Ltetutorial 100126072043-phpapp01

LTE tutorial- Looking forward beyond HSPA+

[email protected] System Engineer

All rights reserved @ 2009

Outline

• Beyond HSPA+• LTE: motivation and expectations• E-UTRAN overview & initial performance evaluation• OFDMA and SC-FDMA fundamentals• LTE physical layer• LTE transmission procedures


Beyond HSPA evolution – 3GPP path

Rel-99WCDMA

Rel-7

HSPA+ (HSPA Evolution)

DL: 14.4 MbpsUL: 5.76Mbps

HSDPA/HSUPA

DL: 28 MbpsUL: 11 Mbps



DL: 100+ MbpsUL: 23+ Mbps

Rel-8 Rel-9 Beyond Rel-9

LTE specification process ~ 2007Q4

E-UTRAN

UTRAN

Rel-6Rel-5

LTE-A

DL:300 MbpsUL: 75 Mbps

DL: 1 GbpsUL: 100 Mbps

deployment& service

enhancement


LTE - background• Motivation:

– Based on HSPA success story(274* commercial HSPA networks worldwide)

– Uptake of mobile data traffic upon cellular networks enforces:

• Reduced latency• Higher user data rate• Improved system capacity and coverage• Cost-reduction per bit

• Expectation:– Detailed requirements captured

in 3GPP TR 25.913– NGMN formally released requirements

on next generation RAN in late 2006**

*source: www.gsacom.com“ mobile broadband evolution: roadmap from HSPA to LTE” UMTS forum White paper**http://www.ngmn.org/nc/de/downloads/techdownloads.html

http://www.gsacom.com/

http://www.ngmn.org/nc/de/downloads/techdownloads.html


LTE - background• Motivation:

– Based on HSPA success story(274* commercial HSPA networks worldwide)

– Uptake of mobile data traffic upon cellular networks enforces:

• Reduced latency• Higher user data rate• Improved system capacity and coverage• Cost-reduction per bit

• Expectation:– Detailed requirements captured

in 3GPP TR 25.913– NGMN formally released requirements

on next generation RAN in late 2006**

*source: www.gsacom.com“ mobile broadband evolution: roadmap from HSPA to LTE” UMTS forum White paper**http://www.ngmn.org/nc/de/downloads/techdownloads.html

http://www.gsacom.com/

http://www.ngmn.org/nc/de/downloads/techdownloads.html


LTE feature overview

• Flexible and expandable spectrum bandwidth

• Simplified network architecture

• High data throughput (Macro eNodeB & Home eNodeB)

• Support for multi-antenna scheme (up to 4x4 MIMO in Rel-8)

• Time-frequency scheduling on shared-channel

• Soft(fractional) frequency reuse

• Self-Organizing Network (SON)


LTE spectrum flexibility

FDD Pair

uplink downlink

5 MHz20 MHz

• Operating bands– Flexible carriers: from 700MHz to

2600MHz– Extensible bandwidth: from 5MHz to

20MHz

active RBs

Transmission bandwidth configuration(RBs)

Channel bandwidth (MHz)


LTE basic parameters

Frequency range UMTS FDD bands and TDD bands defined in 36.101(v860) Table 5.5.1

channel bandwidth (MHz)1.4 3 5 10 15 20

Transmission bandwidth NRB:(1 resource block = 180kHz in 1ms TTI)

6 15 25 50 75 100

Downlink: QPSK, 16QAM, 64QAMModulation Schemes:

Uplink: QPSK, 16QAM, 64QAM(optional)

downlink: OFDMA (Orthogonal Frequency Division Multiple Access)Multiple Access:

uplink: SC-FDMA (Single Carrier Frequency Division Multiple Access)

downlink: TxAA, spatial multiplexing, CDD ,max 4x4 arrayMulti-Antenna Technology

Uplink: Multi-user collaborative MIMO

Downlink: 150Mbps(UE Category 4, 2x2 MIMO, 20MHz bandwidth)300Mbps(UE category 5, 4x4 MIMO, 20MHz bandwidth)Peak data rate

Uplink: 75Mbps(20MHz bandwidth)


LTE Peak throughput w.r.t UE categories

UE Category Maximum number of DL-SCH transport block bits received

within a TTI

Maximum number of bits of a DL-SCH transport

block received within a TTI

Total number of soft

channel bits

Maximum number of supported layers for spatial multiplexing

in DL

Category 1 10296 10296 250368 1

Category 2 51024 51024 1237248 2

Category 3 102048 75376 1237248 2

Category 4 150752 75376 1827072 2

Category 5 299552 149776 3667200 4

3GPP TS 36.306 v850 “User Equipment (UE) radio access capabilities“

Table 4.1-1: Downlink physical layer parameter values set by the field ue-Category

Table 4.1-2: Uplink physical layer parameter values set by the field ue-CategoryUE

Category

Maximum number of bits of an UL-SCH transport block transmitted within a TTI

Support for 64QAM in

UL

Category 1 5160 No

Category 2 25456 No

Category 3 51024 No

Category 4 51024 No

Category 5 75376 Yes

Peak rate 150Mbps with

2x2 MIMO

Peak rate 300Mbps with 4x4 MIMO

Peak rate 75Mbps


LTE UE category

UE Category 1 2 3 4 5

DL 10 50 100 150 300

UL 5 25 50 50 75

RF bandwidth 20 MHz

DL QPSK, 16QAM, 64QAM

UL QPSK, 16QAMQPSK,

16QAM,64QAM

2 Rx Diversity Assumed in performance requirements

2x2 MIMO Optional Mandatory

4x4 MIMO Not supported Mandatory

Modulation

Peak rate(Mbps)

