turn an OFF cell back to service, X2 signalling was standardized to allow an eNodeB to request a neighbouring eNodeB to switch on the OFF cell. This limits the achievable ON/OFF transition times to a timescale of hundreds of milliseconds, limiting the usefulness of these schemes to situations with very low network load such as during the night-time. One of the features discussed intensively and adapted to LTE Rel-12 standards was the small cell on/off operation and associated cell discovery. The feature is used to, for example, reduce network energy consumption and interference during the times when the network load is low by allowing fast on/off switching of cells, with the energy saving potential analyzed more in the next section. Rel-12 On/Off Mechanism The work done in Rel-12 improves the efficiency of on/off mechanism by facilitating shorter transition times to/from eNodeB dormant (i.e. OFF) state. There are two mechanisms for achieving this: LTE Small Cell Optimization Cell on On-off transition Discovery signal Off-on transition Cell on Dormancy period Figure 4.10 Illustration of eNodeB on-off operation 1. Transmission of DL Discovery Reference Signals to facilitate fast discovery of dormant cells. UE RRM measurements of DRS are also introduced. 2. Configuration of an OFF-cell as a deactivated SCell. Using SCell activation/deactivation for on/off-switching reduces the transition times between the ON and dormant states. The transmission of discovery signals for cell on/off is illustrated in Figure 4.10. First (during e.g. a low network load) the network decides to turn a cell off. The decision is then followed by an on-off transition period, during which the network releases the UEs in the cell to be turned OFF, using, for example, handover, connection release, redirecting RRC_IDLE mode UEs to different frequency layers etc. Once the network is satisfied that there are no longer UEs camping in the cell it can then make the final decision to turn the cell off and start a dormancy period. During the dormancy p