Capacity of Secondary Users Exploiting Multispectrum and Multiuser Diversity in Spectrum-Sharing Environments

In spectrum-sharing environments, secondary users are permitted to share the primary user´s spectrum only if limited interference to the primary user can be guaranteed. Hence, the capacity of the secondary link is limited by interference constraints given by the primary user. This fact motivated us to investigate selection diversity as a way of enhancing secondary link capacity. Selection diversity in conventional licensed wireless communication systems can be achieved only by selecting the user with the strongest channel gain. On the other hand, spectrum-sharing environments allow the secondary user to select not only the best secondary receiver with the strongest channel gain multiuser diversity (MUD) but the best primary spectrum with the weakest interference link gain multispectrum diversity (MSD) as well, thus doubling the opportunities for the secondary user. In this paper, we analyze the capacity gain of a secondary user exploiting MSD and MUD in spectrum-sharing environments in the form of a closed-form expression in a Rayleigh-fading channel. We then separately extract the MSD and MUD capacity gains, which results in an asymptotic capacity expression. Our results show that MSD and MUD play different roles in capacity enhancement in the spectrum-sharing environment, with capacity enhancement for secondary receivers as a whole and transmit capacity enhancement for the secondary transmitter.