UNID Narrowband 2400 Bd High-Order Constellation Waveform

These signals were captured on 14430.0 kHz (USB) by my colleague ANgazu, who kindly shared them with me. Special thanks go to EA1FAQ for providing access to his SDR via the AirSpy Server Network.
The intercepted transmission consists of short time-synchronized bursts, occurring either as discrete pulses or in clusters of five. Occasionally, longer-duration bursts occur with no apparent correlation to the primary pulse train. These signals exhibit a bandwidth of approximately 3 kHz and a symbol rate of 2400 Baud. Spectral analysis indicates a high SNR. Figure 1 illustrates an example of this behavior (abridged-timeline view).

Fig. 1

Analysis of the Autocorrelation Function (ACF) exhibits strong peaks every 62.5 ms. At a 2400 Baud rate, this confirms a frame length of exactly 150 symbols (2400×0.0625=150). The structure likely consists of 122 unknown data symbols followed by 28 known probe symbols. This indicates that the signal is not a standard MIL-STD/STANAG waveform; based on available non-restricted documentation, none of the established standards utilize a 150-symbol framing structure.

Fig. 2: autocorrelation analysis

Figure 3 shows a rhomboidal (rotated) signaling constellation, as observed during the analysis of the longer bursts. This pattern strongly resembles High-Order QAM modulation. Notably, the symbols cluster toward the edges of the grid rather than forming a uniform "checkerboard" layout. This effect is likely attributable to Non-linear Distortion or Gain Compression (often termed AM-AM distortion). Such effects typically occur when a signal is recorded with excessive gain (clipping) or processed through an amplifier operating near its saturation point. Similarly, in a high-power transmitter, this occurs when the power amplifier is driven close to its physical limits to maximize efficiency, causing the outer constellation points to compress.

Fig. 3: rhomboidal (rotated) signalling constellation

In QAM, outer constellation points possess higher amplitudes; under saturation, these points are "compressed" inward or flattened against the boundaries, while inner points remain relatively unaffected. Furthermore, in the case of 64- or 256-QAM, the high symbol density means that without precise Symbol Timing Recovery, the constellation points converge into a blurred mass near the decision boundaries.
I analyzed one of the long bursts using an Octave script from my repository [1], and the result of Figure 4 closely approximates the constellation analysis shown in Figure 3.

 

Fig. 4

Then I used a different Octave script [2] to mitigate the effects of clipping and saturation and employing various decimation factors across several bursts. As demonstrated in Figure 5, this approach recovered the (suspected) QAM constellations, resulting in stable amplitudes, minimal IQ imbalance, and accurate frequency offset estimations.

 

Fig. 5

Subsequently, I specifically tested the signal from Figure 3 using a dedicated 256-QAM analysis script; the results are presented in Figure 6.

 

Fig. 6

According to these findings, the signal employs high-order QAM/APSK modulation.
The interpretation of 64/256-QAM is consistent with MS-110D Waveform IDs 10-12 and STANAG-4539, which support:

- 2400 baud in a 3 kHz bandwidth
- High-order QAM (64-QAM, 256-QAM)
- high spectral efficiency, with raw data rates of approximately 14.4 kbps (64-QAM) and 19.2 kbps (256-QAM), calculated at a symbol rate of 2400 baud with 6 and 8 bits/symbol respectively.

Although the constellation alone aligns with both MS-110D and STANAG 4539 standards, the observed 150-symbol periodic structure is inconsistent with the framing specified by either protocol (MS-110D 64/256-QAM modulations utilize 288- and 384-symbol frames, while STANAG 4539 employs a 287-symbol frame).
In HF environments, the Channel Coherence Time — the window during which the ionosphere remains quasi-stationary — typically ranges between 50 and 100 ms. The observed 62.5 ms probe interval offers significantly enhanced robustness compared to the MS-110D and STANAG 4539 standards (which feature durations of 120 ms, 160 ms, and 119.5 ms, respectively). By inserting a training sequence (probe) every 62.5 ms, the system enables the receiver to update its equalizer coefficients more frequently, allowing for more effective tracking of rapid channel variations and Doppler spreads that would otherwise degrade high-order QAM performance.

The analysis tends to confirm the detection of a highly optimized narrowband (3 kHz) waveform rather than a standard MS-110D or STANAG-4539 implementation. While it utilizes a single-carrier 2400 baud rate with high-order modulation, its unique 150-symbol periodic structure distinguishes it from established military protocols. This emission may represent experimental testing or a next-generation waveform currently restricted to non-public documentation or classified reference libraries. At the time of writing, I have no reports of further interceptions, at least not on 14430 kHz.

 

https://disk.yandex.com/d/X0hzag4Z5WUgrQ 

 

[1] https://disk.yandex.com/d/6ZqFXE9ULW59SQ
[2] https://disk.yandex.com/d/g0OwfkL3KBlyvQ