When multiple carriers in our FDM modem are close to a peak of their waveform simultaneously, we can get a momentary output power that is a large multiple of the average. For our FDMDV-derived modem, the ratio is 13.5 dB.
We don't actually operate our SSB amplifiers in a way that avoids clipping, because we can tolerate /some/ clipping. Clipping increases the bit error rate but increasing power reduces the bit error rate generally. In the 925 mile test, the peak power spikes that should be visible aren't, they're either buried in noise or were clipped, and the bit error rate is still /zero/ for much of the transmission. It sounds to me like the amplifier was operated with about 6 dB headroom, but we don't have a measurement and there is much unknown about how the amplifier clips.
While PAPR is managible on an SSB radio. It is potentially a problem if we want to use the FDMDV modem on an HT. We need power efficiency on handhelds, which means operating the power amplifier very close to full power at all times. We can't afford to have lots of headroom there.
So, I have some serious questions about how well our FDMDV modem would work on an HT, and whether some alternative is required. We could use some more science on this topic, and unfortunately I'm not sufficiently clueful to provide it. Sorry.
* How much clipping can we tolerate, and how does BER diminish with
clipping?
* How does spurious emission increase as we drive the PA into clipping?
* Should we compress in software before feeding the amplifier, so that
clipping is more graceful than the hardware might make it?
* What am I missing?
Thanks
Bruce
<<attachment: bruce.vcf>>
smime.p7s
Description: S/MIME Cryptographic Signature
------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/
_______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
