Cooperative communications in a spectrum sharing environment

Abstract

Demands for wider frequency spectra have been dramatically increasing due to new wireless applications and increased traffic loads, while radio frequency is a very limited resource and the spectrum utilization is generally very low due to exclusive, single-purpose use of primary licensees. Some measurement reports clearly indicated that some portion of the allocated spectrum is never accessed or is accessed for only a fraction of time in a certain area. In order to mitigate this spectrum shortage problem and to improve low spectrum utilization, the spectrum policy task force (SPTF) of the federal communications commission (FCC) in U.S. proposed a concept of spectrum sharing. Based on this spectrum sharing concept, secondary users can share spectrumbands already allocated to primary users as long as the primary users` reliable communications are guaranteed. Generally, the quality of primary user`s communications is degraded by the interference power from secondary users sharing the primary users` spectrum. Thus, the SPTF introduced a concept of interference regulation in order to limit the interference power from secondary users. There are two different approaches to limit the interference from secondary users to primary users: overlay and underlay models. An overlay spectrum sharing model allows secondary users to share spectrum bands licensed to primary users only when the primary users do not occupy them. On the other hand, in an underlay spectrum sharing model, secondary users can always share the primary users` spectrum bands as long as primary users` reliable communication is secured. Thus, secondary users` transmit power should be strictly regulated in order to guarantee reliable operation of primary users. In this dissertation, we focus on the underlay model because the underlay model is more general and can achieve higher spectral efficiency than the overlay model if some practical problems are cleared. In the underlay spectrum sharing model, the second...