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.

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

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!

SA: work with OFDM module, FFT spectrum. Tutorial part II.

OFDM module, FFT spectrum. Tutorial part II.

Let's consider new abilities of SA OFDM module in version 6.1.0.3.

In the description of the previous version and in the first part of OFDM module tutorial, we have mentioned "the magic triangle", and have defined, that detection of this triangle says, that the analyzed signal belongs to OFDM class with CP. We have also concerned the bases of OFDM creation principles, and we had also have defined, that the triangle is closely linked with the transferred OFDM symbol. Let's go further, Let's take a view on the triangle more attentively.

For start we will consider schematically structure of the magic triangle, and we will try to understand, why only triangle shape is the right sign and What is so good in it? As i was already spoken, the magic triangle - it is known result of correlation of two equal rectangles, but nevertheless, we will look hardly more deeply on this triangle. Here seven is(conditional) main steps of results of calculation of correlation, we assume that LS and LG are received precisely.

It is perfectly visible, that the triangle's peak precisely points on an absolute coincidence of controll LS and LS of the signal. So It is perfectly visible that the triangle left-hand side "tells" us: that LU of the controll window window of LS, has completely entered into LS of the signal(It is very importent momemnt). Increasing of the amplitudes (rising) of the left-hand side of the triangle shows to us that: the controll LU is closer and closer to it's "native position" in LS.

Right side of the triangle says that: the controll LU, does not get any more completely in the signal's LS, and it goes away further and further, thereupon right side becomes not valuble for analysis. It is impoertent to to understand why the left side is good for analysis , and right is not good. We do not want that the user thoughtlessly would place the marker on "the good side" without understanding why it should be so and what hiddens behind it.

Ок, let's look on CP (LG), what does CP give?

CP increases length of the symbol, by so much countings, how many for CP is taken. What in general it means? As CP it is a copy of a part of the end of OFDM symbol, CP possesses very important property, it "is fastened" in the beginning of the symbol precisely in the phase. Here follows awesome conclusion:

The quanitity of correct LU values is not just one in the symbol with CP.

Quanitity of the correct LU = LG+1.

More visually it looks so:

It allows us to use LU freely enough at the analysis, because any LU, which we receive on "the good" side of the triangle, is basically correct.

Let's move to practice now. I will only mark that, for getting quantity of channeles and their amplitudes, all LU are equal. We will check up everything, as well as it is necessary, on an ideal signal. Let's synthesise for example such test signal.

Everything, what has been told earlier, is approved.

Move the specified slider and be convinced that "the good side" is really good, and that it's lengh takes precisely LG size, and that on this side all channels, which are entering into LU have stable amplitude and location.

Certainly you will notice that the real signals, behave a bit differently than an ideal ones, but you will also notice that the negative triangle does not promise anything good, and that it is useless to search for anything on "the bad side of triangle", because there simply nothing to search for. You will see, that, in general, all OFDM signals with CP submit to the common laws, considered by us that. Thus as soon as the marker leaves "the good side", FFT spectrum starts to behave strange, unpredictable etc.

Let's consider some practical nuances:

Please Pay attention, on the field LU (FFT Size) = X, where is X - os the FFY block sizeFT, or size of LU in countings. On real signals, X can be any. Actually, only the even size can be considered as the correct. Odd size of LU says that, sampling rate, or signal's shift by frequency, or all that have together led to the fact, that LU precisely lays down on odd quantity of countings of sampling rate. It is not forbidden, but it is not correct. Althogh, even in this case, the program works, as we use FFT algorithm working with any quantity of countings.

Why the yellow marker sometimes is a one pixel thick, and sometimes it is rather wide bar? Here all is very simple:

The size of LS is not limited formally, there can be signals both with LS=20 and with LS=2000, and for more comfortable observation of "the magic triangle", the map is scaled, and sometimes, there are 20-30 or more of countings in one pixel, in this case, then the marker is a thin line, and sometimes one counting is expanded on several pixels, and then the marker is wide.

You can consider the results of spacing and speed of manipulation as correct or atleast very close to real ones only, in case when:

1 - your triangle is motionless at symbol-by-symbol moving,

2 - LU is even,

3 - peak is positive

Otherwise it is necessary to speak about conditional enough accuracy.

Actually, you will notice that OFDM signals, allow rough enough inaccuracies. The very good spectra can be seen even at the big errors, moreover, it is also possible to receive the quite good constellations in the channels, but it concerns only to relative sorts of manipulation. The absolute constellations in channels, such as QAM, demand very serious measures of bringing on parametres of a signal to true ones. But this is already future...for now I suggest to be mastered with this tool SA OFDM module, this tool allows to do very many things, for example, like this:

It is DAB signal. FFT spectrum of this signal do not fit into the mapping window of the module on my screen monitor:) You do not have chances to receive exact pricese parametres of this signal by another way. SA allows to get pricese parameters.

Good luck~

Version 6.1.0.3: SA OFDM Module expansion.

SA update to version 6.1.0.3

In version 6.1.0.3 we continue to expand possibilities of SA OFDM module.

New components are added in the module(new components are selected with red color), old ones are moved to more convenient positions. The general view of the module:

Description of the new elements:

1 - regulation of FFT image scale by vertical

2 - moving of FFT image

3 - shift/displacement of the whole signal, for the fast correction of inaccurate position of the signal on frequency. Accuracy is in 1 hrz.

