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:

    2

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    Chapter rror! "se the #ome ta$ to apply 1 to the te%t that yo& want to appear here. rror! "se the #ome ta$

    to apply1 to the te%t that yo& want to appear here.

    here

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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

    4

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    Chapter rror! "se the #ome ta$ to apply 1 to the te%t that yo& want to appear here. rror! "se the #ome ta$

    to apply1 to the te%t that yo& want to appear here.

    *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"

    5

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

    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 s