statistics from the site-specific scenarios reveals that those errors primarily occur at street intersections, especially when the UE is turning a corner. The former is a result of the more detailed modelling for the site-specific scenarios with explicit representation of buildings, and the related radio propagation characteristics in street canyons, where a user passing through street intersection is more likely to experience fast changes of both the desired and interfering signalling strength. Furthermore, the HOF events are primarily observed when attempting intra-frequency handover between two small cells or when conducting inter-frequency handovers between the two layers. SON-based techniques LTE Small Cell Optimization Europe Tokyo 3 kmph 30 kmph 60 kmph Figure 8.14 Handover failure (HOF) percentage for different scenarios with DC such as MRO can help further reduce the HOF probability as a result of tuning the handover- related parameters [3]. It is observed from the simulations that the HOF percentage is significantly lower with DC enabled when comparing the results in Figures 8.14 and 8.13, thus demonstrating that the use of DC offers benefits in terms of mobility robustness also. The improved HOF performance from using DC comes from always having the PCell at the macro layer, while utilizing the small cells by configuring those as SCell whenever possible for the user. Keeping the PCell at the macro layer essentially means that the HOF probability is corresponding to the macro-only scenario, and therefore not affected by the small cells. Figure 8.15 shows the empirical cumulative distribution function (cdf) of the small cell time-of-stay, assuming the use of DC. Here it is worth noticing that the small cell Europe Tokyo Time-of-stay in small cells (s) Figure 8.15 Small cell time-of-stay for different scenarios, assuming 3 kmph user speed and use of DC Small Cell Optimization time-of-stay is orders of magnitudes higher for the site-specific scenarios as compared to the generic 3GPP simulation case. The re