OFDM features and nuances: Models of OFDM signals and possibilities of SA and OCG new versions.

OCG update to version 1.0.2.7

SA update to version 6.1.2.7

Both OCG and SA are modernized.

The matter is that we will refer on OCG, we will consider various models of the signals, and any interested person should have possibility to repeat or check up everything what we are doing. Old version OCG 1.0.2.5 hadn’t abilities to provide it.

From the beginning of OCG creation, the main program operating mode was "Synthese" mode. "Calculate" mode had rather relative character, owing to limitations, which were put in there initially, and it's main purpose was demonstrating of that the synthesized signal corresponds to settled parameters.

In version OCG 1.0.2.7 possibility to form signals of three types is added.

We have detected at least three models of OFDM signals. All three models are presented widely enough, and we see the necessity of synthesis of any of these models. As the last several months, we received a lot of questions about the problems of OFDM analysis, the part of these questions is directly linked to type of model of a signal. Let's talk about it.

Conditionally, OFDM signals can be divided into two main types, by the way of their channelization (method of channel forming). However, by the way of creation of the working channels and the pilot tones, we can observe three models. Actually there are can be more models, but we will consider the most key basis models.

Model A:

All channels are formed "as is", including the pilot tones. In this case, the pilot tone cannot be selected randomly and settled from the limited number of suitable candidates. The typical representative of this model is signal WINDRM 51-Tone COFDM Modem

Model B:

All channels are formed as potential pilot tone, any channel can be settled as the pilot tone. The typical representative of this model is CIS 12-tones PSK-2,4 120(240) bps per channel .

Model C:

Has mixed type of creation, all channels are formed "as is" by "A" model. But the pilot tones are formed specially by "B" model. In this model, any channel or channels can be settled as the pilot-tone. The typical representative of this model, signal MIL-188-110B-39 tone .

It is necessary to mark that:

model B is characteristic for signals CIS

model C for NATO signals

But, of course this is rather relative classification, although it is steady enough.

I cannot tell that this is iron rule, not so many signals has been passed through our hands to make such conclusions. Nether less, there is no special problems, to generate this or that model by the standard algorithms FFT/IFFT.

Moreover, signal MIL-STD-188-110A 16 Channels is generated by the model B. Most likely, as a hypothesis, signals of models A and C, are formed with using FFT/IFFT algorithms of dimension 2^n, and signals of model B are formed without this limitation. But I will repeat, there is no problems to generate any model in any dimension FFT/IFFT, OCG does it. :-)

Accordingly in SA, at the analysis, there is the possibility of a choice of this or that type of creation of the channel. There are two types as it has already been marked. These are three (and more) models of signals can be generated on the basis of these two types.

The classical analysis does not provide authentic definition of modulation mode/manipulation in the channel, even if you will select somehow the channel more or less purely. This phenomenon is directly linked to type of OFDM signal, and it is better to consider it on an example of synthesis of the signal with a good rating/spacing of channels, for example CIS-12 tone.

But, preliminary, it is necessary to consider some important points.

Parameters CIS-12 tone are well-known, these are LU = 36, LG = 24. These values characterize OFDM signals, and these values are the ultimate goal of analysis.

One more great value is even more universal k = LG/LU, this coefficient is the universal constant for OFDM with CP, and it allows to receive all necessary, for implementation/demodulation of a concrete signal and-or concrete conditions.

Certainly in the real signals, there is at least one additional moment: this is the quantity of used channels, but it is secondary parameter. The knowledge of quantity of channels, without knowledge of coefficient k, does not speak about anything, while only knowledge of k value, almost completely characterizes the signal.

I absolutely meaningly ignore such "the important" values, as sampling rate and-or clock frequency of manipulation. The matter is that these values, in the conditions of a priori uncertainty of the analyzed signal, especially on records, have no special sense. Because they can be easily distorted, and do not represent the real facts at all, while k is a fundamental constant for OFDM, and does not depend on external factors.

