4) Ofdma and Scfdma

8/14/2019 4) Ofdma and Scfdma

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MobileCommProfessionals, Inc.

Your Partner for Wireless Engineering Solutions


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Objective

Understand LTE Duplexing

Single Transmitter FDMA Principle

Multi carrier principle

OFDMA and SC FDMA PrincipleMultipath Propagation

Cyclic Prefix

OFDMA and SC FDMA

Transmitter Receiver

OFDM and SC FDMA Key Parameters

Resource Block


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Duplexing and Multiple Access


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Legacy- Single Transmitter


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FDMA Principle


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LTE: Multi-Carrier Principle


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The Rectangular Pulse

Advantages:

Simple to implement: there is nocomplex filter system required to

detect such pulses and to generatethem.

The pulse has a clearly definedduration. This is a major advantagein case of multi-path propagationenvironments as it simplifies handling

of inter-symbol interference.

Disadvantage:

It allocates a quite huge spectrum

However the spectral power densityhas null points exactly at multiplesof the frequency fs = 1/Ts.

This will be important in OFDM.

time

amplitude

Ts fs1

Ts

Time Domain

frequency f/fs

spectralpowerdensity

Frequency Domain

fs

FourierTransform

InverseFourierTransform


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OFDMA Principle

Transmits hundreds or even thousands of separately modulatedradio signals using orthogonal subcarriers spread across a

wideband channel

Orthogonality:

The peak (centrefrequency) of onesubcarrier

intercepts thenulls of theneighbouringsubcarriers

15 kHzin LTE: fixedTotal transmission bandwidth


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OFDM Basics

Data is sent in parallelacross the set of subcarriers, each subcarrieronly

transports a part of the whole transmission

The throughputis the sum of the data rates of each individual (or used)

subcarriers while the power is distributed to all used subcarriers

FFT( Fast Fourier Transform) is used to create the orthogonalsubcarriers.

The number of subcarriers is determined by the FFT size ( by the bandwidth)

Power

Frequency

Bandwidth


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OFDM Signal


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OFDM: Nutshell


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Frequency-Time Representation


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FFT/IFFT

It can be shown that the OFDM signal may be obtained by transformingLdata symbols by the IFFT, where L is the number of subcarriers.

Therefore, OFDM transmitter and receiver are implemented using IFFTand FFT respectively.

The size of the FFT should be chosen carefully as a balance betweenprotection against multipath (i.e. ISI), temporal variations (i.e. ICI), anddesign cost/complexity.

LTE FFT period is 66.67 usec, corresponding to the 15 KHz subcarrierseparation.

IFFT FFT

d1d2

dL

d1d2

dL

Time-domain(to be transmitted)


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Motivation for OFDMA

Good performance in frequency selective fadingchannels

Low complexity of base-band receiver

Good spectral properties and handling of multiplebandwidths

Link adaptation

Frequency domain scheduling

Compatibility with advanced receiver and antenna

technologies.


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Challenges


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1) ISI


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Solution: CP


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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 toomuch 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


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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 !

PowerDen

sity

PowerDens

ity

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

SavedBandwidth


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3)Inter-Carrier Interference (ICI)

The price for the optimum subcarrier spacing is the sensitivity of OFDMto frequency errors.

If the receivers frequency slips some fractions from the subcarrierscenter frequencies, then we encounter not only interference betweenadjacent carriers, but in principle between all carriers.

This is known as Inter-Carrier Interference (ICI) and sometimes alsoreferred to as LeakageEffectin the theory of discrete Fouriertransform.

One possible cause that introduces frequency errors is a fast movingTransmitter or Receiver (Doppler effect).


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f0 f1 f2 f3 f4

P

I3

I1I4I0

ICI=Inter-CarrierI

nterference

Frequency Drift

Two effects begin to work: Subcarrier has no longer its

power density maximum- soloose of signal energy.

The rest of subcarriers haveno longer a null point here.So we get some noise fromthe other subcarrier.

OFDM T itt


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LowPass

cos(2fct)

-sin(2fct)

I

Q

ModulationMapper

IFFT

s0

ModulationMapper

s1

ModulationMapper

sN-1

b10 ,b11,

Serial toParallel

Converter(Bit

Distrib.)

b20 ,b21,

bN-1 0

BinaryCodedData

.

.

.

