Realization of Random Access Channel-less Layer 1 / Layer 2 Trigger Mobility

Abstract

Generally, the subject matter of the present disclosure relates to implementing random access channel-less (RACH-less) layer 1 / layer 2 triggered mobility (LTM). In some implementations, implementing RACH-less LTM can include receiving information indicating whether a handover (HO) of services for a user equipment (UE) currently served by a serving DU of a base station can be a random access channel-less (RACH-less) HO to at least one target cell or a RACH-based HO to at least one target cell in at least one LTM target cell of the base station, and preparing at least one LTM target cell for one of a RACH-less HO and a RACH-based HO based on the information received in at least one LTM target cell.

Description

Technical Field

[0001] Cross-reference to Related Applications This application claims priority to Indian Patent Application No. 202221056932, filed on October 4, 2022, entitled "Method and system to realize RACH-less HO for LLM in inter gNB-DU scenario", the entire content of which is incorporated herein by reference.

[0002] In some implementations, the subject matter of the present disclosure relates to a telecommunication system, and more particularly, to realizing random access channel layer-less (RACH-less) layer 1 / layer 2 triggered mobility (LTM).

Background Art

[0003] In today's world, cellular networks provide on-demand communication capabilities to individuals and enterprises. Typically, a cellular network is a wireless network that can be distributed over a terrestrial area called a cell. Each cell is served by at least one fixed-position transceiver called a cell site or base station. Each cell can use a different set of frequencies from its neighboring cells to avoid interference and provide improved services within each cell. When cells are combined, they provide wireless coverage over a large geographical area, thereby enabling a large number of mobile telephone devices, and / or other wireless devices or portable transceivers to communicate with each other, as well as with some fixed transceiver and telephone device anywhere in the network. Such communication is carried out via a base station and is achieved even when the mobile transceiver is moving through two or more cells during transmission. Major wireless communication providers have deployed such cell sites around the world, thereby enabling communication mobile telephone devices and mobile computing devices to connect to the public switched telephone network and the public Internet.

[0004] A mobile phone device is a portable telephone device that can receive and / or transmit telephone and / or data communications via a cell site or transmission tower by using radio waves to transfer signals to and from the device itself (the mobile phone device). Considering a large number of mobile phone device users, the current mobile phone network provides limited shared resources. In that regard, cell sites and handsets can change frequencies and use low-power transmitters to enable simultaneous use of the network by more callers with less interference. Coverage by cell sites can depend on a particular geographical location and / or the number of users potentially able to use the network. For example, in a city, a cell site can have a range of up to about 1 / 2 mile, in rural areas the range can be up to 5 miles, and in some areas, users can receive signals from cell sites up to 25 miles away.

[0005] The following are some examples of digital cellular technologies used by communication providers, namely, Global System for Mobile Communications (GSM) for mobile communication, General Packet Radio Service (GPRS), cdmaOne, CDMA2000, Evolution-Data Optimized (EV-DO), Enhanced Data Rates for GSM Evolution (EDGE), Universal Mobile Telecommunications System (UMTS), Digital Enhanced Cordless Telecommunications (DECT), Digital AMPS (IS-136 / TDMA), and Integrated Digital Enhanced Network (iDEN). Long Term Evolution, i.e., 4G LTE developed by the 3rd Generation Partnership Project (3GPP™) standards body, is a standard for wireless communication of high-speed data for mobile phone devices and data terminals. Currently, 5G standards are being developed and deployed. 3GPP cellular technologies such as LTE and 5G NR are evolved from early generation 3GPP technologies such as GSM / EDGE and UMTS / HSPA digital cellular technologies, enabling increased capacity and speed by using different radio interfaces along with improvements to the core network.

[0006] A cellular network can be divided into a radio access network and a core network. The radio access network (RAN) may include network functions capable of handling radio layer communications processing. The core network may include network functions capable of handling upper layer communications, such as Internet Protocol (IP), transport layer, and application layer. In some cases, the RAN functions may be divided into a baseband unit function and a radio unit function. For example, a radio unit connected to a baseband unit via a fronthaul network can be responsible for lower layer processing of the radio physical layer, and the baseband unit can be responsible for upper layer radio protocols, such as MAC, RLC, etc.

[0007] A base station for a 5G cellular network may include a centralized unit (CU), one or more distributed units (DUs) communicatively coupled to the CU, and one or more radio units (RUs) each communicatively coupled to at least one of the one or more DUs and each configured to be communicatively coupled to one or more mobile phone devices and / or other user equipment (UE). The CU may be logically divided into a control plane portion CU-CP and one or more user plane portions (CU-UP). During the process of the UE communicatingly coupling with the base station, the DU supporting the UE may change. Time synchronization is established in the random access channel (RACH) procedure between the UE and the new supporting DU so that appropriate communication is performed between the UE and the new supporting DU. However, such a handover from one DU to another requires more time when a contention based RACH procedure has to occur instead of a contention free RACH procedure. Such an increase in time to achieve the handover increases the handover latency, thereby increasing the user plane interruption time. Summary of the Invention

[0008] In some implementations, the subject matter of the present disclosure relates to a computer-implemented method. The method may include receiving, at at least one layer 1 / layer 2 triggered mobility (LTM) target distributed unit (DU) of a base station, information indicating whether a handover (HO) of a service for a user equipment (UE) currently served by a serving DU of the base station can be a random access channel-less (RACH-less) HO to at least one LTM target cell of the target DU or can be a RACH-based HO to the at least one LTM target cell. The method may also include preparing at least one LTM target cell for one of the RACH-less HO and the RACH-based HO based on information received in the at least one LTM target cell.

[0009] The method may enable an HO from one cell of a base station to another cell of the base station to take less time. Because the HO from a serving cell to one of the at least one target cells may already be prepared in each of the one or more target cells and may include either a contention-free RACH procedure (for RACH-less HO) or a contention-based RACH procedure (for RACH-based HO). Thus, the handover latency can be reduced, thereby reducing the user plane interruption time.

[0010] In some implementations, the subject matter of the present disclosure may include one or more of the following optional features.

[0011] In some implementations, the information may include the timing advance (TA) information of the UE in the serving DU, and the TA information of the UE in the serving cell being either (a) the same TA as the TA in at least one LTM target cell or (b) a TA of 0 (zero) may indicate that the HO to at least one LTM target cell can be a RACH-less HO. The TA information of the UE in the serving cell being neither (a) the same TA as the TA in at least one LTM target cell nor (b) a TA of 0 may indicate that the HO to at least one LTM target cell can be a RACH-based HO. Further, at least one LTM target DU can receive the TA information of the UE in the serving DU in a message from the serving DU, and at least one LTM target DU of the method may indicate to the serving DU whether the prepared LTM target cell configuration is a RACH-less configuration or a RACH-based configuration. Alternatively, at least one LTM target DU can receive the TA information of the UE in the serving DU in a message from the centralized unit control plane (CU-CP) of the base station, and at least one LTM target DU of the method may indicate to the CU-CP whether the prepared LTM target cell configuration is a RACH-less configuration or a RACH-based configuration. Further, the CU-CP can receive the TA information of the UE in the serving DU from the serving DU during the setup of the F1 communication interface between the serving DU and the CU-CP, or the CU-CP can receive the TA information of the UE in the serving DU from the serving DU after the F1 communication interface is set up between the serving DU and the CU-CP.

[0012] In some implementations, at least one LTM target cell may be included in at least one DU of a base station that is not the serving DU.

[0013] In some implementations, the serving DU can select one of at least one prepared LTM target cell for handover of services for the UE from the serving DU, and the serving DU can trigger a handover of services for the UE to the selected LTM target cell from the serving DU. Further, the serving DU can determine which of the at least one LTM target cell(s) has a radio quality that exceeds a predetermined threshold radio quality, the selection can be among one or more determined LTM target cells, and / or triggering can include the serving DU sending a Medium Access Control (MAC) control element (CE) message to the UE.

[0014] In some implementations, the base station can have a disaggregated architecture.

[0015] In some implementations, the base station can include a Next Generation Radio Access Network (NG-RAN) node. Further, the NG-RAN node can include a gNodeB or an ng-eNodeB.

[0016] In some implementations, the base station can include at least one processor and at least one non-transitory storage medium storing instructions that, when executed by the at least one processor, cause the at least one processor to perform the method.

[0017] A non-transitory computer program product (i.e., a physically embodied computer program product) is also described that, when executed by one or more data processors of one or more computing systems, causes the one or more data processors to store instructions for performing the operations herein. Similarly, a computer system is also described that may include one or more data processors and a memory coupled to the one or more data processors. The memory can store instructions, either temporarily or persistently, that cause at least one processor to perform one or more of the operations described herein. Further, the method may be implemented by one or more data processors within a single computing system or distributed across two or more computing systems. Such computing systems may be connected via one or more connections and can exchange data and / or commands or other instructions, etc., and this connection includes, but is not limited to, a connection via a network (such as the Internet, a wireless wide area network, a local area network, a wide area network, a wired network, etc.) such as a direct connection between one or more of the plurality of computing systems.

[0018] Details of one or more variations of the disclosed subject matter described herein are set forth in the accompanying drawings and the description below. Other features and advantages of the disclosed subject matter will be apparent from the description and drawings, and from the claims.

[0019] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate specific aspects of the disclosed subject matter and, together with the description, serve to explain some of the principles associated with the disclosed implementations. BRIEF DESCRIPTION OF THE DRAWINGS

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[0040] The subject matter of the present disclosure can provide systems and methods that can be implemented in a wireless communication system. The system can include various wireless communication systems, including a 5G new radio communication system, a long term evolution communication system, and the like.

[0041] Generally, the subject matter of the present disclosure relates to implementing RACH-less LTM.

[0042] In some implementations of the subject matter of the present disclosure, the timing advance (TA) of the serving cell of a base station in a wireless communication system can be used when preparing at least one target cell of the base station for LTM. The at least one target cell can be informed of the TA of the serving cell, for example, by receiving the TA of the serving cell from the CU-CP of the base station communicatively coupled to the serving cell and the at least one target cell. By informing the at least one target cell of the TA of the serving cell, each of the one or more target cells can prepare itself for either a RACH-less handover (HO) (which may also be referred to as RACH-free) from the serving cell or a RACH-based HO for a user equipment (UE) communicatively coupled to the base station. Thus, a HO from one cell of the base station to another cell of the base station may take less time because the HO from the serving cell to one of the at least one target cells is already prepared in each of the one or more target cells to include either a contention-free RACH procedure (for RACH-less HO) or a contention-based RACH procedure (for RACH-based HO). Thus, the handover latency can be reduced, thereby reducing the user plane interruption time.

[0043] Layer 1 / Layer 2 Triggered Mobility (LTM) is an updated (latest) term for Lower Layer Mobility (LLM). RAN2 has agreed on the definition of LTM. In general, LTM is a mobility procedure that enables the network to switch the UE from a source cell to a target cell without necessarily requiring reconfiguration by sync. In particular, based on the received L1 measurement values, the network can indicate, in L2 signaling (e.g., messages such as MAC CE), the beam to which the LTM candidate cell belongs for the UE to execute the LTM cell switching procedure. The UE is provided with at least one LTM candidate cell configuration by the network before executing the LTM cell switching procedure.

[0044] The 3GPP specifications that define one or more aspects related to the subject matter of this disclosure include 3GPP TS 38.321, "NR; Medium Access Control (MAC) Protocol Specification", 3GPP TS 38.331, "NR; Radio Resource Control (RRC) Protocol Specification", 3GPP TS 38.463, "NG-RAN; E1 Application Protocol (E1AP)", and 3GPP TS 38.473, "NG-RAN F1 Application Protocol (F1AP)". The specifications of the O-RAN Alliance may also be related to one or more aspects of the subject matter of this disclosure.

[0045] One or more aspects of the subject matter of this disclosure may be incorporated into the transmitter and / or receiver components of a base station (e.g., gNodeB, eNodeB, etc.) within such a communication system. The following is a general discussion of the Long-Term Evolution communication system and the 5G New Radio communication system.

[0046] I. Long-Term Evolution Communication System Figures 1a through 1c and FIG. 2 illustrate an exemplary conventional Long Term Evolution (LTE) communication system 100 along with its various components. The LTE system or 4G LTE is governed by a standard for wireless communication of high-speed data for mobile telephone devices and data terminals, as is commercially known. This standard is an evolution of GSM / EDGE (Global System for Mobile Communications / Enhanced Data rates for GSM Evolution) as well as UMTS / HSPA (Universal Mobile Telecommunications System / High Speed Packet Access) network technologies. This standard was developed by the 3GPP (3rd Generation Partnership Project).

[0047] As shown in FIG. 1a, system 100 can include an evolved universal terrestrial radio access network (EUTRAN) 102, an evolved packet core (EPC) 108, and a packet data network (PDN) 101, and EUTRAN 102 and EPC 108 provide communication between user equipment 104 and PDN 101. EUTRAN 102 can include a plurality of evolved Node Bs (eNodeB or ENODEB or enodeb or eNB) or base stations 106 (106a, 106b, 106c) (as shown in FIG. 1b) that provide communication capabilities to a plurality of user equipment 104 (104a, 104b, 104c). User equipment 104 can be a mobile telephone device, a smartphone, a tablet, a personal computer, a personal digital assistant (PDA), a server, a data terminal, and / or any other type of user equipment, and / or any combination thereof. User equipment 104 can connect to EPC 108, and ultimately to PDN 101, via any eNodeB 106. Typically, user equipment 104 can connect to the eNodeB 106 that is closest in terms of distance. In the LTE system 100, EUTRAN 102 and EPC 108 cooperate to provide connectivity, mobility, and services for user equipment 104.

[0048] Figure 1b shows further details of the network 100 shown in Figure 1a. As described above, the EUTRAN 102 includes a plurality of eNodeBs 106, also known as cell sites. The eNodeB 106 provides radio functions and performs important control functions including scheduling of air link resources or radio resource management, active mode mobility or handover, and admission control for services. The eNodeB 106 is responsible for selecting which mobility management entity (such as the MME shown in Figure 1c) provides services to the user equipment 104, and protocol features (functions) such as header compression and encryption. The eNodeBs 106 that make up the EUTRAN 102 cooperate with each other for radio resource management and handover.

[0049] The communication between the user equipment 104 and the eNodeB 106 is performed via the air interface 122, also known as the LTE-Uu interface. As shown in Figure 1b, the air interface 122 provides communication between the user equipment 104b and the eNodeB 106a. The air interface 122 uses orthogonal frequency division multiple access (OFDMA) and single carrier frequency division multiple access (SC-FDMA), an OFDMA variant, on the downlink and uplink, respectively. OFDMA enables the use of multiple known antenna technologies such as multiple input multiple output (MIMO).

[0050] The air interface 122 uses various protocols, including radio resource control (RRC) for signaling between the user equipment 104 and the eNodeB 106, and the non-access stratum (NAS) for signaling between the user equipment 104 and the MME (as shown in FIG. 1c). In addition to signaling, user traffic is transferred between the user equipment 104 and the eNodeB 106. Both signaling and traffic in the system 100 are carried by physical layer (PHY) channels.

[0051] Multiple eNodeBs 106 can be interconnected with each other using the X2 interface 130 (130a, 130b, 130c). As shown in FIG. 1b, the X2 interface 130a provides an interconnection between the eNodeB 106a and the eNodeB 106b, the X2 interface 130b provides an interconnection between the eNodeB 106a and the eNodeB 106c, and the X2 interface 130c provides an interconnection between the eNodeB 106b and the eNodeB 106c. The X2 interface can be established between two eNodeBs to provide for the exchange of signals, which may include information related to load or interference, as well as information related to handover. The eNodeB 106 communicates with the evolved packet core 108 via the S1 interface 124 (124a, 124b, 124c). The S1 interface 124 can be split into two interfaces, one for the control plane (shown as the control plane interface (S1-MME interface) 128 in FIG. 1c) and the other for the user plane (shown as the user plane interface (S1-U interface) 125 in FIG. 1c).

[0052] The EPC 108 establishes and enforces Quality of Service (QoS) for user services and enables the user equipment 104 to maintain a consistent (stable) Internet Protocol (IP) address while in motion. Note that each node within the network 100 has its own IP address. The EPC 108 is designed to interact with legacy wireless networks. The EPC 108 is also designed to separate the control plane (i.e., signaling) and the user plane (i.e., traffic) in the core network architecture, which allows for more flexibility in implementation forms as well as independent scalability of control and user data functions.

[0053] The EPC 108 architecture is dedicated to packet data and is shown in more detail in FIG. 1c. The EPC 108 includes a Serving Gateway (S-GW) 110, a PDN Gateway (P-GW) 112, a Mobility Management Entity (MME) 114, a Home Subscriber Server (HSS) 116 (subscriber database for the EPC 108), and a Policy Control and Charging Rules Function (PCRF) 118. Some of these (such as S-GW, P-GW, MME, and HSS) are often combined into nodes according to the manufacturer's implementation form.

[0054] The S-GW 110 functions as an IP packet data router and is the bearer path anchor of user equipment within the EPC 108. Therefore, when a user equipment moves from one eNodeB 106 to another during a mobility operation, the S-GW 110 remains the same, and the bearer path towards the EUTRAN 102 is switched to communicate with the new eNodeB 106 that provides services to the user equipment 104. When the user equipment 104 moves to the domain of another S-GW 110, the MME 114 will transfer all of the user equipment's bearer paths to the new S-GW. The S-GW 110 establishes a bearer path for the user equipment to one or more P-GWs 112. When downstream data is received for an idle user equipment, the S-GW 110 buffers the downstream packets and requests the MME 114 to identify and re-establish the bearer path to the EUTRAN 102 and the bearer path via the EUTRAN 102.

[0055] The P-GW 112 is the gateway between the EPC 108 (and the user equipment 104 and the EUTRAN 102) and the PDN 101 (shown in FIG. 1a). The P-GW 112 functions as a router for user traffic and performs functions on behalf of the user equipment. These include IP address assignment to the user equipment, packet filtering of downstream user traffic to ensure that the downstream user traffic is placed on the appropriate bearer path, and enforcement of downstream QoS including data rate. Depending on the services used by the subscriber, there may be multiple user data bearer paths between the user equipment 104 and the P-GW 112. A subscriber can use services on PDNs served by different P-GWs, in which case the user equipment has at least one bearer path established for each P-GW 112. During a handover of the user equipment from one eNodeB to another, if the S-GW 110 is also changing, the bearer path from the P-GW 112 is switched to the new S-GW.

[0056] The MME 114 manages the user equipment 104 within the EPC 108. This management includes managing subscriber authentication, maintaining a context for the authenticated user equipment 104, establishing a data bearer path within the network for user traffic, and tracking the location of idle mobiles that have not detached from the network. In the case of an idle user equipment 104 that needs to reconnect to the access network to receive downstream data, the MME 114 initiates paging to identify the location of the user equipment, and re-establishes the bearer path to the EUTRAN 102 and the bearer path through the bearer path. The MME 114 for a particular user equipment 104 is selected by the eNodeB 106 when the user equipment 104 initiates system access. The MME is typically part of a set of MMEs within the EPC 108 for load sharing and redundancy purposes. In establishing the user's data bearer path, the MME 114 is responsible for selecting the P-GW 112 and the S-GW 110, which constitute the termination of the data path through the EPC 108.

[0057] The PCRF 118 is responsible for policy control decision-making and controlling the flow-based charging functionality within the Policy Control Enforcement Function (PCEF) residing within the P-GW 110. The PCRF 118 provides QoS authorization (QoS Class Identifier (QCI) and bit rate), which determines how a certain data flow is handled within the PCEF and ensures that this is in accordance with the user's subscription profile.

[0058] As described above, the IP service 119 is provided by the PDN 101 (as shown in Figure 1a).

[0059] Figure 1d shows an exemplary structure of eNodeB 106. eNodeB 106 may include at least one Remote Radio Head (RRH) 132 (typically, three RRHs 132 may exist) and a Baseband Unit (BBU) 134. The RRH 132 may be connected to an antenna 136. The RRH 132 and the BBU 134 may be connected using an optical interface compliant with the Common Public Radio Interface (CPRI) / Extended CPRI (eCPRI) 142 standard specification, either using an RRH-specific custom control and user plane framing method or an O-RAN Alliance compliant Control and User plane framing method. The operation of eNodeB 106 can be characterized using the following standard parameters (and specifications), namely, high-frequency bands (Band 4, Band 9, Band 17, etc.), bandwidths (5, 10, 15, 20 MHz), access methods (downlink: OFDMA, uplink: SC-OFDMA), antenna technologies (single-user and multi-user MIMO, uplink: single-user and multi-user MIMO), number of sectors (maximum 6), maximum transmission speeds (downlink: 150 Mb / s, uplink: 50 Mb / s), S1 / X2 interfaces (1000Base-SX, 1000Base-T), and mobile environments (maximum 350 km / h). The BBU 134 can be responsible for digital baseband signal processing, S1 line termination, X2 line termination, call processing, and monitoring control processing. IP packets received from the EPC 108 (not shown in Figure 1d) can be modulated into digital baseband signals and transmitted to the RRH 132. Conversely, digital baseband signals received from the RRH 132 can be demodulated into IP packets for transmission to the EPC 108.

