LTE Air Interface-1

Objective

LTE Duplex Modes & Frequency Bands OFDMA Feature, Principles and Challenges SC-FDMA Principle OFDM DATA Processing

• Subcarrier Principle• Time Domain and Frequency Domain

LTE Air Interface Protocol Architecture• RRC Layer and Functionality • PDCP Layer and Functionality• RLC Layer and Functionality• Physical Layer Functionality

LTE Channel Concept• Logical Channel• Transport Channel• Physical Channel

Duplex Modes and Frequency BandsDuplex Modes and Frequency Bands

Duplexing and Multiple Access

ULULULULULULULULULUL

DLDLDLDLDLDLDLDLDLDL

Frequency

Time

Bandwidth

up to 20MHz Bandwidth

up to 20MHz

DLSFULDLDLDLSFULDLUL

Frequency

Time

Bandwidth

up to 20MHz

Harmonization Differences

Both are included in same specification

Same radio interface schemes for both uplink and downlink

Same sub-frame formats

Same network architecture

Same air interface protocols

Same physical channels procedures

FDD developed in the paired 3GPP spectrum

TDD developed in the unpaired 3GPP spectrum

Small differences in the physical channels design

Different frame formats

FDD mode has commonalities with 3G UMTS

TDD mode has commonalities with TD-SCDMA (developed in China)

FDD and TDD

FDD

FDDFDDFDD

FDDFDDFDD

FDDFDDFDDFDDFDD

FDDFDDFDDFDDFDD

FDDFDDFDDFDDFDD

FDDFDDFDDFDDFDD

E-UTRA Operating Band Frequency UL/DL MHz Duplex Mode12

1920-1980/2110-21701850-1910/1930-1990

3456789101112131415161718

20212223

19

2425262728

1710-1785/1805-18801710-1755/2110-2155

824-849/869-894830-840/875-885

2500-2570/2620-2690880-915/925-960

1749.9-1784.9/1844.9-1879.91710-1770/2110-2170

1427.9-1447.9/1475.9-1495.9699-716/729-746777-787/746-756788-798/758-768

ReservedReserved

704-716/734-746815-830/860-875

832-862/791-8211447.9-1462.9/1495.9-1510.9

830-845/875-890

3410-3490/3510-3590

703-748/758-803 FDD

2000-2020/2180/22001626.5-1660.5/1525-1559

1850-1915/1930-1995814-849/859-894807-824/852-869

FDD Bands

TDD Bands

TDDTDDTDD

TDDTDDTDDTDD

TDDTDDTDDTDDTDD

E-UTRA Operating Band Frequency UL/DL MHz Duplex Mode3334

1900-19202010-2025

35363738394041424344

1850-19101930-19901910-19302570-26201880-19202300-24002496-26903400-36003600-3800

703-803

OFDMA and SC-FDMAOFDMA and SC-FDMA

Multiple Access

Time

•

1 2 3 4 5

•2

12345

4 2

1

23

45

31

15

53

3

24

1

Pow

er

Frequency

TDMATime Division

Multiple Access,2G e.g. GSM,

PDC

FDMAFrequency

DivisionMultiple Access1G e.g. AMPS,

NMT, TACS

CDMACode Division

Multiple Access3G e.g. UMTS,

CDMA2000

1 2 3UE 1 UE 2 UE 3 4 UE 4 UE 55

OFDMAOrthogonal

Frequency Division Multiple Access

e.g. LTE

Multiple Access

Motivation for OFDMA

OFDM Basics

Power

Frequency

Bandwidth

OFDM Signal

The Rectangular Pulse

Advantages:Simple to implement: there is no complex filter system required to detect such pulses and to generate themThe pulse has a clearly defined duration. This is a major advantage in case of multi-path propagation environments as it simplifies handling of inter-symbol interference

