Random access channel-less Layer 1 / Layer 2 triggered 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 entitled "Method and system to realize RACH-less HO for LLM in inter gNB-DU scenario", filed on October 4, 2022, which is incorporated herein by reference in its entirety.

[0002] In some implementations, the subject matter of the present disclosure relates to telecommunications systems, and in particular to enabling random access channel layer-less (RACH-less) Layer 1 / Layer 2 triggered mobility (LTM). [Background technology]

[0003] In today's world, cellular networks provide on-demand communication capabilities to individuals and businesses. Typically, cellular networks are wireless networks that can be distributed over terrestrial areas called cells. Each cell is served by at least one fixed-location transceiver called a cell site or base station. Each cell may use a different set of frequencies from its neighboring cells to avoid interference and provide improved service within each cell. When cells are combined, they provide radio coverage over a wide geographic area, allowing numerous mobile telephone devices and / or other wireless devices or portable transceivers to communicate with each other and with fixed transceivers and telephone devices anywhere in the network. Such communication is performed through base stations and is achieved even when the mobile transceiver is traveling through two or more cells during transmission. Major wireless communication providers have deployed such cell sites worldwide, allowing communicating mobile telephone devices and mobile computing devices to connect to the public switched telephone network and the public Internet.

[0004] A mobile telephone device is a portable telephone device that can receive and / or transmit telephone and / or data communications through a cell site or transmission tower by using radio waves to transmit signals to and from the device (mobile telephone device). Given the large number of mobile telephone device users, current mobile telephone networks offer limited shared resources. In that regard, cell sites and handsets may change frequencies and use low-power transmitters to allow simultaneous use of the network by many callers with less interference. Cell site coverage may depend on the particular geographic location and / or the number of users that can potentially use the network. For example, in urban areas, a cell site may have a range of up to about 1 / 2 mile; in rural areas, the range may be as much as 5 miles; and in some areas, users may receive signals from cell sites as far as 25 miles away.

[0005] The following are some examples of digital cellular technologies used by communication providers: Global System for Mobile Communications (GSM), 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). 4G LTE, developed by the Long Term Evolution, or 3rd Generation Partnership Project (3GPP®) standards organization, is a standard for high-speed data wireless communication for mobile phone devices and data terminals. 5G standards are currently being developed and deployed. 3GPP cellular technologies such as LTE and 5G NR are an evolution of earlier generations of 3GPP technologies such as GSM / EDGE and UMTS / HSPA digital cellular technologies, and allow for increased capacity and speeds by using different air interfaces along with improvements to the core network.

[0006] A cellular network may 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 higher layer communications, e.g., Internet Protocol (IP), transport layer, and application layer. In some cases, the RAN functions may be divided into baseband unit functions and radio unit functions; for example, a radio unit connected to a baseband unit via a fronthaul network may be responsible for lower layer processing of the radio physical layer, and the baseband unit may be responsible for higher layer radio protocols, e.g., 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 a UE communicatively coupling with a base station, the DU supporting the UE may change. To ensure proper communication between the UE and the new supporting DU, time synchronization is established in a random access channel (RACH) procedure between the UE and the new supporting DU. However, such a handover from one DU to another DU takes more time when a contention-based RACH procedure must occur instead of a contention-free RACH procedure. Such an increase in the time to accomplish 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 this disclosure relates to a computer-implemented method. The method may include receiving, at at least one layer 1 / layer 2 triggered mobility (LTM) target distribution unit (DU) of a base station, information indicating whether a handover (HO) of 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. The method may also include preparing the at least one LTM target cell for one of the RACH-less HO and the RACH-based HO based on the information received at the at least one LTM target cell.

[0009] The method may enable handover from one cell of a base station to another cell of a base station to take less time because the handover from a serving cell to one of the at least one target cell 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, handover latency may be reduced, thereby reducing user plane interruption time.

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

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

[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 the at least one prepared LTM target cell for handover of service for the UE from the serving DU, and the serving DU can trigger handover of service for the UE from the serving DU to the selected LTM target cell. Further, the serving DU can determine which LTM target cell(s) of the at least one LTM target cell have a radio quality above a predetermined threshold radio quality, and the selection can be among the one or more determined LTM target cells, and / or the 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 may have a disaggregated architecture.

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

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

[0017] Non-transitory computer program products (i.e., physically embodied computer program products) that store instructions that, when executed by one or more data processors of one or more computing systems, cause at least one data processor to perform the operations described herein are also described. Similarly, computer systems that may include one or more data processors and memory coupled to the one or more data processors are also described. The memory may store, on a temporary or permanent basis, instructions that cause at least one processor to perform one or more of the operations described herein. Furthermore, methods 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 may exchange data and / or commands or other instructions, etc., including, but not limited to, connections over a network (e.g., the Internet, a wireless wide area network, a local area network, a wide area network, a wired network, etc.), such as via a direct connection between one or more of the computing systems.

[0018] The 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 described herein 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 certain aspects of the disclosed subject matter described herein and, together with the description, serve to explain some of the principles associated with the disclosed implementations. [Brief explanation of the drawings]

[0020] [Figure 1a] FIG. 1 illustrates an exemplary conventional Long Term Evolution (LTE) communication system.

[0021] [Figure 1b] FIG. 1b illustrates further details of the exemplary LTE system shown in FIG. 1a.

[0022] [Figure 1c] FIG. 1B illustrates further details of the evolved packet core of the exemplary LTE system shown in FIG. 1A.

[0023] [Figure 1d] FIG. 1B illustrates an exemplary evolved Node B of the exemplary LTE system shown in FIG. 1a.

[0024] [Figure 2] FIG. 2 shows further details of the evolved Node B shown in FIGS. 1a to 1d.

[0025] [Figure 3] FIG. 1 illustrates an example virtual radio access network in accordance with some implementations of the subject matter of this disclosure.

[0026] [Figure 4] FIG. 1 illustrates an exemplary 3GPP split architecture for providing use of higher frequency bands to its users.

[0027] [Figure 5a] FIG. 1 illustrates an exemplary 5G wireless communication system.

[0028] [Figure 5b] FIG. 1 illustrates an example layer architecture of a split gNB and / or a split ng-eNB (e.g., a next-generation eNB that may be connected to 5GC).

[0029] [Figure 5c] A diagram illustrating an exemplary functional division in the gNB architecture shown in Figures 5a-5b.

[0030] [Figure 6a] A diagram showing an exemplary system according to some implementations of the subject matter of the present disclosure.

[0031] [Figure 6b] A diagram showing an exemplary alternative configuration of the system of FIG. 6a according to some implementations of the subject matter of the present disclosure.

[0032] [Figure 7] A diagram showing an exemplary method according to some implementations of the subject matter of the present disclosure.

[0033] [Figure 8a] A diagram showing another exemplary system according to some implementations of the subject matter of the present disclosure.

[0034] [Figure 8b] A diagram showing another exemplary system according to some implementations of the subject matter of the present disclosure.

[0035] [Figure 8c] A diagram showing yet another exemplary system according to some implementations of the subject matter of the present disclosure.

[0036] [Figure 8d] A diagram showing yet another exemplary system according to some implementations of the subject matter of the present disclosure.

