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This paper is concerned with spectrum sharing for wireless communication, where a secondary or cognitive radio (CR) link communicates over the same bandwidth that has been assigned to an existing primary radio (PR) link. It is assumed that the CR transmitter has perfect channel state information (CSI) on the channels from it to both the PR and CR receivers (as usually assumed in the literature), as well as the channel from the PR transmitter to PR receiver (a new assumption made). With known PR CSI, we study the optimal power control for the CR fading channel to maximize its ergodic capacity subject to the CR's transmit power constraint as well as the constraint on the maximum ergodic capacity loss of the PR link due to the CR transmission. It is shown that the new power-control policy performs better than the conventional one that does not depend on the primary user CSI, but is based on the average interference-power constraint at the PR receiver, in terms of both the PR and CR channel ergodic capacities.