Modulasi Multicarrier

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Modu l 6 Modulasi Mult icarr ier

Modul 6 Multicarrier Modulation

Faculty of Electrical Engineering

Bandung 2015

Wireless Communication System


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Subject

a. Konsep OFDM

b. Konsep OFDMA

c. Pengenalan SC-FDMA

Modul 6 Mult icarr ier Modulat ion


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Konsep OFDM



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Propagation Concept: NLOS Performance


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Propagation Concept: Mutipath Propagation

Sinyal-sinyal multipath datang pada waktu yang berbeda dengan

amplitudo dan pergeseran fasa yang berbeda, yang menyebabkan

pelemahan dan penguatan daya sinyal yang diterima.

Propagasi multipath berpengaruh terhadap performansi link dan coverage.

Selubung (envelop) sinyal Rx berfluktuasi secara acak.


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ro ems oMutipath Propagation & High Data Rate


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Multicarrier Transmission - Basic Concep

Orthogonal Frequency Division Multiplexing(OFDM) is a

multi-carrier modulation scheme First break the data into small portions

Then use a number of parallel orthogonal sub-carriers

to transmit the data

Conventional transmission uses a single carrier, which ismodulated with all the data to be sent

Single Carrier Company

Multi Carrier Company



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OFDM Concept: Mengapa OFDM

Sinyal OFDM (Orthogonal Frequency Division

Multiplexing) dapat mendukung kondisi NLOS

(Non Line of Sight) dengan mempertahankan

efisiensi spektral yang tinggi dan memaksimalkan

spektrum yang tersedia. Mendukung lingkungan propagasi multi-path.

Scalable bandwidth: menyediakan fleksibilitas dan

potensial mengurangi CAPEX (capital expense).


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OFDM

Peak of spectrum for each sub-band must

correspond to zero crossings for all othermodulated sub-carriers.

Interference now avoided in frequency-domain.

Adjacent sub-carriers spaced exactly 1/T Hz apart

when sub-band pulse-rate is 1/T pulses/second.

Sub-carriers are as close together as they can

possibly be without introducing spectral

interference. Each modulated sub-carrier is orthogonal to all

others which means that they do not interfere with

each other.



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OFDM Basic Concept

OFDM is a special case ofFrequenc y Division

Mult ip lexing(FDM) For FDM

No special relationshipbetween the carrierfrequencies

Guard bands have to beinserted to avoid AdjacentChannel Interference(ACI)

For OFDM

Strict relation betweencarriers: fk = kf where f =

1/TU(TU - symbol period)

Carriers are orthogonal toeach other and can bepacked tight



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Suatu pulsa kotak

merupakan opsi sinyal yangbaik dan mudah

diimplementasikan.

Pulsa kotak dalam kawasanfrekuensi memiliki spktrum

dengan null terletak pada

kelipatan integer dari

1/durasi simbol.

Puncak daya terletak

pada center frequency .

OFDM


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OFDM secara sederhana menempatkan next carrier

persis di null pertama dari carrier sebelumnya.

Sehingga tidak diperlukan pulse shaping

Antar carrier OFDM carrier menggunakan durasi simbol yang sama

Ts, tanpa guard band.

OFDM


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Combining OFDM s b bands


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t

T/2-T/2

1

rectT(t)

ModulateF

t

T/2-T/2

1

rectT(t)

ModulateF+2/T

t

T/2-T/2

1

rectT(t)

ModulateF+1/T

T.sinc1/T(..)

f

T

F+2/T

F

T.sinc1/T(f-F)

f

T

F+1/T

F

T.sinc1/T(..)

f

F+2/T

F+1/T F+3/T

Assume purely

real spectrum

Combining OFDM sub-bands


O


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OFDM spectrum

Fourier transformSUM

f

1/T 3/T

Combine

real

spectraAssume

purely real

spectra



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Tipe Sub Carrier OFDM


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Tipe Sub-Carrier OFDM

Data Sub-carriers

Membawa simbol BPSK, QPSK, 16QAM, 64QAM

Pilot Sub-carriers

Untuk memudahkan estimasi kanal dan demodulasi koheren pada receiver.

Null Subcarrier

Guard Sub-carriers

DC Sub-carrier

OFDM Si l ti


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OFDM Signal properties

Time domain

Frequency domain

Power Spectrum for OFDM symbol

frequency


Ch l E ti ti il t b l


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Frequency/subcarrier

Pilot carriers /reference signals

Data carriers

Channel Estimation_pilot symbols

The channel parameters can be estimated based on known

symbols (pilot symbols)

The pilot symbols should have sufficient density to provide

estimates with good quality (tradeoff with efficiency)

Different estimation methods exist

Averaging combined with interpolation Minimum-mean square error(MMSE)

