Prioritize random access channel-less Layer 1 / Layer 2 triggered mobility

Abstract

In general, the present subject matter relates to prioritizing random access channel layer-less (RACH-less) Layer 1 / Layer 2 triggered mobility (LTM). In some implementations, prioritizing RACH-less LTM may include receiving, at a serving distribution unit (DU) of a base station, information indicating RACH-less handover configurations for multiple LTM target cells of the base station or timing advance information for each of the multiple LTM target cells from an aggregation unit control plane of the base station; identifying, at the serving DU, at least one target cell to which a RACH-less handover of a service for a user equipment (UE) may be performed based on the received information; and selecting, at the serving DU, one of the identified at least one target cell for handover from the serving DU of the service for the UE and triggering a handover of the service to the selected target cell.

Description

[Technical Field]

[0001] In some implementations, the present subject matter relates to communication systems, and in particular to prioritizing random access channel layer-less (RACH less) Layer 1 / Layer 2 triggered mobility (LTM). [Background technology]

[0002] In today's world, cellular networks provide on-demand communication capabilities to individuals and business entities. Typically, cellular networks are wireless networks that can be distributed over a terrestrial area called a cell. Each such cell is served by at least one fixed-location transceiver, referred to as a cell site or base station. Each cell can use a different set of frequencies from its neighboring cells to avoid interference and provide improved service within each cell. Together, the cells provide wireless coverage over a wide geographic area, allowing numerous mobile phones and / or other wireless devices or portable transceivers to communicate with each other and with fixed transceivers and phones anywhere in the network. Such communication is performed through base stations, even when a communicating mobile transceiver passes through more than one cell. Major wireless communication providers deploy such cell sites worldwide, allowing communicating mobile phones and mobile computing devices to connect to public switched telephone networks and the public Internet.

[0003] A mobile phone is a portable telephone that can receive and / or make telephone and / or data calls through a cell site or communication tower by using radio waves to transmit signals to and from the mobile phone. From the perspective of a large number of mobile phone users, current mobile phone networks offer limited and shared resources. In this regard, cell sites and handsets can change frequencies and use low-power transmitters to allow simultaneous use of the network by many callers with less interference. Coverage by a cell site can depend on the particular geographic location and / or the number of users that can potentially use the network. For example, in a city, a cell site may have a range of up to about 1 / 2 mile. In suburban areas, the range may be as long as 5 miles. In some areas, users can receive signals from cell sites as far away as 25 miles.

[0004] The following are some examples of digital cellular technologies used by communications 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"). Long Term Evolution or 4G LTE, developed by the Third Generation Partnership Project ("3GPP") standards organization, is a standard for high-speed data wireless communications for mobile phones and data terminals. 5G standards are currently being developed and deployed. 3GPP cellular technologies such as LTE and 5G NR are evolutions of previous 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 core network improvements.

[0005] 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 communication processing. The core network may include network functions capable of handling higher layer communication (e.g., Internet Protocol (IP), transport layer, and application layer). In some cases, the RAN function may be divided into a baseband unit function and a radio unit function. Here, the radio unit connected to the baseband unit via a fronthaul network may be responsible for lower layer processing, for example, of the radio physical layer, and the baseband unit may be responsible for higher layer radio protocols (e.g., MAC, RLC, etc.). Summary of the Invention [Problem to be solved by the invention]

[0006] 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 phones 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 a UE's communicative coupling with the base station, the DU supporting the UE may change. Time synchronization is established in a random access channel (RACH) procedure between the UE and the new supporting DU so that communication between the UE and the new supporting DU can proceed properly. However, such a handover from one DU to another DU requires more time if a contention-based RACH procedure must be used instead of a contention-free RACH procedure. This increased time to achieve the handover increases handover latency and user plane interruption time. [Means for solving the problem]

[0007] In some implementations, the present subject matter relates to a computer-implemented method. The method may include receiving, at a serving distribution unit (DU) of a base station, information indicating random access channel-less (RACH-less) handover configurations for multiple Layer 1 / Layer 2 triggered mobility (LTM) target cells of the base station from an aggregation unit control plane (CU CP) of the base station, or timing advance (TA) information for each of the multiple LTM target cells of the base station. The method may also include, at the serving DU, identifying, based on the received information, at least one target cell from which a RACH-less handover of a service for a user equipment (UE) currently served by the serving DU can be performed. The method may also include, at the serving DU, selecting one of the at least one identified target cells for a handover from the serving DU of a service for the UE and triggering a handover from the serving DU of the service for the UE to the selected target cell.

[0008] The method may enable a RACH-less handover from one cell to another cell to be prioritized over a RACH-based HO for a UE communicatively coupled with a base station of the wireless communication system.

[0009] In some implementations, the current subject may include one or more of the following optional features:

[0010] In some implementations, identifying may include determining whether one or more of the multiple target cells have (a) a TA that is the same as the TA of the serving cell, (b) a zero TA, or (c) a RACH-less handover configuration prepared therein.

[0011] In some implementations, the received information may also include a handover priority for each of the multiple target cells. The identifying may identify more than one target cell to which a RACH-less or RACH-based handover may be performed. The selecting may include selecting one of the more than one target cells having a RACH-less configuration and a highest priority.

[0012] In some implementations, the method may also include determining, at the serving DU, which one or more of the plurality of target cells have a radio quality above a predetermined radio quality threshold. A selection may be made from the one or more determined target cells.

[0013] In some implementations, the CU-CP may receive information indicating RACH-less handover configurations for multiple LTM target cells or TA information for each of the multiple LTM target cells from one or more target DUs of a base station that includes the multiple LTM target cells.

[0014] In some implementations, the triggering may include sending a media access control (MAC) control element (CE) message from the serving DU to the UE.

[0015] In some implementations, a base station may have a disaggregated architecture.

[0016] 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 eNodeB.

[0017] In some implementations, a base station may include at least one processor and at least one non-transitory storage medium capable of storing instructions that, when executed by the at least one processor, cause the at least one processor to perform a method.

[0018] Non-transitory computer program products (i.e., physically embodied computer program products) storing 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 are described that may include one or more data processors and memory coupled to the one or more data processors. 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. In addition, 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 to exchange data and / or commands or other instructions, etc., over one or more connections (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 multiple computing systems.

[0019] Details of one or more variations of the subject matter described herein are set forth in the accompanying drawings and the description below. Other features and advantages of the subject matter described herein will be apparent from the description and drawings, and from the claims. [Brief explanation of the drawings]

[0020] The following accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate certain aspects of the presently disclosed subject matter and, together with the description, serve to explain some of the principles associated with the disclosed implementations.

[0021] FIG. 1a illustrates an exemplary conventional "long term evolution" ("LTE") communications system.

[0022] FIG. 1b shows further details of the exemplary LTE system shown in FIG. 1a.

[0023] FIG. 1c shows additional details of the "evolved packet core" of the exemplary LTE system shown in FIG. 1a.

[0024] FIG. 1d illustrates an exemplary "evolved Node B" for the exemplary LTE system shown in FIG. 1a.

[0025] FIG. 2 illustrates further details of the "evolved Node B" shown in FIGS. 1a-d.

[0026] FIG. 3 illustrates an exemplary virtual radio access network according to some implementations of the current subject matter.

