Taking full advantages of both heterogeneous networks (HetNets) and cloudaccess radio access networks (CRANs), heterogeneous cloud radio access networks(H-CRANs) are presented to enhance both the spectral and energy efficiencies,where remote radio heads (RRHs) are mainly used to provide high data rates forusers with high quality of service (QoS) requirements, while the high powernode (HPN) is deployed to guarantee the seamless coverage and serve users withlow QoS requirements. To mitigate the inter-tier interference and improve EEperformances in H-CRANs, characterizing user association with RRH/HPN isconsidered in this paper, and the traditional soft fractional frequency reuse(S-FFR) is enhanced. Based on the RRH/HPN association constraint and theenhanced S-FFR, an energy-efficient optimization problem with the resourceassignment and power allocation for the orthogonal frequency division multipleaccess (OFDMA) based H-CRANs is formulated as a non-convex objective function.To deal with the non-convexity, an equivalent convex feasibility problem isreformulated, and closedform expressions for the energy-efficient resourceallocation solution to jointly allocate the resource block and transmit powerare derived by the Lagrange dual decomposition method. Simulation resultsconfirm that the H-CRAN architecture and the corresponding resource allocationsolution can enhance the energy efficiency significantly.
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