[0037] [Figure 8e] A diagram showing another exemplary system according to some implementations of the subject matter of the present disclosure. [

[0038] [Figure 9] A diagram showing yet another exemplary system according to some implementations of the subject matter of the present disclosure. <氧

[0039] [Figure 10] A diagram showing yet another exemplary method according to some implementations of the subject matter of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

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

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

[0042] In some implementations of the subject matter of this disclosure, a timing advance (TA) of a serving cell of a base station of a wireless communication system can be used in 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 a CU-CP of a base station communicatively coupled to the serving cell and the at least one target cell. Informing the at least one target cell of the TA of the serving cell can enable each of the one or more target cells to prepare itself for either RACH-less (which may also be referred to as RACH-free) handover (HO) or RACH-based handover (HO) from the serving cell for a user equipment (UE) communicatively coupled with 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 cell has already been 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 reducing handover latency and thereby reducing 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 allows the network to switch a UE from a source cell to a target cell without necessarily requiring a synchronous reconfiguration. In particular, based on received L1 measurements, the network can indicate in L2 signaling (e.g., messages such as MAC CE) the beams belonging to LTM candidate cells on which the UE should perform the LTM cell switch procedure. The UE is provided with at least one LTM candidate cell configuration by the network before performing the LTM cell switch procedure.

[0044] 3GPP standards that define one or more aspects that may be 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)." O-RAN Alliance standards 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 transmitter and / or receiver components of base stations (e.g., gNodeB, eNodeB, etc.) within such communication systems. The following is a general discussion of Long Term Evolution communication systems and new 5G wireless communication systems.

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

[0047] As shown in FIG. 1a, system 100 may include an evolved universal terrestrial radio access network (EUTRAN) 102, an evolved packet core (EPC) 108, and a packet data network (PDN) 101, where EUTRAN 102 and EPC 108 provide communication between user equipment 104 and PDN 101. EUTRAN 102 may include multiple evolved Node Bs (eNodeBs or ENODEBs or enodeb or eNBs) or base stations 106 (106a, 106b, 106c) (as shown in FIG. 1b) that provide communication capabilities to multiple user equipment 104 (104a, 104b, 104c). User equipment 104 may be mobile telephone devices, smartphones, tablets, personal computers, personal digital assistants (PDAs), servers, data terminals, and / or any other type of user equipment, and / or any combination thereof. User equipment 104 can connect to the EPC 108 and ultimately to the PDN 101 via any eNodeB 106. Typically, user equipment 104 can connect to the nearest eNodeB 106 in terms of distance. In the LTE system 100, the EUTRAN 102 and the 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 mentioned above, the EUTRAN 102 includes multiple eNodeBs 106, also known as cell sites. The eNodeBs 106 provide radio functionality and perform important control functions, including air link resource scheduling or radio resource management, active mode mobility or handover, and admission control for services. The eNodeBs 106 are responsible for selecting which mobility management entity (MME, as shown in Figure 1c) will serve the user equipment 104, as well as protocol features 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] Communication between the user equipment 104 and the eNodeB 106 occurs over an air interface 122 (also known as the LTE-Uu interface). As shown in FIG. 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 allows 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 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 may be interconnected with each other using X2 interfaces 130 (130a, 130b, 130c). As shown in FIG. 1b, the X2 interface 130a provides interconnection between the eNodeBs 106a and 106b, the X2 interface 130b provides interconnection between the eNodeBs 106a and 106c, and the X2 interface 130c provides interconnection between the eNodeBs 106b and 106c. The X2 interfaces may be established between two eNodeBs to provide for the exchange of signals, which may include information related to loading or interference, as well as information related to handover. The eNodeBs 106 communicate with the evolved packet core 108 via the S1 interfaces 124 (124a, 124b, 124c). The S1 interface 124 can be divided into two interfaces, one for the control plane (shown in FIG. 1c as control plane interface (S1-MME interface) 128) and the other for the user plane (shown in FIG. 1c as user plane interface (S1-U interface) 125).

[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 moving. Note that each node in the network 100 has its own IP address. The EPC 108 is designed to interwork 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 and 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 Figure 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 (a subscriber database for the EPC 108), and a Policy Control and Charging Rules Function (PCRF) 118. Some of these (such as the S-GW, P-GW, MME, and HSS) are often combined into nodes according to manufacturer implementations.

[0054] The S-GW 110 functions as an IP packet data router and is the bearer path anchor for the user equipment within the EPC 108. Thus, when the user equipment moves from one eNodeB 106 to another during 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 serving the user equipment 104. If the user equipment 104 moves to the domain of a different S-GW 110, the MME 114 will transfer all of the user equipment's bearer path 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 and through the EUTRAN 102.

[0055] The P-GW 112 is the gateway between the EPC 108 (and user equipment 104 and EUTRAN 102) and the PDN 101 (shown in FIG. 1a). The P-GW 112 acts as a router for user traffic and performs functions on behalf of the user equipment. These include IP address allocation for the user equipment, packet filtering of downstream user traffic to ensure that it is placed on the appropriate bearer path, and enforcement of downstream QoS, including data rate. Depending on the services a subscriber is using, there may be multiple user data bearer paths between the user equipment 104 and the P-GW 112. A subscriber may use services on PDNs served by different P-GWs, in which case the user equipment has at least one bearer path established to each P-GW 112. During handover of a 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 context for 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 locate the user equipment and reestablishes a bearer path to and through the EUTRAN 102. The MME 114 for a particular user equipment 104 is selected by the eNodeB 106 from which the user equipment 104 initiates system access. An MME is typically part of a collection of MMEs within the EPC 108 for load sharing and redundancy purposes. In establishing a 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 points of the data path through the EPC 108.

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

[0058] As mentioned above, IP services 119 are provided by PDN 101 (as shown in FIG. 1a).

[0059] 1d shows an example structure of the eNodeB 106. The eNodeB 106 may include at least one remote radio head (RRH) 132 (typically, there may be three RRHs 132) and a baseband unit (BBU) 134. The RRHs 132 may be connected to an antenna 136. The RRHs 132 and BBU 134 may be connected using an optical interface compliant with the Common Public Radio Interface (CPRI) / enhanced CPRI (eCPRI) 142 standard specification, either using an RRH-specific custom control and user plane framing method or using an O-RAN Alliance compliant Control and User plane framing method. The operation of the eNodeB 106 can be characterized using the following standard parameters (and specifications): radio frequency band (Band 4, Band 9, Band 17, etc.), bandwidth (5, 10, 15, 20 MHz), access method (downlink: OFDMA, uplink: SC-OFDMA), antenna technology (single-user and multi-user MIMO, uplink: single-user and multi-user MIMO), number of sectors (up to 6), maximum transmission speed (downlink: 150 Mb / s, uplink: 50 Mb / s), S1 / X2 interface (1000Base-SX, 1000Base-T), and mobile environment (up to 350 km / h). The BBU 134 can be responsible for digital baseband signal processing, S1 line termination, X2 line termination, call processing, and monitoring and control processing. IP packets received from the EPC 108 (not shown in FIG. 1d) can be modulated into digital baseband signals and transmitted to the RRH 132. Conversely, digital baseband signals received from the RRH 132 may 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 (using a converter (CONV) 140) digital baseband signals from the BBU 134 to radio frequency (RF) signals and power amplify them (using an amplifier (AMP) 138) for transmission to the user equipment 104 (not shown in FIG. 1d). Conversely, RF signals received from the user equipment 104 are amplified (using AMP 138) and converted (using CONV 140) to digital baseband signals for transmission to the BBU 134.

