5G Advanced positioning: a technical overview of key 3GPP enhancements

Positioning in 5G new radio (NR) has come a long way since the first NR specifications in 3GPP Rel. 15.

Over the years, successive 3GPP releases have resulted in significant developments to both time- and angle-based positioning methods for indoor- and outdoor 5G networks. The introduction of new 5G positioning reference signals, measurements, and procedures has facilitated the progress.

The era of 5G Advanced, supported by the latest 3GPP Rel. 18, introduces several enhancements, narrowing the degree of accuracy significantly across a broader range of use cases.

Ericsson, as the co-rapporteur for the work item, played a prominent role in bringing home the successful finalization of the positioning enhancements in the latest release. This included deep technical contributions across all positioning features, leading simulation campaigns, chairing and moderating technical discussions, as well as coordinating and preparing specification developments for introducing all the Rel. 18 positioning features into the specification.

Below, we present a technical summary of some of Rel. 18’s most significant positioning enhancements.

Overview of positioning enhancements in 3GPP Rel. 18

Rel. 18 work item on new radio (NR) positioning had as its objectives to specify the following:

• positioning of RedCap devices,
• support for bandwidth aggregation for positioning measurements,
• enhancements to enable low power and high accuracy positioning,
• support for carrier phase measurement for positioning, and
• sidelink based positioning.

Positioning of reduced capability (RedCap) devices

Introduced in 2022 as part of 3GPP Rel. 17, RedCap devices are made to be simple with lower complexity and lower cost thanks to lower bandwidth and fewer antennas. RedCap devices can reduce modem complexity by about 65 percent for low- or mid-band (FR1) devices, and by about 50 percent for high-band (FR2) devices. This produces significant cost reduction, scalability, and time to market benefits making RedCap devices optimal for deployments that do not require the full capabilities of regular 5G devices. Examples include industrial wireless sensor networks, video surveillance, and wearables. These use cases largely benefit from positioning. However, delivering decent positioning accuracy requires processing of large bandwidth signals, among other factors, something that RedCap devices do not have the luxury of. To address this, positioning of RedCap devices was considered as one of the objectives of the Rel. 18 work item on NR positioning.

Figure 1. RedCap will enable new service opportunities on 5G networks.

Reference signals, measurements, and the procedures for positioning introduced in Rel. 16 are adopted by the Rel. 18 specification to support positioning of RedCap devices. Based on Rel. 18, RedCap devices can be configured to perform downlink (DL) positioning measurements on the positioning reference signal (PRS) and report the measurements to a positioning node or location management function (LMF) via the LTE positioning protocol (LPP). RedCap devices can also be configured to transmit sounding reference signal (SRS) for uplink (UL) positioning measurements to be performed by the network node such as gNB, known as the 5G base station in simpler terms.

To ensure that measurements performed by RedCap devices guarantee a decent positioning accuracy, requirements for the positioning measurements performed by RedCap devices are defined as a part of the Rel. 18 radio resource management (RRM) specification.

An extensive simulation campaign was run to evaluate and determine the achievable accuracy of the positioning measurements by a device with 1 Rx branch in FR1 with limited bandwidth processing capability in both line-of-sight and non-line-of-sight scenarios.

Based on these simulation studies, new requirements for minimum achievable positioning accuracy for RedCap devices with 1 Rx branch were defined as a part of Rel. 18 NR positioning RRM specification. For RedCap UEs capable of supporting 2 Rx branches, existing accuracy requirements were reused.

Frequency hopping (FH) for high accuracy positioning measurements

Positioning measurements with reduced bandwidth on a limited set of resources in the frequency domain is one of the key limitations for high accuracy positioning of RedCap devices. Reduced bandwidth for positioning measurements limits the accuracy of positioning measurements which adversely impacts the performance of positioning methods such as DL and UL time difference of arrival (DL-TDoA, UL-TDoA) and multi-round-trip time (multi-RTT).

To improve the achievable positioning accuracy of a RedCap device, a technique called frequency hopping (FH) is introduced in Rel. 18 for NR positioning. This method is designed to allow RedCap devices to perform positioning measurements on PRS resources with an effective bandwidth wider than the maximal RF bandwidth supported by RedCap devices.

