Mobility robustness optimization (MRO) requires an appropriate selection of handover (HO) parameters such as the time-to-trigger (TTT) and offset margin to balance HO failures and ping-pong HOs. Existing stochastic geometry-based analyses for MRO have treated the angular position of the target base station (BS) as uniformly distributed over a feasible region. However, this treatment does not explicitly capture the spatial distribution of the target BS dynamically selected as a user equipment (UE) moves through the network. In this paper, we develop a stochastic geometry-based analytical framework for MRO in sub-6 GHz cellular networks. We derive the distribution of the HO triggering time and the spatial distribution of the dynamically selected target BS under straight-line UE mobility. Based on these distributions, we formulate too-late HO and ping-pong HO events as mutually exclusive events and analytically derive their probabilities. Numerical results validate the analysis, demonstrate improved accuracy over the conventional uniform-angle model, and reveal the tradeoff between the two HO events and the dependence of the optimal TTT on BS density.

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