[0061] Figure 2 shows additional details of an exemplary eNodeB 106. The eNodeB 106 includes multiple layers: LTE Layer 1 (202), LTE Layer 2 (204), and LTE Layer 3 (206). LTE Layer 1 includes the physical layer (PHY). LTE Layer 2 includes medium access control (MAC), radio link control (RLC), and packet data convergence protocol (PDCP). LTE Layer 3 includes various functions and protocols, including radio resource control (RRC), dynamic resource allocation, eNodeB measurement configuration and provisioning, radio admission control, connection mobility control, and radio resource management (RRM). The RLC protocol is an automatic repeat request (ARQ) fragmentation protocol used over the cellular air interface. The RRC protocol handles LTE Layer 3 control plane signaling between 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 radio bearer sequence number maintenance. The BBU 134 shown in FIG. 1d may include LTE layers L1-L3.

[0062] One of the primary functions of the eNodeB 106 is radio resource management, including scheduling of both uplink and downlink air interface resources for the user equipment 104, control of bearer resources, and admission control. As an agent for the EPC 108, the eNodeB 106 is responsible for forwarding paging messages used to locate a mobile when it is idle. The eNodeB 106 also communicates common control channel information over the air, performs header compression, encryption and decryption of user data sent over the air, and establishes handover reporting and trigger criteria. As mentioned above, the eNodeB 106 can cooperate with other eNodeBs 106 via the X2 interface for handover and interference management purposes. The eNodeB 106 communicates with the MME of the EPC via the S1-MME interface and with the S-GW using the S1-U interface. Additionally, the eNodeB 106 exchanges user data with the S-GW via the S1-U interface. The eNodeBs 106 and the EPC 108 have a many-to-many relationship to support load sharing and redundancy between the MMEs and the S-GWs. The 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 this disclosure relates to 5G new radio (NR) communication systems. 5G NR is the next communication standard beyond the 4G / IMT-Advanced standard. 5G networks offer higher capacity than current 4G, allowing for a larger number of mobile broadband users per unit area, and enabling consumption of more and / or unlimited data amounts in gigabytes per month and per user. This may enable users to stream high-definition media for many hours per day using their mobile devices, even when Wi-Fi networks do not allow them to do so. 5G networks have improved support for device-to-device communication, lower costs, lower latency and lower battery consumption than 4G equipment, etc. Such a network would have data rates of tens of megabits per second for many users, data rates of 100 Mb / s for metropolitan areas, 1 Gb / s simultaneously to users within a limited area (e.g., an office floor), many simultaneous connections for wireless sensor networks, enhanced spectral efficiency, improved coverage, increased signaling efficiency, 1-10 ms latency, and reduced latency compared to existing systems.

[0064] 3 illustrates an exemplary virtual radio access network 300. The network 300 can provide communication between various components, including a base station (e.g., eNodeB, gNodeB) 301, radio equipment 303, a centralized unit 302, a digital unit 304, and wireless devices 306. The components in the system 300 can be communicatively coupled to a core using a backhaul link 305. The centralized unit (CU) 302 can be communicatively coupled to a distributed unit (DU) 304 using a midhaul connection 308. The radio frequency (RU) component 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 capabilities 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 Figure 3). In 5G communication systems, compressed CPRI over Ethernet frames is referred to as eCPRI and is the recommended fronthaul network. This architecture can enable standardization of fronthaul / midhaul, which can include upper layer splitting (e.g., Option 2 or Option 3-1 (upper / lower RLC split architecture)) and fronthaul using L1 split architecture (Option 7).

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

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

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

[0070] 5a illustrates an exemplary 5G wireless communication system 500 according to some implementations of the subject matter of this disclosure. The system 500 may be configured to have a lower-layer split architecture according to Option 7-2. The system 500 may include a core network 502 (e.g., 5G Core) and one or more gNodeBs (or gNBs), where the gNBs may have a centralized unit (gNB-CU). The gNB-CU may be logically divided into a control plane portion (gNB-CU-CP) 504 and one or more user plane portions (gNB-CU-UP) 506. The control plane portion 504 and the user plane portion 506 may be configured to be communicatively coupled using an E1 communication interface 514 (as defined in the 3GPP standard). The control plane portion 504 may be configured to be responsible for executing the RRC and PDCP protocols of the radio stack.

[0071] The control plane portion 504 and user plane portion 506 of the centralized unit of the gNB may be configured to be communicatively coupled to one or more distributed units (DUs) 508, 510 according to an upper layer split architecture. The distributed units 508, 510 may be configured to execute upper portions of the RLC, MAC, and PHY layer protocols of the radio stack. The control plane portion 504 may be configured to be communicatively coupled to the distributed units 508, 510 using an F1-C communication interface 516, and the user plane portion 506 may be configured to be communicatively coupled to the distributed units 508, 510 using an F1-U communication interface 518. The distributed units 508, 510 may 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 communicate with one or more user equipment (not shown in FIG. 5a). The remote radio unit 512 may be configured to execute lower portions of the PHY layer protocol and provide antenna capabilities to the remote unit for communication with user equipment (similar to the description above in connection with Figures 1a-2).

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

[0073] The gNB-DU 508 may include RLC, MAC, and PHY layers, as well as various communications sublayers. These may include an F1-Application Protocol (F1-AP) sublayer, a GPRS Tunneling Protocol (GTPU) sublayer, a Stream Control Transmission Protocol (SCTP) sublayer, a User Datagram Protocol (UDP) sublayer, and an Internet Protocol (IP) sublayer. As described above, the distributed unit 508 may be communicatively coupled to the control plane portion 504 of the centralized unit, which may also include the F1-AP, SCTP, and IP sublayers, as well as the Radio Resource Control and PDCP Control (PDCP-C) sublayer. Furthermore, the distributed unit 508 may also be communicatively coupled to the user plane portion 506 of the centralized unit of the gNB. The user plane portion 506 may include a Service Data Adaptation Protocol (SDAP), a PDCP User (PDCP-U), a GTPU, a UDP, and an IP sublayer.

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

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

[0076] The MAC sublayer of Layer 2 may perform beam management, random access procedures, mapping between logical channels and transport channels, concatenation of multiple MAC service data units (SDUs) belonging to one logical channel into transport blocks (TBs), multiplexing / demultiplexing of SDUs belonging to logical channels to / from TBs passed to / from the physical layer on transport channels, scheduling information reporting, error correction via HARQ, priority handling between logical channels for one UE, priority handling between UEs via dynamic scheduling, transport format selection, and other functions. The RLC sublayer functions may include forwarding upper layer packet data units (PDUs), error correction via ARQ, reordering of data PDUs, duplication and protocol error detection, reestablishment, etc. The PDCP sublayer may be responsible for forwarding user data, various functions during reestablishment procedures, retransmission of SDUs, discarding SDUs in the uplink, forwarding of control plane data, etc.

