sed by drifts in local oscillators in the UE and in the base station. 3GPP has defined the maximum frequency error in the UE as +0.1 ppm and in the base station as +0.05 ppm The worst-case Doppler shift is then the combination of the UE movement and the UE and base station frequency error. Table 10.25 shows the worst-case Doppler shift when the UE is moving at 500 km/h and when the UE and base station frequency error is included. In order to avoid interference between OFDM subcarriers, as a rule of thumb, the subcarrier spacing should be 10-20 times higher than the maximum Doppler shift. Fol- lowing this assumption, 3.5 GHz must utilize at least 30 kHz subcarrier spacing, and 28/39 GHz must utilize at least 240 kHz. If the Doppler effect is compensated by the Figure 10.71 Change of Doppler shift when the UE UE moving passes a base station. Doppler Doppler shift > 0 shift < 0 10.13 Mobility and High-Speed Trains Table 10.25 Worst-case Doppler shift with UE moving at 500 km/h. Doppler shift 3.5 GHz 4.5 GHz 28 GHz 39 GHz Doppler due to UE mobility (kHz) Doppler due to UE + BTS error (kHz) Total Doppler (kHz) Link throughput with 16QAM 2 DMRS 3 DMRS 4 DMRS 120 km/h 240 km/h 500 km/h Figure 10.72 Relative link performance with different frequency of Demodulation Reference Signals (DMRSs) [14]. physical layer solutions, a lower subcarrier spacing should be possible. DMRS can be used for the channel estimation and for the Doppler compensation. For low UE speeds, single front-loaded DMRS may provide a sufficiently accurate channel estimate for the entire slot. For high velocities, additional DMRSs need to be included also later in the slot, as the channel estimate done based on the DMRS in the beginning may be already inaccurate due to the short coherence time. We can estimate the maximum UE speed with a single DMRS by assuming that the channel coherence time is 0.4 X Doppler shift. Using the 0.5 ms slot at 3.5 GHz enables 300 km/h, and using the 0.125 ms slot at 28 GHz enables 150 km/h. These calculations assume no freq