LTE in high speed

LTE in high speed

  November 15, 2020    tls123

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 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.

Fig.1: Non-Single Frequency Network (SFN) high speed scenario

 

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.

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UE Power Saving in NR

UE Power Saving in NR

  November 07, 2020    Mr. Standards

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UE battery life is an important aspect of the user’s experience.   The RAN1 study of the Rel-16 UE power saving had shown substantial power saving gain comparing to considered Rel-15 NR features such as DRX operation, with UE adaptation in frequency domain, time domain, antenna domain, tight control of DRX operations, and reducing PDCCH monitoring with different traffic types. 

The Rel-16 UE power saving in NR includes the power saving techniques, such as DRX adaptation, cross-slot scheduling, and maximum MIMO layer adaptation in CONNECTED state, fast transition out of CONNECTED state, and reduced RRM measurements in idle/inactive states.  The UE assistance information is part of the work to enable the UE to feedback its preferred configuration to achieve desired power saving. 

The UE power saving work in Rel-16 focuses on the power saving techniques  in CONNECTED state, which includes DRX adaptation, cross-slot scheduling, maximum MIMO layer adaptation, and fast transition out of CONNECTED state.  The RRM measurement reductions are the power saving techniques specified in idle/inactive states.   UE assistance information is supported for the UE to feedback its preferred configuration of the specific power saving technique.  

Power Saving Techniques in CONNECTED state

The power saving techniques are dynamically triggered by L1 signaling indicated from PDCCH-based power saving signal/channel or semi-statically configured by RRC signaling.   The PDCCH-based power saving signal/channel reuses the existing PDCCH search space and CORESET configurations with dynamic TCI states with DCI field indicating the adaptation to achieve UE power saving, such as UE wakeup in the DRX operation, cross-slot scheduling, and maximum MIMO layer adaptation through BWP switching.   

-    DRX adaptation

The DRX adaptation power saving technique is to configure the PDCCH-based power saving signal/channel at the active BWP before the beginning of DRX ON for UE monitoring with the indication of UE wakeup or not depending on whether there is data for UE to receive.    A new DCI format 2_6 is introduced with CRC scrambled by PS-RNTI (DCP) which contains the wakeup indication as well as SCell dormancy indication if configured.   A PS-offset is semi-statically configured before DRX ON defining the start of the interval for the DCP monitor occasion as shown in Figure 1.   More than one monitoring occasions could be configured for DCP on PCell for CA and SpCell for DC based on the search space and CORESET configurations.   Minimum time gap is specified as the UE processing time as shown in Figure 1.  UE is not required to monitor DCP at the interval of minimum time gap and within Active Time.

When DCP monitoring occasion collides with other procedures with higher priority in PDCCH monitoring, the monitoring occasion is considered invalid.   UE follows legacy behavior when all the configured monitoring occasions are invalid.   UE is configured by RRC to wake up or not when no DCP is detected with valid monitoring occasions.   One DCP can be configured to control PDCCH monitoring during on-duration for one or more UEs independently.  UE is also configured by RRC whether to report periodic L1-RSRP or periodic CSI/L1-SINR when UE is not indicated to wake up at the DRX ON.  

-    Cross slot scheduling

Power saving technique with cross-slot scheduling facilitates UE to achieve power saving with the assumption that it won’t be scheduled to receive PDSCH, triggered to receive A-CSI or transmit a PUSCH at the scheduling slot within Active Time.  A 1-bit minimum scheduling offset in DCI format 1_1 and 0_1 enables dynamic switching of DL and UL minimum scheduling offset values.  

-    Maximum MIMO Layer Adaptation

UE power saving techniques with the adaptation to the DL maximum number of MIMO layers could be achieved by dynamic switching of BWPs, which the DL maximum number of MIMO layers are configured to be different.

-       Fast transition out of CONNECTED state

UE can feed back the assistance information of its preference to be released/suspended for gNB to get UE transitioning out of CONNECTED state quickly when there is no further data arrival. 

 

Power Saving Techniques in idle/inactive state

-    Reduced RRM measurements in idle/inactive state

Power saving in RRC_IDLE and RRC_INACTIVE can also be achieved by UE relaxing neighbour cells RRM measurements when it meets the criteria determining it is in low mobility and/or not at cell edge.

 

UE assistance information

UE assistance information allows the UE to feedback its preferred configuration, such as c-DRX configuration, aggregated bandwidth, SCell configuration, MIMO configuration, RRC state, minimum scheduling offset values in order for network to assist UE achieving power saving gain. 

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