lte ofdm principle 32

8/13/2019 LTE OFDM Principle 32

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OFDM Fundamentals

Course Objectives:

Understand the basic OFMD concepts

Understand the OFMD fundamentals

Understand the advantages and disadvantages of OFDM

Understand the key technologies of OFDM

Understand the application of OFDM in the uplink and donlink


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Contents

1 System Overview..........................................................................................................................................1

!"! #ireless Channel $ropagation Characteristics""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!

!"!"! %arge &cale Fading""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""'

!"!"' &hado Fading""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""(

!"!"( Multipath Fading"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""(

!"!") *ime +arying ,ature and Doppler &hift of #ireless Channels""""""""""""""""""""""""""""""""""""""""""""""-

!"' .asic Concepts /bout OFDM"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""0

!"( /dvantages and Disadvantages of OFDM"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!1

2 Key Technologies of OFDM......................................................................................................................13

'"! 2uard 3nterval and Cyclic $refi4"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!(

'"' &ynchroni5ation *echnology""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!-

'"'"! Carrier &ynchroni5ation"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!6

'"'"' *imed &ymbol &ynchroni5ation""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!0

'"( Channel 7stimate"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!8

'") $/$9 9eduction *echnology"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""'1

'")"! /mplitude %imiting"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""'1

'")"' Compression and 74pansion"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""'!

3 OFDM Applications...................................................................................................................................23

("! OFDM /pplications in Donlink""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""'(

("' OFDM /pplications in Uplink"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""'

("'"! DF*;spread OFDM""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""'

("'"' &C;FDM/""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""'6

i


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1 System Overview

Knowledge points:

#ireless channel propagation characteristics

.asic OFDM concepts

/dvantages and disadvantages of OFDM

1.1 Wireless Channel Propagation Charateristis

Compared ith other communication channels< the mobile channel is one of the most

complicated channels" 7lectromagnetic aves propagate mainly in the form of space

aves< including the direct ave< refracted ave< scattered ave< and their composite

ave" Due to the motion of the Mobile &tation =M&>< the ireless channel beteen the

M& and the .ase &tation =.&> becomes variable and hard to control" &ignals suffer

different fading effects hen passing through a ireless channel" 2enerally< the

received signal poer is given by:

$=d> ? @d@;n &=d>9=d>

here< dindicates the distance vector beteen the M& and the .&< and |d|indicates the

distance beteen the M& and the .&" /ccording to the eAuation above< the impacts of

ireless channels on signals can be classified into three types:

!" *he path loss |d|-nof electromagnetic aves in free space is also called large

scale fading< here the value range of nis usually ()"

'" *he shado fading S(d)indicates the fading due to blocking or shadoing from

buildings or other obstacles or topographic relief in propagation environments"

(" *he multipath fading 9=d>< knon as small scale fading< is a common

phenomenon in radio signal transmission" #hen radio signals travel in space is given by:

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%*7FDDe,.C!! %*7 OFDM $rinciple

Figure !"!;' Multipath propagation of radio signals

9adio aves take different time and phases to travel different distances on differing

paths" Multiple signals in different phases are superimposed each other at the receiver"

Constructive superposition results in signal amplitude increase< hile destructive

superposition results in signal amplitude decrease" Dramatic amplitude changes of

received signals ill lead to fading"

For e4ample< if the transmitter sends a narroband pulse signal< the receiver can

receive multiple narroband pulses< hich have different fading conditions< time

delays and Auantities< and each corresponds to one transmitted pulse signal" *he

folloing figure shos the signals received by the receiver" *his results in ireless

channel time dispersion< here Ema4denotes the ma4imum delay spread"

Figure !"!;( 9eceived multipath signals

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*he aveform of a symbol in the received signals may e4pand to other symbols due to

delay spread during transmission< resulting in inter;symbol interference =3&3>" *o avoid

the 3&3< the symbol rate must be larger than the reciprocal of the ma4imum delay

spread" &ince the delay spreads measured at different time vary ith geographical

locations in comple4 mobile environments< e need to adopt the average value of large

amounts of statistical data"

*he folloing table lists delay spreads in different channel environments:

*able !"!;! Delay spreads in different channel environments

7nvironment Ma4imum Delay &pread Ma4imum $ath Difference3ndoor )1ns'11ns !'m!-m

Outdoor !Gs'1Gs (11m111m

/nother important concept related to delay spread in freAuency domain is coherence

bandidth hich is defined to be the reciprocal of the ma4imum delay spread:

