OFDM MC-CDMA

OFDM and MC-CDMA: An Implementation using

MATLAB

Arjun R. Kurpad 1PI99EC 014Ashish Uthama 1PI99EC 017Saptarshi Sen 1PI99EC 089Shounak Mondal1PI99EC 096Internal 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 OFDMUsing this system to transfer files between computers using the sound cardSimulation of wireless environment using fading channelsMulti-User Capability using MC-CDMAPlotting 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 FDMIncreased 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 ModulesModulator module:

BPSK, QPSK, 8QAM

Channel module:

AWGN, Fading, Physical

Equalizer module

MC-CDMA module

FTX (The Transmitter)

Data Parameters

Modulator Module

Channel Module

Equalizer Module FRX (The Receiver)

Demodulator Module

DataParameters

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

Applications1. PC to PC file transfer using the ubiquitous

sound card2. Digital audio broadcasting (DAB), wireless CD-

quality sound transmission3. Digital video broadcasting (DVB), specifically,

digital terrestrial television broadcasting (DTTB)

4. Wireless LAN 802.11a5. ADSL (asymmetric digital subscriber line), also

called DMT (Digital Multi Tone)

Demonstration of the OFDM system…