OFDM pilot subcarriers

[thavamaran]

Hi guys, I am designing my OFDM system, actually have designed, I am using 128 subcarriers, 72 of it carriers my data, and 56 are zero padded, 27 subcarriers of zeros at the beginning which are at DC, then 36 subcarries of data, then 2 subcarries of zeros in the middle, then again with 36 subcarriers of data, at the end another 27 subcarries with zeros.

So 27 beginning, 2 in the middle and 27 at the end (Nyquist frequency). Correct me if I padded it wrongly.

Then now I am having problem at the end of my constellation, I have a phase shift because Im using a nonlinear laser model in between the channel, so to do the channel estimation now, I need to add Pilot symbol. I went through a few books and papers, but I still dont get the idea of Pilot symbol.

Can anyone rigorously explain to me why pilot tone, how to include them in subcarrier and a bit of explanation on that, please, thank you very much.

 

[Tom Duke]

A pilot symbol is a symbol which all or a part of its subcarriers are previously known. they are used to perform channel estimation. In order to include them in your design you can add them periodically. It will be very helpful to understand the meaning of the "preamble", then you will fully understand them.

 

[thavamaran]

Hi Duke, thanks a lot. Can I also know how to get real-valued output out of IFFT? It says that you data must be conjugate-symmetric, I dont get this concept.

 

[Tom Duke]

In general,the IFFT values are coplex values. In order to get the real part of it you can use the real() command from Matlab (I guess your design is in Matlab or a similar EDA tool).
Unfortunatelly I do not fully understand your question about conjugate symmetric. I guess that you probbly want this : Since that the IFFT has as inputs frequencies, you need to describe your input as a symmetric signal around the zero frequency (DC) before you feed it to IFFT.

 

[thavamaran]

YES!! exactly, you got the point, I have to arrange my data in terms of hermitian symmetry to meet the requirement to achieve real value at the output of IFFT. I have been trying that for so many times and its going crazy!! Please advice on this.

What I did was, I generated 36 subcarriers with data, and then did a complex conjugate of that and placed them together, 1 until 36 subcarries is the data and 37 until 72 is complex conjugate of the data, then zero pad it in the beginning, in between the data and its conjugate and at the end and make it 128 subcarries, 128 is N^2 and meets the requirement of FFT algorithm. Then I performed IFFT, yet its crazy!!