With the proliferation of radio services in the past decades, most of the spectrum available below 2GHz has been allocated, resulting in the spectrum scarcity problem. However, many studies have revealed that vast portions of the licensed spectra are rarely used. This triggered the concept of cognitive radio (CR) [1], [2], [3], where secondary users are allowed to ‘borrow’ parts of the licensed spectra by sensing and opportunistically accessing spectral holes in temporal, frequency, and spatial domains. However, to avoid adjacent-channel interference (ACI), the out-of-band (OOB) radiation of secondary users must be strictly suppressed. As specified by FCC, for example, the adjacent-channel leakage power ratio (ACLR) should no higher than −72.8dB when the television white space (TVWS) are opportunistically utilized.