Wireless virtualization for next generation mobile cellular networks

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WIRELESS VIRTUALIZATION FOR NEXT GENERATION MOBILE CELLULAR NETWORKS CHENGCHAO LIANG AND F. RICHARD YU

ABSTRACT With wireless virtualization, the overall expenses of mobile cellular network deployment and operation can be significantly reduced by abstracting and sharing infrastructure and radio spectrum resources. Moreover, wireless virtualization can provide easier migration to newer products or technologies by isolating part of the network. Despite the potential vision of wireless virtualization, several significant research challenges remain. In this article, we provide a brief survey on some of the work that has been done on wireless virtualization in cellular networks. We also present the motivations and business models of wireless virtualization. Furthermore, we present a framework that enables wireless virtualization. In addition, we discuss a number of challenges that need to be addressed for the deployment of wireless virtualization in next generation mobile cellular networks.

INTRODUCTION

The authors are with Carleton University.

In wired networks, virtualization has occurred for decades. Two early forms of virtualization are virtual private networks (VPNs) over wide area networks (WANs) and virtual local area networks (VLANs) in enterprise networks. Recently, network virtualization has been actively used in Internet research testbeds, such as 4WARD [1]. It aims to overcome the resistance of the current Internet to fundamental architecture changes. Network virtualization has been considered one of the most promising technologies for the future Internet [2]. With the tremendous growth of traffic and services in cellular networks, it will be necessary and beneficial to extend virtualization to cellular networks. Using wireless virtualization, cellular network infrastructure can be decoupled from the services it provides so that differentiated services can share the same infrastructure, maximizing its utilization [3]. Consequently, the capital expenses (CapEx) and operation expenses (OpEx) of radio access networks (RANs), as well as core networks (CNs), can be significantly reduced. Meanwhile, wireless virtualization provides easier migration to newer products or technologies while supporting legacy products by isolating part of the network[4].

IEEE Wireless Communications • February 2015

Although wireless virtualization is a promising technology for next generation cellular networks, many significant research challenges remain to be addressed before the widespread deployment of wireless virtualization in mobile cellular networks. These research challenges include isolation, control signaling, resource discovery and allocation, mobility management, network management and operation, and security, as well as non-technical issues such as government regulations. Particularly, different from wired networks, where bandwidth resource abstraction and isolation can be done on a hardware (e.g., port and link) basis, radio resource abstraction and isolation are not straightforward, due to the inherent broadcast nature of wireless communications and stochastic fluctuation of wireless channel quality. Another significant challenge of wireless network virtualization is resource allocation, which decides how to embed a virtual wireless cellular network on physical networks. In addition, a large number of intelligent devices/nodes with self-adaptation/context awareness capabilities induce nontrivial security challenges to wireless network virtualization. These challenges need to be studied carefully by comprehensive research efforts. In this article, we present a brief survey on some of the work that has been done to achieve wireless virtualization in cellular networks. We also present the motivations for and business models of wireless virtualization. Moreover, we present a framework that enables wireless virtualization in cellular networks. In addition, we discuss a number of challenges that need to be addressed for the deployment of wireless virtualization in next generation mobile cellular networks. The rest of this article is organized as follows. The next section presents the business models and motivations of wireless virtualization. A framework of wireless virtualization is then presented. Following that, we discuss some research challenges of wireless virtualization. We conclude this study in the final section.

OVERVIEW OF WIRELESS VIRTUALIZATION In this section, we discuss the business models and motivations of wireless virtualization in cellular networks. Current projects on wireless virtualization are also introduced.

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Figure 1. Business models of wireless network virtualization: a) a two-level model; b) a three-level model. SP: service provider; MNO: mobile network operator; MVNO: mobile virtual network operator; InP: infrastructure provider.

DEFINITION OF WIRELESS NETWORK VIRTUALIZATION Similar to wired network virtualization, wireless virtualization needs physical resources to be abstracted and isolated to a number of virtual resources, which then can be offered to different service providers. Nevertheless, the distinctive properties of the wireless environment make the problem more complicated. In this article, we consider wireless virtualization in cellular networks as the technologies in which physical cellular network infrastructure resources and physical radio resources can be abstracted and sliced into virtual cellular network resources holding certain corresponding functionalities, and shared by multiple parties through isolating each other. In other words, virtualizing mobile cellular networks is to realize the process of abstracting, slicing, isolating, and sharing mobile cellular networks. Generally speaking, the physical resources in cellular networks comprise licensed spectrum resource and infrastructure resources, including RANs, CNs, and transport networks.

