Handover from a mobile base station

Abstract

The invention relates to handover from a mobile base station, in particular a method in a cellular telecommunication network having a first base station and a second base station, wherein the first base station is moving with respect to a reference object and serves a user equipment, UE, the method comprising the steps of: identifying a relative speed of the UE with respect to the reference object; preparing a message to transfer the UE to the second base station, the message comprising one or more information elements to indicate the relative speed of the UE with respect to the reference object, wherein a mobility state of the UE can be estimated from the one or more information elements indicating the relative speed of the UE with respect to the reference object; and initiating the transfer of the UE from the first base station to the second base station by sending the message to the second base station, wherein the message comprises the one or more information elements to indicate the relative speed of the UE with respect to the reference object.

Description

[0001] This application is a divisional application of patent application filed on January 11, 2019, with application number 201980014411.7 (international application number PCT / EP2019 / 050677) and invention title "Handover from Mobile Base Station". Technical Field

[0002] This invention relates to methods in cellular telecommunications networks. In particular, this invention relates to methods for determining mobility state estimates of user equipment (UE) in cellular telecommunications networks. Background Technology

[0003] Cellular telecommunications networks consist of multiple base stations, each with its own coverage area. User equipment (UE) is typically mobile and travels between the coverage areas of multiple base stations. Cellular telecommunications networks provide several processes, called cell reselection and handover, to allow a UE to move from the coverage area of ​​one base station to the coverage area of ​​another without losing service.

[0004] One parameter in cellular telecommunications networks is the UE's speed (or its estimate), which is referred to as "mobility state estimate" in the Long Term Evolution (LTE) protocol. When the UE is in connected mode, the serving base station estimates this parameter to classify the UE into one of three mobility states (e.g., "normal," "fast," or "very fast"). Based on this mobility state, the base station configures several mobility-related parameters for the UE (e.g., measurement report triggering).

[0005] When a UE first connects to a base station in a cellular telecommunications network (e.g., after power-on), the base station classifies the UE as having a "normal" mobility state estimate and uses associated mobility-related parameters. Subsequently, as the UE moves between the corresponding coverage areas of the base stations, it will transfer between them. After several such handovers, the base station will estimate the UE's mobility state more accurately based on information contained in the handover request message. The handover request message from the serving base station to the target base station includes a "UE History" field, which indicates the last 16 serving base stations for the UE, including the current serving base station: the identifier of each serving base station, the time the UE stayed in the coverage areas of those serving base stations, and the approximate size of each coverage area. The target base station can then estimate the UE's mobility state based on this historical information.

[0006] Modern cellular telecommunications networks also use mobile base stations, which can be self-powered or moved by fixed-to-mobile vehicles (such as trains or cars). They are particularly useful in areas where demand suddenly increases and / or where fixed infrastructure is difficult or expensive to deploy. When a UE connects to a mobile base station, if the UE moves at the same or similar speed relative to the mobile base station's frame of reference, the UE can be correctly identified as having a "normal" mobility state estimate. However, if both the mobile base station and the UE move relatively quickly relative to the non-mobile base station's frame of reference, problems arise if the non-mobile base station is the target base station from which the UE is being handed over. That is, if the UE's historical information indicates that the UE has been within the mobile base station's coverage area for a long time, the target base station will classify the UE as having a "normal" mobility state estimate and set its mobility-related parameters based on that estimate (e.g., measurement report triggering). However, if both the UE and the serving base station are moving and moving relatively quickly relative to the target base station's frame of reference, such that the UE is more accurately described as "fast" or "very fast" relative to the target base station's frame of reference, then these mobility-related parameters will be inappropriate. The UE will then experience poorer Quality of Service (QoS).

[0007] Therefore, it is desirable to alleviate some or all of the above problems. In particular, it is desirable that the UE's mobility state estimate be correctly classified after the transfer from the mobile serving base station to the target base station. Summary of the Invention

[0008] According to a first aspect of the invention, a method is provided in a cellular telecommunications network having a first base station and a second base station, wherein the first base station moves relative to a reference object and serves a user equipment (UE), the method comprising the steps of: identifying the relative velocity of the UE relative to the reference object; preparing a message to transfer the UE to the second base station, the message including one or more information elements to indicate the relative velocity of the UE relative to the reference object, wherein the mobility state of the UE can be estimated based on the one or more information elements indicating the relative velocity of the UE relative to the reference object; and initiating a transfer of the UE from the first base station to the second base station by sending a message to the second base station, wherein the message includes one or more information elements to indicate the relative velocity of the UE relative to the reference object. The reference object may be stationary (e.g., the Earth).

