Cell Complexity Impact on Railway 5G Performance: Measurements Along Tallinn–Tartu Corridor

Fifth-generation (5G) networks enable railway digitalization but face signal degradation challenges in high-mobility environments. While the existing literature attributes degradation primarily to Doppler frequency shifts, this study presents empirical evidence challenging this paradigm. Analysis of 13.7 million 5G New Radio measurements across 370 km of Estonian railway reveals that visible cell density, not velocity, dominates signal quality degradation. Nine geographic hotspots exhibit 5.4-18.0 dB degradation at moderate velocities (54-66 km/h, mean 60.2 km/h) with zero high-speed measurements, excluding the Doppler effect as the reason behind service quality degradation. Cell complexity demonstrates a 3.25× stronger correlation with degradation (r = -0.390) than velocity (r = -0.120), consistent with automatic frequency control tracking instability under high cell ID churn rates (40-115 visible cells per location), though direct confirmation of this mechanism requires access to internal modem frequency-lock state data. Frequency band analysis shows that 700 MHz is optimal at 98.1% of locations, with a 19 dB advantage over 3.5 GHz. Degradation mechanism decomposition reveals within-cell effects (60%, 7.85 dB) and handover boundary effects (40%, 2-6 dB). The findings challenge velocity-centric optimization paradigms and recommend network planning focused on cell overlap reduction rather than Doppler compensation enhancement. Practical recommendations include 700 MHz prioritization, handover parameter optimization, and geographic targeting of identified hotspots for European railway 5G deployment.