[0027] FIG. 4 shows an exemplary 3GPP split architecture for providing its users with use of higher frequency bands.

[0028] FIG. 5a illustrates an exemplary 5G wireless communication system.

[0029] FIG. 5b shows an example layer architecture for a split gNB and / or a split ng-eNB (e.g., a “next generation eNB” that may be connected to 5GC).

[0030] Figure 5c shows an exemplary functional split in the gNB architecture shown in Figures 5a-b.

[0031] FIG. 6a illustrates an exemplary system according to some implementations of the current subject matter.

[0032] FIG. 6b shows an exemplary alternative configuration of the system of FIG. 6a, according to some implementations of the current subject matter.

[0033] FIG. 7 illustrates an exemplary method according to some implementations of the current subject matter.

[0034] FIG. 8a illustrates another exemplary system according to some implementations of the current subject matter.

[0035] FIG. 8b illustrates another exemplary method according to some implementations of the current subject matter.

[0036] FIG. 9 illustrates yet another exemplary system according to some implementations of the current subject matter.

[0037] FIG. 10 illustrates yet another exemplary method according to some implementations of the current subject matter. DETAILED DESCRIPTION OF THE INVENTION

[0038] The present subject matter can provide systems and methods that can be implemented in wireless communication systems. Such systems can include various wireless communication systems, including 5G New Radio communication systems, "long term evolution" communication systems, and the like.

[0039] In general, the current topic concerns the preference for RACH-less LTM.

[0040] In some implementations of the present subject matter, a RACH-less (which may also be referred to as "RACH-free") handover (HO) from one cell to another may be prioritized over a RACH-based HO for a user equipment (UE) communicatively coupled to a base station of a wireless communication system. The HO from one cell to another requires less time because a contention-free RACH procedure may be taken instead of a contention-based RACH procedure. This reduces handover latency and can reduce user plane interruption time.

[0041] 3GPP standards that define one or more aspects that may be related to the current subject matter 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 current subject matter.

[0042] One or more aspects of the present subject matter may be integrated into transmitter and / or receiver components of a base station (e.g., gNodeB, eNodeB, etc.) in such communication systems. The following is a general discussion of long-term evolution communication systems and 5G New Radio communication systems. I. "Long Term Evolution" Communication Systems

[0043] 1a-c and 2 illustrate an exemplary conventional long-term evolution ("LTE") communication system 100 along with its various components. LTE systems, or 4G LTE, as they are commercially known, conform to standards for high-speed data wireless communications for mobile phones and data terminals. The standards are 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 standards were developed by 3GPP ("3rd Generation Partnership Project").

[0044] 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 through which EUTRAN 102 and EPC 108 provide communications 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(a, b, c) (as shown in FIG. 1b) that provide communications capabilities to multiple user equipment 104(a, b, c). User equipment 104 may be a mobile phone, a smartphone, a tablet, a personal computer, a personal digital assistant (“PDA”), a server, a data terminal, and / or any other type of user equipment, and / or any combination thereof. User equipment 104 can connect to the EPC 108 and therefore the PDN 101 via any eNodeB 106. Typically, user equipment 104 can connect to the eNodeB 106 that is closest in distance. In the LTE system 100, the EUTRAN 102 and the EPC 108 cooperate to provide connectivity, mobility, and services to user equipment 104.

[0045] Figure 1b illustrates further details of the network 100 shown in Figure 1a. As previously mentioned, the EUTRAN 102 includes multiple eNodeBs 106, also known as cell sites. The eNodeBs 106 provide radio functionality and perform key control functions, including scheduling of air link resources 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) serves the user equipment 104, and 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.

[0046] 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 enables the use of multiple known antenna technologies, such as "Multiple Input Multiple Output" ("MIMO").

[0047] 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.

[0048] Multiple eNodeBs 106 may be interconnected with each other using X2 interfaces 130(a, b, c). As shown in FIG. 1b, X2 interface 130a provides interconnection between eNodeB 106a and eNodeB 106b, X2 interface 130b provides interconnection between eNodeB 106a and eNodeB 106c, and X2 interface 130c provides interconnection between eNodeB 106b and eNodeB 106c. The X2 interfaces may be established between two eNodeBs to provide for the exchange of signals that may include load- or interference-related information and handover-related information. The eNodeBs 106 communicate with the “evolved packet core” 108 via S1 interfaces 124(a, b, c). The S1 interface 124 may be divided into two interfaces: One is the control plane (shown in FIG. 1c as control plane interface (S1-MME interface) 128) and the other is the user plane (shown in FIG. 1c as user plane interface (S1-U interface) 125).

[0049] The EPC 108 establishes and enables "Quality of Service" ("QoS") for user services and allows the user equipment 104 to maintain a consistent 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 work with legacy wireless networks. The EPC 108 is also designed to separate the control plane (i.e., signaling) and user plane (i.e., traffic) in the core network architecture, allowing for greater flexibility in implementation and independent scalability of control and user data functions.

[0050] The architecture of the EPC 108, which is for packet data, 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 (S-GW, P-GW, MME, HSS, etc.) are often combined into a node according to manufacturer implementation.

[0051] The S-GW 110 functions as an IP packet data router and is the user equipment's bearer path anchor in the EPC 108. Thus, as 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 talk to the new eNodeB 106 serving the user equipment 104. If the user equipment 104 moves to the domain of another S-GW 110, the MME 114 forwards all of the user equipment's bearer paths to the new S-GW. The S-GW 110 establishes bearer paths 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.

[0052] The P-GW 112 is the gateway between the EPC 108 (and thus the 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 it is placed on the appropriate bearer path, and enabling 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 can use services on PDNs served by different P-GWs. In this 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 also changes, the bearer path from the P-GW 112 is switched to the new S-GW.

[0053] The MME 114 manages the user equipment 104 in the EPC 108 (including managing subscriber authentication, maintaining context for authenticated user equipment 104, establishing a data bearer path in the network for user traffic, and keeping track of the location of idle mobiles that have not detached from the network). For idle user equipment 104 that needs to reconnect to the access network to receive downstream data, the MME 114 initiates paging to identify the user equipment and reestablish 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. The MME is typically part of a collection of MMEs in 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 S-GW 110 that constitute the ends of the data path through the EPC 108.

[0054] The PCRF 118 is responsible for policy control decision making, controlling the flow-based charging functionality in the Policy Control Activation Function ("PCEF") residing in the P-GW 110. The PCRF 118 determines how a particular data flow is treated in the PCEF and provides QoS authorization (QoS Class Identifier ("QCI") and bitrate) ensuring this is in line with the user's subscription profile.

[0055] As previously mentioned, IP services 119 are provided by PDN 101 (shown in FIG. 1a).

[0056] 1d shows an example configuration of an 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 specification, using RRH-specific custom control and user plane framing methods or O-RAN Alliance-compliant control and user plane framing methods. The operation of the eNodeB 106 may 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 scheme (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 rate (downlink: 150 Mb / s; uplink: 50 Mb / s), S1 / X2 interface (1000Base-SX, 1000Base-T), mobile environment (up to 350 km / h). The BBU 134 may 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) may 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.