D

Ax0, x1, , xN-1 IQSplit

LowPass

D

A

R

freq.f1f2f0 fN-1

s0

s1 sN-1s2

FrequencyDomain

timet1t2t0 tN-1

x0x1

xN-1

x2

TimeDomain

CP/Guard

Generation

I

Q

Time Domain Signal

FrequencyDomain Signal:(Collection ofSinusoids)

Each entry to the IFFT modulecorresponds to a different sub-carrier

Each sub-carrier is modulatedindependently by ModulationSchemes:

BPSK,QPSK, 16QAM, 64QAM

OFDM Transmitter


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reference(pilot)

ChannelCorrection

Demodulator

Bit Mapping

j

I

Q

A

D

ChannelEstimation

RF

LowNoise

Amp.

+Bandp

ass

A

D

AGCAutomatic

Gain Control

De-rotator

signalstrength

LNA gain

Frequency And Timing Sync

signalautocorreation

pha

secorrection

timee

adjust

.

.

.

s0

s1

sN-1

channel

response

s0

Bit Mappings1

Bit MappingsN-1

.

.

.

.

.

.

.

.

.

B10 ,B11,

B20 ,B21,

BN-1 0

BitDistrib

ution

Soft BitCodedData

freq.f1f2f0 fN-1

s0s1 sN-1

s2

Frequency Domain

Time Domain

timet1t2t0 tN-1

y0y1yN-1

x2

QPSK

Im

Re

10

11

00

01

skd11

d10

OFDM Receiver

Windowing+

FFT

FrequencyD

omain


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OFDM Key Parameters

2) Subcarrier Spacing (f = 15 KHz)The Symbol time is

Tsymbol = 1/f = 66,7s

f

TSYMBOL

TCP SYMBOL

TCP

TS

Frequency

Time

Powerdensity

Amplitude

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

Frequency


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3) The number of Subcarriers Nc

If BW = 20MHz Transmission BW = 20MHz2MHz = 18 MHz

the number of subcarriers Nc = 18MHz/15KHz = 1200 subcarriers

TransmissionBandwidth [RB]

Transmission Bandwidth Configuration [RB]

Channel Bandwidth [MHz]

Resourceblock

Channeledge

Channeledge

DC carrier (downlink only)Active Resource Blocks

OFDM Key Parameters


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4) FFT (Fast Fourier Transform) size Nfft

For a bandwidth BW = 20 MHzNc = 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 f

Example: 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 inUMTS!!

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

OFDM Key Parameters


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Bandwidth

(NCf)

1.4 MHz 3 MHz 5 MHz 10 MHz 15 MHz 20 MHz

Subcarrier Fixed to 15 kHz Spacing (f)

Symbol Tsymbol= 1/f = 1/15kHz = 66.67s

duration

Sampling rate,

fS(MHz)

1.92 3.84 7.68 15.36 23.04 30.72

Data

Subcarriers (NC)

72 180 300 600 900 1200

NIFFT

(IFFT Length)

128 320 512 1024 1536 2048

Number ofResource Blocks 6 15 25 50 75 100

Symbols/slot Normal CP=7; extended CP=6

CP length Normal CP=4.69/5.12sec., Extended CP= 16.67sec

OFDM Recap

OFDMA Ch ll


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OFDMA Challenges

1) Tolerance to frequency offset(Inter carrier Interference-ICI)

2) High Peak-to-Average Power Ratio

(PAPR)

Frequency

ICI


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SC FDMA


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SC FDMA nd OFDMA


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SC-FDMA and OFDMA

OFDMA transmits data in parallelacross 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 1/6thof the modulation symbol in OFDMA

OFDMA SC-FDMA

SC FDMA and OFDMA


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OFDM SC-FDMA

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

which should spread the input modulation symbols over all theallocated subcarriers

SC-FDMA and OFDMA

OFDMA vs SC FDMA: QPSK


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OFDMA vs SC-FDMA: QPSK

From: TS 36.211.

SC FDMA P i i l


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SC-FDMA Principles

PAPR is the same as that used for the input modulation symbols

This could be achieved by transmitting Nmodulation symbols in series at N times therate.

One can see that the SC-FDMA symbol which ishaving 66.66s is containing N sub-symbols

N = 6 in the example shown

In Time domain only one modulation symbolis transmitted at a time.

The number of subcarriers which could be allocated for transmissionshould be multiple of 2,3 and/or 5

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

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

SC FDMA P inciples


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The FFT output size is always smaller than the IFFT input size

FFT

IFFT

.

.

.

Subcarriers

allocated for oneUE

Subcarriersallocated to

other users orset to zero

This is because a typical cells uplinkcapacity will be greaterthan 180kHz

Other UEs will be assigned other groups ofsubcarriers to use across the uplink channelbandwidth.