3GPP TS 36.306 v850 “User Equipment (UE) radio access capabilities“


Channel dependent scheduling

• Time-frequency scheduling

UE #1

UE #2


Soft (fractional) frequency reuse• Soft Frequency Reuse(SFR):

– inner part of cell uses all subbands with less power;– Outer part of cell uses pre-served subbands with higher power;

MS 21

MS 11

BS 1

BS 3

BS 2

Power density

sub-

carri

er

power density

Power

dens

ity

Sub-carriers

sub-

carrie

r

MS 31MS 12

MS 22

MS 32

3GPP R1-050841 “Further Analysis of Soft Frequency Reuse Scheme “


E-UTRAN overview


E-UTRAN architecture

S1 S1

S1 S1X2X2


E-UTRAN architecture


E-UTRAN radio protocol

Paging System information

Dedicated Control and information transfer

PCCH

SRB0 SRB1 SRB2 DRB1 DRB2

PCH

BCCH

BCH

CCCH

RACH

DCCH 1 DCCH 2 DTCH 1 DTCH 2

DL-SCH UL-SCH

PBCH PRACH PDSCH PUSCH

PHY layer functions

Multiplexing and HARQ control

Integrity and ciphering

ARQ

Integrity and ciphering

ARQ

ciphering and ROHC

ARQ

ciphering and ROHC

ARQ

PDCP

RLC

MAC

RRC

radiobearers

logicalchannels

transportchannels

physicalchannels

notifications common dedicated


E-UTRAN radio channels

PCCH BCCH CCCH DCCH DTCH MCCH MTCH

PCH BCH DL-SCH MCH

PDCCH PBCH PDSCH PMCH

CCCH DCCH DTCH

RACH

PRACH

UL-SCH

PUCCH PUSCH

downlinkLogical

channels

Transport channels

Physical channels

uplink

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

•Transport Channels – no per-user dedicated channels!Define how is something transmitted over the air, e.g. what are encoding, interleaving options used to transmit data

•Physical ChannelsDefine where is something transmitted over the air, e.g. first N symbols in the DL frame


E-UTRAN bearersMAC

RLCLTE

L1

PDCPRRC

IP

NAS

UDPRTP

TCPHTTP

UE

MACRLC

LTE L1

PDCPRRC

PHYLayer

2IP

S1 AP

SCTP

GTP-u

UDP

eNodeB

L2

PHY

IPSCTP

S1 AP

MME

NAS L2

PHY

IPUDP

GTP-u

S-GW

L2

PHY

IPUDP

GTP-uIP

P-GW

L2

PHY

IPUDP

GTP-u

SRB: internal E-UTRAN signalings such as RRC signalings, RB management signalings

NAS signalings: such as tracking area update and mobility management messagesdata traffic: E-UTRAN radio bearer + S1 bearer +S5/S8 bearer

E-UTRAN radio bearer S1 bearerS5/S8 bearer

EPS bearer

L1/L2 control channel


E-UTRAN – Control plane stack

RRC

PDCP

RLC

MAC

PHY

RRC

PDCP

RLC

MAC

PHY

S1APX2AP

SCTP

L2

L1

IP

S1APX2AP

SCTP

L2

L1

IP

NAS NAS

UE

eNodeB

MME/eNodeB

24.301

36.331

36.323

36.322

36.321

36.211~36.214

36.41336.423

36.41236.422

S1-MME/X2-CLTE-Uu


E-UTRAN – User Plane Stack

IP

PDCP

RLC

MAC

PHY

PDCP

RLC

MAC

PHY

GTP-u

UDP

L2

L1

IP

GTP-u

UDP

L2

L1

IP

Application

UE

eNodeB

PDN/S-GWeNodeB

36.323

36.322

36.321

36.211~36.214

S1-U/X2-uLTE-Uu

IP

29.274


Radio resource management

RLC

MAC

RRC

PHY

PDCP

QoS managementAdmission

control Semi-persistentscheduling

Hybrid ARQmanager

Dynamicscheduling Link adaptation

PDCCH adaptation CQI manager

Interferencemanagement

Loadcontrol

L2

L1

L3 mobilitymanagement

“An overview of downlink radio resource management for LTE”, Klaus Ingemann Pedersen, et al, IEEE communication magazine, 2009 July


E-UTRAN mobility• Simplified RRC states• Idle-mode mobility (similar as HSPA)

• Connected-mode mobility – handover controlled by network

RRC-connectedRRC-idle

• Cell reselection decided by UE• Based on UE measurements• Controlled by broadcasted parameters• Different priorities assigned to frequency

layers

• Network controlled handovers• Based on UE measurements

MME/SGW

Target cell signal quality meets

reporting threshold

SourceeNodeB

HO decision

targeteNodeB

Call Admission

Mobility difference between UTRAN and E-UTRAN UTRAN E-UTRAN

Location area (CS core) Not relevant since no CS connections

Routing area Tracking area

SHO No SHO

Cell_FACH, Cell_PCH,URA_PCH No similar RRC states

RNC hides most of mobility Core network sees every handover

Neighbour cell list requiredNo need to provide cell-specific

information, only carrier-frequency is required.