[0057] The RRH 132 can transmit and receive wireless signals using an antenna 136. The RRH 132 can convert digital baseband signals from the BBU 134 to radio frequency ("RF") signals (using a converter ("CONV") 140) and can power amplify (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.

[0058] 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 media 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 sequence number maintenance for radio bearers. The BBU 134 shown in FIG. 1d may include LTE layers L1-L3.

[0059] 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 identify idle mobiles. The eNodeB 106 also communicates over-the-air common control channel information, header compression, encryption and decryption of over-the-air user data, and establishes handover reporting and trigger criteria. As previously mentioned, the eNodeB 106 can cooperate with other eNodeBs 106 over 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 over the S1-U interface. Furthermore, the eNodeB 106 exchanges user data with the S-GW over the S1-U interface. The eNodeBs 106 and the EPC 108 have a many-to-many relationship to support load sharing and redundancy among the MMEs and 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. II. 5G NR wireless communication network

[0060] In some implementations, the current subject matter relates to 5G new radio ("NR") communication systems. 5G NR is the next communication standard after the 4G / IMT-Advanced standard. 5G networks offer higher capacity than current 4G, allowing more mobile broadband users per unit area and enabling consumption of higher and / or unlimited data volumes in gigabytes per month and per user. This may enable users to stream high-resolution media using their mobile devices much of each day (even when it is not possible to do the same with Wi-Fi networks). 5G networks have improved support for device-to-device communication, lower cost, lower latency than 4G equipment, lower battery consumption, etc. Such networks have data rates of tens of megabits per second for many users, data rates of 100 Mb / s for metropolitan areas, simultaneous 1 Gb / s for users in a restricted area (e.g., an office floor), many simultaneous connections for wireless sensor networks, improved spectral efficiency, improved coverage, improved signaling efficiency, and 1-10 ms latency, a reduced latency compared to existing systems.

[0061] 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, aggregation units 302, digital units 304, and wireless devices 306. The components in the system 300 can be communicatively coupled to the core using backhaul links 305. The aggregation units (“CUs”) 302 can be communicatively coupled to the distributed units (“DUs”) 304 using midhaul connections 308. The radio frequency (“RUs”) components 306 can be communicatively coupled to the DUs 304 using fronthaul connections 310.

[0062] 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.

[0063] In a lower layer split architecture environment, the CPRI bandwidth requirements for NR can be several hundred Gb / s. CPRI compression can be implemented in the DU and RU (shown in Figure 3). In 5G communication systems, compressed CPRI over Ethernet frames is denoted as eCPRI and is the recommended fronthaul network. The architecture can enable standardization of fronthaul / midhaul, which can include fronthaul with higher layer split (e.g., "Option 2" or "Option 3-1" (higher / lower RLC split architecture)) and L1 split architecture ("Option 7").

[0064] In some implementations, a lower layer split architecture (e.g., "Option 7") may include a receiver in the uplink, joint processing across multiple transmission points (TPs) for both DL / UL, and transport bandwidth and latency requirements for ease of deployment. Additionally, the subject lower layer split architecture may include a split between cell-level and user-level processing, which may include cell-level processing in a remote unit ("RU") and user-level processing in a DU. Additionally, using the subject lower layer split architecture, frequency-domain samples may be transported over the Ethernet fronthaul, which may be compressed to reduce fronthaul bandwidth.

[0065] 4 illustrates an example communication system 400 that can implement 5G technology and provide users with access to higher frequency bands (e.g., above 10 GHz). The system 400 can include a macrocell 402 and small cells 404, 406.

[0066] The mobile device 408 may be configured to communicate with one or more of the small cells 404, 406. The system 400 may enable separation of the control plane (C-plane) and user plane (U-plane) between the macrocell 402 and the small cells 404, 406 using different frequency bands. In particular, 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-effective, and 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.

[0067] FIG. 5a illustrates an exemplary 5G wireless communication system 500 according to some implementations of the present subject matter. 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), one or more gNodeBs (or gNBs), which may have an aggregation unit (gNB-CU). The gNB-CU may be logically separated 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 (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.

[0068] The control plane and user plane portions 504, 506 of the aggregation unit of the gNB may be configured to be communicatively coupled to one or more distributed units (DUs) 508, 510 according to a higher layer split architecture. The distributed units 508, 510 may be configured to execute the upper 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 of a switch, a link, etc.) and may communicate with one or more user equipment (not shown in FIG. 5a). The remote radio unit 512 may be configured to execute the lower portions of the PHY layer protocol and provide antenna capabilities to the remote unit for communication with user equipment (similar to the discussion above with respect to FIGS. 1a-2).

[0069] Figure 5b shows an example layer architecture 530 for a split gNB. The architecture 530, which may be configured as a virtualized and disaggregated radio access network (RAN) architecture (layers L1, L2, L3 and radio processing may be virtualized and disaggregated in the aggregation unit, distributed unit, and radio unit), may be implemented in the communication system 500 shown in Figure 5a. As shown in Figure 5b, the gNB-DU 508 may 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 may be configured to include one or more layers.

[0070] The gNB-DU 508 may include RLC, MAC, and PHY layers, as well as various communication 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 previously described, the distribution unit 508 may be communicatively coupled to the control plane portion 504 of the aggregation unit, which may include the F1-AP, SCTP, and IP sublayers, the radio resource control, and the PDCP Control (PDCP-C) sublayers. Furthermore, the distribution unit 508 may be communicatively coupled to the user plane portion 506 of the aggregation unit of the gNB. The user plane portion 506 may include the Service Data Adaptation Protocol (SDAP), the PDCP User (PDCP-U), the GTPU, the UDP, and the IP sublayers.

[0071] Figure 5c shows an example functional split in the gNB architecture shown in Figures 5a-b. 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. A higher 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.

[0072] Some of the functions of the PHY layer in a 5G communication network may include error detection on transport channels and suggestion 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 onto physical channels, power weighting of physical channels, modulation and demodulation of physical channels, frequency and time synchronization, radio characteristic measurements and suggestion to higher layers, MIMO antenna processing, digital and analog beamforming, RF processing, and other functions.

[0073] 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 delivered on transport channels to / from the physical layer, scheduling of reporting information, 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's 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, and other functions.

[0074] The RRC sublayer of Layer 3 may perform functions such as broadcasting system information to NASs and ASs, establishing, maintaining and releasing RRC connections, security, establishing, configuring, maintaining and releasing point-to-point radio bearers, mobility functions, reporting, and other functions. III. Prioritize RACH-less LTM

[0075] In some implementations of the current subject matter, a RACH-less handover (HO) from one cell to another may be prioritized for a user equipment (UE) communicatively coupled with a base station of a wireless communication system.

[0076] A Layer 1 / Layer 2 triggered mobility (LTM) inter-cell handover at a base station can occur by performing a Serving Cell Change (SSC) from a serving cell to a target cell. Multiple target cells may satisfy the radio conditions or handover criteria required for the UE to receive SSC and become potential target cell options for handover. One or more of the potential target cells may require HO based on a RACH with a contention-based or contention-free RACH procedure, for example, because the timing advance (TA) of the serving cell and the TA of the target cell are different. The timing advance represents a time offset between the start times of the received downlink subframe and the transmitted uplink subframe at the UE. This offset at the UE is necessary to ensure that the downlink and uplink subframes are synchronized at the base station. This is a media access control (MAC) layer (Layer 2) control element (CE) from the base station to the UE used in controlling the timing of uplink (e.g., from the UE to the base station) signal transmissions. One or more of the other potential target cells may allow RACH-less HO, for example because the target cell TA is zero or the serving cell TA is the same as the target cell TA. Prioritizing RACH-less HO over RACH-based HO is configured to allow handover to one of the target cells that allows the RACH-less HO procedure.

