Underlay cognitive radios (UCRs) allow a secondary user to enter a primary user�s spectrum through intelligent utilization of\r\nmultiuser channel quality information (CQI) and sharing of codebook. The aim of this work is to study two-user Gaussian UCR\r\nsystems by assuming the full or partial knowledge of multiuser CQI. Key contribution of this work is motivated by the fact that\r\nthe full knowledge of multiuser CQI is not always available. We first establish a location-aided UCR model where the secondary\r\nuser is assumed to have partial CQI about the secondary-transmitter to primary-receiver link as well as full CQI about the other\r\nlinks. Then, new UCR approaches are proposed and carefully analyzed in terms of the secondary user�s achievable rate, denoted\r\nby C2, the capacity penalty to primary user, denoted by ?C1, and capacity outage probability. Numerical examples are provided\r\nto visually compare the performance of UCRs with full knowledge of multiuser CQI and the proposed approaches with partial\r\nknowledge of multiuser CQI.
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