Special Research and Analysis of SMTC

Author: Wang Rui, unit: Hebei Mobile

Labs Guide

As 5G networks continue to mature, NR2.6G and 700M inter-frequency networking has also begun to take shape. When mobile users move between 2.6G and 700M networks, a large number of inter-frequency switching will occur. If the inter-frequency parameters in the network are not set properly, it will eventually lead to measurement failures, which will affect the perception of data and voice users. In 4G networks, the reference signal CRS is continuously transmitted, and it is easy for end users to perform neighboring cell measurements. However, there is no CRS reference signal in 5G networks, but SSB signal strength is used for measurement. End users do not need to measure cell signals periodically like SSB, and can configure appropriate measurement cycles according to channel conditions. This helps avoid unnecessary measurements and reduce power consumption on mobile devices (UEs). Therefore, the protocol defines an SSB-based RRM Measurement Timing Configuration window (SMTC window) to notify UE of the measurement cycle and the timing of UE measurement of SSB. To this end, we have conducted special research and analysis on the principle and related parameters of inter-frequency SMTC. By optimizing the parameters, we can avoid unnecessary network complaints caused by measurement failures, thereby further enhancing the brand image of mobile networks.

1SMTC principle

In 5G, the SSB beam is sent periodically and the period is configurable, but the UE does not need to measure the cell signal periodically like SSB. To avoid unnecessary measurements and reduce UE power consumption, the protocol defines the SMTC window to notify the UE of the measurement period and the timing of measuring SSB. That is, when the UE is notified of the SMTC window by the base station, it detects and measures the SSB within the window and reports the measurement results to the base station.

The SMTC time window can be set the same as that of SSB, i.e. 5, 10, 20, 40, 80 or 160ms; the window duration can be set to 1, 2, 3, 4 or 5ms according to the number of SSBs in the measured cell. The measurement (Gap) should be greater than the SMTC window duration length, and the SMTC duration should include the SSB duration, otherwise it cannot be measured.

2 Conventional SSB and GAP Configuration

2.1 NR2.6G (SCS 30KHz) adopts CASE C scenario, and the SSB configuration position is {2, 8} + 14*n (n=0,1,2,3);

2.2 NR700M (SCS 15KHz) adopts CASE A scenario, and the SSB configuration position is {2, 8} + 14*n (n=0,1);

2.3 GAP Configuration

The start time of the measurement in GAP mode is determined by gapOffset (gapOffset message is obtained in the road test signaling RRCReconfigration). The currently commonly used GAP pattern1 is 80ms for MGRP and 6ms for MGL. The subframe position of the GAP start measurement is calculated by gapOffset and other related parameters, and is generally automatically obtained by the network side.

3. Case of measuring missed

When a certain city was verifying the VONR switching from Ericsson NR 2.6G to Huawei 700M, the base station triggered the A2 event multiple times, but the mobile phone did not report the 700M measurement report.

Parameter setting: The SMTC period and SSB period of Ericsson and Huawei are consistent, both are 20ms, SMTC duration is 5ms, and offset is 0. Huawei's gap measurement start position can be adaptively adjusted according to the SSB position of the measurement cell, and Ericsson's gapoffset needs to be calculated through smtcduration and offset.

Problem analysis: The 2.6G and 700M frame headers are not aligned, and the heterodyne SMTC measurement window configured for 2.6G does not include the 700M cell SSB, resulting in the terminal measuring "in vain" and the UE cannot measure the 700M SSB.

Solution: Set the Huawei 700M frame deviation to 70728TS. After the frame header is aligned, the 2.6G SMTC inter-frequency measurement window includes the 700M SSB. After the inter-frequency measurement is triggered normally, the switching can be performed normally.

4 Special Research

4.1 Frame Header Alignment

For the alignment of 2.6G and 700M frame headers of different manufacturers in the province, SMTC duration is verified with different settings of 1~5ms. When the duration is 1, the 700M frequency point cannot be measured because the SSB of Huawei and ZTE 700M cells is configured in the second subframe. When it is 2~5ms, it can be measured normally and switched.

4.2 Frame header misalignment (NR2.6G delay 2.3 or 3ms, and between ZTE and Huawei from the same manufacturer, ZTE 2.6G and Nokia 700M)

When the frame header is not aligned to 2.3ms or 3ms, when ZTE 2.6G and 700M are from the same manufacturer or ZTE 2.6G and Nokia 700M are from different manufacturers, when SMTC Duration is set to 5ms, no matter how smtc offset is set, 2.6G can measure 700M, gapoffset can be displayed, and the switching is basically normal. As shown in the following table:

Huawei SMTC offset parameters do not support modification. When the 2.6G frame offset is set to 70728TS/92160TS, the 700M frame offset is set to 0, and the SMTC Duration is set to 5ms, the GAPoffset value is displayed, and the mobile phone can measure the NR700M signal when occupying NR2.6G and can switch normally.

5. Lessons Learned

After comprehensive verification of the above topics, the following are the configuration suggestions for the measurement parameters of different manufacturers' heterodyne networks in the scenarios of frame header alignment and frame header misalignment: