Merry Christmas!

Merry Christmas and Happy Holidays!

SA: How to work with OFDM module. How to apply new features of OFDM module on practice. Practical examples on real signals from the air.

SA - how to work with OFDM module.

Several practical examples on the real signals.

Full version of this really interesting tutorial is here.

After version 6.1.0.5 and article SA OFDM module, constellations in channels has appeared, we received several letters with questions about how to use OFDM module and what are practical possibilities of constellations in the channels.

Even taking into account those difficulties, which have been described in previous article, SA OFDM module represents powerful tool. Operation with this tool we will be considered more detailed in this article.

In this small tutorial we took 4 real signals from the air to work with in SA OFDM Module.

Here is the signals from the air, which are used for analysis in this tutorial:

1) DAB

2) OFDM-59

3) WinDRM

4) Q15X25

I do not post the whole artilce in the blog, because it will take too much space, not because of text, but because of pictures.

Please, follow the link to see whole article with all pictures, and interesting features inside.

My personal opinion: It will be very interesting to read for everyone.

WINDRM 51-Tone COFDM Modem

WINDRM 51-Tone COFDM Modem

Amateur mode, Br-37.5, Sh-46.875, Ch-51, QAM-64

Here is the example of WINDRM signal

Author: SergUA6

Band Width 2400 Hz

Low Range 300 Hz

Baud Rate 37.5 Hz

Count of Carriers 51

Step between carriers 46.875 Hz

Pilot tone(s) 3 pilot tones 750, 1500 and 1875 Hz

ACF 400 ms, brightly expressed period

RX mode SSB

Sonograms:

pic.1 General view

pic.2 Graphical spectrum

pic.3 Allocation of channeles

Diagrams:

pic.4 Constellation of the lower channel, start.

pic.5 Constellation of the lower channel

pic.6 Constellations in other channeles

Pictures:

pic.7 ACF

WINDRM 51-TONE - difficult enough sort of modulation mastered by hams, the "industrial" basis is certainly visible, many things are taken from standard DRM. 400 msec frames and the circuit of a rating/spacing of pilots-tones are made by analogy of "big DRM", all accessible frequency resource of 2400 Hz is completely used. SA possibilities allow to receive both signal parametres, and constellation in any channel. It is well visible that QAM-64 is with various service "design", there are also channels with "pure" QAM-64. Maximum technical speed is 10800 bps.

SA OFDM module improvement: constellations in channels.

Phase constellations in the channels of OFDM signals.

As usually, we will try to apply new options on the qualitative signal, with well-known parameters. In the first, it will allow us to estimate possibilities of the new method basically, and in the second, will give accurate enough representation of how to apply this method on real signals. For this purpose we have synthesized signal OFDM test2.wav.

Parameters of the test2.wav signal:

Number of channels - 30

Manipulation in the channels - pi/4 DQPSK

Step between channels (frequency net/grid)- 80 Hz

Manipulation speed - 66.6(6) Hz

Sampling rate of signals formation - 6400 Hz

Symbols length (LS) with CP, in samplings - 96

Length of defensive interval (LG), in samplings- 16

Symbols length (LU), block size IFFT/FFT, in samplings - 80

Preliminary, I wish to pay attention to one widespread enough mistake at the work with OFDM module. Very often in records of OFDM signals, there is a pause at the beginning. This pause it is not visible on a spectrum, because it is short enough and simply masks. Selecting a fragment of a signal for the analysis in OFDM module and capturing this pause, will strongly reduce efficiency of calculations, if the fragment is short enough. For example: we have 40 symbols for calculation of correlation triangle, while in reality we can have only 10 or less symbols, because the main body will get on absolutely not informative pause fragment. We do not speak about zero level, it can be pause in signal transmission, which somehow has gotten into record.

Example:

Such situations should be considered, it Is necessary be simply attentive. Especially it concerns to short signals. SA Free inserts a zero fragment into beginning of signals always, while SA Full in this respect, works more correctly.

Let's open in SA the test signal and select a fragment, please pay attention, we select the fragment from the beginning of the file, because we know that the signal is synthesized, and absolutely precisely does not contain any pauses. Now, let’s call OFDM module with default settings on, and start search for correlative triangle. There is no necessity to wait for the end of calculations, very often the triangle, which is not calling doubts, can be formed very fast, the program can be stopped at any moment. Although, in doubtful cases, it is better wait until the end of procedure.

