WO2025136334A1 - A delay difference based group handover method in non-terrestrial networks - Google Patents

Abstract

A method realized by a NTN base station (110) in a non-terrestrial network where NTN base stations (110) provide communication services to plurality of user equipment (150). Characterized in that comprising steps of: acquiring measurement reports comprising information that a handover of a plurality of user equipment (150) currently connected to a source cell (121) is needed to a target cell (131); determining a delay for each user equipment (150) where said delay is defined by delay between a user equipment (150) and the NTN base station (110); selecting a reference user equipment from user equipment (150) that sent measurement reports; calculating a delay difference between each remaining user equipment and said reference user equipment; grouping user equipment (150) having delay differences that fall within a predetermined delay window into a handover group; initiating handover for the handover group.

Description

A DELAY DIFFERENCE BASED GROUP HANDOVER METHOD IN NON-TERRESTRIAL NETWORKS

TECHNICAL FIELD

A method realized by a NTN base station in a non-terrestrial network where NTN base stations provide communication services to plurality of user equipment.

PRIOR ART

Handover, is a critical process in mobile communication systems where an ongoing call or data session is seamlessly transferred from one cell site to another as the user equipment moves across different cell boundaries. This mechanism is essential to maintain uninterrupted service and consistent communication quality. The handover process involves assessing signal strengths, determining the optimal target cell, and reallocating network resources to facilitate the transition, ensuring that the user experiences minimal disruption during mobility. When user equipment realize handover individually, signaling between network devices increase significantly. Concept of group handover is introduced to reduce unnecessary signaling.

Group handover is a handover method that is not included in 3GPP standards but is currently in use. Since the processes of multiple users are combined in a single handover process, it reduces the signaling traffic and processing load on the network. However, the main advantage of group handover compared to individual handovers of users occurs when user mobility is simultaneous between cells. It groups user equipment having same movement pattern, such as multiple user equipment on a bus.

In non-terrestrial networks, the mobility of base stations presents a unique dynamic, contrasting sharply with terrestrial networks where base stations are stationary. This mobility factor is so significant that, in comparison, the movement of users within these networks becomes almost negligible. Consequently, the primary catalyst for handover conditions in nonterrestrial networks stems not from user mobility, but rather from the movement of the base stations themselves. This scenario is markedly different from traditional networks and impacts not just individual users, but groups of users simultaneously. Therefore, employing a group- based handover strategy in non-terrestrial networks is not only more practical but also potentially more efficient, as it accounts for the collective mobility patterns of users influenced by the dynamic positioning of the base stations, n non-terrestrial networks, strategies that focus on grouping and handover based on current location and movement patterns of individual users tend to be less effective. Handover methods in non-terrestrial networks require a different approach, one that accounts for the collective dynamics of user groups in relation to the movement of base stations, rather than relying on traditional, individual-based mobility patterns.

US2015245255A1 discloses an NR-based, LEO Non-Terrestrial Networks (NTN) to improve handover process by sending broadcast or group-east downlink handover (HO) Command message to all (or group of) UEs in the coverage area (cell or beam-spot) of the LEO satellite. Subsequently, on receiving the broadcast (or group-east) HO Command message, all the UEs in the source cell transmit HO Complete message to the source cell of the LEO-NTN. In order to reduce or avoid heavy Random Access (RA), generating from all the UEs, the LEO-NTN can either send Contention Free Random Access (CFRA), or instruct the UE to perform a random backoff before sending the uplink HO Complete message. Moreover, improvements to configure Conditional Handover (CHO) and delaying the transmission of HO Complete message are also provided.

US2015245255A1 discloses a method, including acquiring, by a network node of a source cell, information that a handover of a plurality user terminals currently connected to the source cell is needed to a target cell; determining at least one attribute with respect to each of the plurality of user terminals; selecting at least some of the plurality of user terminals to form a group of user terminals such that the user terminals in the group share at least one common attribute; and indicating the at least one common attribute of the group to a target node of the target cell in a group handover request message.

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 a method and a NTN base station to eliminate the above- mentioned disadvantages and bring new advantages to the relevant technical field. An object of the invention is to reduce signaling between network components while managing handovers.

An object of the invention is to increase successful handovers in a group.

To achieve all the objects mentioned above and that will emerge from the following detailed description, the present invention relates to a method realized by a NTN base station in a nonterrestrial network where NTN base stations provide communication services to plurality of user equipment. Accordingly, it comprises following steps:

- acquiring measurement reports comprising information that a handover of a plurality of user equipment currently connected to a source cell is needed to a target cell.

- determining a delay for each user equipment where said delay is defined by delay between a user equipment and the NTN base station,

- selecting a reference user equipment from user equipment that sent measurement reports,

- calculating a delay difference between each remaining user equipment and said reference user equipment,

- grouping user equipment having delay differences that fall within a predetermined delay window into a handover group,

- initiating handover for the handover group. Thus, grouping is realized without the need of tracking locations of user equipment. In a surprise effect unnecessary handovers is reduced significantly.