From the frequency domain perspective, delay spread of multipath

signals can result in freAuency;selective fading" #ireless channels make different

random responses for freAuency components in signals" Different freAuency

components e4perience different fading< so signal aveforms are distorted after fading"

#hen signals are transmitted at a high freAuency and the signal bandidth e4ceeds the

coherence bandidth of the ireless channel< freAuency components undergo different

changes after the signals pass the ireless channel< hich results in signal aveform

distortion and 3&3" *his phenomenon is called freAuency;selective fading" #hen signals

are transmitted at a lo freAuency and the signal bandidth is less than the coherence

bandidth of the ireless channel< freAuency components e4perience the same fading

after the signals pass the ireless channel" &uch fading does not lead to signal

aveform distortion or 3&3< and it is deemed flat fading or non;freAuency;selective

fading" Coherence bandidth is a characteristic of ireless channels" #hether signals

ill e4perience flat fading or freAuency;selective fading hen passing a ireless

channel is subject to the signal bandidth"

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1.1.$ %ime &arying 'ature and Doppler Shi(t o( Wireless Channels

*he change of the freAuency of received signals caused by motion of an M& is called

Doppler effect hich is a common feature in any ave process" *ake visible light as an

e4ample" /ssume that a luminous object gives off light at a fi4ed freAuency in the

distance< our received freAuency needs to be the same ith the freAuency of this object"

#hen this object moves toard us and produces the second ave crest< the distance

from the object to us is shorter than that from the first ave crest to us" *herefore< the

time interval beteen the arrivals of these to ave crests becomes short and our

received freAuency increases correspondingly" #hen the luminous object moves aay

from us< our received freAuency diminishes"

/stronomers judge that other gala4ies are moving aay from us according to the

Doppler effect< hich leads us to an interpretation that the universe is e4panding" *he

relationship beteen freAuency and rate in Doppler effect is Auite familiar to us" For

e4ample< hen a car approaches< its siren gets louder =the sound freAuency increases>H

hen the car drives aay< its sirenIs pitch gets loer =the sound freAuency decreases>"

*ime varying nature of channel indicates that the transfer function of a channel varies

over time" 3n other ords< received signals are different for the same signals sent at

different moments< as shon in the figure belo"

Figure !"!;) Time varying nature caused by multipath propagation

/n embodiment of time varying nature in mobile communications systems is Doppler

shift" #hen monophonic signals pass a time varying fading channel< they have a

certain bandidth and freAuency envelop as shon in the figure belo" *his property is

also called freAuency dispersion"

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Figure !"!; Frequency dispersion caused by Doppler shift

#hen an M& moves toard the incident ave direction< the Doppler shift is positive

and the freAuency of the signals received by the M& ill increase" #hen an M& moves

in the opposite direction of the incident ave< the Doppler shift is negative and the

freAuency of the signals received by the M& ill decrease" Due to the Doppler shift reaches the receiver< its spectrum is no longer the

function J located at Kf1on the freAuency a4is but the spectrum distributed in = > and

ith a certain bandidth" *able !"!;' lists the ma4imum Doppler shifts of to carriers

at different moving speeds"

*able !"!;' Ma4imum Doppler shift =5>

&peed

Carrier

!11 kmLh 6 kmLh 1 kmLh ' kmLh

811 M5 0( -' )' '!

' 25 !0 !(8 8( )-

From the time domain perspective< another concept related to Doppler shift is

coherence time"

Coherence time is an average time duration over hich the channel impulse response is

invariant< and in hich to signals have strong potential for amplitude correlation" 3f

the reciprocal of the baseband signal bandidth< usually the symbol bandidth< is

greater than the coherence time of ireless channels< signal aveform may e4perience

some changes< leading to signal distortion and time;selective fading< also called fast

fading" 3f the symbol bandidth is less than the coherence time< the signal goes non;

time;selective fading< also called slo fading"

Free space propagation loss and shado fading mainly affect the coverage in radio

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areas< hich can be eliminated by appropriate design" 9adio communications systems

need to focus on removing the impact of time;selective fading and freAuency;selective

fading hich can be achieved by using the OFDM technologies"

