5G Mobility: Idle and Connected Modes

Mobility in 5G Networks

Mobility has been a cornerstone of cellular systems since their inception, and its relevance continues in 5G. In this article, we’ll explore how mobility works in both RRC Idle and RRC Connected modes, and how these procedures differ across Standalone (SA) and Non-Standalone (NSA) 5G networks.

For a deeper look at the architectural differences between SA and NSA, check out my newsletter: Note: 5G Standalone (SA) vs Non-Standalone (NSA)

A new dimension introduced in 5G is beam-based coverage, which gives rise to beam mobility, adding another layer of complexity (and opportunity) in mobility management.

Inter-Cell and Intra-Cell Mobility

Let’s begin by exploring how 5G handles inter-cell mobility, the process of a UE moving from one cell to another. The behavior varies depending on whether the UE is in NSA or SA mode, and whether it’s in idle or connected state.

NSA vs SA Mode

In NSA (Non-Standalone) mode, the mobility behavior is tightly coupled with LTE, as 5G idle mode is not applicable. All idle mode procedures are managed via LTE.

In Connected Mode, however, both NR and LTE mobility come into play. This means handovers may occur between NR cells, LTE cells, or between both, depending on the movement and coverage areas.

In contrast, SA (Standalone) mode must handle both idle and connected mobility internally, since there is no LTE anchor. Idle mode procedures such as cell selection, reselection, and IRAT (Inter-Radio Access Technology) handovers are all managed by the 5G system itself.

In connected mode, SA networks support both intra-frequency and inter-frequency handovers between NR cells.

IRAT handovers in SA mode are supported towards LTE, but not to 2G or 3G in the initial 3GPP releases. Support for legacy technologies like 2G and 3G became available in Release 16 and later.

Visualizing Mobility

Let’s look at a visual summary of idle and connected mode mobility across NSA and SA networks.

In NSA networks, idle mode mobility is fully handled by LTE. These procedures are nearly identical to legacy LTE systems.

In SA networks, 5G handles idle mode mobility directly. Upon power-up, the UE searches for a suitable PLMN and synchronizes with a selected NR cell. If no RRC connection is active, or if it was released, the UE resides in RRC Idle mode and engages in cell reselection as it moves between cells.

Connected Mode, Beam vs Cell Mobility

Now, let’s explore connected mode mobility, which is applicable in both SA and NSA modes.

There are two main types:

1. Cell Mobility (Inter-Cell Handover)

This occurs when the UE moves out of one cell’s coverage and into another’s. For example, a UE traveling from Cell A to Cell B requires a handover. This is managed primarily by the gNodeB’s RRC layer, which oversees coordination between the cells.

2. Beam Mobility (Intra-Cell Mobility)

This scenario is specific to 5G. Here, the UE remains within the same cell, but moves between beams of the same gNodeB.

For instance, a UE may initially receive its strongest signal from Beam 1, but as it moves, Beam 2 becomes the dominant source. At this point, the UE must switch to Beam 2 for continued reliable communication.

Beam mobility is managed primarily by the MAC and PHY layers, ensuring the UE stays connected to the best beam available without needing a full handover between cells.

Summary

• Idle mode mobility is only applicable in SA networks (not in NSA).

• Connected mode mobility applies in both SA and NSA, including:

-Inter-cell mobility (handover between cells)

-Intra-cell or beam mobility (switching between beams within the same cell)

• NSA UEs rely on LTE for idle mode and may perform NR and LTE handovers in connected mode.

• SA UEs handle all mobility within the 5G system, including idle mode cell reselection and connected mode handovers.

5G mobility introduces not just an evolution of legacy procedures but a whole new set of mechanisms like beam management, making it a fascinating and essential area of study for telecom professionals.

References

Qualcomm - 5G NR Technical Training:

https://academy.qualcomm.com/course-catalog/5G-Technical-Training