[0060] The RRH 132 can transmit and receive wireless signals using the antenna 136. The RRH 132 can convert the digital baseband signals from the BBU 134 (using the converter (CONV) 140) into radio frequency (RF) signals and power-amplify them (using the amplifier (AMP) 138) for transmission to the user equipment 104 (not shown in FIG. 1d). Conversely, the RF signals received from the user equipment 104 are amplified (using the AMP 138) and converted into digital baseband signals (using the CONV 140) for transmission to the BBU 134.

[0061] Figure 2 shows additional details of an exemplary eNodeB 106. The eNodeB 106 includes multiple layers, namely, an LTE layer 1 (reference numeral 202), an LTE layer 2 (reference numeral 204), and an LTE layer 3 (reference numeral 206). The LTE layer 1 includes a physical layer (PHY). The LTE layer 2 includes a media access control (MAC), a radio link control (RLC), and a packet data convergence protocol (PDCP). The LTE layer 3 includes various functions and protocols, including a radio resource control (RRC), a dynamic resource allocation, an eNodeB measurement configuration and provisioning, a radio admission control, a connection mobility control, and a radio resource management (RRM). The RLC protocol is an automatic repeat request (ARQ) fragmentation protocol used over the cellular air interface. The RRC protocol processes the LTE layer 3 control plane signaling between the user equipment and the EUTRAN. The RRC includes functions for connection establishment and release, system information broadcast, radio bearer establishment / reconfiguration and release, RRC connection mobility procedures, paging notification and release, and outer loop power control. The PDCP performs IP header compression and decompression, user data transfer, and maintenance of the sequence numbers of the radio bearers. The BBU 134 shown in FIG. 1d may include the LTE layers L1 - L3.

[0062] One of the main functions of eNodeB 106 is radio resource management, which includes scheduling of uplink and downlink air interface resources for user equipment 104, control of bearer resources, and admission control. eNodeB 106, as an agent for EPC 108, is responsible for transferring paging messages used to identify the location of a mobile when it is idle. eNodeB 106 also communicates common control channel information over the air, communicates header compression, encryption and decryption of user data sent over the air, and establishes handover reports and trigger criteria. As described above, eNodeB 106 can cooperate with other eNodeB 106s via the X2 interface for the purposes of handover and interference management. eNodeB 106 communicates with the MME of the EPC via the S1-MME interface and communicates with the S-GW using the S1-U interface. Further, eNodeB 106 exchanges user data with the S-GW via the S1-U interface. eNodeB 106 and EPC 108 have a many-to-many relationship to support load sharing and redundancy between the MME and the S-GW. eNodeB 106 selects an MME from a group of MMEs so that the load can be shared by multiple MMEs to avoid congestion.

[0063] II. 5G NR Wireless Communication Network In some implementations, the subject matter of the present disclosure relates to a 5G new radio (NR) communication system. 5G NR is the next communication standard beyond the 4G / IMT-Advanced standard. 5G networks provide higher capacity than current 4G, enabling more mobile broadband users per unit area and allowing for consumption of more and / or unlimited amounts of data in gigabytes per month and user. This may enable users to stream high-definition media for hours a day using a mobile device, even when this cannot be done with Wi-Fi networks. 5G networks have improved support for device-to-device communication, lower cost, lower latency, and lower battery consumption than 4G devices. Such networks have data rates of tens of megabits per second for a large number of users, a data rate of 100 Mb / s for metropolitan areas, 1 Gb / s simultaneously to users within a limited area (e.g., an office floor), a large number of simultaneous connections for wireless sensor networks, enhanced spectral efficiency, improved coverage, enhanced signaling efficiency, a latency of 1 to 10 ms, and reduced latency, compared to existing systems.

[0064] FIG. 3 shows an exemplary virtual wireless access network 300. Network 300 can provide communication between various components including base stations (e.g., eNodeB, gNodeB) 301, wireless devices 303, a central unit 302, a digital unit 304, and a radio device 306. The components in system 300 can be communicatively coupled to the core using a backhaul link 305. The centralized unit (CU) 302 can be communicatively coupled to the distributed unit (DU) 304 using a midhaul connection 308. The radio frequency (RU) components 306 can be communicatively coupled to the DU 304 using a fronthaul connection 310.

[0065] In some implementations, the CU 302 can provide intelligent communication functions to one or more DU units 304. The units 302, 304 can include one or more base stations, macro base stations, micro base stations, remote radio heads, etc., and / or any combination thereof.

[0066] In a lower layer split architecture environment, the CPRI bandwidth requirement for NR can be several hundred Gb / s. CPRI compression can be implemented in the DU and RU (as shown in FIG. 3). In a 5G communication system, the compressed CPRI over an Ethernet frame is called eCPRI and is the recommended fronthaul network. This architecture can enable the standardization of fronthaul / midhaul, which can include upper layer split (e.g., option 2 or option 3-1 (upper / lower RLC split architecture)) and fronthaul using an L1 split architecture (option 7).

[0067] In some implementations, a lower layer split architecture (e.g., Option 7) may include a receiver in the uplink, joint processing over multiple transmission points (TPs) for both DL / UL, and transport bandwidth and latency requirements to facilitate deployment. Further, the lower layer split architecture of the subject matter of the present disclosure can include a split between cell-level processing and user-level processing, which can include cell-level processing at a remote unit (RU) and user-level processing at a DU. Further, using the lower layer split architecture of the subject matter of the present disclosure, frequency domain samples can be transported via an Ethernet front haul, and the frequency domain samples can be compressed for reduced front haul bandwidth.

[0068] FIG. 4 shows an exemplary communication system 400 that can implement 5G technology and provide its users with access to higher frequency bands (e.g., greater than 10 GHz). System 400 can include a macro cell 402 and small cells 404, 406.

[0069] Mobile device 408 may be configured to communicate with one or more of small cells 404, 406. System 400 can enable splitting of the control plane (C-plane) and user plane (U-plane) between macro cell 402 and small cells 404, 406, where the C-plane and U-plane utilize different frequency bands. Specifically, small cells 404, 406 may be configured to utilize a higher frequency band when communicating with mobile device 408. Macro cell 402 can utilize an existing cellular band for C-plane communication. Mobile device 408 may be communicatively coupled via U-plane 412, where a small cell (e.g., small cell 406) can provide a higher data rate and more flexible / cost / energy-efficient operation. Macro cell 402 can maintain good connectivity and mobility via C-plane 410. Further, in some cases, LTE and NR may be transmitted on the same frequency.

[0070] FIG. 5a shows an exemplary 5G wireless communication system 500 according to some implementations of the subject matter of the present disclosure. System 500 may be configured to have a lower layer split architecture according to Option 7-2. System 500 can include a core network 502 (e.g., 5G core) and one or more gNodeBs (or gNBs), where the gNB can have a centralized unit gNB-CU. The gNB-CU may be logically split into a control plane part gNB-CU-CP504 and one or more user plane parts gNB-CU-UP506. The control plane part 504 and the user plane part 506 may be configured to be communicatively coupled using an E1 communication interface 514 (as defined in the 3GPP standard). The control plane part 504 may be configured to perform the execution of the RRC and PDCP protocols of the radio stack.

[0071] The control plane part 504 and the user plane part 506 of the gNB's central unit can be configured to be communicatively coupled to one or more distributed units (DUs) 508, 510 according to the upper layer split architecture. The distributed units 508, 510 can be configured to execute the upper parts of the RLC, MAC, and PHY layer protocols of the radio stack. The control plane part 504 can be configured to be communicatively coupled to the distributed units 508, 510 using the F1-C communication interface 516, and the user plane part 506 can be configured to be communicatively coupled to the distributed units 508, 510 using the F1-U communication interface 518. The distributed units 508, 510 can be coupled to one or more remote radio units (RUs) 512 via a fronthaul network 520 (which may include one or more switches, links, etc.), which in turn communicates with one or more user devices (not shown in FIG. 5a). The remote radio unit 512 can be configured to execute the lower part of the PHY layer protocol and provide antenna capabilities to the remote unit for communication with user devices (similar to the above description related to FIGS. 1a - 2).

[0072] FIG. 5b shows an exemplary layer architecture 530 of a split gNB. The architecture 530 can be configured as a virtualized disaggregated radio access network (RAN) architecture and can be implemented within the communication system 500 shown in FIG. 5a, whereby the layers L1, L2, L3, and radio processing can be virtualized and disaggregated within the central unit, distributed units, and radio units. As shown in FIG. 5b, the gNB-DU 508 can be communicatively coupled to the gNB-CU-CP control plane part 504 (also shown in FIG. 5a) and the gNB-CU-UP user plane part 506. Each of the components 504, 506, 508 can be configured to include one or more layers.

[0073] The gNB-DU 508 may include the RLC, MAC, and PHY layers, as well as various communications sublayers. These may include the F1 application protocol (F1-AP) sublayer, the GPRS tunneling protocol (GTPU) sublayer, the stream control transmission protocol (SCTP) sublayer, the user datagram protocol (UDP) sublayer, and the internet protocol (IP) sublayer. As described above, the distributed unit 508 can be communicatively coupled to the control plane portion 504 of the central unit, and the central unit may also include the F1-AP, SCTP, and IP sublayers, as well as the radio resource control and PDCP control (PDCP-C) sublayer. Further, the distributed unit 508 can also be communicatively coupled to the user plane portion 506 of the central unit of the gNB. The user plane portion 506 may include the service data adaptation protocol (SDAP), the PDCP user (PDCP-U), the GTPU, the UDP, and the IP sublayers.

[0074] Figure 5c shows an exemplary functional split in the gNB architecture shown in Figures 5a - 5b. As shown in Figure 5c, the gNB-DU 508 can be communicatively coupled to the gNB-CU-CP 504 and the gNB-CU-UP 506 using the F1-C communication interface. The gNB-CU-CP 504 and the gNB-CU-UP 506 can be communicatively coupled using the E1 communication interface. The upper part of the PHY layer (or layer 1) can be executed by the gNB-DU 508, and the lower part of the PHY layer can be executed by the RU (not shown in Figure 5c). As shown in Figure 5c, the RRC and PDCP-C parts may be executed by the control plane portion 504, and the SDAP and PDCP-U parts may be executed by the user plane portion 506.