[0009] A handover can be one of the following groups: handover and cell reselection.

[0010] The one or more information elements may include a first information element and a second information element, wherein the first information element represents the size of the coverage area of ​​the serving base station, and the second information element represents the time during which the UE has been located within the coverage area of ​​the first base station, wherein the time value may be modified based on the relative speed of the UE relative to a reference object.

[0011] The first base station may be able to connect to the core networking entity via a donor connection to the donor base station, and the one or more information elements may include a first information element and a second information element, the first information element indicating the size of the coverage area of ​​the donor base station and the second information element indicating the time during which the donor base station has been used as a donor to the first base station.

[0012] The relative movement of the UE with respect to the first base station can make it possible that if the first base station is stationary relative to the reference object, the UE is located within the coverage area of ​​the first base station for a shorter time, and the time value can be reduced.

[0013] The relative movement of the UE with respect to the first base station allows the UE to be positioned within the coverage area of ​​the first base station for a longer period of time if the first base station is stationary relative to the reference object, and the time value can be increased.

[0014] The method may also include the following steps: the second base station estimates the mobility state of the UE based on one or more information elements indicating the relative speed of the UE with respect to a reference object.

[0015] According to a second aspect of the invention, a computer program product including instructions is provided, which, when executed by a computer, cause the computer to perform the method of the first aspect of the invention. The computer program can be stored on a computer-readable data carrier.

[0016] According to a third aspect of the invention, a network node for a cellular telecommunications network is provided, the cellular telecommunications network including a base station adapted to move relative to a reference object and serve a user equipment (UE), wherein the network node includes a memory and a processor configured to implement the steps of the method according to a first aspect of the invention, wherein the network node is a first base station or a UE. Attached Figure Description

[0017] To better understand the present invention, embodiments thereof will now be described by way of example only, with reference to the accompanying drawings, wherein:

[0018] Figure 1 This is a schematic diagram of a first embodiment of the cellular telecommunications network of the present invention;

[0019] Figure 2 yes Figure 1 A schematic diagram of the first ground base station of the network;

[0020] Figure 3 yes Figure 1 A schematic diagram of a mobile base station in a network;

[0021] Figures 4a to 4c Examples Figure 3 mobile base stations relative to Figure 1 The first and second ground base stations of the network move;

[0022] Figure 5 This is a flowchart of the first embodiment of the method of the present invention;

[0023] Figures 6a to 6c A mobile base station according to a second embodiment of the present invention is illustrated; and

[0024] Figure 7 This is a flowchart of a second embodiment of the method of the present invention. Detailed Implementation

[0025] Now refer to Figures 1 to 3 A first embodiment of the cellular telecommunications network 1 of the present invention is described. The cellular telecommunications network 1 includes a first ground base station 10, a second ground base station 20, and a mobile base station 30. In this embodiment, the mobile base station 30 is an unmanned aerial vehicle (UAV) and serves a user equipment (UE) 40.

[0026] exist Figure 2 The first ground base station 10 is shown in more detail below. The first ground base station 10 includes a first transceiver 11, a processor 13, a memory 15, and a second transceiver 17, all connected via a bus 19. In this embodiment, the first transceiver 11 is an antenna configured for wireless communication via the Long Term Evolution (LTE) protocol, and the second transceiver 17 is an optical fiber connection for wired communication with the core network (not shown) of the cellular telecommunications network 1 and other base stations. The first ground base station 10 is configured to use the first transceiver 11 to communicate with a UE (e.g., UE 40) of the cellular network 1 to provide voice and / or data communication services.

[0027] The first ground base station 10 stores a "UE history" database in memory 15. For each UE served by the first ground base station 10, this data indicates information about the first ground base station 10 and up to fifteen previously serving base stations of the UE. This information includes their identifiers (e.g., E-UTRAN Cell Global Identifier, eCGI), the approximate size of their coverage areas (e.g., very small, small, medium, or large), and the time the UE remained within their coverage areas. As will be described in more detail in an embodiment of the method of the present invention described below, this information can be used to define a UE's mobility state estimate (i.e., speed estimate).