No two UEs will be assigned the same180KHz block to use simultaneously.

As not all sub-carriers are used by themobile station, many of them are set to zeroin the diagram

Notethat if the output size of the FFT is equalto the size of the IFFT input then the overalleffect is nullsince the two operations (FFT andIFFT are complementary)

SC-FDMA Principles

SC FDMA Principles


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Adjusting the data rate in SC-FDMA

Halved SC-FDMAsub-symbol

duration

Initialbandwidth

SC-FDMAsub-

symbolduration

Doubledbandwidth

If the data rate increases more bandwidth is needed to transmit more modulationsymbols (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 inthe 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 s

Double the datarate

SC-FDMAsymbol 67s

SC-FDMAsymbol 67s

SC-FDMA Principles

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

SC FDMA: Multiplexing


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SC-FDMA: Multiplexing

One user always continuous in frequencySmallest uplink bandwidth, 12 subcarriers: 180 kHz (same for OFDMA in downlink)

Largest uplink bandwidth: 20 MHz (same for OFDMA in downlink)

In time domain the granularity for resource allocation is 1 msfor one user (same for OFDMA in downlink)

User 2 f

User 1 f

f

Receiver

User 1 User 2

Bandwidth Distribution


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Bandwidth Distribution

CarrierBandwidth

(MHz)

Number ofSub-Carriers

1.4 72

3 198

5 330

10 660

15 990

20 1320

Resource: Element Block Grid


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Resource: Element, Block, Grid

[source: 3GPP TR 25.814]

LTE Reference Signals (R)areInterspersed Among ResourceElements

The Usage of RE


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The Usage of RE

Resource elements

reserved for

reference symbols

Control Channel

Region (1-3 OFDM symbols)

One subframe (1ms)

12

sub

carriers

Frequency

TimeData

Region

Duplexing FDD/TDD


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DuplexingFDD/TDD

FDD

..

..

..

..

Downlink Uplink

Frequency band 1

Frequency band 2

.. ..Single frequency

band

TDD

Frame Structure: Generic


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Frame Structure: Generic

Radio Frame Type 1 FDD


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Radio Frame Type 1 - FDD

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

radio frame 10 msec

subframe 1 msec Type 1

0

0 1 2 3 4 5 67 OFDM symbols (short CP)

Radio Frame Type 2 TDD


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Slot

f

time

UL/DLcarrier

radio frame 10 ms

subframe 0

DwPT

S

GP

UpPT

S

half frame

subframe 2subframe 3subframe 1 subframe 4 subframe 5

DwPT

S

GP

UpPT

S

half frame

subframe 7subframe 8subframe 6 subframe 9

Slot

f

time

UL/DLcarrier

radio frame 10 ms

subframe 0

DwPTS

GP

UpPTS

half frame

subframe 2subframe 3subframe 1 subframe 4 subframe 5

half frame

subframe 7subframe 8subframe 6 subframe 9

Downlink Slot Uplink or Downlink Special SlotUplink Slot

Radio Frame Type 2 - TDD

Special Subframe


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Special Subframe

DwPTS (Downlink Pilot Timeslot Channel) Can contain synchronization, PDSCH and PDCCH.

The DwPTS is used for downlink synchronization.

Primary synchronization signal transmitted in the first OFDM symbol of theDwPTS.

Secondary synchronization signal transmitted in the last OFDM symbol ofsubframe 0 (immediately preceding to the DwPTS).

Resources not used for synchronization signals can be used for data,reference signals and control signaling.

UpPTS (Uplink Pilot Timeslot Channel) Used by eNB to determine the received power level and the received

timing from the UE.

Resources not used for reference signals(sounding and/or demodulationreference signals) can be used for random access.

No PUCCHis transmitted in UpPTS.

GP (Guard Period) The guard period between DwPTS and UpPTS determines the maximum cell

size.

TDD Frame Configurations


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TDD Frame Configurations

Configuration1 DL:UL=2:2 (or 3:2)

Configuration2 DL:UL=3:1 (or 4:1)

D S UDownlink Special Uplink

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 D6 5 ms D S U U U D S U U D


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Summary


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Summary

Understand LTE Duplexing

Single Transmitter FDMA Principle

Multi carrier principle

OFDMA and SC FDMA PrincipleMultipath Propagation

Cyclic Prefix

OFDMA and SC FDMA

Transmitter Receiver

OFDM and SC FDMA Key Parameters

Resource Block


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HAPPY LEARNING

MobileCommProfessionals, Inc.www.mcpsinc.com

Documents

4) Ofdma and Scfdma