Overview of a PS call – control plane• UE activities after power-on

Initialcell search

Derive system information

RandomAccess Data Tx/Rx

Power up

PDCCH

PDSCH

PCFICH/PHICHPSS/SSS

BCH

Rnadom Access

PUSCH/PUCCH

UE E-UTRAN

Radom Access procedure

Security procedures

paging

RRC Connection Request

RRC Connection Setup

RRC Connection Setup Complete

RRC Connection Reconfiguration

RRC Connection Reconfiguration Complete

Connection establishment

Radio bearer establishment


Overview of a PS call – control plane• UE activities after power-on

Initialcell search

Derive system information

RandomAccess Data Tx/Rx

Power up

DL data transmission

ACK & channel status report

UL data transmission

ACK & uplink scheduling grantUE E-UTRAN

Radom Access procedure

Security procedures

paging

RRC Connection Request

RRC Connection Setup

RRC Connection Setup Complete

RRC Connection Reconfiguration

RRC Connection Reconfiguration Complete

Connection establishment

Radio bearer establishment


Overview of a PS call – user plane

1 resource block:180 kHz = 12 subcarriers

1 resource block pair1 TTI = 1ms = 2 slots

PS data via S1 interface

Multiplexingper user

scheduling

RLC(Segmentation, ARQ)

PDCP(Ciphering

Header Compression,)

HARQ

OFDM SignalGeneration

coding

data modulator

resourcemapping

eNodeB

Tx

to RF

UE


Overview of a PS call – user plane


1 resource block pair1 TTI = 1ms = 2 slots

PS data via S1 interface

Multiplexingper user

scheduling

RLC(Segmentation, ARQ)

PDCP(Ciphering

Header Compression,)

HARQ

OFDM SignalGeneration

coding

data modulator

resourcemapping

eNodeB

Tx

to RF

UEOccupying different radio resources across TTIsadapts to time-varying radio channel condition!


LTE initial deployment scenario

• Similar coverage as 3G HSPA on existing 3G frequency bands– LTE radio transmission technology itself does not provide coverage boost.– Lower frequency (e.g, 900MHz) provides better coverage but demands large-

size antennas.

• “Over-layed” initial deployment on hot-spot area– Spectrum availability– Backhaul capacity– Handset maturity (multi-mode)

urban(0.6 ~ 1.2km)

sub-urban(1.5 ~ 3.4km)

Rural(26 ~ 50 km)


LTE initial trial performance• LTE data rates

– Peak rate measured in lab and trial align with3GPP performance targets

– In reality, user throughputs are impacted by• RF conditions & UE speed• Inter-cell interference & multiple users sharing the capacity• Application overhead

Source: www.lstiforum.org

Active users per cell

Peak rate measured with a single user in unloaded, optimal radio condition

Top 5%, loaded

Average

Cell edge

Active users per cell

Average: 10 active users with 3Mbps

throughput per user

1Mpbs throughput at cell edge


Macro Cellular network: peak rate Vs average rate• Unlike circuit-switched network design, live network throughput

is not fixed any more, being dependent on many environmental factors such as CQI,Tx buffer status,etc.

• In macro cellular network, network average throughput falls behind peak rate by 10x.

• Cellular booster for Mobile broadband– Ubiquitous coverage– High capacity & data rate – Low cost>> “FemtoCell” – Home eNodeB!

Tput (Mbps)

0

8

4

2

-3

2

10

25

15

G-factor (dB)HSPA cell throughput

3GPP TS 25.101 Table 9.8D3, 9.8D4, 9.8F3 for PA3


LTE initial trial performance

• User plane latency– 3GPP RTT target is 10ms for short IP packet– Field trial results:

• 10~13ms with pre-scheduled uplink• <25ms with on-demand uplink

• Control plane latency– Short latency helps to keep “always on” user experience– Field trial results

• Measured idle to active latency: 70~ 100ms

* Measurement taken with one UE in unloaded case* Source: www.lstiforum.org

EPCApp Server

air interface RTT

End-to-End Ping

Camped-state (idle)

Active (Cell_DCH)

Dormant (Cell_PCH)

Less than 100msec

Less than 50msec


OFDMA and SC-FDMA rationale


OFDM fundamentals – frequency spectrum

…f

FDM

f

OFDM

No Inter-Carrier Interference!

fΔfΔ−fΔ− 2 0 fΔ2

)sin(ff

Δ⋅π

fTu Δ

=1

Time domain

frequency domain


OFDM fundamentals – multicarrier modulation

“+1”

“+1”

“-1”

f1

f2

f3

+

Modulatedsubcarriers

110 ,...,, −cNaaa

0a

S/P 1a

1−Nca

tfje 02π

tfje 12π

tfj Nce 12 −π…

)(0 tx

)(1 tx

)(1 txNc−

+)(tx

110 ,...,, −cNaaa0a

S/P

1a

1−Nca

…

IFFT

0

0

P/S

X0

X1

XN-1

…

…

∑∑−

=

Δ−

=

==1

0

21

0)()(

Nc

k

ftkjk

Nc

kk eatxtx π

Specifying system sampling rate: fNTf ss Δ⋅== /1

We get:

∑ ∑

∑−

=

−

=

−

=

Δ

′==

==

1

0

1

0

22

1

0

2)(

Nc

k

N

k

Nnkj

kNnkj

k

Nc

k

fnTskjkn

eaea

eanTsxx

ππ

π


OFDM fundamentals- Cyclic Prefix

ττ

1−ka 1+kauT

Integration interval of direct path

directed path:

reflected path:

guard time FFT integration time=1/Carrier spacing

OFDM symbol time

ka

τ

τ>cpT

directed path:

reflected path:

Guard time: Cyclic Prefix Vs Padding Zeroes


OFDM fundamentals- Cyclic Prefix

ττ

1−ka 1+kauT

Integration interval of direct path

directed path:

reflected path:

guard time FFT integration time=1/Carrier spacing

OFDM symbol time

ka

τ

τ>cpT

directed path:

reflected path:

Guard time: Cyclic Prefix Vs Padding Zeroes

IFFT

0a1a

1−Nca

… addCyclicPrefixTu Tu+Tcp

an OFDM symbol

P/S


OFDM fundamentals – general link level chains

“Digital communications: fundamentals and applications” by Bernard Sklar, Prentice Hall, 1998. ISBN: 0-13-212713-x“OFDM for Wireless Multimedia Communications” by Richard van Nee & Ramjee Prasad, Artech house,2000, ISBN: 0-89006-530-6

