Chinese Mixed Mode (MFSK-4+OFDM-20/1) MFSK-4 Br-100, Shift-400 OFDM Br~72.2, Shift~100

The signal is here

Author: SergUA6 6.0
Band Width ~2200-2250 Hz
Low Range ~450 Hz, the shift is possible. Recieveing is in SSB
Baud Rate For MFSK ~100 Hz, For OFDM ~72.2 Hz
N-Ary (PSK/MPSK) mixed, pi/4 dqpsk and PSK-4
Count of Carriers For MFSK 4, for OFDM 20 + pilot
Step between carriers For MFSK ~400, for OFDM ~100 Hz
Pilot tones: spacing, frequencies (Pilot tone(s)) One pilot ~513 Hz, the shift is possible, Recieveing in SSB
The modulation in which the signal was recieved (RX mode) SSB

Sonograms:

pic.1 the signal's generall look

pic.2 Manipulation speed for MFSK

pic.3 Manipulation speed for OFDM

Diagrams:

pic.4 The graphical spectrum of the signal

pic.5 Detalization of the preamble

Pictures:

pic.6 Constellation of the upper channel OFDM

pic. 7 Constellation of the lower channel OFDM

Chinese Mixed(MFSK-4+OFDM-20/1) - Chineese modem. The classical OFDM signal with CP duration about ~ 3.85 msec. The interesting moment is that this signal is the classical OFDM with MFSK preamble. Nevertheless It is characteristic for China, and it is not big surprise. The record's quality is excellent, that allows to define easily all parametres of the signal with high reliability.

Analysis of the real Chinese Serial Modem signal.

The signals analysis. Practical approach.

The real signal 7970.wav.

It is always necessary to spent estimation of the current record's quality before the analysis of the signal. It is very essential and important step. This step, in many cases, allows to avoid useless

work with problematical records as: the records with overloads, with the distortion of amplitudes, compressions and etc. There is lot of information about records quality, but as practice, the very small amount of people take this information into attention.

In our case, any of specific problems are not observed. The signal is "live" and it "breathes", the noise level is small enough, that allows to hope for the successful analysis. The best fragments by

the spectrum are the first and the third. The second and the fourth fragments have the visible selective distortions, althoguht it is the not the reason for denial of 2 and 4 fragments, but we are choosing the best fragments. By the first look, this signal seems like founded OFDM. The typical mistake in such cases, is the orientation on the searches of the signal's parametres as OFDM, that is the other versions are not even considered. It is the mistake in the sense, that even if any disparities will be detected, it will be very hard to stop consider the signal as OFDM. All the attention will be paid to signs in favour of the OFDM version, and the signs, which are not in favour of this version, most likely will be recognised as accidental and ignored. As I always say, even if
the signal seems very "familiar" or "clear" the approach to it should be always standard and same as to absolutely unfamiliar and unknown signal, to which all arsenal of knowledge and abilities is
applied. And only then, having gathered the various data, it is possible to do conclusions and to check all the versions, co-ordinating the results, received earlier. Lets gather, without going deep, the data in favour of OFDM.

The spectrum's sort/look on the sonogram, especially in the first part of the signal is very similar to OFDM, it is possible to distinguish "channels" and "pilots-tones". The second part is lesser
similar to OFDM, but nevertheless is does create the strong enough impression of the block-channel transmission, and the "pilots-tones" between the blocks. ACF is bad and does not give the clear answer, besides, it is visible on the waveform, that the signal "is slightly beaten" by the impulse interferences. So it is not clear what ACF is that. The graphical spectrum also does not give any special pluses in favour of OFDM, the whole spectrum has not clear ejections through each ~50 Hz, and it is hard to to say is that the result of creation of the signal or something else.
In that way, there is no any forcible arguments in OFDM favour, except the first impression. The situation with "pilots-tones" brings even more suspicions: the pilot-tone, even in OFDM without CP, should be located in relatively pure space. In this signal, we hardly can recognise that condition, but ofcourse it is possible to recognize if You have great desire to believe that the first OFDM
impression is correct, and if You ignore all the disparities, which are not in OFDM favour.

Lets take envelope of the signal.

There is bright enough and obvious line on the position of expected line of the clock frequency. That line can say that the signal is already not OFDM, but FSK, PSK or QAM. The FSK is denied very fast
after using of Phase/frequency detector. But to define is the signal is PSK or QAM is problemcaticaly enoguh. Methods allowing to detect QAM, in current SA version do not exist, whether I'm also not
assured if these methods are exist in general. It is possible to define QAM or PSK on the signal's constellation, but it assumes the absolute high quality of the signal's record, and knowledge of the
carrier frequency of the signal.
It is not possible to receive the carrier frequency on this signal assured, because the signal has distortions. SA allows to restore the signal, by the clock frequency, from supposition that it is PSK.
That is, the semi-automatic corrector, in AM mode, attempts to restore the signal being orinted not on the concrete PSK order, but on the signal's belonging to PSK class basically, and also oriented on value of the clock frequency. It is not such trivial task, it also does work far not in all cases, but this only thing that we have. To check this version, I have taken the third fragment.