BUSINESS MODELS As shown in Fig. 1a, two logical roles can be identified after virtualization: mobile network operator (MNO) and service provider (SP). MNOs own and operate infrastructures and radio resources of physical substrate wireless networks, including licensed spectrum, RANs, backhaul, transmission networks, and CNs. MNOs execute the virtualization of the physical substrate networks into some virtual mobile network resources. For brevity, we use virtual resources to indicate the virtual mobile network resources. SPs lease, operate, and program these virtual resources to offer end-to-end services to mobile users. The roles in the business model can be further decoupled into more specialized roles, including SP, infrastructure provider (InP), and mobile virtual network operator (MVNO) [5], as shown in Fig. 1b. The functions of them in this model are described as follows:

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• SP: concentrates on providing services to its subscribers based on the virtual resources provided by MVNOs. • InP: owns the physical cellular network infrastructure resources and physical radio resources. In some special cases, the physical radio resources may not be owned by InP. • MVNO: leases the network resources from InP, creates virtual resources based on the requests from SPs, and operates the virtual resources and assigns them to SPs. The rise of MVNOs breaks the value chain dominated by traditional MNOs [6]. According to different levels of control on mobile network resources, there are several types of MVNOs in the current market [6], which are described in the following.

Resellers — This model is the classic model of MVNOs, where MVNOs do not own any infrastructure of mobile networks, but resell subscriptions from MNOs to end users. In this case, MVNOs are dependent of MNOs in terms of running networks and providing services. One example is Tesco Mobile in the United Kingdom. Service Providers — In this model, MVNOs still do not have any substrate mobile network. However, unlike resellers, they have their own authentication centers (e.g., home location register), billing systems, and mobile network codes (used to uniquely identify a mobile phone operator), so they are able to get some independence from MNOs and provide differentiated services to end users. Virgin Mobile, one of the biggest MVNOs in the world, is an example of this model. Full MVNOs — Full MVNOs own all the elements of networks except the RANs and spectrum resources. In the current market, compared to other MVNO models, full MVNOs have the highest bargaining power with MNOs, since they are able to independently operate without MNOs at CNs and transport networks. MVNOs can design different services, pricing policies, switching strategies, and so on. Tele2 is a typical full MVNO operating in Europe. From resellers to full MVNOs, MVNOs are changing, and getting more and more independent from MNOs. Nevertheless, MNOs still dominate the market, because the current MVNOs have little control over the RANs (including spectrum), which are the most important and pivotal part of cellular networks. With wireless network virtualization, future MVNOs can lease and operate mobile network resources from InPs, and they have the power to access all the parts of cellular networks including RANs. Moreover, MVNOs can deploy customized wireless protocols and create more flexible virtual networks. The above business models can be summarized using the emerging concept of X as a service (XaaS) in cloud computing. Infrastructure as a service (IaaS) is provided by InPs; network as a service (NaaS) is operated by MVNOs. Moreover, SPs can provide software as a service (SaaS) or cloud as a service (CaaS).



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Figure 2. An example framework of mobile cellular network virtualization. Here, the physical mobile cellular substrate network is provided by an MNO or InP. Through the cellular network controller, the substrate network is virtualized into virtual resources, so SPs can lease and manage them to provide specific services to end users.

BENEFITS OF WIRELESS NETWORK VIRTUALIZATION In commercial markets, CapEx and OpEx can be lowered significantly due to the sharing enabled by wireless network virtualization. The authors of [7] estimate that up to 40 percent of US$60 billion in OpEx and CapEx can be saved by operators worldwide over a five-year period. Over the past years, MVNOs and over-the-top (OTT) SPs have become strong players in mobile network markets and brought their featured services to impact the ecotope of the traditional market dominated by MNOs. Fortunately, wireless network virtualization brings a win-win situation for both MVNOs and MNOs [4]. MVNOs or other types of SPs can lease virtual networks from MNOs, and MNOs can attract greater numbers of customers from MVNOs and SPs. For MNOs themselves, since the network can be isolated into several slices, any upgrading and maintenance in one slice will not affect other running services. For SPs, leasing virtual networks helps them “get rid of” the control of MNOs, so customized and more flexible services can be provided more easily, and quality of service (QoS) can be enhanced as well. This also brings revenues to MNOs, because SPs need to pay MNOs for the leased virtual networks.


