Layer 1 / layer 2 triggered mobility

Abstract

A method includes receiving, by a source CU from a user device, a L3 measurement report; determining that a handover should be performed for the user device from a serving cell associated with the source centralized unit to a first target cell associated with a target CU, wherein a handover configuration for the first target cell has not yet been prepared; determining to provide to a source DU associated with the serving cell, a LTM handover configuration with a relaxed handover requirement for a LTM handover of the user device to a second target cell associated with the source CU; transmitting, to the source DU, the LTM handover configuration for the second target cell; and transmitting, to the target CU associated with the first target cell, a handover request to prepare a handover configuration for the user device to perform a handover to the first target cell.

Description

[0001] LAYER 1 / LAYER 2 TRIGGERED MOBILITY

[0002] TECHNICAL FIELD

[0003] This description relates to wireless communications.

[0004] BACKGROUND

[0005] A communication system may be a facility that enables communication between two or more nodes or devices, such as fixed or mobile communication devices. Signals can be carried on wired or wireless carriers.

[0006] An example of a cellular communication system is an architecture that is being standardized by the 3rd Generation Partnership Project (3GPP). A recent development in this field is often referred to as the long-term evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) radio-access technology. EUTRA (evolved UMTS Terrestrial Radio Access) is the air interface of 3GPP's Long Term Evolution (LTE) upgrade path for mobile networks. In LTE, base stations or access points (APs), which are referred to as enhanced Node AP (eNBs), provide wireless access within a coverage area or cell. In LTE, mobile devices, or mobile stations are referred to as user equipments (UE). LTE has included a number of improvements or developments. Aspects of LTE are also continuing to improve.

[0007] 5G New Radio (NR) development is part of a continued mobile broadband evolution process to meet the requirements of 5G, similar to earlier evolution of 3G and 4G wireless networks. In addition, 5G is also targeted at the new emerging use cases in addition to mobile broadband. A goal of 5G is to provide significant improvement in wireless performance, which may include new levels of data rate, latency, reliability, and security. 5G NR may also scale to efficiently connect the massive Internet of Things (loT) and may offer new types of mission- critical services. For example, ultra-reliable and low-latency communications (URLLC) devices may require high reliability and very low latency. 6G and other networks are also being developed.

[0008] SUMMARY

[0009] A method may include receiving, by a source centralized unit from a user device via a source distributed unit, a layer 3 measurement report that provides results of radio signal measurements of at least one cell; determining, by the source centralized unit based on the layer 3 measurement report, that a handover should be performed for the user device from a serving cell associated with the source centralized unit to a first target cell associated with a target centralized unit, wherein a handover configuration for handover of the user device to the first target cell has not yet been prepared, wherein the target centralized unit is different than the source centralized unit; determining, by the source centralized unit based on the received layer 3 measurement report, to provide to the source distributed unit associated with the serving cell, a layer 1 / layer 2 triggered mobility (LTM) LTM handover configuration with a relaxed handover requirement for a LTM handover of the user device to a second target cell, wherein the second target cell is also associated with the source centralized unit; transmitting, by the source centralized unit to the source distributed unit, the LTM handover configuration for the second target cell; and transmitting, by the source centralized unit to the target centralized unit associated with the first target cell, a handover request to prepare a handover configuration for the user device to perform a handover to the first target cell.

[0010] An apparatus may include means for receiving, by a source centralized unit from a user device via a source distributed unit, a layer 3 measurement report that provides results of radio signal measurements of at least one cell; means for determining, by the source centralized unit based on the layer 3 measurement report, that a handover should be performed for the user device from a serving cell associated with the source centralized unit to a first target cell associated with a target centralized unit, wherein a handover configuration for handover of the user device to the first target cell has not yet been prepared, wherein the target centralized unit is different than the source centralized unit; means for determining, by the source centralized unit based on the received layer 3 measurement report, to provide to the source distributed unit associated with the serving cell, a layer 1 / layer 2 triggered mobility (LTM) LTM handover configuration with a relaxed handover requirement for a LTM handover of the user device to a second target cell, wherein the second target cell is also associated with the source centralized unit; means for transmitting, by the source centralized unit to the source distributed unit, the LTM handover configuration for the second target cell; and means for transmitting, by the source centralized unit to the target centralized unit associated with the first target cell, a handover request to prepare a handover configuration for the user device to perform a handover to the first target cell.

[0011] An apparatus may include at least one processor; and at least one memory including computer program code; the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to: receive, by a source centralized unit from a user device via a source distributed unit, a layer 3 measurement report that provides results of radio signal measurements of at least one cell; determine, by the source centralized unit based on the layer 3 measurement report, that a handover should be performed for the user device from a serving cell associated with the source centralized unit to a first target cell associated with a target centralized unit, wherein a handover configuration for handover of the user device to the first target cell has not yet been prepared, wherein the target centralized unit is different than the source centralized unit; determine, by the source centralized unit based on the received layer 3 measurement report, to provide to the source distributed unit associated with the serving cell, a layer 1 / layer 2 triggered mobility (LTM) LTM handover configuration with a relaxed handover requirement for a LTM handover of the user device to a second target cell, wherein the second target cell is also associated with the source centralized unit; transmit, by the source centralized unit to the source distributed unit, the LTM handover configuration for the second target cell; and transmit, by the source centralized unit to the target centralized unit associated with the first target cell, a handover request to prepare a handover configuration for the user device to perform a handover to the first target cell.

[0012] A method may include receiving, by a source distributed unit associated with a serving cell from a source centralized unit, a layer 1 / layer 2 triggered mobility (LTM) LTM handover configuration with a relaxed handover requirement for a LTM handover of a user device to a target cell, and a timer value to be used for a validity timer for the LTM handover configuration, wherein the target cell is associated with the source centralized unit and with a different distributed unit; starting, by the source centralized unit, the validity timer for the LTM handover configuration indicating a validity period for the LTM handover configuration; receiving, by the source distributed unit from the user device, a layer 1 measurement report before expiration of the validity timer for the LTM handover configuration; determining, by the source distributed unit based on the layer 1 measurement report, that the relaxed handover requirement of the LTM handover configuration has been fulfilled; and transmitting, by the source distributed unit to the user device, a media access control (MAC) control element to trigger the user device to perform a LTM handover to the target cell.

[0013] An apparatus may include means for receiving, by a source distributed unit associated with a serving cell from a source centralized unit, a layer 1 / layer 2 triggered mobility (LTM) LTM handover configuration with a relaxed handover requirement for a LTM handover of a user device to a target cell, and a timer value to be used for a validity timer for the LTM handover configuration, wherein the target cell is associated with the source centralized unit and with a different distributed unit; means for starting, by the source centralized unit, the validity timer for the LTM handover configuration indicating a validity period for the LTM handover configuration; means for receiving, by the source distributed unit from the user device, a layer 1 measurement report before expiration of the validity timer for the LTM handover configuration; means for determining, by the source distributed unit based on the layer 1 measurement report, that the relaxed handover requirement of the LTM handover configuration has been fulfilled; and means for transmitting, by the source distributed unit to the user device, a media access control (MAC) control element to trigger the user device to perform a LTM handover to the target cell. An apparatus may include at least one processor; and at least one memory including computer program code; the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to: receive, by a source distributed unit associated with a serving cell from a source centralized unit, a layer 1 / layer 2 triggered mobility (LTM) LTM handover configuration with a relaxed handover requirement for a LTM handover of a user device to a target cell, and a timer value to be used for a validity timer for the LTM handover configuration, wherein the target cell is associated with the source centralized unit and with a different distributed unit; start, by the source centralized unit, the validity timer for the LTM handover configuration indicating a validity period for the LTM handover configuration; receive, by the source distributed unit from the user device, a layer 1 measurement report before expiration of the validity timer for the LTM handover configuration; determine, by the source distributed unit based on the layer 1 measurement report, that the relaxed handover requirement of the LTM handover configuration has been fulfilled; and transmit, by the source distributed unit to the user device, a media access control (MAC) control element (MAC CE) to trigger the user device to perform a LTM handover to the target cell.

[0014] Other example embodiments are provided or described for each of the example methods, including: means for performing any of the example methods; a non-transitory computer- readable storage medium comprising instructions stored thereon that, when executed by at least one processor, are configured to cause a computing system to perform any of the example methods; and an apparatus including at least one processor, and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform any of the example methods.

[0015] The details of one or more examples of embodiments are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.

[0016] BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. l is a block diagram of a wireless network.

[0018] FIG. 2 is a diagram illustrating movement of a UE within a wireless network according to an example embodiment.

[0019] FIG. 3 is a flow chart illustrating operation a source centralized unit according to an example embodiment.

[0020] FIG. 4 is a flow chart illustrating operation a source distributed unit according to an example embodiment. FIGs. 5A-5C are diagrams illustrating operation of a network according to an example embodiment.

[0021] FIGs. 6A-6C are diagrams illustrating operation of a network according to another example embodiment.

[0022] FIGs. 7A-7C are diagrams illustrating operation of a network according to another example embodiment.

[0023] FIGS. 8A-8D illustrate operation of a network according to yet another example embodiment. FIG. 9 is a block diagram of a wireless station or node (e.g., network node (such as gNB), user node or UE, relay node, or other node).

[0024] DETAILED DESCRIPTION

[0025] FIG. 1 is a block diagram of a wireless network 130. In the wireless network 130 of FIG. 1, user devices 131, 132, 133 and 135, which may also be referred to as mobile stations (MSs) or user equipment (UEs), may be connected (and in communication) with a base station (BS) 134, which may also be referred to as an access point (AP), an enhanced Node B (eNB), a gNB or a network node. The terms user device and user equipment (UE) may be used interchangeably. A BS may also include or may be referred to as a RAN (radio access network) node, and may include a portion of a BS or a portion of a RAN node, such as e.g., such as a centralized unit (CU) and / or a distributed unit (DU) in the case of a split BS or split gNB. At least part of the functionalities of a BS (e.g., access point (AP), base station (BS) or (e)Node B (eNB), gNB, RAN node) may also be carried out by any node, server or host which may be operably coupled to a transceiver, such as a remote radio head. BS (or AP) 134 provides wireless coverage within a cell 136, including to user devices (or UEs) 131, 132, 133 and 135. Although only four user devices (or UEs) are shown as being connected or attached to BS 134, any number of user devices may be provided. BS 134 is also connected to a core network 150 via a SI interface 151. This is merely one simple example of a wireless network, and others may be used.

[0026] A base station (e.g., such as BS 134) is an example of a radio access network (RAN) node within a wireless network. A BS (or a RAN node) may be or may include (or may alternatively be referred to as), e.g., an access point (AP), a gNB, an eNB, or portion thereof (such as a centralized unit (CU) and / or a distributed unit (DU) in the case of a split BS or split gNB), or other network node.

[0027] Some functionalities of the communication network may be carried out, at least partly, in a central / centralized unit, CU, (e.g., server, host or node) operationally coupled to distributed unit, DU, (e.g., a radio head / node). Thus, 5G networks architecture may be based on a so- called CU-DU split. The gNB-CU (central node) may control a plurality of spatially separated gNB-DUs, acting at least as transmit / receive (Tx / Rx) nodes. In some embodiments, however, the gNB-DUs (also called DU) may comprise e.g., a radio link control (RLC), medium access control (MAC) layer and a physical (PHY) layer, whereas the gNB-CU (also called a CU) may comprise the layers above RLC layer, such as a packet data convergence protocol (PDCP) layer, a radio resource control (RRC) and an internet protocol (IP) layers. Other functional splits are possible too.