[0075] Some of the functions of the PHY layer in a 5G communication network include error detection on the transport channel and indication to the upper layer, FEC encoding / decoding of the transport channel, hybrid ARQ soft combining, rate matching of the coded transport channel to the physical channel, mapping of the coded transport channel to the physical channel, power weighting of the physical channel, modulation and demodulation of the physical channel, frequency and time synchronization, radio characteristic measurement and indication to the upper layer, MIMO antenna processing, digital and analog beamforming, RF processing, and other functions.

[0076] The MAC sublayer of layer 2 can perform beam management, random access procedure, mapping between the logical channel and the transport channel, concatenation of multiple MAC service data units (SDUs) belonging to one logical channel into a transport block (TB), multiplexing / demultiplexing of SDUs belonging to the logical channel to / from the TB passed between the physical layer on the transport channel, scheduling information reporting, error correction by HARQ, priority handling between logical channels of one UE, priority handling between UEs by dynamic scheduling, transport format selection, and other functions. The functions of the RLC sublayer may include transfer of upper layer packet data units (PDUs), error correction by ARQ, rearrangement, duplication and protocol error detection of data PDUs, re-establishment, etc. The PDCP sublayer can be responsible for transfer of user data, various functions during the re-establishment procedure, retransmission of SDUs, SDU discard in the uplink, transfer of control plane data, etc.

[0077] The RRC sublayer of layer 3 can perform functions such as broadcasting system information to NAS and AS, establishing, maintaining, and releasing RRC connections, security, establishing, configuring, maintaining, and releasing point-to-point radio bearers, mobility functions, reporting, and other functions.

[0078] III. Realization of RACH-less LTM In some implementations of the subject matter of this disclosure, it is possible to realize RACH-less HO from one cell to another cell for a UE communicatively coupled to a base station of a wireless communication system.

[0079] Layer 1 / Layer 2 Triggered Mobility (LTM) inter-cell handover in the base station can be performed by executing a serving cell change (SSC: Serving Cell Change) from the serving cell to the target cell. Multiple target cells meet the radio state (radio conditions) or handover criteria required for the UE to receive the SSC, and thus can be viable target cell options for the HO. One or more of the viable target cells may require a RACH-based HO that includes contention-based or contention-free RACH procedures, for example, because the timing advance (TA) of the UE's serving cell is different from the TA of the target cell. The timing advance refers to the time offset at the UE between the start of the received downlink subframe and the transmitted uplink subframe. This offset at the UE is necessary to ensure that the downlink and uplink subframes are synchronized at the base station. This is a medium access control (MAC) layer (Layer 2) control element (CE) from the base station to the UE that is used, for example, when controlling the uplink, i.e., the signal transmission timing from the UE to the base station. For example, one or more of the other viable target cells may enable RACH-less HO because the TA of the UE's target cell is 0 (zero), or because the TA of the serving cell is the same as the TA of the UE's target cell. Enabling RACH-less HO on a RACH-based HO is configured to enable a handover to one of the target cells that enables the RACH-less HO procedure.

[0080] As an initial access of a UE to a wireless communication system including a base station, or as a handover from one cell to another cell, the base station can receive signals on a communication channel, for example, an uplink communication channel established between the base station and one or more UEs, using one or more antennas such as antenna 136 in FIG. 1d. The interface may be, for example, the air interface 122 in FIG. 1 using OFDMA and SC-FDMA, or may be another communication channel. The signal can include a frame including a plurality of symbols including one or more symbol groups. Generally, an OFDM symbol can include a cyclic prefix (CP) portion and a random access preamble including data symbols. The symbol group can include a CP portion and a plurality of data symbols, which can reduce overhead by allowing one CP portion to be used for a plurality of data symbols instead of using one CP portion for one data symbol only. Random access is based on the UE transmitting a random access preamble on a random access channel (RACH) for access to the base station, either for initial access or handover. For a certain cell, only a specific number of random access preambles may be available. For example, in 3GPP, an LTE cell may have 64 available preambles. Thus, it is possible for multiple UEs to each randomly select the same preamble and for the cell to receive signals from multiple UEs simultaneously for each signal including the same preamble, thereby causing a collision referred to as "contention". The contention-based RACH procedure can be executed to resolve the collision, which can delay HO, for example, because the collision must be resolved and each UE must have a different preamble for use before both UEs can communicate properly with the cell.

[0081] Conversely, in the contention-free RACH procedure, the UE does not select a preamble. Instead, the cell selects a preamble for the UE and transmits the preamble to the UE. Thus, the cell can ensure that no collision occurs by allocating the preamble such that a particular preamble is assigned to only one UE, which can save the uplink synchronization time that is less than 20 ms, so that the HO latency can be reduced compared to the contention-based RACH procedure.

[0082] In some implementations of the subject matter of this disclosure, the timing advance (TA) of a UE served by a serving cell of a base station in a wireless communication system can be used when preparing the configuration of at least one target cell of at least one target DU of the base station for LTM. At least one target DU can be informed of the TA of the serving cell of the UE. By informing the TA of the serving cell to at least one target cell, each of one or more target DUs can be prepared for either a RACH-less HO or a RACH-based HO from the serving cell for a UE communicatively coupled to the base station. Thus, a HO from one cell of the base station to another cell of the base station may take less time because the HO from the serving cell to one of at least one target cells is already prepared in each of one or more target DUs such that it includes either a contention-free RACH procedure (for RACH-less HO) or a contention-based RACH procedure (for RACH-based HO). Thus, the handover latency is reduced, thereby reducing the user plane interruption time.

[0083] In some implementations of the subject matter of this disclosure, at least one target DU may be informed of the TA of the UE's serving cell by receiving the TA of the UE's serving cell from the CU-CP of a base station communicatively coupled to the serving cell and at least one target cell. The TA information is layer 1 (L1) information and thus not information used by the CU.

[0084] In some implementations of the subject matter of this disclosure, a base station (e.g., a next-generation RAN (NG-RAN) node such as a gNodeB, eNodeB, or gNodeB in FIG. 5a) of a wireless communication system (e.g., a 5G wireless communication system, a 6G or later-generation wireless communication system, etc.) can have a non-agglomerated architecture in which the base station includes one gNB-CU-CP (e.g., gNB-CU-CP 504 in FIGS. 5a-5c), two or more CU-UPs (e.g., gNB-CU-UP 506 in FIGS. 5a-5c), and gNB-DUs (e.g., gNB-DUs 508, 510 in FIGS. 5a-5c). The base station can be configured to implement a RACH-less LTM when a UE is handed off from one cell (serving cell) of the base station to another cell (target cell) of the base station.

[0085] FIG. 6a shows an exemplary system 600 configured to implement RACH-less LTM in an inter-DU scenario. The base station 624 in this illustrated implementation is a gNB configured to be in a 5G wireless communication system similar to the 5G wireless communication system 500 of FIG. 5a described above, but other base stations can be configured similarly and can be used when implementing RACH-less LTM. In the illustrated implementation of FIG. 6a, the base station 624 includes a plurality of CU-UPs 606a, 606b, 606c. The base station 624 includes three CU-UPs 606a, 606b, 606c in this illustrated implementation, but can include a different plurality of CU-UPs. The CU of the base station 624 including the plurality of CU-UPs 606a, 606b, 606c is configured to be communicatively coupled to a core network (not shown in FIG. 6a), such as the 5GC 502 of FIG. 5a, etc.

[0086] The CU of the base station 624 also includes a CU-CP 604 configured to be communicatively coupled to the user plane portions 606a, 606b, 606c of the CU using an E1 communication interface 614. The E1 interface 614 includes three communication links in this illustrated implementation to reflect that there are three CU-Up 606a, 606b, 606c to which the CU-CP 604 can be configured to communicate.

[0087] The base station 624 also includes a plurality of DUs 608, 610. The base station 624 includes two DUs 608, 610 in this illustrated implementation form, but can include another plurality of DUs. The CU-CP 604 is configured to be communicatively coupled to the DUs 608, 610 using the F1-C communication interface 616. The CU-UPs 606a, 606b, 606c are configured to be communicatively coupled to the DUs 608, 610 using the F1-U communication interface 618. The F1-U interface 618 associated with each of the DUs 608, 610 includes three communication links in this illustrated implementation form to reflect that there are three CU-UPs 606a, 606b, 606c to which each DU 608, 610 can be configured to communicate.

[0088] The base station 624 also includes a plurality of RUs 612. The base station 624 includes five RUs 612 in this illustrated implementation form, but can include another plurality (a plurality other than five) of RUs. The RUs 612 are configured to be communicatively coupled to the DUs 608, 610 via the fronthaul network 620. Further, each of the RUs 612 is configured to be communicatively coupled to one or more UEs 622. In this illustrated implementation form, two of the RUs 612 are shown to be communicatively coupled to one UE 622, two of the RUs 612 are shown to be communicatively coupled to two UEs 622, and one of the RUs 612 is shown to be communicatively coupled to three UEs 622, but each of the RUs 612 can be coupled to a different number of UEs, either the same as or different from any of the other RUs 612.

[0089] The realization of RACH-less LTM in the DU scenario can be configured such that when one of the UEs communicatively coupled to the base station 624 is handed off from one of the DUs 608, 610 of the base station 624 to the other of the DUs 608, 610 of the same base station 624. One of the DUs 608, 610 currently providing service to the UE 622 is referred to as the "serving DU" because it is currently providing service to the UE 622. One of the DUs 608, 610 to which the service of the UE is being handed off is referred to as the "target DU" because it aims to provide service to the UE 622.

[0090] A system that can be configured to implement RACH-less LTM in a DU scenario is further described with respect to FIG. 6b. FIG. 6b shows the CU-CP 604 and CU-UPs 606a, 606b, 606c of FIG. 6a, but in the illustrated implementation of FIG. 6b, the base station 624 includes three or more DUs. In the illustrated implementation of FIG. 6b, the base station 624 includes 66 DUs. Three of the DUs 628a, 628b, 628c are macro cells (labeled macro1, macro2, and macro3 in FIG. 6b), and 63 of the DUs 626 are small cells (nine of which are labeled gNB-DU10, gNB-DU20, gNB-DU30, gNB-DU40, gNB-DU50, gNB-DU60, gNB-DU70, gNB-DU80, and gNB-DU90 in FIG. 6b). The base station 624 may include a different number of macro cells and / or a different number of small cells. Twenty-one of the small cell DUs 626, including macro1 DU 628a, macro2 DU 628b, and gNB-DU10, gNB-DU20, and gNB-DU30, are configured to be served by a first CU-UP 606a (labeled CU-UP1 in FIG. 6b). Twenty-one of the small cell DUs 626, including macro1 DU 628a, macro2 DU 628b, macro3 DU 628c, and gNB-DU40, gNB-DU50, and gNB-DU60, are configured to be served by a second CU-UP 606b (labeled CU-UP2 in FIG. 6b). Twenty-one of the small cell DUs 626, including macro2 DU 628b, macro3 DU 628c, and gNB-DU70, gNB-DU80, and gNB-DU90, are configured to be served by a third CU-UP 606c (labeled CU-UP3 in FIG. 6b).

