Why does the phone clearly show 5G signal but is occupying the 4G cell?

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This article is reprinted from the WeChat public account "Network Optimization Mercenary", the author is in correspondence. To reprint this article, please contact the WeChat public account "Network Optimization Mercenary".

Some time ago, a netizen raised the question:

What is the reason for this?

We have to start with 5G NSA networking.

Early 5G deployments adopted option 3, which is commonly known as NSA (non-standalone networking).

The so-called NSA (non-independent networking), as the name suggests, means that the 5G system (5G base station and 5G core network) is not networked independently, but is dual-connected with 4G LTE.

As shown in the figure above, in the Option 3 deployment mode, whether it is Option 3 or Option 3a/3x, only 5G base stations are introduced into the existing 4G LTE infrastructure. The core network still uses the 4G core network, and the 5G base stations are connected to the 4G base stations and the 4G core network.

In such an architecture, the 4G base station Master is the main node and the control anchor point of the 5G base station. The control signaling of the 5G mobile phone's network access, connection establishment, switching, etc. is handled by the 4G base station.

Simply put, in the NSA network, the 4G base station is the boss and the 5G base station is the younger brother, and all actions are under the command of the boss.

So, here comes the question.

In the 2G, 3G, and 4G eras, mobile phones only access one type of network. If they are resident on and connected to a 2g network, "g" or "e" will be displayed; if they are resident on and connected to a 3g network, "3g" or "h" will be displayed; if they are resident on and connected to a 4G network, "4G" or "4G+" will be displayed. Everyone has their own responsibilities, with clear division of labor, and the icon display is very simple.

But now we are entering the 5G era. Under NSA networking, 4G base stations and 5G base stations serve 5G mobile phones at the same time. This is a mess. How should the icons be displayed?

To address this issue, standards organizations have provided unified standards.

The first problem to be solved is that in idle state, the mobile phone does not dual-connect to 4G base stations and 5G base stations. Even if there is 5G signal coverage, the mobile phone only resides on the 4G LTE network. At this time, how to let 5G mobile phone users know that 5G services are available in their location?

To this end, the standard organization defines the "upper layer indication switch (upperLayerIndication)" in the system information (SIB2) broadcast by LTE.

The function of this "upper-layer indication switch" is equivalent to the "5G status indicator". When set to True (on), the UE can display the 5G icon, so that 5G users can know that 5G services are available in their location when they are idle.

By the way, even if you are using a 5G-supported mobile phone now, the 5G icon will not be displayed if you have not subscribed to a 5G package.

This is because when a mobile phone that has not subscribed to a 5G package is attached to the 4G core network, the system will notify the mobile phone through the "RestrictDCNR" parameter in the "Attach Accept" message that it is not allowed to use the 5G network.

The “upper-layer indicator switch” solves the above problems, but next, there is a series of problems on “how to specifically display the 5G icon” in idle and connected states.

For example, under the NSA networking architecture, the coverage of 4G base stations (cells) and 5G base stations (cells) is not completely overlapping due to the different working frequency bands and antenna heights. When the mobile phone is in different signal coverage areas, how to display the 5G icon?

According to different specific situations, GSMA defines the following four options:

It is not difficult to see that Config D is the most radical. Under Config D, as long as the phone occupies a 4G LTE cell under NSA networking, the 5G icon will be displayed even if no 5G NR signal coverage is detected at this time. Config D can easily make users mistakenly think it is "fake 5G".

Config C is second. The 5G icon will only be displayed when the phone detects 5G NR signal coverage.

For Config B, in the connected state, if only connected to LTE, the 5G icon will not be displayed even if 5G NR coverage is detected. This means that when the user makes a call using VoLTE, regardless of whether there is 5G coverage at this time, the 5G icon will not be displayed, but only the 4G icon will be displayed.

Config A is the most conservative. The 5G icon is displayed only when the phone is in connected state and is dual-connected to LTE and NR cells.

Why are there multiple options?

The reason is that global operators have not reached a consensus on a single option configuration, so multiple options are adopted, which allows flexibility in displaying the 5G icon according to their preferences.

For the above four options, operators can choose different configuration schemes according to actual needs. If they are more aggressive, they can choose Config D; if they are more conservative, they can choose Config C or B. However, Config A is indeed too conservative and is rarely used.

To avoid confusion in the display of 5G icons, regulators can specify configurations, or operators can require mobile phone manufacturers to specify configurations.

In short, sometimes when you see that your phone clearly displays a 5G signal but is occupying a 4G cell, this is not "fake", but is just because of the 5G NSA networking characteristics.