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In this paper, the transmission capacities of two coexisting wireless networks (primary network and secondary network) in the same geographic region and sharing the same spectrum are derived. The primary (PR) network has a higher priority to access the spectrum, while the secondary (SR) network limits its interference to the PR network by controlling its transmission intensity. Considering spread-spectrum transmission and general wireless channel with path-loss and small-scale Raleigh fading, we derive the transmission capacities for PR and SR networks, which incorporate no spreading as a special case. Extend our results in and to a more general case. Our results show that the sum transmission capacity of the two networks (i.e., the overall spectrum efficiency per unit area) is boosted significantly over that of a single network, and the transmission capacity gain is larger than that of no spreading case.