Where sk(t) is the transmitted signal for the kth information symbol ak. 1/(N)1/2 ensures symbol energy of
unity. Δ f=1/Ts (Ts is one OFDM symbol duration) to ensure orthogonality among carriers, and (2π /N) ki is
the phase offset used to generate the spreading code for ak which ensures the orthogonality among the N
information symbols. fc is the carrier frequency.
After transmitted over a frequency selective fading channel, the received signal at receiver side is