The blogger has not authorized any person or organization to reprint any original articles of the blogger, thank you for your support for the original!
blogger link
I work for an internationally renowned terminal manufacturer and am responsible for the research and development of modem chips.
In the early days of 5G, he was responsible for the development of the terminal data service layer and the core network. Currently, he is leading the research on technical standards for 6G computing power networks.
The content of the blog mainly revolves around:
5G/6G protocol explanation
computing power network explanation (cloud computing, edge computing, end computing)
advanced C language explanation
Rust language explanation
Article directory
1. Introduction of NR measurement Gap
Due to manufacturing cost and form factors, there is often only one RF module in a device, which is responsible for both performing all measurements and transmitting and receiving data .
While sending and receiving data from the serving cell, the mobile device can easily measure the signals of neighboring cells on the same frequency . When measuring cells working under different frequencies and different systems, the mobile device must suspend communication with the serving cell (including Tx and Rx), and need to adjust the RF module to the frequency to be measured (the frequency parameter comes from the configured Meas object), And restore the connection with the serving cell after a period of time .
The time during which the mobile device suspends communication with the serving cell to measure inter-frequency neighbors or inter-system neighbors is called the measurement interval（Meas Gap）。
2. Measure the Gap configuration
2.1 LTE
In the LTE system, the measurement gap length (MGL, Meas Gap Length) is fixed, so that at least one synchronization signal (PSS, SSS) is included in one gap. The LTE synchronization signal is transmitted at a period of 5 milliseconds, so the MGL of LTE is 6 milliseconds, allowing the radio module to have 0.5ms to readjust when the measurement gap starts and ends. The terminal will detect the synchronization signal in the measurement gap, identify the physical cell ID (PCI) and receive timing, and then the terminal will use cell specific reference signals (Cell specific Reference Signals, CRS) for measurement .
2.2 NR
- MGL：1.5 ms、3ms、3.5 ms、4ms、5.5 ms和6ms；
- Measurement Gap Repeat Period (MGRP): 20 ms, 40 ms, 80 ms and 160 ms;
3. Different types of measurement Gap in NR system
In 5G NR, there are three different measurement gap configurations as follows.
3.1 gapFR1
This means that this measurement gap configuration can only be applied to FR1 . gapFR1 cannot be configured together with gapUE. For example, when UE is in RRC connection state (EN-DC connection) and needs to measure FR1 frequency, gNB will configure gapFR1 or gapUE.
3.2 gapFR2
This means that this measurement gap configuration can only be applied to FR2 . Similar to gapFR1, gapFR2 cannot be configured together with gapUE. For example, if UE is in RRC connected state (EN-DC connected) and FR2 frequency needs to be measured, then gNB will configure gapFR2 or gapUE.
3.3 gapUE
This measurement gap configuration can be applied to all frequencies , namely FR1 and FR2. If gapUE is configured, neither gapFR1 nor gapFR2 can be configured. Using this measurement gap configuration, UE can measure FR1, FR2 and non-NR systems.
4. Measurement Gap parameter configuration
Similar to LTE, in 5G NR, RRC is also responsible for providing measurement gap mode configuration to UE . This is done using the MeasGapConfig IE in the MeasConfig IE, configured by the RRC Reconfiguration message. It is divided into two parts, the first part defines the type of measurement gap, and the second part defines the parameter configuration under the corresponding measurement gap type. The details of this IE are as follows:
The meaning of its parameters is as follows:
- gapOffset : It can be defined as the offset of the Gap mode. There are about 160 offset values, but not all of them are available for all periods. The offset value points to the start subframe in the period , and the value range is 0 ~ MGRP-1. For example, if the period is 20ms, the offset range is 0 ~ 19;
- mgl : the measurement gap length is 1.5, 3, 3.5, 4, 5.5, 6ms;
- mgrp : It defines the repetition period (in milliseconds) for measuring Gap. Configurable as 20ms, 40ms, 80ms and 160ms;
- mgta (Measurement Gap Timing Advance): If this parameter is configured, the UE will start measurement before the Gap subframe occurs. The timing advance can be 0.25 ms (FR2) or 0.5 ms (FR1);
5. Why is the measurement Gap configurable?
As mentioned earlier, 5G NR MGL is not fixed but configurable. Because the fixed measurement gap may cause the throughput of the serving cell to decrease . The SMTC window and window duration can be set to match SSB transfers and corresponding MGLs. For example, if we set the SMTC window duration to 2ms and the MGL to 6ms, the remaining 4ms here will not be used for data transmission and reception in the serving cell, which will result in reduced DL/UL throughput .
To reduce this unnecessary throughput drop, NR introduces a configurable MGL whose value can be 5.5 ms, 4 ms, 3.5 ms, 3 ms or 1.5 ms as well as the traditional 6ms.
As can be seen in case1 and case2 in the figure below, case1 uses an SMTC window of 2ms and an MGL of 4ms, while case2 uses an SMTC window of 4ms and an MGL of 6ms.
6. How to calculate and measure SFN and SF of Gap
The starting SFN and SF for measuring Gap meet the following conditions:
- SFN mod （MGRP/10）= FLOOR（gapOffset/10）；
- SF = gapOffset mod 10；
For example, when the MGRP is configured as 40 ms and the gapOffset is configured as 35, the SFN and SF for measuring the Gap can be calculated according to the above formula.
- SFN mod （MGRP/10）= FLOOR（gapOffset/10）
1 mod(40/10)= FLOOR (35/10) dissatisfied 3 mode 4 = 3 satisfy 7 mod 4 =3 satisfy 11 mod 4 = 3 satisfy - SF = gapOffset mod 10
SF = 35 mod 10 SF = 5
Therefore, the SFN for measuring Gap can be 3, 7, 11, 15, 19, etc., and the measurement starts from subframe 5 under the configuration of MGL of 4ms, as shown below.
Reference:
3GPP TS38.133 NR, TS38.331, TS38.321
