diff options
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)
parent637ca87627c3e3347a03f1031b9a444091c9d920 (diff)
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
- * 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},
\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 \
+\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