[0077] For a UE's initial access 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) via one or more antennas, such as antenna 136 of FIG. 1d. The interface may be, for example, air interface 122 of FIG. 1 using OFDMA and SC-FDMA, or another communication channel. The signal may include a frame including multiple symbols, each of which includes one or more symbol groups. Typically, 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 may reduce overhead by allowing one CP portion to be used for multiple data symbols instead of using one CP portion for 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 (initial access or handover). Only a certain number of random access preambles may be available to the cell. For example, under 3GPP, an LTE cell may have 64 available preambles. Thus, multiple UEs may each randomly select the same preamble, and the cell may receive signals containing the same preamble from multiple UEs at the same time, resulting in a collision referred to as "contention." A contention-based RACH procedure may be performed to resolve the collision, but this may delay HO because the collision must be resolved before the UE can properly communicate with the cell, e.g., each UE must use a different preamble.

[0078] In contrast, 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. In this way, the cell can ensure that no collisions occur by assigning preambles such that a particular preamble is assigned to only one UE, and can reduce the HO latency compared to a contention-based RACH procedure by saving uplink synchronization time of at least 20 ms.

[0079] In some implementations of the current subject matter, 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 disaggregated architecture in which the base station includes one gNB-CU-CP (e.g., gNB-CU-CP 504 in FIGS. 5a-5c, etc.), more than one CU-UP (e.g., gNB-CU-UP 506 in FIGS. 5a-5c, etc.), and a gNB-DU (e.g., gNB-DU 508, 510 in FIGS. 5a-5c, etc.). The base station may be configured to prioritize 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).

[0080] FIG. 6a shows an example system 600 configured to provide RACH-less LTM. The base station 624 in this illustrated implementation is a gNB configured in a 5G wireless communication system similar to the previously described 5G wireless communication system 500 of FIG. 5a, although other base stations may be similarly configured and used to provide 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 to a core network (not shown in FIG. 6a), such as the 5G GC 502 of FIG. 5a.

[0081] The CU of the base station 624 also includes a CU-CP 604 that is 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, in this illustrated implementation, includes three communication links, reflecting the fact that there are three CU-UPs 606a, 606b, 606c with which the CU-CP 604 may be configured to communicate.

[0082] 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, reflecting the three CU-UPs 606a, 606b, 606c with which each DU 608, 610 may be configured to communicate.

[0083] The base station 624 also includes multiple RUs 612. The base station 624 includes five RUs 612 in this illustrated implementation, but may include other multiple RUs. The RUs 612 are configured to be communicatively coupled to the DUs 608, 610 via the fronthaul network 620. In addition, 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 as being communicatively coupled to one UE 622, two of the RUs 612 are shown as being communicatively coupled to two UEs 622, and one RU 612 is shown as being communicatively coupled to three UEs 622, but each of the RUs 612 may be coupled to other numbers of UEs, the same or different from any other RU 612.

[0084] A preference for RACH-less LTM may be configured to occur when one of the UEs communicatively coupled to the base station 624 is handed off from one DU 608, 610 of the base station 624 to another DU 608, 610 of the same base station 624. The one DU 608, 610 currently providing service to the UE 622 is referred to as the "serving DU" because it currently provides service to the UE 622 (e.g., currently serves the UE 622). The other DU 608, 610 to which the UE's service is handed off is referred to as the "target DU" because it is the target for providing service to the UE 622.

[0085] A system in which RACH-less LTM priority can be configured is further described with respect to Figure 6b. Figure 6b illustrates the CU-CP 604 and CU-UPs 606a, 606b, 606b of Figure 6a, but in the illustrated implementation of Figure 6b, the base station 624 includes more than two DUs. In the illustrated implementation of Figure 6b, the base station 624 includes 66 DUs 626, 628. Three of the DUs 628a, 628b, and 628c are macro cells (labeled Macro 1, Macro 2, and Macro 3 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 other numbers of macro cells and / or other numbers of small cells. The 21 small cell DUs 626, including Macro 1 DU 628a, Macro 2 DU 628b, gNB-DU10, gNB-DU20, and gNB-DU30, are configured to be served by the first CU-UP 606a (labeled CU-UP1 in FIG. 6b). The 21 small cell DUs 626, including Macro 1 DU 628a, Macro 2 DU 628b, Macro 3 DU 628c, gNB-DU40, gNB-DU50, and gNB-DU60, are configured to be served by the second CU-UP 606b (labeled CU-UP2 in FIG. 6b). Twenty-one small cell DUs 626, including Macro 2 DU 628b, Macro 3 DU 628c, gNB-DU 70, gNB-DU 80, and gNB-DU 90, are configured to be served by a third CU-UP 606c (labeled CU-UP3 in FIG. 6b).

[0086] 6b, each CU-UP 606a, 606b, 606c serves a subset of the DUs 626, 628a, 628, 628c for all services, although a CU-UP can serve all DUs of 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)).

[0087] Figure 7 illustrates an exemplary method 700 according to some implementations of the present subject matter. Method 700 will be described with reference to exemplary system 800 shown in Figure 8a, but may be similarly implemented in other systems, such as, for example, system 100 of Figures 1a-1c and 2, system 400 of Figure 4, system 500 of Figure 5a, and the systems of Figures 6a and 6b. System 800 of Figure 8a is a 5G system, but as noted above, the RACH-less LTM preference described herein may also be implemented in other types of wireless communication systems, such as LTE wireless communication systems, 6G or later generation wireless communication systems, etc.

[0088] In system 800, a UE 802 (e.g., UE 104 of FIGS. 1a-1c, UE 622 of FIG. 6a, etc.) is configured 810 with LTM with one or more target cells in 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, DU 628 of FIG. 6b, etc.) of a base station, such as a gNB (e.g., gNodeB of FIG. 5a, gNodeB 624 of FIGS. 6a and 6b, etc.). For ease of explanation, system 800 is shown in FIG. 8a with one UE 802 communicatively coupled to the base station and a base station including two DUs 804, 806, although more than one UE may be communicatively coupled to the base station and / or a base station may include more than two DUs. The base station of the system 800 also includes a CU, which includes a CU-CP 808 (e.g., gNB-CU-CP 504 of FIGS. 5a-5c, CU-CP 604 of FIGS. 6a and 6b, etc.) and one or more CU-UPs (e.g., gNB-CU-UP 506 of FIGS. 5a-5c, CU-UPs 606a, 606b, 606c of FIGS. 6a and 6b, etc.) (not shown in FIG. 8a), and multiple RUs (e.g., RU 512 of FIG. 5a, RU 612 of FIG. 6a, etc.) (not shown in FIG. 8a). The UE 802 is currently served by the serving DU 804. In addition, the base station of FIG. 8a is communicatively coupled to a core network (e.g., EPC 108 of FIGS. 1a-1c and 2, 5GC 502 of FIG. 5a, etc.) (not shown in FIG. 8a).

