4 June 2024

does wideband Akula use a FBMC-SS waveform?

The idea for this post came to me while talking with my friend ANgazu (from radiofrecuencias.es) about an emerging Spread Spectrum (SS) technique that uses MultiCarrier waveforms (MC-SS). The question that came up was whether the so-called "wideband" Akula (15 × 500Bd DBPSK) used this type of spread spectrum technique, specifically a Filter Bank based multicarrier waveform (FBMC-SS).

I demodulated the 15 channels and found that they carry the same information carried by the following "usual" FSK 500Bd/1000 transmission (Figs. 1,2,3).

Fig. 1 - channels 1-6

Fig. 2 - channels 7-12
Fig. 3 - channels 13-15 and FSK segment

Channel separation is 2 Khz, quite enough to allow a easy detection and filtering of the subcarriers, for a total bandwidth of 30 KHz (Figure 4).  As one can see, wideband Akula's spectrum is very different from other  multicarrier waveforms like OFDM or mPSK (if only for the used bandwidth).

Fig. 4 - wideband Akula and its spettral occupancy

Two popular spread spectrum systems in usage today are frequency-hopping spread spectrum (FH-SS) and direct-sequence spread spectrum (DS-SS). The basic idea of the multicarrier spread spectrum (MC-SS) is to transmit redundant information on multiple subcarriers with a slight phase variation on each one. The Filter Bank MultiCarrier Spread Spectrum (FBMC-SS) waveform, as its name implies, makes use of a filter bank to develop a spread spectrum technique. With this waveform, data symbols  are spread across a number of non-overlapping adjacent subcarriers unlike in DS-SS, where spreading is performed across time, as it happens using Walsh Direct Sequence Spread Spectrum (Walsh DS-SS). The carriers are positioned in a way that the receiver can isolate a single channel by means of selective filtering without interchannel interference. One unique feature of this FBMC-SS construction is that it can easily mask portions of the band that are corrupted by interference or jamming intended by a foe: indeed, a narrow band interference stays well isolated and does not affect more than a few subcarriers (it is no coincidence that I heard wideband Akula using a remote SpyServer receiver located in Ukraine).
I don't have the tools to say for sure that they use a FBMC-SS waveform, but there are some elements that lead to this conclusion. In the links below you can download, in addition to the signal and the channel demodulations, interesting documentation about FBMC-SS so that people more skilled than me can comment or deny our hypothesis.

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