Digital Signal Processing Reference
In-Depth Information
Sin /Cos
f
1
S
0
Dig.filter
2
Sin /Cos
f
2
S
1
Dig.filter
2
2
Mapper
S
DMT
Sin/Cos
f
n
S
n
Dig.filter
2
2
Real
part
Imaginary
part
I
nphase
Q
uadr atur phase
t
0
f
1
f
2
f
3
f
4
... fn
Frequency
t
1
t
2
Frequency
Frequency
Illustration 270:
4-PSK-DMT as an example of a discrete multiple carrier system
The block diagram at the top shows the mapper (which in this case is very straightforward) which
“parallelizes” two successive bits of the incoming bit stream (real and imaginary part). It connects the two
bits with twice the symbolic length with two circuits (real and imaginary part). The demultiplexer then
distributes these bits to, for example, 1000 channels with two circuits each, which additionally increases
the symbolic length by the factor 1000. These bits need to be buffered by the demultiplexer and output
synchronously so that they are time-synchronous. Then the real part is multiplied by a cosine and the
imaginary part by sinusoidal oscillation of the frequency f
k
(0 < k < 1000) per channel. All signals are
added in the adder; s
DMT
is composed of 1000 frequencies of the same amplitude which can take on just
four phase positions: 45, 135, 225 and 315 degrees (4-PSK).
The bottom of the Illustration shows the instantaneous phase position of four of these 1000 carrier
frequencies at three successive points in time. The vertical axis is the time axis, the horizontal axis is the
frequency axis.
