  In this paper, we investigate the issue of spectrum assignment in CRNs and
examine various opportunistic spectrum access approaches proposed in the
literature. We provide insight into the efficiency of such approaches and their
ability to attain their design objectives. We discuss the factors that impact
the selection of the appropriate operating channel(s), including the important
interaction between the cognitive linkquality conditions and the time-varying
nature of PRNs. Protocols that consider such interaction are described. We
argue that using best quality channels does not achieve the maximum possible
throughput in CRNs (does not provide the best spectrum utilization). The impact
of guard bands on the design of opportunistic spectrum access protocols is also
investigated. Various complementary techniques and optimization methods are
underlined and discussed, including the utilization of variablewidth spectrum
assignment, resource virtualization, full-duplex capability, cross-layer
design, beamforming and MIMO technology, cooperative communication, network
coding, discontinuousOFDM technology, and software defined radios. Finally, we
highlight several directions for future research in this field.