Disadvantage: It allocates a quite huge spectrumHowever the spectral power density has null points exactly at multiples of the frequency fs = 1/Ts This will be important in OFDM

time

ampl

itude

Ts

fs 1Ts

Time Domain

frequency f/fs

spec

tral

pow

er d

ensit

y

Frequency Domain

fs

FourierTransform

Inverse FourierTransform

OFDMA Principle

Transmits hundreds or even thousands of separately modulated radio signals using orthogonal subcarriers spread across a wideband channel

Orthogonality:

The peak (centre frequency) of one subcarrier …

…intercepts the ‘nulls’ of the neighbouring subcarriers

15 kHz in LTE: fixed

Total transmission bandwidth

ChallengesChallenges

1. Multi-Path Propagation and Inter-Symbol Interference

Inter Symbol Interference

BTSTime 0 Ts

d1(Direct path)

d3

d2

d1< d2 < d3

Time 0 Tt Ts+Tt

Tt

Multi-Path Propagation and the Guard Period

2

time

TSYMBOL

Time Domain

1

3

time

TSYMBOL

time

TSYMBOL

Tg

1

2

3

Guard Period (GP)

Guard Period (GP)

Guard Period (GP)

(Direct path)

Obviously when the delay spread of the multi-path environment is greater than the guard period duration (Tg), then we encounter inter-symbol interference (ISI)

Propagation Delay Exceeding the Guard Period

12

34

time

TSYMBOLTime Domain

time

time

Tg

1

2

3

time

4

Cyclic Prefix

symbolCP

time

Tsymb

12

3

1

2

3

Tcp

symbolCP symbolCP

symbolCP symbolCP symbolCP

symbolCP symbolCP symbolCP

12

3

1

2

3

Cyclic Prefix

2. Multi-Carrier Modulation

The center frequencies must be spaced so that interference between different carriers, known as Adjacent Carrier Interference ACI, is minimized; but not too much spaced as the total bandwidth will be wasted.

Each carrier uses an upper and lower guard band to protect itself from its adjacent carriers. Nevertheless, there will always be some interference between the adjacent carriers.

frequency

∆fsubcarrier

f0 f1 f2 fN-1fN-2

∆fsub-used

ACI = Adjacent Carrier Interference

Solution: OFDM Multi-Carrier

OFDM allows a tight packing of small carrier called the subcarriers - into a given frequency band

No ACI (Adjacent Carrier Interference) in OFDM due to the orthogonal subcarriers !

Pow

er D

ensit

y

Pow

er D

ensit

y

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

3.Inter-Carrier Interference (ICI)

f0 f1 f2 f3 f4

∆P

I3I1I4I0

ICI =

Inte

r-Car

rier I

nter

fere

nce

Frequency Drift

Two effects begin to work:Subcarrier has no longer its power density maximum- so loose of signal energy.

The rest of subcarriers have no longer a null point here. So we get some noise from the other subcarrier.

OFDM Key Parameters

2) Subcarrier Spacing (Δf = 15 KHz) Tsymbol = 1/ Δf = 66.7μs

Δf

TSYMBOL

•TCP SYMBOL

TCP

TS

Frequency

Time

Powerdensity

Amplitude

1) Variable Bandwidth options: 1.4, 3, 5, 10, 15 and 20 MHz

Frequency

3) The number of Subcarriers Nc

If BW = 20MHz → Transmission BW = 20MHz – 2MHz = 18 MHzthe number of subcarriers Nc = 18MHz/15KHz = 1200 subcarriers

TransmissionBandwidth [RB]

Transmission Bandwidth Configuration [RB]

Channel Bandwidth [MHz]

Resource block

Channel edge

Channel edge

DC carrier (downlink only)Active Resource Blocks

OFDM Key Parameters

4) FFT (Fast Fourier Transform) size Nfft

For a bandwidth BW = 20 MHz Nc = 1200 subcarriers not a power of 2 The next power of 2 is 2048 → the rest 2048 -1200 848 padded with zeros

