ofdm technology

Committed to connecting the world

4G Mobile (IMT Advanced) System and Applications

Core Technologies for 4G: OFDMCore Technologies for 4G: OFDM

Pusan, HAEUNDAE CENTUM HOTEL. KoreaApril 2011

Jongseob BaekJ b b k@ il

April 2011

[email protected]

April, 2011 1


Contents Backgrounds Backgrounds

Broadband wireless channels

Basic concept of FDM Basic concept of FDM

SC System

OFDM System

Application of OFDM System (OFDMA)

CP-SC System

Application of CP-SC System (SC-FDMA)

April, 2011 2


Digital communication systems require each channel to operate at a specific

Backgrounds Digital communication systems require each channel to operate at a specific

frequency and with a specific bandwidth.

In fact, communication systems have evolved so that the largest amount of

data can be communicated through a finite frequency range.

This lecture will focus on the recent evolution of communication systems into

using various mechanisms for effectively using the frequency spectrum.

More specifically, the lecture will describe how frequency division multiplexing

(FDM) and orthogonal frequency division multiplexing (OFDM) are able to (FDM) and orthogonal frequency division multiplexing (OFDM) are able to

effectively utilize the frequency spectrum.

In addition, this lecture will compare the properties of single-carrier (SC) and , p p p g ( )

OFDM, and then it describe why OFDM systems are currently being

implemented in some of the newest and most advanced communication

April, 2011

systems.

3


Broadband Wireless Channel What is the broadband wireless channel? What is the broadband wireless channel?

Broadband wireless channel could be defined by understanding a delay

spread (multipath propagation) and Doppler spread (a fading

phenomenon).

Under a delay spread consideration, a radio signal travels over two or more

paths from a transmitter to a receiver paths from a transmitter to a receiver.

Thus, delay spread can cause changes in the received signal level by either

adding or subtracting delayed signals (reflected signals) from the received

signal level.

Delay spread is frequency dependent, which means that it’s properties will vary

depending on a used frequency( band)depending on a used frequency(-band).

Delay spread is important for lower frequencies and near line of sight (NLOS)

transmission.

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Broadband Wireless Channel Whereas, Doppler spread is not usually a challenge on systems that use higher Whereas, Doppler spread is not usually a challenge on systems that use higher

frequencies as these systems tend to use highly directional (high-gain)

antennas for LOS transmission.

Under Doppler spread consideration, a radio signal level varies according to

mobility of either mobile terminal or surrounding environments.y g

In usual, a fading degree depends on a mobile speed of such associated

terminals and the used frequency(-band). It is also depends on a angle of

signal reception from transmitter.

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Broadband Wireless Channel Delay spread determines a frequency-selectivity Delay spread determines a frequency selectivity

Selective vs. Non-selective

Doppler spread determines a time-selectivity pp p y

Fast fading vs. Slow fading

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Broadband Wireless Channel Frequency selectivity (caused by multipath)Frequency selectivity (caused by multipath)

Multipath channel is usually modeled by a finite impulse response (FIR) filter

Ts < maximum delay (Ts: symbol duration) which causes an inter-symbol

interference (ISI)

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Broadband Wireless Channel Frequency selectivity (caused by multipath)Frequency selectivity (caused by multipath)

Ts >> maximum delay, in which the ISI effect could be mitigated

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Broadband Wireless Channel Time selectivity (caused by Doppler effect)Time selectivity (caused by Doppler effect)

Doppler frequency:

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Broadband Wireless Channel Frequency-selective fading (doublely selectivity) channel Frequency selective fading (doublely selectivity) channel

Frequency selectivity + time selectivity

4-path Rayleigh fading channel- time domain

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Broadband Wireless Channel Frequency-selective fading channel Frequency selective fading channel

4-path Rayleigh fading channel- frequency domain

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The concept of FDM involves the allocation of each channel to a unique

The basic concept of FDM The concept of FDM involves the allocation of each channel to a unique

frequency range.

This frequency range prescribes both the center frequency and channel width

(bandwidth), which means these channels are non-overlapping.

Consequently, multiple channels (or users) can operate concurrently by using

different channels in terms of the frequency domain.

Due to such a simple property, FDM is commonly used in a variety of

communications such as Global System for Mobile Communications protocol communications such as Global System for Mobile Communications protocol

(GSM), Time division multiple access (TDMA), code division multiple access

(CDMA), WIMAX, and LTE.

