Wireless spectrum is typically considered as a scarce resource, and thus is often allocated through static licensing. Field measurements show that, however, most spectrum bands are often under-utilized even in densely populated urban areas. To resolve this dilemma, people have proposed using the cognitive radio technology, which enables radio devices or networks to flexibility change the operational parameters to match the radio environment. The devices or networks can then efficiently sense and utilize the otherwise wasted “spectrum holes” in time and space without violating the rights of the licensed users.

We have been studying a wide range of models in this research area, focusing on optimization of network performance and analysis of new network architecture.

• Open Access: all users are allowed to share the spectrum with equal rights. Example: the industrial, scientific and medical (ISM) radio bands.

• Spectrum underlay: unlicensed users are allowed to transmit concurrently with the licensed users as long as the interference generated is sufficiently low. Example: Ultra Wide Band or spread spectrum transmission techniques with an interference temperature limit.

• Spectrum overlay: unlicensed users are allowed to transmit at the time, location, and frequency bands where no licensed users are active. Examples: unlicensed transmissions in the 5 GHz RADAR bands and 400 MHz -900 MHz TV Bands.

• Dynamic exclusive use: the spectrum owner (temporally) transfers the spectrum usage rights to unlicensed users in exchange of monetary return or performance improvement of the licensed users.

• Hybrid model: a combination of two or more of the above models.