3GPP TR 25892-600 feasibility study for OFDM in UTRAN

Coding Interleaving QAM mapping

Pilot Insertion S/P IFFT P/S add CP

Pulse shapingDACRF Tx

Timing andfrequency SyncADCRF Rx

de-coding de-interleaving

QAM de-mapping Equalizer P/S FFT S/P CP

removal

Binary input data

Binary output data

…

Sub-carriersFFT

Time

Symbols

5 MHz Bandwidth

Guard Intervals

…Frequency


OFDM fundamentals – frequency domain equalizer

)(τh)(tS

transmitter

)(τw+

)(tn

)(tr )(~ ts

receiverChannel model

)()( * ττ −= hw1)()( =⊗ ττ wh

})()(ˆ{ 2tstsE −=ε

D D D

W0 W1 WL-1

+

nr

nsDFT

⊗0R0W

0S

⊗1−NR 1ˆ

−NS1−NW IDFT

)(tr )(ˆ ts

MRC filter:Zero Forcing:MMSE:

Time domain frequency domain

“Frequency domain equalization for single carrier broadband wireless systems”, David Falconer , et.al,IEEE Communication magazine, 2002 April

Frequency domain equalizer outperforms with much less complexity!


OFDM fundamentals• Advantages:

– OFDM itself does not provide processing gains, but provides a degree of freedom in frequency domain by partitioning the wideband channel intomultiple narrow “flat-fading” sub-channels.

– Channel coding is mandatory for OFDM to combat frequency-selective fading.

– Efficiently combating multi-path propagation in term of cyclic prefix– OFDM receiver (frequency domain equalizer) has less complexity than that of

Rake receiver on wideband channels.– OFDM characterizes flexible spectrum expansion for cellular systems.

• Drawbacks:– high peak-to-average ratio.– Sensitive to frequency offset, hence to Doppler-shift as well

f

f


OFDM fundamentals – downlink OFDMA

f


1 slot = 0.5 ms

PDSCH

PDCCH

• OFDMA provides flexible scheduling in time-frequency domain.• In case of multi-carrier transmission, OFDMA has larger PAPR than traditional

single carrier transmission. Fortunately this is less concerned with downlink.• Does OFDMA suits for uplink transmission?

– Uplink being sensitive to PAPR due to UE implementation requirements– With wider bandwidth in operation, OFDMA in uplink will have lower power per pilot

symbol which in turn leads to deterioration of demodulation performance.


Wideband single carrier transmission -frequency domain equalizer (SC-FDE)

• While time-domain discrete equalizer has effect of “linear convolution” on channel response; frequency domain equalizer actually serves as “cyclic convolution” thereof.

• The difference will make first L-1 symbols “incorrect” at the output of FDE.

• Solution could be either “overlapped processing” or “cyclic prefix”added in transmitter.

“Adaptive Frequency-Domain Equalization and Diversity Combining for Broadband Wireless Communications,” M. V. Clark, IEEE J. Sel. Areas Commun., vol. 16, no. 8, Oct. 1998“Linear Time and Frequency Domain Turbo Equalization,” M. Tüchler et al., Proc. IEEE 53rd Veh. Technol. Conf. (VTC), vol. 2,

May 2001“Block Channel Equalization in the Frequency Domain,” F. Pancaldi et al., IEEE Trans. Commun., vol. 53, no. 3, Mar. 2005

CPinsertionN samples N+Ncp samples

Single carriersignal

generation

PulseShaping

x(t)

block-wise generation

transmitter


SC-FDMA – multiple access with FDE

“Introduction to Single Carrier FDMA”, Hyung G Myung, 2007 EURASIP

Coding Interleaving QAM mapping add CP

Pulse shapingDACRF Tx

Timing andfrequency SyncADCRF Rx

de-coding de-interleaving

QAM de-mapping

Freq Domain Equalizer

CPremoval

DFT (size M)

IFFT(size N) P/S

Subcarriermapping

IDFT(Size M) P/S FFT

(size N) S/P

Binary input data

Binary output data

Single Carrier: sequential transmission of the symbolsover a single frequency carrier

FDMA: user multiplexing in frequency domain


SC-FDMA – multiple access with SC-FDE

• Multiple access in LTE uplink

DFT OFDM

0

Pulse Shaping

data stream

DFT OFDM

0Pulse

Shapingdata stream

Terminal B

Terminal A

f

f

Orthogonal uplink design in frequency domain!


SC-FDMA – multiple access with SC-FDE

• Multiple access in LTE uplink

DFT OFDM

0

Pulse Shaping

data stream

DFT OFDM

0Pulse

Shapingdata stream

Terminal B

Terminal A

f

f

Orthogonal uplink design in frequency domain!


SC-FDMA – multiple access with FDE

block-wisesignals

DFT(M)

IFFT(N)

CPinsertion

D/A conversion/pulse shaping

RF

Also called DFT-Spread OFDM!

Adopted by LTE uplink!

DFT(M)

IFFT(N)A B C D

DFT(M)

A B C D

……

……

… IFFT(N)

Distributed FDMA:Localized FDMA:

A B C D A B C D A B C D A B C D

Upsampling in freq domain makesrepeated sequence at time domain output

A * * * B * * * C * * * D * * *

OverSampling in freq domain results in interpolation at time domain output

time domain:

frequency domain:


OFDMA Vs SC-FDMA

ttime domain

ffrequency domain

Input data symbols

OFDM symbol

SC-FDMA symbol *

* Assuming bandwidth expansion factor Q=4 in distributed FDMA.

