Abstract: Traditional cognitive radio employs Opportunistic Spectrum Access (OSA), the quality of cognitive transmission could not be guaranteed. By exploiting the spatial signal processing ability provided by multiple antennas, a Space Division Multiplexing based OSA (SDM-OSA) scheme for cognitive radio networks is proposed. When spectrum holes exist traditional OSA is employed. While there is no idle spectrum available cognitive system utilizes spatial information to implement transmit precoding and receive filtering, so that cognitive transmission is carried out using SDM. Antenna requirements in cognitive system are further discussed. When this demand is met, cognitive transmission could coexist with the primary (licensed) in an occupied authorized frequency channel with mutual interference eliminated. Compared with traditional cognitive radio which employs OSA to utilize the temporally spare frequency resource, the proposed scheme could effectively improve the performance of cognitive system and impose no interference on the primary.