[0089] The method 700 illustrates an implementation of an intra-DU LTM serving cell change scenario, including 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. In accordance with 3GPP standards, the intra-frequency L1 measurement report may include Layer 1 (L1) measurement results that may be analyzed by the serving DU 804 when making resource control decisions, which may include a serving cell change where the UE 802 is served by a DU different from the serving DU 804 (e.g., a target DU 806) for at least one service.

[0090] 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., MAC CE) to the UE 802.

[0091] 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. In this manner, the notification 704 may identify the UE 802 to the CU-CP 808, such as by an identifier that uniquely identifies the UE 802 to the CU-CP 808 and that is known to the serving DU 804, in accordance with 3GPP standards. As shown in FIG. 8a, the notification 704 to the CU-CP 808 may include the serving DU 804 sending 818 a serving cell change notification message to the CU-CP 808 using the F1 communication interface. Also as shown in FIG. 8a, the serving cell change notification message includes a cell identification (ID) that uniquely identifies the UE 802 that has undergone the serving cell change.

[0092] 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) reconfiguration confirm message to the CU-CP 808. The CU-CP 808 recognizes from the RRC reconfiguration confirm message that the UE 802, uniquely identified to the CU-CP 808 by the serving DU 804, has confirmed the success (completion) of the serving cell change.

[0093] Also, in response to receiving the Layer 3 RRC measurement configuration, the UE 802 sends 822 an RRC measurement report to the CU-CP 808 in accordance with 3GPP standards. In accordance with 3GPP standards, the RRC measurement report may include Layer 3 (L3) measurement results 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 so 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.

[0094] In response to determining 824 to prepare at least one target cell for LTM, the CU-CP 808 prepares 706 the at least one target cell for LTM. As shown in FIG. 8a, in this illustrated embodiment, each of the at least one target cell is an inter-DU target cell (e.g., part of the same base station as the serving DU 804, but a different DU from the serving DU 804). Also, in this illustrated embodiment, 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 may include more than two target cells. Currently, according to the 3GPP standard, up to eight LTM target cells may be prepared for a given UE.

[0095] Preparing 706 the at least one target DU cell for LTM may include notifying the at least one target DU that the at least one target DU may later be notified to begin providing service to the UE 802 for the at least one service, so that 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 embodiment is only the target DU 806, may include the CU-CP 808 sending 826 a UE CONTEXT SETUP REQUEST message to the target DU 806 using the F1 communication interface in accordance with 3GPP standards.

[0096] The CU-CP 808 is assumed to be aware of the timing advance (TA) of each of the base station's DUs 804, 806 in accordance with the 3GPP standard. Therefore, the CU-CP 808 can determine whether each of the at least one target cell is a candidate for RACH-less HO from the serving DU 804 based on whether the target cell's TA is zero or the serving cell's TA is the same as the target cell's TA. Alternatively, the CU-CP 808 can request the serving DU 804 to provide the TA of the UE in the serving cell. This can be passed transparently to the target DU 806 to enable determination of the possibility of RACH-less handover. If the target cell's TA is zero or the serving cell's TA is the same as the target cell's TA, the target cell is a candidate for RACH-less HO. Preparing 706 of the at least one target cell can include notifying each of one or more target cells whether the target cell is a candidate for RACH-less HO. 8a, this notification may include a UE Context Setup Request message indicating, e.g., an information element (IE), RACH-less ON (the target cell is a candidate for RACH-less HO) or RACH-less OFF (the target cell is not a candidate for RACH-less HO). In this way, each of the one or more target cells may be aware of the necessary RACH procedures if the target cell is later notified by the UE 802 to initiate service to the UE 802 on behalf of the serving DU 804 for at least one service.

[0097] In response to receiving the UE context setup request message from the CU-CP 808, the target DU 806 prepares each of the at least one target cell for LTM (828). For example, the target DU 806 reserves necessary resources for the UE 802 and notifies the CU-CP 808 that the preparation 828 is complete. In this illustrated embodiment, the at least one target cell includes only one target DU 806 that prepares the target cell (828). As shown in FIG. 8a, the notification to the CU-CP 808 may include the target DU 806 sending (830) a UE context setup response message to the CU-CP 808 using the F1 communication interface in accordance with the 3GPP standard. Also as shown in FIG. 8a, the UE context setup response message includes integrated cell group configuration information for the target cell prepared in the target DU 806.

[0098] The CU-CP 808 identifies each of the one or more LTM-prepared target cells and notifies the serving DU 804 of the at least one LTM-prepared target cell by including information about each of the one or more target DUs (708). In some implementations, one or more of the at least one LTM-prepared target cell may belong to a different DU than the serving DU 806. For example, with reference to the system of FIG. 6b, the serving DU may be a small cell 626 of the Macro 1 DU 628a, and one or more of the target cells may be one or more small cells 626 of the Macro 2 DU 628b and / or the Macro 3 DU 628c. In some implementations, one or more of the at least one target cell may belong to the same DU as the serving DU 806. For example, with reference to the system of FIG. 6b, the serving DU may be the gNB-DU 70, and the at least one target cell may include the gNB-DU 80 and the gNB-DU 90.

[0099] As shown in FIG. 8a, the notification 708 to the serving DU 804 may include the CU-CP 808 sending a UE context modification request (UE CONTEXT MODIFICATION REQUEST) message to the serving DU 804 using the F1 communication interface (832). Also shown in FIG. 8a, 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, TA information for the target DU (e.g., a TA value for the target DU 806), and priority information. In this manner, the serving DU 804 can be aware of the TA for one or more prepared target cells, which is information that a serving DU does not traditionally have. Knowing the TA for each of one or more target cells identified for the serving DU 804 enables the serving DU 804 to identify which of the one or more target cells are candidates for RACH-less HO, as discussed further below. The notification 708 to the serving DU 804 may include information indicating the RACH-less HO configuration for the target DU 806 instead of the TA information. As described above, the CU-CP 808 is aware of the target DU's candidacy for RACH-less handover. Because the serving DU 804 has learned from the CU-CP 808 whether RACH-less HO is possible through the information about the RACH-less HO configuration, the serving DU 804 does not need the target cell TA information to determine whether RACH-less HO is possible.

[0100] The priority information provided by the CU-CP 808 to the serving DU 804 indicates a ranking of one or more target cells configured for RACH-less handover for selection by the serving DU 804 as a target DU for handover. Thus, the priority information for a particular target cell may include a ranking (e.g., 1, 2, 3, etc.) indicating the rank of the target cell in a ranking of all RACH-less handover eligible target cells identified by the CU-CP 808 to the serving DU 804. The CU-CP 808 can determine the ranking in any of a variety of ways. In some implementations, the CU-CP 808 can determine the ranking 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 one or more target cells for RACH-less HO, as discussed further below.

[0101] If there is only one target cell eligible for RACH-less handover, as in this illustrated embodiment where the target DU 806 is the only option, the priority information may be omitted from the message from the CU-CP 808 to the serving DU 804 since it is the candidate for serving cell change and there is only one possible option for HO specified for the serving DU 804.

