Switching off small cells brings an additional benefit in terms of minimization of inter-cell interference. While switching off the small cells during low load is simple, more intelligence is required to identify when to switch on the small cells again. When the load in the macro cell increases, some of the small cells should be switched on again. A number of different proposals can be considered: Discovery signal in 3GPP Release 12: The small cell in dormant state can transmit the new discovery signal with low density allocation in time and in frequency domain. The network informs UE about the discovery signal and the timing information, which allows UE to detect the discovery signal of multiple small cells at the same time. Small cell Small cell Macro cell Figure 8.20 Switching off small cells when the load is low Small Cell Optimization (copy) UE search window Macro cell Small cell no. 1 Small cell no. 2 Small cell no. 3 Discovery signal Figure 8.21 Release 12 discovery signal Pre-configuration: The macro cell includes a predefined list of small cells that should be switched on first. That information is based on the earlier statistical learnings. Small cell uplink measurements: The small cells can measure the uplink interference levels even if they do not transmit any data. If there is high uplink interference, it implies that there must be UEs close to that cell. UE measurements: Activation of reference signal transmission in small cells and requesting UE measurements to find out of those UEs can receive the small cell signal. Location measurements: UE positioning information relative to the small cell locations can be used to define which small cells would best serve the UEs. The discovery signal solution enables UEs to find small cells that are in dormant state. Figure 8.21 illustrates the concept. The macro cell can provide assistance information to UE to indicate the measurement windows where the UE can find the small cell discovery signals. The benefit of the solution is that UE does not need to search 