  Multiband spectrum access presents the next generation of cognitive radio
networks (CRNs), where multiple bands are sensed and accessed to enhance the
network's throughput, improve spectrum's maintenance, and reduce handoff
frequency and data transmission interruptions due to the activities of the
primary users. In this paper, we discuss the challenges and limitations of the
major multiband spectrum sensing techniques. Particularly, we highlight the
edge-detection problem and examine several issues of the state-of-the-art
wavelet-based techniques. We also study the compressive sensing problem.
Finally, we highlight the promises of utilizing the angle-domain for the CRNs.
