Digital Signal Processing Reference
In-Depth Information
7,5
5,0
Q
5,0
2,5
2,5
0,0
0,0
-1
I
1
-2,5
-2,5
-5,0
-7,5
-5,0
-7,5
-5,0
-2,5
0,0
2,5
5,0
7,5
-5,0
-2,5
0,0
2,5
5,0
V
V
Signal space of QPSK
Eye diagram of QPSK
Illustration 264:
Signal space and eye diagram for QPSK
In the signal space on the left-hand side you can see four discrete states represented as points. The
connecting lines between them show possible transitions from one state to another.
In this particular eye diagram the phase shifting of 90 degrees between the four different sinusoidal
oscillations - which correspond to the four points of the signal space - can clearly be seen.
This explains the structure of the QPSK-circuit. Real and imaginary parts now need to be
added. Please keep in mind that the sum of a pure cosine and a pure sinusoidal oscillation
always results in a sinusoidal oscillation shifted by a particular phase angle.
By adding two sinusoidal oscillations which are phase -shifted by
90 degrees to each other (sine and cosine) sinusoidal oscillations
of any phase shift can be generated provided the relationship of
the amplitudes towards each other can be arbitrarily selected.
This means that any point on the GAUSSian numerical plane can
be reached if the two relevant amplitudes are selected.
As dibit channels have only two states, A and -A (bipolar), there are four different phase
angles. Illustration 264 shows the four states and the possible transitions from one state to
the next one as connecting lines between the points.
The eye diagram not only shows the four sinusoidal oscillations shifted by 90 degrees
towards each other (corresponding to the four points in the signal space), but also the 45
degree angle of the diagonal.