[0102] In response to being notified of at least one LTM-prepared target DU cell (708), the serving cell 804 stores (834) the received information regarding the at least one LTM target cell. For example, the serving cell 804 stores a list of LTM-prepared target cells and their respective TAs (or RACH-less HO configurations), 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 sends (836) a UE CONTEXT MODIFICATION RESPONSE message to the CU-CP 808 using the F1 communication interface. As shown in FIG. 8a, the UE CONTEXT MODIFICATION RESPONSE message may include integrated 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. In this way, the serving DU 804 can receive information about at least one target cell from the CU-CP 808 and acknowledge receipt to the CU-CP 808 using messages already sent for HO according to the 3GPP standard.

[0103] In response to receiving the UE Context Modification Response message, the CU-CP 808 sends 838 an RRC Reconfiguration message to the UE 802 in accordance with 3GPP standards. Also, as shown in FIG. 8a, the RRC Reconfiguration message includes the LTM target cell configuration information (e.g., provided to the CU-CP 808 by the target DU 806 in the UE Context Setup Response message sent 830).

[0104] In response to receiving the target cell configuration in the RRC reconfiguration message, the UE 802 sends 840 an L1 measurement report to the serving DU 804 in accordance with the 3GPP standard. The L1 measurement report provides the serving DU 804 with radio condition information measured by the UE for the configured target cell.

[0105] In response to receiving the L1 measurement report transmitted (840) from the UE 802, the serving cell 804 selects (710, 842) a target cell capable of RACH-less handover from among one or more LTM-prepared target cells identified for the serving DU 804. In this illustrated embodiment, the serving cell selection (710, 842) is simple (the serving DU 804 selects (710, 842) the target DU 806) because only one target cell (the target DU 806) has been identified by the CU-CP 808 as an LTM-prepared target cell for the serving DU 804. 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 (if available) that is configured for RACH-less handover, as discussed herein. If there are no LTM-prepared target cells that are candidates for RACH-less HO, a target cell may be selected according to conventional 3GPP procedures.

[0106] In implementations where there are multiple target cells identified by the CU-CP 808 for the serving DU 804, the serving cell's target cell selection (710, 842) may include determining which one or more of the multiple target cells have a radio quality above a predetermined radio quality threshold. The predetermined radio quality threshold is determined by the UE radio conditions received by the serving DU 804 from the UE 802 in an L1 measurement report. In this manner, the serving DU 804 can consider the specific needs of the particular UE 802 involved in the HO when selecting (710, 842) a target cell for HO. Additionally, the L1 measurement report sent (840) by the UE 802 to the serving DU 804 reports L1 measurement results, which may include reference signal received power (RSRP), as defined by 3GPP, for each of the multiple target cells known to the UE 802, as provided by the CU-CP 808 to the UE 802 in an RRC reconfiguration message. Thus, the serving DU 804 can analyze the L1 measurement reports received from the UE 802 to determine which one or more of the multiple target cells has a radio quality above a predetermined radio quality threshold.

[0107] If only one of the target cells satisfies the UE's radio conditions, e.g., if only the radio quality of one target cell exceeds a predetermined radio quality threshold, the serving cell 804 selects the target cell (710, 842). If more than one target cell satisfies the UE's radio conditions, e.g., if each of the target cells' radio qualities exceeds a predetermined radio quality threshold, any one of these target cells can serve the UE's needs. The serving cell 804 then considers the timing advances of these target cells, transmitted (832) from the CU-CP 808 to the serving DU 804, to determine whether RACH-less handover is possible for any of these target cells. Such a determination may include determining whether one or more of these target cells have (a) the same TA as the serving cell 804's TA, (b) a zero TA, or (c) a RACH-less handover configuration prepared therein. If TA information is included in the notification (708) from the CU-CP 808 to the serving DU 804, then (a) or (b) can occur. If the notification 708 from the CU-CP 808 to the serving DU 804 includes RACH-less HO configuration information, (c) may occur.

[0108] If RACH-less HO is possible for only one of the target cells that satisfies the UE's radio conditions, the serving DU 804 selects that target cell (710, 842). In this way, the serving DU 804 does not need to select a target cell with a better reported radio quality than the selected target cell (710, 842) to prioritize RACH-less handover. The implementation illustrated in Figure 8a provides an example of this scenario by showing that the first cell A may be selected instead of the second cell B (710, 842) because both the first and second cells A and B satisfy the UE's radio conditions, but RACH-less HO is possible for the first cell A but not for the second cell B.

[0109] If RACH-less HO is possible for more than one target cell that satisfies the UE's radio conditions, the serving DU 804 considers the priority information received from the CU-CP 808 (priority information is sent from the CU-CP 808 to the serving DU 804 (832) because there is more than one target cell prepared for LTM). In particular, the serving DU 804 selects the target cell with the highest priority among the target cells that have radio quality above a predetermined radio quality threshold and are candidates for RACH-less handover (710, 842). In this way, the serving DU 804 does not need to select a target cell with a reported radio quality better than the selected target cell (710, 842) to prioritize RACH-less handover. In an implementation in which the serving DU 804 does not receive priority information from the CU-CP 808, the serving DU can randomly select a target cell from among the target cells that have radio quality above a predetermined radio quality threshold and are candidates for RACH-less handover (710, 842).

[0110] The serving DU 804, having selected (710, 842) a target cell (e.g., the target DU 806 in the illustrated implementation of FIG. 8a), triggers (712) a serving cell change to the selected (710, 842) target cell. As shown in FIG. 8a, triggering (712) the serving cell change may include the serving DU 804 sending (844) a MAC CE to the UE 802 that includes a serving cell change command and identifies the selected (710, 842) target cell to the UE 802, such as by a PCI. FIG. 8a reflects the example described above by showing the PCI identifying the first cell A sent (844) to the UE 802.

[0111] Receipt of the MAC CE by the UE indicates to the UE 802 that an LTM serving cell change (SCC) to the identified target cell (e.g., the target DU 806 in the illustrated implementation of FIG. 8a) should be performed. Thus, in response to receiving the MAC CE from the serving cell 804, the UE 802 initiates RACH-less HO to the target cell (714). As shown in FIG. 8a, the UE 802 initiating RACH-less HO to the target cell (714) may include the UE 802 accessing the target cell for RACH-less HO without performing a RACH procedure for the target DU 806 in accordance with the 3GPP standards (846). In response to the UE 802 accessing the target cell, the target DU 806 transmits a serving cell change notification to the CU-CP 808 over the F1 communication interface (848), identifying the target DU 806 as the new current serving cell for the UE 802 for at least one service, such as by a unique identifier in accordance with 3GPP. Also, in response to receiving the MAC CE from the serving cell 804, the UE 802 sends 850 an RRC reconfiguration confirm message to the CU-CP 808. In this manner, the CU-CP 808 receives confirmation from both the UE 802 (via the RRC reconfiguration confirm 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, handed over from the serving DU 804, is serving the UE 802 for at least one service).

[0112] In some implementations, such as that illustrated in FIG. 8a, the CU-CP 808 receives TA information from each of one or more candidate target cells as part of the HO process (particularly after the CU-CP 808 determines (824) to prepare one or more candidate target cells for LTM). In this manner, the CU-CP 808 receives TA information on-demand when needed for a possible HO. Such UE-HO-associated reception of TA information on-demand may reduce storage requirements for the CU-CP 808, since the TA information is received only when needed and does not necessarily need to be stored for later use.

