Method and apparatus for managing mobility in wireless communication system

Abstract

The present disclosure relates to a 5G or 6G communication system for supporting a data transmission rate higher than that of a 4G communication system such as LTE. More particularly, this method performed by a distributed unit (DU) in a wireless communication system may comprise the steps of: receiving, from a centralized unit (CU), information about a preamble index to which a random access response message is not transmitted; receiving, from a terminal, a preamble related to early synchronization with a candidate target DU of the terminal; and determining whether the preamble index of the received preamble matches the preamble index to which the random access response message is not transmitted, wherein, if the preamble index of the received preamble matches the preamble index to which the random access response message is not transmitted, the random access response message for the received preamble cannot be transmitted.

Description

Method and device for mobility management in a wireless communication system

[0001] The present disclosure relates generally to wireless communication systems, and more specifically to methods and devices for managing mobility.

[0002] Looking back at the evolution of wireless communication over successive generations, technologies have primarily been developed for human-facing services such as voice, multimedia, and data. With the commercialization of the 5G (5th Generation) communication system, an explosive increase in connected devices is expected to be connected to communication networks. Examples of networked objects include vehicles, robots, drones, home appliances, displays, smart sensors installed in various infrastructures, construction equipment, and factory equipment. Mobile devices are also expected to evolve into diverse form factors, such as augmented reality glasses, virtual reality headsets, and holographic devices. In the 6G (6th Generation) era, efforts are being made to develop improved 6G communication systems to connect hundreds of billions of devices and objects and provide diverse services. For this reason, 6G communication systems are often referred to as "beyond 5G."

[0003] The 6G communication system, expected to be realized around 2030, will have a maximum transmission speed of terabytes (i.e., 1,000 gigabits) per second (bps) and a wireless latency of 100 microseconds (μsec). In other words, compared to 5G, the transmission speed in a 6G communication system will be 50 times faster and the wireless latency will be reduced to one-tenth.

[0004] To achieve these high data rates and ultra-low latency, 6G communication systems are being considered for implementation in the terahertz (THz) band (e.g., from 95 gigahertz (GHz) to 3 terahertz (THz)). Compared to the millimeter wave (mmWave) band introduced in 5G, the terahertz band is expected to have more severe path loss and atmospheric absorption, making it more important to develop technologies that can guarantee signal reach, or coverage. Key technologies to ensure coverage include Radio Frequency (RF) components, antennas, new waveforms that offer better coverage than Orthogonal Frequency Division Multiplexing (OFDM), beamforming, and multiple antenna transmission technologies such as massive Multiple-Input and Multiple-Output (MIMO), Full Dimensional MIMO (FD-MIMO), array antennas, and large-scale antennas. In addition, new technologies such as metamaterial-based lenses and antennas, high-dimensional spatial multiplexing using Orbital Angular Momentum (OAM), and Reconfigurable Intelligent Surface (RIS) are being discussed to improve the coverage of terahertz band signals.

[0005] In addition, in order to improve frequency efficiency and system network, 6G communication systems are developing full duplex technology that utilizes the same frequency resources at the same time for uplink and downlink; network technology that integrates satellites and HAPS (High-Altitude Platform Stations); network structure innovation technology that supports mobile base stations and enables optimization and automation of network operation; dynamic spectrum sharing technology through collision avoidance based on spectrum usage prediction; AI-based communication technology that utilizes AI (Artificial Intelligence) from the design stage and internalizes end-to-end AI support functions to realize system optimization; and next-generation distributed computing technology that realizes services with complexity that exceeds the limits of terminal computing capabilities by utilizing ultra-high-performance communication and computing resources (Mobile Edge Computing (MEC), cloud, etc.). In addition, efforts are being made to further strengthen connectivity between devices, further optimize networks, promote softwareization of network entities, and increase the openness of wireless communications through the design of new protocols to be used in 6G communication systems, the implementation of hardware-based security environments, the development of mechanisms for the safe use of data, and the development of technologies for maintaining privacy.

[0006] Research and development of these 6G communication systems are expected to enable a new level of hyper-connected experience through the hyper-connectivity of 6G communication systems, which encompass not only connections between things but also connections between people and things. Specifically, 6G communication systems are expected to enable services such as truly immersive eXtended Reality (XR), high-fidelity mobile holograms, and digital replicas. Furthermore, services such as remote surgery, industrial automation, and emergency response, which are provided through 6G communication systems through enhanced security and reliability, will be applied in diverse fields such as industry, medicine, automobiles, and home appliances.

[0007] Based on the discussion described above, the present disclosure seeks to provide a device and method capable of performing effective signal transmission and reception in a wireless communication system.

[0008] The present disclosure provides a device and method for performing an LTM procedure.

[0009] The technical problems to be achieved in the present disclosure are not limited to the technical problems mentioned above, and other technical problems not mentioned can be clearly understood by a person having ordinary skill in the technical field to which the present invention belongs from the description below.

[0010] The present disclosure has the effect of providing a device and method capable of effectively providing a service in a wireless communication system.

[0011] The present disclosure has the effect of reducing random access response message transmission resources wasted in the LTM procedure in a wireless communication system.

[0012] The present disclosure has the effect of reducing preamble resources wasted in an LTM procedure in a wireless communication system.

[0013] The effects that can be obtained from the present disclosure are not limited to the effects mentioned in the various embodiments, and other effects that are not mentioned can be clearly understood by a person having ordinary skill in the art to which the present disclosure belongs from the description below.

[0014] FIG. 1 illustrates a wireless environment network in a wireless communication system according to various embodiments of the present disclosure.

[0015] FIG. 2 illustrates a functional configuration of a base station in a wireless communication system according to various embodiments of the present disclosure.