If everything has been done correctly, the triangle will be positioned with it's top precisely on the yellow marker automatically, because we are working right now with the ideal synthesized signal.

Let's select the first channel (the lower channel) by double click on the graphic. Now let's include constellation mapping also by double click on the necessary record in the list. Actions order does not play a special role. Further, it is possible to observe constellation in the lower channel moving on the signal symbol-by-symbol. Now it is visible, that in absolute corners it is PSK-8, in relative PSK-4, all is correct, it is classical pi/4 DQPSK.

Selecting other channels on the graphic's spectrum by single click, and moving on the signal symbol-by-symbol, it is possible to receive constellation of any channel.

In the video clip - this procedure is shown visually. Sometimes it is much better to see something once, for better understanding.

While we were making this video clip there was small problem: As far as constellation's points, on an ideal signal are the real points in one pixel, they weren't visible in the video clip. Thus, it was necessary to change mapping algorithm. Thereupon, for everyone who has version 6.1.0.5, it is necessary to update it to version 6.1.0.5a.

There is some thin moment, both absolute, and the relative constellations will be always precisely oriented on axes on any channel. Stable orientation of constellations, is a result of ideality of conditions of analyzed signal. These conditions are unstable, it is enough to shift the marker from triangle's top on the left slope, and from exact orientation on axes for absolute angles remains nothing. While for relative constellation everything stays former.

Why it occurs? Shift on the good side of triangle, as you should remember from the previous articles, guarantees a choice of one of correct LU. The quantity of LU in OFDM signal with CP is equal to LG+1. Remember the picture:

All LU differ from each other by phase shifts. Initially, while the marker was precisely at top of triangle, the latest LU was processed, and as this LU is "parent" of all other LU on it, at observance of all conditions, it is possible to demodulate the symbol and the whole signal. Actually this LU is the purpose of demodulator, because other LU mostly are spent on distortions in the signal.

That is why, as soon as we go away from the "true LU", we receive casual phase shift in absolute angles, while it is not reflected anyhow on relative angles.

To complete the picture, it is necessary to consider one more moment. We loaded the signal's fragment precisely from the first symbol, from beginning of the file. What if we will load file from random fragment? In this case, everything remains same: absolute angles will be randomly shifted, even if the yellow marker will stand precisely at triangle top. Relative remain stable.

What has happened? Isn't LU now "not native", after all, it is last LU in the complete symbol? LU is still "native", but if to speak more correctly, LU is the most authentic from all others, because distortions will reach it in the last moment. But now we got "not native" symbol!:) The program considers it as first symbol, while in practice we do not really know it. Yes, yes, in OFDM everything is so coordinated: symbol number, and number of LU, both sampling rate, and shift on frequency all that is coordinated. If one of these parameters is not that one, which should be, then chaos begins in absolute angles:)

Fortunately we have strong enough order in relative angles. Although, it is also not very stable.

For this reason, relative sorts of manipulations are so widespread in OFDM.

In reality, all is not so terrible as it is represented, the matter is that even not native symbol, and not native LU, at exact observance of all other parameters(such as the sampling rate and shift on frequency) do not threaten with anything deadly, except, a turn of absolute constellation relatively axes, of what we were convinced just now. The constellation is drawn correctly, and to calculate the angle of an error and to compensate it for each channel is possible. It is not an easy task, but this task has decisions.

In practice, any not distorted LU suits for the analysis, basically it is not important which number has the symbol. The maximum that it is possible to lose, in this case, is the correct orientation of absolute constellation. This loss is not too great for the analyzer.

Let's look, what results will bring not precise sampling rate. As minimum - the triangle will slide concerning to marker in this or that side, that automatically means that at symbol-by-symbol moving, there is almost no chance to stay on same LU. It pulls behind itself complete chaos in absolute angles, in this case, we cannot speak about any stable constellation. Also depending on speed of moving of the triangle, there can be order distortions in relative angles. There also can be different problems at the channel changing.

Let's check it up. We will change sampling rate on 1 Hz.

As it was anticipated, there is complete chaos in absolute angles. The constellation is not simply developed, it permanently turns around. In relative angles we have something looks like an order, but, at the big errors there also will be problems, with one difference from absolute angles, the constellation will not turn around constantly. At more serious changing of sampling rate, we observe more bright problems.

Let's watch what will give shift by on frequency in 0.1 Hz.

Everything will be same, but sensitivity to shift will become stronger, then to changing of sampling rate.

We will not bring examples, what will happen at shift of the signal by frequency and not correct sampling rate at same time, it is obvious that very serious problems will appear. The purpose of all this - is to show level of sensitivity of OFDM signals to these parameters.

And here it is best time to remember, in what conditions, we are situated, when we have got record of really unknown OFDM signal. Generally, we do not know anything:

1) neither the sampling rate of creation of the signal

2) nor sampling rate of the record

3) we also don't know exact frequency of signal and etc.

As we saw signal's shift only in 0.1 Hz, practically disorganizes an absolute constellation. It means, that without auto regulation systems, chances to get exact parameters of modulation are extremely not great. But there is one more problem that is : to make the self tuning system work, it is necessary to know what exact values it is necessary to support. As far as it is impossible to receive exact parameters, as in general nothing is known exactly.:) The vicious circle. This is the main complexity of OFDM analysis, in difference from demodulation, when we have the signal with completely known parameters.

Really everything is so bad? To tell the truth, all is not too good, and at this moment all we have - is the possibility to observe convincingly enough, only relative constellations, what it gives? Unfortunately nothing very great. Even having good relative constellation, without accurate absolute constellation, it is not possible to guarantee neither a modulation mode in the channel, nor value of clock frequency, neither accuracy of position of a signal on frequency, nor an exact rating of a grid of channels. All it will be: something, somewhere, in a range of something.

Why the relative constellation, does not allow to define manipulation parameters precisely? The matter is that, for example, within the limits of relative manipulation pi/4 DQPSK, it is possible, and very often it used to be done, that both PSK-2 and PSK-4 are used. And there are two variants of PSK-2, with different orientation concerning axes. Nothing prevents to use relative manipulation 3*pi/8 DPSK-8, and it is used often. This sort of relative manipulation, has in it's arsenal, even more variants and types. Generally, there is no possibility to tell precisely, only by the look of relative constellation, if it is really relative manipulation or absolute is used.

Random phase shifts in the channels are not limited, and they very easily can create illusion of obvious and very strong constellation in absolute angles.

Is it possible to prove that in this channel is not absolute PSK-4 or QPSK, and all the same pi/4 DQPSK, while the triangle is not sliding at symbol-by-symbol moving, its peak positive and LU size is even? But it is possible to make convincing absolute constellation almost in any channel.

Basically, it is possible to guarantee something only on ideal, or qualitative records yet. Thus, it is necessary concern results, received in the current version of OFDM module carefully enough.

The problem is very hard and interesting; we are planning to go further with our developments in this direction

Digital Scrambler Daxon D600 PSK-4.

Scrambler Daxon, D600 PSK-4.

The record of Scrambler Daxon D600. Send by Maxwel.

Author: SergUA6

Band Width 3100 Hz

Low Range 300 Hz

Baud Rate 2400 Hz

n-Ary (PSK/MPSK) 4

Carrier frequency 1800 Hz

ACF 45 ms

RX mode NFM

Sonograms:

pic.1

pic.2

pic.3

Diagrams

pic.1

Pictures

pic.1

Scrambler Daxon D600, uses a principle of conversion of speech into digital stream. After this conversion, this stream is transferred (also probably coded) to the consumer in the form of binary information. Modulation PSK-4 is used, with speed of manipulation 2400 it provides bit speed in 4800 bps.

New Year's Eve is coming.

New Year's Eve is coming.

In light of this event, we are giving reduction for Signals Analyzer price in 15% until 1st Janary 2010.

Hope this small card will pleasure your eye:-)

MSM Group.

SA update to version 6.1.0.5: OFDM module expansion. Mapping of phase constellation in the channel.

SA update to version 6.1.0.5

Updates in version 6.1.0.5 are connected with OFDM module.

In this version accuracy of signal's shift by frequency is increased to 0.1 Hertzs.

Possibility to observe phase constellations in selected channel, in absolute and relative corners is added.

Detected errors and minor problems are solved, thanks to users who inform us about them.

For mapping of phase constellation, it is necessary to double-click on required record in the list to turn on this option. It is also nessecary to double-click on any record, in case, if You need to switch off this option. Double-cliques in any area of constellation mapping, clears picture. Constellation is drawning at symbol-by-symbol moving on the signal.

In closest time, the detailed enought tutorial about how to use option of constellation mapping in OFDM module will be published. Necessity of such article, is cauased by the fact, that there is no ability to describe all nuances and features whithin limits of SA update description. Nevertheless, even now it is possible to use this function successfully.

Good luck!

FSK: Italian Military 1200 FSK-2.

Italian Military 1200

FSK-2

Italian Mil FSK example

Author: SergUA6

Band Width ~ 1400-1500 Hz

Low Range ~ 1000 HZ, shift is possible, recievein is in SSB

Baud Rate 1200 Hz

Count of Carriers 2 manipulation frequency, the are not distinguishable on the spectrum

Step between carriers 800 Hz

RX mode SSB

Sonograms

pic.1 General view

pic.2 Detalization

pic.3 Manipulation speed

Diagrams:

pic.4 Precise spacing

pic.5 Unique feature of Italian Mil 1200 FSK-2

Italian Mil - Does not have obvious distinctive characteristics, but... Basically on such speeds, as a rule, frequency manipulation without phase rupture is used. But this signal is formed by another way. Classically, speed 1200, and spacing is 800, in this sense it same as well as AX.25 1200 in way of it's creation, but not in way of protocols. In that sense, it is unique enough feature, and it is possible to use this unqiue feature at an identification of this signal. With SA VMW help interesting enough feature of this signal is detected. This feature is shown on pic. 5 - almost regular changing of polarity through eight clock ticks (bit). That is a good distinctive characteristic.

New article: I/Q format. SA and I/Q records.

I/Q format records. SA and I/Q records.

Author: SergUA6.

We have received some letters, concerning I/Q format and usage of I/Q records in SA.

Let's try understand, what is I/Q record, why is it good and what possibilities it represents for the analysis and signal’s identification.

Why is I/Q format good and what possibilities it represents?

Standardly, 90-80 % of usual receivers, produce, as result of their operation, an audiostream. It is the real signal, which is standardly written in wav file, and after that, it can be used for analysis and identification, or placed somewhere on the Internet. Quality of such signal is completely depends on receiver’s characteristics.

The receiver in this case, is fully realizing processing of the signal, and in most cases, receiver "solves" for you, what and how to do with the signal. Of course, it is supposed, that the receiver is managed/controlled/used by competent user, however, practice shows that it is far not so. High enough number of people, still, do not suppose, what is going on inside of receiver, and what is going on at the recording process. That is why Internet is full of records with the very low quality, or even "non-liquid" for analysis/identification record. In most of such records, we do not have the real signal, but something which is only a bit similar to it, or even not similar at all, in very bad cases. The main problem, is that, far not each user wants to gain an understanding of the very big diversity of various classes of signals and special sorts of modulations/manipulations.

Luckily, today, old enough and known ideas, thanks to existing computer technologies and its power, are successfully realized in the form of SDR technologies. What is SDR technology? SDR technology means, linear qualitative transfer of required segment of a spectrum, into area of low frequencies, and its further processing.

All charm of this technology, is that for this technology it doesn't matter what sort of modulation in the signal, and what are features of the signal, how high the skill of concrete user - the signal will be transferred on relatively low frequencies without distortions, in it's initial form. After that, the signal can be saved for the detailed analysis.

Please, pay attention, that we are speaking not about the signal after demodulation and processing, as it becomes in usual receivers, but about the signal in that sort as it is. In what sort the signal is represented in this case? It is I/Q format. Sense and method of I/Q format forming is represented below:

The signal (necessary range) :

1) is selected, if it is necessary, by the entry bandpass filter

2) and it is transferred on zero IF in the form of two quadrature channels I and Q

3)then each channel is filtered by LPF filter and moved on ADC.

As a result of such transferring, in each channel, there is a folded up/convoluted half-and-half spectrum of the main signal and phase shift between channels is equal exactly 90 degrees.

(Let's not go deep into Mathematics, all this is well described in the theory).

Such approach allows:

in the first, to reduce sampling rate in each channel, at the expense of spectrum's convolution

in the second, to develop the signal into "complete growth" at any time, in that sort, in which it was, when it is necessary, at the expense of phase shift in 90 degrees between channels.

If digit capacity of ADC is selected correctly, and own level of noise of conversions is small, then having saved channels I and Q in the record form, we are getting an absolutely exact initial signal, without any distortions.

Standardly, for these purposes the stereo format of wav files is used. In one of channels samples

I are written, and in another one samples Q, or on the contrary, it does not play a special role.

Thus, the record of format I/Q is the standard stereo file, which demands special processing further.

How to work with I/Q records in SA? Easy!

SA it positioned as software for the qualitative analysis (signals correct identification), and, of course, I/Q format takes a special place in SA, because I/Q format is potentially guarantying an absolute quality of initial material.

That is why, there is special mechanism in SA, which allows to receive the real, present signal from I/Q record, for the further operation with this signal.

So how it looks on practice? It is very simple. At loading of standard I/Q record, SA is warning you, that the record is the stereo record, and SA suggests :

to select the channel

to load all samples

You need only select the point "load all samples", and converting of I/Q record into the real signal will begun.

That's it! Now, it is possible to work with the signal in complete volume of SA possibilities. You can save the file, filter it, re-sample, demodulate it and etc etc etc.

It is noticeable that the sampling rate of the file with the real signal is twice more than initial I/Q, and it is normal. Because now the signal is presented completely as the real one, and instead of two channels as earlier, we got one. However, data volume is remained precisely same: data is not reduced or increased in the file with real signal, the data in such file is simply represented in other sort, which is more convenient for human.

Why some people are still deny I/Q format, while it is obvious that I/Q is the most suitable for analysis and identification?

It is obvious that format of I/Q records is the most preferable for analysis, and it seems like it is clear enough. However, on practice, we often meet complete not understanding of this fact.

And there are some reasons for it. Let's consider them:

The first and the main reason, is that, almost all owners of SDR receivers, are confident, that various demodulators, which are included in software, which is supplied with these receivers are miracle.

Please do not get under self-deception, everything is not so is easy, and mostly, everything is not easy at all.

Necessity to work in real-time, orientation on radio signals listening, and many other factors force to use solutions, which are far from the ideal. In result, after demodulation, such records become nothing better, and sometimes even worse than, the records, which are done by usual receivers.

SDR receiver, which does not allow to write necessary bar or a signal in I/Q form, is lame by default

If you want to correctly identify any signal, first of all, You should be interested it the signal’s initial form only. And such form can be provided only in pure I/Q format.

Some people, who already faced with I/Q files in SA, have noticed that, sometimes, level of I/Q record is very low.

It is not the problem of SA. It is the problem of SDR receivers and their circuitry.

Maximum, what it is possible recommend in such situations, is to try to increase amplitude of a signal as soon as possible, before any manipulations with the file.

For example, after loading I/Q record, we can, at once, estimate and increase signal level if it is necessary:

Then it is necessary to save the file as usual wav, and after it is loaded again to process it as I/Q. It can compensate weak level of initial file in some measure.

Although it will not completely solve this problem, I underline, this is not SA problem, and this problem should be solved on the stage of the record's forming, because these are questions of SDR equipment competence.

The second reason why some people still do not consider I/Q as the most suitable for analysis format is that: there is an opinion that I/Q record is too plentiful and takes unfairly big volume. It not so truly.

I/Q assumes, that signal is transferred on zero frequency and if, to write only the signal with a reasonable store, then the volume/size of the record will be exactly such as necessary, no more and not less.

Another matter, if the signal has a bar in 10 kHz, but whole segment of range of the signal radiation (for example 1Mhrz) is fully transformed in I/Q, then of course, in this case, we have an absolutely redundant record. This kind of records says only, that the user who have done this record, simply do not understand he was doing.

Do not compare size of records of the signal after demodulation and the size I/Q record of a complete signal before demodulation.

In the first case, You work only with image of signal, and You do not know how was this image gotten, and if this image is even correct.

In the second case, You work really with the present signal, and only You solve when and how should it be demodulated.

Surely, in the second case, the chances of correct identification of the signal are incommensurably higher then in the first case.

It is very strange to hear that I/Q has too big unjustified volume from the people, who are making demodulated signals with bar in 3 KHz, in wav files with sampling rate 44100 Hz, in stereo format, and after that these people contrived to compress it by mp3 codec!

After all they are sincerely surprised that nobody have any interest in their records.:)