contiguous bands as well as the spectrum allocated to different standards. As noted, the depth of virtualization is different in these three frameworks and they are complementary to each other. From an implementation perspective, the flow-based virtualization is the most feasible approach with immediate benefits. It connects virtual resources and provides a more flexible and efficient traffic manage- ment. In all three cases, a flow-based virtualization is required to integrate the data. However, in the flow-based approach, the depth of virtualization is not sufficient for more advanced wireless communi- cation techniques, such as the coordinated multipoint transmission and reception [34, 40, 41]. As a potential enabler for future radio access network, wireless vir- tualization is gaining increasing attention. However, virtualization of wireless networks, especially efficient spectrum Virtualization, is far 5 By means of a wireless NIC, which is basically a Wi-Fi card, a computer workstation can be configured to act as an 802.11 access point. As a result, 802.11 virtualization techniques can be applied to the 802.11 wireless NIC. Virtualization of WLAN, known as VirtualWiFi (previously MultiNet [36, 37]), is a relatively old technology. It abstracts a single WLAN card as multiple virtual WLAN cards, each to connect to a different wireless network. Therefore, it allows a user to simultaneously connect his machine to multiple wireless networks using a single WLAN card. 2.4 Wireless Virtualization more complicated than that of a wired network. It faces some unique challenges that are not seen in wired networks and data centers. Virtualization of the wireless link is the biggest challenge in this domain [34, 42, 43]. Some other key issues in wireless virtualization are as follows: Isolation: Isolation is necessary to guarantee that each operator can make independent decision on their resources [44]. Also, since resources are shared in a virtualized environment, there must be effective techniques for ensuring that the