Re: Understanding APSK and QAM Rohde and Schwartz video #VARA
Hi Kristoff ,toggle quoted messageShow quoted text
Without getting into the complex details as to why, In the Military as well as a number of Commercial and even some Amateur Radio waveforms these days, as you strive to increase the Over the Air data rate throughput you need to work with more complex forms of modulation to either increase the number of data bits per symbol or more densely pack it, especially with Serial (Single) Tone modems which are now predominate I Military communications as they are superior to Parallel tone modems. With Parallel tone modems you can increase the throughput by just adding more carriers or increasing the symbol rate for a given form of modulation, But many designers have been maintaining the number of carriers and symbol rate and implementing more complex modulation.
There are many challenges and tradeoffs involved. The Military basically hit the wall with Serial tone modems as they did with Parallel tone waveforms. At 256-QAM they did something new and old again, they sort of dusted off the Parallel waveform approach without actually going there and add a few new twists. They took the serial tone modem and started running it on ISB, then 2-ISB and then 4-ISB. Then they went to what is called Wideband, they allocate 24kHz and now 48kHz channels where the actual in use channel starts where not occupied, say the first or 2nd 3kHz portion and extends continuously where not occupied until the full bandwidth id reached iif possible. The PSK carrier is placed where appropriate for the resulting bandwidth occupied. This is the approach taken with MIL-STD-188-110D Appendix D and 4G ALE to MIL-STD-188-141D Appendix G.
Bach to a less than 3kHz channel in the Commercial modem world, some dozen years ago now SCS seen the futility iwth Parallel modems and went to serial tone for PACTOR IV, implementntig aspects found in a number of Military waveforms as they had done with PACTOR III prior. They get into QAM for the highest throuhput as does STANAG 4539 and use Burst waveform techniques as does 3G ALE in STANAG 4538. They lowered the symbol and center of the PSK carrier from that of the Military to fit Commercial and Amateur SSB filter bandwidth. This was all achieved under the guidance of the most reknowned German STANAG modem engineer. That coupled withe SCS Memory AQR provided SCS with the then best and still best in a 3kHz only channel HF modem available, better than the STANAG and MIL-STD modems in a 3kHz channel in my opinion.
In Amateur Radio, at least here in the U.S. we are ham strung by the 300 baud symbol rate limitation imposed by the FCC. Thus serial tone modems cannot be exploited, Parallel tone modems are the only only approach to achieve high throughput.
/s/ Steve, N2CKH
At 03:13 PM 8/9/2021, you wrote: