms

One subframe = 1 ms

One slot = 0.5 ms


Slot Structure and Resource Elements

DLsymbN

slotT

0=l 1DLsymb −= Nl

RB scDL RB

NN

× RB scN

RBsc

DLsymb NN ×

),( lk

0=k

1RBsc

DLRB −= NNk

Transmission BW (MHz) 1.4 3 5 10 15 20

Slot duration 0.5 ms

Subcarrier spacing 15 kHz / 7.5 kHz (MBSFN)

Sampling frequency (MHz)

1.92 3.84 7.68 15.36 23.04 30.72

FFT size 128 256 512 1024 1536 2048

# downlink RBs(NDL

RB)6 15 25 50 75 100

# subarriers / RB (NRB

sc)12 / 24 (MBSFN)

# OFDM symbols / slot (NDL

symb)

7 (normal CP) 6 (extended CP)

3 (extended CP in MBSFN)

CP length (μs) 1 x 5.21, 6 x 4.69 (normal)16.6 (extended)


Downlink Physical Channels

►Physical downlink shared channel (PDSCH)• Carries the downlink shared channel (DL-SCH) and paging channel (PCH)

►Physical downlink control channel (PDCCH)• Informs the UE about the resource allocation of PCH and DL-SCH, and Hybrid

ARQ information related to DL-SCH• Carries the uplink scheduling grant

►Physical control format indicator channel (PCFICH)• Informs the UE about the number of OFDM symbols used for the PDCCHs;• Transmitted in every subframe.

►Physical broadcast channel (PBCH)• The coded broadcast channel (BCH) transport block is mapped to four

subframes within a 40 ms interval• 40 ms timing is blindly detected• Each subframe is assumed to be self-decodable, i.e. the BCH can be decoded

from a single reception, assuming sufficiently good channel conditions.►Physical Hybrid ARQ Indicator Channel (PHICH)

• Carries Hybrid ARQ ACK/NAKs in response to uplink transmissions.►Physical multicast channel (PMCH)

• Carries the multicast channel (MCH) transport channel


Channel Coding of Physical Channels

►Cyclic redundancy check• Error detection capability for transport block and each code block

►Code-block segmentation• Segment the transport block into smaller code blocks of roughly equal length

►Channel coding• Rate 1/3 turbo code – Downlink shared, paging, and multicast channels• Rate 1/3 tail-biting convolutional code – Broadcast channel and downlink control

information• Rate 1/16 block code – Control format indicator• Rate 1/3 repetition code – HARQ indicator

►Rate-matching• Includes sub-block interleaving, bit collection, and bit selection and pruning• Support hybrid ARQ transmissions

CRC

Code block

segment-ation

Channelcoding

Ratematching

UncodedTransport Channel bits

CodedPhysicalChannel bits


Baseband Modulation of Downlink Physical Channels

►Scrambler breaks long strings of 1s and 0s of the coded bits►Modulation mapper converts scrambled bits into complex-valued symbols►Layer mapper and precoder performs symbol transformations to enable

multi-antenna transmission techniques►Resource element mapper maps the physical channel symbols to the

appropriate locations in the time-frequency grid ►OFDM signal generation converts the frequency domain symbols to time

domain complex baseband signals for each antenna port for transmission

ScramblerOFDM Signal

Generation

ModulationMapper

LayerMapper

+Precoder

ResourceElementMapper

Reference Signal Synchronization Signal

Reference & synch signalgenerator

Coded PhysicalChannel bits Complex

OFDM signal


Downlink Reference Signals

► Transmitted in all subframes not used for MBSFN transmission

► Primarily used for channel estimation and tracking purposes

► Pseudo-random QPSK signals generated using length-31 gold sequence

► Orthogonal across time, frequency, and antennas within a cell

► Orthogonal in the code-domain among neighboring cells

One antenna port

Two antenna ports


Downlink Synchronization Signals

►Primary synch signal• Half-frame timing reference• Frequency domain Zadoff-Chu

sequence• Occupies only middle 62

subcarriers in frequency►Secondary synch signal

• Frame timing reference• Interleaved concatenation of two

length-31 m-sequences

DL - RS

PSCH

1st SSCH

2nd SSCH

RS: Reference SignalPSCH: Primary Synchronization ChannelSSCH: Secondary Synchronization Channel

Frame (10ms)

Subframe(1ms)

1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7

Slot(0.5ms)