4 - regulation of FFT image scale by horizontal. The regulation is discrete.

Other elements are same as in previous version.

Detailed description:

The working window is divided into two parts.

Top part: is intended for output of the image of a correlative triangle and the various additional information. It is also used for exact positioning of the signal's LS concerning to the marker (the yellow line in the middle).

Bottom part: for output of image of FFT spectrum.

Let's consider the common example of operation with the new elements of OFDM module.

Let’s take an already known signal DRM from our site. As we already know, clock frequency of manipulation is about 37 Hertz, thus it is possible to reduce strongly time of calculations, by having installed lower limit of speed at search of LS and LG at 36-37 a Hertz, and upper limit at 38-39.

"The magic triangle" has two sides, left is the "good one", right is the "bad one" (an explanation of this feature will be described in the OFDM module tutorial part II).

Here we consider the common approaches.

For this purpose that Operation FFT would be spent really over "correct" LU, it is necessary to locate the signal so, that the marker would be situated or on "the good" side or exactly on the peak.

Actually, it is better to locate the signal on the middle of the good side.

For exact positioning of the signal concerning to marker, it is simply necessary to click on point of a slope of the triangle, and the signal automatically will be taken up into needed position. For position specification, it is possible to do smaller steps in hand-held, through the left and right buttons of the slider of moving on a signal.

After the signal will be exposed in the necessary:

to regulate view of FFT image,

to correct signal's shift by maximum of the triangle's peak

and, in general, to estimate that purely it has turned out.

The main purpose of all these operations is to receive FFT picture of representation LU of a part of the symbol, this part is pure information itself. Moving on the signal symbol-by-symbol, we can learn a lot of interesting and useful information. For example:

how many frequencies are used in the concrete symbol,

how many of them are used in general

and others undetectable before/earlier things.

For example, on this signal to learn: how many channels are used, we need to count them in hand-held, or to use special markers to count them. If precisely to take place on the most left frequency of FFT image, and to make double cliques, this frequency will be marked by two (cliques double) markers; now, if to click precisely on the most right frequency, one of markers "will go" into it.

In the window of correlative triangle it is possible to read, how many frequencies are between the markers, what are the frequencies for the lower channel and for the upper one, what common spectrum is occupied with working channels and etc.

Concerning to this signal as result we have :

1)in total, there are 229 working channels

2)there are three, from them, which are located exactly on centre are not used( it is visible on all symbols) from the remained 226 channels, there are three more are pilots-tones.

Thus 223 channels participate in information transmission. The signal’s spectrum occupies the bar in 9500 Hrz. In reality, it is a bit wider, as the channel is not a line, the line is in frequency area after FFT and this line is only for one symbol, instead of whole signal in time.

Taking into account, all information from the previous update SA to version 6.1.0.0/1 and article SA - operation with OFDM module, these new tools are certainly essentially expand possibilities of analysis OFDM in SA. Detailed tutorial about how to use new possibilities of SA OFDM module, and further development and improvement of SA OFDM Module will appear soon.

Good luck~

Signals Analyzer: SA - OFDM module detailed tutorial. Part I.

SA - OFDM module detailed tutorial. Part I.

This tutorial is intented to help with solving difficult enough tasks of OFDM analysis.

The purpose of this tutorial - is to show what is possible to learn/understand/define with the help of current implementation of SA OFDM module and small "magic triangle":).

Click here to read full OFDM module SA tutorial by MSM Group.

From this SA OFDM module tutroial you will know:

• how to define the magic OFDM triangle
• how to recognize the right one triangle, in case, when there are several triangles in the signal
• when triangle can be negative and what does it mean
• what the correct "magic triangle" should look like
• specifics of "behavour" of the correct "magic OFDM triangle"
• and more interesting and usefull features for effective analysis of OFDM signals with help of SA OFDM module.

Soon the second part of OFDM module tutorial will appear.

Yours sincerely, Maria and MSM Group.

The unqiue OFDM module is added: Signals Analyzer updated. version 6.1.0.0/1.

Signals Analyzer new updated version 6.1.0.0/1.

In SA version 6.1.0.0/1 OFDM module is added. New SA OFDM module is the unqiue tool, which do not have analogues.

The detailed description of OFDM module is available in our site. Click here to read detailed OFDM module tutorial.

This tutorial is "must read", it is short, but infromative description of princples of OFDM analysis and principles of work with SA OFDM module.

Everyday more OFDM signals are appearing, and they are always problematical for the analysis. SA OFDM Module is intented to help and greatly simplify the analysis of OFDM signals.

From our description of version 6.1.0.0/0 You will know:

• basics and princples of OFDM analysis
• structure of OFDM signals
• "Magic traingle" as the sign of OFDM presence
• how to recognize "Magic triangle"
• easy steps of how to use OFDM module
• the results of SA OFDM module work.

Current implementation of SA OFDM module is only the first step in this direction. Further we will expand and develop OFDM module to receiving of constellation of the separate channel, to compensation of inaccuracies of sampling rate and displacement by frequency, and if it ill be possible, to OFDM module full automation of putting to rights parameters of record.

We are also planning to write very detailed and usefull article about work in SA OFDM Module, because possibilities of SA OFDM module are great, and some part of very vexed questions(which are often discussed on forums), can be easily enough solved with SA OFDM module help.

Sincerely, Maria and MSM Group.