[0028] According to an illustrative example, a BS node (e.g., BS, eNB, gNB, CU / DU, . . .) or a radio access network (RAN) may be part of a mobile telecommunication system. A RAN (radio access network) may include one or more BSs or RAN nodes that implement a radio access technology, e.g., to allow one or more UEs to have access to a network or core network. Thus, for example, the RAN (RAN nodes, such as BSs or gNBs) may reside between one or more user devices or UEs and a core network. According to an example embodiment, each RAN node (e.g., BS, eNB, gNB, CU / DU, ...) or BS may provide one or more wireless communication services for one or more UEs or user devices, e.g., to allow the UEs to have wireless access to a network, via the RAN node. Each RAN node or BS may perform or provide wireless communication services, e.g., such as allowing UEs or user devices to establish a wireless connection to the RAN node, and sending data to and / or receiving data from one or more of the UEs. For example, after establishing a connection to a UE, a RAN node or network node (e.g., BS, eNB, gNB, CU / DU, ...) may forward data to the UE that is received from a network or the core network, and / or forward data received from the UE to the network or core network. RAN nodes or network nodes (e.g., BS, eNB, gNB, CU / DU, ...) may perform a wide variety of other wireless functions or services, e.g., such as broadcasting control information (e.g., such as system information or on-demand system information) to UEs, paging UEs when there is data to be delivered to the UE, assisting in handover of a UE between cells, scheduling of resources for uplink data transmission from the UE(s) and downlink data transmission to UE(s), sending control information to configure one or more UEs, and the like. These are a few examples of one or more functions that a RAN node or BS may perform.

[0029] A user device or user node (user terminal, user equipment (UE), mobile terminal, handheld wireless device, etc.) may refer to a portable computing device that includes wireless mobile communication devices operating either with or without a subscriber identification module (SIM), including, but not limited to, the following types of devices: a mobile station (MS), a mobile phone, a cell phone, a smartphone, a personal digital assistant (PDA), a handset, a device using a wireless modem (alarm or measurement device, etc.), a laptop and / or touch screen computer, a tablet, a phablet, a game console, a notebook, a vehicle, a sensor, and a multimedia device, as examples, or any other wireless device. It should be appreciated that a user device may also be (or may include) a nearly exclusive uplink only device, of which an example is a camera or video camera loading images or video clips to a network. Also, a user node may include a user equipment (UE), a user device, a user terminal, a mobile terminal, a mobile station, a mobile node, a subscriber device, a subscriber node, a subscriber terminal, or other user node. For example, a user node may be used for wireless communications with one or more network nodes (e.g., gNB, eNB, BS, AP, CU, DU, CU / DU) and / or with one or more other user nodes, regardless of the technology or radio access technology (RAT). In LTE (as an illustrative example), core network 150 may be referred to as Evolved Packet Core (EPC), which may include a mobility management entity (MME) which may handle or assist with mobility / handover of user devices between BSs, one or more gateways that may forward data and control signals between the BSs and packet data networks or the Internet, and other control functions or blocks. Other types of wireless networks, such as 5G (which may be referred to as New Radio (NR)) may also include a core network.

[0030] In addition, the techniques described herein may be applied to various types of user devices or data service types, or may apply to user devices that may have multiple applications running thereon that may be of different data service types. New Radio (5G) development may support a number of different applications or a number of different data service types, such as for example: machine type communications (MTC), enhanced machine type communication (eMTC), Internet of Things (loT), and / or narrowband loT user devices, enhanced mobile broadband (eMBB), and ultra-reliable and low-latency communications (URLLC). Many of these new 5G (NR) - related applications may require generally higher performance than previous wireless networks. loT may refer to an ever-growing group of objects that may have Internet or network connectivity, so that these objects may send information to and receive information from other network devices. For example, many sensor type applications or devices may monitor a physical condition or a status and may send a report to a server or other network device, e.g., when an event occurs. Machine Type Communications (MTC, or Machine to Machine communications) may, for example, be characterized by fully automatic data generation, exchange, processing and actuation among intelligent machines, with or without intervention of humans. Enhanced mobile broadband (eMBB) may support much higher data rates than currently available in LTE.

[0031] Ultra-reliable and low-latency communications (URLLC) is a new data service type, or new usage scenario, which may be supported for New Radio (5G) systems. This enables emerging new applications and services, such as industrial automations, autonomous driving, vehicular safety, e-health services, and so on. 3 GPP targets in providing connectivity with reliability corresponding to block error rate (BLER) of 10-5 and up to 1 ms U-Plane (user / data plane) latency, by way of illustrative example. Thus, for example, URLLC user devices / UEs may require a significantly lower block error rate than other types of user devices / UEs as well as low latency (with or without requirement for simultaneous high reliability). Thus, for example, a URLLC UE (or URLLC application on a UE) may require much shorter latency, as compared to an eMBB UE (or an eMBB application running on a UE).

[0032] The techniques described herein may be applied to a wide variety of wireless technologies or wireless networks, such as 5G (New Radio (NR)), cmWave, and / or mmWave band networks, loT, MTC, eMTC, eMBB, URLLC, 6G, etc., or any other wireless network or wireless technology. These example networks, technologies or data service types are provided only as illustrative examples.

[0033] A user device (or UE) may measure various signals and may transmit one or more measurement reports to the network. For example, a UE may measure reference signals received from one or more network nodes (e.g., gNBs or DUs), including channel state information-reference signals (CSLRSs) and / or synchronization signal block (SSB) reference signals, demodulation references signals, and / or other reference signals. Based on received reference signals, the UE may measure various signal parameters, e.g., such as reference signal received power (RSRP), reference signal received quality (RSRQ), signal to interference plus noise ratio (SINR), received signal strength indicator (RS SI), or other signal parameter.

[0034] The PHY (physical) layer may refer to layer 1 (LI) and MAC (media access control) may refer to layer 2 (L2). RSRP, RSRQ, SINR and RSSI are signal quantities measured at LI. The UE may send LI measurement reports (e.g., CSLRS reports, which measure one or more signal parameters for one or more cells) to a gNB, source DU or serving cell. These LI measurement reports may be sent periodically, for example, or aperiodically. L1 / L2 measurement reports may include no averaging or filtering of measurement values or may include less averaging or filtering than what is performed for L3 measurement reports. LI (or L1 / L2) measurement reports may be transmitted by a UE to a serving network node or source DU and may cause the network node to trigger or initiate a L1 / L2 triggered mobility (LTM) handover of the UE to another cell. LI measurements (e.g., RSRP RSRQ, RSSI) may be provided or reported periodically to the DU (MAC / PHY).

[0035] L3 measurement reports are event-based measurement reports, e.g., which are triggered when a cell (e.g., a neighbour cell) has a RSRP or RSSI (or other signal parameter) that meets some HO event criteria. For example, a L3 measurement report may be transmitted by a UE to a serving cell or network node if, e.g., a serving cell measured signal becomes / is worse than a threshold, a neighbour cell signal measurement is or becomes better than a first threshold and / or serving cell signal measurement becomes an offset threshold worse than a neighbouring cell’s signal measurement, etc. Thus, L3 measurement reports may be event-triggered measurement reports.

[0036] The L3 measurement report may include or may be based on averaging and / or filtering of more signal samples, or averaging over a longer period of time, as compared to the LI measurement report. Also, L3 measurement reports are transmitted to a CU, and are event-triggered measurement reports and may trigger a network node to initiate or trigger a handover (e.g., L3 handover, which may be basic handover or conditional handover) of the UE to another cell. A L3 handover (L3 HO) may be based on a L3 measurement report. In response to receiving a L3 measurement report from a UE, a source network node (e.g., gNB, or source CU) may send a handover request to a target network node (e.g., target CU). The source network node may transmit to the UE a RRC reconfiguration message including a HO command to cause the UE to perform handover to the target cell, or a CHO configuration (to configure the conditional handover conditions that will trigger or cause the UE to initiate CHO to the target cell). Because the L3 HO (either HO or CHO) is based on a L3 measurement report, e.g., which may require more time to obtain or measure (e.g., based on more signal measurement samples and / or filtering and / or averaging of the measurement samples) as compared to a LI measurement report, the L3 measurement report may be transmitted by a UE well after radio conditions with a serving cell / serving network node have already degraded. If a handover configuration is not already prepared for a HO / CHO to the target cell, then the source network node may need to send a message to the target node to request and prepare the HO configuration for the UE, which may cause significant delay, e.g., of 100ms or more, before the UE can perform a L3 HO or CHO to the target cell. This significant delay may increase the likelihood the UE will suffer a radio link failure (RLF) or loss in connectivity.

[0037] In 3GPP Rel.18, a new type of inter-cell mobility is introduced, referred to as a L1 / L2 based inter-cell mobility (L1 / L2 triggered mobility, LTM). As the UE changes its location in the network, its radio link to the serving cell can experience significant fluctuations or degradations. To ensure radio connectivity, the network may trigger a cell change or handover. In legacy procedures (e.g., L3 HO or CHO, described above), the cell switch or HO is either triggered by the network based on the reported measurements from the UE (e.g., as in the triggered L3 HO), or it is triggered by the UE when a certain set of conditions (evaluated locally at the UE) provided by the network to the UE as a CHO configuration is met (e.g., in CHO). In either case, the HO decision is made based on L3 measurements, which may introduce significant latency or delay before the UE can perform the L3 HO or CHO.

[0038] In LTM, the source CU may receive a L3 measurement report via a source DU, and then on that basis, may prepare one or more HO configurations for UE handover. The source CU may transmit to the UE (via a source DU that is serving the UE), a RRC reconfiguration message to provide a measurement configuration to configure the UE to perform LI measurements of cells, and a configuration (or handover configuration) of one or more prepared cells. A HO configuration for each prepared cell may include, e.g., beam information of the target cell, and bandwidth, access information, such as a random access preamble information, for the target cell and / or other configuration information for the UE to communicate with the target cell. The UE may then measure a set of cells (e.g., UE may measure RSRP or RSRQ of reference signals received from the set of cells) and may periodically send LI measurement reports to the source DU that is serving the UE. On the basis of these LI measurement reports from the UE, the source DU associated with the source CU for the UE may change the UE’s serving cell(s) by a cell switch command by sending to the UE a MAC control element (MAC CE), which indicates a LTM candidate (target) cell configuration that the gNB or CU previously prepared and provided to the UE through RRC signaling. Thus, the MAC CE for LTM HO may identify 1 of 8 (or other number) of previously configured LTM target cell configurations the UE should use for LTM handover. The UE may then perform a random access procedure to establish a connection with the indicated target cell.

[0039] Thus, a cell switch for LTM HO is triggered by the network node selecting a LTM candidate cell configuration (e.g., having a strongest RSRP or RSRQ or other signal parameter based on the received LI measurement report) as the target configuration by the network node and indicating this target cell or HO configuration for this target cell to the UE via MAC CE. A LTM candidate cell configuration can typically only be added, modified, and / or released by network via RRC signaling (RRC messages), which have significant delay. The LTM procedure can be used to reduce the mobility (or HO) latency, since the LTM HO is triggered by a LI measurement report (which has lower latency than the L3 measurement report).