1." )asi Conepts *+out OFDM

*he total signal bandidth< in a classical parallel data system< can be divided into ,

non;overlapping freAuency subchannels" 7ach subchannel is modulated ith a separate

symbol and then the , subchannels are freAuency;multiple4ed" *he general practice of

avoiding spectral overlap of subchannels as applied to eliminate inter;carrier

interference =3C3>< but this resulted in insufficient utili5ation of the e4isting spectrum"From this constraint the idea of Orthogonal FreAuency Division Multiple4ing =OFDM>

as born" OFDM is a multi;carrier transfer mode that fully utili5es spectral resources"

Figure !"';- shos the channel distribution in classical FDM and OFDM" From this

figure< the OFDM mode can save at least a half of spectral resources"

Figure !"';6 Channel distribution in classical FDM and OFDM

OFDM principle: Divide a channel into several orthogonal subchannels" Convert high;

rate data signals into lo;rate parallel substreams and modulate them on each

subchannel< as shon in the figure belo"

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Figure !"';0 OFDM principle

The OFDM utili5es 3nverse Fast Fourier *ransform =3FF*> and Fast Fourier

*ransform =FF*> to implement modulation and demodulation as shon in the figure

belo"

Figure !"';8 OFDM modulation and demodulation processes

OFDM modulation and demodulation processes are as follos:

!" *he transmitter converts high;rate serial data into lo;rate parallel data for data

transmission by using multiple orthogonal subcarriers"

'" 7ach subcarrier adopts an independent modulator and a demodulator"

(" *hese subcarriers are completely orthogonal to each other and synchronous in

transmission and reception"

)" *he transmitter and the receiver must be accurately co;channel and

synchronous< and sample bits precisely"

" *he receiver performs bit sampling at the backend of the demodulator to acAuire

and convert data into high;rate serial data"

/s a key role in the evolution to .(2L)2< the OFDM can ma4imi5e system

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performance by integrating diversity< spatiotemporal coding< interference< inter;channel

interference suppression< and intelligent antenna technologies"

The beginning of OFDM can be dated to !81s< but it is practically impossible to

implement orthogonal subcarriers ith traditional analog techniAues due to constraints

in steps ' and ( above" #ith advancement of digital signal processing technologies< although subcarriers can still maintain orthogonal< the receiver locks and traces the target" Dividing the

synchroni5ation task into to phases has advantages: *he algorithm in each phase only

needs to consider the tasks to be e4ecuted in a specific period< hich brings sufficient

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freedom for synchronous structure design" 3n phase 3< e only need to consider ho to

roughly estimate the carrier freAuency in a large acAuisition scope ithout considering

the tracing performance" 3n phase 33< e only need to consider ho to achieve better

tracing performance"

"."." %imed Sym+ol Synhroni/ation

/ cyclic prefi4 guard interval is inserted beteen OFDM symbols< so the start time of

synchroni5ation beteen OFDM symbols can change ithin the guard interval ithout

causing any 3C3 or 3&3< as shon in the figure belo"

Figure '"';'1 &tart time of synchroni5ation beteen OFDM symbols

*he 3C3 and 3&3 e4ist only hen the FF* calculation indo goes beyond the symbol

boundary or falls in the symbol amplitude roll;off 5one" *his means that the OFDM

system does not reAuire strict timed synchroni5ation beteen symbols" oever< e

must identify the optimal symbol timing in order to achieve the best system

performance in a multipath propagation environment" /lthough the start time of

synchroni5ation can be selected at discretion ithin the guard interval< any symbol

timing change may enhance the sensitivity of the OFDM system to delay spread and

thus the tolerant delay spread of the system ill be loer than the preset value" *o

minimi5e this negative impact< e need to reduce the deviation of timed symbol

synchroni5ation as much as possible"

3n most cases< timed symbol synchroni5ation and carrier synchroni5ation are

implemented by inserting a pilot symbol< hich may result in aste of bandidth and

poer resources and system effectiveness decrease" 3n fact< almost all multi;carrier

systems eliminate the 3&3 by inserting a guard interval" *o avoid resource aste< e

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usually utili5e the information carried by the guard interval to implement the ma4imum

likelihood estimation for symbol synchroni5ation and carrier synchroni5ation"

Figure '"';'! OFDM block diagram used in carrier synchroni5ation and symbol synchroni5ation

&ynchroni5ation is an essential issue for the OFDM system< and synchroni5ation

performance directly determines hether the OFDM technology can be applied to the

ireless communications field" *here are several synchroni5ation modes in the OFDM

system: carrier synchroni5ation< timed symbol synchroni5ation< and sample

synchroni5ation" *hey all affect the OFDM system performance" #e can choose an

appropriate synchroni5ation method to lay a solid foundation for OFDM application in

ireless communications systems"

