  In spectrum-sharing networks, primary users have the right to preempt
secondary users, which significantly degrades the performance of underlying
secondary users. In this paper, we use backup channels to provide reliability
guarantees for secondary users. In particular, we study the optimal white
channel assignment that minimizes the amount of recovery capacity (i.e.,
bandwidth of backup channels) needed to meet a given reliability guarantee.
This problem is shown to be coupled by two NP-hard objectives. We characterize
the structure of the optimal assignment and develop bi-criteria approximation
algorithms. Moreover, we investigate the scaling of the recovery capacity as
the network size becomes large. It is shown that the recovery capacity is
negligible as compared to the total traffic demands in a large-scale network.