[0077] The RRC sublayer of Layer 3 may perform the broadcasting of system information to the NAS and AS, establishment, maintenance, and release of RRC connections, security, establishment, configuration, maintenance, and release of point-to-point radio bearers, mobility functions, reporting, and other functions.

[0078] III. Realization of RACH-less LTM Some implementations of the subject matter of this disclosure may provide RACH-less HO from one cell to another for a UE communicatively coupled with a base station of a wireless communication system.

[0079] A Layer 1 / Layer 2 triggered mobility (LTM) inter-cell handover at a base station can be performed by performing a Serving Cell Change (SSC) from a serving cell to a target cell. Multiple target cells meet the radio conditions or handover criteria required for the UE to receive SSC and therefore may be viable target cell options for HO. One or more of the viable target cells may require RACH-based HO, including a contention-based or contention-free RACH procedure, for example, because the timing advance (TA) of the UE's serving cell differs from that 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 the medium access control (MAC) layer (Layer 2) control element (CE) from the base station to the UE used in controlling the uplink, e.g., signal transmission timing from the UE to the base station. For example, one or more of the other viable target cells may allow RACH-less HO because the TA of the UE's target cell is 0 (zero) or the TA of the serving cell is the same as the TA of the UE's target cell. Implementing RACH-less HO over RACH-based HO is configured to allow handover to be to one of the target cells that allow the RACH-less HO procedure.

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

[0081] Conversely, in a 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. Therefore, the cell can ensure that collisions do not occur by assigning preambles so that a particular preamble is assigned to only one UE. This can save uplink synchronization time, which can be no less than 20 ms, and therefore reduce HO latency compared to a contention-based RACH procedure.

[0082] In some implementations of the subject matter of the present disclosure, a timing advance (TA) of a UE served by a serving cell of a base station of a wireless communication system can be used in preparing the configuration of at least one target cell of at least one target DU of the base station for LTM. The at least one target DU can be informed of the TA of the serving cell of the UE. Informing the at least one target cell of the TA of the serving cell can enable each of the one or more target DUs to prepare itself for either RACH-less HO or RACH-based HO from the serving cell for a UE communicatively coupled with the base station. Thus, HO from one cell of the base station to another cell of the base station may take less time because HO from the serving cell to one of the at least one target cell has already been prepared in each of the one or more target DUs to include either a contention-free RACH procedure (for RACH-less HO) or a contention-based RACH procedure (for RACH-based HO). Thus, handover latency can be reduced, thereby reducing 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 a CU-CP of a base station communicatively coupled to the serving cell and the at least one target cell. The TA information is Layer 1 (L1) information and therefore is 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.) may have a non-aggregated architecture in which the base station includes one gNB-CU-CP (e.g., gNB-CU-CP 504 in FIGS. 5a-5c) and 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 may be configured to implement RACH-less LTM when a UE is handed off from one cell of the base station (a serving cell) to another cell of the base station (a target cell).

[0085] FIG. 6a shows an example 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 in a 5G wireless communication system similar to the 5G wireless communication system 500 of FIG. 5a described above, although other base stations may be similarly configured and used in implementing RACH-less LTM. In the illustrated implementation of FIG. 6a, the base station 624 includes multiple CU-UPs 606a, 606b, and 606c. The base station 624 includes three CU-UPs 606a, 606b, and 606c in this illustrated implementation, but may include multiple other CU-UPs. The CUs of the base station 624, including the multiple CU-UPs 606a, 606b, and 606c, are configured to be communicatively coupled with a core network (not shown in FIG. 6a), such as the 5G GC 502 of FIG. 5a.

[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-Ups 606a, 606b, 606c with which the CU-CP 604 may be configured to communicate.

[0087] The base station 624 also includes multiple DUs 608, 610. The base station 624 includes two DUs 608, 610 in this illustrated implementation, but may include multiple other DUs. The CU-CP 604 is configured to be communicatively coupled to the DUs 608, 610 using an F1-C communication interface 616. The CU-UPs 606a, 606b, 606c are configured to be communicatively coupled to the DUs 608, 610 using an 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 to reflect the three CU-UPs 606a, 606b, 606c with which each DU 608, 610 may be configured to communicate.

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

[0089] An implementation of RACH-less LTM in an inter-DU scenario can be configured to occur 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 another of the DUs 608, 610, also of the same base station 624. One of the DUs 608, 610 currently serving the UE 622 is referred to as the "serving DU" because it is currently serving the UE 622, e.g., currently serving the UE 622. One of the DUs 608, 610 to which the UE's service is being handed off is referred to as the "target DU" because it is targeted to serve the UE 622.

[0090] A system that can be configured to implement RACH-less LTM in an inter-DU scenario is further described with respect to Figure 6b. Figure 6b shows the CU-CP 604 and CU-UPs 606a, 606b, and 606b of Figure 6a, but in the illustrated implementation of Figure 6b, the base station 624 includes three or more DUs. In the illustrated implementation of Figure 6b, the base station 624 includes 66 DUs. Three of the DUs 628a, 628b, and 628c are macro cells (labeled macro1, macro2, and macro3 in Figure 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 Figure 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 cells DUs 626, including the macro1 DU 628a, the 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 cells DUs 626, including the macro1 DU 628a, the macro2 DU 628b, the 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 cells DU626, including macro2 DU628b, macro3 DU628c, and gNB-DU70, gNB-DU80, and gNB-DU90, are configured to be served by a third CU-UP606c (labeled CU-UP3 in FIG. 6b).

[0091] 6b, each CU-UP 606a, 606b, 606c serves a subset of DUs 626, 628a, 628, 628c for all services. However, a CU-UP can serve all DUs at a base station for one service (e.g., enhanced mobile broadband (eMBB)) while serving a subset of DUs for another service (e.g., vehicle-to-everything (V2X) or ultra-reliable low latency communication (URLLC)).

[0092] Figure 7 illustrates an example method 700 according to some implementations of the subject matter of this disclosure. Method 700 is described with respect to example system 800 shown in Figure 8a, but may similarly be implemented in other systems, such as system 100 of Figures 1a-1c and 2, system 400 of Figure 4, system 500 of Figure 5a, systems 6a and 6b, etc. System 800 of Figure 8a is a 5G system, but as noted above, implementations of RACH-less LTM in DU-to-DU scenarios described herein may be performed in other types of wireless communication systems, such as an LTE wireless communication system or a 6G or later generation wireless communication system.

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

[0094] The method 700 illustrates an implementation of an inter-DU LTM serving cell change scenario that includes a serving DU 804 determining 702 that a cell change is necessary for a UE 802. The serving DU's determination 702 may include the serving DU 804 analyzing 814 an intra-frequency L1 measurement report sent 812 by the UE 802 to the serving DU 804 in accordance with 3GPP standards. According to the 3GPP standards, the intra-frequency L1 measurement report may include Layer 1 (L1) measurements that may be analyzed by the serving DU 804 when making resource control decisions, which may include a serving cell change in which the UE 802 will be served by a DU other than the serving DU 804, e.g., a target DU 806, for at least one service.

