present an XaaS taxonomy, discuss the enabling technologies and challenges.

Radio-Access-Network-as-a-Service (RANaaS)
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First and foremost, no matter how the network will evaluate, it is still a radio-based network, where the wireless infrastructure and resources are the basis for all kinds of network operations. Therefore, the fundamental of a service-oriented wireless system should be able to offer Radio Access-Network-as-a-Service (RANaaS) [@Sabella2]. It can be noticed that the current proposals usually consider RANaaS is one of the products of cloud-based networks, i.e., cloud-RAN, where all the RAN functionalities are centrally operated. In this part, we revisit the concept of RANaaS and further decouple the RANaaS to different categories, i.e., Infrastructure-as-a-Service (IaaS), Radio-Resources-as-a-Service (RaaS) and Radio-Access-as-a-service (RAaaS), where the hardware, software and resource can be treated separately and properly towards a flexible and programmable 5G.

### Infrastructure-as-a-Service (IaaS)

Unlike the current BS-dominated 4G system or behind, it is expected that the 5G infrastructure concept will be significantly enriched, due to the massive deployment of different types of BSs and their ultra-density [@Chang1], data caching entities, computing centers, machines, sensors and energy harvesting units. The explosive growth of these network elements can also consequently increase the demands for supporting hardware, such as backhaul, 