[0040] Note, for example, that LTM supports intra-CU mobility, including mobility to a target cell that is not a current serving cell (but the target cell is associated with or provided by the same CU as the serving cell (that is currently serving the UE), as described hereinbelow). As mentioned above, different from the existing L3 HO or CHO handover procedures, in LTM procedure, cell change triggering happens via a MAC CE command sent from the source DU to the UE (based on the reported LI measurements). Despite the trigger for the process being in the DU, the setup (or requesting and obtaining of LTM HO configurations for target cells) for LTM HO / cell switch is performed via the CU and transmitted to the UE via an RRC Reconfiguration message. The transmitted RRC Reconfiguration message contains information about all the cells that have been configured with LTM for the UE. FIG. 2 is a diagram illustrating movement of a UE within a wireless network according to an example embodiment. In this example, the UE has been configured for LTM handover for cells 1, 2 and 3, which are associated with (e.g., controlled by or provided by) source CU 210. Thus, cells 1, 2 and 3 are provided by the same CU. LTM in NR rel. 18 is defined only for intra-CU mobility, where LTM mobility for a UE may be performed only between cells of the same CU (not between cells of different CUs). Thus, because LTM is not available to the UE to perform HO to a cell of (or associated with, or controlled by) a different CU, the UE must use a L3 HO or CHO to perform handover to a cell that is controlled by or associated with a different CU than its current or source CU, which may require much more time to accomplish a L3 handover or CHO, as compared to a LTM handover that has already been prepared.

[0041] Thus, as shown in FIG. 2, as the UE moves between cells 1, 2 and 3 (an example path of UE shown with dashed line 206), the UE can perform LTM mobility (e.g., LTM cell change or HO may be performed for the UE within any of the cells of source CU 210, e.g., from cell 1 to cell 2, cell 2 to cell 1, cell 2 to cell 3, or cell 3 to cell 2,...), e.g., triggered by a source DU (and associated with source CU 210) serving the UE (not shown) based on the LI measurement report(s) received from the UE, since all of cells 1, 2 and 3 are provided by or associated with the source CU 210.

[0042] Referring to FIG. 2, at 208, the UE (as shown by example UE path 206) is in an intersection of cells 2, 3, 4. At point 208, the UE is connected to cell 3 (current source cell, associated with source CU 210), but is headed or traveling in a direction that is deeper into cell 4, and away from cells 3 and 2 (and into a region where there is overlap between cells 2 and 4). Based on the UE movement along example path 206 deeper into cell 4, a L3 measurement report may be triggered and transmitted by the UE, e.g., based on the RSRP or RSRQ of cell 4 (for which a handover event is met, e.g., RSRP of cell 4 is threshold better than RSRP of current / serving cell 3). Note that cell 4 is associated with (controlled by or provided by) target CU 220, while the current serving cell 3 is associated with (controlled by or provided by) source CU 210. Thus, a LTM HO of the UE from cell 3 to cell 4 cannot be performed, since cells 3 and 4 are associated with different CUs. Rather, based on the L3 measurement report, the source CU 210 may trigger a L3 HO or CHO of the UE from cell 3 to cell 4. In order for the UE to perform HO / CHO to cell 4, the new cell needs to be prepared and a handover command (which may include a CHO configuration) needs to be sent to the UE. This procedure is time consuming and it may take up to 100 ms or more to complete. During this time, the radio conditions between the UE and the serving cell may continue to degrade and the UE may end up in radio link failure (RLF) or connectivity loss, e.g., due to deterioration of radio conditions in the current serving cell (e.g., cell 3) or losing the handover command, even though the UE already has one or more LTM HO configurations (e.g., for cells 1, 2, 3, . ..) which could enable the UE to maintain the connectivity.

[0043] For example, in the example shown in FIG. 2, at the time the UE travels deeper into cell 4 at point 208, the UE already has the LTM HO configurations for cells 1, 2 and 3.2. And, based on path / movement of the UE into the overlapping region of cells 2 and 4, the UE could, in this example, perform a temporary LTM HO to cell 2 (as an intermediate HO step) in order to reduce the likelihood of a radio link failure, before eventually performing the L3 HO or CHO to the preferred (stronger) cell 4. For example, performing a LTM HO to cell 2 as an intermediate or desperate HO step may avoid a RLF, and may provide additional time for a HO configuration to be prepared for L3 HO or CHO for cell 4.

[0044] Thus, for example, while a LTM HO of the UE from cell 3 to cell 2 is not the ideal or preferred HO or cell change (the direct HO from cell 3 to cell 4 would be preferred, since cell 4 provides a stronger RSRP than cell 2), the temporary or intermediate HO of the UE from cell 3 to cell 2 (e.g., in the event that cell 2 is a better cell for the UE than cell 3) may be a desperate HO by the network, based on degraded radio conditions of the radio link between the UE and cell 3, e.g., to avoid a RLF. Such a desperate HO may be an intermediate or temporary HO performed for the UE to temporarily change the UE to a better cell (if available) than the currently serving cell, e.g., to reduce the risk of RLF. Such a desperate HO or intermediate HO may be performed, for example, in the event the radio signal measurement(s) of the L3 measurement report received by source CU from the UE indicates that UE is under radio conditions with respect to the serving cell that are lower than a predetermined threshold (e.g., RSRP of reference signals from serving cell are less than a threshold, indicating significant risk of RLF). As noted, because a LTM HO configuration for HO to this better cell (e.g., cell 2 in FIG. 2 is better than current serving cell 3), has already been provided to the UE, and because such HO is based on LI measurement report, such a desperate or intermediate HO (e.g., performed as LTM HO) may advantageously be performed very quickly, as compared to a L3 HO or CHO, thereby hopefully avoiding RLF, or at least decreasing the risk of RLF. Furthermore, at least in some cases, by performing this LTM HO (e.g., desperate HO or intermediate HO) to the better cell (e.g., cell 2) that is associated with the source CU 210, this may provide additional time (while maintaining RLF and maintaining connectivity for the UE) for the HO command and / or CHO configuration for the L3 HO to the preferred cell 4 to be prepared.

[0045] As noted in the example of FIG. 2, the L3 measurement report provided by the UE may provide L3 cell measurements that indicate a preferred HO of the UE to cell 4 (e.g., cell 4 signal measurement meets the HO event criteria) that is associated with (e.g., controlled by) target CU 220, which is different from the current CU 210 of the UE. However, in this example, a LTM HO has not or is not triggered yet at the source DU based on the LI measurement report from the UE because, while the cell 2 is better (has a stronger RSRP) than cell 3, cell 2 RSRP (as measured by the UE) may not be sufficiently stronger than cell 3 RSRP. In this case, the LTM HO from cell 3 to cell 2 would not be triggered at the source DU based on LI measurement report. According to an example embodiment, in order to facilitate a likely LTM HO of the UE to a better cell (e.g., cell 2 of FIG. 2), the source CU 210 may provide the source DU with a LTM HO configuration with a relaxed (or reduced) HO requirement for the UE, as described in greater detail hereinbelow.

[0046] Therefore, according to an example embodiment, the source CU 210 may: 1) receive the L3 measurement report from the UE, and based on this L3 measurement report, the source CU 210 determines that a HO / CHO should be performed for the UE to cell 4 (since HO event criteria met for cell 4, e.g., cell 4 RSRP is more than a threshold better than RSRP of current serving cell 3), and source CU 210 also knows or has determined that cell 4 is not associated with source CU 210, but is associated with target CU 220 (thus, a LTM HO of the UE is not possible to cell 4); and 2) the source CU 210 may determine, based on the L3 measurement report, that the current radio conditions (e.g., RSRP or RSRQ) of the radio link between the UE and current serving cell 3 is less than a threshold (e.g., indicating a significant or enhanced risk of RLF for the UE), and thus, source CU 210 should adjust the source DU LTM HO behavior by configuring a desperate LTM HO of UE using a relaxed (or reduced) LTM HO requirement that encourages or makes it more likely that a UE handover is performed to a cell (e.g., cell 2 or cell 1) associated with the source CU 210 (to a cell associated with same CU as the serving cell), or to a cell for which the UE already has a LTM HO configuration.

[0047] The source CU 210 may then transmit to a source DU associated with the serving cell, a (e.g., an updated) LTM HO configuration for the UE, where the (e.g., updated) LTM HO configuration includes a relaxed HO requirement for a LTM HO of the UE to a cell associated with the source CU 210. The relaxed HO requirement makes it more likely that the source DU (serving the UE, and associated with source CU 210) will trigger a LTM HO to one of the cells that have been already configured for LTM HO (e.g., cell 1 or cell 2) and / or to a cell that is associated with the source CU (same CU associated with serving cell, cell 3). The source DU would then evaluate any LI measurement reports received from the UE, and would trigger a UE LTM HO of the UE to a cell (e.g., cell 2) if the relaxed HO requirement is met for that cell.

[0048] In further operations, which may be performed substantially (or partially) in parallel to the above operations for LTM HO configuring / updating, the source CU 210 may transmit to the target CU 220 associated with the target cell (e.g., cell 4) a handover request to prepare a L3 handover configuration (HO or CHO) for the UE to the (more preferred) target cell (e.g., cell 4). In this example, even though signal measurements for cell 4 (the target cell) are stronger than the signal measurements from cell 2 (thus, cell 4, associated with target CU 220 is the more preferred target cell), the LTM HO from cell 3 to cell 2 may nonetheless be performed (if LI measurements for cell 2 and / or for cell 3, are sufficient or meet the relaxed HO requirement to cell 2), as an intermediate HO step for the UE, e.g., to decrease the likelihood that a RLF would be experienced by the UE. In the event that the LTM HO is performed by the UE from the current serving cell (e.g., cell 3) to another cell associated with the source CU 210 (e.g., cell 2), this may provide (or buy) additional time (e.g., without UE suffering RLF) for the L3 HO or CHO configuration to be prepared for the UE and then provided to the UE to allow a L3 HO or CHO of the UE to the target cell (e.g., cell 4) to be successfully performed, and thus, make a RLF or disconnection for the UE less likely.

[0049] In addition, a validity timer may be used by the source CU and / or source DU to indicate a validity period of the (e.g., updated) LTM HO configuration (including the relaxed HO requirement) transmitted by the source CU 210 to the source DU. The validity period is a period of time for which the (e.g., updated) LTM HO configuration with relaxed HO requirement is valid. The source DU would use the (e.g., updated) received LTM HO configuration with relaxed HO requirement as long as the validity timer has not expired, to determine whether to trigger a LTM HO to a cell via MAC CE. Once the validity timer expires, the source DU would no longer use the (e.g., updated) received LTM HO configuration with relaxed HO requirement, but would resume using the previous or a default or a non-relaxed LTM HO configuration to determine whether a LTM HO should be performed based on LI measurements from the UE. The source CU 210 may provide the source DU with both the LTM HO configuration with relaxed HO requirement and a timer value for the validity timer.

[0050] As noted, the source CU 210 providing the source DU with the LTM HO configuration with relaxed HO requirement (e.g., which may have a lower threshold for RSRP for LTM HO to a cell) may make it more likely that a LTM HO is performed for the UE. This (providing of the LTM HO configuration with relaxed HO requirement to the source DU) may be advantageously performed or provided by the source CU 210 in response to the source CU 210 determining, for example, based on the L3 measurement report from the UE, that the current radio conditions (e.g., RSRP or RSRQ) of the radio link between the UE and current serving cell 3 is less than a threshold (e.g., radio conditions indicating a significant or enhanced risk of RLF), and thus, a desperate or intermediate LTM HO should be configured based on a relaxed HO requirement to a cell that is associated with the source CU, in order to encourage or make it more likely that a temporary or intermediate HO of the UE will be performed to a cell that is associated with the source CU 210, before the UE performs the L3 HO or CHO to the (e.g., more preferred) target cell (e.g., cell 4) that is associated with the target CU 220. Additional features, operations and / or examples are described herein. The description and figures provided herein, including the figures and description hereinbelow, provide further details, description and / or illustrative examples.