In each hop, the RedCap device receives a part of wide bandwidth PRS resources and performs positioning measurements after PRS resources in multiple hops are received and aggregated. By aggregating the PRS resources received in all hops (up to 100 MHz in FR1 and 400 MHz in FR2), the RedCap device can process a combined reference signal with wider bandwidth to perform time of arrival (ToA) measurements with higher accuracy (see Figure 2 for an illustration). This way, the RedCap UE can perform positioning measurements on wide bandwidth PRS resources such that the accuracy of positioning measurements performed is on par with the positioning measurements performed by NR devices that can process wide bandwidth PRS resources.

Figure 2: Frequency hopping for RedCap positioning.

For effective implementation of frequency hopping for positioning measurements, three factors are fundamental. One is the RF switching time, which is the time it takes for the UE to switch between hops during reception of resources for positioning measurements. The others are the RRM requirements set to ensure predictable and acceptable UE behavior, and the performance requirements.

To ensure that positioning measurement accuracy is actually enhanced by Rx frequency hopping, the time for a RedCap device to perform all hops for PRS reception during positioning measurements needs to be within the channel coherence time. For this purpose, requirements for RF retuning time are defined, {70μs, 140μs, and 210 μs} for FR1 and {35 μs, 70 μs, and 125 μs} for FR2.

A RedCap device capable of performing sufficiently fast RF switching between the hops can complete a whole PRS reception across target wide bandwidth within a time slot. This is called intra-slot PRS hopping and is suitable for the use cases with requirements on high positioning accuracy and short latency (see Figure 3).

For use cases with high positioning accuracy requirements but less strict latency requirements, the RedCap device can use a longer time to perform the PRS reception FH on PRS resources that are transmitted by a transmission and reception point (TRP) over multiple slots as shown in Figure 3.

Figure 3: Frequency hopping for RedCap positioning.

Rx frequency hopping for the purpose of positioning measurements is a new capability. To define RRM performance requirements for this new feature, there was a need to understand the achievable accuracy of the positioning measurements when a RedCap UE is configured to aggregate PRS resources from multiple narrowband hops and perform positioning measurements on PRS resources with effective bandwidth wider than the maximal bandwidth supported by the RedCap UE.

To understand achievable accuracy of the positioning measurements performed with Rx frequency hopping in different scenarios, a new simulation campaign was run in 3GPP. Ericsson drove this activity as moderator and, in doing so, ensured that a minimum performance requirement to guarantee the intended gain by Rx frequency hopping was defined as a part of the latest release.

Bandwidth aggregation for positioning measurements

In addition to RedCap positioning support, Rel. 18 introduces new measurement procedures to further enhance positioning accuracy of regular NR devices. This includes a new positioning framework that allows the network to configure UEs to perform positioning measurements by aggregating positioning reference signal (PRS) resources from up to 3 positioning frequency layers (PFLs).

Accuracy of the positioning measurement is proportional (and thus limited) to the bandwidth of the PRS resources on which the measurements are performed. Aggregating PRS resources from intra-band and contiguous PFLs allows UE to perform positioning measurements on PRS resources with bandwidths larger than 100 MHz in FR1 and 400 MHz in FR2 to further enhance the accuracy of positioning measurements.

Up until Rel. 17, positioning measurement procedures and the accuracy requirement were defined to be met by the UE on a per PFL basis on PRS resources up to 100 MHz bandwidth in FR1 and up to 400 MHz bandwidth in FR2. The aggregation of PRS resources from multiple PFLs for positioning measurements is a new scenario for which new requirements had to be defined.

An extensive simulation campaign was run to determine the achievable positioning accuracy based on the measurements by aggregating PRS resources transmitted in up to 3 PFLs from the same TRP in both line-of-sight and non-line-of-sight scenarios. Based on the results from the simulation campaign, new accuracy requirements were specified in Rel. 18. These will ensure that achievable positioning accuracy based on NR is further enhanced and capable of supporting use cases that demand more stringent positioning accuracy than previous specifications.

Figure 4: Bandwidth aggregation for positioning measurements.

Low power and high accuracy positioning

As an energy-efficient generation, 5G has introduced a range of energy-efficient features that are designed to reduce the power consumption of both network resources and connecting devices. This includes the introduction of UE operational states: connected, inactive, and idle.

