Mention CFR and DPD

3 files changed, 36 insertions, 1 deletions

diff --git a/README.md b/README.md
index 82c8961..386dc5b 100644
--- a/README.md
+++ b/README.md
@@ -19,5 +19,5 @@ http://opendigitalradio.github.io/mmbtools-doc/mmbtools.pdf
 
 Acknowledgements
 ----------------
- * Rash for proof-reading
+ * Rash and Adam for proof-reading and corrections
  * Wim Nelis for the section about Monitoring using Xymon
diff --git a/mmbtools.tex b/mmbtools.tex
index 0f52a4b..70cf572 100644
--- a/mmbtools.tex
+++ b/mmbtools.tex
@@ -172,6 +172,7 @@ clock_config_t},
 \addcontentsline{toc}{section}{Acronyms}
 \begin{acronym}[mmbTools]
     \acro{1PPS}{One pulse per second}
+    \acro{CFR}{Crest Factor Reduction}
     \acro{CIF}{Common Interleaved Frame}
     \acro{CRC}{Communications Research Centre Canada}
     \acro{DAB}{Digital Audio Broadcasting}
@@ -181,10 +182,12 @@ clock_config_t},
     \acro{FIC}{Fast Information Channel}
     \acro{HE-AAC}{High Efficiency Advanced Audio Codec}
     \acro{mmbTools}{Mobile Multimedia Broadcasting Tools}
+    \acro{MER}{Modulation Error Rate}
     \acro{MNSC}{Multiplex Network Signalling Channel}
     \acro{NTP}{Network Time Protocol}
     \acro{OCXO}{Oven-Controlled Crystal Oscillator}
     \acro{OFDM}{Orthogonal Frequency-Division Multiplexing}
+    \acro{PAPR}{Peak-to-Average Power Ratio}
     \acro{PRBS}{Pseudo-Random Bit Sequence}
     \acro{SFN}{Single-Frequency Network}
     \acro{TCXO}{Temperature-Compensated Crystal Oscillator}
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