The thought for this post came to me while talking with my friend ANgazu (from radiofrecuencias.es) about an emerging Spread Spectrum (SS) method that uses MultiCarrier waveforms (MC-SS).
The question that came up was whether the alleged "wideband" Akula (15 × 500Bd DBPSK) utilized this kind 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).
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Fig. 1 - channels 1-6
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Fig. 2 - channels 7-12
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Fig. 3 - channels 13-15 and FSK segment
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Channel separation is 2 Khz, rather adequate to let a easy detection and filtering of the subcarriers, for a full bandwidth of 30 KHz (Figure 4). As 1 can see, wideband Akula's spectrum is very different from another multicarrier waveforms like OFDM or mPSK (if only for the utilized bandwidth).
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Fig. 4 - wideband Akula and its spettral occupancy
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Two popular spread spectrum systems in usage present are frequency-hopping spread spectrum (FH-SS) and direct-sequence spread spectrum (DS-SS). The basic thought 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 usage of a filter bank to make 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 utilizing Walsh Direct series 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. 1 unique feature of this FBMC-SS construction is that it can easy 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 fewer subcarriers (it is no coincidence that I heard wideband Akula utilizing a distant SpyServer receiver located in Ukraine).
I don't have the tools to say for certain that they usage a FBMC-SS waveform, but there are any 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.
