“…Existing literature mainly focused on the pico CRE bias, ABS power and other parameters’ optimization for the network capacity maximization or link reliability improvement in HetNets [ 5 , 9 , 16 , 17 ], and recent works began to shift to the network EE optimization with the consideration of inter-tier data channel interference coordination and mitigation [ 18 , 19 , 20 , 21 , 22 , 23 ], including spectrum allocation [ 18 , 19 ], power control [ 20 , 21 , 22 ], and cognitive sensing based on the inter-cell co-channel downlink interference coordination [ 23 ]. In [ 24 ], the authors maximized the EE of pico cells in HetNets by means of some non-convex methods.…”