A possible embodiment of the invention is characterized in that comprising steps of:

- receiving post handover parameters from user equipment (150) in the handover group;

- determining a handover success rate based on post handover parameters for consecutive time slots,

- increasing delay window if it is determined that handover success rate is increased on consecutive time slots and decreasing delay window if it determined that handover success rate is decreased on consecutive time slots. Thus, delay window changes dynamically in order to achieve optimum handover success rate and reduced signaling.

Another possible embodiment of the invention is characterized in that comprising steps of:

- periodically receiving signal strength measurements from user equipment,

- determining user equipment having signal strength decrease rate higher than a predetermined threshold, - removing determined user equipment from handover group. Thus, user equipment causing group to underperform are excluded in order to prevent it from effecting the delay window of the group.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 2 is a drawing illustrating handover groups having different delay windows.

Figure 3 is a graphic showing handover success rate comparison of dynamic and static window size group handover for 15ms.

REFERENCE NUMBERS GIVEN IN THE FIGURE

110 NTN Base station

120 Source base station

121 Source cell

130 Target base station

131 Target cell

150 User equipment

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.

Present invention is a method to be realized by a NTN (non-terrestrial network) base station (110) of a non-terrestrial network. The method groups user equipment (150) based on their delays from NTN base station (1 10) and realizes handover of groups. Method also provides adaptive parameters in order to increase efficiency of grouping.

Referring to figure 1 , system comprises plurality of NTN base stations (1 10) and plurality of user equipment (150) receiving network services from NTN base stations (1 10). Base stations move relatively to ground. For instance, a target base station (130) and a source base station (120) is moving to right hand side. Thus, some of the user equipment (150) that are at the edge of source cell (121 ) will need to handover to target cell (122).

NTN base stations (1 10) may be provided on high altitude platform stations (HAPS), low earth orbit satellites (LEO) or other mobile NTN components. NTN Base stations (1 10) are well known in the art. NTN Base stations (1 10) are equipped to communicate with user equipment (150), each other and other network components. NTN Base stations (110) are well known in the art so further details are not explained herein.

User equipment (150) may be mobile phones, smart phones, tablet computers and etc. User equipment (150) may be any equipment that receive network service from non-terrestrial network. User equipment (150) are configured and equipped to communicate with other nonterrestrial network components via cells. User equipment (150) are configure to measure certain parameters such as signal strength (reference signal received power (RSRP)). User equipment (150) are configured to measure current signal power of a target cell (132) and signal power of a source cell.

User equipment (150) are configured to monitor conditions for conditional handover. Handover measurement events involve events such as A1 to A5 in 3GPP (3rd Generation Partnership Project). For instance, event A3 is used to determine the required signal strength for a handover. Unlike other events, A3 uses an offset to prevent rapid switches and doesn't need signal threshold adjustments for different conditions. In simple terms, when the signal from a target base station consistently exceeds the offset compared to the source base station, A3 triggers the user equipment to switch to the target base station.

In order to trigger measurement events, it is necessary to wait for the Time-to-trigger (TTT) period after the expected changes in signal strength occur. In this way, ping-pong handovers that may occur due to instantaneous signal strength changes are prevented.

Subject matter inventions groups user equipment (150) in order to realize handover of the group.

Subject matter method is realized by a NTN base station (1 10). The method comprises the steps of: - Acquiring measurement reports comprising information that a handover of a plurality of user equipment (150) currently connected to a source cell (121 ) is needed to a target cell (131 ).

As previously mentioned, user equipment (150) send measurement reports when handover conditions are met. Measurement reports comprises signal strength measurements relating to target cell (130) and source cell (120). Measurement reports may further comprise reference signal received power (RSRP), reference signal receiver quality (RSRQ), signal to interference plus noise ratio (SINR) and received signal strength indicator (RSSI) etc.

Determining a delay for each user equipment (150) where said delay is defined by delay between a user equipment (150) and the NTN base station (110).

For instance, the delay between a user equipment (150) and a base station may be determined using a process known as Timing Advance (TA) in LTE and 5G networks. This process may involve the user equipment (150) sending a signal to the base station, and base station calculating delay.

- Selecting a reference user equipment from user equipment (150) that sent measurement reports.

- Calculating a delay difference between each remaining user equipment (150) and said reference user equipment. For instance if reference user equipment have tO ms and a first user equipment have tO-t1 ms and a second user equipment have t0-t2 ms delay. First and second user equipment would have t1 ms and t2 ms delay difference.

- Grouping user equipment (150) having delay differences that fall within a predetermined delay window into a handover group. For instance delay window may be 0 to t2 ms. In this case first user equipment and second user equipment would be grouped together with the reference user equipment according to above example assuming t2 is higher than t1 .