[0051] FIG. 3 is a flow chart illustrating operation a source centralized unit according to an example embodiment. Operation 310 includes receiving, by a source centralized unit (e.g., source CU 210, FIG. 2) from a user device (e.g., UE) via a source distributed unit (source DU), a layer 3 (L3) measurement report that provides results of radio signal measurements of at least one cell. Operation 320 includes determining, by the source centralized unit (e.g., source CU 210, FIG. 2) based on the layer 3 measurement report, that a handover should be performed for the user device from a serving cell associated with the source centralized unit (e.g., source CU 210) to a first target cell (e.g., HO should be performed for UE to target cell 4) associated with a target centralized unit (e.g., target cell 4 is associated with target CU 220, FIG. 2), wherein a handover configuration for handover of the user device to the first target cell has not yet been prepared (e.g., a HO configuration for L3 HO or CHO of the UE to target cell 4 has not yet been prepared), wherein the target centralized unit is different than the source centralized unit (e.g., target CU 220 is different than the source CU 210). Operation 330 includes determining, by the source centralized unit (e.g., source CU 210) based on the received layer 3 measurement report, to provide to the source distributed unit (e.g., source DU) associated with the serving cell (source DU associated with serving cell 3, FIG. 2), a layer 1 / layer 2 triggered mobility (LTM) LTM handover configuration with a relaxed handover requirement for a LTM handover of the user device to a second target cell (e.g., target cell 2), wherein the second target cell is also associated with the source centralized unit (e.g., target cell 2 is also associated with source CU 210). Operation 340 includes transmitting, by the source centralized unit (e.g., source CU 210) to the source distributed unit (source DU), the LTM handover configuration for the second target cell (LTM HO configuration for target cell 2). And, operation 350 includes transmitting, by the source centralized unit (source CU 210, FIG. 2) to the target centralized unit (target CU 220) associated with the first target cell (target cell 4 is associated with target CU 220), a handover request to prepare a handover configuration for the user device to perform a handover to the first target cell.

[0052] With respect to the method of FIG. 3, the method may further include starting, by the source centralized unit, a validity timer for the LTM handover configuration indicating a validity period for the LTM handover configuration.

[0053] With respect to the method of FIG. 3, the method may further include upon making the determination to provide the LTM handover configuration to the source distributed unit, starting, by the source centralized unit, the validity timer for the LTM handover configuration. With respect to the method of FIG. 3, the method may further include wherein the handover of the user device to the first target cell is performed after either: 1) an expiration of the validity timer, or 2) a LTM handover of the user device to the second target cell based on the LTM handover configuration for the second target cell.

[0054] With respect to the method of FIG. 3, the method may further include receiving, by the source centralized unit, a LTM handover indication indicating that a LTM handover of the user device to the second target cell has been triggered; stopping the validity timer for the LTM handover configuration based on the reception of the LTM handover indication; and transmitting, by the source centralized unit to the source distributed unit associated with the second target cell, a radio resource control (RRC) message including a handover command or conditional handover configuration to instruct the user device to perform a handover or conditional handover to the first target cell associated with the target centralized unit.

[0055] With respect to the method of FIG. 3, the method may include detecting expiration of the validity timer for the LTM handover configuration before a LTM handover indication indicating that a LTM handover of the user device to the second target cell has been triggered; and transmitting, by the source centralized unit to the source distributed unit associated with the second target cell, a radio resource control (RRC) message including a handover command or conditional handover configuration to instruct the user device to perform a handover or conditional handover to the first target cell associated with the target centralized unit.

[0056] With respect to the method of FIG. 3, the method may further include the RRC message includes a delta configuration including one or more updated parameters for the handover of the user device to the first target cell, the one or more updated parameters being with reference to a full configuration known to the user device, wherein the full configuration corresponds to the configuration of the serving cell.

[0057] With respect to the method of FIG. 3, the RRC message may include a delta configuration including one or more updated parameters for the handover of the user device to the first target cell, the one or more updated parameters being with reference to a full configuration known to the user device, wherein the full configuration corresponds to the configuration of the second target cell.

[0058] With respect to the method of FIG. 3, wherein the user device uses the delta configuration for the handover to the first target cell, after the user device performs the handover from the serving cell to the second target cell. With respect to the method of FIG. 3, the LTM handover configuration may include values or updated values for one or more of: a time to trigger value, a delta value, or a threshold value for a handover of the user device.

[0059] With respect to the method of FIG. 3, the relaxed handover requirement for the LTM handover configuration of the user device is more easily met, as compared to a non-relaxed handover requirement, to more easily trigger the LTM handover to the second target cell based on a LI measurement report, or the relaxed handover requirement for the LTM handover configuration triggers the LTM handover of the user device to the second target cell based on a wider range of LI measurements as compared to a non-relaxed handover requirement.

[0060] With respect to the method of FIG. 3, LTM handover configuration is user device-specific.

[0061] With respect to the method of FIG. 3, the method may include detecting, by the source centralized unit based on the reception of the L3 measurement report, that the user device is under radio conditions with respect to the serving cell that are lower than a predetermined threshold; and transmitting, by the source centralized unit to the source distributed unit based on the detecting, the LTM handover configuration for the second target cell.

[0062] With respect to the method of FIG. 3, the method may include detecting, by the source centralized unit based on the reception of the L3 measurement report, that the user device is under radio conditions with respect to the serving cell that are lower than a predetermined threshold; confirming, by the source centralized unit, that the source centralized unit has not received an indication of a LTM handover for the user device from the serving cell to the second target cell; and transmitting, by the source centralized unit to the source distributed unit based on the detecting and the confirming, the LTM handover configuration for the second target cell.

[0063] With respect to the method of FIG. 3, the LTM handover configuration also comprises a timer value to be used for the validity timer.

[0064] With respect to the method of FIG. 3, the LTM handover configuration for the LTM handover of the user device to the second target cell is an interim or intermediate step handover to reduce a likelihood of a radio link failure for the user device, before the user device performs a handover to the first target cell based on the handover configuration for the first target cell.

[0065] FIG. 4 is a flow chart illustrating operation a source distributed unit according to an example embodiment. Operation 410 includes receiving, by a source distributed unit associated with a serving cell from a source centralized unit, a layer 1 / layer 2 triggered mobility (LTM) LTM handover configuration with a relaxed handover requirement for a LTM handover of a user device to a target cell, and a timer value to be used for a validity timer for the LTM handover configuration, wherein the target cell is associated with the source centralized unit and with a different distributed unit. Operation 420 includes starting, by the source centralized unit, the validity timer for the LTM handover configuration indicating a validity period for the LTM handover configuration. Operation 430 includes receiving, by the source distributed unit from the user device, a layer 1 measurement report before expiration of the validity timer for the LTM handover configuration. Operation 440 includes determining, by the source distributed unit based on the layer 1 measurement report, that the relaxed handover requirement of the LTM handover configuration has been fulfilled. And, operation 450 includes transmitting, by the source distributed unit to the user device, a media access control (MAC) control element to trigger the user device to perform a LTM handover to the target cell.

[0066] FIGs. 5A-5D (5 A, 5B, 5C and 5D) are diagrams illustrating operation of a network according to an example embodiment. In FIGs. 5A-5D, an intermediate LTM handover of the UE is performed to a second target cell associated with a source CU 210, before the UE performs a L3 HO or CHO to a cell associated with a target CU 220.

[0067] With reference to FIGs. 5 A - 5D, a UE 510 may be served initially by a source distributed unit (source DU 512), which is associated with or controlled by source CU 210. A target DU 514 is also associated with or controlled by source CU 210. A target DU 516 is associated with or controlled by target CU 220. At Steps 1-2, a L3 measurement report is transmitted by UE 510 to source DU 512 and forwarded to source CU 210. At Step 3, the source CU 210 performs LTM candidate preparation, including transmitting a UE context setup request to target DU 514, and receiving a UE context setup response. At Steps 6-7, the source CU 210 transmits a UE context modification request to source DU 512, and receives a UE context modification response. At Step 8, source CU 210 generates a RRC reconfiguration message including: 1) a measurement configuration of LI cell change that provides measurement configuration for the UE to perform LI measurements and 2) a configuration of prepared candidate LTM cells (e.g., where each configuration may include beam of candidate / target cell, bandwidth of candidate cell, access information for candidate cell such as random access preamble, and / or other information or parameters the UE may need to communicate with and / or perform handover to the cell). At Steps 9-10, the source CU 210 transmits the generated RRC reconfiguration message to UE 510 via source DU 512. At Steps 11-12, source CU 210 receives a RRC reconfiguration complete message from the UE 510. At Step 13, the UE transmits a LI measurement report to the source DU 512. At Steps 14-15, the UE performs downlink (DL) synchronization with candidate cells based on the LTM candidate configurations, and performs timing advance (TA) acquisition with candidate cells. So, at this point, LTM HO configurations have been prepared for the UE 510 for a set (e.g., a set of 8) LTM HO candidate / target cells associated with source CU 210.

[0068] With respect to FIGs. 5A-5D, at Steps 16-17, source CU receives a L3 measurement report from UE 510. The L3 measurement report may, for example, include signal measurements where a strongest or best signal measurement is indicated for a first target cell (e.g., cell 4, FIG. 2) associated with target DU 516 of target CU 220. Thus, in this example, the first target cell (cell 4) is not associated with the source CU 210, but is associated with target CU 220. Thus, the L3 measurement report provided by the UE may provide L3 cell measurements that indicate (or source CU 210 may determine based on the L3 measurements) a preferred HO of the UE to the first target cell (e.g., cell 4 signal measurement meets the HO event criteria, and / or provides the best cell for the UE) that is associated with (e.g., controlled by) target CU 220, which is different from the current source CU 210 of the UE (thus, LTM HO to this first target cell, or cell 4, is not possible, and HO of UE to the first target cell or cell 4 should be performed via L3 HO or CHO). However, in this example, the source CU 210 may also confirm that a LTM HO of the UE has not yet been triggered for the UE to a cell associated with the source CU 210. Therefore, according to an example embodiment, the source CU 210 may: a) receive the L3 measurement report from the UE, and based on this L3 measurement report, the source CU 210 determines that a HO / CHO should be performed for the UE to the first target cell (since HO event criteria is met for cell 4, e.g., cell 4 RSRP is more than a threshold better than RSRP of current serving cell 3, and source CU 210 also knows that cell 4 is not associated with source CU 210, but is associated with target CU 220, thus, LTM HO is not possible to cell 4); and b) the source CU 210 determines, based on the L3 measurement report, that the current radio conditions (e.g., RSRP or RSRQ) of the radio link between the UE and current serving cell 3 (cell 3, FIG. 2) is less than a threshold (e.g., indicating a significant or enhanced risk of RLF for the UE), and thus, source CU 210 should perform or configure a desperate (or intermediate) LTM HO of UE using a relaxed LTM HO requirement that encourages or makes it more likely that a UE handover is performed to a second target cell (e.g., cell 2, or other cell that meets such relaxed HO requirement) associated with the source CU 210 (to a cell associated with same CU (source CU 210) as the serving cell), or to a cell for which the UE already has a LTM HO configuration.

