Hi Ranjeet,
Your example is correct, and it is the same as my explanation. because
OFDM symbol period= 1/frequency spacing (dalta F).
so if you take frequency spacing (dalta F)= Bit rate/(log2(M)*Nsc), then
OFDM symbol period= (log2(M)*Nsc)/Bit rate, as per your example
OFDM symbol period= (2*128)/40Gbps
Kind regards
Mohamed