•Time domain: •Frequency domain- OFDM symbol is a sum of all data symbols by IFFT- SC-FDMA symbol is repeated sequence of data “chips”

- OFDM modulates each subcarrier with one data symbol- SC-FDMA “distributes” all data symbols on each subcarrier.


OFDMA Vs SC-FDMA• Similarities

– Block-wise data processing and use of Cyclic Prefix– Divides transmission bandwidth into smaller sub-carriers– Channel inversion/equalization is done in frequency domain– SC-FDMA is regarded as DFT-Precoded or DFT-Spread OFDMA

• Difference– Signal structure: In OFDMA each sub-carrier only carries information related

to only one data symbol while in SC-FDMA, each sub-carrier contains information of all data symbols.

– Equalization: Equalization for OFDMA is done on per-subcarrier basis while for SC-FDMA, equalization is done over the group of sub-carriers used by transmitter.

– PAPR: SC-FDMA presents much lower PAPR than OFDMA does.– Sensitivity to freq offset: yes for OFDMA but tolerable to SC-FDMA.


LTE Physical layer and transmission procedures


LTE physical layer – a vertical view• What kind of information is transmitted?

– Upper layer SDUs plus additional L1 control information in transmission, e.gReference Signals, Sync signals,CQI, HARQ,etc

• How is it transmitted?– Downlink OFDMA and uplink SC-FDMA – Channel dependent scheduling, HARQ,etc– multiple antenna support

• Related L1 procedures– random access, power control, time alignment, etc

coding Scrambling multiplexmodulation

reference signals

control information

time

frequency

PDCP

RLC

MAC

Transport blocks

control informationor user data

signals from other channels


LTE physical layer - a horizontal view

• PBCH: carries system broadcast information• PCFICH: indicates resources used for PDCCH• PHICH: carries ACK/NACK for HARQ operation.• PDCCH: carriers scheduling assignments and other control information• PDSCH: conveys data or control information• PMCH: for MBMS data transmission• Reference signal• Synchronization signal (PSS,SSS) • PUCCH: carries control information

• PRACH: to obtain uplink synchronization• PUSCH: for data or control information• Reference Signals (Demod RS & SRS)

data transmissionPDCCH notifies how to demodulate data

Feedback CQIs,


Fundamental Downlink transmission scheme

1 resourrc block = 12 sub-carriers = 180KHz

1 slot = 0.5 ms =7 OFDM symbols

1 sub-frame = 1 ms1 resource

element

1 radio frame = 10 ms

1 radio frame = 10 sub-frames = 10 ms

1 sub-frame = 2 slot = 14 OFDM symbols*

*An alternative slot structure for MBMS is 6 OFDM symbols per slot where extended CP is in use.

⎩⎨⎧

=,7.4,2.5

ss

Tcpμμ

66.7 us

66.7 us

Tcp

Tcp-e

for first OFDM symbol

for remaining symbols

seTcp μ7.16_ =


System information broadcast• System information

– MIB: transmitted on PBCH (40msTTI)• information about downlink bandwidth• PHICH configuration• SFN

– SIB: transmitted on PDSCH(DL-SCH)• SIB1: operator infor & access restriction infor• SIB2: uplink cell bandwidth, random access parameters• SIB3: cell-reselection• SIB4~SIB8: neighbor cell infor

Synchronization signal

PBCH: the first 4 OFDM symbol in 2nd Slot per

10ms frame

10MHz600 subcarriers

10ms frame

1.08 MHz

10ms frame

CRC insertion

scrambling

modulation

antennamapping

De-multiplexing

1/3 conv. coding

One BCH transportation block


Downlink control channels – PCFICH,PHICH• PCFICH:

– tells about the size of the control region.– Locates in the first OFDM symbol for each sub-frame.

• PHICH: – acknowledges uplink data transfer– Locates in 1st OFDM symbol for each sub-frame

inferior to PCFICH allocation

1/16 block code Scrambling QPSK mod

2 bits 32 bits 32 bits16 symbols

PCFICH-to-resource-element mapping depends on cell identity so as to avoid inter-cell interference.

3xrepetition BPSK mod

1 bit 3 bits

scrambling3x

repetition BPSK mod1 bit 3 bits

Orthogonal code

Orthogonal code

I

Q

12 symbols…

One PHICH group contains 8 PHICHs


Downlink control channels - PDCCH

• Downlink control information (DCIs)– Downlink scheduling assignments– Uplink scheduling assignments– Power control commands

• Control region size indicated by PCFICH• Blind decoded by UE in its “search space” and common “search

space” – allows UE’s micro-sleep even in active state• QPSK always used but channel coding rate is variable

R1-073373 “ Search space definition ofr L1/L2 control channels.“Downlink control channel design for 3GPP LTE”, Robert Love, Amitava Ghosh, et,al. IEEE WCNC 2008.

reference signals

control information

1 sub-frame = 1 ms

control region


Downlink control channels – PDCCH

• How to map DCIs to physical resource elements– Control Channel Elements(CCEs), consisting of 36 REs, are used to

construct control channels.– CCE aggregated at pre-defined level(1,2,4,8) to ease blind detections.

• Usually 5MHz bandwidth system renders 6 UL/DL scheduling assignments within a sub-frame.

Control Channel Element 0






Control channel candidates on which the UE attempts to decode the information

(10 decoding attempts in this example)

Control channel candidate set Or search space

CC

H c

andi

date

1

CC

H c

andi

date

2

CC

H c

andi

date

3

CC

H c

andi

date

4

CC

H c

andi

date

5

CC

H c

andi

date

6

CC

H c

andi

date

7

CC

H c

andi

date

8

CC

H c

andi

date

9

CC

H c

andi

date

10

R1-070787 “Downlink L1/L2 CCH design”


Downlink control channels - PDCCH• Each PDCCH carries one DCI message.

