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An opportunistic spectrum sharing scheme for a pair of secondary users co-existing with a pair of primary users is proposed in this letter. The primary user is considered to exploit the limited feedback of channel quality information (CQI) to allocate its transmission power and rate. By overhearing the limited feedback of the primary CQI, the secondary user accesses the channel with proper transmission power and rate while causing interference to the primary user. Under the primary rate loss constraint, the optimal transmit power and transmission rate for the secondary is obtained to maximize the secondary user throughput. Numerical results show that, with only 3 or 4 bits of feedback, the effective throughput of the secondary user is comparable to the case when both primary and secondary transmitters have the perfect CQI of the primary link.