constellation diagram for QAM 16 OFDM system

[Priti Garg]

Hi
I am designing a QAM 16 based OFDM system using bidirectional fiber. if I use 1 or 2 sanmples get the expected constellation diagram if i take 4 samples or higher i get an unexpected constellation diagram. I am attaching the screen shots. please help me as to what change i should do to get a proper constellation diagram.
Regards

[Priti Garg]

I am attaching the global parameters used as i was not able to attach more than one file. please help me find my mistake.

[Abhilash Achanta]

Hi,

Even I am simulating OFDM along with different modulation schemes.

There are two things that you can do:
1. Go to Project browser->OFDM demodulation component->Graphs->Contellation diagram

OR

2. How many subcarriers(S) and total carriers(T) are you using?
Go to the properties of constellation diagram visualizer and replace bitrate with bitrate*S/T.

[Priti Garg]

Hi Abhilash
Thanks for your reply.
1). I want to know why i do not get the perfect QAM 16 constellation diagram, why i am getting the noise?

2). If i take 2 samples per bit i get the perfect constellation diagram but any no greater than 2 gives the diagram like I attached earlier

Please help.

[Abhilash Achanta]

I don’t think the constellation diagram that you are seeing is due to noise. This is because the constellation looks orderly.
Did you try checking the constellation in OFDM demodulation component in project browser ?

If you are still facing same problem. Please send the simulation file so that I can take a look at it.

[Priti Garg]

hi Abhilash

thanks for your reply.
i receive the same constellation as the one I send
As you said it is orderly and exactly same as one i send.
but at the QAM sequence generator i dont see the perfect constellation as expected for QAM 16

[Abhilash Achanta]

Did check the graphs in OFDM demodulation component?

[Dr. Dhiman Kakati]

Hi Preeti,
As per the constellation is concerned, I don’t think the received power is sufficient for detection as there is no noise (represented by blue dots). So, I would suggest you to measure the optical power after the OFDM demodulator using optical power meter. There must be very low power level for detection. Moreover for higher order QAM the SNR requirement is higher.

Regards,
Dhiman

[Priti Garg]

Hi Dhiman
1. please guide me on the range for power so that it can be detected at the receiver.
as pointed by you I get very low power at receiver, it is -58 dBm at input to coherent receiver.

2. Also in a bidirectional fiber link should i try to boost power using amplifier or will it further degrade BER?

3. Which kind of amplifier should i use?

Regards

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