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| -rw-r--r-- | scenarios.tex | 77 |
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diff --git a/scenarios.tex b/scenarios.tex index bccc8a3..2648ac1 100644 --- a/scenarios.tex +++ b/scenarios.tex @@ -131,32 +131,93 @@ it in steps of $3$dB or smaller while measuring the output signal, until the correct power is reached. \subsubsection{Other Hardware} -The HackRF can also be used as a transmit platform. It's interfacing is not -integrated with ODR-DabMod, and it only supports 8-bit samples. The -configuration is a bit different. First, we must output signed 8-bit interleaved -I/Q samples instead of 32-bit interleaved floats, and second, we cannot use UHD, -but must go through a pipe to HackRF. The output settings inside the -configuration are as shown: +ODR-DabMod supports other radio interfaces using either standard output, or via +a fifo -- the latter must be created prior to runtime with the \texttt{mkfifo} command. +Due to limitations in the UHD driver library, \texttt{/dev/stdout} will only +function correctly if ODR-DabMod is configured at compilation time with the +following argument: \begin{lstlisting} +--disable-output-uhd +\end{lstlisting} + +ODR-DabMod has been tested to work with HackRF from Great Scott Gadgets on i386 +and x86 architectures.\footnote{HackRF has not been tested to any degree of + success with ARM-based computers at this time as they do not (yet) capable + of resampling to the required higher rates as the process is highly CPU + intensive.} +The unit is an entry level yet versatile SDR which +provides for $\approx10$MHz to $6$GHz frequency coverage. To use HackRF one must +configure the output of ODR-DabMod to produce 8-bit signed integers, rather +than the default complex floats. + +HackRF has selectable baseband filters, however the lowest filter setting +($1.75$MHz) does not provide adequate image rejection at the native sampling rate of +$2048$k samples per second. An appropriate rate to start with is $4096$k, and for +some purposes this may well be adequate as this moves the image signals +generated within the radio far enough into the stop-band of filter to attenuate +them significantly. The digital gain in the ODR-DabMod configuration file should +be set to a maximum of $2.4$ at this rate to avoid digital clipping on modulation +peaks. + +Example of the settings in the \texttt{mod.ini} file suitable for use with HackRF: + +\begin{lstlisting} +[remotecontrol] +telnet=1 +telnetport=2121 + +[input] +transport=file +source=myfirst.eti +loop=1 + +[modulator] +gainmode=2 +digital_gain=2.4 +rate=4096000 + +[firfilter] +enabled=1 +filtertapsfile=filtertaps.txt + [output] output=file + [fileoutput] format=s8 -filename=/tmp/ofdm.fifo +filename=/dev/stdout + \end{lstlisting} The output fifo has to be created beforehand, and the \texttt{hackrf\_transfer} utility is then used to transmit the signal to the device. The options needed for the transmission utility are not complete yet. The rough idea would be: +Depending on the capabilities of the host computer, using higher sampling rates +($6144$k, and even $8192$k) may be possible. This oversampling is desirable as +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. It is also important to note that the digital gain value +must also be scaled accordingly as the sampling rate is increased. + +HackRF has been measured with shoulder performance at $\approx 35$dB, which is roughly +equivalent to that obtained from first generation commercial modulators. This +can be increased to a relatively respectable $\approx 40$dB by enabling the FIR filter +in ODR-DabMod, and an appropriate coefficient file provided. + + +Example of using ODR-DabMod with the \texttt{hackrf\_transfer} utility: + \begin{lstlisting} mkfifo /tmp/ofdm.fifo odr-dabmod mod.ini & -hackrf_transfer -f <frequency> -t /tmp/odr.fifo +hackrf_transfer -t /tmp/ofdm.fifo -f 216928000 -x 47 \ + -a 1 -s 4096000 -b 1750000 \end{lstlisting} + \subsection{Audio Sources} Preparing a DAB multiplex with different programmes requires that we are able to read and encode several audio sources. This audio data can reach the multiplexer |