Introduction
 OFDM can be seen as either a modulation technique or a
multiplexing.
 In a classical parallel data system, the total signal frequency
band is divided into N nonoverlapping frequency subchannels.
Each subchannel is modulated with a separate symbol and then the N
subchannels are frequency-multiplexed with guard band. This leads to
inefficient use of the available spectrum.
 To cope with the inefficiency, the idea of OFDM, in which
each carrying a signaling rate RS symbol/s is spaced RS Hz
apart in frequency is proposed from the mid-1960s.
It can be shown that the carriers are noncoherent orthogonal if the carrier
spacing is a multiple of 1/T, where T is the symbol period and equal the
value of RS .
Wireless Communication Technologies 2.5.1
1
Introduction
 From Figure, it can be seen that using OFDM save almost
50% of bandwidth.

Orthogonal
|  || Sinc (Tf ) |
1
T
Conventionarl multicarrier technique
2
T
3
T
f
Frequency
Saving of bandwidth
OFDM technique
Frequency
Wireless Communication Technologies 2.5.1
2
Introduction
 Figure shows the spectrums of the OFDM signal. Note that
there is no crosstalks from other channels.
 Therefore, if we use DFT at the receiver, we recover the
transmitted data.
 In addition, frequencydivision multiplex is
achieved not by bandpass
filtering but by
basedband processing.
Wireless Communication Technologies 2.5.1
3
Introduction
 To eliminate the banks of subcarrier oscillators and coherent
demodulators, completely digital implementations could be
built by the fast Fourier transform (FFT).
FFT is an efficient implementation of the DFT. Using this method in both
transmitter (IFFT) and receiver (FFT), the number of operations from N 2 in
DFT down to N log 2 N .
 In the recent years, OFDM was exploited for digital audio
broadcasting (DAB), high-definition terrestrial broadcasting
(HDTV), Wireless LAN (ex. IEEE802.11a) , UWB system
(ex, MB/OFDM), WiMAX system (ex. IEEE802.16e) and so
on.
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GROUP 5
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3432
MHz
3960
MHz
4488
MHz
5016
MHz
5544
MHz
6072
MHz
6600
MHz
7128
MHz
7656
MHz
8184
MHz
8712
MHz
9240
MHz
9768
MHz
10296
MHz
Wireless Communication Technologies 2.5.1
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