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Cognitive radio technology is widely accepted as an efficient approach to solve the problem that the wireless spectrum has been getting scarcer and scarcer due to the rapid growth in the ubiquitous wireless applications. Several cognitive medium access control (MAC) protocols have been proposed for the secondary users (non-licensed users) to take advantage of the vacant channels whenever they are not occupied by the primary users (licensed users) in the wireless time division multiple access (TDMA)-based networks. In this paper, by exploiting the unique property of the wireless fading channel and cross-layer design technique, we develop a packets scheduling scheme for the primary users in the context of wireless TDMA networks, which is set up to operate friendly towards the secondary users in terms of vacant-channel probability. Our proposed scheme can be implemented with just slight modification on the traditional TDMA scheduling algorithm. We develop a rigorous queuing model and then quantitatively analyze the tradeoff among multiple performance metrics to identify when and where the cost for favoring the secondary users is worthy. The analytical results show that our proposed scheme can generate more vacant-channel opportunities for secondary users, at the expense of little increasing packet delay, as compared with the traditional wireless TDMA scheduling algorithm. In addition, since the implementation of our proposed scheme only needs little modification on the existing TDMA scheduling algorithm, our proposed scheme is a practical and cost-effective approach to increase the wireless spectrum utilization.