[0095] In response to determining 702 that a serving cell change should occur, the serving DU 804 notifies 704 the UE 802 of the serving cell change. As shown in FIG. 8a, notifying 704 the UE 802 may include the serving DU 804 sending 816 a serving cell change command, e.g., a MAC CE, to the UE 802.

[0096] Also, in response to determining (702) that a cell service change should occur, the serving DU 804 notifies (704) the CU-CP 808 that a serving cell change has occurred for the UE 802. Thus, the notification (704) may identify the UE 802 to the CU-CP 808 using an identifier known to the serving DU 804, such as an identifier according to a 3GPP standard, that uniquely identifies the UE 802 to the CU-CP 808. As shown in FIG. 8a, the notification to the CU-CP 808 (704) may include the serving DU 804 sending (818) a serving cell change notification message to the CU-CP 808 using the F1 communication interface. As also shown in FIG. 8a, the serving cell change notification message includes a cell identity (ID) that uniquely identifies the UE 802 that has undergone the serving cell change.

[0097] In response to receiving the serving cell change command 704 from the serving DU 804, the UE 802 sends 820 a radio resource control (RRC) Reconfigure Acknowledgement message to the CU-CP 808. The CU-CP 808 will recognize from the RRC Reconfigure Acknowledgement message that the UE 802, uniquely identified to the CU-CP 808 by the serving DU 804, acknowledges the completion of the successful serving cell change.

[0098] Also, in response to receiving the layer 3 RRC measurement configuration, the UE 802 sends an RRC measurement report to the CU-CP 808 in accordance with 3GPP standards (822). According to 3GPP standards, the RRC measurement report may include layer 3 (L3) measurements that may be analyzed by the CU-CP 808 in 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 the target DU 806 is ready to serve the UE 802 instead of the serving DU 804 for at least one service.

[0099] In response to determining to prepare at least one target cell for LTM (824), the CU-CP 808 prepares at least one target cell for LTM (706). As shown in FIG. 8a, in this illustrated implementation, each of the at least one target cell is an inter-DU target cell, e.g., part of a DU different from the serving DU 804 where the same CU (e.g., the CU including the CU-CP 808) serves each DU 804, 806. Also, in this illustrated implementation, because there are only two gNB-DUs, the at least one target cell includes only the target DU 806; however, as described above, a base station can include three or more target cells. Currently, according to the 3GPP standard, up to eight LTM target cells can be prepared for a given UE.

[0100] Preparing 706 the at least one target cell for LTM may include notifying the at least one target DU 806 that the at least one target DU 806 may be later notified to start serving the UE 802 for at least one service. Thus, the target DU 806 can reserve necessary resources for the UE 802. As shown in FIG. 8a, preparing 706 the at least one target cell, which in this illustrated implementation is only the target DU 806, may include the CU-CP 808 sending 832 a UE context setup request message to the target DU 806 using the F1 communication interface in accordance with 3GPP standards.

[0101] Preparing (706) the at least one target cell may include providing information to at least one target DU 806 indicating whether the HO may be RACH-less HO or RACH-based HO (contention-free RACH (CFRA) or contention-based RACH (CBRA)). In an example implementation, the information includes UE timing advance (TA) information for the serving DU 804. If the UE's TA information for the serving cell 804 is either (a) the same TA as the target cell receiving the TA information, or (b) zero, it indicates that the HO to the target cell may be RACH-less HO, thereby enabling the target cell to prepare for RACH-less HO. If the UE's TA information for the serving cell 804 is neither (a) the same TA as the target cell receiving the TA information, nor (b) zero, it indicates that the HO to the target cell may be RACH-based HO, thereby enabling the target cell to prepare for RACH-based HO. In an example where the 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 rest of the target cells. Information indicating whether HO may be RACH-less HO or RACH-based HO may be provided to the at least one target DU 806 in various ways.

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

[0103] In some implementations in which the CU-CP 808 receives the serving cell TA information before target cell preparation (706), the CU-CP 808 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 DU 806. Thus, the fetching (826) of the serving cell (804) TA information can occur after the F1 setup procedure in which the F1 communication interface is set up between the CU-CP 808 and the serving cell 804. Figure 8a illustrates such an implementation.

[0104] As shown in FIG. 8a, the CU-CP 808 fetching (826) the serving cell (804) TA information may include the CU-CP sending (828) a message to the serving cell 804 requesting that the serving cell 804 provide the CU-CP 808 with its TA information, and the serving cell 804 responding by sending (830) the TA information to the CU-CP 808. The CU-CP 808 determining (824) to prepare at least one target cell may trigger the CU-CP sending (828) a message requesting the TA information because the CU-CP 808 is now aware of the HO situation. Obtaining the serving cell (804) TA information after the need for UE HO is known may enable the CU-CP 808 to have the most up-to-date TA information about the serving cell. Receiving TA information related to UE-HO upon request when the need for UE HO is known can ease the storage requirements of the CU-CP 808, as the TA information is received only when needed and does not necessarily need to be stored for later use. As shown in Figure 8a, the message sent by the CU-CP 808 (828) requesting the serving cell TA information can be a UE context modification request message, and the message sent by the serving cell 804 in response (830) can be a UE context setup response message. Because the UE context modification request message and the UE context setup response message are defined by 3GPP, the process can utilize existing CU and DU functions.

[0105] In response to receiving the UE context setup request message from the CU-CP 808, the target DU 806 prepares 834 each of the at least one target cell for LTM. In this illustrated implementation, the at least one target cell includes only the target DU 806 preparing 834 the one target cell. As shown in FIG. 8a, the preparation 834 may include the target cell reserving the necessary resources for the UE 802 and verifying the RACH-less feasibility of HO. The target DU receiving the serving cell TA information from the CU-CP 808 enables such verification because the target DU already knows the TA of its own cell and now knows the TA of the serving cell.

[0106] The target DU 806 notifies the CU-CP 808 that the preparation (834) is complete. As shown in FIG. 8a, the notification to the CU-CP 808 may include the target DU 806 sending a UE context setup response message to the CU-CP 808 using the F1 communication interface in accordance with the 3GPP standard (836). As also shown in FIG. 8a, the UE context setup response message includes consolidated cell group configuration information of the target cell prepared by the target DU 806. The consolidated cell group configuration information includes information about the configuration type, for example, a RACH-based or RACH-less configuration.

[0107] The CU-CP 808 identifies each of one or more LTM-prepared target cells and notifies the serving DU 804 of the LTM-prepared at least one target cell and its configuration type by including information about each of the one or more target DUs (708). In an inter-DU LTM scenario, one or more of the at least one LTM-prepared target cell belongs to a DU different from the serving DU 806. For example, referring to the system of FIG. 6b, the serving DU may be a small cell 626 of the macro1 DU 628a, and one or more of the target cells may be one or more small cells 626 of the macro2 DU 628b and / or macro3 DU 628c.

[0108] 8a, the notification to the serving DU 804 (708) may include the CU-CP 808 sending a UE context modification request message to the serving DU 804 using the F1 communication interface (838). The UE context modification request message may include, for each of one or more target DUs, cell identification information (e.g., a unique cell ID that identifies the target DU, such as a physical cell identifier (PCI)) or an index corresponding to the cell ID, and priority information. The notification to the serving DU 804 (708) may include information indicating the RACH-less HO configuration of the target DU 806. As described above, the CU-CP 808 is aware of the target DU candidates for RACH-less handover.

