Transcript Orthogonal Frequency Division Multiplexing

OFDM and MC-CDMA:
An Implementation using MATLAB
Arjun R. Kurpad
Ashish Uthama
Saptarshi Sen
Shounak Mondal
Internal Guide:
External Guide:
1PI99EC
1PI99EC
1PI99EC
1PI99EC
014
017
089
096
Ms. Geetha Prakash, Lecturer, PESIT
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 intercarrier 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
Modulator module:
Data
Parameters
FTX (The Transmitter)
Channel Module
BPSK, QPSK, 8QAM
Modulator Module
Channel module:
AWGN, Fading, Physical
Demodulator Module
Equalizer module
MC-CDMA module
Equalizer Module
FRX (The Receiver)
Parameters
Data
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 CDquality 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…