Pre-correction with CRC-DabMod and USRP1+WBX ?

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Ulrik Brinck - Kanal Plus

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May 25, 2011, 7:48:27 PM5/25/11
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Dear friends,
I work at a local radio station in Denmark, where we are setting up a local DAB+ multiplex for trial purposes. For this, we will use CRC-DabMod with Ettus USRP1 and WBX, and then we need a power amplifier (and, yes, a cavity filter, an antenna and a license, but we already got those).

To get an appropriate power amplifier, a dealer has now asked me to find out whether or not the modulator makes pre-correction, thus as "to make correction of the distortion introduced by the non-linearities of the power amplifier devices in the near saturation region".

I don't see anything about this on the mmbtools page, and I don't know much about this subject. When I Google-search it, I do however see it mentioned in descriptions of various hardware modulators.

So my question is, does someone know: With CRC-DabMod and Ettus USRP1 and WBX, do we have such pre-correction?

Best wishes,
Ulrik.

Pascal Charest

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May 27, 2011, 8:57:13 AM5/27/11
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Hi Ulrik,

No, CRC-Dabmod does not do any pre-correction for the power amplifier.
The power amplifier should be used in its linear region.

On another hand, CRC-Dabmod does some pre-corretion for the CIC filter
of the USRP. This filter is used on the FPGA for up-sampling to 128MHz.
This pre-correction filter is only enabled when the -c option is used.
You should use -c128000000 for regular USRP1 only.

In addition, the -g option control how CRC-Dabmod computes the symbol
gain. 0 uses a fixed factor and is really not recommended. It is more
useful on an academic perspective for people trying to understand the
DAB modulation. The gain mode 1 is the normalization of every OFDM
symbol. No overshoot, no truncating, but varying output power (around
3dB) which might not be the best for some power amplifier. The last gain
mode is mode 2 and uses the method specified in ETSI 300 798. This
method normalizes to 4 times the standard deviation for an approximation
of the RMS power. So around 6/100000 samples will be truncated and will
introduce some really minor distortion. But this mode also maximizes the
output power. This is the gain mode recommended for real world operation
as it is based on a DAB standard; the only difference is that CRC-Dabmod
uses a better resolution with 16 bits in place of 8 bits.

In our labs, when we use a power amplifier, we usually adjust the USRP
output power to keep some backoff to make sure that the amplifier
operates only in its linear band. The backoff is similar to the
peak-to-power-ratio of a DAB signal.

Regards,

Pascal

Ulrik Brinck - Kanal Plus

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May 27, 2011, 10:20:54 AM5/27/11
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Thank you for your reply, Pascal. I have passed it on to the dealer.

And also thank you for your tips about the -c and -g option. I'm getting wiser and wiser about DAB/DAB+ and what it takes to set up such a transmitter. :)

Best wishes,
Ulrik.

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