OFDM with pulse shaping

							    OFDM with pulse shaping


                        Gang LIN
     Department of Electronics and telecommunications
                        August 26, 2004




1
    Traditional Frequency Division
    Multiplex (FDM)
    • Sub-channels are completely separated in
      frequency




2
    Orthogonal Frequency Division
    Multiplex (OFDM)
    • There exists intersection between sub-channels but
      sub-channels are orthogonal to each other.




3
    Why we use OFDM in wireless channel?

    • OFDM is much more robust in wireless fading
      channel than single carrier system




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    Two main categories of OFDM

    • OFDM/QAM with guard interval:
    - The most common type of OFDM
    - Used in ADSL, DVB, IEEE 802.11, etc.
    • OFDM/OQAM with pulse shaping:
    - Improved power and bandwidth efficiency
    - Lower side lobes in frequency domain




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    OFDM/QAM with guard interval




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    Guard interval to eliminate ISI in
    OFDM/QAM




    If the maximum delay of multi-path channel is less
    than the guard interval , there will be no inter-
    symbol interference (ISI) and inter-channel
    interference (ICI); The guard interval can be
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    implemented by inserting cyclic prefix or suffix.
    Efficient scheme for OFDM/QAM
    based on DFT




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      Some characteristics of OFDM/QAM

    • The DFT based scheme has low complexity;
    • The receiver pulse is not matched with transmitter
      pulse. SNR loss:
                                       ∆
                 SNRloss   = 10 log(1 + )
                                       T
    • The guard interval will reduce the information rate:

                 BWloss = 1 (1 + ∆ T )


    • Usually, ∆ = 0.2T
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     Basic block diagram of OFDM/OQAM




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             Pulse design for OFDM/OQAM
     Principles for the design of pulse:
     - Zero ISI and ICI
     - Concentrated in time and/or frequency
     Example:
     (1) Square root raised cosine pulse with a roll off factor less
         than 1, which is strictly band limited to [-1/T,1/T];
     (2) Optimal finite duration pulses with maximum energy
         concentration in [-1/T,1/T].




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     Modulator of OFDM/OQAM based on DFT




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     Demodulator of OFDM/OQAM based on DFT




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        Equalization for OFDM/OQAM
     • OFDM/OQAM need equalizer in multi-path
       channel;
     • If the equalizer eliminates ISI, then ICI is also
       eliminated;
     • If the number of sub-channel is large enough, the
       channel can be viewed as flat fading. Then a one-
       tap equalizer is sufficient.




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            A rough Comparison of OFDM/QAM
            and OFDM/OQAM

                                OFDM/QAM            OFDM/OQAM

     Total computations        5 N (log 2 N − 1)   4 P + 5 N (log 2 N − 1)
     per complex
                                                             s
                                                   (Cariolano’ scheme)
     symbol period
     Equalizer               One-tap equalizer              Yes
     Information rate                                  No reduction
                                1 (1 + ∆ T )
     reduction
     SNR loss (dB)                           ∆
                                10 log(1 +     )              0
                                             T
            P : the length of shaping pulse with respect to
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                 an over sampling interval T/N
                    Work done so far

     • Implementation of OFDM/OQAM;
     • Equalization for OFDM/OQAM over multi-path
       fading channel;
     • Design of pulses robust to carrier frequency offset.




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                   Future research

     • Synchronization;
     - Timing and frequency offset estimation
     - Joint timing and frequency offset and channel estimation


     • A comparison of OFDM/QAM and
       OFDM/OQAM;
     • Wireless MIMO system based on OFDM/OQAM.


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