What is Pilot ? & What is synchronization in OFDM?

[mazdaspring]

I am reading OFDM and I came across a word "Pilot". What is it? and what is it for?

Also what is synchronization? and what is frequency synchronization for?

 

[ckshivaram]

Pilots are used to track the residual phase error if present after frequency correction. Without this correction the constellation points starts rotating either +ve/-ve angle. It is very much sensitive at higher constellation.
We estimates phase say Θ , from pilots, then we corrects by multiplying exp(-j*2*pi*Θ) before demapping........

Pilot sequences are the unmodulated data we are transmitting along with the data.Pilots are used for syncronization and Channel estimation purposes. Channel estimation is the main thing we need to concentrate for improving the channel capacity .
More the number of pilots more efficeintly we can estimate the channel and increase the capacity . But as pilots increases overhead increases. so the selection of pilots is such that tradeoff should be considered between pilots symbols and spectral efficiency. In IEEE 802.11 OFDM standard we are using 64 subcarreirs per OFDM symbol in these 64 , 48 are for data and 4 are Pilot and remaining are for zeropadding .

http://www.signal.uu.se/Publications/pdf/c0502.pdf

---------- Post added at 13:18 ---------- Previous post was at 13:17 ----------

check this discussion also
https://www.edaboard.com/threads/165874/

 

[mazdaspring]

Thank you so much for your reply. That's very clear. Would you mind give me a brief summary of what "frequency Synchronization" is? (something to do with OFDM)

Also I had a read through the thread you gave me. What is DC offset? There is a guy said something about the pilot ---> "Unused subcarriers in the middle is needed in order to avoid the problem of DC-offset in the analog quadrature demodulator."

Thank you so much for your help again. Very kind of you

 

[ckshivaram]

check this .. it maybe useful

**broken link removed**
**broken link removed**

 

[Ahmed Alaa]

Pilots are subcarriers of known data (known modulation symbols), those are used to detect the channel changes and thus used for equalization, frequency offset estimation and many other baseband algorithms.

Synchronization is determining the start (trigger point) of an OFDM symbol, it is also called "symbol timing".

Frequency synchronization, arises from the different frequencies of local oscillators of the transmitter and the receiver, this will destroy the OFDM demodulation, so we have to determine the difference between carrier frequencies to compensate for it. Many algorithms based on pilots are used to detect for the frequency offset.

 

[emeto]

Hallo ,

I am a student and I have to estimate the time delay of three OFDM frames sent from three cells (Matlab). My idea is first to find the integer delay during the synchronisation using the P-SYNC and S-SYNC pilots and after that to do a more precisely estimation in the frequency domain using the phase angle of the channel.
Can somebody helps me to understand the steps of the synchronisation with the pilot signals? it is urgent...

frequency offset = 0

RECEIVER:
1. cross-correlation between the received signal and a local replica of the P-SYNC and S-SYNC
2. find the max of the cross-correlation which is the integer delay
3. Fit FFT window (I am not sure how to do this part)

Thank you!