[0028] The second ground base station 20 is substantially the same as the first ground base station 10, enabling the second ground base station 20 to communicate with the UE (e.g., UE 40) using the LTE protocol and to communicate with the core network via an optical fiber connection.

[0029] exist Figure 3 The mobile base station 30 is shown in more detail, and it is similar to the first terrestrial base station 10, including a first transceiver 31 and a second transceiver 37, a processor 33, and a memory 35, all connected via a bus 39. However, the second transceiver 37 is a wireless communication interface for communicating with the core network of the cellular network 1, which in this embodiment is implemented via the terrestrial base station. Therefore, the mobile base station 30 can serve the UE 40 via its first transceiver 31, and any data traffic associated with the UE is transmitted between the UE and the core network via the mobile base station 30 and the first terrestrial base station 10 or the second terrestrial base station 20 (that is, the first terrestrial base station 10 or the second terrestrial base station 20 can be used as a "donor" base station for the mobile base station 30).

[0030] In this embodiment, the mobile base station 30 includes multiple rotors enabling it to generate lift, multiple actuators and sensors controlling its flight (which operate under the control of the autonomous flight function of the processor 33), and a power supply. Furthermore, the mobile base station 30 includes: Global Navigation Satellite System (GNSS) functionality (such as Global Positioning System (GPS) functionality) to determine its position (and its time derivatives, such as velocity and acceleration); a compass to determine its orientation; and an altimeter to determine its altitude.

[0031] Now refer to Figure 4a , Figure 4b , Figure 4c and Figure 5 A first embodiment of the method of the present invention is described. In this embodiment, mobile base station 30 serves UE 40 and uses first ground base station 10 as donor base station. Figure 4a As shown in the schematic diagram, the mobile base station 30 moves at a first speed (e.g., Figure 4a (As indicated by the corresponding arrow), UE40 also moves at a first speed or at a speed substantially the same as the first speed (e.g., ...). Figure 4a (As indicated by the corresponding arrows in the diagram), and both the first ground base station 10 and the second ground base station 20 are stationary. Therefore, the UE 40 remains substantially stationary relative to the mobile base station 30, while both the UE 40 and the mobile base station 30 move relative to the first ground base station 10 and the second ground base station 20. Figure 4a , Figure 4b and Figure 4cThe network at times t, t+1, and t+2 illustrates the movement of mobile base station 30 and UE 40 toward the location of the second ground base station 20.

[0032] At time t+2, mobile base station 30 and UE 40 move to a position within the coverage area of ​​the second ground base station. In step S1, as... Figure 5 As shown, the conditions are met to initiate a handover of UE 40 from mobile base station 30 to second ground base station 20. Therefore, mobile base station 30 prepares a handover request message, which in this embodiment is prepared according to the following steps.

[0033] In step S3, mobile base station 30 retrieves data from its UE history database for UE 40. This data is shown in the table below:

[0034]

[0035] Table 1: An example of UE history records for UE 40 stored in the memory of mobile base station 30.

[0036] In step S5, the mobile base station 30 compiles a handover request message including a first part and a second part. The first part includes a request to the second ground base station 20 to accept the handover for UE 40 and subsequently provide voice and / or data communication services to UE 40. The second part is a UE history information element that includes the UE history information retrieved in step S3.

[0037] In step S7, mobile base station 30 modifies one or more values ​​of the UE history information element for UE 40. In this embodiment, mobile base station 30 modifies the value of "time the UE stayed in the base station's coverage area" to indicate the time period during which UE 40 stayed in the base station's coverage area if mobile base station 30 was stationary. This can be achieved by the following method:

[0038] Mobile base station 30 uses GPS measurements to determine its own speed v MBS This can also be expressed as the relative velocity of a mobile base station with respect to a stationary object.

[0039] • The mobile base station 30 determines its relative speed with the UE 40 based on the angle of arrival and / or the UE location report. as well as

[0040] ·Mobile base station 30 according to Calculate the relative velocity of the UE with respect to a stationary object.

