Your smartphone can pinpoint your location within a few meters outdoors. Walk into a building, and that level of precision becomes harder to maintain. This gap between outdoor and indoor positioning isn't just inconvenient – it has real implications for emergency services, industrial operations, and increasingly, everyday applications.

GPS works by measuring the time it takes signals to travel from satellites orbiting Earth. It's highly effective when your device has a clear line of sight to those satellites, but that line of sight can weaken as it passes through materials like concrete and steel. Inside buildings, especially multi-story structures, this can lead to reduced accuracy or less reliable location estimates.

The wireless industry has recognized indoor positioning as a priority for years. The U.S. FCC mandates that wireless emergency calls provide location accuracy within 50 to 100 meters.

Meeting that requirement indoors hasn’t been easy; the industry has been developing alternatives, such as Multi-Round Trip Time (Multi-RTT) positioning. With Multi-RTT, operators use the cellular network itself rather than satellites to determine more accurate positioning.

How Multi-RTT positioning works

Multi-RTT positioning represents a fundamentally different approach. Instead of relying on satellites, it uses the 5G network infrastructure already in place.

The basic mechanism involves measuring signal travel time between a device and multiple cell sites. But unlike earlier methods that only measured downlink signals (from tower to phone), multi-RTT measures both directions.

Here's the technical sequence (see Figure 1 below):

⦁ Uplink measurements: The user equipment (UE, or phone) transmits uplink reference signals. In 3GPP Release 16, specifications introduced a dedicated Sounding Reference Signal (SRS) specifically for positioning, called SRS-PosResource (SRS-POS). Multiple gNBs (5G base stations) measure when these signals arrive and calculate the round-trip time.

⦁ Downlink measurements: The network transmits Positioning Reference Signals (PRS) from neighboring cell sites. These signals use different comb offsets among intra-frequency neighbor cells to avoid interference. The UE measures the receive-transmit time difference for signals from multiple cells.

⦁ Location calculation: All measurements are fed into the Location Management Function (LMF), the network element responsible for calculating position. In 4G, the UE used LTE Positioning Protocol (LPP) to provide Time Difference of Arrival (TDOA) information to the LMF. In 5G, along with LPP, the UE provides SRS-POS to the gNB and the gNB then provides RTT information to the LMF over NRPPa, a newly introduced protocol, which is then used by the LMF to compute more accurate positioning information. By combining round-trip times from multiple cell sites, the LMF triangulates the device's position in two dimensions – to identify its horizontal location.

What makes this more accurate than older methods like TDOA? Multi-RTT measures the actual round-trip time rather than just arrival-time differences. This approach provides absolute distance measurements from multiple points, thereby improving positioning accuracy.

Adding the third dimension

Vertical positioning has been missing in past solutions. However, many places can hardly be located without this info. This necessity is where angle measurements come in.

Massive MIMO radio systems can measure the Angle of Arrival (AoA) for uplink signals. This feature provides vertical dimension data that supplements the horizontal position from RTT measurements. The combination transforms 2D positioning into 3D positioning.

This capability matters in practical scenarios. Emergency responders need to know not just that a 911 caller is in a building, but which floor they're on. Warehouse systems tracking inventory movement need vertical position data in facilities with multiple levels.

Angle measurements also help in complex radio environments. Urban canyons with tall buildings create multipath conditions where signals bounce off surfaces. Having both timing and angle data gives the positioning system more information to work with, improving accuracy even in challenging RF conditions.

A combination of NR introduced SRS-POS along with a 4G PRS mechanism and AoA measurements supported by Massive MIMO radios enables the RAN network to provide not only horizontal positioning information but also vertical positioning information which is not available through GPS-based positioning. As explained earlier, GPS has limitations in indoor environments. Because it does not provide vertical position data, it’s used to compute accurate position information is limited in urban and multi-story buildings. This limitation is now easily resolved using the new positioning technique introduced in 5G NR.

Resource management challenges and solutions

Network-based positioning has always faced a resource allocation problem. Positioning requires dedicated reference signals, which consume radio resources. Allocate too many resources to positioning, and you impact network capacity for regular voice and data traffic.

The solution involves on-demand resource allocation. Rather than continuously reserving positioning resources for all devices, the network allocates SRS positioning resources only when needed. These resources are allocated when a user triggers the position information. At this point, gNB will assign SRS-POS resources to the UE for a certain duration (typically less than 3 seconds). Once the positioning measurement is complete, those resources are released and returned to the pool.

This approach keeps positioning activity separate from regular uplink capacity. The SRS resources used for positioning don't impact resource allocation for normal calls or data sessions. For network operators, this means positioning capability doesn't come at the expense of overall network performance.

Accuracy and applications

⦁ Multi-RTT positioning achieves horizontal meter level accuracy within 10 meters, significantly better than the 50–100-meter FCC requirement for emergency calls. Combined with vertical estimation from AoA measurements, the system can handle complex environments that challenge other positioning methods.

⦁ That level of accuracy opens applications beyond emergency services. Industrial sites need meter-level positioning to track equipment and materials across large facilities. Hospitals require precise location data for asset management and patient care coordination. Any indoor environment where GPS fails but location data is essential, becomes a potential use case.

Samsung's implementation and vision

Samsung has implemented multi-RTT positioning and is one of the first network vendors to provide a terrestrial 5G network high-precision positioning solution using the multi-RTT method. Our system measures RTT for intra-frequency neighbor cells and provides results to the LMF via the NRPPa interface. The Massive MIMO units add AoA measurement capability, delivering the 3D positioning data.

Samsung positions this as terrestrial 5G network positioning, but we see the measurement principles extending further in the future. The same techniques will apply to Non-Terrestrial Networks - satellite-based 5G systems and future 6G deployments. Network-based RTT positioning is a foundational technology that works regardless of whether the network infrastructure is ground- or space-based.

Looking ahead, this work supports the development of the Integrated Sensing and Communication (ISAC) network for 6G. The vision involves networks that simultaneously provide communication services and sensing capabilities, with positioning as one core function.

The technology is operational now. What comes next is deployment at scale, gathering performance statistics from real-world use cases, and building the positioning capabilities into standard network operations. Indoor positioning moves from being a special-case problem to a standard network service.