Slot 0 Slot 10

* Figure courtesy of Taeyoon Kim, Freescale


Freescale’s Radio Link Simulator Architecture

PHY EncoderTS 36.212 –

Multiplexing and Channel Coding

Transportchannel raw bits

(generated by encoder) …

Physical channel coded bits

PHY ModulatorTS 36.211 –

Physical Channels and Modulation

PDSCHPDCCH

PBCH

TD-transmitted signal

Mobile MIMOChannel

PHY DemodulatorTS 36.211 –

Physical Channels and Modulation

Noise/Interference…

PDSCHPDCCH

PBCH

PHY DecoderTS 36.212 –

Multiplexing and Channel Coding

Decoded bits

Physical channel LLRs

Statistics Calculator (Coded/Uncoded BER, PER, Throughput)

PhyChanBits.txTrChanBits.tx

TrChanBits.rx PhyChanBits.rx

ReceiverImpairments


Example RAN4 PDSCH Simulation Results

FDD 1x2 SIMO, 10MHz, 16-QAM 1/2, ETU300 channel

SIMO 16QAM 1/2 ETU300

0

2000000

4000000

6000000

8000000

10000000

12000000

14000000

16000000

-8 -6 -4 -2 0 2 4 6 8 10 12 14

SNR (dB)

Thro

ughp

ut (b

ps)

EricssonFreescaleInterDigitalLGEFujitsuNokiaNXPMotorolaNECTI


Example RAN 4 PDSCH Simulation Results

►FDD 2x2-SFBC, 10MHz, ►1 codeword, 2 layers►16-QAM rate 1/2►EVA5, non-ideal channel

estimation

►FDD 2x2-SM with precoderfeedback for whole BW,

►10MHz MMSE receiver ►2 x 64QAM 3/4, ►EVA5, non-ideal channel

estimation

MIMO-SFBC 16QAM 1/2 EVA5

0

2000000

4000000

6000000

8000000

10000000

12000000

14000000

-8 -6 -4 -2 0 2 4 6 8 10 12 14 16

SNR (dB)

Thro

ughp

ut (b

ps)

EricssonFreescaleInterDigitalLGEFujitsuNokiaNXPMotorolaNECTI

MIMO-SM 64QAM 1/3 EPA5

0

10000000

20000000

30000000

40000000

50000000

60000000

70000000

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32

SNR (dB)

Thro

ughp

ut (b

ps)

Ericsson

Freescale

InterDigital

LGE

Fujitsu

Nokia

NXP

Motorola

NEC

TI


Example RAN4 PDCCH/PCFICH Simulations

0.10%

1.00%

10.00%

100.00%

-10 -8 -6 -4 -2 0 2 4 6

SNR [dB]PC

FIC

H/P

DC

CH

BLE

R

Ericsson

Freescale

TI

LGE

NTT DoCoMo

Motorola

Marvell

Nokia

Fujitsu

InterDigital

NEC

CATT

Qualcomm

AVERAGE

Sim 46.6: FDD 1x2 10MHz 8CCE Format1 ETU70 low

0.10%

1.00%

10.00%

100.00%

-10 -8 -6 -4 -2 0 2 4 6

SNR [dB]

PCFI

CH

/PD

CC

HB

LER

Ericsson

Freescale

TI

LGE

NTT DoCoMo

Motorola

Marvell

Nokia

Fujitsu

InterDigital

NEC

CATT

Qualcomm

Samsung

AVERAGE

Sim 47.10: FDD 2x2 SFBC 1.4MHz 2CCE Format2 EPA5 low


References

►[1] Francois Coureau, “3GPP Evolution: LTE and SAE,” Beijing, China, 2004

►[2] TR 25.913 7.3.0, “Requirements for Evolved-UTRA and Evolved UTRAN,” March 2006

►[3] TS 36.201 v. 8.1.0, “E-UTRA: LTE Physical Layer – General Description,” Nov. 2007

►[4] TS 36.211 v. 8.2.0, “E-UTRA: Physical Channels and Modulation,” March 2008

►[5] TS 36.212 v. 8.2.0, “E-UTRA: Multiplexing and Channel Coding,”March 2008

►[6] TS 36.300 v. 8.4.0, “E-UTRA and E-UTRAN: Overall Description Stage 2,” March 2008


Related Session Resources

SessionsSession ID

DemosPedestal ID

PM101 LTE: MIMO Techniques in 3GPP-LTE

Title

Demo Title

Session Location – Online Literature Libraryhttp://www.freescale.com/webapp/sps/site/homepage.jsp?nodeId=052577903644CB

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