[0091] In the implementation shown in FIG. 6b, each of the CU-UPs 606a, 606b, 606c provides services to a subset of the DUs 626, 628a, 628, 628c for all services. However, the CU-UP can provide services to all DUs of the base station for one service (e.g., enhanced mobile broadband (eMBB)), while providing services to a subset of the DUs for another service (e.g., vehicle-to-everything (V2X) or ultra-reliable low latency communication (URLLC)).

[0092] FIG. 7 shows an exemplary method 700 according to some implementations of the subject matter of the present disclosure. The method 700 is described with respect to the exemplary system 800 shown in FIG. 8a, but can be similarly implemented in other systems, such as the systems 100 of FIGS. 1a-1c and 2, the system 400 of FIG. 4, the system 500 of FIG. 5a, and the systems of FIGS. 6a and 6b. Although the system 800 of FIG. 8a is a 5G system, as described above, the realization of RACH-less LTM in the DU-to-DU scenario described herein can be performed in other types of wireless communication systems, such as an LTE wireless communication system or a wireless communication system of the 6G or later generations.

[0093] In system 800, a UE 802 (e.g., UE 104 in FIGS. 1a - 1c, UE 622 in FIG. 6a, etc.) is configured (810) with an LTM having one or more target cells within one or more DUs 804, 806 (e.g., DU 508 in FIGS. 5a - 5c, DU 510 in FIG. 5a, DU 608 in FIG. 6a, DU 610 in FIG. 6a, DU 626 in FIG. 6b, DU 628a, 628b, 628c in FIG. 6b, etc.) of a base station, e.g., a gNB (e.g., gNodeB in FIG. 5a, gNodeB 624 in FIGS. 6a and 6b, etc.). For ease of explanation, system 800 is shown in FIG. 8a with one UE 802 communicatively coupled to a base station that includes two DUs 804, 806, but two or more UEs can be communicatively coupled to the base station, and / or the base station can include three or more DUs. The base station of system 800 also includes a CU - CP 808 (e.g., gNB - CU - CP 504 in FIGS. 5a - 5c, CU - CP 604 in FIGS. 6a and 6b, etc.), and a CU including one or more CU - UPs (e.g., gNB - CU - UP 506 in FIGS. 5a - 5c, CU - UP 606a, 606b, 606c in FIGS. 6a and 6b, etc.) (not shown in FIG. 8a), and a plurality of RUs (e.g., RU 512 in FIG. 5a, RU 612 in FIG. 6a, etc.) (not shown in FIG. 8a). The UE 802 is currently served by the serving DU 804. Further, the base station in FIG. 8a is communicatively coupled to a core network (e.g., EPC 108 in FIGS. 1a - 1c and 2, 5GC 502 in FIG. 5a, etc.) (not shown in FIG. 8a).

[0094] Method 700 shows an implementation form of an inter-DU LTM serving cell change scenario that includes a serving DU804 determining (702) that a cell change for UE802 is necessary. The determination (702) by the serving DU may include the serving DU804 analyzing (814) an in-band L1 measurement report transmitted (812) to the serving DU804 by UE802 in accordance with 3GPP specifications. According to 3GPP specifications, the in-band L1 measurement report may include layer 1 (L1) measurement values that can be analyzed by the serving DU804 when making resource control decisions, which may include a serving cell change where UE802 is to be served by a DU other than the serving DU804, such as target DU806, for at least one service.

[0095] In response to determining (702) that a serving cell change should occur, the serving DU804 notifies (704) UE802 of the serving cell change. As shown in Figure 8a, the notification (704) to UE802 may include the serving DU804 transmitting (816) a serving cell change command, such as a MAC CE, to UE802.

[0096] Also, in response to determining (702) that a cell service change should occur, the serving DU804 notifies (704) CU-CP808 that a serving cell change has occurred for UE802. Thus, the notification (704) can identify UE802 to CU-CP808 using, for example, an identifier known to the serving DU804 and in accordance with 3GPP specifications that uniquely identifies UE802 to CU-CP808. As shown in Figure 8a, the notification (704) to CU-CP808 may include the serving DU804 transmitting (818) a serving cell change notification message to CU-CP808 using the F1 communication interface. As also shown in Figure 8a, the serving cell change notification message includes cell identification information (ID) that uniquely identifies UE802 that has received the serving cell change.

[0097] In response to receiving a serving cell change command 704 from serving DU804, UE802 transmits (820) a Radio Resource Control (RRC) reconfiguration acknowledgement message to CU-CP808. CU-CP808 will recognize from the RRC reconfiguration acknowledgement message that UE802, uniquely identified to CU-CP808 by serving DU804, approves the completion of a normal serving cell change.

[0098] Also, in response to receiving a layer 3 RRC measurement configuration, UE802 transmits (822) an RRC measurement report to CU-CP808 in accordance with 3GPP specifications. According to 3GPP specifications, the RRC measurement report may include layer 3 (L3) measurement values that can be analyzed by CU-CP808 when making resource control decisions, which may include determining (824) to prepare at least one target DU cell for LTM such that at least one target cell from target DU806 is ready to serve UE802 instead of serving DU804 for at least one service.

[0099] In response to determining (824) to prepare at least one target cell for LTM, CU-CP808 prepares (706) at least one target cell for LTM. As shown in Figure 8a, in this illustrated implementation, each of the at least one target cells is an inter-DU target cell, for example, part of a DU different from serving DU804 that is served by the same CU (e.g., the CU including CU-CP808) for each of DUs 804, 806. Also, in this illustrated implementation, since there are only two gNB-DUs, the at least one target cell includes only target DU806, but as described above, the base station can include three or more target cells. Currently, according to 3GPP specifications, up to eight LTM target cells can be prepared for a given UE.

[0100] Preparing at least one target cell for LTM (706) may include notifying at least one target DU806 that it may be notified later to start providing services to UE802 for at least one service by at least one target DU806. Thus, the target DU806 can reserve the resources required for UE802. As shown in Figure 8a, in this illustrated implementation, preparing at least one target cell (706), which is only the target DU806, may include the CU-CP808 sending a UE context setup request message to the target DU806 (832) using the F1 communication interface and in accordance with 3GPP specifications.

[0101] The preparation (706) of at least one target cell may include providing at least one target DU 806 with information indicating whether the HO can be a RACH-less HO or a RACH-based HO (Contention-Free RACH (CFRA) or Contention-Based RACH (CBRA)). In an exemplary implementation, the information includes the UE's Timing Advance (TA) information for the serving DU 804. The fact that the UE's TA information for the serving cell 804 is either (a) the same TA as the TA of the target cell that receives the TA information or (b) a TA of 0 indicates that the HO to that target cell can be a RACH-less HO, thereby enabling the target cell to be prepared for RACH-less HO. The fact that the UE's TA information for the serving cell 804 is neither (a) the same TA as the TA of the target cell that receives the TA information nor (b) a TA of 0 indicates that the HO to the target cell can be a RACH-based HO, thereby enabling the target cell to be prepared for RACH-based HO. In an example where at least one target cell includes two or more target cells, RACH-less HO may be possible for zero or more of the target cells, and RACH-based HO may be possible for the remaining target cells. The information indicating whether the HO can be a RACH-less HO or a RACH-based HO can be provided to at least one target DU 806 in various ways.

[0102] In some implementations, the CU-CP 808 transmits information, such as the UE's serving cell (804) TA information, as a parameter within, for example, the UE context setup request message transmitted (832) to the target DU 806, in the preparation (706) of at least one target cell 806. The CU-CP 808 can receive the TA information prior to the target cell preparation (706).

[0103] In some implementations where the CU-CP808 receives serving cell TA information before target cell preparation (706), the CU-CP808 can fetch (826) the serving cell TA information from the serving cell 804 before sending (832) the UE context setup request message to the target DU806. Thus, the fetching (826) of the serving cell (804) TA information can be performed after the F1 setup procedure in which the F1 communication interface is set up between the CU-CP808 and the serving cell 804. Figure 8a shows such an implementation.

[0104] As shown in FIG. 8a, the fetching of the serving cell (804) TA information by the CU-CP 808 (826) may include the CU-CP sending a message to the serving cell 804 requesting that the serving cell 804 provide the CU-CP 808 with the TA information of the serving cell 804 (828), and the serving cell 804 sending the TA information to the CU-CP 808 in response (830). The determination by the CU-CP 808 to prepare at least one target cell (824) can trigger the CU-CP to send (828) a message requesting TA information because the CU-CP 808 is currently aware of the HO situation. By obtaining the serving cell (804) TA information after the need for UE HO is known, the CU-CP 808 may be able to have the latest TA information about the serving cell. In response to the request, the reception of the TA information related to UE-HO when the need for UE HO is known is only received when the TA information is needed and does not necessarily have to be stored for later use, thus relaxing the storage requirements of the CU-CP 808. As shown in FIG. 8a, the message requesting the serving cell TA information, sent by the CU-CP 808 (828), can be a UE context change request message, and the message sent by the serving cell 804 in response (830) can be a UE context setup response message. Since the UE context change request message and the UE context setup response message are defined by 3GPP, the process can utilize the existing CU and DU functions.

[0105] In response to receiving a UE context setup request message from CU-CP808, target DU806 prepares (834) each of at least one target cell for LTM. In this illustrated implementation, the at least one target cell includes only target DU806 that prepares (834) one target cell. As shown in Figure 8a, the preparation (834) may include the target cell securing the resources required for UE802 and verifying the RACH-less realizability of HO. Since the target DU already recognizes the TA of its own cell and currently recognizes the TA of the serving cell by receiving the serving cell TA information from CU-CP808, such verification becomes possible.

[0106] Target DU806 notifies CU-CP808 that the preparation (834) has been completed. As shown in Figure 8a, the notification to CU-CP808 may include target DU806 transmitting (836) a UE context setup response message to CU-CP808 using the F1 communication interface and in accordance with 3GPP specifications. As also shown in Figure 8a, the UE context setup response message includes the consolidated cell group configuration information of the target cell prepared at target DU806. The consolidated cell group configuration information includes information regarding the configuration type, e.g., RACH-based or RACH-less configuration.