PROJECTS ON WIRELESS NETWORK VIRTUALIZATION Recently, several research projects have been started around the world in the area of wireless network virtualization, including Global Environment for Network Innovations (GENI) in the United States, Smart Applications on Virtual Infrastructure (SAVI) in Canada, Virtualized Distributed Platforms of Smart Objects (VITRO) in Europe, and so on. Software defined networking (SDN) is an approach to computer networking where network control is decoupled from forwarding functions [8]. SDN is considered as one of the most promising technologies to realize virtual networks, especially in network control. OpenFlow is a standard communications interface defined between the control and forwarding layers of an SDN architecture.

A FRAMEWORK OF MOBILE CELLULAR NETWORK VIRTUALIZATION In this section, a framework is presented for wireless virtualization with four main components: radio spectrum resource, mobile network

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Figure 3. The paradigms of network sharing. In spectrum sharing, the licensed spectra can be shared by multiple operators. In infrastructure sharing, the infrastructures can be shared. In full network sharing, both spectra and infrastructures can be shared.

infrastructure, mobile virtual resource, and mobile virtualization controller. Moreover, network sharing that can enhance virtualization in cellular networks is also introduced. In addition, we present some works that have been done on wireless virtualization in Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) systems. An example framework of wireless virtualization is shown in Fig. 2. In this example, there are three SPs (a video SP, a game SP, and a voice MVNO) that request virtual networks from an MNO (InP), and the MNO virtualizes the substrate network and radio spectrum into three virtual resources. End users logically connect to the virtual network through which they subscribe to the service, while they physically connect to the cellular network. A cellular network controller needs to be deployed at the cellular network to realize the virtualization process.

RADIO SPECTRUM RESOURCE Radio spectrum resource is one of the most important resources in wireless communications. Usually, radio spectrum resource refers to the licensed spectrum or some dedicated free spectrum. As cognitive radio emerges, radio spectrum extends its range from dedicated spectrum to white spectrum, which means the idle spectrum unused by the owner can be used by others. With spectrum sharing, all or part of the licensed spectra owned by operators can be utilized by multiple operators based on agreements. For example, operator A and operator B have a contract to share their spectra with each other so that they have more flexible frequency scheduling and diversity gain, which can improve the efficiency and capacity of networks. Actually, inter-operator spectrum sharing has been proposed for many years. However, due to policies and markets rather than technologies, spectrum sharing is not popular in current cellular net-

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The infrastructure components are the “foundation” of cellular networks and occupy the majority of the investment of MNOs. In modern cellular networks, a single whole cellular network may be possessed by one MNO, or some parties may only own part of the whole network; for example, some parties own CNs, while others only have transport networks. In some cases, MNOs may be competitors in a certain geographical area, and no sharing or limited sharing (e.g., roaming) exists among them. In this case, virtualization can be realized within a single MNO. The term network sharing refers to the scenario where multiple MNOs share the infrastructure of the same physical network with each other. From the business perspective, network sharing can be considered as an agreement that two or more MNOs pool their physical network infrastructure and radio resources together and share with each other. Network sharing can also be considered as an important step to enable IaaS. One of the network sharing approaches, spectrum sharing, was introduced in the previous subsection. Thus, the other two approaches, infrastructure sharing and full network sharing, as shown in Fig. 3, are discussed below.

Infrastructure Sharing — Unlike spectrum sharing, only infrastructures are shared in this case. Infrastructure sharing can be classified into two categories: passive sharing and active sharing. Passive sharing refers to the scenario in which passive infrastructures (e.g., building premises, sites, and masts) can be shared among multiple operators. Active sharing refers to sharing of the network elements of a whole mobile network, such as: • RF antennas and evolved nodeBs (eNodeBs) included in the RAN • Backhaul and backbone transmission included in the transmission network • Routers, switches, and register (e.g., evolved packet core [EPC]) included in the CN An example is given in Fig. 3, where there are two MNOs, MNO 1 and MNO 2, in the same area. MNO 1 (green) and MNO 2 (blue) have their own infrastructure and licensed spectrum. They can share part of their networks (either RANs or CNs) or whole networks (both RANs and CNs). In addition, MNOs can also share their infrastructure based on their agreements and operate them on their own spectrum. A coordinator is needed to enable sharing. Virtualization-based infrastructure sharing can be called cross-infrastructure virtualization, which means wireless network virtualization is possible both across MNOs (InPs) and within MNOs (InPs). Full Network Sharing — Full network sharing is the combination of spectrum sharing and infrastructure sharing, which means both radio



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Wireless virtual resources are created by slicing wireless network infrastructure and spectrum into multiple virtual slices. Ideally, a single slice should include all the virtual entities sliced by each element in the wireless network infrastructure. In other words, a completed slice is an universal wireless virtual network.

