WO2025029226A1

WO2025029226A1 - Cell reselection method in non-terrestrial network

Info

Publication number: WO2025029226A1
Application number: PCT/TR2023/051480
Authority: WO
Inventors: Hanadi Mohammad Yousef SALMAN; Huseyin Arslan
Original assignee: Ulak Haberlesme AS
Current assignee: Ulak Haberlesme AS
Priority date: 2023-08-02
Filing date: 2023-12-06
Publication date: 2025-02-06
Anticipated expiration: 2026-02-02

Abstract

A cell reselection method for a non-terrestrial network realized by a user equipment (100) being served by a serving NTN transmitter (210) in a serving cell (310) characterized in that comprising following steps: determining at least a target NTN transmitter (220) providing at least a target cell (320) and which the user equipment (100) is in coverage of, measuring at least two of following NTN parameters of serving NTN transmitter (210) and at least a target NTN transmitter (220); a Doppler parameter which is defined by Doppler shift of received signal from NTN transmitters (200), a distance parameter which is defined by the distance between said serving cell (310) and a target cell (320); a visibility window parameter which is defined by the conditions whether there is a direct line of sight between user equipment (100) and NTN transmitter (200) and whether a predetermined Doppler spread value of NTN transmitter (200) meets at least a predetermined condition; a signal strength parameter which is defined by received signal strength; selecting one of the NTN transmitters (200) and a cell (300) according to measured NTN parameters meeting predetermined constraints.

Description

CELL RESELECTION METHOD IN NON-TERRESTRIAL NETWORK

TECHNICAL FIELD

Invention relates to cell reselection methods for a non-terrestrial network realized by a user equipment being served by a serving NTN transmitter in a serving cell.

BACKGROUND

Non-Terrestrial Network (NTN) comprises NTN transmitters (Nodes) which refer to the individual communication entities within the NTN. These nodes can include satellites, spacecraft, landers, rovers, or any other non-terrestrial infrastructure that acts as a communication endpoint. Non-terrestrial nodes facilitate data transmission, reception, and routing within the network. NTN comprises User Equipment (UE) which are the end-user devices that connect to the NTN for communication. UE is served by Cells that are provided by NTN transmitters.

Cell reselection in NTNs refers to the process by which a UE determines and selects the most suitable NTN cell for connectivity and communication. In NTNs, which include satellite-based networks or other non-terrestrial infrastructure, cell reselection plays a crucial role in maintaining optimal network performance and seamless connectivity.

Existing Third Generation Partnership Project (3GPP) systems typically base cell selection on signal strength measurements (e.g., reference signal received power (RSRP) measurements) and do not consider NTN-specific aspects in the selection. In some cases, RSRP measurements across a set of multiple cells in an NTN network may display limited signal level variations due to the typical line of sight conditions between the device and the NTN transmitters. As a result, a reliable cell selection ranking entirely on RSRP may be difficult to determine accurately. As another example, NTN transmitters may move relative to the surface of the earth, inducing a significant Doppler component on the link.

When utilizing NTN to extend 5G services, the mobility mechanisms, in particular the handover strategies, plays an important role in ensuring NTN service capability and continuity. In terrestrial network, a user can trigger handover by an obvious change in signal strength from the cell center to cell edge. However, this signal strength variation is reduced in NTN. Such an effect may lead to a small difference of signal strength in an overlap with two or more cells. If the handover mechanisms in terrestrial networks are copied directly, user may have difficulty distinguishing the better target. Hence more effective handover conditions need to be designed in NTN.

W02020165675A1 obtains one or more NTN-related metrics and the selected cell for camping is based in part on one or more NTN-related metrics. These NTN-related metrics can be the geographical distance between the user and the respective cell, round-trip time offered by the satellites, the velocity of the satellites, tracking area code, and signal strength quality offset. In WO2020165675A1 , the distance is referred to distance between NTN node and UE. The cell selection based on said distance and signal strength can be beneficial for low altitude platforms such as UAVs and balloons. Since the distance varies significantly from one location to another. Therefore, the disclosed parameters will be failed for high altitude platforms such as aircrafts and satellites.

W02022060896A1 discloses a handover technique based on cell type (i.e., LEO cell, GEO cell, moving cell, fixed cell, temporarily fixed LEO cell, permanently fixed LEO cell). The selected cell type is based on the requirements needed to be fulfilled. The handover conditions in NTN changes very quickly due to the high mobility. Method disclosed comprises complex processes which causes processing latency. So such latency will affect severely the cell selection decisions.

