  Underlay cognitive radios (UCRs) allow a secondary user to enter a primary
user's spectrum through intelligent utilization of multiuser channel quality
information (CQI) and sharing of codebook. The aim of this work is to study
two-user Gaussian UCR systems by assuming the full or partial knowledge of
multiuser CQI. Key contribution of this work is motivated by the fact that the
full knowledge of multiuser CQI is not always available. We first establish a
location-aided UCR model where the secondary user is assumed to have partial
CQI about the secondary-transmitter to primary-receiver link as well as full
CQI about the other links. Then, new UCR approaches are proposed and carefully
analyzed in terms of the secondary user's achievable rate, denoted by $C_2$,
the capacity penalty to primary user, denoted by $\Delta C_1$, and capacity
outage probability. Numerical examples are provided to visually compare the
performance of UCRs with full knowledge of multiuser CQI and the proposed
approaches with partial knowledge of multiuser CQI.
