path: root/scenarios.tex
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authorMatthias P. Braendli <matthias.braendli@mpb.li>2017-09-28 17:54:32 +0200
committerMatthias P. Braendli <matthias.braendli@mpb.li>2017-09-28 17:54:32 +0200
commit76f9d9107072b5844ea0d6fc0ad646cba93a3d79 (patch)
treedf0ca3be3a50a8ede658d539a58bb717a66e2a2f /scenarios.tex
parent637ca87627c3e3347a03f1031b9a444091c9d920 (diff)
Mention CFR and DPD
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+\subsection{Advanced Signal Processing}
+\subsubsection{Crest Factor Reduction}
+ODR-DabMod contains a prototype for crest factor reduction (CFR), which allows you to
+reduce peak-to-average power ratio (PAPR), trading off with modulation error
+rate (MER). The DAB OFDM signal has a very high PAPR, which directly translates
+to a decrease in power amplifier efficiency. The power amplifier has to be
+driven such that the peaks do not drive it into compression, but the overall
+efficiency is calculated from the average power.
+Reducing the PAPR makes it possible to drive the amplifier more.
+The CFR algorithm works in the following way: all the generated OFDM samples go
+through a limiter, which clips the amplitude to a configurable value. This
+directly reduces PAPR, but impacts both amplitude and phase of the constellation
+points, thereby degrading MER. To compensate for this, a second step of the
+algorithm calculates the error vector for each constellation point, and clips
+the error to some maximum amplitude. The clipped error is then subtracted from
+the signal, which corrects part of the distortion created by the limiter.
+To summarise: a low clipping value distorts the signal more; a high error
+clipping value corrects the distortion again.
+In the constellation plot view, the first is seen as an increase in spread of
+the points; the second is visible because it pulls the constellation points back
+into a circle of radius proportional to the error clipping value.
+Settings and some statistics are available through the remote control.
+\subsubsection{Digital Pre-Distortion}
+An ongoing activity is the development of a method to characterise a power
+amplifier and predistort the I/Q samples to invert the distortion behaviour of
+the amplifier. More information about this work is in the \verb+dpd/README.md+
+file in ODR-DabMod.
\subsection{Audio Sources}
Preparing a DAB multiplex with different programmes requires that we are able to