Final Project

OFDMOrthogonal frequency-division

multiplexing

PROJECT MENTOR : PRESENTED BY:MR.KESHAV JHA GERIJ SHARMA LOVEPREET SINGH

VIJAY KUNIYAL

CONTENTS

WHY OFDM ?INTRODUCTIONTRANSMITTER AND RECEIVERPROBLEMS WITH OFDM INTODUCTION TO PAPRTECHNIQUES TO OVERCOME PAPRFUTURE WORK

WHY OFDM ?• The Orthogonal Frequency Division Multiplexing (OFDM)

transmission scheme is the optimum version of the multicarrier transmission scheme. In the past, as well as in the present, the OFDM is referred in the literature Multi-carrier, Multi-tone and Fourier Transform.

• OFDM was first proposed as a way of dealing with multipath. One of the problems with single carrier modulations (SCM) is that, in a given environment, the symbol interval becomes much shorter than the delay spread at high data rate condition. To resolve this problem with multi-carrier modulation formats, the data rate on its subcarrier is less than the original one. The basic idea is that if you take a signal and send it over multiple low-rate carriers instead of a single high-rate carrier, then inter-symbol interference (ISI) is eliminated and multipath effects can be compensated with a much simpler equalizer.

• OFDM was invented more than 40 years ago.• OFDM has been adopted for several technologies:– Asymmetric Digital Subscriber Line (ADSL)

services.– IEEE 802.11a/g, IEEE 802.16a.– Digital Audio Broadcast (DAB).– Digital Terrestrial Television Broadcast: DVD in

Europe, ISDB in Japan– 4G, IEEE 802.11n, IEEE 802.16, and IEEE

802.20.

INTRODUCTION

SPREADING THE DATA OVER LARGE NUMBER OF SUB-CARRIERS

OFDM = FDM + ORTHOGONALITY

SPECTRAL EFFICIENCY

TRANSMITTER

RECEIVER

PROBLEMS WITH OFDM

ORTHOGONALITY

PAPR(Peak to average ratio)

PAPR (power to average ratio)

• There are some obstacles in using OFDM in transmission system in contrast to its advantages. A major obstacle is that the OFDM signal exhibits a very high Peak to Average Power Ratio (PAPR). Therefore, RF power amplifiers should be operated in a very large linear region. Otherwise, the signal peaks get into non-linear region of the power amplifier causing signal distortion.

• The other limitation of OFDM in many applications is that it is very sensitive to frequency errors caused by frequency differences between the local oscillators in the transmitter and the receiver. Carrier frequency offset causes a number of impairments including attenuation and rotation of each of the subcarriers and inter carrier interference (ICI) between subcarriers.

TECHNIQUES FOR REMOVAL OF PAPR

ACTIVE CONSTELLATION EXTENSIONCONSTANT ENVELOPEPTS TECHNIQUEREPEATED CLIPPING & FREQUENCY DOMAIN

FREQUENCYSLM

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