29-01-2013, 04:49 PM
Orthogonal Frequency Division Multiplexing (OFDM):
Tutorial and Analysis
Orthogonal Frequency Division Multiplexing.doc (Size: 217 KB / Downloads: 68)
INTRODUCTION
Purpose
Efficient use of radio spectrum includes placing modulated carriers as close as possible without causing Inter-Carrier Interference (ICI). Optimally, the bandwidth of each carrier would be adjacent to its neighbors, so there would be no wasted spectrum. In practice, a guard band must be placed between each carrier bandwidth to provide a space where a filter can attenuate an adjacent carrier’s signal. These guard bands are wasted bandwidth.
In order to transmit high data rates, short symbol periods must be used. The symbol period is the inverse of the baseband data rate (T = 1/R), so as R increases, T must decrease. In a multi-path environment, a shorter symbol period leads to a greater chance for Inter-Symbol Interference (ISI). This occurs when a delayed version of symbol ‘n’ arrives during the processing period of symbol ‘n+1’.
Orthogonal Frequency Division Multiplexing (OFDM) addresses both of these problems. OFDM provides a technique allowing the bandwidths of modulated carriers to overlap without interference (no ICI). It also provides a high date rate with a long symbol duration, thus helping to eliminate ISI. OFDM may therefore be considered as a candidate modulation technique in a broadband, multi-path environment.
The purpose of this report is to provide the following information
concerning OFDM:
• theory of operation
• analysis of important characteristics
• implementation example (matlab)
OFDM Overview
OFDM is a modulation technique where multiple low data rate carriers are combined by a transmitter to form a composite high data rate transmission. Digital signal processing makes OFDM possible. To implement the multiple carrier scheme using a bank of parallel modulators would not be very efficient in analog hardware. However, in the digital domain, multi-carrier modulation can be done efficiently with currently available DSP hardware and software. Not only can it be done, but it can also be made very flexible and programmable. This allows OFDM to make maximum use of available bandwidth and to be able to adapt to changing system requirements.
Each carrier in an OFDM system is a sinusoid with a frequency that is an integer multiple of a base or fundamental sinusoid frequency. Therefore, each carrier is like a Fourier series component of the composite signal. In fact, it will be shown later that an OFDM signal is created in the frequency domain, and then transformed into the time domain via the Discrete Fourier Transform (DFT).