 a result of using too high TTT and handover offset, which essentially results in too late outbound handover from the pico. Furthermore, allowing the users moving at 60 kmph to be handed over to the picos is often not attractive, as they quickly leave the pico coverage again, resulting in an unnecessary high handover rate with an undesirable short time-of-stay in the pico. For additional results see References 2 and 6. However, by using differentiated TTT settings for the different handover settings as sup- ported for Release 12, the desired mobility robustness can also be maintained for the co-channel HetNet cases. This is illustrated in Figure 8.3, where the average experienced HOF percent- ages are reported for different settings of the TTT, assuming a handover offset of 2 dB for the A3 event. In these results the TTT for P-M is set to 160 ms, while the TTT for M-M is 256 ms. As can be seen, good performance is achieved by using a relatively long TTT for pico inbound handovers (M-P), as this significantly lowers the probability of high-speed users from being handed over to picos (i.e. will also not experience P-M handover). For users that do experience P-M handovers, the TTT should take a low value to ensure quick outbound handovers, while the TTT should take medium values for M-M to low HOF and ping-pong probabilities. Finally, the HOF probability can be further reduced by enabling self-organizing network (SON)-based techniques such as mobility robustness optimization (MRO) for whether Small Cell Optimization 30 kmph 60 kmph M-P TTT (ms) Figure 8.3 Handover failure (HOF) percentage for different TTT settings the cell individual offset is further adjusted for each cell boarder to avoid too late or two earlier handovers; see more detail in Reference 3. For HetNet deployments with macros and small cells on different carrier frequencies, the mobility performance does not naturally suffer from interference between macro and small cells. However, for such cases the main challenge is for macro-UEs to discover small 