[0113] In some implementations, instead of the CU-CP 808 receiving TA information from one or more candidate target cells as part of the HO process, the CU-CP 808 receives the TA information from the cell during the F1 setup procedure, in which the F1 communication interface is set up between the CU-CP 808 and the cell. This procedure may be more granular and correspond to each beam or beam group. Thus, a single cell may have multiple TAs, each corresponding to one or more beams / beam groups of the cell. This procedure can also be performed for smaller cells where the TAs do not vary greatly. Thus, the CU-CP 808 receives and retains the TA information before the UE's HO needs are known, ready to send to the serving cell during the HO process, similar to that described above with respect to Figures 7 and 8a.

[0114] 8b illustrates the system of FIG. 8a configured to provide TA information from the cell to the CU-CP 808 during an F1 setup procedure, according to some implementations of the present subject matter. The F1 communication interface is set up between the DU and the CU-CP 808 in accordance with 3GPP standards.

[0115] As shown in FIG. 8b, 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 of all beams / beam groups for each cell of the serving DU 804 (852). Although the serving DU 804 is shown in FIG. 8b as having multiple cells including cell 1, cell 2, etc., the serving DU 804 may include a single cell or two or more cells. 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 (854).

[0116] Also shown in FIG. 8b, 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 (856). Although the target DU 806 is shown in FIG. 8b as having multiple cells including cell 1, cell 2, etc., the target DU 806 may include a single cell or two or more cells. 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 (858).

[0117] Although FIG. 8b shows the F1 communication interface being 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.

[0118] In some implementations, the present subject matter may be configured to be implemented in a system 900, as shown in FIG. 9 . The system 900 may 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 may be interconnected using a system bus 950. The processor 910 may be configured to process instructions for execution within the system 600. In some implementations, the processor 910 may be a single-threaded processor. In alternative implementations, the processor 910 may be a multi-threaded processor. The processor 910 may further be configured to process instructions stored in the memory 920 or the storage device 930, including receiving or transmitting information through the input / output device 940. The memory 920 may store information within the system 900. In some implementations, the memory 920 may be a computer-readable medium. In alternative implementations, the memory 920 may be a volatile memory unit. Further, in some implementations, memory 920 may be a non-volatile memory unit. Storage device 930 may provide 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 for 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.

[0119] 10 illustrates an exemplary method 1000 for prioritizing RACH-less LTM according to some implementations of the present subject matter. Method 1000 may be performed, for example, using the implementations shown and described with respect to FIGS. 1-8b.

[0120] The method 1000 includes receiving, at a serving DU (e.g., serving DU 804 of FIGS. 8a and 8b, etc.) of a base station (e.g., eNodeB 106 of FIGS. 1b-2, gNodeB 5a, gNodeB 624 of FIGS. 6a and 6b, etc.), information indicating RACH-less handover configurations for a plurality of LTM target cells of the base station or timing advance (TA) information for each of a plurality of target cells of the base station (e.g., DUs 508, 510 of FIG. 5a, DUs 608, 610 of FIG. 6a, cells 626, 628 of FIG. 6b, target DU 806 of FIGS. 8a and 8b, etc.) from an aggregation unit control plane (e.g., gNB-CU-CP 504 of FIGS. 5a-5c, CU-CP 604 of FIGS. 6a and 6b, CU-CP 808 of FIGS. 8a and 8b, etc.) of the base station. The method may also include, at the serving DU, identifying, based on the received information, at least one target cell to which a RACH-less handover of a service for a user equipment (e.g., UE 104 in FIGS. 1a-1c, UE 622 in FIG. 6a, etc.) currently served by the serving cell can be performed. The method may also include, at the serving DU, selecting one of the at least one identified target cell for a handover from the serving DU of the service for the UE, and triggering a handover from the serving DU of the service for the UE to the selected target cell.

[0121] In some implementations, the current subject may include one or more of the following optional features:

[0122] In some implementations, identifying may include determining whether one or more of the multiple target cells have (a) a TA that is the same as the TA of the serving cell, (b) a zero TA, or (c) a RACH-less handover configuration prepared therein.

[0123] In some implementations, the received information may also include a handover priority for each of the multiple target cells. The identifying may identify more than one target cell to which a RACH-less or RACH-based handover may be performed. The selecting may include selecting one of the more than one target cells having a RACH-less configuration and a highest priority.

[0124] In some implementations, the method may also include determining, at the serving DU, which one or more of the plurality of target cells have a radio quality above a predetermined radio quality threshold. A selection may be made from the one or more determined target cells.

[0125] In some implementations, the CU-CP may receive information indicating RACH-less handover configurations for multiple LTM target cells or TA information for each of the multiple LTM target cells from one or more target DUs of a base station that includes the multiple LTM target cells.

[0126] In some implementations, the triggering may include sending a MAC CE message from the serving DU to the UE.

[0127] In some implementations, a base station may have a disaggregated architecture (eg, as shown in FIG. 6a or 6b).

[0128] In some implementations, the base station may include a Next Generation Radio Access Network (NG-RAN) node. Furthermore, the NG-RAN node may include a gNodeB (e.g., the gNodeB in FIG. 5a, the gNodeB 624 in FIG. 6a or FIG. 6b, the gNodeB in FIGS. 8a and 8b, etc.) or an eNodeB (e.g., the eNodeB 106 in FIGS. 1b-2, etc.). The serving cell and the multiple target cells may each include a distributed unit of the base station (e.g., the DU 304 in FIG. 3, the DU 508 in FIGS. 5a-5c, the DU 510 in FIG. 5a, the DU 608 in FIG. 6a, the DU 610 in FIG. 6a, the DU 626 in FIG. 6b, the DU 628 in FIG. 6b, the DU 804, 806 in FIGS. 8a and 8b, etc.).

[0129] In some implementations, a base station may include at least one processor and at least one non-transitory storage medium (e.g., memory 920, storage device 930, etc., of FIG. 9 ) that can store instructions that, when executed by the at least one processor (e.g., processor 910, etc., of FIG. 9 ), cause the at least one processor to perform method 1000.

[0130] 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 disclosed implementations may be implemented in various environments. Such environments and associated applications may be specially configured to perform the various processes and operations of 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 devices may be used with programs written in accordance with the teachings of the disclosed implementations, or it may be more convenient to configure specialized devices or systems to perform the required methods and techniques.

[0131] The systems and methods disclosed herein may 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, computer, or multi-computer)). The computer program may be written in any form of programming language, including compiled or interpreted languages, and may be deployed in any form, including a stand-alone program or a module, component, subroutine, or other unit suitable for use in a computing environment. The computer program may 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.

[0132] As used herein, the term "user" may refer to any entity, including a person or a computer.

[0133] Although ordinal numbers such as first, second, etc. may refer to an order in some circumstances, ordinal numbers used in this document do not necessarily imply an 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 does not necessarily imply a chronological order or a fixed reference system (just as the first event in one paragraph of a description may be different from the first event in another paragraph of the description).

[0134] The above description is intended to be illustrative, but not limiting, of the scope of the invention, which is defined by the appended claims. Other implementations are within the scope of the following claims.

