I got a suggestion from Mark, Optiwave support as follows: Find it worthy to share here
It’s basically a trial and error process (there is no set way to allocate the pilot tones). I suggest trying a small number of pilot tones and then increasing to see if the performance improves.
In terms of where you can place the pilot tones, these can only be on any of the allocated sub-carrier locations. So if you have sub-carriers from FFT points 25-104, the pilot symbols can be placed on any of the FFT points from 25 to 104 (for example 30, 50, 60, 80, 100)
I hope this helps