# Using Pilot tones to estimate carrier frequency offset in ofdm

I have been studying carrier frequency offset estimation techniques in OFDM systems. I have understood several of them. when using Pilot tones, as stated in paper (1), the process involves two methods namely: acquisition and tracking modes. a large range CFO is estimated in the acquisition mode while in the tracking mode, only fine CFO is estimated. I have confusion, What are the nature of those Pilot tones? how are they being generated? the authors of the paper wrote equation (18) without any derivation? can I get the system model for this technique? the questions may sound easy, but I really need it. Thanks in advance.

Reference: (1) Praween Kumar Nishad, P. Singh, "Carrier Frequency Offset Estimation in OFDM Systems", National Institute of technology http://dspace.nitrkl.ac.in/dspace/bitstream/2080/1928/1/ID-451_praween.pdf

• There is a citation in the paper for eq. 18. Have you looked at the cited paper? – jan Mar 22 '14 at 12:13

I am familiar with 802.11a, so I'll answer your questions with regard to how 802.11a does it.

What are the nature of those Pilot tones?


There are 64 sub-carriers in 802.11a. Four of them are used as the pilot channels. The pilots themselves are simply known data- if I remember correctly they are either (1 + i) or -(1 + i).

how are they being generated?


The transmitter just puts the appropriate data into the pilot sub-channels and then ifft's the entire symbol (i.e. with all of the other sub-channels).

the authors of the paper wrote equation (18) without any derivation?


This isn't really a question.

can I get the system model for this technique?


What do you mean by a system model? We don't normally provide code.

• About the equation, I was just hoping that you may illuminate me the equation and how it works. and just forget about the system model. Thanks – Frah Mar 23 '14 at 7:03

I may add that the tones, are exactly that, a sinusoid with no data. Its received amplitude and possibly phase as compared to what was sent is then used by the receiver to "estimate" the channel through which the signal has traveled, resulting in better SNR for decoding.