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| Fig. 1 - Link-22 transmission |
Recently my friend ANgazu from radiofrecuencias.es sent me a large catch of a Link-22 transmission - Figure 1 - recorded at 11128.0 KHz/USB. Identifying the mode (Link-22) is reasonably straightforward by examining the bitmap depicting the framing period utilized by the waveform (Figure 2).
The 112.5 ms duration is typical of the waveforms utilized by Link-22 and described by STANAG-4539 standard (non-hopping TDMA traffic waveforms). Note that in this case the framing consists of 3 data blocks (DATA) interspersed with 3 mini-probes (MP) blocks.
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| Fig. 2 - typical Link-22 framing |
Honestly, examining the individual bursts I expected to find QPSK or PSK8 modulations: I was very amazed erstwhile the phase plane revealed bursts with QPSK and even 12QAM constellations within the same transmission (Figure 3): clear sign that the header must so encode the modulation mode.
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| Fig. 3 - QPSK and 12QAM constellations |
A 12QAM modulation is employed in non-hopping TDMA traffic waveform #13 (Annex G to STANAG-4539):
12QAM is different due to the fact that QAM typically uses powers of 2 (so each symbol cleanly represents an integer number of bits). 12 is not a power of 2, so it cannot map bits straight since log2(12) ≈ 3.585 bits per symbol, which is not an integer.
However, any NATO HF modem implementations of STANAG-4539 usage a circular 12QAM layout (which uses 2 concentric rings) alternatively than the rectangular 1 (3x4 grid). Figure 4 shows an approximate layout:
- interior ringing 4 points (like QPSK), offset for symmetry
- outer ringing 8 points evenly spaced (like 8-PSK)
This plan gives a radial + angular separation, making it easier to decode under fading and sound than a rectangular grid.
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| Fig. 4 - circular 12QAM (STANG-4539 style) |
This variation of a constellation is not "new" in STANAG-4539 as, for example, the 64-QAM constellation described in paragraph #4.2.2.1.6: "This constellation is simply a variation on the standard 8 x 8 square constellation, which achieves a better peak-to-average ratio without sacrificing the very good pseudo-Gray code properties of the square constellation".
As seen, with 12QAM circular, the goal is 3.5 bits/symbol (b/sym). There are at least 3 applicable approaches:
1) 4-bit labeling + FEC rate 7/8
2) Multilevel/TCM ( trellis-coded modulation): 3 info bits + 1 structural bit
3) Probabilistic Amplitude Shaping (PAS)
The most common is the 4-bit labeling + FEC rate 7/8:
- delegate a 4-bit mapping to the 12 points (16 combos → 12 used, 4 unused).
- apply channel coding with rate R=7/8 (e.g., LDPC or turbo). Net efficiency: 4×7/8=3.5 b/sym.
I can give an Octave example [1] for specified method with a 12QAM circular constellation. Since Octave doesn’t have built-in LDPC/RS at hand, the code illustrates it utilizing a simple block code to emulate the R=7/8 effect (i.e., 7 info bits + 1 parity). The rule is the same as utilizing LDPC/Turbo in practice.
What it does:
1. Defines the 12QAM circular constellation (4+8 rings) with 4-bit Gray-like labels.
2. Implements a (7,8) block encoder (simple parity, emulating FEC with rate 7/8).
3. Transmits random data: 7 info bits → +1 parity → 8 coded bits → mapped into 2 QAM symbols.
4. Sends through an AWGN channel.
5. At the receiver: nearest-neighbour detection, recover labels.
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| Fig. 5 - 4-bit labeling + FEC rate 7/8 simulation |
Must be noted that:
* the STANAG-4539 12QAM waveform uses a channel coding with rate R=4/5 and TBCC (Tail-Biting Convolutional Code). Net efficiency: 4×4/5=3.2 b/sym.
* STANAG-4539 is not the same as Link-22, but Link-22 can usage STANAG-4539 as 1 of its underlying HF bearers.
* the Octave code is just an example/simulation just to prove that ~3.58 bits per symbol fits nicely with the interleaving, coding, and frame structures utilized in the waveform: I do not know what is the method for the 12QAM utilized in the analyzed signal.
The choice of 12 points is not arbitrary, it’s a trade-off between spectral efficiency and robustness: it can deliver intermediate data rates where 8-PSK is besides low and 16-QAM is besides fragile under HF channel conditions. In military waveforms like STANAG-4539, robustness and flexibility substance more than mapping convenience.
