On Mon, Aug 9, 2021 at 08:13 PM, Kristoff Bonne wrote:
use a combination of QAM and APSK constelationKristoff
Its never immediately obvious , from the video, the issues round the pk-pk levels of the vara modem are perhaps , a little more translucent [as opposed to transparent ] as are the various displays ..
The hybrid serves to maximise the pay load of a single carrier , perhaps more fragile, but , that's linked to the rate , the faster , with the same energy , then the higher becomes the errors , either reduce the rate , more energy/time ..or increase the energy , again more energy , just the rate of supply increases ..
So, is the issue with single high speed carriers , quite incompatible with turbulent conditions encountered on HF
one single jamming signal in the pass band and everything stop's ,higher rates fall over due to delays and multi path etc ..
then comes the maximum rate compatible with HF ,
Its reasonable , to take a guide from the vara modem , the maximum rate for HF is nearly x10 slower than the 300 baud limit , quoted as a limiting factor , 300 providing a b/w limitation , for single carrier systems , under ideal conditions , anything and everything works , DRM broadcast requires , direct or single hop , anything after , then it fails to 'lock'
[ as the sun spot cycle progresses , the opportunity for multiple hop connections will increase]
B/W utilisation , Much the same as say a Tank with tracks , and multiple wheeled vehicle , one track off , stops, one tyre off , keeps going ..
Taking the vara modem has the optimum single carrier rate for HF ,
Then , as can be seen, in the OFDM levels, multiple low rate carriers , are combined , to provide , maximum rate, maximum ''in channel '' jamming / qrm rejection , where as the on-air carrier spectrum's are provided with 16 ? fixed patterns , not seen, in the coding of the modem is the mathematical 'engine' that seeks to distribute the payload , making best use the the available channel b/w and recover it post demodulation .
.selective qsb can remove sections the b/w, but as long as the minimum criteria is met , for the recovered data , the pay-load can be extracted if not , a more robust level can be selected , or vice versa
NB: Reaching the limitation of psk, the lower levels revert to mfsk , simply as during trials , high latitude users
encountered link survival issues coupled to psk modulation , mfsk provides ''the'' most robust and energy efficient
Hope that fills in some of the gaps ,