CRC attachment

1/3 Conv Coding

Rate mattching

Control information

RNTI CRC attachment

1/3 Conv Coding

Rate mattching

Control information

RNTI

……CRC attachment

1/3 Conv Coding

Rate mattching

Control information

RNTI

CCE aggragation and PDCCH multiplexing

Scrambling

QPSK

Interleaving

Cell specific Cyclic shift


Downlink shared channel: PDSCH

• Support up to 4 Tx antennas*• Resource block allocation:

– Localized: with less signaling overheads– Distributed: benefits from frequency diversity

• Channelization (location):

reference signals

control information

1 sub-frame = 1 ms

User A

User B

User C

unused

CRC

Segmentation

FEC

RM+HARQ

Scrambling

Modulation

CRC

Segmentation

FEC

RM+HARQ

Scrambling

Modulation

Antenna mapping

Transport blockfrom MAC

Transport blockfrom MAC

RB mapping

To OFDM modulation for each antenna

data region

Cell-specific, bit-level scrambling for interference

randomization **

* For MBSFN, antenna diversity scheme does not apply. ** For MBSFN, it’s MBSFN-area-specific scrambling.


Downlink reference signals• Cell-specific reference signals are length-31 Gold sequence,

initialized based on cell ID and OFDM symbol location.• Each antenna has a specific reference signal pattern, e.g 2

antennas– frequency domain spacing is 6 sub-carriers– Time domain spacing is 4 OFDM symbols– That is, 4 reference symbols per Resource Block per antenna

time

frequency

Antenna 0 Antenna 1

3GPP TS 36.211 “ physical channels and modulation“ section 6.10.1.1


LTE Multiple antenna scheme

3210 ,,, SSSSSTTD UE

3210 ,,, SSSS

*2

*3

*0

*1 ,,, SSSS −−

NodeB transmitter

OFDM modulation

0a1a2a3a

…

OFDM modulation

*1a

*0a−

*2a

*3a−

…

UE

eNodeB transmitter

OFDM modulation

0a1a2a3a

…

OFDM modulation

tfjea Δ⋅Δπ21

…

UE

eNodeB transmitter

0a

tfjea Δ⋅Δ 222

π

tfjea Δ⋅Δ 323

π

WCDMA STTD scheme:

LTE SFBC (space frequency block coding): LTE CDD (cyclic delay diversity):


LTE Multiple antenna scheme• Downlink SU-MIMO

– Transmission of different data streams simultaneously over multiple antennas – Codebook based pre-coding: signal is “pre-coded” at eNodeB before transmission

while optimum pre-coding matrix is selected from pre-defined codebook based on UE feedback.

– Open-loop mode possible for high speed

• Uplink MU-MIMO: collaborative MIMO– Simultaneous transmission from 2UEs on

same time-frequency resource– Each UE with one Tx antenna– Uplink reference signals are coordinated

between UEs

Pre-coding

SICreceiver

S1

S2

Sr

r1

r2

γr

H

eNodeB UEPMI, RI, CQI


LTE Multiple antenna schemeLTE channels Multiple Antenna Schemes comments

open-loop spatial multiplexing large delay CDD/ SFBC

closed-loop spatical multiplexing SU-MIMO

multi-user MIMO MU-MIMO

UE specific RS beam-forming Applicable > 4 Antennas

PDCCH SFBC

PHICH SFBC

PCFICH SFBC

PBCH SFBC

Sync Signals PVS

receiver diversity MRC/IRC

multi-user MIMO MU-MIMO

PUCCH receiver diversity MRC

PRACH receiver diversity MRCUL control channel

UL data channel PUSCH

open-loop transmit diversityDL control channel

DL data channel PDSCH


Synchronization and Cell Search• LTE synchronization design considerations:

– high PSR (Peak to side-lobe ratio: the ratio between the peak to the side-lobes of its aperiodic autocorrelation function) to ease time-domain processing

– low PAPR for coverage– Generalized Chirp Like (GCL) sequences overwhelm Golay and Gold sequences!

• Synchronization signals– PSS: length-63 Zadoff-Chu sequences

• Auto-correlation/cross-correlation/hybrid correlation based detection– SSS: an interleaved concatenation of two length-31 binary sequences

• Alternative transmission (SSS1 and SSS2) in one radio frame

0 1 2 3 4 5 6 7 8 91 radio frame = 10 ms SSS

PSS

3GPP TS 36.211 “physical channels and modulation ““Cell search in 3GPP LTE systems”, by Yingming Tsai etal, JUNE 2007 | IEEE VEHICULAR TECHNOLOGY MAGAZINE


Synchronization and Cell Search• LTE synchronization design considerations:

– high PSR (Peak to side-lobe ratio: the ratio between the peak to the side-lobes of its aperiodic autocorrelation function) to ease time-domain processing

– low PAPR for coverage– Generalized Chirp Like (GCL) sequences overwhelm Golay and Gold sequences!

