OFDM 20 out of 26 tones modem (possibly a Chinese waveform?)

The transmission has been spotted by ANgazu on 4102.0 KHz/USB by using a remote Kiwi SDR in Sweden. The signals consists of 4500 msec bursts, sent contiguously, and occupies a bandwidth of 2000 Hz (Figure 1).

Fig. 1

The waveform uses OFDM technology for 26 channels although 20 of these are used, ie the six tones 5,8,9,13,18,23 are off (at least in this sample). Modulation is PSK-2 in each channel with a symbol rate of 60Bd. Speed and modulation are confirmed by analyzing the whole signal and also a single channel (Figs 2,3).

Fig. 2 - analysis of the full OFDM spectrum

Fig. 3 - analysis of a single OFDM tone

KarapuZ suggets that it's likely a Chinese waveform, in this case the signal should be tuned on LSB. On my side I tried to synthetize a similar waveform although the results obtained (bandwidth and speed) are slightly different as shown in Figure 4.

Fig. 4 -synthetized OFDM 20-of-26 waveform

In another recording I also found bursts with 24 out of 34 tones up and PSK-2 modulation at 50 Bd (Fig. 5) , these short bursts are in between the groups of 20/26 bursts.

Fig. 5

A distinctive feature of the 20/26 waveform is its period that is formed by 14040 bits (Fig. 6): thanks to KarapuZ for the demodulated bitstream.

Fig. 6 - 14040 bits period

07 June 2018 - update
The OFDM has been spotted on 4816 (cf) by AngazU, in this sample they use only one out of two sequences. OFDM Parameters are the same.

https://yadi.sk/d/etaTfAa73SFnKm

a CIS-20 variant (37.5Bd, 60Hz)

the so-called CIS-20, in its standard format, is an OFDM 20-tone waveform characterised by π/4-DQPSK modulation at 75 symbols per second and 120Hz channel separation, as described here. CIS-20 is supposed to be the successor of CIS-12.
This variant exhibits the same modulation format but speed and channel separation are exactly the half of the ones of the standard version, i.e. 37.5Bd and 60Hz channel step. The signal bandwidth is unchanged (~2800Hz).

CIS-20 variant - OFDM parameters

CIS-20 single channel analysis

Note, once again, the differences between PSK-8 and π/4-DQPSK constellations (in absolute and relative mode, Diff=0 and Diff=1):

in a real PSK-8 the paths cross the origin of axis

CIS-20 in idle state

The main features of the waveform, as reported in radioscanner.ru, are: k = 3/5, 20 channels + 1 pilot, channel modulation pi/4 DQPSK, speed 75 Baud and channel step 120 Hz.

In this sample the modem was heard when in idle state: modulation switched to BPSK (4 positions in the absolute constellation) but same manipulation speed and channel separation (75 Baud and 120 Hz). Notice the resampling at its native frequency (7200 Hz as shown here). ACF in idle state is 200 mSec.

 

AT-3104D waveform (idle mode) OFDM parameters

AT-3104D waveform (idle mode) ACF

The recordered signal does not have a good quality and modulation is a bit confused, so I isolated a single channel to confirm the relative constellation (BPSK).

AT-3104D waveform (idle mode) single channel inspection

 As other characteristic, at least in this sample, is the carrier #13 that is not modulated

According to this post in radioscanner CIS-20 is a new version of CIS-12 so probably sourced from the same AT-3104 modem (or the older AT-3004D). Both the two modems were designed to have many different devices plugged into them to form the modem’s input – most often teleprinters unique to the different military services and civilian agencies that use them, and also, vocoders and speech encipherment systems, but all are transported in the same fashion – using one of these 2 waveforms.