ithout cell identity information). Target cell-dependent TTT [Release 12]: The network can configure UEs to use different TTTs depending on the target cell. This allows to use large TTT for small cell inbound handover to reduce the probability of fast moving users to be handed over to small cells. Still, users served by small cells can have low values of TTT to ensure safe outbound handover back to the macro layer. This is supported by the specifications as follows: The network can signal to the UE a list of PCIs for each measurement object (i.e. carrier frequency) for which an alternative TTT can be used. Default TTT is used for the target cells not in the list. With the above aggregates, it is also possible to ensure efficient and robust mobility for HetNet co-channel cases. As an example, the UE in Figure 8.1 with peripheral small cell crossings should only hand off to those small cells if it is moving at low to moderate speed, while otherwise be kept at the macro layer if it is travelling at high speed. LTE Small Cell Optimization 30 kmph 60 kmph Figure 8.2 Handover failure (HOF) percentage for macro-macro (M-M), macro-pico (M-P) and pico-macro (P-M) co-channel handovers In order to further illustrate the differences in handover performance for co-channel HetNet cases, Figure 8.2 shows the handover failure (HOF) percentage separately for macro-macro (M-M), macro-pico (M-P) and pico-macro (P-M) handovers. These results are obtained for the case with two picos per macro cell area. HOF event is declared if radio link failure (RLF) occurs after TTT expires, during the handover execution time as defined in [8]. The results in Figure 8.2 are obtained under the assumption of using the event A3 for triggering handovers, using a conservative setting with 3 dB handover offset and a TTT of 480 ms. As can be seen from these results, the performance of P-M handovers is significantly worse than the M-M and M-P handover cases - especially for the higher UE speeds. The worse handover performance for the P-M case is primarily