Select the correct statement(s) regarding direct sequence spread spectrum (DSSS) and orthogonal frequency division multiplexing (OFDM).
a. OFDM is not classified as a spread spectrum technique, although OFDM has the effect of spreading the signal over a larger frequency spectrum
b. OFDM has greater spectral efficiency compared to DSSS
c. DSSS relies upon a PN code that is only shared by the transmitter and receiver pair
d. all statements are correct