".# Channel 0stimate

)n OFDM system ith a cyclic prefi4 is eAuivalent to , separate parallel subchannels"

9eceived signals on , subchannels ? *ransmitted signals on subchannels 4 &pectral

feature of channel< if channel noise is not taken into account" 3f the spectral feature of

channel is knon through estimation< received signals can be correctly demodulated by

dividing the received signals on subchannels by the spectral feature of channel"

Channel estimate methods are commonly based on pilot channel and pilot symbol

=reference symbol>" *he multi;carrier system has a time;freAuency to;dimensional

='D> structure< so pilot symbol assisted channel estimate is more fle4ible" *his estimate

method is to insert some knon symbols and seAuences in several fi4ed locations of

transmitter signals< so that the receiver can perform channel estimate using applicable

algorithms based on these pilot symbols and seAuences" 3n a single;carrier system< pilot

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symbols and seAuences can be inserted in the time a4is only and the receiver e4tracts

pilot symbols to estimate the channel pulse response" 3n a multi;carrier system< pilot

symbols can be inserted in both time and freAuency a4es and the receiver e4tracts pilot

symbols to estimate the channel transfer function" #e can insert filters in a 'D structure

to estimate the channel transfer function only if the interval of pilot symbols in time

and freAuency directions is small enough as opposed to the channel bandidth"

".$ P*P edution %ehnology

.esides lo sensitivity to freAuency deviation< the OFDM system has another

disadvantage: overhigh $/$9" Compared ith the single;carrier system< an OFDMsymbol is the sum of many independent modulated signals" &uch signals may produce

larger peak poer hich ill cause a high $/$9"

&ignal pre;distortion is the simplest ay to reduce the $/$9" .efore sent to an

amplifier< signals undergo non;linear processing hich aims to perform pre;distortion

for the signals ith a high $/$9 to prevent them from going beyond the dynamic

change scope of the amplifier" *he most popular signal distortion technologies include

amplitude limiting and compression and e4pansion"

".$.1 *mplitude !imiting

/mplitude limiting before signals pass the non;linear components can reduce the peak

signal level to be loer than the e4pected ma4imum level" /mplitude limiting is Auite

simple but also brings problems for the OFDM system" First< distortion of the OFDM

symbol amplitude ill cause interference to the system itself< leading to .79

performance degradation" &econd< non;linear distortion of OFDM signals ill increase

the outband radiation poer" /mplitude limiting can be deemed as a process of

multiplying the OFDM sample symbol by a rectangular indo function" #hen the

amplitude of OFDM signals is less than the threshold value< the amplitude value of this

rectangular indo function is !" #hen the signal amplitude needs to be limited< the

amplitude value of this rectangular indo function must be less than !" /s e all

kno< time multiplied by freAuency is eAuivalent to freAuency domain convolution"

*herefore< the limited OFDM symbol spectrum eAuals the convolution of the original

OFDM symbol spectrum and the indo function spectrum" Outband spectral features

are determined by the signal ith a large spectral bandidth< that is< by the spectrum of

the rectangular indo function"

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To prevent overhigh outband radiation caused by the rectangular indo

function< other non;rectangular indo functions can be adopted< as shon in the

figure belo"

Figure '");'' indo# adding for time domain OFDM symbols

*he principle for selecting a indo function: 7nsure that the indo function has

good spectral features and does not stay an overlong time in the time domain to prevent

bringing an impact to more time domain sampling signals"

".$." Compression and 0pansion

.esides amplitude limiting< signal compression and e4pansion is another choice for

signal pre;distortion" 3n a classical e4pansion method< symbols of smaller amplitude

are amplified hile those of larger amplitude remain unchanged" *his increases the

average system transmit poer but also makes the symbol poer be closer to the non;

linear change area of the poer amplifier< easily resulting in signal distortion"

*o avoid signal distortion< this document presents an improved compression and

e4pansion means through hich large;poer transmit signals are compressed and

small;poer transmit signals are amplified to keep the average poer of transmit

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signals relatively unchanged" *his not only reduces the system $/$9< but also

enhances anti;interference capability of small;poer signals" G;la compression and

e4pansion can be employed in this means" 3mplement compression and e4pansion on

signals at the transmitter< and carry out reverse operations at the receiver to restore

original data signals" Figure '");'( shos the OFDM system baseband diagram ith

compression and e4pansion changes"