Initiating handover for the handover group.

In a possible embodiment the method may comprise steps of:

- Receiving post handover parameters from user equipment (150) in the handover group. - Determining a handover success rate based on post handover parameters for consecutive time slots.

- Increasing delay window if it is determined that handover success rate is increased on consecutive time slots and decreasing delay window if it determined that handover success rate is decreased on consecutive time slots (fig 2).

Figure 3 shows a graphic showing successful handover rate on a simulation where a constant time window is used and a dynamic simulation is used. Dynamic delay window significantly increases successful handover rate.

Each user equipment (150) may have different TTT values before triggering measurement events. Because of these differences, specifying a fixed delay window for grouping users may cause the handover procedure to not work as desired in some cases. For this reason, it is necessary to determine a variable window size based on feedback. NTN base station (1 10) uses the post handover measurements received in the previous time slot to calculate handover success rate. When calculating the success rate of the grouping process, the ratio of grouped users who successfully handover and unsuccessfully grouped user equipment (150) can be used.

Unsuccessful handover may be defined as start time of handover process exceeding a predetermined time or falling below another predetermined time.

User equipment that get into target cell (131 ) after base station reference user equipment are grouped together depending on the delay window. If delay window is too large, reference user equipment maybe late for a desirable handover and users that grouped last would be subjected to handover that is too early for a desirable handover.

Successful handover rate may be calculated using the number of successful handovers and the number of Radio Link Failure (RLF) users.

If the success rate increases according to this parameter, the window size is increased by multiplying by an Alpha coefficient. Said alpha coefficient can be determined according to the type of base station used (LEO Satellite or HAPS), characteristics of the base station (altitude, coverage radius, movement speed, etc.) and environmental characteristics. If the success rate decreases according to this parameter, the window size is reduced by multiplying by a Theta coefficient. Theta coefficient can be determined according to the type of base station used (LEO Satellite or HAPS), characteristics of the base station (altitude, coverage radius, movement speed, etc.) and environmental characteristics.

If the success rate falls below a certain threshold, the window size is reduced to initial window size. Initial window size is may be the smallest window size. This window size can be enlarged by multiplying it by the alpha coefficient, but it cannot be reduced by multiplying it by the theta coefficient. Initial window size can be determined according to the type of base station used (LEO Satellite or HAPS), characteristics of the base station (altitude, coverage radius, movement speed, etc.) and environmental characteristics.

In another possible embodiment in addition to steps in previous embodiment the method may comprise following steps:

- Periodically receiving signal strength measurements from user equipment (150).

- Determining user equipment having signal strength decrease rate higher than a predetermined threshold.

- Removing determined user equipment (150) from handover group. Thus, user equipment (150) that effect delay window negatively is removed from group. The NTN base station (1 10) monitors the changes in signal strength of grouped users. If the rate of change in signal strength is high and new users are expected to join the group and the start of the handover procedure will be delayed, so these users must be handed over to another base station without leaving the coverage area. Due to this situation, users are dropped from the group and individual handovers of these users are performed by the NTN base station (1 10). Said predetermined threshold may be a Kappa coefficient and it can be determined according to the type of base station used (LEO Satellite or HAPS), characteristics of the base station (altitude, coverage radius, movement speed, etc.) and environmental characteristics.

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 method realized by a NTN base station (110) in a non-terrestrial network where NTN base stations (110) provide communication services to plurality of user equipment (150) characterized in that comprising steps of

- acquiring measurement reports comprising information that a handover of a plurality of user equipment (150) currently connected to a source cell (121 ) is needed to a target cell (131 ),

- determining a delay for each user equipment (150) where said delay is defined by delay between a user equipment (150) and the NTN base station (110),

- selecting a reference user equipment from user equipment (150) that sent measurement reports,

- calculating a delay difference between each remaining user equipment (150) and said reference user equipment,

- grouping user equipment (150) having delay differences that fall within a predetermined delay window into a handover group,

- initiating handover for the handover group.

2. The method according to claim 1 , characterized in that comprising steps of

- receiving post handover parameters from user equipment (150) in the handover group;

- determining a handover success rate based on post handover parameters for consecutive time slots,

- increasing delay window if it is determined that handover success rate is increased on consecutive time slots and decreasing delay window if it determined that handover success rate is decreased on consecutive time slots.

3. The method according to claim 2, characterized in that comprising steps of

- periodically receiving signal strength measurements from user equipment (150),

- determining user equipment having signal strength decrease rate higher than a predetermined threshold, removing determined user equipment (150) from handover group.

- time slots.

4. The method according to claim 1 , wherein first user equipment (150) to enter target coverage area is selected as reference user equipment.

5. A NTN base station (110) which is configured to realize a method from any one of the claims 1-4.