Let's come back to practice. Basically, everything that is necessary is already known. It is possible to start study distinctions among the models.

Let's synthesize OFDM signal, model A, with parameters LU = 36, LG = 24. But for descriptive reasons all channels we will settle as the pilot-tones. There is no sense to take many channels, 6-7 will be enough.

It is well visible that, despite the fact of an absence of manipulation in channels, the channels on the spectrum look as if manipulation is present. The exception makes every third channel. This is absolutely normal phenomenon, characteristic for OFDM signals, which channels are formed "as is". The phenomenon is linked to including CP, which is equal LG, and all this has a close connection with coefficient k. As it is easy to calculate, the coefficient k is equal in this model 24/36 = 2/3 = 0.66(6), and k*3 there is precisely/exact integer value, that gives the period(3) of pure/clear pilot-tones on the spectrum.

This phenomenon, does not leave chances for correct definition of manipulation/modulation in the channel by the classical method, even in case of complete selection of the channel.

Now we will check this up.

Let's switch off in current model all channels except the third, and after synthesis we will select it. As it will be one, it can be easily selected.

Now it becomes absolutely clear - we can detect, everything, besides, that is actually presents. As we remember, manipulation in the channel is not present, the channels are settled as the pilot-tones.

However, introduction/including of CP provides exact phasing in the symbol, it does not guarantee saving of this phasing among characters/symbols at all. And the malfunction of intercharacter/symbol phasing leads to such results for the classical analysis.

We synthesize now the initial signal both as model A and model C, for matching. We will set the third channel as the pilot-tone, the others we will leave as working ones.

In model C, at the expense of saving of intercharacter/symbol phasing on channels, which are settled as the pilot/tones, we obtain possibility to select as the pilot any channel. But in this model, on the working channels intercharacter/symbol phasing is broken with CP introduction/including, same as in model A, that does not give guarantees in definition of manipulation mode by the classical methods.

We will not synthesize model B, interested persons are able to do it, but I will mark that in this model, intercharacter/symbol phasing is saved for all channels at once by default in the course of signal’s creation. And for this type of signals, the classical analysis allows to define precisely and successfully manipulation mode in the channel, in case of its qualitative selection is finite.

And here is the most interesting.

Hardly possible that the model B was oriented to make the analysis of this type of signals easier, and the models A and C to make it more difficult, of course no. But there are two different approaches in solution of the same tasks.

The problem is that even having on hands special OFDM analyzer, without having possibility to consider these different approaches in creation of the signals, or simply without knowing about types of OFDM signals, chances of successful analysis are not great. In this case, problems with the following tasks are guaranteed:

- with definition of manipulation mode

- with pilot/tones

- with detection of nonexistent phase shifts between channels etc., etc.

In this situation, everything may be good on one signals, while on others it will become worse than ever.

In the last SA version, OFDM mode supports two modes of channelization.

Mode A: demodulation of the selected channel is realized "as is" (by model A)

Mode B: demodulation of the selected channel is realized in a special mode (by model B).

As it is possible to select any mode on any channel, possibility of the analysis of all considered above models of signals is provided.

Ignoring or absence of knowledge about existing features of creation of signals OFDM, cause gross errors in the analysis. For example like this. :-)

Good Luck~

QAM: Telemetry of service of high-voltage lines.

Telemetry of service of high-voltage lines. Main channel: QAM-32, Br - 3200. Two add channels: QAM-X, Br - 66.7

The record of this signal, send by sergo2007

Author: SergUA6

Band Width 4000 Hz

Low Range ~560 Hz, in this example

Baud Rate 3200 Main Ch, 66.7 add ch Hz

n-Ary (PSK/MPSK) QAM-32 Main Ch, QAM-? add ch

Count of Carriers 3 One main channel & two additionaly

RX mode Source I/Q record

Sonograms:

pic.1 Generall view

Diagrams:

pic.2 Phase constellation of the main channel

Telemetry of service of high-voltage lines - an example of the signal of Russian equipment AWS1,2,3-CM - in russian abbriviation(transcrition), equaly on english as Digital Modify Special System Service & Communication-1,2,3. This equipment is used to service the high-voltage air-lines, working in range LW/LF. Production of "Neptune" factory. In the resulted example, the main channel works in mode QAM-32 with the maximum technical speed 16000 bps. There are also two relatively low speed technical auxiliary channeles, in which QAM modulation can be used also.