• Synchronization signals– PSS: length-63 Zadoff-Chu sequences

• Auto-correlation/cross-correlation/hybrid correlation based detection– SSS: an interleaved concatenation of two length-31 binary sequences

• Alternative transmission (SSS1 and SSS2) in one radio frame

0 1 2 3 4 5 6 7 8 91 radio frame = 10 ms SSS

PSS

62 CentralSub-carriers

3GPP TS 36.211 “physical channels and modulation ““Cell search in 3GPP LTE systems”, by Yingming Tsai etal, JUNE 2007 | IEEE VEHICULAR TECHNOLOGY MAGAZINE


Synchronization and Cell Search• Hierarchical cell ID(1 out of 504):

– Cell ID = 3* Cell group ID + PHY ID :

• PSS structure

• SSS structure

)2()1(3 IDIDCELLID NNN +⋅=

⎪⎩

⎪⎨⎧

=

== ++−

+−

61,...,32,31

30,...,1,0)(63

)2)(1(

63)1(

ne

nend nnuj

nunj

u π

π25=μ29=μ34=μ

0)2( =IDN

1)2( =IDN

2)2( =IDN

IFFT

…

0pssx1pssx62pssx

CPinsertion

PSS sequences f

62 sub-carriers excluding DC carrier

… …

f

slot 0 slot 10

odd sub-carriers

even sub-carriers

…

+

)0(0mS

0C

1SSC

+

)1(1mS

1C

2SSC

+

)0(1mZ

+

)1(1mS

0C

1SSC

+

)0(0mS

1C

2SSC

+

)1(1mZ

The indices (m0, m1) define the cell group identity.


LTE Cell Search Vs WCDMA cell search• PSS detection

– Slot timing– Physical layer ID (1 of 3)

• SSS detection– Radio frame timing– Cell group ID (1 of 168)– CP length

• PBCH decoding– PBCH timing– System information access

• P-SCH detection– Slot boundary

• S-SCH detection– frame timing– code group ID

• CPICH detection– Cell-specific scrambling code

identified

• BCH reading

“cell searching in WCDMA”,Sanat Kamal Bahl, IEEE Potential 2003;


LTE uplink• SC-FDMA: fundamental uplink radio parameters are aligned with

downlink scheme, e.g frame structure, sub-carrier spacing, RB size.…

• Multiplexing of uplink data and control information– Combination of FDM and TDM are adopted in LTE uplink

• Uplink transmission are well time-aligned to maintain orthogonality (no intra-cell interference)

• PRACH will not convey user data like WCDMA does, but serve to obtain uplink synchronization


Fundamental uplink transmission scheme

• Uplink transmission frame aligned with downlink parameterizationto ease UE implementation.

f

1 radio frame = 10 ms

⎩⎨⎧

=,7.4,2.5

ss

Tcpμμ

66.7 us

66.7 us

Tcp

Tcp-e

for first OFDM symbol

for remaining symbols

seTcp μ7.16_ =

1 slot = 0.5 ms =7 OFDM symbols

1 sub-frame = 1 ms

under eNodeB scheduling


Uplink reference signal• Uplink reference signals

– Mostly based on Zadoff-Chu sequences (cyclic extensions)– Pre-defined QPSK sequences for small RB allocation

• Demodulation Reference Signal (DRS) in a cell– Each cell is assigned 1 out of 30 sequence groups– Each sequence group contains 1(for less than 5 RB case) or 2 (6RB+ case) RS

sequence across all possible RB allocations– Sequence-group hopping is configurable in term of broadcasting information where the

hopping pattern is decided by Cell ID– Cyclic time shift hopping applies to both control channel and data channel

• DRS on PUSCH

DFT(size M)

OFDMmodulator add CPblock of

data symbols

…

00

00

One DFTS-OFDM symbol

Instantaneous bandwidth

(M sub-carriers)

interference randomization

across intra-cell and inter-cells

…R

S sequence3GPP TS 36.101 “physical channels and modulation” section 5.5.1


Uplink reference signal• DRS on PUCCH

– See next slides

• Sounding Reference Signal (SRS)– Not regularly but allows eNodeB to estimate uplink channel quality at alternative

frequencies– UE’s SRS transmission is subject to network configuration– Location: always on last OFDM symbol of a sub-frame if available

one sub-frame

wideband, non-frequency hopping SRS narrowband, frequency hopping SRS


Uplink control channel transmission - PUCCH• Uplink control signaling

– Data associated: transport format, new data indicator, MIMO parameters– Non-data associated: ACK/NACK, CQI, MIMO codeword feedback

• Channelization– In the absence of uplink data transmission: in reserved frequency region on

band edge– In the presence of uplink data transmission: see multiplexing with data on

PUSCH

f

downlinkdata transmission

downlinkdata transmission

Uplink control TDM

with data

standaloneuplink control

no explicit tranmissionfrom UE as it follows eNodeB scheduling!

…..

1 ms sub-frame

Control region 1 Control region 2

total uplink system bandwidth


Uplink control channel transmission - PUCCH

• To cater for multiple downlink transmission mode, while preserving single-carrier property in uplink, multiple PUCCH formats exist.

• PUCCH is thus mainly classified by PUCCH format 1 & 2– PUCCH format 1/1a/1b: 1 or 2 bits transmitted per 1ms, for ACK/NACK/SR– PUCCH format 2/2a/2b: up to 20 bits transmitted per 1ms, for CQI/PMI/RI

…..

1 ms sub-frame

CQI referencesignal

…..

1 ms sub-frame

ACK/NACK referencesignal


Multiuser transmission on PUCCH• In PUCCH format 1, multiple PUCCHs are distinguished by cyclic

shift of ZACAC sequences plus orthogonal cover sequence• In PUCCH format 2, multiple PUCCHs are distinguished by cyclic

shift of ZACAC sequences.

