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Diffstat (limited to 'scenarios.tex')
| -rw-r--r-- | scenarios.tex | 102 |
1 files changed, 51 insertions, 51 deletions
diff --git a/scenarios.tex b/scenarios.tex index 12eaf7a..d7a53eb 100644 --- a/scenarios.tex +++ b/scenarios.tex @@ -268,57 +268,6 @@ device-utility --arguments \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{OFDM Symbol Windowing} -One technique to improve the shoulder performance is to avoid generating abrupt -transitions between the OFDM symbols, but apply cross-fading from one symbol to -the next using a windowing function.\footnote{As of ODR-DabMod v1.0.1, this feature is -still in the \texttt{next} development branch, and not part of a released -version.} - -This feature can be enabled by setting the number of samples to use for -overlapping one symbol with the next one, the default value is $10$ (out of a -total symbol length of $2552$ samples in Transmission Mode I). Refer to the -parameter \verb+ofdmwindowing+ in the file \verb+doc/example.ini+ for -instructions. - -As this windowing feature modifies samples that are in the guard interval, the -trade-off is reduced resilience against delayed reflections, reduced maximum -transmitter delay difference in an SFN scenario, and potentially diminished -robustness against Doppler spread. - -\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 read and encode several audio sources. We have seen in @@ -395,4 +344,55 @@ This is illustrated in ``Studio C'' in figure~\ref{fig:txchain-with-encoders}. If ``Studio C'' is able to prepare slides for MOT Slideshow and text to be sent as DLS, ODR-PadEnc can be used to prepare the PAD data for ODR-AudioEnc. + +\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{OFDM Symbol Windowing} +One technique to improve the shoulder performance is to avoid generating abrupt +transitions between the OFDM symbols, but apply cross-fading from one symbol to +the next using a windowing function.\footnote{As of ODR-DabMod v1.0.1, this feature is +still in the \texttt{next} development branch, and not part of a released +version.} + +This feature can be enabled by setting the number of samples to use for +overlapping one symbol with the next one, the default value is $10$ (out of a +total symbol length of $2552$ samples in Transmission Mode I). Refer to the +parameter \verb+ofdmwindowing+ in the file \verb+doc/example.ini+ for +instructions. + +As this windowing feature modifies samples that are in the guard interval, the +trade-off is reduced resilience against delayed reflections, reduced maximum +transmitter delay difference in an SFN scenario, and potentially diminished +robustness against Doppler spread. + +\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. + % vim: spl=en spell tw=80 et |