[0107] The CU-CP808 identifies each of one or more LTM-prepared target cells and notifies (708) the serving DU804 of at least one LTM-prepared target cell and its configured type by including information regarding each of one or more target DUs. In an inter-DU LTM scenario, one or more of at least one LTM-prepared target cell belong to a DU different from the serving DU806. For example, referring to the system of FIG. 6b, the serving DU can be the small cell 626 of macro1 DU628a, and one or more of the target cells can be one or more small cells 626 of macro2 DU628b and / or macro3 DU628c.

[0108] As shown in FIG. 8a, the notification (708) to the serving DU804 can include the CU-CP808 transmitting a UE context change request message to the serving DU804 using the F1 communication interface (838). The UE context change request message can include, for each of one or more target DUs, cell identification information (e.g., a unique cell ID such as a physical cell identifier (PCI) that identifies the target DU) or an index corresponding to the cell ID, and priority information. The notification (708) to the serving DU804 can include information indicating the RACH-less HO configuration of the target DU806. As described above, the CU-CP808 recognizes candidates for target DUs for RACH-less handover.

[0109] The priority information provided by the CU-CP808 to the serving DU804 indicates a ranked order of one or more target cells selected by the serving DU804 as target DUs for handover, configured for RACH-less handover. Thus, the priority information for a particular target cell may include a ranking number (e.g., 1, 2, 3, etc.) indicating the rank of the target cell in the ranking of all target cells eligible for RACH-less handover, identified by the CU-CP808 to the serving DU804. The CU-CP808 can determine the ranked order in any of various ways. In some implementations, the CU-CP808 can determine the ranked order based on resource availability, load, slice compatibility, and / or any other RRM criteria. The priority information can help the serving DU804 select one of one or more target cells for RACH-less HO, as further discussed below.

[0110] As in the illustrated implementation where the target DU806 is the only option, if there is only one target cell eligible for RACH-less handover that is a candidate for serving cell change, the priority information can be omitted from the message from the CU-CP808 to the serving DU804 because there is only one possible option for HO identified for the serving DU804.

[0111] In response to at least one LTM-prepared target DU cell being notified (708), serving cell 804 stores the received information regarding at least one LTM target cell, for example, stores a list of LTM-prepared target cells, and stores their respective priority information if provided to serving DU804. Also, in response to at least one target DU being notified (708), serving cell 804 transmits (840) a UE context change response message to CU-CP808 using the F1 communication interface. The UE context change response message may include integrated cell group configuration information for each of one or more target cells identified to UE802 by CU-CP808. The UE context change request message and the UE context change response message are each defined by 3GPP. Thus, serving DU804 can receive information regarding at least one target cell from CU-CP808 and can approve that reception to CU-CP808 using the messages already transmitted for HO according to 3GPP specifications.

[0112] In response to receiving the UE context change response message, CU-CP808 transmits (842) an RRC reconfiguration message to UE802 according to 3GPP specifications. As shown in Figure 8a, the RRC reconfiguration message includes LTM target cell configuration information, for example, as provided from target DU806 to CU-CP808 in the transmitted (830) UE context setup response message. Thus, the UE can be informed whether RACH-less HO (LTM SCC) is possible. If RACH-less HO is not possible, the UE can perform CBRA for target cell 806 since the preamble is not reserved for CFRA.

[0113] In response to receiving the target cell configuration within the RRC reconfiguration message, UE 802 transmits an L1 measurement report to serving DU 804 (844) in accordance with 3GPP specifications. The L1 measurement report provides the UE measurement radio state information of the configured target cell to serving DU 804.

[0114] In response to receiving the (844) L1 measurement report transmitted from UE 802, serving cell 804 selects (710, 846) a target cell from among the one or more LTM-prepared target cells identified for serving DU 804 that is capable of RACH-less HO. In the illustrated implementation, since there is only one target cell (target DU 806) identified for serving DU 804 as an LTM-prepared target cell by CU-CP 808, the selection of the serving cell (710, 846) is straightforward and serving DU 804 selects target DU 806 (710, 846). If there are multiple LTM-prepared target cells that meet the handover criteria at serving DU 804, serving DU 804 is configured to select a target cell configured for RACH-less handover, if available, as described herein. If there are no LTM-prepared target cells that are candidates for RACH-less HO, a target cell can be selected according to conventional procedures in accordance with 3GPP.

[0115] In an implementation where there are multiple target cells identified for the serving DU804 by the CU-CP808, the serving cell target cell selection (710, 846) may include determining which of the multiple target cells (one or more) have a radio quality that exceeds a predetermined threshold radio quality. The predetermined threshold radio quality is defined by the radio state of the UE that the serving DU804 received from the UE802 in the L1 measurement report. Thus, the serving DU804 can take into account the specific needs of a particular UE802 involved in the HO when selecting (710, 846) a target cell for the HO. Further, the L1 measurement report transmitted (844) by the UE802 to the serving DU804 may report L1 measurement values including the reference signal received power (RSRP) defined by the 3GPP for each of the multiple target cells, and the identification information is known to the UE802 as provided by the CU-CP808 to the UE802 in the RRC reconfiguration message. Thus, the serving DU804 can analyze the L1 measurement report received from the UE802 to determine which of the multiple target cells (one or more) have a radio quality that exceeds the predetermined threshold radio quality.

[0116] If only one of the multiple target cells meets the radio state of the UE, for example, if only one of the radio qualities of the target cells exceeds the predetermined threshold radio quality, the serving cell 804 selects (710, 846) that target cell. If two or more of the multiple target cells meet the radio state of the UE, for example, if the radio qualities of the target cells each exceed the predetermined threshold radio quality, any one of these target cells can meet the needs of the UE, and one of these target cells can be selected (if received) according to the priority information, randomly, or according to another desired criterion.

[0117] When a target cell (e.g., target DU806 in the illustrated implementation of FIG. 8a) is selected (710, 846), the serving DU804 triggers (712) a serving cell change to the selected (710, 846) target cell. As shown in FIG. 8a, triggering a serving cell change (712) may include the serving DU804 sending to the UE802 a MAC CE that includes a serving cell change command and identifies the selected (710, 846) target cell for the UE802, for example, by PCI.

[0118] The reception of the MAC CE by the UE indicates to the UE 802 that an LTM serving cell change (SCC) must be performed for the identified target cell, e.g., the target DU 806 in the illustrated implementation of FIG. 8a for UE 802. Thus, in response to receiving the MAC CE from the serving cell 804, the UE 802 initiates a RACH-less HO to the target cell (714, 850). As shown in FIG. 8a, the UE 802's initiation of a RACH-less HO to the target cell (714, 850) may include the UE 802 accessing the target cell without a RACH procedure, in accordance with 3GPP specifications, to the target DU 806 for the RACH-less HO (852). In response to the UE 802 accessing the target cell, the target DU 806 transmits, via the F1 communication interface, a serving cell change notification identifying the target DU 806, e.g., by a unique identifier per 3GPP, as the new current serving cell for the UE 802 for at least one service (854). Also, in response to receiving the MAC CE from the serving cell 804, the UE 802 transmits an RRC reconfiguration positive acknowledgment message to the CU-CP 808 (856). Thus, the CU-CP 808 receives an affirmative response from both the UE 802 via the RRC reconfiguration positive acknowledgment message indicating a normal RRC reconfiguration in the UE 802 and the target DU 806 via the serving cell change notification that the target DU 806 is currently serving the UE 802 for at least one service that has been handed over from the serving DU 804.

[0119] In some implementations, the serving DU 804 that has already been notified by the CU-CP 808 regarding the RACH-less or RACH-based configuration of the plurality of target cells can still configure the UE 802 to perform uplink (UL) synchronization to the selected (710, 846) target cells while still serving as the serving cell, prior to the serving cell change. The serving DU 804 can configure the UE 802 to perform uplink synchronization, for example, by sending a command to the UE 802 before the transmission 848 of the MAC CE to the UE 802, and thus before the serving cell change occurs. The UE 802 performing UL sync before the serving cell change can help the UE 802 obtain the Timing Advance of the selected (710, 846) target cells and adjust in case there are any changes to the UE's serving cell TA due to further mobility of the UE 802 between the time of target cell preparation and the execution of the serving cell change. The UE 802 can be configured to report the target cell TA obtained by the UE 802 during the UL sync procedure with the target DU 806 to the serving DU 804. The serving DU 804 can compare the TA of the UE 802 in the serving cell (determined in the serving DU 804) with the selected (710, 846) target cells (obtained during UL sync), and thus can be configured to determine whether the previously configured RACH-less HO can be performed for a given target cell. In case there is a serving cell TA change that leads to the impossibility of realizing RACH-less HO, the serving DU 804 can be configured to indicate to the UE 802 in the sent (848) serving cell change command that RACH-based HO should be performed in the target cell.

[0120] In some implementation forms, based on a target cell preparation request from CU-CP808, for example, a (832) UE context setup request sent from CU-CP808, target DU806 can be configured to prepare a RACH rest target cell configuration and secure a RACH preamble for RACH-based serving cell change. In a scenario where UE802 experiences continuous mobility in the serving DU / cell that leads to the impossibility of realizing RACH rest HO due to a change in the TA of the UE in serving DU804, UE802 can use the RACH preamble secured for contention-free RACH access (CFRA) as a fallback solution.

[0121] As described above, the CU-CP808 can receive serving cell TA information before target cell preparation (706). In some implementations, the CU-CP808 can receive serving cell TA information from a cell during the F1 setup procedure in which an F1 communication interface is set up between the CU-CP808 and the cell. This procedure is more granular and can correspond to each beam or beam group. The cell is divided into different beams / beams groups or areas to which a specific TA can be assigned. Then, when it is found that the UE reports a specific beam group or area with its L3 measurement values, a fixed TA can be mapped to the UE. Thus, a single cell can have multiple TAs, each corresponding to one or more beams / beams groups of the cell. This procedure is also feasible for smaller cells where the TA does not change much. Thus, the CU-CP808 can receive TA information before the need for UE HO is known and thus has TA information that is available and ready to be sent to the target cell. FIGS. 8b and 8c illustrate such implementations. FIG. 8b is similar to FIG. 8a, except that instead of fetching (826) serving cell TA information from serving cell 804 during the HO process after the CU-CP808 has been notified (818) of a serving cell change, the CU-CP808 checks (858) the previously received serving cell TA information to identify RACH restarget cell candidates.

[0122] FIG. 8c shows the system of FIG. 8b configured to provide TA information from a cell to the CU-CP808 during the F1 setup procedure, according to some implementations of the subject matter of the present disclosure. This can be updated using the F1:gNB-DU configuration update procedure. The F1 communication interface is set up between the DU and the CU-CP808 in accordance with 3GPP specifications.