Figure 4. Spectrum-level slicing and network-level slicing. Here, both MVNO1 and MVNO2 request virtual spectrum and virtual RAN from the MNO. MVNO1 has its own physical CN, while MVNO2 requests a virtual CN from the MNO. resource and network infrastructure can be shared among multiple MNOs based on agreements. In 3GPP specifications (e.g., [9]), full network sharing supports two identified architectures, which are the multi-operator core network (MOCN) and gateway core network (GWCN) configurations. In MOCN, the shared parts are only the RANs including radio resources themselves. GWCN allows sharing not only the RANs but also MSCs and SGSNs, which can be considered as the entities in core networks. Three scenarios are given in [10] where full network sharing is used. In scenario 1, some operators are allowed to access a RAN that covers a specific geographical area. This RAN is hosted by a third party (may be another operator) presented in Fig. 3 (orange). Scenario 2 is called common spectrum network sharing, where one operator shares its licensed spectrum with other operators, or a group of operators gather their licensed spectrum into a pool and share the total spectrum together. In this scenario, all operators in the group may first connect to a controller, then to the shared RAN. They can also combine with each other to form a common core network, then connect to the shared RAN. In scenario 3, multiple RANs may share a common core network, where the elements or nodes have different functions that belong to different RAN operators. These three scenarios are illustrated in Fig. 3. Full network sharing is enabled among MNO 1 and MNO 2, which means all the infrastructure can be shared among them. Unlike the infrastructure sharing case described in the previous subsection, both MNO 1 and MNO 2 pool their spectrum, so MNO 1 and MNO 2 can access the spectrum pool.


WIRELESS VIRTUAL RESOURCE Wireless virtual resources are created by slicing wireless network infrastructure and spectrum into multiple virtual slices. Ideally, a single slice should include all the virtual entities sliced by each element in the wireless network infrastructure. In other words, a completed slice is an universal wireless virtual network. For example, an SP requesting a slice from an MNO means that this SP wants to have a virtual network from CN to air interface, and is able to customize all the virtual elements in this slice. However, in reality, this ideal slice may not always be necessary. Specifically, some MVNOs who have their own CN but do not have radio coverage only need RAN slices [9], while some SPs only need the slices at a specific area or time. In another scenario, emerging over-thetop (OTT) SPs may want to pay more to MNOs to ensure guaranteed QoS service to their end users. The MNOs need to allocate a certain number of resource slices [4] to these OTT SPs, and these resource slices can be customized by OTT SPs according to their own requirements. Thus, based on different requirements, wireless virtual resources imply different levels of virtualization. Here we present the main three levels of wireless virtual resources, which are spectrum-level slicing, network-level slicing, and flow-level slicing.

Spectrum-Level Slicing — Spectrum-level slicing can be considered as an extension of dynamic spectrum access and spectrum sharing. In this paradigm, through time multiplexing, space multiplexing, or overlaid access (a spectrum reuse method in cognitive radio networks), spectra are sliced and assigned to MVNOs or SPs. In addition, spectrum virtualization is link virtualization

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Unlike the network-level slicing case, the SP cares more about the link between the SP and its end users, and the capability of bearing and customizing flows on these virtual slices, instead of detailed networks. The topologies and components of cellular networks are totally transparent to the SP in this example.

where the focus is on the data bearer in this link instead of physical layer technologies. As shown in Fig. 4, there are two virtual spectra (V_Spectrum, green/gray), which are two slices (e.g., in the time domain) of a whole spectrum. The other slices of this spectrum may be used for other virtual resources. Roughly, we can say that spectrum-level slicing is an application of spectrum sharing and dynamic access in the virtualization environment. It should be noted that while the licensed spectrum is currently not shared, there is considerable experience in spectrum sharing for unlicensed spectrum (e.g., WiFi), which can be beneficial for designing spectrum-level slicing in wireless virtualization. In addition, recent advances in cognitive radio technologies can also be considered in spectrumlevel slicing.