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The basic concept of FDM FDM with/without guard-band which involves assigning non-overlapping FDM with/without guard band which involves assigning non overlapping

frequency ranges to different signals (or to each "user" of a medium)

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SC System Let us consider that a single channel (individual channel) is utilized by SC Let us consider that a single channel (individual channel) is utilized by SC

system

Sequential signal is transmitted with a short Ts (< max. delay spread)

Signal are transmitted with a constant power level, which leads to a

low peak-to-average power ratio (PAPR)

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SC System The standard structure of SC transmissionThe standard structure of SC transmission

Pulse-shaping filter implemented with a square root-raised cosine (SRRC)

filter (Finite impulse response)

It is used to eliminate interference from adjacent channels in the frequency

domain

Whereas, it causes an inter-symbol interference (ISI) by overlapping

subsequent symbols on the same channel

Non-linear amplifier Non linear amplifier

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The structure of SC reception

SC SystemThe structure of SC reception

The receiver performs reverse process to the SC transmission

Receive signal suffers from ISI effect due to short Ts

Basically, ISI effect can be mitigated through a channel equalizer

implemented with a linear/non-linear FIR filter

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SC System Understanding of channel equalization in terms of frequency-domainUnderstanding of channel equalization in terms of frequency domain

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Equalizer properties used for SC system

SC SystemEqualizer properties used for SC system

Time-domain equalizer (TDE) implemented a FIR filter has a high

computational complexity

The total tap-length depends on the maximum delay spread and multipath

power

Efficient implementation could be archived through an adaptive filtering Efficient implementation could be archived through an adaptive filtering

scheme

Adaptive filtering would degrade the equalizer performance over a multipath

fading channel, since the adaptation could not follow the fading degree

The such problems could be resolved by using a frequency-domain equalizer

(FDE) which requires additional receiver signal processing (FDE), which requires additional receiver signal processing.

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Summary

Key properties of SC System Summary

SC signal guarantees a low PAPR by controlling the transmit power in linear

region of amplifier

which means it can cover a wide-range service area with a high power emission

and without additional repeater, such as relay

Channel equalization has a high computational complexity and low Channel equalization has a high computational complexity and low

tracking capability to multipath fading channel

SC receiver is robust to frequency and timing offset effects, since it just uses

single carrier

April, 2011 19


OFDM System OFDM is a subset of FDM in which a single channel utilizes multiple sub-OFDM is a subset of FDM in which a single channel utilizes multiple sub

carriers on adjacent frequencies

In addition, the sub-carriers in an OFDM system are overlapped to maximize

spectral efficiency

Ordinarily, overlapping adjacent channels can interfere with one another.

However, sub-carriers in an OFDM system are precisely orthogonal to one

another

April, 2011 20


OFDM System Thus sub-carriers are able to be overlapped without interfering and to Thus, sub carriers are able to be overlapped without interfering and to

maximize spectral efficiency without causing adjacent channel interference

(in ideal system condition)

In addition, these sub-carriers are able to be detected correctly, since the

maximum power of each sub-carrier corresponds directly with the minimum

f h dj h lpower of each adjacent channel.

April, 2011 21


OFDM System Basic concept of OFDM: Rectangle pulse-shaping on the time-domainBasic concept of OFDM: Rectangle pulse shaping on the time domain

(a) DC centered spectrum with equally spaced zeros

(b) Shift spectrum with linear phase on DC pulse: move spectrum to first

spectral zero

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Real and imaginary parts of complex exponential time series: Integer number

of cycles per interval

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Spectra of complex exponential time series: Integer number of cycles per

interval

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OFDM System Basic concept of OFDM: orthogonal transmissionBasic concept of OFDM: orthogonal transmission

Serial-to-parallel (S/P): once the bit-stream composing of N data symbols has been

divided among the individual sub-carriers, each sub-carrier is modulated as if it was an

individual

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OFDM System Basic concept of OFDM: orthogonal receptionBasic concept of OFDM: orthogonal reception

April, 2011 26


Efficient OFDM transmission using inverse discrete Fourier transform (IDFT)

OFDM System Efficient OFDM transmission using inverse discrete Fourier transform (IDFT)

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OFDM System OFDM transceiver structure using inverse fast Fourier transform (IFFT) and OFDM transceiver structure using inverse fast Fourier transform (IFFT) and

FFT at transmitter and receiver, respectively

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OFDM System OFDM converts the frequency-selective channel to frequency-flat channel in OFDM converts the frequency selective channel to frequency flat channel in

terms of each frequency-bin.

