[Discuss-gnuradio] ofdm_frame_acquisition module

   in gr_ofdm_frame_acquisition.cc file ,the coarse frequency compensation is gr_expj(-2*PI*freq_delta*d_cplen/d_fft_length*symbol_count)
 .i don't know why ,could somebody give me an explanation or tell me which peper i can refer to ?
  thanks in advance