Ofdma for Lte

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    LTE FDD vs. TDD

    -Yatin Jadhav

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    LTE-FDD FRAME

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    LTE-TDD FRAME

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    Comparison between FDD-LTE and TDD-LTE

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    Comparison between FDD-LTE and TDD-LTE

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    OFDMA

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    What is Orthogonal signal

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    FDM

    10/4/2013 ALUMS TEC 8

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    OFDM

    10/4/2013 ALUMS TEC 9

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    Working of OFDM (Example)

    Suppose we are having following bits for transmission1,1,-1,-1,1,1,1,-1,1,-1,-1,-1,-1,1,-1,-1,-1,1,1,-1,-1,-1,1,1

    Convert this high speed serial stream to low speed parallel stream.

    C1 C2 C3 C4

    1 1 -1 -1

    1 1 1 -1

    1 -1 -1 -1

    -1 1 -1 -1

    -1 1 1 -1

    -1 -1 1 1

    These low speed streams will be transmitted on different sub-carriers

    10/4/2013 ALUMS TEC 10

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    Modulated signal for C1 Modulated signal for C2

    Modulated signal for C3 Modulated signal for C4

    Each signal will be modulated using BPSK.

    10/4/2013 ALUMS TEC 11

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    Final OFDMA wave, sum of all signals.

    10/4/2013 ALUMS TEC 12

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    OFDM Block Diagram

    10/4/2013 ALUMS TEC 13

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    OFDMA

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    OFDMA

    OFDM can be easily used as multiple access technology, namedOFDMA.

    Total spectrum space is divided into sub-channels(SC).

    One SC is a sub-set of available channels. It contains more thanone sub-carrier.

    Users may occupy more than one SC depending upon user QOS

    profile.

    10/4/2013 ALUMS TEC 15

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    Subset is also called as OFDMA traffic

    channel.

    user1

    user2

    user3

    user4

    10/4/2013 ALUMS TEC 16

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    Sub-channel implementation techniques

    In first technique, sub channels are grouped in same frequency range.

    In second technique, sub channels are spread over whole spectrum

    Second technique is more immune to narrow band fading as it willEffect fraction of sub channels.

    10/4/2013 ALUMS TEC 17

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    10/4/2013 ALUMS TEC 18

    OFDMA Illustration

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    10/4/2013 ALUMS TEC 19

    Frequency Re-use

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    10/4/2013 ALUMS TEC 20

    OFDM and OFDMA illustration

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    Cyclic Prefix

    10/4/2013 ALUMS TEC 21

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    In OFDM, multipath causes loss of orthogonality .Delayed paths

    cause overlap between symbols

    Cyclic Prefix (CP) insertion helps maintain orthogonality

    Reduces efficiency (or Usable Symbol time, Tu)CP is repetition of modulation symbol

    10/4/2013 ALUMS TEC 22

    Cyclic Prefix

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    How OFDMA saves bandwidth

    10/4/2013 ALUMS TEC 23

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    Synchronization Tight Synchronization between users are required forFFT in receiver

    Pilot signals are used for synchronizations Co-channel interference

    Dealing with this is more complex in OFDM than inCDMA

    Dynamic channel allocation with advancedcoordination among adjacent base stations

    Disadvantages

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    10/4/2013 ALUMS TEC 25

    Peak to average power ratio (PAPR)

    The large amplitude variation increases in-band noise and increasesthe BER when the signal has to go through amplifier nonlinearities.

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    10/4/2013 ALUMS TEC 27

    Why SC-FDMA?

    Low Peak to average ratio.

    Carries all advantages of OFDMA such as low ISI, multipath fading.

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    10/4/2013 ALUMS TEC 29

    SC-FDMA Signal generation

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    10/4/2013 ALUMS TEC 30

    SC-FDMA Generation

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    10/4/2013 ALUMS TEC 31

    THANK YOU