Network-Level Slicing — Network-level slicing is the ideal case, as mentioned above. Particularly in LTE-based next generation cellular networks, it has attracted much attention [11]. Here we give two examples, as shown in Fig. 4. First, MVNO 1, which has its own CN but no RAN in this area, requests a virtual RAN from the MNO. Based on this request, the MNO virtualizes a specific virtual RAN (V_RAN 1) to MVNO 1. Assume that MVNO 1 wants to have more control over the network: a virtual eNB (V_eNB), a virtual relay (V_Relay), and a virtual femtocell (V_Femto) are created and assigned to MVNO 1. MVNO 1 may operate this virtual RAN on virtual spectrum and connect it to its own CN. Second, MVNO 2 wants a virtual cellular network including spectrum, a RAN, and a CN. Therefore, the MNO creates another virtual RAN (V_RAN 2) and a virtual CN (V_CN), and assigns them to MVNO 2. Different from MVNO 1, MVNO 2 may only need a virtual BS (V_BS) instead of a full RAN. In other words, the MNO can virtualize one or some of the access points (e.g., eNB, relay, or femto) in this area to be one virtual BS based on the location and channel state of user equipment. In addition to LTE-based cellular networks, network-level slicing can be found in multiple radio access technologies [12]. Flow-Level Slicing — The main idea of flow-level slicing virtualization was first proposed in FlowVisor [13]. In flow-level virtualization, the definition of slice can be different, but usually it should be a set of flows belonging to an entity that requests virtualized resources from MNOs [4]. Some work has been done toward this architecture (e.g., [4, 7]). In this architecture, the physical resources that belong to one or more MNOs are virtualized and split into virtual resource slices. The resource slices can be bandwidth-based (e.g., data rate) or resource-based (e.g., time slots) [4]. A typical example is an MVNO that does not have physical infrastructures and spectrum resource (but has its own customers) to serve video calls to its customers. This MVNO may request a specific slice based on a certain data rate from the MNO that actually operates the physical networks. Figure 5 shows an example of flow-level slicing, where a virtual slice (V_Slice) is created

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by the MNO based on the requirements of an SP (which may provide HD video streaming services, e.g., Amazon or Youtube). Unlike the network-level slicing case, the SP cares more about the link between the SP and its end users, and the capability of bearing and customizing flows on these virtual slices, rather than detailed networks. The topologies and components of cellular networks are totally transparent to the SP in this example.

WIRELESS VIRTUALIZATION CONTROLLER A wireless virtualization controller is used for realizing customizability, manageability and programmability of virtual resources available to SPs. The architecture of a wireless virtualization controller is presented in Fig. 6. Through the wireless virtualization controller, the control plane is decoupled from the data plane, and SPs can customize the virtual resources within their own virtual slices. As shown in Fig. 6, there are two parts of the wireless virtualization controller, the substrate controller and virtual controller. The substrate controller is used for MNOs or InPs to virtualize and manage the substrate physical network. The virtual controller is used for MVNOs and SPs to manage the virtual slices or networks. Specifically, MNOs use the wireless virtualization controller to create virtual slices and embed the virtual slices onto wireless physical substrate networks. This process includes physical resource allocation, abstraction, virtualization, slicing, isolation, and assignment. Through the virtual controller, an SP can customize their own end-to-end protocols and services, such as scheduling and forwarding. As shown in Fig. 6, different protocol stacks, such as evolved packet system (EPS), traditional TCP/IP, or even new protocols, are running at different virtual resources. Since SDN and OpenFlow have been considered as the most promising and effective technologies in the network management domain, applying SDN in wireless networks has attracted some attention [12].