[0069] Thus, at Step 18 of FIGS. 5A-5D, the source CU 210 makes a determination to modify (or to provide) a LTM HO configuration at source DU 512 (e.g., to change the behavior of DU 512), with a relaxed HO requirement, for a desperate (or intermediate) LTM HO to a target cell (e.g., cell 2, FIG. 2) associated with source CU 210, e.g., to make it more likely that an intermediate or desperate LTM HO for the UE may be performed to a cell associated with the source CU 210 (for which the UE 510 already has prepared LTM HO configuration). At the same time, the source CU 210 makes a determination to prepare candidate cell(s) for L3 HO or CHO, e.g., including for the first target cell (cell 4) (see Steps 21-25 to prepare HO / CHO configuration for L3 HO or CHO for a first target cell, e.g., cell 4). Also, at this time, the source CU 210 starts a validity timer CU-X that indicates a validity period of the (e.g., updated) LTM HO configuration with the relaxed HO requirement.

[0070] At Step 19, the source CU 210 may transmit to source DU 512 associated with the serving cell, a (e.g., an updated) LTM HO configuration for the UE including a relaxed HO requirement for a LTM HO of the UE to a cell associated with the source CU 210. The relaxed HO requirement makes it more likely that the source DU 512 (serving the UE) will trigger a LTM HO, based on LI measurement report, to one of the cells that have been already configured for LTM HO (e.g., cell 1 or cell 2, associated with source CU 210) and / or otherwise to a cell that is associated with the source CU 210.

[0071] At Step 20, the source DU 512 receives the LTM HO configuration with relaxed HO requirement, and starts a validity timer DU-X (which is synchronized with validity timer CU- X used by source CU 210), indicating a validity period for the received LTM HO configuration that includes the relaxed LTM HO requirement.

[0072] The preparation of the L3 HO or CHO configuration to the first target cell (e.g., cell 4) for L3 HO or CHO is performed via messages 21-25, in which the source CU 210 transmits a handover request (for HO or CHO) to the target CU 220. At Step 26, based on the preparation of the HO command or CHO configuration of at least the target cell (e.g., cell 4) the source CU 210 now has prepared a RRC reconfiguration message for L3 HO or CHO to a first target cell (e.g., cell 4) associated with target CU 220. In a first example embodiment, the CU 210 may immediately transmit or forward the RRC reconfiguration message (including HO command or CHO configuration) to source DU 512 and / or target DU 514. However, in a second example embodiment, the source CU 210 does not yet forward (but waits to forward) the RRC reconfiguration message (including the HO command or CHO configuration) to the source DU 512 (associated with serving cell of UE) or the target DU 514 (associated with the second target cell, e.g., cell 2 in FIG. 2). Rather, in this second example embodiment, the source CU 210 may wait for either of the following before forwarding the RRC reconfiguration message: 1) an expiration of the validity timer (before receiving an LTM HO indication) (in this case the RRC reconfiguration message including L3 HO command or CHO configuration will be transmitted to the source DU 512 associated with the source CU 210, since LTM HO to a second target cell, such as to cell 2, based on relaxed HO requirement was not performed, and this case is shown in FIGs. 6A-6C); or 2) source CU 210 receives from source CU 210 a LTM HO (or LTM cell change) indication indicating a LTM HO of the UE to the second target cell (e.g., cell 2, FIG. 2) associated with the source CU 210 has been triggered (in this case the RRC reconfiguration message including L3 HO command or CHO configuration will be transmitted to the target DU 514 (the DU associated with the cell to which the UE performed LTM HO) associated with the source CU 210, since LTM HO to the second target cell, or cell 2, was performed, and this case is shown in FIGs. 5A-5D). Thus, according to an example embodiment, the source CU 210 may wait to forward the RRC reconfiguration message with HO command / CHO configuration for L3 HO or CHO until the source CU 210 knows where to forward the HO command / CHO configuration for L3 HO or CHO (until the source CU 210 knows whether the LTM HO was performed or not).

[0073] At Step 27, the source DU 512 may receive and evaluate any LI measurement reports received from the UE 510, to determine if the relaxed HO requirement is met for any of the cells that are associated with the source CU 210. At Step 28, the source DU 512 determines that the relaxed HO requirement is met for a cell, e.g., cell 2 that is associated with source CU 210, and then triggers a UE LTM HO of the UE to that cell (e.g., cell 2) (since the relaxed HO requirement is met for this cell). The validity timer DU-X is stopped.

[0074] At Step 29, the source DU 512 transmits to source CU 210 a LTM HO indication indicating that a LTM HO of the UE (e.g., to the second target cell or cell 2) has been triggered (the target cell to which the LTM HO was performed, and / or DU (DU 514) may be provided or indicated to the source CU 210), and at Step 30 the source CU 210 stops its validity timer CU- X. At Step 31, the source DU 512 transmits a MAC control element (MAC CE) to trigger the UE 510 to perform a LTM HO to the indicated second target cell (e.g., to perform LTM HO to cell 2) associated with the source CU 210 and target DU 514. At Step 32 the UE 510 performs random access to the second target cell (e.g., cell 2) associated with the source CU 210. At Steps 33-34, the UE notifies the source CU 210 via the target DU 514 that RRC reconfiguration is completed (LTM HO of UE to second target cell is completed).

[0075] At Steps 33-34, the UE 510 provides a RRC reconfiguration complete message to the source CU 210. At Steps 35-36, after source CU 210 receives (at Step 29) the LTM HO indication indicating that a LTM HO of the UE (e.g., to the second target cell or cell 2) has been triggered, the source CU 210 transmits the RRC reconfiguration message including L3 HO command or CHO configuration to the target DU 514 associated with the source CU 210, since LTM HO to the second target cell, or cell 2 (associated with target DU 514 and source CU 210), was performed. UE 510 acknowledges receipt of the RRC reconfiguration message at Steps 37- 38, and UE 510 releases contexts with source CU 210 for previous connection to cell of source DU 512 at Steps 39-40. After completing the intermediate or desperate HO to the second target cell (e.g., HO to cell 2), the RRC reconfiguration message including HO command or CHO configuration for L3 HO or CHO causes the UE 510 to perform a L3 HO or CHO to the first target cell (e.g., cell 4, shown in FIG. 2) via the target CU 220, shown as Step 41.

[0076] The steps of FIGs. 5A-5D may be summarized based on the following:

[0077] Steps 1-15: LTM Preparation and Early sync performed / UE also provides LI measurements.

[0078] Steps 16-17: UE sends L3 Measurement report for new cells (belonging possibly to other CU) as radio condition deteriorated and LTM condition is not met for LTM.

[0079] Step 18: Source CU decides to modify the LTM configuration at source DU for desperate LTM and same time prepares candidate cells for Handover (for e.g., CHO). At the same time, it starts timer CU-X.

[0080] Step 19: CU indicates S-DU with new thresholds for LTM along with a timer value X(DU- X). New LTM threshold is valid till the timer expiry.

[0081] Step 20: S-DU starts the timer DU-X. Note, timers CU-X and DU-X may have the same value and they target CU and DU to have the coordinated operation.

[0082] Steps 21-26: Handover Preparation initiated to all selected new inter cu candidate cells in parallel (started at Step 18).

[0083] Step 27: UE sends LI Measurement report to S-DU (May happen in parallel to Steps 21 to 26).

[0084] Step 28: S-DU takes decision (desperate) for servicing cell change considering the new LTM thresholds for candidate cells. Stops the timer DU-X.

[0085] Step 29: Informs Source CU about cell switch command triggered.

[0086] Step 30: Stop the timer CU-X after receiving Cell change triggered indication.

[0087] Steps 31-34: LTM Handover Execution.

[0088] Steps 35-38: Reconfiguration procedure for CHO or BSO.

[0089] Steps 39-40: UE context release at S-DU. Step 41 : UE does CHO with target CU.

[0090] FIGs. 6A-6C (FIGs. 6A, 6B and 6C) are diagrams illustrating operation of a network according to another example embodiment. In FIGs. 6A-6C, an intermediate or desperate LTM handover of the UE is configured but not performed to a second target cell associated with a source CU 210, and then, after expiration of the validity timer for the relaxed LTM HO requirement, the UE performs a L3 HO or CHO to a cell associated with a target CU 220. The steps in FIGs. 6A-6C will be described briefly that are different from the steps in FIGs. 5A-5D. Steps 1-26 of FIGs. 6A-6c are generally the same as Steps 1-26 of FIGs. 5A-5D. At Step 27, the validity timer DU-X at source DU 512 expires, and a LTM HO to the second target cell was not performed, and the source DU 512 reverts back to a LTM HO configuration (e.g., previous or default configuration) that uses a non-relaxed HO requirement. Similarly, at Step 28, the validity timer CU-X at source CU 210 expires (without source CU 210 receiving a LTM HO indication for the UE 510 (thus, no LTM HO of the UE to the second target cell was performed). At Steps 29-30, after expiration of the validity timer CU-X, the source CU 210 transmits the RRC reconfiguration message including L3 HO command or CHO configuration to the source DU 512 associated with the source CU 210, since LTM HO to the second target cell, or cell 2 (associated with DU 514 and source CU 210), was not performed, and therefore, the UE 510 is still served by serving cell associated with source DU 512. The RRC message including a HO command or CHO configuration that is transmitted by source CU 210 to the source DU 512 may include a full configuration, e.g., with values for all parameters of the configuration. And / or, the RRC message may include a delta configuration including one or more updated parameters (e.g., only including configuration parameters that are changed or different, with respect to the reference configuration) for the HO of the UE to the first target cell, where the one or more updated parameters being reference to a full configuration known by the UE, wherein the full configuration corresponds to the configuration of the serving cell associated with the source DU 512 (that is currently serving the UE 510).

[0091] The operation of FIGs. 6A-6C may be summarized as follows:

[0092] Steps 1-15: LTM Preparation and Early sync performed / UE also provides LI measurements.

[0093] Steps 16-17: UE sends L3 Measurement report for new cells (belonging possibly to other CU) as radio condition deteriorated and LTM condition is not met for LTM.

[0094] Step 18: Source CU decides to modify the LTM configuration at source DU for desperate LTM and same time prepares candidate cells for Handover (for e.g., CHO with delta / full config) on optionally considering serving cell as reference for CHO / other Base Handover (BHO). At same time starts the timer CU-X.

[0095] Step 19: CU indicates S-DU with new thresholds for LTM along with a timer value X(DU- X). New LTM threshold is valid till the timer expiry.

[0096] Step 20: S-DU starts the timer DU-X. Note, timers CU-X and DU-X have the same value and they target CU and DU to have the coordinated operation.

[0097] Steps 21-26: Handover Preparation initiated to all selected new inter cu candidate cells in parallel with full / delta config (started at Step 18).

[0098] Step 27: Timer DU-X expires. S-DU discards the desperate LTM threshold and consider the previously sent LTM thresholds.

[0099] Step 28: Timer CU-X expires at S-CU. RRC Configuration (with delta if available else full config) for CHO / BHO sent to UE via S-DU.

[0100] Steps 29-32: RRC Reconfiguration procedure for CHO / other BHO through source DU.

[0101] Step 33: CHO with target CU. In case BHO, RRC reconfiguration at Step 30 should be sent to target CU via target DU under target CU.

[0102] FIGs. 7A-7C (FIGs. 7A, 7B and 7C) are diagrams illustrating operation of a network according to another example embodiment. The steps of FIGs. 7A-7C may be summarized as follows:

[0103] Steps 1-15: LTM Preparation and Early sync performed / UE also provides LI measurements.

[0104] Steps 16-17: UE sends L3 Measurement report for new cells (belonging possibly to other CU) as radio condition deteriorated and LTM condition is not met for LTM.

[0105] Step 18: Source CU decides to modify the LTM configuration at source DU for desperate LTM and same time prepares candidate cells for Handover (for e.g., CHO). At the same time it starts timer CU-X.