[0041] • Mobile base station 30 uses a stored value of "the size of the base station's coverage area" (=Size1) and the UE's speed relative to a stationary object. The system calculates the time the UE would remain within the coverage area of ​​the mobile base station if the mobile base station 30 were stationary. Since the stored value for the size of the base station's coverage area is one of "very small," "small," "medium," or "large," the base station first converts this stored value into an appropriate distance metric (e.g., a normalized distance value for each size). Then, the mobile base station 30 divides this distance by the UE's velocity relative to a stationary object. To calculate the modified time value.

[0042] In step S9, mobile base station 30 sends a handover request message to second ground base station 20. In step S11, second ground base station 20 determines that the handover can proceed. In step S13, second ground base station 20 populates its corresponding UE history database with data from the UE history information element in the handover request message, including a modified time value for the "time the UE stayed in the coverage area of ​​the base station" for mobile base station 30, and in step S15, estimates the mobility state of UE 40 based on this information. Because the time value has been modified to represent the speed of the UE relative to a stationary object rather than relative to mobile base station 30, second ground base station 20 estimates the mobility state of UE 40 to accurately reflect its speed relative to (also stationary) second ground base station 20. In step S17, second ground base station 20 sets mobility-related parameters, such as measurement reporting parameters, based on this mobility state estimate. These parameters are now more appropriate for the speed of the UE relative to second ground base station 20 compared to parameters set if the mobility state were estimated from the original time value "Time1". In one example, the second ground base station 20 may set a measurement reporting frequency suitable for a rapidly moving UE that may move out of its coverage area within a short time frame, whereas if the mobility state is estimated from raw time values, the measurement frequency may be set too low (or even not at all). Alternatively, the second ground base station 20 may use event-based measurement reports, and these events may be set based on mobility state estimates.

[0043] In step S19, the target base station 20 (via mobile base station 30) sends a handover request confirmation message to the UE. This includes RRC connection reconfiguration parameters, which include (or derive from) the mobility-related parameters determined in step S17.

[0044] In the above embodiments, mobile base station 30 and UE 40 have speeds in the same direction. However, those skilled in the art will understand that the method is equally applicable to other situations, such as when UE 40 is served by mobile base station 30 traveling in the opposite direction relative to UE 40. That is, the step of determining the relative speed between UE 40 and mobile base station 30 will take this into account by summing the two relative speeds together (wherein one value will naturally be negative due to the opposite direction). This will result in the time value being modified to indicate that the UE has been in the coverage area of ​​mobile base station 30 for a longer period if mobile base station 30 is stationary. The target base station can then interpret the modified UE history information elements to estimate a mobility state (e.g., "normal") that is more suitable for a slower-moving UE compared to the estimated mobility state (e.g., "fast") if the time value is not modified.

[0045] In the example above, the UE and mobile base station 30 move at the same speed (e.g., they are both traveling on the same moving vehicle, such as a train), making it possible to implement the invention by determining the speed only along a single axis (in fact, it is possible to implement the invention using only scalar speed). Those skilled in the art will also understand that the above method can be extended to cover cases where the UE and mobile base station 30 move not only along a single axis. That is, the method outlined above can be extended to cover motion in a two-dimensional coordinate system. In this way, the relative speed of the UE 40 relative to a stationary object can be resolved on two axes, and the time value can then be modified based on the larger of the two relative speeds.

[0046] Those skilled in the art will also understand that the target base station can use additional factors to estimate the UE's mobility state. That is, if the target base station itself is a mobile base station, it can estimate the UE's mobility state using both the UE's relative velocity relative to a stationary object, its own relative velocity to the UE (e.g., based on UE location reports), and the size of the target base station's coverage area. Therefore, it is advantageous for the serving base station to modify the time value based on the UE's relative velocity relative to a stationary object (such as the Earth), allowing the target base station to interpret the time value in any suitable manner. The serving base station can also modify the time value to indicate the UE's relative velocity relative to any reference object, which both the serving and target base stations can calculate to implement the methods described above. However, relative velocity relative to a stationary object (such as the Earth) requires fewer processing steps.

[0047] In the above embodiments, the serving base station modifies the time value of the UE remaining within its coverage area. However, those skilled in the art will understand that the serving base station can also modify the size of its coverage area to achieve the same technical benefit. In this way, the serving base station can convey the relative speed between the UE and a stationary object by modifying its coverage area to be larger or smaller than its actual coverage area (e.g., "small" to "medium" or "large" to "medium"), indicating that the relative speed of the UE relative to the stationary object is greater or less than its speed relative to the serving base station. However, modifying the time value is beneficial because there is a finite set of options for coverage area size, and this method loses accuracy.

