Orthogonal Frequency Division Multiplexing - PowerPoint

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					        OFDM and MC-CDMA:
   An Implementation using MATLAB

        Arjun R. Kurpad              1PI99EC   014
        Ashish Uthama                1PI99EC   017
        Saptarshi Sen                1PI99EC   089
        Shounak Mondal               1PI99EC   096




Internal Guide:    Ms. Geetha Prakash, Lecturer, PESIT
 External Guide:     Dr. Girish Chandra, Scientist, NAL
Motivation…
 High transmission rates over wireline and
 wireless channels with protection from
 multipath fading

 Predicted technique in fourth Generation (4G)
 mobile phones

 Ideal for bandwidth hogging applications like
 Video Conferencing, DAB, DVB, etc

 Multiuser capacity possible using MC-CDMA
Objectives
  Simulation of a baseband communication
  system using OFDM
  Using this system to transfer files
  between computers using the sound card
  Simulation of wireless environment using
  fading channels
  Multi-User Capability using MC-CDMA
  Plotting of BER curves and constellations
Advantages of OFDM
 Allows carriers to overlap (no guard band), resulting
 in lesser wasted bandwidth without any Inter Carrier
 Interference (ICI)
 High data rate distributed over multiple carriers
 resulting in lower symbol rate (more immune to ISI)
 Permits higher data rate as compared to FDM
 Increased security and bandwidth efficiency possible
 using CDMA – OFDM (MC-CDMA)
 Simple guard intervals make the system more robust
 to multipath effects.
What is OFDM ??
Orthogonality in time domain…
  What is OFDM ??
  Orthogonality in frequency domain…




• Each carrier is modulated using BPSK / QPSK / M-ary QAM
• Frequency response for each carrier is a Sinc(X)
• Overlap of frequency response is possible as against FDM where inter-
carrier spacing is a must
• Frequency responses of the carriers overlap at zero crossings avoiding
Inter Carrier Interference
OFDM Methodology
Understanding Multipath
Why we need the Guard Period…
Equalization   Performed in frequency
               domain

               FFT compulsorily
               performed in Receiver

               Time domain
               convolution replaced
               with frequency domain
               multiplication –
               computationally simpler
               code

               Adaptive filters used
Multicarrier Code Division Multiple
Access (MC-CDMA)
  Combination of CDMA-OFDM to provide
  multi user capability

  Frequency Diversity avoids deep fades

  Simple receiver structure
MC-CDMA Transmitter
MC-CDMA Receiver
Design and Implementation

  The Environment:
           Windows OS
           MATLAB (R12) + Toolboxes




  The design approach:
           Modular
           Tandem development of Tx and Rx
The Modules                 Data         Parameters



Modulator module:        FTX (The Transmitter)        Channel Module

 BPSK, QPSK, 8QAM
                           Modulator Module



Channel module:
 AWGN, Fading, Physical
                                                   Demodulator Module



Equalizer module         Equalizer Module        FRX (The Receiver)



                                              Parameters       Data
MC-CDMA module
The Physical Channel
  Separate channel module

  Uses DAC/ADC capacity of the sound card

  Unique synchronization scheme:

      Time synchronization
      Gain correction
The Physical Channel

     Receiver Triggers here   OFDM wave
Results
  Constellation diagrams

  Performance of Physical channel

  BER curves for various combinations
Constellation diagram
BER Curve
Screen Shots
SS1.bmp
Applications
1. PC to PC file transfer using the ubiquitous
   sound card
2. Digital audio broadcasting (DAB), wireless CD-
   quality sound transmission
3. Digital video broadcasting (DVB), specifically,
   digital terrestrial television broadcasting
   (DTTB)
4. Wireless LAN 802.11a
5. ADSL (asymmetric digital subscriber line), also
   called DMT (Digital Multi Tone)
Demonstration of the
  OFDM system…

				
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posted:9/4/2011
language:English
pages:23