The work with the semi-automatic corrector, was described earlier, both in the program's Help and in other articles, so i won't consider the work of the semi-automatic corrector in the detals. I will result screenshots after attempts of restoring the signal by the semi-autmatic corrector. Here is also the video-clip, which shows the process course of how semi-mode corrector works with this signal.

Literally after four-five passeges of the semi-automatic corrector in AM mode, the classical picture is appeared in the eighth degree. After getting of the classical picture, it is possible both to
continue correction in AM mode, and to switch into X^N mode, or to combine both modes. This is the deal of technique results are usually reached very fast.

The question is: maybe it is just the fortuity or the comdination of incredible coincidences, and the corrector has distorted the signal into PSK? Theoretically it is possible. But in practice, it is very
improbable, athough sometimes it happens. For detection of such cases, it is necessary, periodically to inspect/controll waveform of the signal in the first degree: the absence of deep failures in the
signal's bar or the absence of obvious distortion of the waveform, atleast guarantee the absence of obvious distortions of the signal, unless guarantee 100% total accuracy of the corrector's work.

It is necessary to save the signal and to compare it with initial signal, essential differences should not be present. Ofcourse after the semi-automatic corrector's work the signal will be another, with
more correct spectrum, with its much more regular allocation. But the signal after corrector's work should not be the absolutely another signal, the main features and structure should stay same.

Good luck!

The author of this article is SergUA6
All rights reserved © http://signals.radioscanner.ru

Updating SA to 6.0.5.3 version : Now SA updates will be also described in our BLOG.

SA updating to 6.0.5.3 version.

This isn't final update in 6.0.5.3 series, not all things which were planned are finished yet, but there is cathegory of errors, which were removed as fast as possible. Once of such mistakes is reflected on the screenshots, which were sent by several users.



Under Windows Vista, the transgression of SA windows structure is observed at some user settings.

It makes impression of the extremely careless appearance of the program. In our opinion it is simply not acceptable for commercial products.

In version 6.0.5.3 this problem is solved.

Some additions are added in the work of WaveForm module aswel.




1) The filter switch - You have just to click simply on it
The default mode is LowPassFilter, the frequency cutoff of it is manually specified from the list.

In MedianeFilter mode, the Mediane filter does work. The mediane filter has no equivalents in the analogue technique, it has special characteristics. Part of users insisted to add this Filter. The Area of Application of Mediane filter is relative enough. Generally that filter allows effectively to suppress sharp and strong impulse interferences, without distortion of fronts of the signal. In practice, on the Real signals it is necessary to estimate results
visually.


The principle of work of Mediane filter is simple enough. The signal is passed through the window, the size (2) of Which is setted by user and that size should be odd. Then the received array is sorted and as output counting the average one is taken.

Thus, if the impulse-interference has the duration, which is lesser then the window size/2, then this impulse is fully supressed.

For the absolutely qualitative work of Mediane filter it is necessary that impulses-interferences would have distance among themselves not lesser than the window size, in practice it is occuring very seldom, but nevertheless, sometimes the results of Mediane filter work are very impressing. The maximum size of the window is limited by 301 counting, further this size will be increased.

It is possible to enter any number of countings, automatically it will be added to the nearest odd number(to the bigger side) or installed at the level of 301 limit. On the big sizes of the window
speed of processing is decelerated, however it will be optimised in the near future, so this problem is not strongly serious and has the solution.


3) The oprative stepback into intial state is added. That is cancellation of all actions spent earlier with the signal. It allows quickly and effectively to pick up the necessary filter, to choose the necessary cutoff frequency and etc.

As distinct from the previous versions, the work of filters in WF module is optimised, and the resulting signal takes of precisely the same position in the displaying window as well as the initial signal.

It is nessesary to show some care In LPF mode. The frequency of a cutoff filter, as it is accepted, is setted at level -3 Db from the pass-band. For the decision of the problem of signals preservation on former positions the double-sided scheme of filtration is applied. That scheme leads to weakening on the cutoff frequency on 6 Db already. In some cases, it is not acceptable, that's why it is better to take the cutoff frequency upper and to do some passages, before recieveing the necessary result of filtration, in other it is obvious things.