OFDM analysis module in SA version 6.1.2.5

OFDM analysis module in SA version 6.1.2.5

Extended description.

The main difference of the current version from previous ones, is the presence of the extremely effective clock/symbol synchronization, and as consequence, possibility to view dedicated/selected channel in dynamics.

Let's consider module elements in more details.

The first step in analysis OFDM, is the search of a correlative triangle.

There are only free key parameters for search of a correlative traingle:

These are:

- The Min Br-LS field, search lower bound

- The Max Br-LS field, search higher bound

- The Symbols field, quantity of characters/symbols end result is averaging out

Search is carried on from the higher bound to the lower bound, as the required/assumed result is located in range, and speed of calculations is strongly reduced at increasing of the symbol's length.

The symbol's length is directly linked to clock frequency OFDM, Br = Fd/LS. Where Fd - is sampling rate of creation of a signal or record.

It is better to set boundaries of search as value Br, but keep in mind that only the whole values are admissible. It means, if we want that 56.625 value Hz would be precisely counted, it is necessary to expose value 55 in the lower bound, or put 54 would be even better.

It is desirable to narrow down boundaries of search from above as well. Although the higher bound has no special value, because on short lengths speed of calculations is rather high. Nevertheless, on big sampling rates it is necessary to adjust it.

The quantity of symbols used in calculation at searches of a correlative triangle has the great importance only on short records, in those cases, when the value of Symbols field is essentially lower 40.

By default the program tries to expose this value in 40 symbols. But as, in attention taken only the lower bound (as the most expected/investigated symbol), then frequently on the short records it becomes impossible, then this value is lesser then 40, that indirectly speaks, about possibility to get not fully correct results.

In general, using of fixed quantity of symbols for an averaging-out can't be admitted as the correct solution, because, for the short LS and short CP, more symbols are required than for long LS and long CP. But these are questions of future optimizations.

By default, the program exposes a lower bound proceeding from sampling rate. The higher sampling rate, the higher lower bound. Seldom enough we can meet OFDM signals with clock frequency below 20 Hz.

As it was already spoken many times, analysis is the complex the task, and if there is a possibility to receive preliminary parameters of the researched signal through the classical analysis, it is necessary to do it. Frequently, it sharply reduces time for finding of a correlative triangle, at the expense of reduction of boundaries of search.

For an example, we will take signal MIL-STD-188-110A 16 Channels

After selection by vertical markers of a fragment of the signal in working window SA, we call OFDM module, and leaving default settings, we start the program of triangle searches.

Approximately in 1-2 seconds the triangle will be found, and the program can be stopped. If calculations will not be interrupted, searches of triangle will be continued until the lower bound, this will take much more time, and most likely more candidates on "triangle" will be found. If we would receive preliminary clock frequency, through the classical analysis (on this signal it can be easily defined through ADP function) then we would avoid superfluous loss of time and superfluous results.

It can be considered as advantage of the preliminary classical analysis.

As we can see, the triangle is negative, and it’s polarity should be changed. It can be realized through Shift Frequency. It is possible to make shift in any side, but it is better to do it in that side, at which the amplitude of the negative triangle will decrease, as the necessary positive maximum in this case is closer. It is necessary to mark, that it is not fully correct method, and the result can have no single meaning.

After signal's shift by frequency is done, recalculation of a triangle for the purpose of specification of parameters is necessary. Because the previous results are concerned to another signal, which has been not shifted. In this version, repeated recalculation is spent only for those values LS and LG, which are selected in the text box, it raises efficiency and allows to avoid superfluous expenses of time.