[0123] As shown in FIG. 8c, in the F1 setup procedure where the F1 communication interface is set up between the CU-CP 808 and the serving DU 804, the serving DU 804 transmits (860) an F1: setup request to the CU-CP 808 that includes TA information for all beams / beam groups for each cell of the serving DU 804. Although the serving DU 804 is shown in FIG. 8c as having a plurality of cells, the serving DU 804 can include a single cell. In response to receiving the F1: setup request from the serving DU 804, the CU-CP 808 transmits (862) an F1: setup response to the serving DU 804.

[0124] As also shown in FIG. 8c, in the F1 setup procedure where the F1 communication interface is set up between the CU-CP 808 and the target DU 806, the target DU 806 transmits (864) an F1: setup request to the CU-CP 808 that includes TA information for all beams / beam groups for each cell of the target DU 806. Although the target DU 806 is shown in FIG. 8b as having a plurality of cells, the target DU 806 can include a single cell. In response to receiving the F1: setup request from the target DU 806, the CU-CP 808 transmits (866) an F1: setup response to the target DU 806.

[0125] FIG. 8c shows that the F1 communication interface is set up between the serving DU 804 and the CU-CP 808 before the F1 communication interface is set up between the target DU 806 and the CU-CP 808. However, the F1 communication interface can be set up between the target DU 806 and the CU-CP 808 before the F1 communication interface is set up between the serving DU 804 and the CU-CP 808.

[0126] In some implementations where the CU-CP808 receives TA information before target cell preparation (706), the serving DU804 can proactively provide the CU-CP808 with TA information without receiving a request for TA information from the CU-CP808. Thus, the CU-CP808 can store the serving cell TA information and have it on hand for later provision to the target DU806. FIG. 8d shows such an implementation. FIG. 8d is similar to FIG. 8a, except that instead of fetching serving cell TA information from the serving cell 804 (826) during the HO process after the CU-CP808 has been notified (818) of a serving cell change, the CU-CP808 checks (876) the previously received serving cell TA information to identify RACH-less target cell candidates.

[0127] As shown in FIG. 8d, in response to receiving the L1 measurement report transmitted (870) from the UE802 after the serving SU804 has determined (702) that a cell change is required for the UE802, the serving DU804 identifies (872) one or more potential candidate target cells for a RACH-less HO from an adjacent DU that in the illustrated implementation of FIG. 8d includes only one DU806. The serving DU804 then transmits (874) a message identifying the one or more potential candidate target cells to the CU-CP808 via the F1 communication interface. The serving DU804 can be configured to transmit such a message to the CU-CP808 (874) only if one or more potential candidate target cells have been identified.

[0128] In some implementation forms such as the various implementation forms described above with respect to FIGS. 8a to 8d, each of one or more candidate target cells receives UE serving cell TA information from the CU-CP. In other implementation forms, DUs of the base station can directly share those TA information with each other. Such DU-to-DU communication is not possible in a 5G wireless communication system, but may be possible in a 6G or later wireless communication system. FIG. 8e shows such an implementation form. FIG. 8e is the same as FIG. 8a, except that the serving DU 804 executes various functions described as being executed by the CU-CP 808 in the implementation form of FIG. 8a. The functions to implement this solution include the termination of the RRC protocol in both the DU and the CU-CP, the distribution of L3 measurement values within the gNB to the DU, the distribution of inter-gNB and inter-radio access technology (RAT) measurement values to the CU-CP, and the autonomous determination by the DU to execute an intra-gNB handover.

[0129] As shown in FIG. 8e, in response to receiving a serving cell change command 704 from the serving DU 804, the UE 802 transmits an RRC reconfiguration positive acknowledgment message to the serving DU 804 (878). The serving DU 804 will come to recognize from the RRC reconfiguration positive acknowledgment message that the UE 802 approves the completion of a normal serving cell change.

[0130] Also, in response to receiving a layer 3 RRC measurement configuration, the UE 802 transmits an RRC measurement report to the serving DU 804 (880). The RRC measurement report may include L3 measurement values that can be analyzed by the serving DU 804 when making a resource control decision, which may include determining to prepare at least one target DU cell for LTM such that at least one target cell from the target DU 806 is ready to serve the UE 802 instead of the serving DU 804 for at least one service (882).

[0131] In response to determining (882) to prepare at least one target cell for LTM, serving DU 804 prepares at least one target cell for LTM. Preparing at least one target cell for LTM may include notifying at least one target DU 806 that it may later be notified to start providing services to UE 802 for at least one service. Accordingly, target DU 806 can reserve the resources required for UE 802. As shown in FIG. 8e, in this illustrated implementation, the preparation of at least one target cell that is only target DU 806 may include serving DU 804 sending a UE context setup request message to target DU 806 (884).

[0132] The preparation of at least one target cell may include providing at least one target DU 806 with information indicating whether the HO can be a RACH-less HO or a RACH-based HO (CFRA or CBRA), similar to what was described above. As shown in FIG. 8e, the information includes serving cell TA information sent from serving DU 804 to target DU 806 in a UE context setup request message (884).

[0133] In response to receiving a UE context setup request message from serving DU804, target DU806 prepares each of at least one target cell for LTM (886), similar to the preparation (834) described above with respect to FIG. 8a. Target DU806 notifies serving DU804 that the preparation (886) is complete. As shown in FIG. 8e, the notification to serving DU804 may include the target DU806 transmitting a UE context setup response message to serving DU804 (888). As also shown in FIG. 8e, the UE context setup response message includes integrated cell group configuration information of the target cells prepared at target DU806. In response to being notified that at least one LTM-prepared target DU cell, serving cell 804 stores received information regarding at least one LTM target cell, for example, stores a list of LTM-prepared target cells and, if provided to serving DU804, stores their respective priority information.

[0134] In response to receiving a UE context change response message, serving DU804 transmits an RRC reconfiguration message to UE802 (890). As shown in FIG. 8e, the RRC reconfiguration message includes LTM target cell configuration information, such as provided from target DU806 to serving cell 804 in the transmitted (888) UE context setup response message. Thus, the UE can be informed whether RACH-less HO (LTM SCC) is possible. If RACH-less HO is not possible, the UE can perform CBRA for target cell 806 because the preamble is not reserved for CFRA. In response to receiving the target cell configuration in the RRC reconfiguration message, UE802 transmits an L1 measurement report to serving DU804 (844), and the process continues as described above with respect to FIG. 8a.

[0135] In some implementations, the subject matter of the present disclosure can be configured to be implemented in a system 900, as shown in FIG. 9. The system 900 can include one or more of a processor 910, a memory 920, a storage device 930, and an input / output device 940. Each of the components 910, 920, 930, and 940 can be interconnected using a system bus 950. The processor 910 can be configured to process instructions for execution within the system 600. In some implementations, the processor 910 can be a single-threaded processor. In alternative implementations, the processor 910 can be a multi-threaded processor. The processor 910 can be further configured to process instructions stored in the memory 920 or the storage device 930, which can include receiving or sending information through the input / output device 940. The memory 920 can store information within the system 900. In some implementations, the memory 920 can be a computer-readable medium. In alternative implementations, the memory 920 can be a volatile memory unit. In still some implementations, the memory 920 can be a non-volatile memory unit. The storage device 930 can be capable of providing mass storage for the system 900. In some implementations, the storage device 930 can be a computer-readable medium. In alternative implementations, the storage device 930 can be a floppy disk device, a hard disk device, an optical disk device, a tape device, a non-volatile solid state memory, or any other type of storage device. The input / output device 940 can be configured to provide input / output operations to the system 900. In some implementations, the input / output device 940 can include a keyboard and / or a pointing device. In alternative implementations, the input / output device 940 can include a display unit for displaying a graphical user interface.

[0136] FIG. 10 shows an exemplary method 1000 for implementing RACH-less LTM according to some implementations of the subject matter of the present disclosure. The method 1000 can be executed using, for example, the implementations shown in FIGS. 1-8b and described with respect to FIGS. 1-8b.

[0137] The method 1000 includes receiving (1002) information indicating whether a handover (HO) of service for a UE currently served by a serving DU (such as serving DU 804 in FIGS. 8a-8e, etc.) of a base station (such as eNodeB 106 in FIGS. 1b-2, gNodeB in FIG. 5a, gNodeB 624 in FIGS. 6a and 6b, base station in FIGS. 8a-8e, etc.) can be a RACH-less HO to at least one target cell (such as DU 508, 510 in FIG. 5a, DU 608, 610 in FIG. 6a, cells 626, 628a, 628b, 628c in FIG. 6b, target DU 806 in FIGS. 8a-8e, etc.) of at least one LTM target cell of the base station or can be a RACH-based HO to at least one target cell. The method may also include preparing (1004) at least one LTM target cell for one of a RACH-less HO and a RACH-based HO based on the information received in the at least one LTM target cell.

[0138] In some implementations, the subject matter of the present disclosure can include one or more of the following optional features.

[0139] In some implementations, the information may include the timing advance (TA) information of the UE in the serving DU, and the TA information of the UE in the serving cell being either (a) the same TA as the TA in at least one LTM target cell, or (b) a TA of 0 may indicate that the HO to at least one LTM target cell may be a RACH-less HO. The TA information of the UE in the serving cell being neither (a) the same TA as the TA in at least one LTM target cell nor (b) a TA of 0 may indicate that the HO to at least one LTM target cell may be a RACH-based HO. Further, at least one LTM target cell may receive the TA information of the UE in the serving DU (such as the serving DU804 in FIG. 8e, etc.) in a message from the serving DU, and the method may include at least one LTM target cell indicating to the serving DU whether the prepared LTM target cell configuration is a RACH-less configuration or a RACH-based configuration. Or at least one LTM target cell may receive the TA information of the UE in the serving DU in a message from the CU-CP of the base station (such as the gNB-CU-CP504 in FIGS. 5a-5c, the CU-CP604 in FIGS. 6a and 6b, the CU-CP808 in FIGS. 8a-8d, etc.), and the method may include at least one LTM target cell indicating to the CU-CP whether the prepared LTM target cell configuration is a RACH-less configuration or a RACH-based configuration. Further, the CU-CP may receive the TA information of the UE in the serving DU from the serving DU during the setup of the F1 communication interface between the serving DU and the CU-CP, or the CU-CP may receive the TA information of the UE in the serving DU from the serving DU after the F1 communication interface is set up between the serving DU and the CU-CP.

[0140] In some implementations, at least one LTM target cell may be included in at least one DU of a base station that is not the serving DU.

