“…Different from previous works in [14], [39], [40] where a single type of SBS is deployed, in this work, we consider a UDHN deployment where different types of SBSs (remote radio head (RRH), micro, pico, and femto BSs) are deployed under the coverage of the macro BSs (MBSs). Moreover, unlike the previous works [14], [39], [40] where a SBS is selected as the cluster head and is responsible for controlling the switching off/on of other SBSs, here the MBSs are responsible for controlling the switching off/on of the different SBSs because it has more global information about all the SBSs under its coverage. Also, we consider a scenario where horizontal traffic offloading is not always possible, as a result, vertical traffic offloading is employed in this work.…”