Multiuser diversity (MD), as a fundamental property of fading wireless networks, has been widely applied in today's wireless communication systems. In this letter, we consider the newly emerging cognitive radio (CR) wireless network that communicates by sharing the spectrum with the existing primary radio (PR) network. Under the assumption that the CR transmit terminal protects the PR transmission by applying the interference-power constraint at the PR receiver, this letter demonstrates several new forms of multiuser diversity in the spectrum sharing CR networks, named {\it multiuser interference diversity} (MID), which are resulted by the mutual interferences between the CR and PR networks and are thus not present in the conventional wireless networks. Three types of CR multiuser networks are considered, including the multiple-access channel (MAC), broadcast channel (BC), and parallel-access channel (PAC), for which this letter characterizes the transmit MID (T-MID), receive MID (R-MID), and combined T-MID and R-MID, respectively. It is shown that thanks to the new discovered MID, the MD in each CR network of interest can be substantially larger than that in the conventional networks, while their resultant diversity gain ratio is upper-bounded by some determinable value that is constant regardless of the number of CR users.
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