WIRELESS VIRTUALIZATION IN 3GPP LTE SYSTEMS The authors of [11] investigate eNodeB virtualization in 3GPP LTE. A hypervisor is physically added to the LTE eNodeB, and logically allocated between physical resource and virtual eNodeB. The LTE hypervisor takes the responsibility of virtualizing the eNodeB into a number of virtual eNodeBs, such as virtual machines (e.g., CPU, memory, I/O devices), which can be used by different SPs or MVNOs. Moreover, the LTE hypervisor is also responsible for scheduling the radio resources, such as orthogonal frequency-division multiple access (OFDMA) subcarriers, between the virtual eNodeBs. Bandwidth estimation is calculated and sent back to the hypervisor from each virtual operator. The hypervisor schedules the PRBs based on these estimations, the remaining (unused) PRBs, and a fairness index. A similar hypervisor mechanism is used in [14], with an extension to address multiple specific issues. In [15], load balancing techniques are proposed. Using the dynamic load balance



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Figure 5. Flow-level slicing that originates from the OpenFlow concept. With the slices of virtual resource, SPs can manage the QoS of each end user by scheduling end users.

mechanism, highly loaded virtual eNBs can offload excessive traffic to a low loaded virtual eNB, which brings a significant performance gain. Unlike other approaches, [14] introduces a bankruptcy game into the resource allocation problem in LTE virtualization. The authors model the InPs and MVNOs as the bankrupt company and players in the game, respectively. By solving the bankrupt game, the InPs guarantee relative fairness among VMOs when allocating the PRBs. Since the concept of SDN was proposed, the idea of decoupling the control plane and data plane has been applied in LTE virtualization. Moreover, OpenFlow, as a technology to realize SDN, is also introduced in LTE virtualization [12]. It should be noted that SDN and OpenFlow are not equal to network virtualization. SDN is a mechanism that can be applied in network virtualization. In other words, it is possible to use SDN to realize a network virtualization but not necessary.

CHALLENGES OF WIRELESS VIRTUALIZATION FOR NEXT GENERATION MOBILE CELLULAR NETWORKS Although wireless virtualization is a promising technology for next generation mobile cellular networks, many significant research challenges remain to be addressed. In this section, we present some of these challenges.


Isolation and Control Signaling — In virtualization, isolation is the basic issue that enables abstraction and sharing of resources among different parties. While isolation is relatively easier in wired networks, isolation in mobile cellular networks is challenging. Different from wired networks, where bandwidth resource abstraction and isolation can be completed on a hardware (e.g., port and link) basis, radio resource abstraction and isolation is more challenging due to the inherent broadcast nature of wireless communications and stochastic fluctuation of wireless channel quality. For example, any change in one cell may introduce high interference to neighbor cells. Also, isolation should be realized at different levels, such as flow level, subchannel or time slot level, or hardware level (antennas and signal processors) [4]. Before a virtual cellular network can be created, connectivity needs to be established between SPs and InPs. With this connectivity, SPs can express their requirements of resources to serve end users. In addition, since virtualization can happen among InPs, a standard language to express explicit sharing information among InPs becomes necessary. Thus, proper control signaling and interface considering delays and reliability need to be designed carefully to enable the communication among different parties involved in wireless virtualization. Due to the particular properties of mobile cellular networks, SPs or end users may require different QoS attributes. In contrast to functional

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Unlike wired networks, resource allocation becomes much more complicated in cellular network virtualization due to the variability of radio channels, user mobility, frequency reuse, power control, interference, coverage, roaming, and so on.

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service features, there is less agreement regarding the specification of QoS attributes. Therefore, the control signaling and interface should be compatible with different kinds of requirements.

neous networks, and each of them has unique and particular properties, some specific solutions and mechanisms are required for provisioning, operation, and maintenance of virtualized cellular networks.

Mobility and Network Management — With wireless virtualization, tracking a user’s location is challenging in mobile cellular networks, since it may perform location update with different VMNOs or InPs. Centralized location management can solve the problem. However, latency is introduced in centralized management; thus, some distributed mechanisms merit further research. In addition, since a user with ongoing communications may switch among multiple VMNOs or InPs, the handoff management problem becomes more complicated than in traditional cellular networks. To maintain service continuity when a user switches among multiple VMNOs or InPs, proper synchronization mechanisms among different networks are necessary. Management of cellular network virtualization is crucial to guarantee the proper operation of the physical infrastructure, the host virtual wireless networks, and the wireless services supported by the virtual networks. As a virtual network may span over multiple underlying physical networks, network management and operation face new challenges. Moreover, since underlying physical networks can be formed by heteroge-