However, at any time complete recalculation "from zero" can be demanded, the reasons can be vary. How to be in this case? It is necessary to select string where LS and LG are not present, such string is always exists and it is the last string. Selection of this string, means that it is required to start searches of a correlative triangle from zero. As in our situation we do not need to start searches from zero, we do fast recalculation.

Fast recalculation leaves one-three results, and deletes all received earlier intermediate results.

Now we only need:

1) to select scale of FFT spectrum for convenient observation

2) to put the yellow marker on "the good" side of triangle,

3) to switch on phase planes

4) to select the channel for observation/studying

5) to start process of mapping by button "Show".

Approximately so everything should look before start "Show", for observation the uppermost channel is selected.

There are some details. The selected channel marked with green or violet marker (if both markers are combined), should be in the window of mapping of a spectrum. As it is possible to move and scale the spectrum, there is also possibility to output the marked channel behind window limits. If such situation arises, then there will not be any constellations. The program ignores channels which are not visible on the screen.

There are still some things, which need to be known.

The field "Current LS" is the field of double assignment. In the mode of search of a correlative triangle, this field mirrors the current length LS, which it is checked. In the signal browse mode, it mirrors number of the current symbol, which it is output on FFT spectrum. Number naturally concerns to the fragment of the signal loaded in the module by its call.

Before we will start automatic process of mapping, I suggest to make some steps in a hand-held mode, it will be well visible how the triangle intensively enough "leaves" aside and sooner or later will leave the marker.

Approximately through 50-60 steps, "the good" side of triangle, in general, will leave from marker, and FFT spectrum will be transformed beyond recognition, it means that the symbol is completely lost. Now, it is possible to return a slider in the beginning, to expose the yellow marker in the necessary position, and to start process of mapping of constellation in dynamics.

As we see, total quantity of the symbols in the fragment is about 540, the triangle has not moved anywhere, and relative constellation more than stable and pure, as clock/symbolical synchronization works.

Dynamic mapping from the symbol (which is displayed before beginning (Current LS floor)) starts. That is, it should not be only the null symbol, it is possible to take any other symbol, sometimes it is useful. The program will not start, if there are less than 10 symbols, such situation is possible, when the positioning of the slider by a fragment is installed in the end, or the record is very short.

There are two ways to solve this problem:

1) if the fragment is long enough, it is necessary to move the positioning slider in earlier position

2) If the fragment is short, and the slider already situated in the beginning of the fragment, then it is necessary to reduce quantity of symbols in the field Symbols.

It is not recommended to reduce quantity of symbols lesser than 6. Because clock synchronization spends an averaging out by this quantity, and though the algorithm is effective, smaller values are fraught with synchronization failures.

Of course an including of clock synchronization is the necessary component for the qualitative OFDM analysis, but it does not solve the main questions of reception of exact parameters of a signal. We still are working in this direction, although we are moving not so fast as it would be desirable we are close enough to the purpose.

Good luck.

SA update version 6.1.2.4: function of dynamic mapping of constellations in the selected channel

SA Update to version v 6.1.2.4/5

The current update is mainly concerned to OFDM module.

Some old control elements and indications are updated/moved: it is selected by the red frame.

Appearance of FFT spectrum is changed. Channels are separated by colour on both even and odd. The width of spectral lines is also increases with a graphic rescaling. It allows to select the necessary channel more comfortably. The former solution demanding absolutely exact hit in a line with the width in one pixel wasn't comfortable.

Perhaps the most important part of update - is an introduction of function of dynamic mapping of constellations in the selected channel, with the common clock/symbolical synchronisation.

In this mode, the programm carries out OFDM demodulation. At least, on the signals where relative phase manipulation is used, relative constellations can be observed practically irrespective of external factors, such as signal offset by frequency, record digitization etc.

An indication of an output of points of constellation behind the boundaries of mapping window is inserted. That speaks about necessity to reduce scale, as a constellation part, or the whole constellation is not displayed fully.

Basically, an order of standard operations in this module is the following:

- fragment of OFDM signal is selected in the working window.