Which means that OFDM system

has more robust transmission

Property than SC system

in such a channel.

April, 2011 29


OFDM System OFDM system still suffers from ISI effect OFDM system still suffers from ISI effect

the ISI affects orthogonality between subcarriers, which leads to severe inter-

(sub)carrier interference (ICI) problem

The advantages of robustness to frequency-selective channel and spectral

efficiency do not be guaranteed anymore.

April, 2011 30


OFDM System Insertion of guard-interval (GI) between OFDM symbols to prevent the ISI Insertion of guard interval (GI) between OFDM symbols to prevent the ISI

effect

The GI length is larger or equal to the maximum delay spread of a channel

GI insertion with zero-padded (ZF) symbol

No ISI, but ZF still affects the orthogonality after FFT operation, since it

broken the continuity of sub-carriers.

April, 2011 31


OFDM System Insertion of cyclic-prefixed (CP) symbol between OFDM symbols instead of ZP Insertion of cyclic prefixed (CP) symbol between OFDM symbols instead of ZP

symbol

It completely eliminates ISI and ICI.

It maintains subcarrier orthogonality.

April, 2011 32


OFDM System Key blocks of OFDM TransceiverKey blocks of OFDM Transceiver

Pilot insertion to estimate channel information and the amount of syn. offset

Non-linear amplifier effect to IFFT output

Relation between symbol timing offset and ISI effect

Relation between frequency offset and orthogonality

One-tap channel equalization on the frequency-domain

April, 2011 33


OFDM System Pilot insertionPilot insertion

Channel information estimation on the time-/-frequency domain

Compensates symbol timing and frequency offsets

Representative pilot patterns

Block-type pilot pattern arrangement

Comb-type pilot pattern arrangement

Scattered-pilot pattern arrangement

Cf) Known-symbol insertion

Known-symbol is sometimes appended in the front of frame block composing Known symbol is sometimes appended in the front of frame block composing

of several OFDM symbols

Known-symbol can be used instead of CP symbol to improve synchronization

April, 2011 34

and channel estimation


OFDM System Block-type pilot pattern arrangement Block type pilot pattern arrangement

The channel estimation can be performed by either periodically inserting pilot

tones into all sub-carriers (frequency axis)

It is usually used in a severe frequency-selective channel

Channel varies slowly enough so that the channel estimation will have a good

accuracy

April, 2011 35


OFDM System Comb-type pilot pattern arrangement Comb type pilot pattern arrangement

The number of pilots used for channel estimation is usually much smaller than

the number of sub-carriers

This method is usually used in systems having significant channel variation

over a short period of time

April, 2011 36


OFDM System Scattered-pilot pattern arrangement Scattered pilot pattern arrangement

Block-type pilot pattern + comb-type pilot pattern

This method is commonly be used in systems having significant doubly

selective channels

April, 2011 37


OFDM System High PAPR problemHigh PAPR problem

IFFT output shows Gaussian distribution, approximately.

High-peak random signals (a high PAPR problem) are often observed, which

are distorted on the non-linear region of amplifier.

April, 2011 38


OFDM System OFDM Input and output of non-linear amplifierOFDM Input and output of non linear amplifier

April, 2011 39


OFDM System ISI effect according to symbol timing offset ISI effect according to symbol timing offset

Symbol timing offset corresponds to the starting position of FFT window.

Cases of timing offset within CP symbol

April, 2011 40


OFDM System

April, 2011 41


OFDM System ICI effect according to frequency offset ICI effect according to frequency offset

Due to oscillator mismatch or Doppler Shift

Breaking orthogonality

-> ICI problem

-> performance degradation

April, 2011 42


OFDM System Efficient receiver processing according to the use of CP symbolEfficient receiver processing according to the use of CP symbol

Convert linear channel matrix to circular matrix after removing CP

symbol

April, 2011 43


OFDM System Simple one-tap frequency-domain equalization (FDE) realizationSimple one tap frequency domain equalization (FDE) realization

Each subcarrier can be processed independently, which means that only one-

tap multiplier is sufficient in term of implementation.