5. Sampling rate fs

This parameter indicates what is the sampling frequency:fs = Nfft x ΔfExample: for a bandwidth BW = 5 MHz (with 10% guard band)The number of subcarriers Nc = 4.5 MHz/ 15 KHz = 300 300 is not a power of 2 → next power of 2 is 512 → Nfft = 512Fs = 512 x 15 KHz = 7,68 MHz → fs = 2 x 3,84 MHz which is the chip rate in UMTS

The sampling rate is a multiple of the chip rate from UMTS/ HSPA. This was acomplished because the subcarriers spacing is 15 KHz. This means UMTS and LTE have the same clock timing!

OFDM Key Parameters

FFT Size and Sampling Rate

SC FDMASC FDMA

OFDM Benefits and Challenges

OFDM benefits:• Good performance in frequency selective fading channels.• Low complexity of base-band receiver.• Good spectral properties and handling of multiple bandwidths.• Link adaptation and frequency domain scheduling.• Compatibility with advanced receiver and antenna technologies.

OFDM Challenges:• Tolerance to frequency offset.• The high Peak-to-Average Power Ratio (PAPR) of the transmitter signal. It requires transmitter with

linear response in a large range. Those “high linear response” amplifier have a low power conversion efficiency and therefore they are not ideal for Mobile Stations. In LTE the problem was solved by adopting SC-FDMA for Uplink, which has better power amplifier efficiency.

The transmitted power is the sum of the powers of all the subcarriers

– Due to large number of subcarriers, the peak to average power ratio (PAPR) tends to have a large range

– The higher the peaks, the greater the range of power levels over which the transmitter is required to work.

– Not best suited for use with mobile ( battery-powered) devices

Peak-to-Average Power Ratio in OFDM

SC-FDMA

• Single Carrier Frequency Division Multiple Access is another variant of OFDMA used to reduce the PAPR for lower RF hardware requirements.

• SC-FDMA is a new hybrid modulation scheme that cleverly combines the low PAR of single-carrier systems with the multipath resistance and flexible subcarrier frequency allocation offered by OFDM.

• This mechanism can reduce the PAPR of 6..9 dB compared to normal OFDMA.

• SC-FDMA is one option in WiMAX (802.16d) and it is the method selected for EUTRAN in the uplink direction.

•SC-FDMA

•OFDMA

SC-FDMA Principles

This could be achieved by transmitting N modulation symbols in series at N times the rate.

One can see that the SC-FDMA symbol which is having 66.66µs is containing N “sub-symbols”

N = 6 in the example shown In Time domain only one modulation symbol is

transmitted at a time.

This limitation is imposed by the input of the FFT block which is before the IFFT. This enables efficient implementation of the FFT.

Note that also the number of Resource Blocks should be multiple of 2,3 or/and 5

Adjusting the data rate in SC-FDMA

Halved SC-FDMA “sub-symbol”

duration

Initial bandwidth

SC-FDMA “sub-

symbol” duration

Doubled bandwidth

If the data rate increases more bandwidth is needed to transmit more modulation symbols (when data rate is doubled the resource allocation in the frequency domain is also doubled). The individual transmission is now shorter in time but wider in the frequency domain.

For double data rate the amount of inputs in transmitter doubles and the “sub-symbol” duration (Time) is halved. Note that the SC-FDMA is still 67 µs

Double the data rate

SC-FDMA symbol 67µs

SC-FDMA symbol 67µs

SC-FDMA Principles

•In the example 6 modulation symbols are sent initially and 12 modulations for double data rate

OFDMA and SC-FDMA

OFDMA transmits data in parallel across multiple subcarriers SC-FDMA transmits data in series employing multiple subcarriers In the example: OFDMA: 6 modulation symbols ( 01,10,11,01,10 and 10) are transmitted per

OFDMA symbol, one on each subcarrier SC-FDMA: 6 modulation symbols are transmitted per SC-FDMA symbol using

all subcarriers. The duration of each modulation symbol is 1/6th of the modulation symbol in OFDMA

OFDMA SC-FDMA

OFDMA SC-FDMA

Difference in transmission: for SC-FDMA there is an extra block on the transmission chain: the FFT block

which should “spread” the input modulation symbols over all the allocated subcarriers

SC-FDMA and OFDMA

OFDMA vs SC-FDMA: QPSK

OFDMA Data ProcessingOFDMA Data Processing

Resource Block and Resource Element

12 subcarriers in frequency domain x 1 slot period in time domain.

