A method of data transmission over guard sub-carriers is provided in a multi-carrier OFDM system. Adjacent radio frequency (RF) carriers are used to carry radio signals transmitted through adjacent frequency channels. A plurality of guard sub-carriers between adjacent frequency channels are aligned and identified for data transmission in a pre-defined physical resource unit. The identified guard sub-carriers do not overlap with normal data sub-carriers of the radio signals transmitted through the adjacent frequency channels. At least one of the identified guard sub-carriers is reserved as NULL sub-carrier. A flexible multi-carrier transceiver architecture is also provided in a multi-carrier OFDM system. Different multi-carrier and/or MIMO/SISO data transmission schemes are implemented by adaptively reconfigure same hardware modules including common MAC layer module, physical layer entities, and RF entities. Furthermore, the flexible multi-carrier transceiver architecture can be used to support data transmission over guard sub-carriers.