  This paper investigates the problem of spectrally efficient operation of a
multiuser uplink cognitive radio system in the presence of a single primary
link. The secondary system applies opportunistic interference cancelation (OIC)
and decode the primary signal when such an opportunity is created. We derive
the achievable rate in the secondary system when OIC is used. This scheme has a
practical significance, since it enables rate adaptation without requiring any
action from the primary system. The \emph{exact} expressions for outage
probability of the primary user are derived, when the primary system is exposed
to interference from secondary users. Moreover, approximated formulas and tight
lower and upper bounds for the ergodic sum-rate capacity of the secondary
network are found. Next, the power allocation is investigated in the secondary
system for maximizing the sum-rate under an outage constraint at the primary
system. We formulate the power optimization problem in various scenarios
depending on the availability of channel state information and the type of
power constraints, and propose a set of simple solutions. Finally, the
analytical results are confirmed by simulations, indicating both the accuracy
of the analysis, and the fact that the spectral-efficient, low-complexity,
flexible, and high-performing cognitive radio can be designed based on the
proposed schemes.
