USE OF EQUALIZER IN OFDM

[prakash jat]

Hi everyone
what is the difference between below two concept
1. use of OFDM using pilot and training symbol in to mitigate ISI in optical communication
2. use of OFDM followed by mlse or electronic equalizer to mitigate ISI in optical communication

or will above two give same think ?

[alistu]

Hi Prakash,

Pilot signals are signals that are used for channel estimation. So if you are using pilot signals, then you should be using a channel estimation method at the receiver. In other words, the equalizer at the receiver makes use of pilot signals in order to estimate the channel in OFDM scheme.

Regards

[prakash jat]

hi alistu
thanks for reply
in terms of performance improvement which is better or equal

[alistu]

You’re welcome Prakash. As I pointed out, they cannot be used separately since they are both parts of the equalization scheme in OFDM. So they cannot be compared together. However, you can search between channel estimation methods (mmse, ml, and so on) to see which suits your purpose best.

Regards

[prakash jat]

how to use ( mmse , ml and so on ) in ofdm?
have you any idea about that
thanks for reply

[alistu]

If you are using OptiSystem 13 or a newer version than 13, the OFDM component already makes use of pilot symbols and channel estimation method. You just need to determine pilot symbols in OFDM components’ properties. If you are using an older OptiSystem version, you should use Matlab for this purpose.

[prakash jat]

thank you for reply
i am using optisystem 14, can i use equalization technique to further reduce error in advance modulation ofdm component?

[alistu]

You’re welcome. Since the OFDM component makes use of equalization techniques as I have mentioned in my comment, you would not need to use the estimation and equalization again. However, you can use dispersion compensating fiber in your channel to reduce ISI caused by dispersion if it is a serious issue in your system.

[prakash jat]

i fully satisfied with your reply
please tell me one think
what optimum way to reduce error in fso link

[alistu]

I believe there are a few things that can be done. the use of a power booster would be really helpful in many cases, if attenuation is an issue (especially in optical wireless communications). If you upload your implementation if it is possible for you, I would be able to provide better help.

Regards

[prakash jat]

hi alistu
i am uploading my file
why it is showing clear constellation while i am getting i percentage of evm?
what is the threshold level of evm for better communication
thanks for reply

[prakash jat]

……

[alistu]

Unfortunately I did not manage to open it since I am using an older version. But the clear constellation may be due to the fact that the new OFDM components have been used in the system implementation as I have explained in the following answer:

Constellation diagram

[prakash jat]

hi alistu
please explain in detail, i am not getting you point (as you said -> which of these main points each constellation point is closest to)

[alistu]

By main points, I mean the ideal constellation points (which can be seen in the transmitter before OFDM modulator using a constellation visualizer). For 4-QAM, these points are 1+j,1-j,-1+j, and -1-j. Now if at the receiver a point was received as -0.5-j1.5, the OFDM demodulator decides it is -1-j and therefore, shows it as -1-j. This is what I meant by “deciding the place of each point based on which of these main points each constellation point is closest to”. Let me know if this explanation helps.

Regards

[prakash jat]

thank you so much alistu , i got it

[alistu]

You’re welcome.

Cheers!

[prakash jat]

cheers

[Ranjeet Kumar]

Equalizer is used in reciever end to compensate the delay caused by dispersion while Pilot signals are signals that are used for channel estimation.

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