- OFDM module is called/started.

- If it is necessary search of boundaries of a correlative triangle and customisation of other settings are installed. In most of cases, you may leave default settings.

- Start of search of a correlative triangle is realized by the button "Find CT"

- After a finding convincing Correlation Trianlge (CT), the signal's position should be adjusted by frequency. The triangle should be positive, and have maximum amplitude. It is controlled by the value of Am field in a window of mapping of triangle.

- After any frequency correction, recalculation is started again by the button "Find CT", as the signal is already another. Repeated calculations in this version are passing very fast, all subresults received earlier, are simply ignored.

- The yellow marker is located on the left ("good") side of a triangle.

- The necessary/convenient scale of FFT spectrum is installed, the necessary/needed channel is selected.

- phase planes are switched on by double-clique on the necessary record, in the window of output of the text data.

- The mode of dynamic mapping is started by button "Show".

It is the basic, generall scheme of operations.

Here is the Videoclip where on an example of signal MIL-STD-188-110A 16 Channels all described actions are shown.

The more detailed description of operation with the module as usually, will appear in the near future in the form of separate article.

Good luck~

APCO-25, is qualitative decoding possible from discriminator's output?

APCO-25, is qualitative decoding possible from discriminator's output?

Author: SergUA6

Appearance of DSD program, which provides decoding of standard APCO-25, has called on ours (and not only) forum active enough discussion. The program processes an audiostream from an output of the discriminator of any FM receiver, and solves demodulation and decoding tasks in real time.

Discussion on our forum is concerned the problem: is it possible to provide qualitative demodulation and decoding of standard APCO-25 from discriminator's output?

We will try to answer this question.

Thanks to high activity of participants of our forum, there is enough of records of standard APCO-25 in I/Q format, IF and the ones which are direct from the discriminator. The quantity of these records is sufficient to be able to give an answer on this question with more or less deep arguments and convincingly.

Modulation C4FM, which is used in APCO-25 standard, is very closely linked with modulation pi/4DQPSK, it allows to demodulate a signal as pi/4 DQPSK.

Example: http://www.radioscanner.ru/info/files/article412/ap_25_1.wav

But in this case, the signal's processing is realized on zero IF and the signal demands representation in the form of I/Q, or, similar results can be received on intermediate frequency (IF).

At the same time, modulation C4FM represents itself four-level FSK, and it can be demodulated as usual FSK (MFSK) modulation with usage of the standard discriminator. It saves compatibility with old analogue standards.

We do not know, how demodulation in concrete models of radio stations APCO-25 is realized, but at least we can estimate, what is possible to take into account with using only discriminator's output.

So now we need:

1) records of APCO-25 signals

2) SA

3) the small program for mapping(displaying) of levels of a signal from the discriminator

4) desire and time.

The small program is written specially for this article. This program imitates the simple demodulator, displaying levels from discriminator's output, with necessary clock frequency.

On the very qualitative signal, after all necessary conversions/procedures in SA, we receive the following results:

An output from the discriminator after the phase detector.

Result of demodulation.

On the upper image the part, which corresponds to exact clock synchronization is selected - it is that what is possible to receive on this signal. Complete synchronization is shown in the lower image.

It is obvious, that errors at demodulation are not avoidable. Although, in general, the result of demodulation is sufficient, but not for the signal of such quality, which is selected for the example.

Of course, considering powerful noise-resistant coding, which is used in standard APCO-25, decoding most likely will be realized qualitatively enough. However, it is obvious, that reception of a bit stream from discriminator's output without any processing is extremely undesirable.

Simple, preliminary, more or less optimal filtering, sharply raises chances for faultless demodulation.

Almost an ideal picture. At well organized clock synchronizations, the signal will be demodulated qualitatively. In the small program-simulator, which results are shown, clock synchronization is very simple.

Under standard APC0-25, after the discriminator, RRC filter is necessary with parameter alpha = 0.2.

However, it is possible to use any optimal filtration instead RRC filter.