All the problems mentioned above have made it necessary to make an innovation in the relevant technical field as a result.

BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to cell reselection method to eliminate the above-mentioned disadvantages and bring new advantages to the relevant technical field.

An object of the invention is to provide a cell reselection method that reduces number of handovers.

Another object of the invention is provide uninterrupted handover between cells. Another object of the invention is to provide reselection of cells that is more suitable for user equipment.

To achieve all the objects mentioned above and that will emerge from the following detailed description, the present invention relates to a cell reselection method for a non-terrestrial network realized by a user equipment being served by a serving NTN transmitter in a serving cell. It is characterized by comprising steps of; determining at least a target NTN transmitter providing at least a target cell and which the user equipment is in coverage of, measuring at least two of following NTN parameters of serving NTN transmitter and at least a target NTN transmitter; a Doppler parameter which is defined by Doppler shift of received signal from NTN transmitter, a distance parameter which is defined by the distance between said serving cell and a target cell; a visibility window parameter which is defined by the conditions whether there is a direct line of sight between user equipment and NTN transmitter and whether a predetermined Doppler spread value of NTN transmitter meets at least a predetermined condition; a signal strength parameter which is defined received by signal strength; selecting one of the NTN transmitters and an Cell according to measured NTN parameters meeting predetermined constraints. Thus, a more suitable cell may be selected for user equipment, or cell reselection may be avoided if serving cell is sufficient according to NTN parameters. This significantly reduces unnecessary handover procedures.

A possible embodiment of the invention is characterized by measuring all of the following NTN parameters; Doppler parameter, distance parameter, visibility window parameter, signal strength parameter.

One aspect of the invention is also includes or provides a UE which is configured to realize above steps.

Another aspect of the Invention is also includes or provides a communication system comprising at least two NTN transmitters and at least a user equipment above mentioned.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a drawing illustrating top schematic view of the system.

Figure 2 is a drawing illustrating schematic view of user equipment. REFERENCE NUMBERS GIVEN IN THE FIGURE

100 User equipment

110 Processor unit

120 Communication unit

200 NTN Transmitter

210 Serving NTN transmitter

220 Target NTN transmitter

300 Cell

310 Serving Cell

320 Target Cell

DETAILED DESCRIPTION OF THE INVENTION

In this detailed description, the subject matter is explained with references to examples without forming any restrictive effect only in order to make the subject more understandable.

Reference is made to Third Generation Partnership Project (3GPP) system, in accordance with embodiments of the present disclosure. The present application employs abbreviations, terms and technology defined in accord with 3GPP technology standards, including the following standards and definitions. 3GPP technical specifications (TS) and technical reports (TR) incorporated in their entirety by reference herein, define the related terms and architecture reference models that follow.

UE: User equipment

NTN: Non-Terrestrial Network

LEO: Low Earth orbit

GEO: Geostationary Equatorial Orbit

Present invention provides a communication system, a user equipment (100) for reselecting cells (300) and a method thereof.

System comprises plurality of NTN transmitters (200) that serve cells to users. NTN transmitter (200) refers to the individual communication entities within the non-terrestrial network. NTN transmitters (200) may be include satellites (e.g. LEO, GEO) spacecraft, landers, rovers, or any other non-terrestrial infrastructure that acts as a communication endpoint. NTN transmitters (200) facilitate data transmission, reception, and routing within the network.

User equipment (100) are end-user devices that connect to the NTN for communication. User equipment (100) may be tablet computers, smart phones, non-terrestrial vehicle communication devices or any other devices designed to access and utilize the services provided by the NTN network. User equipment (100) may comprise a processor unit (1 10) in order to realize cell reselection method and a communication unit (120) in order to transmit signals to NTN transmitter (200) and receive signals from NTN transmitter (200). The communication unit (120) comprises hardware elements, such as antennas and circuitry etc. that is used for measuring, transmitting, and receiving signals. Such hardware elements is well-known in the art and is not explicitly disclosed herein.

Referring to figure 1 , the user equipment (100) is being served by a serving NTN transmitter (210) via a serving cell (310). A target NTN transmitter (210) provides plurality of target cells (320).

User equipment (100) utilizes following parameters in order to reselect cell (300).

A Doppler parameter: Doppler parameter provides information regarding the movement direction of NTN transmitters (200) it is defined by Doppler shift of received signals from NTN transmitter (200). Doppler parameter may be acquired by chirp based method which is a 2 dimensional Fast Fourier Transform (2D-FFT) operation on the received signal beat frequency where the second Fast Fourier Transform (FFT) provides the Doppler occurrence.