Pilot symbol

Time

Frequency


OFDM S h i ti


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OFDM Synchronization

Timing recovery

No problem if offset is within Dt

Frequency synchronization

A carrier synchronization error will

introduce phase rotation, amplitude

reduction and ICI Frequency offsets of up to 2 % of Dfis

negligible

Even offsets of 5 10 % can be

tolerated in many situations

tmax Dt

CP Useful symbol

Integration period, TU


Ch i th OFDM t


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Choosing the OFDM parameters

Symbol time (TU) and subcarrier

spacing (Df) are inverse

TU= 1/Df

Consequences of increasing the

subcarrier spacing

Increase cyclic prefix overhead

Consequences of decreasing the

subcarrier spacing

Increase sensitivity to frequency

inaccuracy

Increasing number of subcarriers

increases Tx and Rx complexity

Increasing

subcarrier spacing

Decreasing

subcarrier spacing

Increase sensitivity to

frequency accuracy

TU

Increase CP

overhead



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OFDM T i i d l


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OFDM Transmission model

Channel, h(t)

Modulator and transmitter

Wireless channel

Receiver and demodulator


OFDM T i


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OFDM Transceiver


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Advantages of OFDM


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Advantages of OFDM

Spectrally efficient because of orthogonality of the 64 carriers.

Good for channels affected by frequency selective fading because:

(i) Effects of fading, affecting a small range of frequencies, can be spread

out using interleaving so that FEC can more easily correct any bit-

errors.

(ii) Cyclic extension as a guard-interval, eliminates ISI caused by multi-

path propagation. Simpler way of eliminating ISI than pulse-shaping

as used in single carrier systems.

(iii) Equalisation is easier than with single carrier systems which use

adaptive filtering. OFDM receiver can amplify real & imag parts ofFFT outputs such that they have same amplitudes.

Possible because of the cyclic extension as explained earlier.


Disadvantages of OFDM


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Disadvantages of OFDM

Peak to mean ratio of symbols can be very large by nature of FFT & Inv-

FFT. (Amplitudes can become very large in comparison to the mean)

Shapes OFDM symbols very complex & must be sent & received

accurately.

With QPSK on each sub-carrier, 1029 shapes & even more with 64-QAM

Transmitter & receiver must be linear to preserve shape.

Definitely not "constant envelope".

Need class A amplifiers: less power efficient than those for constant

envelope transmissions.

Lot of power lost in the amplifiers.

Not ideal for mobile phones, but fine for mobile computers with bigger

batteries that are not sending data continuously.

Sensitive to Doppler frequency shifts.


Effect of Multipath channel


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Effect of Multipath channel

],[00

t

],[11

t

Diffracted and Scattered Paths

Reflected Path

LOS Path

],[kk

t


Multipath channel (cyclic prefix)


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Multipath channel (cyclic prefix)

Time

[t]

Amplitude

[

]

Example multipath profile

t0 t1 t2

The prefix is made cyclic to avoid inter-carrier-interference(ICI)

(maintain orthogonality)

Multipath introduces inter-symbol-interference (ISI)

TU

Prefix is added to avoid ISI

TUTCP



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Cyclic Prefix for Multipath channel


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Cyclic Prefix for Multipath channel

Tcp should cover the maximum length of the time

dispersion Increasing Tcp implies increased overhead in power

and bandwidth (Tcp/ TS)

For large transmission distances there is a trade-off

between power loss and time dispersion

CP Useful symbol CP Useful symbolCP Useful symbol

TUTcp

TS



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Guard Interval (Cyclic Prefix)


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Guard Interval (Cyclic Prefix)

Untuk mengatasi multipath delay spread

Guard Interval (cyclic prefix) : 1/4, 1/8, 1/16 or 1/32


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Multipath channel (frequency diversity)


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Multipath channel (frequency diversity)

=

The OFDM symbol can be exposed to a frequency selective

channel

The attenuation for each subcarrier can be viewed as flat Due to the cyclic prefix there is no need for a complex

equalizer

Possible transmission techniques

Forward error correction (FEC) over the frequency band Adaptive coding and modulation per carrier



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Summary


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Summary

Channel, h(t)PA

CP

Frequency/subcarrier

Pilot carriers /reference signals

Data carriers



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Konsep OFDMA



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OFDM & OFDMA


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OFDM & OFDMA

OFDM

Semua subcarrier dialokasikan untuk satu

user

Misal : 802.16-2004

OFDMA

Subcarrier dialokasikan secara fleksibel

untuk banyak user tergantung pada kondisi

radio.

Misal : 802.16e-2005 dan 802.16m


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LTE Downlink Physical Layer Design: PhysicalResource


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Resource

The physical resource can be seen as

a time-frequency grid

LTE uses OFDM (Orthogonal Frequency Division Multiplexing) as its radio technology in downlink

In the uplink LTE uses a pre=coded version of OFDM, SC-FDMA (Single Carrier Frequency Division

Multiple Access) to reduced power consumption


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Pengenalan SC FDMA



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Documents

Modulasi Multicarrier