LTE in high speed
In Rel-13 and 14, the mobility and throughput performance were enhanced to cover high speeds (up to 350 km/h) by specifying the requirements for UE RRM, UE demodulation and base station demodulation, considering the two types of operator’s practical deployments shown in Figures 1 and 2. Figure 1 shows the case where no specific installation is deployed to handle high-speed trains, i.e. UEs in the train use the "standard" LTE eNBs. Alternatively, figure 2 shows the case where Single Frequency Network (SFN) are deployed. SFNs use so-called "Remote Radio Heads" (RRH), which are dedicated antennas deployed along the train track. In this case, the baseband unit (BBU) is connected to the RRH, e.g. using fiber.
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| Fig.1: Non-Single Frequency Network (SFN) high speed scenario |
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| Fig2: SFN high speed scenario |
These Rel-13 and
14 enhancements were conducted both for non-SFN and for SFN, but only for LTE
single carrier, i.e. not covering Carrier Aggregation (CA).
Rel-16 improves the mobility and throughput performance, now
considering CA and speeds up to 500 km/h. To this aim, it enhances RRM, UE demodulation and base station demodulation: it specifies enhanced RRM core requirements and corresponding RRC signals in respectively
TS 36.133 and TS 36.331.
RRM requirements enhancements:
In Release 14
cases (limited to
350 km/h and single carrier), the latency
requirements under DRX configuration up to 1.28s DRX
cycle were enhanced by reducing the cell identification delay in connected mode and cell reselection delay
in idle mode [1].
In Rel-16,
considering Carrier Aggregation and speeds up
to 500km/h, the following enhanced requirements were introduced to achieve good
mobility performance and less paging outage:
1. Enhanced RRM requirements for active SCells (for 350km/h velocity): The same requirements specified in Rel-14 high speed WI are applied to active SCells.
2. Enhanced RRM requirements for deactivated SCells (for 350km/h velocity): The cell identification delay and measurement period are reduced.
3. Enhanced RRM requirements in DRX in connected mode (for 500km/h velocity): The cell identification delay and measurement period on 1.28s DRX cycle are further reduced from those in Rel-14 high speed WI.
4. Enhanced RRM requirements in idle mode (for 500km/h velocity): The cell detection delay is further reduced from those in Rel-14 high speed WI.
5. Enhanced UL timing adjustment requirements in connected mode (for 500km/h velocity): The larger maximum autonomous time adjustment step is applied when the downlink bandwidth is wider than 10MHz.
Demodulation enhancements
6. For UE and base station demodulation enhancements: In Release 14, UE and base station demodulation requirements were enhanced, for both cases of operator’s practical deployments shown in figures 1 and 2.
In Release 16, regarding the CA case in SFN (figure 2), the requirements specified in Rel-14 are expanded to Dual Connectivity's Secondary Cells (SCells) as defined in TS 36.331. Regarding further high speed up to 500 km/h, additional requirements are introduced to ensure the PDSCH/PUSCH/PRACH demodulation performance with larger Doppler shift.