In a cognitive radio network (CRN), spectrum opportunities should be efficiently utilized through careful coordination between cognitive radio (CR) users. In this paper, we formulate the coordinated channel access as a joint power/rate control and channel assignment optimization problem, with the objective of maximizing the sum-rate achieved by all CRs over all channels. The problem is formulated under a generalized multi-level spectrum opportunity framework, which reflects the microscopic spatial opportunity available to CRs. A centralized polynomial-time approximate algorithm to the problem is developed. We prove the algorithm's correctness and show its accuracy through numerical examples.