Overview of OFDM MODULATION Technique

PRESENTED BY: Dishant Khosla Punjabi University

INTRODUCTION

OFDM is a multicarrier transmission technique which uses the technique of splitting smaller subcarriers of frequencies to deal with multipath problem. OFDM is a special case of FDM. Combination of Modulation and Multiplexing.

Why OFDM

To provide large data rates. To reduce multipath fading effects. Multipath distortion and Radio Frequency interference are minimized through this technique. OFDM has been used to improve the capacity of CDMA systems and satisfies the wireless access method for 4G systems. OFDM is believed to perform in a better way in case of frequency selective fading or narrowband interference. OFDM cuts the size of crosstalk in signal broadcasting.

How OFDM works

It distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this technique which prevents the demodulators from seeing frequencies other than their own.

OFDM PrincipleChannel s a) Frequency Division Multiplexing

Frequency Channels b) Orthogonal Frequency Division Multiplexing

50% Bandwidth saved

Frequency

ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING

OFDM TRANSMITTER AND RECEIVERTransmitterData In Modulation (QPSK, QAM etc.) IFFT D/A

Baseband OFDM Signal

Data Out

Baseband OFDM SignalModulation (QPSK, QAM etc.)FFT A/D

Receiver

OFDM TransceiverRandom Data Generator Serial To Parallel Modula tion Mapping IFFT Guard Period Insertio n Parallel To Serial

Channel

Parallel To Serial

Demodu lation

FFT

Guard Period Removel

Serial To Parallel

Cyclic Prefix Extension

Tg

T Original OFDM Symbol Cyclically Extended OFDM Symbol

Multipath Component - A

Multipath Component - B

Multipath Component - C

max

OFDM ADVANTAGES

Makes efficient use of the spectrum by allowing overlap. OFDM is more resistant to frequency selective fading than single carrier systems are. Eliminates ISI and IFI through use of a cyclic prefix. It is possible to use maximum likelihood decoding with reasonable complexity, as OFDM is computationally efficient by using FFT techniques to implement the modulation and demodulation functions. In conjunction with differential modulation there is no need to implement a channel estimator. Provides good protection against co-channel interference. Low sensitivity to time synchronization errors.

OFDM LIMITATIONS

Sensitive to Doppler shift. Sensitive to frequency synchronization problems. High peak-to-average-power ratio (PAPR), requiring linear transmitter circuitry, which suffers from poor power efficiency. Loss of efficiency caused by Cyclic prefix/Guard interval.

APPLICATIONS Digital Audio and Video Broadcasting

Asymmetric Digital Subscriber Line (ADSL)Wireless Networking

CONCLUSIONOFDM has become one of the most popular air-link technologies in todays broadband wireless communications. OFDM is used to remove the effect of multipath fading efficiently as well as providing large data rates. To further improve its bandwidth efficiency and system performance, adaptive resource allocation and smart antenna techniques have been widely used in the OFDM system.

REFERENCES

R. V. Nee and R. Prasad, OFDM Wireless Multimedia Communications, Norwood, MA: Artech House, 2000. ETS 300 744, "Digital broadcasting systems for television, sound and data services; framing structure, channel coding, and modulation for digital terrestrial television,. European Telecommunication Standard , Doc. 300 744 , 1997. J. A. C. Bingham, "Multi-carrier modulation for data transmission: An idea whose time has come", IEEE Communications Magazine, vol.28, no. 5, pp.5-14, May 1990. Rappaport, T.S., Wireless Communications Principles and Practice, IEEE Press, New York, Prentice Hall, pp. 169-177, 1996. William Stallings, Wireless Communications and Networking, Prentice Hall, 2003.

THANKS