[0135] These computer programs, software, software applications, applications, components, or codes, which may also be referred to as programs, include machine instructions for a programmable processor and may be implemented in a high-level procedural and / or object-oriented programming language and / or assembly / machine language. As used herein, the term "machine-readable medium" refers to any computer program product, apparatus, and / or device (e.g., magnetic disk, optical disk, memory, programmable logic device (PLD)) 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. 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 (e.g., non-transitory solid-state memory or a magnetic hard drive or any equivalent storage medium). Alternatively or additionally, a machine-readable medium may store such machine instructions in a transitory manner (e.g., processor cache or other random access memory associated with one or more physical processor cores).

[0136] To provide for user interaction, the subject matter described herein may 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, a keyboard, and a pointing device, such as a mouse or trackball, through which the user can provide input to the computer. Other types of devices may also be used to provide for user interaction. For example, feedback provided to the user may be any form of sensory feedback, such as visual feedback, auditory feedback, or tactile feedback. Input from the user may be received in any form, including, but not limited to, acoustic, speech, or tactile input.

[0137] The subject matter described herein may be implemented in a computing system that includes back-end components such as one or more data servers, or middleware components such as one or more application servers, or front-end components such as one or more client computers having a graphical user interface or web browser through which a user can interact with an implementation of the 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 a communications network. Examples of communications networks include, but are not limited to, a local area network ("LAN"), a wide area network ("WAN"), and the Internet.

[0138] A computing system may include clients and servers. Clients and servers are generally, but not limited to, 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.

[0139] The implementations presented in the foregoing description do not represent all implementations consistent with the subject matter described herein. Rather, they are merely some examples consistent with aspects related to the described subject matter. While a few 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 presented 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 some additional features disclosed above. In addition, the logic flow depicted in the accompanying figures and / or described herein does not necessarily require the particular order depicted or sequential order to achieve desirable results. Other implementations may also be within the scope of the following claims.

Claims

1. at least one processor; When executed by the at least one processor, receiving, in a serving distribution unit (DU) of a base station, information indicating random access channel-less (RACH-less) handover configurations for a plurality of Layer 1 / Layer 2 triggered mobility (LTM) target cells of the base station from an aggregation unit control plane (CU CP) of the base station, or timing advance (TA) information for each of the plurality of LTM target cells of the base station; - in the serving DU, identifying, based on the received information, at least one of the target cells to which a RACH-less handover of a service for a user equipment (UE) currently served by the serving DU can be performed; selecting, in the serving DU, one of the at least one target cell identified for handover of a service for the UE from the serving DU, and triggering the handover from the serving DU of the service for the UE to the selected target cell; at least one non-transitory storage medium storing instructions that cause the at least one processor to perform operations comprising: An apparatus comprising:

2. 2. The apparatus of claim 1, wherein the identifying includes determining whether one or more of the plurality of target cells has (a) a TA that is the same as a TA of a serving cell, (b) a TA that is zero, or (c) a RACH-less handover configuration prepared therein.

3. the received information also includes a handover priority for each of the plurality of target cells; the identifying step identifies more than one target cell to which a RACH-less or RACH-based handover can be performed; selecting includes selecting one of the more than one target cells having a RACH-less configuration and a highest priority.

10. The apparatus of claim 1.

4. The operations further comprise determining, at the serving DU, whether any one or more of the plurality of target cells has a radio quality above a predetermined radio quality threshold; the selection is made from the one or more determined target cells.

10. The apparatus of claim 1.

5. 2. The apparatus of claim 1, wherein the CU-CP receives the information indicating the RACH-less handover configuration for the plurality of LTM target cells or the TA information for each of the plurality of LTM target cells from one or more target DUs of the base station that includes the plurality of LTM target cells.

6. The apparatus of claim 1 , wherein the triggering includes transmitting a Medium Access Control (MAC) Control Element (CE) message from the serving DU to the UE.

7. The apparatus of claim 1 , wherein the base station has a disaggregated architecture.

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

9. The apparatus of claim 8 , wherein the NG-RAN node comprises a gNodeB or an eNodeB.

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

11. receiving, in a serving distribution unit (DU) of a base station, information indicating random access channel-less (RACH-less) handover configurations for a plurality of Layer 1 / Layer 2 triggered mobility (LTM) target cells of the base station from an aggregation unit control plane (CU CP) of the base station, or timing advance (TA) information for each of the plurality of LTM target cells of the base station; - in the serving DU, identifying, based on the received information, at least one of the target cells to which a RACH-less handover of a service for a user equipment (UE) currently served by the serving DU can be performed; selecting, in the serving DU, one of the at least one target cell identified for handover of a service for the UE from the serving DU, and triggering the handover from the serving DU of the service for the UE to the selected target cell; 1. A computer-implemented method comprising:

12. 12. The method of claim 11, wherein the identifying includes determining whether one or more of the plurality of target cells has (a) a TA that is the same as a TA of a serving cell, (b) a TA that is zero, or (c) a RACH-less handover configuration provisioned therein.

13. the received information also includes a handover priority for each of the plurality of target cells; the identifying step identifies more than one target cell to which a RACH-less or RACH-based handover can be performed; selecting includes selecting one of the more than one target cells having a RACH-less configuration and a highest priority. The method of claim 11.

14. The method further comprises determining, in the serving DU, whether any one or more of the plurality of target cells has a radio quality above a predetermined radio quality threshold; the selection is made from the one or more determined target cells. The method of claim 11.

15. 12. The method of claim 11, wherein the base station comprises a Next Generation Radio Access Network (NG-RAN) node.

16. When executed by at least one processor, receiving, in a serving distribution unit (DU) of a base station, information indicating random access channel-less (RACH-less) handover configurations for a plurality of Layer 1 / Layer 2 triggered mobility (LTM) target cells of the base station from an aggregation unit control plane (CU CP) of the base station, or timing advance (TA) information for each of the plurality of LTM target cells of the base station; - in the serving DU, identifying, based on the received information, at least one of the target cells to which a RACH-less handover of a service for a user equipment (UE) currently served by the serving DU can be performed; selecting, in the serving DU, one of the at least one target cell identified for handover of a service for the UE from the serving DU, and triggering the handover from the serving DU of the service for the UE to the selected target cell; and at least one non-transitory storage medium storing instructions that cause the at least one processor to perform operations comprising:

17. 17. The storage medium of claim 16, wherein the identifying includes determining whether one or more of the plurality of target cells has (a) a TA that is the same as a TA of a serving cell, (b) a TA that is zero, or (c) a RACH-less handover configuration prepared therein.

18. the received information also includes a handover priority for each of the plurality of target cells; the identifying step identifies more than one target cell to which a RACH-less or RACH-based handover can be performed; selecting includes selecting one of the more than one target cells having a RACH-less configuration and a highest priority.

17. The storage medium of claim 16.

19. The operations further comprise determining, at the serving DU, whether any one or more of the plurality of target cells has a radio quality above a predetermined radio quality threshold; the selection is made from the one or more determined target cells.

17. The storage medium of claim 16.

20. 17. The storage medium of claim 16, wherein the base station comprises a Next Generation Radio Access Network (NG-RAN) node.

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