18 June 2016

'Echotel 1810' and 'THALES TRC-1752': same PSK-8 2400Bd, same 768 bits ACF, but two different waveforms

Single tone PSK-8/2400Bd waveforms are very common and it's easy to get mistakes in their identification, sometimes also their ACF values are equal and so things get complicated (it could also happen that the same waveform exhibits different ACFs according to the input data signaling rate). In cases of uncertainty or ambiguity, a careful examination may decide the questions.
Below just an example about  the Thales TRC-1752 and Echotel 1810 "signals": they not only have the same PSK-8 2400Bd single tone features but also the same ACF. Note that althought both Thales TRC-1752 and Echotel 1810 are HF modems (to be clear: MAHRS - Multiple Adaptive HF Radio System is an "operating mode", Arcotel is the radio processor)  I  generically refer to both as "signals".

Looking at their representation in the waterfall, they have a quite similar shape characterized by the same bandwidth, a non-fixed length duration and a short preamble: a first difference is visible just in this part. Looking with a little more attention, the Echotel preamble is composed of 8+8 simmetrical tones while Thales signal exhibits a different header (fig. 1).

fig. 1
The data transfer parts of the two signals share the same features: 8-ary phase-shift keying of a single 1800 Hz carrier and 2400 symbols/sec modulation rate (input data signaling rate is not detected here). It's interesting to note the pronounced BPSK states that, unless their phase shift, appear in both the constellations of the two signals (fig. 2).

fig. 2
These footprints suggest the presence of BPSK segments in the structure of both the two signals. As from literature, they consist of known-data symbols, in contrast with the unknown (user) data, and are are scrambled to appear, on-air, as PSK-8 symbols.
The BPSK insertions, as well as the differences between the two preambles, are more evident reducing the FFT size in the waterfall (fig. 3).

fig. 3
Looking at the two waterfalls, both with the same settings, we can also estimate that the two signals have the same pattern repetition rate (pic.3). Running the ACF and CCF functions for a better accuracy of the repetition rate, we get the same 106.665 ms result for both the signals: this value makes 768 tri-bits, or 256 PSK-8 symbols periods since the manipulation speed is 2400Bd.
At this point, the real difference between these two signals, unless the preamble, can only be found by examining their frame structures.

The frame structure for the Echotel signal is shown in figure 4. The preamble is followed by 256 symbols blocks, each block consisting of 176 unknown data symbols and a mini-probe consisting of 80 symbols of known data.

fig. 4 - frame structure for Echotel
The frame structure for the Thales signal is shown in figure 5. As well as in the frame of Echotel signal, the frame cosists of 256 symbols blocks, each block consisting of 80 symbols preamble followed by 176 symbols data block, each data block consisting of 4 x 32 unknown data symbols and 3 x 16 symbols mini-probes (S4285-like waveform).

fig. 5 - frame structure for THALES TRC-1752
It's worth noting that the two signals have the same length, 80 symbols, for the BPSK modulated  segment.

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