Consequently, it is obvious that the FDE used for OFDM system has lower

computational complexity than TED for SC system

April, 2011 44


OFDM System An estimation of channel frequency response (CFR) using comb-type pilot An estimation of channel frequency response (CFR) using comb type pilot

arrangement

Calculate the channel estimates at the pilot subcarriers

Interpolate the estimates for the other subcarriers

1

Channel, Bandwidth, and Samples

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 10

0.5

1 Zero PackedSpectral Samples

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 10

0.5Spectral Samples

andExtended Reflection

1 InterpolatedSpectral Points

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 10

0.5Spectral Points

0.01Magnitude of Interpolation Error For In-Band Frequencies

April, 2011 45-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 10

0.005

Normalized Frequency


OFDM System Constellations before and after performing FDE Constellations before and after performing FDE

April, 2011 46


Robustness to frequency selectivity one-tap equalizer

Key properties of OFDM System Robustness to frequency selectivity, one tap equalizer

Bandwidth efficiency due to the overlapping orthogonal subcarriers

Simultaneous elimination of ISI and ICI (inter carrier interference) by CP ( ) y

symbol

High peak-to-average power ratio (PAPR)

Sensitivity to Doppler: Channel variation within one OFDM symbol duration

incurs inter-carrier interference (ICI)

Applications:

ADSL, Digital Video Broadcast (DVB), Digital Audio Broadcast (DAB), Digital

terrestrial multimedia broadcasting (DTMB) Wireless LAN (IEEE 802 11a) terrestrial multimedia broadcasting (DTMB), Wireless LAN (IEEE 802.11a),

Wireless MAN (WiMax IEEE 802.16), Down-Link [base station to mobile] 3GPP

LTE, etc

April, 2011 47


Application of OFDM system Orthogonal Frequency-Division Multiple Access (OFDMA)Orthogonal Frequency Division Multiple Access (OFDMA)

A multi-user version of the popular OFDM modulation scheme. Multiple access

is achieved in OFDMA by assigning subsets of subcarriers to individual

This allows simultaneous low data rate transmission from several users.

April, 2011 48


CP-SC System Cyclic-prefixed SC systemCyclic prefixed SC system

It preserves the advantages of SC system, i.e., low PAPR, wide-range service

coverage

This allows efficient receiver processing,

especially the application of FDE,

h h d b h f C b lwhich is caused by the use of CP symbol

Convert linear channel

matrix to circular matrix

after removing CP symbol

April, 2011


Application of CP-SC system Single-carrier FDMA (SC-FDMA)Single carrier FDMA (SC FDMA)

Like other multiple access schemes (TDMA, FDMA, CDMA, OFDMA), it deals

with the assignment of multiple users to a shared communication resource

It has an additional FFT/IFFT processing preceding the conventional OFDMA

processing

M lti l i d ibl b i i diff t Multiple access among users is made possible by assigning different users,

different sets of non-overlapping Fourier-coefficients

The distinguishing feature of SC-FDMA is that it leads to a single-carrier g g g

transmit signal, in contrast to OFDMA which is a multi-carrier transmission

scheme.

Owing to its inherent single carrier structure, a prominent advantage of SC-

FDMA over OFDM and OFDMA is that its transmit signal has a lower PAPR.

April, 2011 50


Application of CP-SC system Single-carrier FDMA (SC-FDMA)Single carrier FDMA (SC FDMA)

April, 2011 51


Some of the key technologies used in 4G communication systems i e

Key Transmission technologies of 4GSome of the key technologies used in 4G communication systems, i.e.,

WiMAX and LTE, include OFDM, frequency reuse, adaptive modulation,

multi-input multi-output (MIMO), and so on.

The use of OFDM reduces the effects of multipath and delay spread, which is

especially important for lower frequencies and near line of sight (NLOS)

i itransmission

The use of OFDM enables high data bandwidths to be transmitted efficiently

The use of OFDM allows efficient access schemes for the uplink and downlink The use of OFDM allows efficient access schemes for the uplink and downlink

WiMAX : Orthogonal Frequency Division Multiple Access (OFDMA) is used in

both the downlink and uplink

LTE: OFDMA is used for the down link, while Single Carrier-Frequency Division

Multiple Access (SC-FDMA) is used in the uplink.

April, 2011 52


LTE: SC-FDMA is used in view of the fact that its peak to average power ratio

Key Transmission technologies of 4G LTE: SC FDMA is used in view of the fact that its peak to average power ratio

is small and the more constant power enables high RF power amplifier

efficiency in the mobile handsets - an important factor for battery power

equipment.

April, 2011 53


Thank you !!

April, 2011 54

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