[0141] In some implementations, the serving DU can select one of at least one prepared LTM target cell for handover of services for the UE from the serving DU, and the serving DU can trigger handover of services for the UE to the selected LTM target cell from the serving DU. Further, the serving DU can determine which LTM target cell(s) of the at least one LTM target cell have a radio quality that exceeds a predetermined threshold radio quality, the selection can be among the one or more determined LTM target cells, and / or triggering can include the serving DU sending a medium access control (MAC) control element (CE) message to the UE.

[0142] In some implementations, the base station may have a non - aggregated architecture.

[0143] In some implementations, the base station may include a next - generation radio access network (NG - RAN) node. Further, the NG - RAN node may include a gNodeB (e.g., the gNodeB in FIG. 5a, the gNodeB624 in FIGS. 6a or 6b, the gNodeB in FIGS. 8a and 8b, etc.) or an ng - eNodeB.

[0144] In some implementations, the base station may include at least one processor (e.g., the processor 910 in FIG. 9, etc.) and at least one non - transitory storage medium (e.g., the memory 920, the storage device 930 in FIG. 9, etc.) that stores instructions that, when executed by the at least one processor, cause the at least one processor to execute method 1000.

[0145] The systems and methods disclosed herein can be embodied in a variety of forms including, for example, a data processor such as a computer, which also includes databases, digital electronic circuits, firmware, software, or combinations thereof. Further, the above features, as well as other aspects and principles of the implementations of this disclosure, can be implemented in a variety of environments. Such environments and related applications can be specially constructed to execute various processes and operations in accordance with the disclosed implementations, or they can include a general-purpose computer or computing platform that is selectively activated or reconfigured by code to provide the required functionality. The processes disclosed herein are essentially independent of a particular computer, network, architecture, environment, or other device, and can be implemented by a suitable combination of hardware, software, and / or firmware. For example, various general-purpose machines can be used with programs written in accordance with the teachings of the disclosed implementations, or it may be more convenient to construct a dedicated device or system for performing the required methods and techniques.

[0146] The systems and methods disclosed herein can be implemented as a computer program product, i.e., a computer program tangibly embodied in an information carrier, such as a machine-readable storage device or 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. The computer program can be written in any form of programming language, including compiled or interpreted languages, and can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. The 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.

[0147] As used herein, the term "user" can refer to a person or any entity including a computer.

[0148] Ordinal numbers such as first, second, etc. may be related to order in some situations, but as used herein, ordinal numbers do not necessarily imply order. For example, ordinal numbers can be used simply to distinguish one item from another. For example, distinguishing a first event from a second event need not imply any chronological order or fixed reference system (such that the first event within one paragraph of the description may be different from the first event within another paragraph of the description).

[0149] The foregoing description is intended to illustrate and not limit the scope of the invention as defined by the appended claims. Other implementations are within the scope of the following claims.

[0150] These computer programs, which may also be referred to as programs, software, software applications, applications, components, or code, include machine instructions for a programmable processor and can be implemented in high-level procedural and / or object-oriented programming languages and / or in assembly / machine language. As used herein, the term "machine-readable medium" refers to any computer program product, apparatus, and / or device used to provide machine instructions and / or data to a programmable processor, including a machine-readable medium that receives the machine instructions as a machine-readable signal, such as, for example, magnetic disks, optical disks, memory, and programmable logic devices (PLDs). The term "machine-readable signal" refers to any signal used to provide machine instructions and / or data to a programmable processor. A machine-readable medium can store such machine instructions non-transitorily, for example, in non-transitory solid-state memory or a magnetic hard drive or any equivalent storage medium. Alternatively or additionally, a machine-readable medium can store such machine instructions temporarily, for example, in a processor cache or other random access memory associated with one or more physical processor cores.

[0151] To provide for interaction with a user, the subject matter disclosed herein can be implemented on a computer having a display device, such as, for example, a cathode ray tube (CRT) or a liquid crystal display (LCD) monitor, for displaying information to the user, and a keyboard and a pointing device, such as, for example, 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, such as, for example, visual feedback, auditory feedback, or tactile feedback, and input received from the user can be received in any form, including, but not limited to, acoustic, speech, or tactile input.

[0152] The disclosed subject matter described herein can be implemented in a computing system that includes back-end components, such as one or more data servers, or includes middleware components, such as one or more application servers, or includes front-end components, such as one or more client computers having a graphical user interface or a web browser through which a user can interact with an implementation of the disclosed subject matter, or can be implemented with any combination of such back-end, middleware, or front-end components. The components of the system can be interconnected by any form or medium of digital data communication, such as, for example, a communication network. Examples of communication networks include, but are not limited to, local area networks (LANs), wide area networks (WANs), and the Internet.

[0153] The computing system can include clients and servers. Clients and servers are generally, but not exclusively, separate from each other and typically interact via a communication network. The relationship between a client and a server arises by computer programs that operate on respective computers and have a client-server relationship to each other.

[0154] The implementation forms described in the foregoing description do not represent all implementation forms that coincide with the disclosed subject matter in this specification. Instead, they are only some examples that coincide with aspects related to the disclosed subject matter to be described. Although some modifications have been described in detail above, other modifications or additions are possible. In particular, in addition to what is described in this specification, further features and / or modifications can be provided. For example, the above-described implementation forms can be directed to various combinations and sub-combinations of the disclosed features, and / or combinations and sub-combinations of some further features disclosed above. In addition, the logical flows shown in the accompanying drawings and / or described in this specification do not necessarily require the specific order or sequential order shown to achieve the desired result. Other implementation forms may be within the scope of the following claims.

Claims

1. An apparatus comprising at least one processor and at least one non-transitory memory storing instructions, wherein when the instructions are executed by the at least one processor, the at least one processor is caused to perform operations including: receiving, at at least one layer 1 / layer 2 trigger mobility (LTM) target distributed unit (DU) of a base station, information indicating whether a handover (HO) of a service for a user equipment (UE) currently served by a serving DU of the base station can be a random access channel-less (RACH-less) HO to at least one LTM target cell of the target DU or a RACH-based HO to the at least one LTM target cell; and preparing, based on the information received in the at least one LTM target cell, the at least one LTM target cell for one of a RACH-less HO and a RACH-based HO. The apparatus according to claim 1, wherein the information includes timing advance (TA) information of the UE within the serving DU, wherein that the TA information of the UE within the serving cell is either (a) the same TA as the TA in the at least one LTM target cell or (b) a TA of 0 indicates that the HO to the at least one LTM target cell can be a RACH-less HO, and that the TA information of the UE within the serving cell is neither (a) the same TA as the TA in the at least one LTM target cell nor (b) a TA of 0 indicates that the HO to the at least one LTM target cell can be a RACH-based HO.

3. The at least one LTM target DU receives the TA information of the UE within the serving DU in a message from the serving DU, wherein the operations further include the at least one LTM target DU indicating to the serving DU whether the prepared LTM target cell configuration is a RACH-less configuration or a RACH-based configuration.

4. ​ ​ ​ The at least one LTM target DU receives the TA information of the UE in the serving DU in a message from the central unit control plane (CU-CP) of the base station, The apparatus according to claim 2, wherein the operation further includes the at least one LTM target DU indicating to the CU-CP whether the prepared LTM target cell configuration is a RACH-less configuration or a RACH-based configuration.

5. The apparatus according to claim 4, wherein the CU-CP receives the TA information of the UE in the serving DU from the serving DU during setup of a set of F1 communication interfaces between the serving DU and the CU-CP.

6. The apparatus according to claim 4, wherein the CU-CP receives the TA information of the UE in the serving DU from the serving DU after an F1 communication interface is set up between the serving DU and the CU-CP.

7. The apparatus according to claim 1, wherein the at least one LTM target cell is included in at least one DU of the base station that is not the serving DU.

8. The apparatus according to claim 1, wherein the serving DU selects one of the at least one prepared LTM target cells for handover of a service for the UE from the serving DU, and the serving DU triggers the handover of the service for the UE from the serving DU to the selected LTM target cell.

9. The serving DU determines which one or more of the at least one LTM target cells have a radio quality that exceeds a predetermined threshold radio quality, The apparatus according to claim 8, wherein the selection is made among the one or more determined LTM target cells.

10. The apparatus according to claim 8, wherein the triggering includes the serving DU transmitting a medium access control (MAC) control element (CE) message to the UE.

11. The apparatus according to claim 1, wherein the base station has a non-integrated architecture.

12. The apparatus according to claim 1, wherein the base station includes a next-generation radio access network (NG-RAN) node.

13. The apparatus according to claim 12, wherein the NG-RAN node includes a gNodeB or an ng-eNodeB.

14. The base station according to claim 1, comprising the at least one processor and the at least one non-transitory memory medium.

15. In at least one layer 1 / layer 2 trigger mobility (LTM) target distributed unit (DU) of a base station, receiving information indicating whether a handover (HO) of a service for a user equipment (UE) currently served by a serving DU of the base station can be a random access channel-less (RACH-less) HO to at least one LTM target cell of the target DU or can be a RACH-based HO to the at least one LTM target cell; Based on the information received in the at least one LTM target cell, preparing the at least one LTM target cell for one of a RACH-less HO and a RACH-based HO; A computer-implemented method comprising.

16. The information includes timing advance (TA) information of the UE in the serving DU, That the TA information of the UE in the serving cell is either (a) the same TA as the TA in the at least one LTM target cell or (b) a TA of 0 indicates that the HO to the at least one LTM target cell can be a RACH-less HO, The computer-implemented method according to claim 15, wherein that the TA information of the UE in the serving cell is neither (a) the same TA as the TA in the at least one LTM target cell nor (b) a TA of 0 indicates that the HO to the at least one LTM target cell can be a RACH-based HO.

17. At least one non-transitory memory medium storing instructions which, when executed by at least one processor, cause the at least one processor to perform operations, the operations comprising In at least one layer 1 / layer 2 trigger mobility (LTM) target distributed unit (DU) of a base station, receiving information indicating whether a handover (HO) of a service for a user equipment (UE) currently served by a serving DU of the base station can be a random access channel-less (RACH-less) HO to at least one LTM target cell of the target DU or can be a RACH-based HO to the at least one LTM target cell. Based on the information received in the at least one LTM target cell, preparing the at least one LTM target cell for one of a RACH-less HO and a RACH-based HO. At least one non-transitory storage medium including the above.

18. The information includes timing advance (TA) information of the UE in the serving DU. The fact that the TA information of the UE in the serving cell is either (a) the same TA as the TA in the at least one LTM target cell or (b) a TA of 0 indicates that the HO to the at least one LTM target cell can be a RACH-less HO. The storage medium according to claim 17, wherein the fact that the TA information of the UE in the serving cell is neither (a) the same TA as the TA in the at least one LTM target cell nor (b) a TA of 0 indicates that the HO to the at least one LTM target cell can be a RACH-based HO.

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