multiple RATs [@Liang]. Moreover, as the powerful computing units are becoming indiscerptible in communications systems, virtualization of the computing resources is an emerging option to efficiently utilize computing units in the wireless networks [@Wen].

Everything-as-a-Service via Virtualization
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![Concept of XaaS[]{data-label="fig:xaas"}](xaasfigure2.pdf){height="10cm" width="18cm"}

An example of XaaS is presented in Fig. \[fig:xaas\], where different types of cells, such as picocell, microcell, femtocell, and many other types of access points will be deployed and other advances, such as cloud-RAN and energy harvesting units, will be merged and utilized in the 5G physical substrate wireless networks. The densitification of heterogeneous wireless networks, together with WNV, can breakthrough the traditional obstacles on the infrastructure and radio resources towards an efficient network management and operation. By such, naturally, our vision is that the network architecture will be purely service-oriented: every component, not only BSs and spectrum, but also data, knowledge, computing units, energy, and security, can be viewed as a type of service that can be provided to whoever needs them and whenever they are needed. For several years now, in the cloud computing, researchers have been proposing and discussing many models for defining anything “as-a-service (aaS)” [@Duan]. Extracted from such a concept, we then 