unable to get good BER in 16-QAM OFDM system

[Mohamed Ben zegalam]

Dear all

I tried to build 16-QAM CO-ODFM point to pint using OFDM OS12 component as per the attached file (project-4.0), the BER set test showing bad BER and the constellation showing . I tried to adjust the cut off frequency in the optical and electrical filters but it did not improve.
also I would like to know what is the different between the three OFDM mod/demod components. and which one is the best for OFDM PON network.

thanks in advance

[alistu]

Hi Mohamed,

The system is working OK. To compensate for the symbol rotation, you can give initial phase to the receiver CW laser in coherent detection block. Please see the following answer(I suggest that you use OFDM 13 components which use channel estimation to avoid the symbol rotation).:

coherent ofdm

To see the answer to you other question, please see the following link:

OFDM-PON

Regards

[–]

Hi Mohamed Ben, OS12 blocks are incapable of sending pilot and training symbols. But OFDM block which was added in Optisystem 13 version is capable of sending pilots and training symbols. So, avoid using OS12 blocks if you need channel estimation in your design.
There are two OS12 blocks. In the block named OFDM Modulator (OS12), you cannot specify the subcarrier locations, perhaps. Whereas, in OFDM Modulator Measured (OS12) block you can specify the location of subcarriers. You can specify which subcarrier is to be null. This can be done in notepad and specifying the path in the OS12 block where the .txt file is located.
Regards.

[Mohamed Ben zegalam]

Hi Alistu, thanks for your reply. when I give initial phase to the receiver laser in the coherent detection, do I have to put the same phase in LO laser at the transmitter. and if not what is the reason behind adding the initial phase shift.

thanks

[alistu]

Hi Mohamed,

You should not apply the initial phase to the first laser, since the difference of the phases is what that matters. You can find more about this in the following topic that I have started for this issue a while ago and you can ask any further questions here:

Symbol Rotation in OFDM

Regards

[Mohamed Ben zegalam]

Hi Alistu,

I tried to set the initial phase in the CW laser at the receiver as sweeps iteration from 0 to 180 deg in 30 iterations. but the BER still pad. did you test my project with the specific initial phase and you get the good result.

thanks

[alistu]

Hi Mohamed,

I had not checked this on your system before. Even though I managed to get the right constellation for some random phases (which you must have obtained if you have used sweep iterations) I couldn’t find any logical basis for this. I have never encountered such a problem and I hope Optiwave helps you with this.

Regards

[Mohamed Ben zegalam]

Hi Alistu,

thanks for your reply, I already used sweep iteration for the initial phase and there isn’t any improvement.
the system work fine when I used 4-QAM, but it is not working when I change it to 16-QAM. I am wondering if there are some global parameters I have to change them when I change from 4-QAM to 16-QAM, also do I have to change the laser power or not.

kind regards

[alistu]

The reason for stating iteration sweep was that I actually managed to get good constellations using random initial phases, although I don’t consider this a solution.

According to the bits per symbol parameter in modulation scheme and the bit rate, the symbol rate should be changed (my experience says as long as you haven’t used symbol rate in script mode, it has no effect on simulation, but you’d better do it anyway).

Since the constellation points are closer 16-QAM, decision region is smaller and therefore more power is needed, but none of this have to do with your problem (symbol rotation in the way you are facing) in my opinion.

Regards

[Author]

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