Re: Understanding APSK and QAM Rohde and Schwartz video #VARA


Hi Kristoff ,

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:

I'm a bit puzzled about this.

After watching the video, I I understand that QAM and APSK are two different systems, each with their advantages and disadvantages, and either with the specific usecase.

So I do not understand why combining QAM and APSK is then a good idea. You mix systems, therefor creating something that has the disadvantages of both.

What is the technical argument for these mil-comm systems to use a combination of QAM and APSK constelation?

Kristoff (on1arf)

On 06.08.21 19:01, ALE wrote:

Hi Graham,

QAM is fun, working to develop Military data modem and ALE serial tone modem QAM waveforms.

In my attached photo the QAM waveform in the constellation display is an MS110B offering, going into MS110C/D Appendix D and 4G ALE get even more complicated, I have not made it to 256-QAM yet, the Appendix waveforms and 3G ALE are in my near future however.

The SCS P4 serial tone modem stops at 32-QAM in less than a 3kHz channel, but the Military waveforms hit a the much higher QAM count and throughput in a 3kHz channel and then can increase the bandwidth and shift the PSK carrier up to incrementally increase the throughput.

My friend Dario maintains an excellent easy to understand technical blog on pretty much all the waveforms to be found on HF if one is interested in those subject matters at:

<>/s/ Steve, N2CKH

At 02:21 PM 7/3/2021, you wrote:
R&S , Useful series of videos covering aspects of radio
This video is applicable to the VARA modem coding ,
makes sense of the displays -


This video provides an introduction to the basic concepts of amplitude and phase shift keying (APSK) and quadrature amplitude modulation (QAM).
You-tube link :-

Understanding APSK and QAM <>

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