[0106] Step 19: CU indicates S-DU with new thresholds for LTM along with a timer value X. New LTM threshold is valid till the timer expiry.

[0107] Step 20: S-DU starts the timer DU-X. Note, timers CU-X and DU-X have the same value and they target CU and DU to have the coordinated operation. Steps 21-26: Handover Preparation initiated to all selected new inter CU candidate cells in parallel (started at Step 18).

[0108] Step 27: UE sends LI Measurement report to S-DU (source DU) (may happen in parallel to Steps 21 to 26).

[0109] Step 28: S-DU takes decision (desperate) for serving cell change considering the new LTM thresholds for candidate cells and stops the timer DU-X.

[0110] Step 29: Informs Source CU about cell switch command triggered.

[0111] Step 30: Stop the timer CU-X after receiving Cell change triggered indication.

[0112] Step 31 : Sends the RRC Reconfiguration in UE Context Modification Request to target DU to send to UE after LTM execution completion (after receiving 1st UL SRB data).

[0113] Steps 32-34: LTM Handover Execution.

[0114] Steps 35-38: RRC Reconfiguration procedure for CHO / Base HO command.

[0115] Steps 39-40: UE context release at S-DU.

[0116] Step 41 : UE does CHO with target CU.

[0117] FIGS. 8A-8D illustrate operation of a network according to yet another example embodiment. The following summarizes the operation of FIGs. 8A-8C.

[0118] Steps 1-15: LTM Preparation and Early sync performed / UE also provides LI measurements.

[0119] Steps 16-17: UE sends L3 Measurement report for new cells (belonging possibly to other CU) as radio condition deteriorated and LTM condition is not met for LTM.

[0120] Step 18: Source CU decides to modify the LTM configuration at source DU for desperate LTM and same time prepares candidate cells for Handover (for e.g., CHO). At the same time it starts timer CU-X.

[0121] Step 19: CU indicates S-DU with new thresholds for LTM along with a timer value X. New LTM threshold is valid till the timer expiry. Step 20: S-DU starts the timer DU-X. Note, timers CU-X and DU-X have the same value and they target CU and DU to have the coordinated operation.

[0122] Step 21 : Source CU predicts the possible LTM target candidate and considers this as reference for CHO / other base line HO. Alternately it can prepare a reference template config for delta config preparation.

[0123] Steps 22-27: Handover Preparation initiated to all selected new inter-CU candidate cells in parallel and target CU prepares the delta and full config for CHO or other base line HO.

[0124] Step 28: UE sends LI Measurement report to S-DU (May happen in parallel to Steps 21 to 26).

[0125] Step 29: S-DU takes decision (desperate) for servicing cell change considering the new LTM thresholds for candidate cells and stops timer DU-X.

[0126] Step 30: Informs Source CU about cell switch command triggered.

[0127] Step 31 : Timer CU-X stopped. RRC reconfiguration should be sent with delta (if predicted LTM candidate is success) or full config (if prediction goes wrong for LTM target candidate).

[0128] Step 32: Sends the RRC Reconfiguration in UE Context Modification Request to target DU to send to UE after LTM execution completion (after receiving 1st UL SRB data).

[0129] Steps 33-35: LTM Handover Execution.

[0130] Steps 36-39: RRC Reconfiguration procedure for CHO / Base HO command.

[0131] Steps 40-41 : UE context release at S-DU.

[0132] Step 42: UE does CHO with target CU.

[0133] In the steps of FIGs. 8 A - 8D, at Step 21, the source CU 210 predicts possible target LTM candidate for desperate or intermediate HO of the UE (e.g., a predicted LTM HO to a second target cell associated with the source CU 210, such as to cell 2). At Step 23, the target CU 220 prepares delta configurations and full configurations considering the serving cell and the predicted LTM HO candidate cell. At Step 30, a cell change / LTM HO indication is received by the source CU. The source CU 210 will transmit the HO command or CHO configuration via RRC reconfiguration message that includes either a full configuration (if the predicted LTM HO cell was incorrect), or includes a delta configuration with respect to the full configuration that the UE has for the predicted LTM HO cell if the predicted cell (e.g., cell 2) for the LTM HO was correct. Thus, the RRC message including a HO command or CHO configuration that is transmitted by source CU may include a delta configuration including one or more updated parameters for the HO of the UE to the first target cell, where the one or more updated parameters being reference to a full configuration known by the UE, wherein the full configuration corresponds to the configuration of the serving cell associated with the target DU 514 (that is now serving the UE 510 after the LTM HO was performed).

[0134] For example, if the source CU 210 has predicted that the UE moves (e.g., makes LTM HO) to cell 2, the source CU 210 may prepare the delta configuration of the target cell 4 with respect to the configuration of cell 2. However, if the UE has not done LTM HO at all or has performed LTM HO to some other cell than cell 2, then the prepared delta configuration of the target cell 4 may not be used (because the UE may have released the configuration of cell 2 when moving out of that cell 2). In such case the source CU 210 sends a full configuration of cell 4 to the UE, which is more resource consuming than sending only the delta configuration. Therefore, benefit of having prepared the delta configuration may be present when the prediction of the UE’s movement is correct. It is noted that the source CU 210 may prepare several delta configurations of cell 4, each with respect to different cell. E.g., the source CU 210 may prepare a first delta configuration of cell 4 indicating offset of parameter(s) with respect to cell 2 and a second delta configuration of cell 4 indicating offset of param eter(s) with respect to cell 3. Then, if the UE either does not perform LTM HO away from cell 3 or makes LTM HO to cell 2, the respective delta configuration may be included in the RRC reconfiguration message configuring the UE with the configuration of cell 4.

[0135] Some examples will now be described, based on the description and figures provided herein.

[0136] Example 1. An apparatus comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the apparatus at least to: receive, by a source centralized unit from a user device via a source distributed unit, a layer 3 measurement report that provides results of radio signal measurements of at least one cell; determine, by the source centralized unit based on the layer 3 measurement report, that a handover should be performed for the user device from a serving cell associated with the source centralized unit to a first target cell associated with a target centralized unit, wherein a handover configuration for handover of the user device to the first target cell has not yet been prepared, wherein the target centralized unit is different than the source centralized unit; determine, by the source centralized unit based on the received layer 3 measurement report, to provide to the source distributed unit associated with the serving cell, a layer 1 / layer 2 triggered mobility (LTM) LTM handover configuration with a relaxed handover requirement for a LTM handover of the user device to a second target cell, wherein the second target cell is also associated with the source centralized unit; transmit, by the source centralized unit to the source distributed unit, the LTM handover configuration for the second target cell; and transmit, by the source centralized unit to the target centralized unit associated with the first target cell, a handover request to prepare a handover configuration for the user device to perform a handover to the first target cell.

[0137] Example 2. The apparatus of Example 1, wherein the apparatus is further caused to: start, by the source centralized unit, a validity timer for the LTM handover configuration indicating a validity period for the LTM handover configuration.

[0138] Example 3. The apparatus of Example 2, wherein the apparatus is further caused to: upon making the determination to provide the LTM handover configuration to the source distributed unit, start, by the source centralized unit, the validity timer for the LTM handover configuration.

[0139] Example 4. The apparatus of any of Examples 2-3, wherein the apparatus is further caused to: wherein the handover of the user device to the first target cell is performed after either: 1) an expiration of the validity timer, or 2) a LTM handover of the user device to the second target cell based on the LTM handover configuration for the second target cell.

[0140] Example 5. The apparatus of any of Examples 2-4, wherein the apparatus is caused to: receive, by the source centralized unit, a LTM handover indication indicating that a LTM handover of the user device to the second target cell has been triggered; and transmit, by the source centralized unit to the source distributed unit associated with the second target cell, a radio resource control (RRC) message including a handover command or conditional handover configuration to instruct the user device to perform a handover or conditional handover to the first target cell associated with the target centralized unit.

[0141] Example 6. The apparatus of Example 5, wherein the apparatus is caused to: stop the validity timer for the LTM handover configuration based on the reception of the LTM handover indication.

[0142] Example 7. The apparatus of any of Examples 2-4, wherein the apparatus is caused to: detect expiration of the validity timer for the LTM handover configuration before a LTM handover indication indicating that a LTM handover of the user device to the second target cell has been triggered; and transmit, by the source centralized unit to the source distributed unit associated with the second target cell, a radio resource control (RRC) message including a handover command or conditional handover configuration to instruct the user device to perform a handover or conditional handover to the first target cell associated with the target centralized unit.

[0143] Example 8. The apparatus of Example 7, wherein the RRC message includes a delta configuration including one or more updated parameters for the handover of the user device to the first target cell, the one or more updated parameters being with reference to a full configuration known to the user device, wherein the full configuration corresponds to the configuration of the serving cell.

[0144] Example 9. The apparatus of Example 7, wherein the RRC message includes a delta configuration including one or more updated parameters for the handover of the user device to the first target cell, the one or more updated parameters being with reference to a full configuration known to the user device, wherein the full configuration corresponds to the configuration of the second target cell.

[0145] Example 10. The apparatus of Example 9, wherein the user device uses the delta configuration for the handover to the first target cell, after the user device performs the handover from the serving cell to the second target cell.

[0146] Example 11. The apparatus of any of Examples 1-10, wherein the LTM handover configuration comprises values or updated values for one or more of: a time to trigger value, a delta value, or a threshold value for a handover of the user device.

[0147] Example 12. The apparatus of any of Examples 1-11, wherein the relaxed handover requirement for the LTM handover configuration of the user device is more easily met, as compared to a non-relaxed handover requirement, to more easily trigger the LTM handover to the second target cell based on a LI measurement report, or the relaxed handover requirement for the LTM handover configuration triggers the LTM handover of the user device to the second target cell based on a wider range of LI measurements as compared to a non-relaxed handover requirement.

[0148] Example 13. The apparatus of any of Examples 1-12, wherein the LTM handover configuration is user device-specific.

[0149] Example 14. The apparatus of any of Examples 1-13, wherein the apparatus is caused to: detect, by the source centralized unit based on the reception of the L3 measurement report, that the user device is under radio conditions with respect to the serving cell that are lower than a predetermined threshold; and transmit, by the source centralized unit to the source distributed unit based on the detecting, the LTM handover configuration for the second target cell.

[0150] Example 15. The apparatus of any of Examples 1-14, wherein the apparatus is caused to: detect, by the source centralized unit based on the reception of the L3 measurement report, that the user device is under radio conditions with respect to the serving cell that are lower than a predetermined threshold; confirm, by the source centralized unit, that the source centralized unit has not received an indication of a LTM handover for the user device from the serving cell to the second target cell; and transmit, by the source centralized unit to the source distributed unit based on the detecting and the confirming, the LTM handover configuration for the second target cell.

[0151] Example 16. The apparatus of any of Examples 1-15, wherein the LTM handover configuration for the LTM handover of the user device to the second target cell is an interim or intermediate step handover to reduce a likelihood of a radio link failure for the user device, before the user device performs a handover to the first target cell based on the handover configuration for the first target cell.

