 frequency is adaptive in 5G. The benefit is that the transmission can be optimized according to the environment and the expected mobile speed. The Doppler frequency Af can be calculated from the mobile speed v, carrier frequency f, and speed of light C: Af=Vf If we want to support 500 km/h with 3.5 GHz, the Doppler frequency is 1600 Hz. The phase of the received signal changes considerably 1600 times per second, and the required reference signal frequency needs to be considerably higher. This means the ref- erence signal transmission must occur multiple times every millisecond. On the other hand, if the mobile speed is lower, like 3 km/h, the Doppler frequency is just 10 Hz, and the channel remains similar for more than 10 ms. LTE radio is designed for a Doppler frequency of up to 750 Hz with a fixed frequency of the reference symbols. The adaptive reference signal frequency is illustrated in Figure 3.14. The frequency of the reference signals also depends on the Doppler spread. When there is a dominant propagation path as in the 3GPP High Speed Train (HST) models, the residual Doppler spread is small after frequency offset correction. The greatest need for a large number of reference sig- nals is for channels that have a U-shaped Doppler spread and an average frequency offset of zero. We also note that the UE synchronizes to the downlink signal including the Doppler spread. Therefore, the experienced Doppler shift can be up to two times higher in the base station receiver than that shown here. 3.4.7 Adaptive UE Specific Bandwidth 5G allows the bandwidth to be configured for each UE differently, which is also known as bandwidth part. Figure 3.15 illustrates an example with a 100 MHz carrier when UE1 uses instantaneously full 100 MHz, UE2 uses 20 MHz, and UE3 10 MHz. The UE with LTE fixed reference signal frequency 4 times every millisecond 5G with high reference signal frequency for high Doppler frequency 5G with low reference signal frequency for low Doppler frequency Figure 3.14 Adaptive reference signal frequ