Sometimes it may happens to come across signals for which - unless of know it a priori - it is hard to correctly specify the utilized modulation; it's the case of the alleged "semi-modes", where any FSK modulations at certain conditions have the form of phase manipulation just due to the fact that specified signals possessing both PSK and FSK issues. specified "dualism" is spread adequate and concerns tightly connected modulations as (G)MSK and OQPSK as well as CPFSK and SDPSK. A signal sent me by a friend of mine just falls into this category.
Let's get any parameters of the signal specified as bandwidth (Bw), baud rate (Br), and shift (Sh): as from Figure 1
Bw = 19000 Hz
Br = 16000 Bd
Sh = 8000 Hz
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| Fig. 1 - main parameters of the signal |
Looking at Figure 2, from "Signals Analyzer - radioscanner.ru" [1], MSK modulation has a bandwidth of about 1.5*Br, GMSK Bw is lesser than this value (in it’s limit is very close to theoretical Br), and SDPSK Bw is more than 1.5*Br. Well, this signal has the value of Sh precisely = Br/2 while Bw is < 1.5*Br: so, judging by these results, it could be a GMSK signal.
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| Fig. 2 - difference between MSK and GMSK |
Another circumstantial feature of the alleged "semi-modes" is the spectrum of their second harmonic: looking at Figure 3, the second harmonic has 2 very clear and defined lines and the spacing between these lines is equal to Br. As from [1], this is the essential condition for their identification, but not the adequate one: indeed, besides both SDPSK and OQPSK modes exhibit 2 spectral lines in the second harmonic. delight note that the carrier in the 4th degree is very weakly expressed, sometimes it is practically invisible at all.
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| Fig. 3 |
That said, as shown in Figure 4, any equalization/autocorrelation is essential to bring out the carrier (a) so that the SA demodulator PLL can lock onto it: this way you have a clearer view of phase plane and constellations. The 4-ary constellation (b) and its transitions pattern (c) regulation out the OQPSK mode (and GMSK too) since it should show an 8-ary like constellation but w/out zero-crossing transitions. The relative/differential view (Diff=1) show alternatively a two-state mode (d,e).
The above considerations propose SDPSK (Simmetrical Diferential PSK) modulation, just like the 1 utilized for Orbcomm series sats [2]. Moreover, note the "Offset mode detected" informing that means a comparative phase shift keying, aka offset keying! Indeed SDPSK is equivalent to π/2 DBPSK or PSK2 with phase rotation: ie, as shown by the transitions in absolute mode (c), SDPSK assumes that the phase is rotated by +π/2 for bit “0” and by -π/2 for bit “1” thus there is not a 180° turn.
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| Fig. 4 - phase plane and constellation of the signal being analyzed |
So, while the mathematical relations among Bw/Br/Sh point to a GMSK modulation, phase plane and constellations seem to point to a SDPSK (or even CPFSK) modulation: the comparative phase planes and constellations are shown in Figure 5 (the SDPSK and CPFSK signals are synthesized).
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| Fig. 5 - phase planes and constellations of (synthesized) SDPSK and CPFSK signals |
However, the comparison between the phase detector results shows a behaviour more akin to a GMSK signal (Figure 6).
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| Fig. 6 - phase detector results (CPFSK, SDPSK, our signal) |
Since specified kinds of signals can be demodulated besides as FSK, I tried both the SA universal PSK and FSK demodulaors: the resulting bitstreams are shown in Figure 8, as you can see they are the same (the 15-bit period is due to the first preamble).
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| Fig. 7 - SA universal PSK and MFSK demodulators |
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| Fig. 8 - bistreams after PSK and FSK demodulations |
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To conclude, "There is simply quite a few information that proves that semi-modes are practically the same from a mathematical point of view but at the receiver side there is no a reliable and easy method to discern the exact kind of modulation. However, if the signal has a good quality, there are any clues that can aid tip the balance 1 way or another... even if they could be not the conclusive" my friend AngazU says.
https://disk.yandex.com/d/WwBwL6tD_CTwLw (.wav signals and bistreams)
[1] http://signals.radioscanner.ru/info/item68/
[2] http://signals.radioscanner.ru/base/signal16/