[0152] Example 17. A method comprising: receiving, by a source centralized unit from a user device via a source distributed unit, a layer 3 measurement report that provides results of radio signal measurements of at least one cell; determining, by the source centralized unit based on the layer 3 measurement report, that a handover should be performed for the user device from a serving cell associated with the source centralized unit to a first target cell associated with a target centralized unit, wherein a handover configuration for handover of the user device to the first target cell has not yet been prepared, wherein the target centralized unit is different than the source centralized unit; determining, by the source centralized unit based on the received layer 3 measurement report, to provide to the source distributed unit associated with the serving cell, a layer 1 / layer 2 triggered mobility (LTM) LTM handover configuration with a relaxed handover requirement for a LTM handover of the user device to a second target cell, wherein the second target cell is also associated with the source centralized unit; transmitting, by the source centralized unit to the source distributed unit, the LTM handover configuration for the second target cell; and transmitting, by the source centralized unit to the target centralized unit associated with the first target cell, a handover request to prepare a handover configuration for the user device to perform a handover to the first target cell. Example 18. The method of Example 17, further comprising: starting, by the source centralized unit, a validity timer for the LTM handover configuration indicating a validity period for the LTM handover configuration.

[0153] Example 19. The method of any of Examples 17-18, further comprising: upon making the determination to provide the LTM handover configuration to the source distributed unit, starting, by the source centralized unit, the validity timer for the LTM handover configuration.

[0154] Example 20. The method of any of Examples 18-19, further comprising: wherein the handover of the user device to the first target cell is performed after either: 1) an expiration of the validity timer, or 2) a LTM handover of the user device to the second target cell based on the LTM handover configuration for the second target cell.

[0155] Example 21. The method of any of Examples 18-20, further comprising: receiving, by the source centralized unit, a LTM handover indication indicating that a LTM handover of the user device to the second target cell has been triggered; and transmitting, by the source centralized unit to the source distributed unit associated with the second target cell, a radio resource control (RRC) message including a handover command or conditional handover configuration to instruct the user device to perform a handover or conditional handover to the first target cell associated with the target centralized unit.

[0156] Example 22. The method of Example 21, further comprising: stopping the validity timer for the LTM handover configuration based on the reception of the LTM handover indication.

[0157] Example 23. The method of any of Examples 18-22, comprising: detecting expiration of the validity timer for the LTM handover configuration before a LTM handover indication indicating that a LTM handover of the user device to the second target cell has been triggered; and transmitting, by the source centralized unit to the source distributed unit associated with the second target cell, a radio resource control (RRC) message including a handover command or conditional handover configuration to instruct the user device to perform a handover or conditional handover to the first target cell associated with the target centralized unit.

[0158] Example 24. The method of Example 23, wherein the RRC message includes a delta configuration including one or more updated parameters for the handover of the user device to the first target cell, the one or more updated parameters being with reference to a full configuration known to the user device, wherein the full configuration corresponds to the configuration of the serving cell. Example 25. The method of Example 23, wherein the RRC message includes a delta configuration including one or more updated parameters for the handover of the user device to the first target cell, the one or more updated parameters being with reference to a full configuration known to the user device, wherein the full configuration corresponds to the configuration of the second target cell.

[0159] Example 26. The method of Example 25, wherein the user device uses the delta configuration for the handover to the first target cell, after the user device performs the handover from the serving cell to the second target cell.

[0160] Example 27. The method of any of Examples 17-26, wherein the LTM handover configuration comprises values or updated values for one or more of: a time to trigger value, a delta value, or a threshold value for a handover of the user device.

[0161] Example 28. The method of any of Examples 17-27, wherein the relaxed handover requirement for the LTM handover configuration of the user device is more easily met, as compared to a non-relaxed handover requirement, to more easily trigger the LTM handover to the second target cell based on a LI measurement report, or the relaxed handover requirement for the LTM handover configuration triggers the LTM handover of the user device to the second target cell based on a wider range of LI measurements as compared to a non-relaxed handover requirement.

[0162] Example 29. The method of any of Examples 17-28, wherein the LTM handover configuration is user device-specific.

[0163] Example 30. The method of any of Examples 17-29, comprising: detecting, by the source centralized unit based on the reception of the L3 measurement report, that the user device is under radio conditions with respect to the serving cell that are lower than a predetermined threshold; and transmitting, by the source centralized unit to the source distributed unit based on the detecting, the LTM handover configuration for the second target cell.

[0164] Example 31. The method of any of Examples 17-30, comprising: detecting, by the source centralized unit based on the reception of the L3 measurement report, that the user device is under radio conditions with respect to the serving cell that are lower than a predetermined threshold; confirming, by the source centralized unit, that the source centralized unit has not received an indication of a LTM handover for the user device from the serving cell to the second target cell; and transmitting, by the source centralized unit to the source distributed unit based on the detecting and the confirming, the LTM handover configuration for the second target cell.

[0165] Example 32. The method of any of Examples 17-31, wherein the LTM handover configuration for the LTM handover of the user device to the second target cell is an interim or intermediate step handover to reduce a likelihood of a radio link failure for the user device, before the user device performs a handover to the first target cell based on the handover configuration for the first target cell.

[0166] Example 33. An apparatus comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the apparatus at least to: receive, by a source distributed unit associated with a serving cell from a source centralized unit, a layer 1 / layer 2 triggered mobility (LTM) LTM handover configuration with a relaxed handover requirement for a LTM handover of a user device to a target cell, and a timer value to be used for a validity timer for the LTM handover configuration, wherein the target cell is associated with the source centralized unit and with a different distributed unit; start, by the source distributed unit, the validity timer for the LTM handover configuration indicating a validity period for the LTM handover configuration; receive, by the source distributed unit from the user device, a layer 1 measurement report before expiration of the validity timer for the LTM handover configuration; determine, by the source distributed unit based on the layer 1 measurement report, that the relaxed handover requirement of the LTM handover configuration has been fulfilled; and transmit, by the source distributed unit to the user device, a media access control (MAC) control element (MAC CE) to trigger the user device to perform a LTM handover to the target cell.

[0167] Example 34. The apparatus of Example 33, wherein the target cell comprises a second target cell, and wherein a handover configuration for the user device has been prepared for a handover to a first target cell associated with a target centralized unit, wherein the LTM handover configuration for the LTM handover of the user device to the second target cell is an interim or intermediate step handover to reduce a likelihood of a radio link failure for the user device, before the user device performs a handover to the first target cell based on the handover configuration for the first target cell.

[0168] Example 35. A method comprising: receiving, by a source distributed unit associated with a serving cell from a source centralized unit, a layer 1 / layer 2 triggered mobility (LTM) LTM handover configuration with a relaxed handover requirement for a LTM handover of a user device to a target cell, and a timer value to be used for a validity timer for the LTM handover configuration, wherein the target cell is associated with the source centralized unit and with a different distributed unit; starting, by the source distributed unit, the validity timer for the LTM handover configuration indicating a validity period for the LTM handover configuration; receiving, by the source distributed unit from the user device, a layer 1 measurement report before expiration of the validity timer for the LTM handover configuration; determining, by the source distributed unit based on the layer 1 measurement report, that the relaxed handover requirement of the LTM handover configuration has been fulfilled; and transmitting, by the source distributed unit to the user device, a media access control (MAC) control element to trigger the user device to perform a LTM handover to the target cell.

[0169] Example 36. The method of Example 35, wherein the target cell comprises a second target cell, and wherein a handover configuration for the user device has been prepared for a handover to a first target cell associated with a target centralized unit, wherein the LTM handover configuration for the LTM handover of the user device to the second target cell is an interim or intermediate step handover to reduce a likelihood of a radio link failure for the user device, before the user device performs a handover to the first target cell based on the handover configuration for the first target cell.

[0170] FIG. 9 is a block diagram of a wireless station or node (e.g., UE, user device, AP, BS, eNB, gNB, RAN node, network node, TRP, or other node) 1300 according to an example embodiment. The wireless station 1300 may include, for example, one or more (e.g., two as shown in FIG. 9) RF (radio frequency) or wireless transceivers 1302 A, 1302B, where each wireless transceiver includes a transmitter to transmit signals and a receiver to receive signals. The wireless station also includes a processor or control unit / entity (controller) 1304 to execute instructions or software and control transmission and receptions of signals, and a memory 1306 to store data and / or instructions.

[0171] Processor 1304 may also make decisions or determinations, generate frames, packets or messages for transmission, decode received frames or messages for further processing, and other tasks or functions described herein. Processor 1304, which may be a baseband processor, for example, may generate messages, packets, frames or other signals for transmission via wireless transceiver 1302 (1302A or 1302B). Processor 1304 may control transmission of signals or messages over a wireless network, and may control the reception of signals or messages, etc., via a wireless network (e.g., after being down-converted by wireless transceiver 1302, for example). Processor 1304 may be programmable and capable of executing software or other instructions stored in memory or on other computer media to perform the various tasks and functions described above, such as one or more of the tasks or methods described above. Processor 1304 may be (or may include), for example, hardware, programmable logic, a programmable processor that executes software or firmware, and / or any combination of these. Using other terminology, processor 1304 and transceiver 1302 together may be considered as a wireless transmitter / receiver system, for example.

[0172] In addition, referring to FIG. 9, a controller (or processor) 1308 may execute software and instructions, and may provide overall control for the station 1300, and may provide control for other systems not shown in FIG. 9, such as controlling input / output devices (e.g., display, keypad), and / or may execute software for one or more applications that may be provided on wireless station 1300, such as, for example, an email program, audio / video applications, a word processor, a Voice over IP application, or other application or software.

[0173] In addition, a storage medium may be provided that includes stored instructions, which when executed by a controller or processor may result in the processor 1304, or other controller or processor, performing one or more of the functions or tasks described above.

[0174] According to another example embodiment, RF or wireless transceiver(s) 1302A / 1302B may receive signals or data and / or transmit or send signals or data. Processor 1304 (and possibly transceivers 1302A / 1302B) may control the RF or wireless transceiver 1302A or 1302B to receive, send, broadcast or transmit signals or data.

[0175] Example embodiments are provided or described for each of the example methods, including: An apparatus (e.g., 1300, FIG. 9) including means (e.g., processor 1304, RF transceivers 1302A and / or 1302B, and / or memory 1306, in FIG. 9) for carrying out any of the methods; a non-transitory computer-readable storage medium (e.g., memory 1306, FIG. 9) comprising instructions stored thereon that, when executed by at least one processor (processor 1304, FIG. 9), are configured to cause a computing system or an apparatus (e.g., 1300, FIG. 9) to perform any of the example methods; and an apparatus (e.g., 1300, FIG. 9) including at least one processor (e.g., processor 1304, FIG. 9), and at least one memory (e.g., memory 1306, FIG. 9) including computer program code, the at least one memory (1306) and the computer program code configured to, with the at least one processor (1304), cause the apparatus (e.g., 1300) at least to perform any of the example methods.

[0176] Embodiments of the various techniques described herein may be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. Embodiments may be implemented as a computer program product, i.e., a computer program tangibly embodied in an information carrier, e.g., in a machine-readable storage device or in a propagated signal, for execution by, or to control the operation of, a data processing apparatus, e.g., a programmable processor, a computer, or multiple computers. Embodiments may also be provided on a computer readable medium or computer readable storage medium, which may be a non-transitory medium. Embodiments of the various techniques may also include embodiments provided via transitory signals or media, and / or programs and / or software embodiments that are downloadable via the Internet or other network(s), either wired networks and / or wireless networks. In addition, embodiments may be provided via machine type communications (MTC), and also via an Internet of Things (IOT).

