Let’s talk about 5G positioning technology

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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".

From 2G to 4G, the main positioning technologies of cellular networks are: E-CID, AoA, ToA, TDOA, etc.

E-CID

Traditional base stations are divided into three sectors, one sector corresponds to one cell, each sector is usually 120 degrees, and each cell has a different identification code (Cell ID).

Since the latitude and longitude of the base station are known, the location of the mobile phone can be roughly locked based on the Cell ID. However, the coverage of a cell is very large, usually hundreds of meters to several kilometers, and the positioning error based on the Cell ID alone is very large, so the E-CID positioning technology was developed.

E-CID, Enhanced Cell-ID, refers to enhanced positioning technology based on Cell ID, including Cell ID+RTT, Cell+RTT+AoA, etc.

Cell ID+RTT

RTT (Round Trip Time) measurement is added based on Cell ID. That is, TA (Time Advance) is used to obtain the time it takes for the signal to reach the base station from the mobile phone, or from the base station to the mobile phone. Then it is multiplied by the speed of light (the speed of wireless signal propagation) to estimate the distance between the mobile phone and the base station.

Under the Cell ID+RTT positioning method, the distances of three nearby base stations can be estimated to improve positioning accuracy.

Cell ID+RTT+AoA

AoA, Angle-of-Arrival, uses the incident angle of the mobile phone signal transmitted to the base station to further determine the location of the mobile phone in the area.

On the basis of Cell ID, adding RTT and AoA assistance can greatly improve positioning accuracy.

E-CID is a positioning method that adds auxiliary information such as TA, AoA, RSRP, RSRQ, etc. on the basis of Cell ID to improve positioning accuracy.

TOA

TOA, Time of Arrival.

It means calculating the distance between different base stations and the mobile phone by measuring the time it takes for the reference signals sent by multiple base stations to reach the mobile phone, and drawing concentric circles with the distance as the radius. Then, the positioning algorithm (three-sided positioning algorithm, least squares algorithm) is used to estimate the location of the mobile phone.

TDOA

TDOA, Time Difference of Arrival.

The disadvantage of the TOA positioning method is that if the time between the base station and the mobile phone is not synchronized, both parties do not know the absolute time when the signal is sent, which will cause calculation and positioning errors.

TDOA makes up for this shortcoming by using relative time (time difference), that is, by measuring the difference in signal arrival time between the mobile phone and two nearby base stations, the distance difference between the mobile phone and the base station is calculated.

From a mathematical point of view, the location of the mobile phone must be located on a hyperbola with the two base stations as the focus and the distance difference as the constant. In this way, three or more base stations around can form two or more hyperbolas in pairs, and the intersection of the hyperbolas is the two-dimensional location coordinates of the mobile phone.

OTDOA, UTDOA and E-OTD in the above table belong to TDOA positioning methods.

A-GNSS

A-GNSS, Assisted GNSS, is a network-assisted satellite positioning system.

A-GNSS requires that both the network and the mobile phone can receive GNSS information. In A-GNSS, the network can determine the GNSS satellites above the area where the terminal is currently located, and provide this information to the terminal, so that the terminal can narrow the satellite search range and shorten the search time based on this information, and complete the search process for available satellites more quickly. After the terminal quickly obtains its own position and then sends the position information to the network's positioning service center, a more accurate position can be calculated.

A-GNSS can meet the needs of fast mobile positioning, but cannot meet the needs of indoor positioning.

Positioning requirements in the 5G era

5G will enable diversified applications in all walks of life. A large number of application scenarios such as Internet of Vehicles, autonomous driving, intelligent manufacturing, smart logistics, drones, and asset tracking require higher positioning capabilities. For example, vehicle platooning and active collision avoidance in the Internet of Vehicles require positioning accuracy of up to 30 cm, and require positioning capabilities that support high-speed movement and ultra-low latency; remote control of drones requires 10-50 cm. At the same time, a large number of applications such as asset tracking, unmanned AGV, AR/VR, etc. are concentrated indoors and cannot be covered by satellite positioning systems. Therefore, 5G must enhance network positioning technology to improve positioning accuracy.

According to the 3GPP R16 definition, 5G positioning capabilities must meet the following minimum requirements:

• For 80% of the terminals, the horizontal positioning accuracy is better than 50 meters and the vertical positioning accuracy is better than 5 meters.
• The end-to-end delay is less than 30 seconds.

For demanding commercial use cases, 5G positioning capabilities must at least meet the following requirements:

• For 80% of the terminals, the horizontal positioning accuracy is better than 3 meters (indoor) and 10 meters (outdoor).
• For 80% of the terminals, the vertical positioning accuracy is better than 3 meters (indoors and outdoors).
• The end-to-end delay is less than 1 second.

5G Positioning Technology

DL-TDOA: 5G R16 introduces a new reference signal, PRS (positioning reference signal), which is used by the UE to perform downlink reference signal time difference (DL RSTD) measurements on the PRS of each base station. These measurement results will be reported to the location server.

UL-TDOA: The 5G R16 version enhances the SRS (channel sounding reference signal) to allow each base station to measure the uplink relative time of arrival (UL-RTOA) and report the measurement results to the location server.

DL-AoD (Downlink Angle of Departure): The UE measures the downlink reference signal received power (DL RSRP) of each beam/gNB and then sends the measurement report to the location server. The location server determines the AoD based on the DL RSRP of each beam and then estimates the UE position based on the AoD.

UL-AOA (Uplink Angle of Arrival): The gNB measures the angle of arrival based on the beam where the UE is located and sends the measurement report to the location server.

Multi-cell RTT: gNB and UE perform Rx-Tx time difference measurement on the signal of each cell. The measurement reports from UE and gNB are reported to the location server to determine the round trip time of each cell and derive the UE location.

E-CID: UE’s RRM measurements (e.g. DL RSRP) for each gNB. The measurement reports will be sent to the location server.

All positioning-related measurement reports must be reported to the location server. These measurement reports include:

Positioning measurement report reported by UE:

• DL RSRP per beam/gNB
• Downlink Reference Signal Time Difference (DL RSTD)
• UE RX-TX time difference

Positioning measurement report reported by gNB:

• Uplink Angle of Arrival (UL-AoA)
• UL-RSRP
• UL-RTOA (UL Relative Time of Arrival)
• gNB RX-TX time difference

In short, based on the previous cellular network positioning technology, 5G R16 introduced a new positioning reference signal (PRS) and adopted multiple positioning technologies such as DL-TDOA, UL-TDOA, DL-AoD, UL-AOA, and E-CID to jointly improve positioning accuracy.

At the same time, since the ultra-dense networks in the 5G era increase the number and diversity of reference points, Massive MIMO multi-beam can make AoA estimation more accurate, and lower network latency can improve the accuracy of time-based measurements, these advantages can further enhance 5G positioning capabilities.

In the future, 5G positioning capabilities will be further enhanced, and the R17 version will also increase 5G positioning accuracy to sub-meter level.