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Recently, network function virtualization（NFV） was proposed as a paradigm shift in the telecommunication industry. Both industry and academia have drawn significant attention in mobile network virtualization.NFV decouples the software implementation of network functions from the underlying hardware, leading to considerable reductions in operating expenses（OPEX） and capital expenses（CAPEX）, and facilitating the network deployment. However, as an emerging technology, NFV brings both challenges and opportunities in developing new architectures, applying and deployment. In this paper, we first survey the related work of NFV, and then propose promising research directions in this area.

The base station（BS） configuration is a key factor to improve energy efficiency（EE）. In this paper,we focus on designing the network deployment parameters（i.e., BS densities） for biased K-tier heterogeneous cellular network（HCN） with quality of service（Qo S） provisioning. Using appropriate approximations, we derive the closed-form expressions of optimal BS density across all tiers to minimize the area power consumption（APC） by applying the stochastic geometry theory, while satisfying the users＇ Qo S requirements. These results are used to obtain some new insights into the EE performance of biased HCN deployment. With the aid of this approach, the best type of BSs to be deployed or switched off for energy saving purposes can be identified from the perspectives of BS transmission power. More precisely, if the BS transmission power ratio between an arbitrary pair of tiers of K-tier HCN, e.g., the small cell BS and macro BS tiers, is higher than a threshold which is a function of path loss exponent, bias factor and power consumption, the small cell BSs are preferred.The opposite situation takes place otherwise. Furthermore, it is also shown that, compared to the unbiased HCN scenario, significant energy savings are possible by appropriately biasing the HCN and optimizing the BS density, subject to the Qo S constraints among all tiers.