802.11n --modulation–> OFDM
802.11B --modulation–> DSSS
Why CWNA book and many of the other documents saying 802.11n is backward compatible with 802.11b???
All 802.11n radios must also support 802.11b modulations, so they are backward compatible (can send and receive older modulation as well).
As (de)modulator design uses quadrature mixer and all signals are generated/processed digitally (using FFT for OFDM), switching between modulations is done on-chip or in radio firmware, there is no difference in hardware design. Same for receive path, it just means there must be dual receiver that can handle both types of signal.