Some minor problems of the whole programm are removed aswel. It is usual process thought and there is no sence to describe all the details, as it is hardly noticeably and does not have the great importance.

Good luck!

CHINESE SERIAL MODEM - is the very interesting signal

Chinese Serial Modem
PSK-8, Br~1630

The signal is here

Author: SergUA6 6.0
Band Width ~1800 Hz
Low Range ~470 Hz, the shift is possible, recieveing is in SSB
Baud Rate ~1627 Hz, possibly that it is 1630 in reality
n-Ary (PSK/MPSK): PSK-8
Carrier frequency ~1415 Hz, the shift is possible, recieveing is in SSB
The modulation, in which the signal was recieved (RX mode): SSB

Sonograms


pic.1 The general look

pic.2 Manipulation speed

pic.3 The signal in 8th degree

Diagrams

pic.4 The phase constellation with accumulation

pic.5 The phase constellation in dynamic

CHINESE SERIAL MODEM - is the extremely interesting signal. As the sample the record restored by corrector is used.
It is difficult enough to believe, that the signal having so obvious OFDM signs and characters, is not OFDM signal in reality.

However the record quality is high enough and allows to identify PSK-8 with the very high probablity.

The very difficult regular data structure is very strongly change the spectrum, in that way, it is creating illusion of the multi-channel transmission.

However the examples of such signals are exist, and it should not be a big surprise.

This signal is the very intresting one. And the detailed analysis of CHINESE SERIAL MODEM will be considered in the next blog post as separate article.

Practical approach: the example analysis of OFDM

The example analysis of OFDM

OFDM – is one of the most difficult sorts of modulation for the analysis. The difficulties are about the fact that, the channels, at OFDM usage, as a rule, do not have clear boundaries, and thereupon it is hard to select any channel or for the detailed analysis. Sometimes, in special cases, it is possible, but not always. OFDM the technology allows to create very not trivial signals, and the classical analysis of such signals is extremely difficult and polysemantic.

The example of such difficult signal(signal1) is here Japaneese Military 8 freq , compare to it this signal(signal2) Mixed mode, PSK-2 and OFDM, system unknown is even more difficult, because it is impossible to select the channel in this signal(signal2).
We will try to analyze the signal2.
But before it we will consider some common principle of analysis on classical signals, as on real ones and on synthesized ones.
The Possibility to synthesise OFDM signal with the specified parameters is very effective, at least it allows to accept some hypothesis or to reject that, it is very useful.

Let’s start.

The link above consist full version of this article, where the principles of OFDM analysis are descrbed step by step on the real example.

The author of this article is SergUA6.

The classical analysis of OFDM signals. Possibilities and limitations.

The classical analysis of OFDM signals

The arctile is written by SergUA6.

Thist article is big enough to put it whole in the blog. Below there will be little bit information from the article, and whole text with pictures and examples you can read here.

The Classical OFDM analysis surely represents difficulty. Before analyze the real signals we will consider the main methods and principles on the synthesized signals. It is typical enough and very productive method.

For synthesis of OFDM signals the OCG program is used. As usually, I underline, you can use any other program both for the analysis and for synthesis, it is important that results would not call doubts, could be verified and have been repeated.

It is supposed that some bases of OFDM creation are known, it does not demand special substantiations as the analysis it is not simple and not only usage of this or that analyzer.

Extensively, it is experience and knowledge of an analyst, it is an ability to generalise, to draw the conclusions etc. Before appearance of OCG, the analysis of OFDM of signals, at least, for me, carried chaotic, unsystematic and extremely not productive character, but the possibility of synthesis of the various by parameters signals has allowed to develop certain techniques and the common approaches. In practice all it does work and does work good enough.

Standardly the task of OFDM analysis is to define:

• the clock frequency of manipulation,
• the value of frequency spacing of channels
• quantities of the channels,
• modulation in the channels
• definition of the presence and length of the cyclic prefix (CP),
• features of the signal if the such are exist.

Advantages of the classical analysis:

• it doesn’t matter how many frequencies in the analyzed OFDM signal, 3-30-300 or 3000
• it is also, generally, doesn’t matter if the signal is generated on even or odd harmonics it does not influence results
• for the score of the very strong averaging-out, the classical analysis is insensitive to separate/single/some anomalies in the signal

Disadvantages of the classical analysis are:

• impossibility to work correctly with short signals
• impossibility of reception of a bit stream (while the universal demodulator can provide it)
• impossibility to allocate the necessary channel in the pure state
• the extremely big problem is to work with the signals having very short CP.

Despite minuses, the classical analysis, in most cases, allows to receive quite good results at the analysis of overwhelming majority of OFDM signals. I hope this article in some measure will help with it.

Auhtor SergUA6 from MSM team.