Digital Signal Processing Reference
In-Depth Information
S
0
S
0
Input bit
sequence
S
1
S
1
Mapper
IDFT
S
OFDM
S
n
S
n
Frequency domain
("Amplitude spectrum")
Time domain
(FOURIER-Synthesis
Input bit sequence
T
Nutz
= 5T
B
t
0
1T
B
2T
B
3T
B
4T
B
5T
B
0,30
0,25
0,20
0,15
0,10
0,05
0,00
1,00
0,50
0,00
-0,50
-1,00
1,00
0,50
0,00
-0,50
-1,00
1,00
0,50
0,00
-0,50
-1,00
1,00
0,50
0,00
-0,50
-1,00
3,00
1,50
0,00
-1,50
-3,00
0,08
Gap
0,06
This Sine is
set on 0
0,04
0,02
S
OFDM
0,00
5 10
20
30
40
50
60
70
80
90
-125
-100
-75
-50
-25
0
25
50
75
100
ms
Hz
Time domain
(FOURIER-Synthesis of the OFDM-signal)
Frequency domain of the OFDM -signal
Illustration 272:
Block diagram and simplified representation of OFDM
In Illustration 270 there are a number of sinusoidal oscillations at the input of the adder. If they are integer
multiples of a basic frequency their sum can be defined as a FOURIER-synthesis. The FOURIER-synthesis
is, however, the result of an Inverted FOURIER transformation IFT (see Illustration 95) or in this case of
an IDFT (Inverted Discrete FOURIER transformation). As a consequence, the bit patterns at the output of
the demultiplexer can be defined as a discrete spectrum (real and imaginary part) which is transformed via
IDFT by FOURIER-synthesis into a DMT-signal in the time domain.
Instead of the many multipliers and oscillators in Illustration 270, an IDFT-block is used in OFDM-
systems. Each bit at the numerous inputs of the IDFT-block has a discrete frequency with a discrete
amplitude and phase position at the output.
To be precise, each input/output consists of 2 lines (real and imaginary part).
