From f99b0216576436a3191c06426120661839db7b13 Mon Sep 17 00:00:00 2001 From: "Matthias P. Braendli" Date: Mon, 22 Jan 2018 02:27:57 +0100 Subject: Explain OFDM windowing --- scenarios.tex | 20 +++++++++++++++++++- 1 file changed, 19 insertions(+), 1 deletion(-) diff --git a/scenarios.tex b/scenarios.tex index 308d50e..7d3d006 100644 --- a/scenarios.tex +++ b/scenarios.tex @@ -222,7 +222,7 @@ it helps to produce a cleaner spectral output. At higher rates one needs to ensure that samples are not being dropped on the USB and that CPU resources are not being contended. -The shoulder performance has been measured with shoulder performance at a little +The shoulder performance has been measured with a value at a little better than $35$dB, which is roughly equivalent to that obtained from first generation commercial modulator equipment. This can be increased to a relatively respectable $\approx 40$dB by enabling the FIR baseband filter in ODR-DabMod. @@ -265,6 +265,24 @@ 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 -- cgit v1.2.3