Rapid convergence of heterogeneous wireless communication technologies such as Wireless Local Area Networks (WLAN), Worldwide Interoperability for Microwave Access (WiMAX), Long Term Evolution (LTE) etc., attract new opportunities for collaborative usage. More and more applications are emerging to benefit from their advantages. However, with the range of approaches that are used to authenticate the wireless devices in such heterogeneous environments, users are skeptical and looking for more user friendly, flexible and reliable ways to interconnect and utilize these different classes of wireless networks. Wireless network users access the different types of wireless networks either independently or cooperatively. In either case, adequate security provision is critical for the successful operation of the networks. Moreover, emerging technologies should provide seamless transition / migration between these networks. Hence the ability to use a single but unique set of credentials to authenticate the wireless devices in heterogeneous wireless network environments would be an anticipated desire of most users. In this paper a number of authentication mechanisms are examined and evaluated for their advantages and limitations. We then propose a unified authentication protocol that can be encapsulated within the RADIUS protocol utilizing the advantages of public key infrastructure. The preliminary experimental results demonstrate that the proposed protocol is feasible and relatively fast.