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156 Chpater 6
Custom Arb Waveform Generator
Working with Modulation Types
To Use a User-Defined Modulation Type (Real Time I/Q Only)
Creating a 128QAM I/Q Modulation Type User File with the I/Q Values Editor
In I/Q modulation schemes, symbols appear in default positions in the I/Q plane. Using the I/Q
Values editor, you can define your own symbol map by changing the position of one or more
symbols. Use the following procedure to create and store a 128-symbol QAM modulation.
NOTE Although this procedure provides a quick way to implement a 128QAM modulation format, it
does not take full advantage of the I/Q modulator’s dynamic range. This is because you begin
with a 256QAM constellation, and delete unwanted points. The remaining points that make
up the 128QAM constellation are closer together than if you had mapped each point
specifically. Additionally, this approach does not enable you to define the bit pattern
associated with each symbol point, as you could if the 128QAM constellation had been
defined one point at a time.
1. Press Preset.
2. Press
Mode > Custom > Real Time I/Q Baseband > Modulation Type > Define User I/Q >
More (1 of 2) > Load Default I/Q Map > QAM > 256QAM.
This loads a default 256QAM I/Q modulation into the I/Q Values editor.
3. Press
More (2 of 2) > Display I/Q Map.
In the next steps, you will delete specific portions of this I/Q constellation and change it into a
128QAM with 128 I/Q states.
4. Press
Return > Goto Row > 0011 0000 > Enter; this is row 48.