[0048] The above embodiments of the present invention also have the technical advantage of solving the problem of incorrect UE mobility state estimation when the UE is transferred from a mobile serving base station in a backward compatible manner with existing base stations. In other words, by modifying the value indicating the UE's speed according to standardized messages (e.g., the UE history record of the handover request message IE), the serving base station can encourage any target base station to estimate the correct UE mobility state.

[0049] In the above embodiments, the serving base station 20 has a connection to the core network via a donor connection to another base station. However, this is not necessary, as the serving base station 20 can connect to the core network via any suitable method (e.g., microwave link, wireless local area network (WLAN), etc.).

[0050] There are other examples of how the serving base station modifies UE history information elements to convey the relative speed of the UE relative to a stationary object, which will now be described.

[0051] Now refer to Figure 6a , Figure 6b , Figure 6c and Figure 7 A second embodiment of the method of the present invention is described. In this embodiment, the cellular telecommunications network further includes a third terrestrial base station 50, which, like the first and second terrestrial base stations, can also be used as a donor base station for the mobile base station 30. In this embodiment, the UE 40 is located within the coverage area of ​​the mobile base station 30 and is served by the mobile base station 30. At time t, as... Figure 6a As shown, both mobile base station 30 and UE 40 are within the coverage area of ​​the first ground base station 10, which acts as the donor base station of mobile base station 30.

[0052] Mobile base station 30 is configured to populate its UE history database with information about the UE's previous serving base stations. However, in this embodiment, the UE history database is modified such that each previous base station requiring a donor connection to the core network (e.g., mobile base station 30 in this second embodiment) is replaced with information about one or more of its donor base stations. Therefore, in this example, the UE history includes information about the mobile base station's donor base station (first ground base station 10) and up to fifteen previous base stations (which previously served the UE directly or acted as donor base stations to base stations that previously served the UE). The information about the donor base stations includes: the donor base station's identifier (eCGI), the approximate size of the donor base station's coverage area (e.g., very small, small, medium, or large), and the duration for which the donor base station acted as a donor to the mobile base station. This information is used in handover messages to prompt the target base station to more accurately estimate the UE's mobility state, as will be described in detail below according to the method of this second embodiment. This will now be described.

[0053] like Figure 6b As shown, both mobile base station 30 and UE 40 move from the coverage area of ​​the first ground base station to the coverage area of ​​the second ground base station at time t+1, and the second ground base station 20 becomes the donor base station of mobile base station 30. In response to mobile base station 30 detecting that it is being served by a new donor base station, it uses the information about the second ground base station 20 to update its UE history database (and the information about the first ground base station 10 is retained in the database as the previous donor base station).

[0054] like Figure 6c As shown, at time t+2, both mobile base station 30 and UE 40 move from the coverage area of ​​the second ground base station to the coverage area of ​​the third ground base station, and the third ground base station 50 becomes the donor base station of mobile base station 30. Again, mobile base station 30 uses the information about the new donor base station to update its UE history database, and information about the first ground base station 10 and the second ground base station 20 is retained in the database. Furthermore, a condition is met that triggers a handover of UE 40 from mobile base station 30 to the third ground base station 50 (step S21).

[0055] Following this trigger, in step S23, the mobile base station 30 retrieves all entries from the UE history database in the memory. As described above, this includes data about the last sixteen previous base stations (which previously served the UE directly or served as donor base stations for the UE) for the mobile base station 30, including their respective identifiers, their respective sizes, and the duration of service provided by each donor base station to the mobile base station 30.

[0056] In step S25, the mobile base station 30 prepares a handover request message having a first part and a second part. The first part includes a request to the third ground base station 50 to accept the handover of the UE 40 and subsequently provide voice and / or data services to the UE 40. The second part includes a UE history information element. In this embodiment, the following processing is used to populate the UE history information element.

[0057] In step S27, mobile base station 30 determines whether the target base station is the current donor base station. If the determination is negative (meaning the target base station is not the current donor base station), the data is used to populate the UE history information element without any changes (step S29a). However, in this embodiment, the determination in step S27 is positive; therefore, in step S29b, mobile base station 30 modifies the data of the last donor base station (third ground base station 50) to use the mobile base station's identifier and coverage area size (keeping the time value the same to indicate when the third ground base station 50 became the donor of mobile base station 30).

