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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 |