[0177] As used in this application, the term ‘circuitry’ or “circuit” refers to all of the following: (a) hardware-only circuit implementations, such as implementations in only analog and / or digital circuitry, and (b) combinations of circuits and soft-ware (and / or firmware), such as (as applicable): (i) a combination of processor(s) or (ii) portions of processor(s) / software including digital signal processor(s), software, and memory(ies) that work together to cause an apparatus to perform various functions, and (c) circuits, such as a microprocessor(s) or a portion of a microprocessor s), that require software or firmware for operation, even if the software or firmware is not physically present. This definition of ‘circuitry’ applies to all uses of this term in this application. As a further example, as used in this application, the term ‘circuitry’ would also cover an implementation of merely a processor (or multiple processors) or a portion of a processor and its (or their) accompanying software and / or firmware. The term ‘circuitry’ would also cover, for example and if applicable to the particular element, a baseband integrated circuit or applications processor integrated circuit for a mobile phone or a similar integrated circuit in a server, a cellular network device, or another network device.

[0178] The computer program may be in source code form, object code form, or in some intermediate form, and it may be stored in some sort of carrier, distribution medium, or computer readable medium, which may be any entity or device capable of carrying the program. Such carriers include a record medium, computer memory, read-only memory, photoelectrical and / or electrical carrier signal, telecommunications signal, and software distribution package, for example. Depending on the processing power needed, the computer program may be executed in a single electronic digital computer, or it may be distributed amongst a number of computers.

[0179] Furthermore, embodiments of the various techniques described herein may use a cyberphysical system (CPS) (a system of collaborating computational elements controlling physical entities). CPS may enable the embodiment and exploitation of massive amounts of interconnected ICT devices (sensors, actuators, processors microcontrollers, ...) embedded in physical objects at different locations. Mobile cyber physical systems, in which the physical system in question has inherent mobility, are a subcategory of cyber-physical systems. Examples of mobile physical systems include mobile robotics and electronics transported by humans or animals. The rise in popularity of smartphones has increased interest in the area of mobile cyber-physical systems. Therefore, various embodiments of techniques described herein may be provided via one or more of these technologies.

[0180] A computer program, such as the computer program(s) described above, can be written in any form of programming language, including compiled or interpreted languages, and can be deployed in any form, including as a standalone program or as a module, component, subroutine, or other unit or part of it suitable for use in a computing environment. A computer program can be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network.

[0181] Method steps may be performed by one or more programmable processors executing a computer program or computer program portions to perform functions by operating on input data and generating output. Method steps also may be performed by, and an apparatus may be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit).

[0182] Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer, chip or chipset. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. Elements of a computer may include at least one processor for executing instructions and one or more memory devices for storing instructions and data. Generally, a computer also may include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magnetooptical disks, or optical disks. Information carriers suitable for embodying computer program instructions and data include all forms of nonvolatile memory, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory may be supplemented by, or incorporated in, special purpose logic circuitry.

[0183] To provide for interaction with a user, embodiments may be implemented on a computer having a display device, e.g., a cathode ray tube (CRT) or liquid crystal display (LCD) monitor, for displaying information to the user and a user interface, such as a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer. Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speech, or tactile input. Embodiments may be implemented in a computing system that includes a backend component, e.g., as a data server, or that includes a middleware component, e.g., an application server, or that includes a frontend component, e.g., a client computer having a graphical user interface or a Web browser through which a user can interact with an embodiment, or any combination of such backend, middleware, or frontend components.

[0184] Components may be interconnected by any form or medium of digital data communication, e.g., a communication network. Examples of communication networks include a local area network (LAN) and a wide area network (WAN), e.g., the Internet. While certain features of the described embodiments have been illustrated as described herein, many modifications, substitutions, changes and equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the various embodiments.

Claims

CLAIMS1. An apparatus comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the apparatus at least to: receive, by a source centralized unit from a user device via a source distributed unit, a layer 3 measurement report that provides results of radio signal measurements of at least one cell; determine, by the source centralized unit based on the layer 3 measurement report, that a handover should be performed for the user device from a serving cell associated with the source centralized unit to a first target cell associated with a target centralized unit, wherein a handover configuration for handover of the user device to the first target cell has not yet been prepared, wherein the target centralized unit is different than the source centralized unit; determine, by the source centralized unit based on the received layer 3 measurement report, to provide to the source distributed unit associated with the serving cell, a layer 1 / layer 2 triggered mobility (LTM) LTM handover configuration with a relaxed handover requirement for a LTM handover of the user device to a second target cell, wherein the second target cell is also associated with the source centralized unit; transmit, by the source centralized unit to the source distributed unit, the LTM handover configuration for the second target cell; and transmit, by the source centralized unit to the target centralized unit associated with the first target cell, a handover request to prepare a handover configuration for the user device to perform a handover to the first target cell.

2. The apparatus of claim 1, wherein the apparatus is further caused to: start, by the source centralized unit, a validity timer for the LTM handover configuration indicating a validity period for the LTM handover configuration.

3. The apparatus of claim 2, wherein the apparatus is further caused to: upon making the determination to provide the LTM handover configuration to the source distributed unit, start, by the source centralized unit, the validity timer for the LTM handover configuration.

4. The apparatus of any of claims 2-3, wherein the apparatus is further caused to:wherein the handover of the user device to the first target cell is performed after either: 1) an expiration of the validity timer, or 2) a LTM handover of the user device to the second target cell based on the LTM handover configuration for the second target cell.

5. The apparatus of any of claims 2-4, wherein the apparatus is caused to: receive, by the source centralized unit, a LTM handover indication indicating that a LTM handover of the user device to the second target cell has been triggered; and transmit, by the source centralized unit to the source distributed unit associated with the second target cell, a radio resource control (RRC) message including a handover command or conditional handover configuration to instruct the user device to perform a handover or conditional handover to the first target cell associated with the target centralized unit.

6. The apparatus of claim 5, wherein the apparatus is caused to: stop the validity timer for the LTM handover configuration based on the reception of the LTM handover indication.

7. The apparatus of any of claims 2-4, wherein the apparatus is caused to: detect expiration of the validity timer for the LTM handover configuration before a LTM handover indication indicating that a LTM handover of the user device to the second target cell has been triggered; and transmit, by the source centralized unit to the source distributed unit associated with the second target cell, a radio resource control (RRC) message including a handover command or conditional handover configuration to instruct the user device to perform a handover or conditional handover to the first target cell associated with the target centralized unit.

8. The apparatus of claim 7, wherein the RRC message includes a delta configuration including one or more updated parameters for the handover of the user device to the first target cell, the one or more updated parameters being with reference to a full configuration known to the user device, wherein the full configuration corresponds to the configuration of the serving cell.

9. The apparatus of claim 7, wherein the RRC message includes a delta configuration including one or more updated parameters for the handover of the user device to the first target cell, the one or more updated parameters being with reference to a full configuration known to the user device, wherein the full configuration corresponds to the configuration of the second target cell.

10. The apparatus of claim 9, wherein the user device uses the delta configuration for the handover to the first target cell, after the user device performs the handover from the serving cell to the second target cell.

11. The apparatus of any of claims 1-10, wherein the LTM handover configuration comprises values or updated values for one or more of: a time to trigger value, a delta value, or a threshold value for a handover of the user device.

12. The apparatus of any of claims 1-11, wherein the relaxed handover requirement for the LTM handover configuration of the user device is more easily met, as compared to a nonrelaxed handover requirement, to more easily trigger the LTM handover to the second target cell based on a LI measurement report, or the relaxed handover requirement for the LTM handover configuration triggers the LTM handover of the user device to the second target cell based on a wider range of LI measurements as compared to a non-relaxed handover requirement.

13. The apparatus of any of claims 1-12, wherein the LTM handover configuration is user device-specific.

14. The apparatus of any of claims 1-13, wherein the apparatus is caused to: detect, by the source centralized unit based on the reception of the L3 measurement report, that the user device is under radio conditions with respect to the serving cell that are lower than a predetermined threshold; and transmit, by the source centralized unit to the source distributed unit based on the detecting, the LTM handover configuration for the second target cell.

15. The apparatus of any of claims 1-14, wherein the apparatus is caused to: detect, by the source centralized unit based on the reception of the L3 measurement report, that the user device is under radio conditions with respect to the serving cell that are lower than a predetermined threshold; confirm, by the source centralized unit, that the source centralized unit has not received an indication of a LTM handover for the user device from the serving cell to the second target cell; and transmit, by the source centralized unit to the source distributed unit based on the detecting and the confirming, the LTM handover configuration for the second target cell.

16. The apparatus of any of claims 1-15, wherein the LTM handover configuration for the LTM handover of the user device to the second target cell is an interim or intermediate stephandover to reduce a likelihood of a radio link failure for the user device, before the user device performs a handover to the first target cell based on the handover configuration for the first target cell.

17. A method comprising: receiving, by a source centralized unit from a user device via a source distributed unit, a layer 3 measurement report that provides results of radio signal measurements of at least one cell; determining, by the source centralized unit based on the layer 3 measurement report, that a handover should be performed for the user device from a serving cell associated with the source centralized unit to a first target cell associated with a target centralized unit, wherein a handover configuration for handover of the user device to the first target cell has not yet been prepared, wherein the target centralized unit is different than the source centralized unit; determining, by the source centralized unit based on the received layer 3 measurement report, to provide to the source distributed unit associated with the serving cell, a layer 1 / layer 2 triggered mobility (LTM) LTM handover configuration with a relaxed handover requirement for a LTM handover of the user device to a second target cell, wherein the second target cell is also associated with the source centralized unit; transmitting, by the source centralized unit to the source distributed unit, the LTM handover configuration for the second target cell; and transmitting, by the source centralized unit to the target centralized unit associated with the first target cell, a handover request to prepare a handover configuration for the user device to perform a handover to the first target cell.

18. An apparatus comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the apparatus at least to: receive, by a source distributed unit associated with a serving cell from a source centralized unit, a layer 1 / layer 2 triggered mobility (LTM) LTM handover configuration with a relaxed handover requirement for a LTM handover of a user device to a target cell, and a timer value to be used for a validity timer for the LTM handover configuration, wherein the target cell is associated with the source centralized unit and with a different distributed unit; start, by the source distributed unit, the validity timer for the LTM handover configuration indicating a validity period for the LTM handover configuration;receive, by the source distributed unit from the user device, a layer 1 measurement report before expiration of the validity timer for the LTM handover configuration; determine, by the source distributed unit based on the layer 1 measurement report, that the relaxed handover requirement of the LTM handover configuration has been fulfilled; and transmit, by the source distributed unit to the user device, a media access control (MAC) control element (MAC CE) to trigger the user device to perform a LTM handover to the target cell.

19. The apparatus of claim 18, wherein the target cell comprises a second target cell, and wherein a handover configuration for the user device has been prepared for a handover to a first target cell associated with a target centralized unit, wherein the LTM handover configuration for the LTM handover of the user device to the second target cell is an interim or intermediate step handover to reduce a likelihood of a radio link failure for the user device, before the user device performs a handover to the first target cell based on the handover configuration for the first target cell.

20. A method comprising: receiving, by a source distributed unit associated with a serving cell from a source centralized unit, a layer 1 / layer 2 triggered mobility (LTM) LTM handover configuration with a relaxed handover requirement for a LTM handover of a user device to a target cell, and a timer value to be used for a validity timer for the LTM handover configuration, wherein the target cell is associated with the source centralized unit and with a different distributed unit; starting, by the source distributed unit, the validity timer for the LTM handover configuration indicating a validity period for the LTM handover configuration; receiving, by the source distributed unit from the user device, a layer 1 measurement report before expiration of the validity timer for the LTM handover configuration; determining, by the source distributed unit based on the layer 1 measurement report, that the relaxed handover requirement of the LTM handover configuration has been fulfilled; and transmitting, by the source distributed unit to the user device, a media access control (MAC) control element to trigger the user device to perform a LTM handover to the target cell.

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