Figure '");') OFDM system baseband diagram ith compression and e4pansion changes

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# OFDM *ppliations

#.1 OFDM *ppliations in Downlin2

%*7 Donlink systems adopt Orthogonal FreAuency Division Multiple /ccess

=OFDM/> hich is an OFDM;based application"

OFDM/ divides transmission bandidth into several subcarrier sets that are

orthogonal and allocates them to different users to fle4ibly achieve resource sharing

among M&s< thus implementing multiple access beteen among users" OFDM/ can be

considered as a multiple access mode combining OFDM< FDM/< and *DM/

technologies"

)s sho#n in Figure ("!;'< there are three diagrams: =a>< =b>< and =c>" 3f e perceive

the OFDM itself as a transfer mode< diagram =a> shos that all resources including

time and freAuency are allocated to a user" Diagram =b> shos an OFDM -FDM/

mode that allos subcarriers to be allocated to different users" One crucially important

difference beteen the traditional FDM/ mode and the OFDMPFDM/ mode is that

adjacent carriers allocated to different users are partially overlapping in the latter mode"

The OFDMPFDM/P*DM/ mode is to dynamically allocate carriers in time domain"

/s sho in diagram =c>< freAuency resources are dynamically allocated based on the

data rate needed by each user and the current channel Auality< here ,QM" .ecause the 3DF* is longer than the DF*< the

e4cess input of the 3DF* is padded ith 5eros"

(" /dd a cyclic prefi4 for this group of data after 3DF* to avoid the 3&3"

/ccording to the modulation process above< DF*&;OFDM implementation has

the same procedureR3DF*Ras OFDM implementation< so DF*&;OFDM can

be deemed as an OFDM process ith precoding"

3f the length M of DF* eAuals the length , of 3DF*< then both DF* and 3DF* effects

ill be cancelled after they are cascaded and the output signal ill be an ordinary

single;carrier modulated signal" #hen ,QM and the 3DF* is padded ith 5eros< the

3DF* output signal has the folloing features:

!" *he $/$9 of the 3DF* output is less than that of the OFDM signal"

'" *he freAuency domain location occupied by output signals can be changed by

varying the mapping from DF* output to 3DF* input"

/cAuire the spectrum of input signals through DF*" *he ,;point 3DF* can be deemed

as an OFDM modulation procedure hich is actually to modulate the spectral

information of input signals to multiple orthogonal subcarriers" OFDM subcarriers in

%*7 donlink carry data symbols directly< so the $/$9 of DF*&;OFDM can maintain

the same $/$9 as the original data symbol" /n e4ample ith ,?M can illustrate this

point< as shon in the figure belo"

Figure ("';(! DF*&;OFDM symbol transmission

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*he spectrum of input data symbols can be moved to a different location by changing

the mapping from DF* output to 3DF* input" Figure ("';) shos to mapping modes:

centrali5ed mapping and distributed mapping"

Figure ("';(' Centrali+ed and distributed DF*&;OFDM modulation schemes

Figure("'; shos the spectral distribution of output signals in these to mapping

modes"

Figure ("';(( 'ignal spectrum modulated in centrali+ed and distributed DF*&;OFDM

schemes

#."." SC4FDM*

&C;FDM/ can be easily implemented by taking advantage of the DF*&;OFDMfeatures above" Multiple access can be achieved by changing the mapping from DF*

output to 3DF* input of different users hen several users reuse spectral resources" 3n

addition< subcarriers are orthogonal to each other< avoiding multi;address interference"

/s shon in Figure ("';-< multiple access can be implemented by changing the

mapping from DF* to 3DF*" &pectral resources can be fle4ibly configured by varying

the data symbol block si5e M of input signals"

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Figure ("';() DF*&;OFDM;based FDM/

/s shon in Figure ("';6< &C;FDM/ supports to resource allocation modes:

centrali5ed mode and distributed mode hich are to uplink access modes as

discussed in (2$$" #e choose the centrali5ed distribution mode in order to obtain a

lo $/$9 and reduce the U7 load" *o acAuire the freAuency diversity gain< e employ

uplink F as an alternative scheme of the uplink distributed transmission mode"

Figure ("';( DF*&;OFDM;based centrali5ed and distributed FDM/

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lte ofdm principle 32

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