Until Rel. 16, NR positioning measurements and procedures could only be performed by a UE in connected state. Rel. 17 introduced a framework that enabled UEs to initiate positioning measurements or positioning transmissions in inactive state. It means that the UE can wake up when PRS resources are transmitted by the network, perform positioning measurements, and establish an active connection with the network only for the purpose of reporting the measurements. It can also mean that the UE can wake up when it is due to transmit the positioning sounding reference signal. This framework allows the UE to save its energy while enganging in positioning. To further enhance the energy efficiency of the UE while performing positioning measurements and to provide support for emerging low power and high accuracy positioning (LPHAP) use cases, Rel. 18 specification allows an NR UE to perform positioning measurements in idle state.

Positioning measurement in IDLE state

When the UE is in idle state extended discontinuous reception (eDRX) cycle is configured by the network. When eDRX cycles are configured, UEs only wake up during the paging time window (PTW) and remain in sleep mode between the two PTWs allowing UEs to significantly preserve their energy. eDRX cycles are primarily designed to ensure that communication needs and requirements are met. Defining positioning measurement procedures for idle state to achieve high accuracy positioning without impacting the existing eDRX cycle framework was one of the challenges overcome during Rel. 18.

For LPHAP use cases, a UE is configured to perform positioning measurements with a reporting interval. When the configured reporting interval overlaps with the eDRX cycle, the UE can perform positioning measurements within PTW and report them to the network. This procedure is aligned with the UE behavior that is well established for the communication purposes. When the reporting interval does not overlap with the configured eDRX cycle, Rel. 18 specification allows UE to wake between the PTWs to perform the positioning measurements to ensure that LPHAP requirements are met without a need to alter the eDRX cycle configuration specifically for positioning. In addition to this, accuracy requirements were defined to ensure that the requirements for high accuracy positioning could be achieved by the UE even when the positioning measurements were performed in an energy efficient manner.

Carrier phase measurement for positioning

Rel. 18 introduces new positioning measurements, namely the reference signal carrier phase difference (RSCPD) and reference signal carrier phase (RSCP) measurements. RSCPD measurement is reported by the UE together with the RSTD measurement, and RSCP measurement is reported by the UE together with the UE Rx-Tx measurement. Similar to the RSTD and UE Rx-Tx time difference measurements, RSCPD and RSCP measurements are performed on the PRS resources and reported to the network for UE positioning. Carrier phase measurement only provides the estimate of phase of the carrier (as shown in Fig.5) when PRS was received by the UE, carrier phase measurement alone cannot be used to derive the UE position. Therefore, a UE is always configured to report carrier phase measurement together with the legacy measurements such as RSTD or UE Rx-Tx measurements. This type of reporting allows location server to use the legacy measurements to resolve the integer ambiguity and use the associated carrier phase measurement to determine the range of the UE leading towards the higher accuracy positioning of the UE under favorable conditions.

Figure 5: Carrier phase measurement for positioning.

To ensure a reasonable positioning accuracy is achieved by exploiting the carrier phase measurements, new performance requirements for RSCPD and RSCP measurements have been defined as part of the latest release based on the extensive simulation results of different UE positioning scenarios driven by Ericsson. In another crucial area, Ericsson provided substantial technical contribution to ensure that the UE’s carrier phase measurement, specifically RSCPD measurement, is based on the PFL that is common between the reference and target TRPs. This is particularly important to ensure carrier phase measurement can be better used for high accuracy UE positioning.

Sidelink-based positioning

In addition to features described above, Rel-18 added support for sidelink positioning to the specification, including sidelink measurements, positioning methods, and corresponding requirements.

Evolution of NR positioning

NR positioning has evolved significantly since its inception and can support positioning of different device types and use cases in a wide range of deployment scenarios. 3GPP is now moving towards embracing AI/ML to support positioning functionality in future releases and next generation of radio access technology (RAT). NR positioning defined and specified until Rel. 18 lays a solid foundation for the anticipated evolution of RAT based UE positioning methods and techniques.

Related reading

Learn more about previous 5G NR positioning specifications in these other insightful articles:

• 5G positioning: What you need to know
• 3GPP GNSS positioning and integrity: The latest trials and developments
• Why is precise indoor positioning your best companion for 5G monetization?

Learn more about the capabilities of 5G Advanced:

• 5G Advanced: Evolution towards 6G
• 5G evolution toward 5G advanced: An overview of 3GPP releases 17 and 18

Learn more about RedCap devices:

• What is reduced capability (RedCap) NR?
• Unlocking RedCap User Equipment performance

Read about the RedCap outlook in Ericsson Mobility Report, June 2024.