IFFT IFFT IFFT IFFT

RS RS RS

ACK/NACK bit

BPSK/QPSK

Length-12 phaserotated sequence

IFFT IFFT IFFT IFFT

RS RS

channel status report

QPSK

Length-12 phaserotated sequence

IFFTLength-4 Walsh sequence

1 slot = 0.5 ms 1 slot = 0.5 ms


Uplink data transmission - PUSCH • In case of PUSCH available, control signaling is multiplexed with

data on PUSCH. – To cater for radio channel variation, link adaptation applies to data part– Control signaling does not adopt adaptive modulation but the size of REs

(resource elements) can change w.r.t varying radio condition

Turbocoding

Ratematching

MUXConv

codingRate

matching

Blockcoding

Ratematching

basebandmodulation IFFTDFT

QPSKBlockcoding

DFTS-OFDMmodulation

UL-SCH

CQI,/PMI

RI

ACK/NACK

CQI/PMI

RSACK/NACK

RIPUSCH data

t


Uplink data transmission - PUSCH• UL-SCH processing chain

– No Tx diversity/spatial multiplexing as downlink does– PUSCH frequency hopping (on slot basis)

• Subband-based hopping according to cell-specific hopping patterns• Hopping based on explicit hopping information in scheduling grant

Transport blockfrom MAC @UE

CRC

Segmentation

FEC

RM+HARQ

Scrambling

ModulationUE-specific,

bit-level scrambling To DFTS-OFDM and map to

assigned frequency resorurce


Random Access• LTE random access serves to obtain uplink synchronization, not

to carry data.– Contention-based random access: preambles based on ZC sequences– Contention-free random access: faster with reserved preambles (e.g, for

handover)

• Random access resources– 64 preambles classified into 3 parts:

– RA area:• 1 in every 1~20 ms(configurable)

UE eNodeB

RA preambles

RA response (timing adjustment, UL grant)

UE terminal ID

Contention resolution

… …Preamble set #0 Preamble set #1 reserved

10 ms frame

1ms

6 RBs random access area

NAS UE ID RRC

Connection Request

temporary C-RNTI; timing advance;

initial uplink grant

early contention resolution


Random Access• PRACH structure

– Preamble sequence: cyclic shifted sequences from multiple root ZC sequences– CP: facilitates frequency-domain prcoessing at eNodeB– Guard time: to handle timing uncertainty

• PRACH format options

Other users CP Preamble Sequence Guard time Other usersnear user

Other users CP Preamble Sequence Other users

timinguncertainty

far user

preamble format RA window (ms) Tcp length (ms) Tseq length (ms) Typical usage

0 1 0.1 0.8 for small~medium cells (up to ~ 14 km)

1 2 0.68 0.8 for larget cells(up to ~ 77km) without link budget problem

2 2 0.2 1.6 for medium cells(up to ~ 29km) supporting low data rates

3 3 0.68 1.6 for very large cells(up to ~ 100km)


Layer 1 procedures – power control

• Uplink power control– WCDMA power control is continuous at 1500Hz; while LTE runs power control

slower at 200Hz– Based on open-loop setting while assisted by close-loop adjustment – Independent power control on PUCCH and PUSCH respectively

• PUCCH power control

• PUSCH power control– Independent of PUCCH power control– UE Power Headroom in use to indicate the true desired Tx power

To increase uplink data rate, LTE would increase user’s bandwidth rather than increase Tx power!{ }δ+Δ++= formatDLT PLPPP 0max ,min

{ }δα +Δ+⋅+⋅+= MCSDLT MPLPPP )(log10,min 100max


Layer 1 procedures – Timing Alignment• To maintain uplink intra-cell orthogonality, timing alignment is

necessary.– The further away from eNodeB, the earlier the UE transmits.– Configurable by eNodeB at granularity of 0.52us from 0 ~0.67 ms

(corresponding to max cell radius of 100km)

Tp1

Ta1

Tp2

Ta2

Timing aligned uplink reception at eNodeB for

different users

Tx

Rx

Tx

Tx

Rx

Rx



Backup - OFDMA Vs SC-FDMA

• Channel equalizer:– OFDMA: divides wideband into multiple narrow “flat-fading” sub-

bands hence equalization done on each sub-band is sufficient.– SC-FDMA: frequency domain equalization on the whole group

bandwidth of sub-carriers in use.

DFT Sub-carrierde-mapping

equalizer

equalizer

equalizer

Detect

Detect

Detect

… … … … …

DFT Sub-carrierde-mapping

… … equalizer IDFT… detect… …

OFDMA:

SC-FDMA:


Backup - OFDMA Vs SC-FDMA

• PAPR: • CM: a better measure of UE PA back-off

“3G evolution, HSPA and LTE for mobile broadband(2nd edition)”, ISBN: 978-0-12-374538-5, page.118,

))((

)(2

2

tsE

tsPAPR =

85.15237.1)(log20)(

)(log203

103

3

10−

=⎥⎥⎦

⎤

⎢⎢⎣

⎡

= rmsnrmsref

rmsn

vF

vv

CM

SC-FDMA has around 2dB CM gain against OFDMA!


Backup - Zadoff-Chu sequence characteristics

• Zadoff-Chu sequences

• Property of ZC sequences:– Constant amplitude, even after Nzc-point DFT.– Ideal cyclic auto-correlation– Constant cross-correlation[=sqrt(1/Nzc)], assuming Nzc is a prime number

⎪⎩

⎪⎨

⎧

=

== ++−

+−

61,...,32,31

30,...,1,0)(63

)2)(1(

63)1(

ne

nend nnuj

nunj

u π

π

“Polyphase codes with good periodic correlation properties”, J.D.C.Chu, IEEE trans on Informaiton theory, ,vol.18, pp.531-532, July 1972“Phase shift pulse codes with good periodic correlation properties”, R.Frank,S.Zadoff and R.Heimiller, IEEE Trans on Information Theory, Vol 8, pp 381-382, Oct 1962.


Backup – mobility: intra-MME handover

UE Source eNodeB Target eNodeB EPC

Measurement reporting

Handoverdecision

Handover requestAdmission

control

Handover request Ack

RRC Connection ReconfigurationDetach from

old cellDeliver packets

to target eNodeB

Data forwardingbuffer packets

From source eNodeB

RRC Connection Reconfiguration completePath switch procedure

UE context releaseFlush buffer

Release resource

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