[0109] The priority information provided by the CU-CP 808 to the serving DU 804 indicates a ranked order of one or more target cells configured for RACH-less handover to be selected by the serving DU 804 as a target DU for 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 a ranking of all target cells eligible for RACH-less handover identified for the serving DU 804 by the CU-CP 808. The CU-CP 808 can determine the ranked order in any of a variety of ways. In some implementations, the CU-CP 808 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 DU 804 select one of the one or more target cells for RACH-less HO, as discussed further below.

[0110] If there is only one target cell eligible for RACH-less handover that is a candidate for serving cell change, such as in this illustrated implementation where the target DU 806 is the only option, then the priority information can be omitted from the message from the CU-CP 808 to the serving DU 804 since there is only one possible option for HO identified for the serving DU 804.

[0111] In response to being notified of at least one LTM-prepared target DU cell (708), the serving cell 804 stores received information regarding the at least one LTM target cell, e.g., stores a list of LTM-prepared target cells and, if provided to the serving DU 804, their respective priority information. Also, in response to being notified of at least one target DU (708), the serving cell 804 transmits a UE context modification response message to the CU-CP 808 using the F1 communication interface (840). The UE context modification response message may include joint cell group configuration information for each of the one or more target cells identified for the UE 802 by the CU-CP 808. The UE context modification request message and the UE context modification response message are each defined by 3GPP. Thus, the serving DU 804 can receive information regarding the at least one target cell from the CU-CP 808 and acknowledge its receipt to the CU-CP 808 using a message already sent for HO according to the 3GPP standards.

[0112] In response to receiving the UE context modification response message, the CU-CP 808 sends 842 an RRC reconfiguration message to the UE 802 in accordance with the 3GPP standard. As shown in FIG. 8a, the RRC reconfiguration message includes the LTM target cell configuration information, for example, as provided by the target DU 806 to the CU-CP 808 in the UE context setup response message sent 830. 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 to the target cell 806 since the preamble is not reserved for CFRA.

[0113] In response to receiving the target cell configuration in the RRC reconfiguration message, the UE 802 sends 844 an L1 measurement report to the serving DU 804 in accordance with 3GPP standards. The L1 measurement report provides the serving DU 804 with UE measured radio condition information of the configured target cell.

[0114] In response to receiving the L1 measurement report transmitted (844) from the UE 802, the serving cell 804 selects (710, 846) a target cell capable of RACH-less HO from among one or more LTM-prepared target cells identified for the serving DU 804. In this illustrated implementation, because there is only one target cell (target DU 806) identified for the serving DU 804 by the CU-CP 808 as an LTM-prepared target cell, the serving cell selection (710, 846) is straightforward, and the serving DU 804 selects (710, 846) the target DU 806. If there are multiple LTM-prepared target cells that satisfy the handover criteria for the serving DU 804, the 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, the target cell can be selected according to conventional procedures in accordance with 3GPP.

[0115] In implementations where there are multiple target cells identified by the CU-CP 808 for the serving DU 804, the target cell selection (710, 846) of the serving cell may include determining which target cell(s) among the multiple target cells have a radio quality above a predetermined threshold radio quality. The predetermined threshold radio quality is defined by UE radio conditions received by the serving DU 804 from the UE 802 in an L1 measurement report. Thus, the serving DU 804 can take into account the specific needs of the particular UE 802 involved in the HO when selecting (710, 846) a target cell for HO. Furthermore, the L1 measurement report sent (844) by the UE 802 to the serving DU 804 reports L1 measurements that may include reference signal received power (RSRP) defined by 3GPP for each of the multiple target cells, the identity of which is known by the UE 802 as provided to the UE 802 by the CU-CP 808 in an RRC reconfiguration message. Therefore, the serving DU 804 can analyze the L1 measurement report received from the UE 802 to determine which target cell(s) among the multiple target cells has a radio quality above a predetermined threshold radio quality.

[0116] If only one of the multiple target cells satisfies the radio conditions of the UE, e.g., if only one of the target cells' radio qualities exceeds a predetermined threshold radio quality, the serving cell 804 selects that target cell (710, 846). If two or more of the multiple target cells satisfy the radio conditions of the UE, e.g., if the target cells' radio qualities each exceed a predetermined threshold radio quality, any one of the target cells can serve the UE's needs, and one of the target cells can be selected according to priority information (if received), randomly, or according to another desired criteria.

[0117] Upon selecting (710, 846) a target cell (e.g., target DU 806 in the illustrated implementation of FIG. 8a), the serving DU 804 triggers (712) a serving cell change to the selected (710, 846) target cell. As shown in FIG. 8a, triggering (712) the serving cell change may include the serving DU 804 transmitting 848, by way of, for example, a PCI, a MAC CE to the UE 802 that includes a serving cell change command and identifies the selected (710, 846) target cell for the UE 802.

[0118] Receipt of the MAC CE by the UE indicates to the UE 802 that an LTM serving cell change (SCC) should occur to the identified target cell, e.g., the target DU 806 in the illustrated implementation of FIG. 8a for the UE 802. Accordingly, in response to receiving the MAC CE from the serving cell 804, the UE 802 initiates RACH-less HO to the target cell (714, 850). As shown in FIG. 8a, the UE 802 initiating 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 standards for RACH-less HO to the target DU 806 (852). In response to the UE 802 accessing the target cell, the target DU 806 sends a serving cell change notification to the CU-CP 808 via the F1 communication interface, identifying the target DU 806, for example 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 sends an RRC reconfiguration acknowledgement message to the CU-CP 808 (856). Thus, the CU-CP 808 receives acknowledgements from both the UE 802 via the RRC reconfiguration acknowledgement message indicating successful RRC reconfiguration at the UE 802 and the target DU 806 via the serving cell change notification, that the target DU 806 is now serving the UE 802 for at least one service handed over from the serving DU 804.

[0119] In some implementations, the serving DU 804, which has already been notified by the CU-CP 808 regarding the configuration of multiple target cells as a RACH-less or RACH-based configuration, can be configured to configure the UE 802 to perform uplink (UL) synchronization (sync) to the selected (710, 846) target cell prior to the serving cell change while still serving as the serving cell. The serving DU 804 can configure the UE 802 to perform uplink synchronization, for example, by sending a command to the UE 802 before transmitting 848 a 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 acquire the timing advance of the selected (710, 846) target cell and adjust if there is any change in the UE's serving cell TA due to further mobility of the UE 802 between the time of target cell preparation and the time of performing the serving cell change. The UE 802 can be configured to report the target cell TA acquired by the UE 802 during the UL sync procedure with the target DU 806 to the serving DU 804. The serving DU 804 can be configured to compare the TA of the UE 802 at the serving cell (determined in the serving DU 804) with the selected (710, 846) target cell (acquired during UL sync) and thus determine whether a previously configured RACH-less HO can be performed for a given target cell. If there is a change in the serving cell TA that leads to the infeasibility of RACH-less HO, the serving DU 804 can be configured to indicate to the UE 802 in the transmitted (848) serving cell change command that RACH-based HO should be performed at the target cell.

