Using Pilot tones to estimate carrier frequency offset in ofdm
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.