From 76f9d9107072b5844ea0d6fc0ad646cba93a3d79 Mon Sep 17 00:00:00 2001
From: "Matthias P. Braendli" <matthias.braendli@mpb.li>
Date: Thu, 28 Sep 2017 17:54:32 +0200
Subject: Mention CFR and DPD

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 scenarios.tex | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

(limited to 'scenarios.tex')

diff --git a/scenarios.tex b/scenarios.tex
index 3b072bb..ba55533 100644
--- a/scenarios.tex
+++ b/scenarios.tex
@@ -264,6 +264,38 @@ hackrf_transfer -t /tmp/ofdm.fifo -f 216928000 -x 47 \
 \end{lstlisting}
 
 
+\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
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