[0120] In some implementations, based on a target cell preparation request from the CU-CP 808, e.g., a UE context setup request sent (832) from the CU-CP 808, the target DU 806 can be configured to reserve a RACH preamble for a RACH-based serving cell change along with preparing a RACH-less target cell configuration. In a scenario where the UE 802 experiences continuous mobility in the serving DU / cell due to a change in the TA of the UE in the serving DU 804, leading to the infeasibility of RACH-less HO, the UE 802 can use the RACH preamble reserved for contention-less RACH access (CFRA) as a fallback solution.

[0121] As mentioned above, the CU-CP 808 can receive serving cell TA information before target cell preparation (706). In some implementations, the CU-CP 808 can receive serving cell TA information from the cell during the F1 setup procedure, when the F1 communication interface between the CU-CP 808 and the cell is set up. This procedure can be more granular and correspond to each beam or beam group. The cell is divided into different beams / beam groups or areas to which a specific TA can be assigned. A fixed TA can then be mapped to the UE once it is determined that the UE reports a specific beam group or area in its L3 measurements. Thus, a single cell can have multiple TAs, each corresponding to one or more beams / beam groups of the cell. This procedure is also feasible for smaller cells where the TA does not change often. Thus, the CU-CP 808 can receive TA information before the need for UE HO is known, and therefore has TA information available and ready to send to the target cell. Figures 8b and 8c illustrate such an implementation. FIG. 8b 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-CP 808 is notified of the serving cell change (818), the CU-CP 808 checks previously received serving cell TA information (858) to identify RACH-less target cell candidates.

[0122] 8c illustrates the system of FIG. 8b configured to provide TA information from the cell to the CU-CP 808 during an F1 setup procedure, according to some implementations of the subject matter of this disclosure. This can be updated using an F1:gNB-DU configuration update procedure. The F1 communication interface is set up between the DU and the CU-CP 808 according to 3GPP standards.

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

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

[0125] Although 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, the F1 communication interface may 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 in which the CU-CP 808 receives TA information before target cell preparation (706), the serving DU 804 can proactively provide the TA information to the CU-CP 808 without receiving a request for TA information from the CU-CP 808. Thus, the CU-CP 808 can store the serving cell TA information and have it on hand to provide to the target DU 806 later. Figure 8d illustrates such an implementation. Figure 8d is similar to Figure 8a, except that instead of fetching the serving cell TA information from the serving cell 804 (826) during the HO process after the CU-CP 808 is notified of the serving cell change (818), the CU-CP 808 checks the previously received serving cell TA information (876) to identify RACH-less target cell candidates.

[0127] As shown in FIG. 8d, the serving UE 802 may determine that a cell change is required. DU After 804 determines (702), in response to receiving (870) an L1 measurement report from UE 802 to serving DU 804, serving DU 804 identifies (872) one or more potential candidate target cells for RACH-less HO from neighboring DUs, which in the illustrated implementation of FIG. 8d includes only one DU 806. The serving DU 804 then transmits (874) a message identifying one or more potential candidate target cells to CU-CP 808 via the F1 communication interface. The serving DU 804 can be configured to transmit (874) such a message to CU-CP 808 only if one or more potential candidate target cells are identified.

[0128] In some implementations, such as the various implementations described above with respect to FIGS. 8a-8d, each of one or more candidate target cells receives the UE's serving cell TA information from the CU-CP. In other implementations, the DUs of a base station can share their TA information directly with each other. Such inter-DU communication is not possible in 5G wireless communication systems, but may be possible in 6G or later wireless communication systems. FIG. 8e illustrates such an implementation. FIG. 8e is similar to FIG. 8a, except that the serving DU 804 performs various functions described as being performed by the CU-CP 808 in the implementation of FIG. 8a. Functions that enable this solution include terminating the RRC protocol at both the DU and the CU-CP, delivering intra-gNB L3 measurements to the DU, delivering inter-gNB and radio access technology (RAT) measurements to the CU-CP, and the DU autonomously deciding to perform an intra-gNB handover.

[0129] 8e, in response to receiving the serving cell change command 704 from the serving DU 804, the UE 802 sends 878 an RRC reconfiguration acknowledgement message to the serving DU 804. The serving DU 804 will recognize from the RRC reconfiguration acknowledgement message that the UE 802 acknowledges the successful completion of the serving cell change.

[0130] Also, in response to receiving the Layer 3 RRC measurement configuration, the UE 802 sends an RRC measurement report to the serving DU 804 (880). The RRC measurement report may include L3 measurements that may be analyzed by the serving DU 804 when making resource control decisions, which may include determining (882) 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 on behalf of the serving DU 804 for at least one service.

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

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

[0133] In response to receiving the UE context setup request message from the serving DU 804, the target DU 806 prepares each of the at least one target cell for LTM (886), similar to the preparation (834) described above with respect to FIG. 8a. The target DU 806 notifies the serving DU 804 that the preparation (886) is complete. As shown in FIG. 8e, the notification to the serving DU 804 may include the target DU 806 sending a UE context setup response message to the serving DU 804 (888). As also shown in FIG. 8e, the UE context setup response message includes integrated cell group configuration information of the target cells prepared in the target DU 806. In response to being notified of at least one LTM-prepared target DU cell, the serving cell 804 stores the received information regarding the at least one LTM target cell, for example, stores a list of LTM-prepared target cells and their respective priority information if provided to the serving DU 804.

[0134] In response to receiving the UE context modification response message, the serving DU 804 sends 890 an RRC reconfiguration message to the UE 802. As shown in FIG. 8e, the RRC reconfiguration message includes LTM target cell configuration information, for example, as provided from the target DU 806 to the 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 to the target cell 806 because the preamble is not reserved for CFRA. In response to receiving the target cell configuration in the RRC reconfiguration message, the UE 802 sends 844 an L1 measurement report to the serving DU 804, and the process continues similarly to that described above with respect to FIG. 8a.

[0135] In some implementations, the subject matter of this 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, including 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. Further, in some implementations, memory 920 may be a non-volatile memory unit. Storage device 930 may be capable of providing mass storage for system 900. In some implementations, storage device 930 may be a computer-readable medium. In alternative implementations, storage device 930 may 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. Input / output device 940 may be configured to provide input / output operations to system 900. In some implementations, input / output device 940 may include a keyboard and / or a pointing device. In alternative implementations, input / output device 940 may include a display unit for displaying a graphical user interface.

[0136] 10 illustrates an example method 1000 for implementing RACH-less LTM in accordance with some implementations of the subject matter of this disclosure. Method 1000 can be performed, for example, using the implementations shown in and described with respect to FIGS. 1-8b.