A distance parameter: Distance parameter is defined by the distance between serving cell (310) and target cell (320). The distance information can be provided by the core network or can be measured directly since the orbits are fixed. Distance between the serving cell (300) and target cell (320) is utilized to minimize the frequency of handovers and maximize the duration of communication with each NTN transmitter (200).

A visibility window parameter is defined by the conditions whether there is a direct line of sight between user equipment (100) and NTN transmitter (200) and whether a predetermined Doppler spread value of NTN transmitter (200) meets at least a predetermined condition. Doppler spread values of NTN transmitters (200) may be acquired from a database. LOS is defined based on the angle between NTN node and UE in 3GPP. It can determined using 3GPP defined methods.

A signal strength parameter: Signal strength parameter is defined by power of received signal of NTN transmitter. It may be received signal strength power (RSRP) as known in the art.

User equipment executes subject matter invention’s steps in order to realize cell reselection.

Cell reselection method comprises following steps: Determining at least a target NTN transmitter (220) which provides at least a target cell (320) where said user equipment (100) is in coverage of said target cell (320). This step may be realized by methods known in the art. Target NTN transmitter (220) is one of the neighbor nodes of serving NTN transmitter (210). All neighbor nodes are provided to serving NTN transmitter (210) by core network using neighbor node list.

Measuring at least two of following NTN parameters of serving NTN transmitter (210) and at least a target NTN transmitter (220); a Doppler parameter which is defined by Doppler shift of received signal from NTN transmitter (200), a distance parameter which is defined by the distance between said serving cell (210) and a target cell (220); a visibility window parameter which is defined by the conditions whether there is a direct line of sight between user equipment (100) and NTN transmitter (200) and whether a predetermined Doppler spread value of NTN transmitter (200) meets at least a predetermined condition; a signal strength parameter which is defined by received signal strength.

In this application the phrase TNT transmitter (200) is used when serving NTN transmitter (210) or target NTN transmitter (220) is directed.

Selecting one of the NTN transmitters (200) according to measured NTN parameters meeting predetermined constraints. Predetermined constrains may be the thresholds predetermined for the parameters. For instance, predetermined constraints for the Doppler may be less than D_th and distance may be less than d_th.

If selected cell (300) is serving cell (310), returning to first step where target NTN transmitters (220) detected. If selected cell (300) is one of the target cells (320), user equipment (100) switches to target cell (300). Switching to target cell (320) may be realized between serving NTN transmitter (210) and target NTN transmitter (220) using a handover procedure.

It is determined that the suitability of a cell (300) is highly dependent on the Doppler shift, in addition to the signal strength. Further, the distance between the serving cells (210) and target cells (220) affect the frequency of handover process. The cell suitability is also dependent on the visibility window (i.e., line of sight conditions). Present invention provides a method that utilizes at least two of the related mentioned TNT parameters in order to realize cell (200) reselection that provides more suitable cells for user equipment.

The scope of protection of the invention is specified in the attached claims and cannot be limited to those explained for sampling purposes in this detailed description. It is evident that a person skilled in the art may exhibit similar embodiments in light of the above-mentioned facts without drifting apart from the main theme of the invention.

Claims

1. A cell reselection method for a non-terrestrial network realized by a user equipment (100) being served by a serving NTN transmitter (210) in a serving cell (310) characterized in that comprising following steps:

- determining at least a target NTN transmitter (220) providing at least a target cell (320) and which the user equipment (100) is in coverage of, measuring at least two of following NTN parameters of serving NTN transmitter (210) and at least a target NTN transmitter (220); a Doppler parameter which is defined by Doppler shift of received signal from NTN transmitters (200), a distance parameter which is defined by the distance between said serving cell (310) and a target cell (320); a visibility window parameter which is defined by the conditions whether there is a direct line of sight between user equipment (100) and NTN transmitter (200) and whether a predetermined Doppler spread value of NTN transmitter (200) meets at least a predetermined condition; a signal strength parameter which is defined by received signal strength;

- selecting one of the NTN transmitters (200) and a cell (300) according to measured NTN parameters meeting predetermined constraints.

2. The cell reselection method according to claim 1 , characterized in that measuring all of the following NTN parameters; Doppler parameter, distance parameter, visibility window parameter, signal strength parameter.

3. A user equipment (100) which is configured to realize steps in claims 1-2.

4. A system comprising at least two NTN transmitters (200) and at least a user equipment (100) such as defined in claim 3.