Physical Resource Block or Resource Block (PRB or RB)

FDD -Frame Structure

FDD Frame structure ( also called Type 1 Frame) is common to both uplink and downlink.

Divided into 20 x 0.5ms slots

10 ms frame

0.5 ms slot

s0 s1 s2 s3 s4 s5 s6 s7s18 s19

1 ms sub-frame

SF0 SF1 SF2 SF9

•sy4

•sy0

•sy1

•sy2

•sy3

•sy5

•sy6

0.5 ms slot

SF3

•Frame length =10 ms•FDD: 10 ms sub-frame for UL 10 ms sub-frame for DL•1 Frame = 20 slots of 0.5ms each•1 slot = 7 ( NCP) or 6 (ECP)

SF: SubFrame

s: slot

Sy: symbol

There are 7 frame configurations, according to different DL/UL partition

1 frame = 10ms1 subframe = 1ms

DL

DL

DL

DL

DL

DL

DL

DL

DLDL

DL DLDL

DL DL DL DL DL

DL

DLDL

DL

DL

DL

DL

DL

DL

DL

DL

DL

DL

DL

DL

DL

DLDL

UL

UL

UL

UL

UL

UL

UL UL UL UL UL

ULUL

UL

UL

UL

UL

UL

UL

UL

UL

UL

UL

SS

SS

SS

SS

SS

SS

SS

0

1

2

3

4

5

6

DL – Downlink subframeUL – Uplink subframeSS – Special Switching subframe

TDD -Frame Structure

SS

SS

SS

SS

TDD has a single frame structure: same as FDD but with some specific fields to enable also TD-SCDMA co-existence (China):

DwPTS, GP, UpPTS Subframe 0 and DwPTS are reserved for downlink; subframe2 and UpPTS are reserved for UL. Remaining fields are dynamically assigned between UL and DL

SF#0

. . .f

time

UL/DL carrier

•subframe 0

DwPT

SGP

UpPT

S SF#2

SF#4

•subframe 2 •subframe 4

SF#0

. . .

DwPT

SGP

UpPT

S SF#2

SF#4

•subframe 0 •subframe 2 •Subframe 4•half frame

DwPTS: Downlink Pilot time Slot

UpPSS: Uplink Pilot Time Slot

GP: Guard Period to separate UL/DL

Downlink SubframeUplink Subframe

TDD -Frame Structure

UE always needs a guard period in order to switch from receiver to transmitter. The guard period includes RTD (Round Trip Delay).

eNodeB

UE

PT PTSP

Downlink

Downlink Uplink

Uplink

eNodeB ends transmitting

End of DL subframe has reached at the UE

UE has switched to transmission and has begun UL subframe

Start of UL subframe reaches at eNodeB

PT = Propagation TimeSP = Switching PeriodRTD = Round Trip DelayGP = Guard PeriodGP

RTD = 2 x PTGP = RTD + SP

Special Subframe

LTE Air Interface ProtocolLTE Air Interface Protocol

Radio Interface

RRC Layer

PDCP Sublayer

RLC Sublayer

RLC Sublayer

MAC Sublayer

LTE ChannelsLTE Channels

Downlink Channels Mapping

DL Logical Channels

DL Logical Channels

DL Transport Channels

DL Transport Channels

DL Physical Channels

DL Physical Channels

Uplink Channels Mapping

UL Logical Channels

UL Transport Channels

Physical Channels