It may seem, that the answer is gotten: it is possible to receive qualitative bit stream and qualitative decoding from discriminator's output. But the answer will be not complete if not to consider other aspects and problems which are surely exist.

The matter is that the signal, which we has been taken for an example, is too good, and does not mirror all problems of demodulation.

Here is another signal http://www.radioscanner.ru/info/files/article412/ap_25_2.wav , such meet very often.

After all conversions and optimal filterings, the result is about the such.:

Surely, there will be demodulation errors, but it is possible to minimize or avoid them, having provided:

1) signal's parameters tracking in demodulator ;

2) correction of levels of decision-making.

Because, hard logic, which perfectly works on qualitative signals, will not work in this case.

We can meet such signals very often, with the bigger or smaller distortions. An origin of these distortions is not known to me, but I assume that it can be result of operation of various repeaters, probably not serviceable or not certificated ones, and/or features of signal's transfer. Signals of such quality, naturally have worst parameters of a noise stability and reliability.

The discriminator's output can be certainly used for decoding APCO-25, but qualitative implementation, is not easy task as it can seem at first sight, this task demands the serious approach.

As we have received some letters, concerning the question of conversion of bit stream APCO-25 received in SA from the phase demodulator, I result the map of conversions to bits. Signal APCO-25 demodulated in SA, in relative mode, same as it is represented in the first image in this article. In this case correspondence of the real dibits and positions of angles is the followin:

0 - 10

1 - 00

2 - 01

3 - 11

Do not forget that signal's inversion is possible, then the values will be:

0 - 00

1 - 10

2 - 11

3 - 01

It is possible to define inversion indirectly by series zero bits, which always are present in APCO-25 bit stream, not less than 15-20 bits in a row. In not inverted signal, in the stream from SA demodulator, it corresponds to series 1, in inverted to series 0. The length of these series is not less than 7-10 characters/symbols (15-20 bits).

That is it.

Good luck~

© MSM Group

PSK: SATCOM Satellite, Telemetry, UHF

SATCOM Satellite, Telemetry, UHF

PSK-2, Br-1200, 15 Channels of data + 1 Synchro

Sample record of SATCOM, Telemetry, UHF, Send by sea2000

The bit stream converted into wav file, for view in VMW

Author: SergUA6

Band Width ~2000-2100 Hz

Low Range ~1452 Hz, only for this record!

Baud Rate General 1200 Hz

n-Ary (PSK/MPSK) 2

Carrier frequency ~2523 Hz, only for this record!

RX mode SSB

Sonograms

pic.1 General view

pic.2 Manipulation speed

Diagrams

pic.3 Phase constellation

Pictures:

pic.4 The bit stream structure in VMW

SATCOM Satellite, Telemetry - Telemetry of sattelites of SATCOM series. The signal has standard parametres. Transmission of 16 channels is organised transparently enough . The structure is well visible on pic.4

Data of all channels is transferred continuously-consistently by ring (circle), without cunnings. That is why, it is enough to build up the bit stream in 16bit "column" after demodulation, to find 0th sinchronization channel on characteristic sequence, and it becomes possible to receive a bit stream of any channel with a binding to its(channel) number. Transmission speed in the channel is 75 Hz.

As it is visible from pic.4, in the signal, in channeles 2, 9, 11-15 only bit=1 are transffered, in other channeles the data is transferred.

The bit stream in channels assumes further processing. Crypto security is not eliminated, and most likely it presents.

SA Update to version v 6.1.2.1

SA Update to version v 6.1.2.1

The problem with mp3 files is corrected. We had messages that sometimes the program opens all mp3 files incorrectly, but we hadn't the detailed description of this situation. Two days ago we have received the deatailed description from one of users, the step-by-step picture of occurrence of the problem.

As it has appeared, after loading wav file, attempts to open any mp3 file cause an error.

Ofcourse, detailed description of a problem, sharply increases chances to find and correct an error fast. That's what we always try to do as fast as possible.

Also the number of minor errors is corrected.

Good luck!