Diagram - Promax 27 User Manual

USER'S MANUAL. PROMAX-27
APPENDIX B.- PRINCIPLE
CONSTELLATION DIAGRAM
The modulation process implies to transfer the information contained in a signal to
a high frequency carrier. Modulation QAM, in concrete uses modulation in quadrature
consisting of two carriers each one of the same frequency, one called-I (in phase) and
another out of phase 90° called Q (quadrature).
Each one of them is modulated in amplitude and phase by a portion of the digital
input signal. The two modulated signals are combined then and they are transmitted as
a single waveform. The receiver only needs to invert the process for generating a digital
output that can be processed to produce images or another useful information also.
The number of levels used in the modulation of each carrier determines the
number of possible symbols and, consequently, the number of bits that can be
transmitted in a certain bandwidth. The DVB-C standard allows 5 types of modulation:
16 QAM, 32 QAM, 64 QAM, 128 QAM and 256 QAM.
For example, if four amplitude levels are applied to each one of the carriers, each
signal could reach the value of -3.0, -1.0, +1.0, +3.0 at a given instant in time, so we
have 16 possible combinations. This is known as 16 QAM. Extending the previous
exposition to four amplitudes, it allows us to generate 8 states for each carrier and 64
possible combinations (64 QAM).
These digital signals can be visualized graphically by means of the Constellation
Diagram. If one imagines on an axis the possible states of the first carrier (signal-I or
signal in phase) we would obtain the image in figure 34.
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OF QAM
Figure 34.- Signal-I states.
Figure 35.- Signal-Q states.
MODULATION.
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