[0058] In step S31, mobile base station 30 sends a handover request message to third ground base station 50. In step S33, third ground base station 50 determines that a handover can proceed. In step S35, third ground base station 50 populates its corresponding UE history database with data from the UE history information element in the handover request message, including information about each previous base station, and in step S37, estimates the mobility state of UE 40 based on this data. Therefore, UE 40 estimates its mobility state as if it were previously served by the first (stationary) base station 10 and the second (stationary) base station 20, rather than by mobile base station 30, using their respective coverage area sizes and the time the UE remained within their respective coverage areas (based on the time mobile base station 30 used each base station as a donor). Since these two previous donor base stations are stationary, this again conveys the UE's velocity relative to a stationary object, allowing the target base station to accurately estimate its mobility state. Then, third ground base station 50 determines the mobility-related parameters of UE 40 based on this more accurate mobility state. In step S39, the third ground base station 50 (via the mobile base station 30) sends a handover request confirmation message to the UE. This includes RRC connection reconfiguration parameters, which include (or derive from) these mobility-related parameters.

[0059] In the second embodiment described above, if the most recent donor is the target base station, the mobile base station 30 uses its own data to modify the entry in the UE history IE, but the time value remains the time when the target base station became the donor of the mobile base station. The advantage of this is that it avoids any confusion caused by future handovers, where the UE history IE might otherwise indicate that a third ground base station is the serving base station, and also the immediately preceding serving base station. However, this is not mandatory. In an alternative scenario, the mobile base station 30 can modify the UE history IE such that the base station identifier includes dummy data.

[0060] Those skilled in the art will also understand that the above-described embodiments can also be applied to situations where the UE is in idle mode and is being transferred from a mobile base station to another base station during a cell reselection process. In the idle mode implementation of the first embodiment of the present invention, the UE can, for example, use GPS to determine its own speed (i.e., its relative speed to a stationary object). The UE can then estimate its mobility state based on the number of cell reselections performed within a certain time period, which is modified to take into account its actual speed. That is, the UE calculates the time it would take to be located within the coverage area of ​​the mobile base station if the mobile base station 30 were stationary (e.g., based on the size of the mobile base station's coverage area and the UE's own speed), rather than using the actual time the UE was located within the mobile base station's coverage area, in order to estimate its mobility state more accurately. In the idle mode implementation of the second embodiment, the mobile base station 30 can also broadcast information about its donor base station, and the UE can use the donor base station information instead of the mobile base station information for its mobility state estimation.

[0061] The above implementation is described for the LTE protocol, in which the UE speed estimation is explicitly referred to as mobility state estimation. However, the term "mobility state" encompasses all forms of UE speed estimation in any cellular communication protocol.

[0062] Those skilled in the art will understand that any combination of features is possible within the scope of the claims of this invention.

Claims

1. A method in a cellular telecommunications network, said cellular telecommunications network having a first base station and a second base station, wherein, The method, in which the first base station moves relative to a reference object and serves a user equipment (UE), includes the following steps: The first base station prepares a message to hand over the UE to the second base station, the message including one or more information elements indicating the relative speed of the UE relative to the reference object, wherein the mobility state of the UE can be estimated based on the one or more information elements indicating the relative speed of the UE relative to the reference object; and The first base station initiates a handover of the UE from the first base station to the second base station by sending the message to the second base station, wherein the message includes one or more information elements indicating the relative speed of the UE relative to the reference object, wherein the first base station is connected to the core networking entity via a donor connection to a donor base station, and the one or more information elements include a first information element and a second information element, wherein the first information element indicates the size of the coverage area of ​​the donor base station, and the second information element indicates the time during which the donor base station has been used as a donor of the first base station.

2. The method according to claim 1, further comprising the following steps: The second base station estimates the mobility state of the UE based on one or more information elements that indicate the relative speed of the UE with respect to the reference object.

3. A network node for a cellular telecommunications network, the cellular telecommunications network including a base station adapted to move relative to a reference object and serve a user equipment (UE), wherein, The network node includes a memory and a processor, the processor being configured to implement the steps of the method according to claim 1 or 2, wherein the network node is the first base station.

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