Uplink Performance of LTE and NR with High-Speed Trains

Cellular network based connectivity for high speed trains (HSTs) is subject to large carrier frequency offset (CFO) due to high Doppler shifts. Large CFO will cause losing orthogonality between OFDM subcarriers which leads to significant performance loss. In this paper, we compare two Doppler estimation methods for HST links to compensate and remove CFO effect in the receiver in the context of 5G New Radio (NR) and long term evolution (LTE) systems. The first considered method to estimate Doppler shifts in LTE systems is based on the cyclic prefix (CP). The second method considered in NR system context is based on the phase tracking reference signal (PTRS). Simulation results shown that NR PTRS based method has higher estimation accuracy compared to LTE CP based method. Moreover, NR PTRS based method has higher signal to noise ratio (SNR) gain to achieve considered link performance target which is set to 70% of the maximum achievable throughput in this study. Additionally, the uplink data channel performance studies shown that, for systems using two demodulation reference signal (DMRS) per subframe for channel estimation, LTE CP based method can support only QPSK modulation scheme. In this case, a significant performance improvement is observed when the number of DMRS symbols per subframe is increased up to four, while almost the same performance is observed in NR systems for both slot patterns. Therefore, NR systems using PTRS based method with two DMRS configuration per subframe can be used with lower system overhead. In addition, block error rate (BLER) performance results show that NR PTRS based method has superior performance compared to LTE CP based method. Overall, these results demonstrate that NR PTRS based Doppler estimation method is more suitable in HST use cases.