Sidelobe suppression for OFDM based cognitive radios in dynamic spectrum access networks

Abstract

As the demand for sophisticated wireless mobile applications incorporating efficient modulation techniques is ever increasing, more bandwidth is needed to support these applications. However, bandwidth is a limited resource. Also, as the existing spectrum allocation policies of the Federal Communications Commission (FCC) allow spectrum access to licensed users only, it has been proven by various spectrum measurement campaigns that, the current licensed spectrum usage across time and frequency is inefficient. Therefore, in order for the unlicensed users to access the unused portions of the licensed spectrum, the concept of "spectrum pooling" has been proposed.

Spectrum pooling is based on dynamic spectrum access (DSA), wherein the secondary user decides on whether or not a particular frequency band is currently being used and transmits the signal in that unused licensed band, while ensuring that the system performance of the primary as well as the secondary is not impacted. Thus, coexistence of the primary and the secondary users is an important criterion that makes DSA a feasible solution for efficient spectrum usage. This thesis investigates an important problem concerning the coexistence of the primary and the secondary users.

Orthogonal frequency division multiplexing (OFDM) has proven to be the prime candidate for spectrum pooling based wireless transmission systems as it can support high data rates and is robust to channel impairments. Even though the secondary transmissions help in improving the spectral efficiency by transmitting in the spectral white spaces left unused by the primary users, the large sidelobes resulting from the use of OFDM result in high out-of-band (OOB) radiation. Thus, the coexistence of the primary and the secondary users in the form of spectrum sharing is dependant on the suppression of the interference from the rental systems to the legacy systems. This thesis presents two novel techniques to suppress the OOB interference from the secondary user to the primary user, while not affecting the other system parameters of the secondary user by a great deal.