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

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

[0139] In some implementations, the information may include 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 the at least one LTM target cell or (b) 0 TA may indicate that the HO to the at least one LTM target cell may be RACH-less HO, and the TA information of the UE in the serving cell being neither (a) the same TA as the TA in the at least one LTM target cell nor (b) 0 TA may indicate that the HO to the at least one LTM target cell may be RACH-based HO. Further, at least one LTM target cell may receive the TA information of the UE in the serving DU (e.g., serving DU 804 in FIG. 8e) in a message from the serving DU, and the method may include the 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 the at least one LTM target cell may receive the TA information of the UE in the serving DU in a message from a CU-CP of the base station (e.g., gNB-CU-CP 504 in FIGS. 5a to 5c, CU-CP 604 in FIGS. 6a and 6b, CU-CP 808 in FIGS. 8a to 8d, etc.), and the method may include the 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. Furthermore, 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 the at least one prepared LTM target cell for handover of service for the UE from the serving DU, and the serving DU can trigger handover of service for the UE from the serving DU to the selected LTM target cell. Further, the serving DU can determine which LTM target cell(s) of the at least one LTM target cell have a radio quality above a predetermined threshold radio quality, and the selection can be among the one or more determined LTM target cells, and / or the 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, which may further include a gNodeB (e.g., the gNodeB of FIG. 5a, the gNodeB 624 of FIG. 6a or 6b, the gNodeB of FIG. 8a and 8b, etc.) or an ng-eNodeB.

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

[0145] The systems and methods disclosed herein may be embodied in various forms, including, for example, a data processor such as a computer, including a database, digital electronic circuitry, firmware, software, or any combination thereof. Furthermore, the above-described features and other aspects and principles of the implementations of the present disclosure may be implemented in a variety of environments. Such environments and associated applications may be specially constructed to perform the various processes and operations in accordance with the disclosed implementations, or they may include general-purpose computers or computing platforms selectively activated or reconfigured by code to provide the required functionality. The processes disclosed herein are not inherently related to any particular computer, network, architecture, environment, or other apparatus, but may be implemented by any suitable combination of hardware, software, and / or firmware. For example, various general-purpose machines may be used with programs written in accordance with the teachings of the disclosed implementations, or it may be more convenient to construct specialized apparatus or systems to perform 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, e.g., 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 type 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 communications network.

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

[0148] Although ordinal numbers such as first, second, etc. may relate to order in some circumstances, as used herein, ordinal numbers do not necessarily imply order. For example, ordinal numbers may 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 frame of reference (just as a first event in one paragraph of description may differ from a first event in another paragraph of description).

[0149] The foregoing description is intended to illustrate, but not to limit, the scope of the invention, which is 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, contain machine instructions for a programmable processor and may be implemented in a high-level procedural and / or object-oriented programming language 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 machine instructions as a machine-readable signal, such as, for example, a magnetic disk, an optical disk, a memory, and a programmable logic device (PLD). 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 may non-transitory store such machine instructions, such as, for example, a non-transitory solid-state memory or a magnetic hard drive or any equivalent storage medium. Alternatively or additionally, a machine-readable medium may temporarily store such machine instructions, such as, for example, a processor cache or other random access memory associated with one or more physical processor cores.

[0151] To provide for user interaction, the disclosed subject matter described herein can be implemented on a computer having a display device, such as a cathode ray tube (CRT) or liquid crystal display (LCD) monitor, for displaying information to the user, and a keyboard and pointing device, such as a mouse or trackball, by which the user can provide input to the computer. Other types of devices can also be used to provide for user interaction. 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 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 may be implemented in a computing system that includes back-end components, such as, for example, one or more data servers, or includes middleware components, such as, for example, one or more application servers, or includes front-end components, such as, for example, one or more client computers having a graphical user interface or web browser through which a user can interact with an implementation of the disclosed subject matter described herein, or any combination of such back-end, middleware, or front-end components. The components of the system may 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, a local area network (LAN), a wide area network (WAN), and the Internet.

[0153] A computing system may include clients and servers. Clients and servers are generally, but not exclusively, remote from each other and typically interact through a communication network. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other.

[0154] The implementations described in the foregoing description do not represent all implementations consistent with the disclosed subject matter described herein. Rather, they are merely some examples consistent with aspects relating to the disclosed subject matter described. While several variations have been described in detail above, other modifications or additions are possible. In particular, additional features and / or variations may be provided in addition to those described herein. For example, the implementations described above may be directed to various combinations and subcombinations of the disclosed features and / or combinations and subcombinations of certain additional features disclosed above. Additionally, the logic flow illustrated in the accompanying figures and / or described herein does not necessarily require the particular order shown, or sequential order, to achieve desirable results. Other implementations may be within the scope of the following claims.

Claims

1. at least one processor; at least one non-transitory storage medium that stores instructions, The instructions, when executed by the at least one processor, cause the at least one processor to perform operations, the operations including: receiving, in at least one Layer 1 / Layer 2 Triggered Mobility (LTM) target distribution unit (DU) of a base station, information indicating whether a handover (HO) of 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; preparing the at least one LTM target cell for one of a RACH-less HO and a RACH-based HO based on the information received at the at least one LTM target cell; An apparatus comprising:

2. the information includes timing advance (TA) information of the UE in the serving DU; The TA information of the UE in the serving DU is either (a) the same TA as the TA in the at least one LTM target cell or (b) a TA of 0, indicating that HO to the at least one LTM target cell may be RACH-less HO; 2. The apparatus of claim 1, wherein the TA information of the UE in the serving DU is neither (a) the same TA as a TA in the at least one long-term mobile (LTM) target cell nor (b) a TA of 0, indicating that HO to the at least one long-term mobile (LTM) target cell may be a RACH-based HO.

3. The at least one LTM target DU receives the TA information of the UE in the serving DU in a message from the serving DU; 3. The apparatus of claim 2, 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 a centralized unit control plane (CU-CP) of the base station; 3. The apparatus of claim 2, wherein the operations further include 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 of claim 4 , wherein the CU-CP receives the TA information of the UE in the serving DU from the serving DU during setup of an F1 communication interface between the serving DU and the CU-CP.

6. The apparatus of 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 of 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. 2. The apparatus of claim 1, wherein the serving DU selects one of the at least one prepared LTM target cell 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 cell have a radio quality above a predetermined threshold radio quality; The apparatus of claim 8 , wherein the selection is made among the one or more determined LTM target cells.

10. 10. The apparatus of claim 8, wherein the triggering includes the serving DU sending a Medium Access Control (MAC) Control Element (CE) message to the UE.

11. The apparatus of claim 1 , wherein the base station has a non-aggregated architecture.

12. The apparatus of claim 1 , wherein the base station comprises a Next Generation Radio Access Network (NG-RAN) node.

13. The apparatus of claim 12 , wherein the NG-RAN node comprises a gNodeB or an ng-eNodeB.

14. The apparatus of claim 1 , wherein the base station includes the at least one processor and the at least one non-transitory storage medium.

15. receiving, in at least one Layer 1 / Layer 2 Triggered Mobility (LTM) target distribution unit (DU) of a base station, information indicating whether a handover (HO) of 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; preparing the at least one LTM target cell for one of a RACH-less HO and a RACH-based HO based on the information received at the at least one LTM target cell; 10. A computer-implemented method comprising:

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

17. At least one non-transitory computer-readable storage medium storing instructions that, when executed by at least one processor, cause the at least one processor to perform operations, including: receiving, in at least one Layer 1 / Layer 2 Triggered Mobility (LTM) target distribution unit (DU) of a base station, information indicating whether a handover (HO) of 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; preparing the at least one LTM target cell for one of a RACH-less HO and a RACH-based HO based on the information received at the at least one LTM target cell; At least one non-transitory computer-readable storage medium comprising:

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

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