New Article: Perspectives of the analysis of OFDM signals with CP.

Perspectives of the analysis of OFDM signals with CP. 

As usual, we assume that reader possesses some basic knowledge concerning principles of formation OFDM, and the reader is acquainted with our previous articles, as terminology and the main ideas remain former. 

There are some general provisions. 

We already were being considered and showed in the previous articles, that at formation of OFDM signals the basic/main/key parameters are LS, LU and LG (CP), which are having dimensional samples. Basically, each one of these two parameters from these three (LS LU LG), unambiguously set the third parameter, as there is a simple correlation, namely: 

LS = LU + LG; 

We add one more, the dimensionless parameter, coefficient k = LG/LU or k = ((Sh/Br) - 1). This parameter allows to receive correct LS, LU and LG for specific conditions, besides k possesses mass of other remarkable properties, but it is beyond this article. 

In a general view the scheme of formation of OFDM signal with CP looks as follows: 

As we already considered, and I once again underline this moment, that there is no any Hertz and-or seconds! LS, LU and LG have at a stage of formation dimensionality of samples/samplings. We name it Fundamental/primary/key parameters of OFDM signal, these parameters are core of OFDM signal. All is equal to these parameters (samples), with what speed they will be transferred, needless to say, that transmission/transfer speed should be a constant.


As soon as some speed of following is given to samples (sampling rate of formation of OFDM signal OFDM), then earlier faceless values LS, LU and LG acquire/get dimensionality in Hertzs and-or seconds. But! They don't lose, and can't lose their main/fundamental properties. For example, as there was duration LS = 100, LU = 60 and LG (CP) = 40 samples, so it remains, irrespective of sampling rate.


There is one nuance in these transparent and logical enough conclusions. All what has been told above, concerns to so-called sampling rate of formation of OFDM signal.


In practice, to form a signal at once on operational frequency it is not convenient, and frequently it is not even possible. Normally, the signal should be formed on sampling rate Х, and further whole spectrum of the signal should be transferred on operational frequency Y. Thus, sampling rate X can't change duration LS, LU and LG, as it (sampling rate) is a consequence of existence of these parameters, and frequency of transfer Y can't destructively influence these parameters as it is not connected to them.


Let's sum up the told...


Full version of this article you may read here.


The arcitle contain examples on the real signals, schemes, detailed pictures, explanations and other interesting information.

Update SA to version 6.2.4.4

 

SA update to the version v 6.2.4.4 

We continue to increase SA power and abilities, we expand possibilities of OFDM module of the analyzer in this update.

 

 

1. In display of OFDM signal spectrum  small pedestal/platform in 10 pixels is added. It allows to see all line of channels current/investigated LU.

2. Possibility to display on phase planes only every second, the third etc. symbols  is added.

3. Possibility to shift/replace a start point is added during dynamic mapping. In a combination with the previous element, it allows to find out/study rather effectively various special channels/symbols in signals.

As an example there is one service channel of signal WinDrm is shown below.

For comparing it is shown as it looks/looked in old versions.

Important! The second and third points of updates, work only in a dynamic mode!

This update considerably expands analysis OFDM  signals, bringing it on qualitatively higher level.

Small manual on the technician of OFDM analysis  by means of current version SA are described in the article the Overview article (manual) about  the analysis of various OFDM signals in SA versions v 6.2.4.4

Good luck.

SA update to version 6.2.3.0

SA Update to version 6.2.3.0.

Changes has been added in OFDM module operation.

Algorithm of search of a correlative triangle has been also changed. Current implementation of this algorithm successfully enough copes with the short signals.

Function of obtaining of exact value of clock frequency is updated. Now besides obtaining of the clock frequency, the task of obtaining of an optimal position of the signal by frequency is solving. It does automatically translates the triangle in the necessary positive polarity. All that demands some time, so no need to be nervous:-)

Some pauses (till 3-5 seconds) in operation of "Get Br" function are possible, The pauses depend on many factors:

from sampling rate of a signal

from its position on frequency

from the sizes LS and LG etc.

In generall, the program solves wide enough range of tasks in an automatic mode. From the user it is required only to situate the triangle in "a good" position and to call function "Get Br", and it is not important if there will be the positive triangle or negative, the program itself will translate the triangle into the necessary type, through search of optimal shift by frequency.

It is necessary to note that shift by frequency isn't precise because precision isn't necessary, while it is very close to optimal value, from the point of view of the program certainly.

All these changes and additions are preliminary preparation of the module for the decision of the primary task: obtaining LS and LG values of concrete signal, within the limits of our ideas, which were described in the article Parameters of OFDM signals.

Some minor problems had been found out by users and solved by us.

The more detailed information of operation with OFDM signals in version 6.2.3.0 will be described in separate article.

Good luck!

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.