Resource Management — In order to realize mobile cellular virtualization, InPs or MVNOs should discover the available active and passive resources in the underlying physical cellular networks. InPs need to decide the physical resources that are used for virtualization. Since resources may be shared among multiple InPs, an efficient coordination mechanism should be designed. Moreover, for better efficiency, InPs need to know the available resources in MVNOs. Therefore, a communication protocol is needed between InPs and MVNOs to discover the available resource in MVNOs. Naming and addressing are important issues in resource discovery as well, since they initialize processes through which VMNOs recognize the physical nodes and links. An MVNO may combine resources from multiple InPs, and end users may also connect to multiple virtual networks simultaneously [1]. Therefore, a global naming and addressing mechanism is necessary for the identification of physical elements and virtual elements. Resource allocation schemes need to decide how to embed a virtual cellular network on physical networks. The procedure includes the selec-



tion of nodes, links, and other resources, as well as the optimization of them. Unlike wired networks, resource allocation becomes much more complicated in cellular network virtualization due to the variability of radio channels, user mobility, frequency reuse, power control, interference, coverage, roaming, and so on. Also, since the properties of uplink and downlink may not be the same in the cellular environment and the traffic is not symmetric in both directions, resource allocation should be considered for both uplink and downlink cases.

Security — With a large number of intelligent devices/nodes with self-adaptation/context awareness capabilities in mobile cellular virtualization, security is a big challenge. Particularly, a compromised party can take advantage of the virtualization mechanisms to misbehave in a malicious manner. Therefore, in addition to the vulnerabilities and threats of traditional mobile cellular networks, the involvement of intelligence in cellular network virtualization present new security challenges. Both prevention-based approaches and detection-based approaches need to be carefully studied for mobile cellular network virtualization.

CONCLUSION Wireless virtualization is becoming an important concept that enables abstraction and sharing of infrastructure and radio spectrum resources, reduced expenses of wireless network deployment and operation, easier migration to newer services and products, and flexible management. In this article, we present the business models and motivations of wireless virtualization in mobile cellular networks. Next, we present a framework of wireless network virtualization for mobile cellular networks. Finally, we discuss some significant research challenges in mobile cellular network virtualization, including isolation, control signaling, mobility management, network management, resource management, and security.

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Wireless virtualization is becoming an important concept that enables abstraction and sharing of infrastructure and radio spectrum resources, reduced expenses of wireless network deployment and operation, easier migration to newer services and products, and flexible management.

BIOGRAPHIES C HENGCHAO L IANG received his B.Eng. in communication engineering and M.Eng. in communication and information systems, both from Chongqing University of Posts and Telecommunications, China, in 2010 and 2013, respectively. From 2011 to 2012 he was a visiting student in the Mobile Telecommunications Research Lab, Inha University, South Korea. He is currently pursuing his Ph.D. degree in the Department of Systems and Computer Engineering at Carleton University, Ottawa, Canada. His research interests include wireless network virtualization, radio resource allocation, interference management for cellular systems, as well as applications of convex optimization in wireless networks. F. R ICHARD Y U [SM] is an associate professor at Carleton University. He received the IEEE Outstanding Leadership Award in 2013, Carleton Research Achievement Award in 2012, Ontario Early Researcher Award (formerly Premier’s Research Excellence Award) in 2011, Excellent Contribution Award at IEEE/IFIP TrustCom 2010, Leadership Opportunity Fund Award from Canada Foundation of Innovation in 2009, and Best Paper Awards at IEEE ICC 2014, IEEE GLOBECOM 2012, IEEE/IFIP TrustCom 2009, and International Conference on Networking 2005. His research interests include cross-layer design, security, green IT, and QoS provisioning in wireless networks. He serves on the Editorial Boards of several journals, including IEEE Transactions on Vehicular Technology and IEEE Communications Surveys and Tutorials. He has served on the Technical Program Committees of numerous conferences, and as TPC Co-Chair of IEEE GLOBECOM ’14, IEEE INFOCOM-MCC ’14, GLOBECOM ’13, GreenCom ’13, CCNC ’13, INFOCOM-CCSES ’12, ICC-GCN ’12, VTC ’12, GLOBECOM ’11, INFOCOM-GCN ’11, INFOCOM-CWCN ’10, IEEE IWCMC ’09, IEEE VTC-Fall ’08, and WiN-ITS’07, and as Publication Chair of ICST QShine ’10 and Co-Chair of ICUMT-CWCN ’09.

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Wireless virtualization for next generation mobile cellular networks

Wireless virtualization for next generation mobile cellular networks

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