Key insights: Early measurements for improved carrier aggregation and dual connectivity setup

Carrier aggregation (CA) is designed to increase the data rate per user by configuring the mobile terminal to be simultaneously connected with multiple cells of the serving base station, which makes the mobile terminal, or user equipment (UE), operate at multiple frequencies at the same time. With Dual Connectivity (DC), the mobile terminal can be simultaneously connected to two serving base stations (known as the master node, MN, and the secondary node, SN).

Dual connectivity and carrier aggregation can also be combined, so that the UE is connected to two base stations and utilize multiple cells in each of them – drastically increasing the maximum bandwidth for the mobile terminal.

Most of the time, a typical UE is actually not transmitting or receiving data to and from the network. Instead, it spends a lot of time in so called dormant states – idle or inactive – to reduce signaling and energy consumption. Only when data arrives, does the terminal go back to a connected state for data transmission, and when it does, it may take some time before it can start operating in CA and/or DC.

That’s because in a typical network, the UE goes first to the connected state with only one serving cell, where it’s then configured to perform measurements of other candidate frequencies/cells. It’s not until the measurement results are reported that the network will configure the UE to start operating with CA and/or DC.

The resulting delay limits the utilization of larger bandwidths for UEs and in turn, the spectrum utilization in the networks, especially in the case of chatty traffic. For us at Ericsson, it has been important to shorten this delay to setup CA and/or DC. As a result, we’ve been driving the work on the early measurement feature in 3GPP Rel-16.

These early measurements make it possible to have the UE up and running with CA and/or DC directly after connection establishment or connection resumption, thereby taking advantage of the available LTE and NR spectrum as soon as possible.

Carrier aggregation and dual connectivity

Carrier aggregation (CA) is one of the most successful features in LTE, and allows up to five component carriers simultaneously to be aggregated for a mobile terminal, effectively increasing the maximum bandwidth fivefold, up to 100MHz. CA has been adopted to 5G New Radio (NR) as well, where up to 16 component carriers can be configured for any given device or UE, giving it up to approximately 1GHz of maximum bandwidth.

Dual connectivity (DC) allows a UE to simultaneously transmit and receive data on multiple component carriers from two serving nodes or cell groups (a master node, MN, and a secondary node, SN). DC is allowed between two serving nodes operating in the same technology (both NR, both LTE, for example), or operating in different radio access technologies (RATs), such as MN operating in LTE while SN is operating in NR, or vice versa. For example, using an LTE MN and NR SN (known as EN-DC) allows networks to employ both 4G and 5G to increase user throughput utilizing the wide 5G spectrum while providing the UE with a wider coverage, thanks to the 4G spectrum.

When a UE is in RRC_IDLE or RRC_INACTIVE, it’s only required to perform measurements for cell reselection, so that it will keep on camping on a single cell on the most suitable frequency and RAT. However, during connection setup or resumption towards the cell that the UE is camping on, neither the UE, nor the network are aware of the radio conditions towards, for example, other potential cells/carriers that the UE could use for CA and/or DC operation. There is therefore an additional delay after the UE has transitioned to RRC_CONNECTED, to configure the UE with new measurement configurations on potential carriers for CA and/or DC, and obtain the measurement results for those, before the UE can be configured with additional carriers to operate with CA and/or DC.

Introducing early measurements

In LTE Rel-15, 3GPP introduced an enhancement to the initial configuration of CA, known as enhanced utilization of carrier aggregation (euCA), where the UE can be configured to perform measurements while in idle/inactive state (also known as early measurements) on carriers that could be configured as additional carriers for CA.

The UE then reports the measurement results when it establishes or resumes the connection again with the so called primary cell (PCell), i.e. when entering connected state. The network can then use these measurements to quickly configure the UE with secondary cells (SCells) to operate in carrier aggregation with the PCell.

A simplified overview of the LTE Rel-15 euCA signaling for early measurement configuration/reporting is shown in Figure 1.

Figure 1: Early measurement configuration/reporting in LTE Rel-15

During connection establishment/resumption, the UE can indicate if it has early measurement results available, which the network can then request the UE to report. The early measurement configuration which the UE receives when entering idle/inactive state includes a list of LTE carriers to be measured, as well as a measurement duration timer (which could be up to five minutes). The measurement duration timer limits the time that the UE is required to perform the measurements while in idle/inactive state to avoid an unnecessary increase in UE power consumption, in case there’s a long interval between connections. 

Further enhancements of early measurements in Rel-16

In 3GPP Rel-16, further enhancements for the early measurements feature have been introduced as part of the CA&DC enhancements work item. There, the feature has been extended to also support NR and early measurements for a quick setup of DC. The LTE Rel-15 euCA feature described above has been used as a baseline for both LTE and NR Rel-16, where the early measurement configuration can now include both LTE and NR carriers. This ensures the UE reports measurement results for a quick setup of multi-Radio Dual Connectivity (MR-DC), such as EN-DC, where the MN is in LTE and the SN is in NR or NR-DC where both MN and SN are in NR.

The earlier the network receives the early measurement results from the UE, the earlier the UE can be configured with CA and/or DC. A restriction however, is that the measurement results cannot be sent until security has been activated so that the information is sent encrypted. This is to avoid external parties being able to determine the position of the UE by reading the transmitted measurement results.

When the UE triggers a connection resumption, i.e. restarts a suspended connection with stored configurations, security will already be activated through the RRC Resume Request message. For the RRC Resume procedure, an even earlier transmission of the early measurement results has therefore been introduced in Rel-16, which is illustrated in Figure 2. Here, the network can request the early measurement results from the UE in the RRC Resume message, and the UE includes these measurement results, if available, in the RRC Resume Complete message. This allows the network to configure the UE with CA and/or DC, based on measurement results, even earlier.

Figure 2: Early measurement configuration/reporting in NR rel-16

In NR, a cell has a number of different synchronization channels (beams), which are in turn related to the resource used by the UE to access the cell. When the UE transitions to a connected state, it will synchronize to the primary cell and determine the best beam in that cell. It will then access the cell using resources that are configured for that beam, thereby allowing the network to transmit to the UE in the direction (beam) of the UE. In order to speed up the access procedure, the UE can be provided with dedicated resources, which then typically will need to be allocated to the UE per beam.

For the network to obtain more precise information about the UE radio condition, and to limit the number of dedicated resources that need to be allocated to the UE, the Rel-16 early measurements support reporting of measurement results on beam level for NR carriers (i.e. either CA or DC candidate NR carriers). This enables the network to configure the UE with resources on the most suitable beams for quick setup of CA and/or DC.

Learn more

Read about Ericsson’s work with standardization and 5G standardization.

Find out more about 3GPP release 16 and 17:

• Read our Ericsson Technology Review article on the 5G New Radio evolution
• Take a look at key innovation areas of 3GPP Rel-17
• Read our blog post on mobility robustness in 5G networks
• Read our blog post on remote interference management for 5G

Connect with Patrik and Jens on LinkedIn.