[0016] FIG. 3 illustrates a functional configuration of a terminal in a wireless communication system according to various embodiments of the present disclosure.

[0017] FIG. 4 illustrates an example of a wireless resource region in a wireless communication system according to embodiments of the present disclosure.

[0018] FIG. 5 is a diagram illustrating an example of a downtime occurring in an L3 handover procedure according to various embodiments of the present disclosure.

[0019] FIG. 6 is a diagram illustrating an example of an LTM procedure according to various embodiments of the present disclosure.

[0020] Figure 7 is a flowchart illustrating an example of an operation for a terminal to obtain a TA for a target cell before receiving a cell change command.

[0021] FIG. 8 and FIG. 9 are diagrams showing examples of situations in which unnecessary RAR transmission occurs as PRACH transmission is performed in a broadcast manner.

[0022] FIG. 10 is a diagram illustrating a problem in which the total available preamble resources become insufficient as non-contention-purpose preambles for PRACH-based early synchronization operations are occupied.

[0023] Figure 11 is a flowchart showing an example of how the LTM procedure is performed from the F1AP perspective.

[0024] FIG. 12 is a diagram illustrating an example of a method for efficiently using limited preamble resources according to one embodiment of the present disclosure.

[0025] FIG. 13 is a diagram for explaining the effect of performing an LTM procedure based on a preamble resource reuse method according to one embodiment of the present disclosure.

[0026] FIG. 14 is a flowchart illustrating an example of an operation in which a source DU transmits a message that can inform a target DU and CU of whether to transmit a preamble (PRACH transmission) according to one embodiment of the present disclosure.

[0027] FIG. 15 is a flowchart illustrating an example of a method of operating a DU (distributed unit) according to various embodiments of the present disclosure.

[0028] FIG. 16 is a flowchart illustrating an example of a method of operating a CU (centralized unit) according to various embodiments of the present disclosure.

[0029] According to various embodiments of the present disclosure, a method performed by a distributed unit (DU) in a wireless communication system includes the steps of: receiving, from a centralized unit (CU), information about a preamble index in which a random access response message is not transmitted; receiving, from a terminal, a preamble related to early synchronization with a candidate target DU of the terminal; and determining whether a preamble index of the received preamble matches a preamble index in which the random access response message is not transmitted, wherein if the preamble index of the received preamble matches a preamble index in which the random access response message is not transmitted, the random access response message for the received preamble may not be transmitted.

[0030] According to various embodiments of the present disclosure, a method performed by a centralized unit (CU) in a wireless communication system may include the steps of: transmitting, to a distributed unit (DU), information about a preamble index for which a random access response message is not transmitted; and receiving, from the DU, information about a timing advance (TA) obtained based on a preamble based on a preamble index that does not match the preamble index for which the random access response message is not transmitted.

[0031] In a wireless communication system, a distributed unit (DU) comprises: a transceiver; and at least one processor coupled to the transceiver, wherein the at least one processor is configured to receive, from a centralized unit (CU), information about a preamble index in which a random access response message is not transmitted, receive, from a terminal, a preamble related to early synchronization with a candidate target DU of the terminal, and determine whether the preamble index of the received preamble matches the preamble index in which the random access response message is not transmitted, wherein when the preamble index of the received preamble matches the preamble index in which the random access response message is not transmitted, the random access response message for the received preamble may not be transmitted.

[0032] In a wireless communication system, a centralized unit (CU) includes a transceiver; and at least one processor coupled to the transceiver, wherein the at least one processor is configured to transmit, to a distributed unit (DU), information about a preamble index for which a random access response message is not transmitted, and receive, from the DU, information about a timing advance (TA) obtained based on a preamble based on a preamble index that does not match the preamble index for which the random access response message is not transmitted.

[0033] The terms used in this disclosure are used only to describe specific embodiments and may not be intended to limit the scope of other embodiments. The singular expression may include the plural expression unless the context clearly indicates otherwise. Terms used, including technical or scientific terms, may have the same meaning as commonly understood by those of ordinary skill in the art described in this disclosure. Terms used in this disclosure that are defined in general dictionaries may be interpreted as having the same or similar meaning as the meaning they have in the context of the relevant technology, and will not be interpreted in an idealized or overly formal sense unless explicitly defined in this disclosure. In some cases, even if a term is defined in this disclosure, it cannot be interpreted to exclude embodiments of the present disclosure.

[0034] The various embodiments of the present disclosure described below illustrate hardware-based approaches. However, since the various embodiments of the present disclosure include technologies utilizing both hardware and software, the various embodiments of the present disclosure do not exclude software-based approaches. Furthermore, terms referring to network entities, terms referring to device components, and the like are provided for convenience of explanation. Therefore, the present disclosure is not limited to the terms described below, and other terms with equivalent technical meanings may be used.

[0035] Additionally, although this disclosure describes various embodiments using terms defined by certain communication standards (e.g., 3rd generation partnership project (3GPP) and European Telecommunication Standards Institute (ETSI)), these are merely illustrative examples. The various embodiments of this disclosure can be easily modified and applied to other communication systems.

[0036] Additionally, in the present disclosure, expressions such as "more than" or "less than" may be used to determine whether a specific condition is satisfied or fulfilled. However, this is merely a description for expressing an example and does not exclude descriptions of more than or less than. Conditions described as "more than" may be replaced with "more than," conditions described as "less than" may be replaced with "less than," and conditions described as "more than and less than" may be replaced with "more than and less than."

[0037] The terms used in the following description, including terms referring to signals, channels, control information, network entities, and device components, are provided for convenience of explanation. Therefore, the present disclosure is not limited to the terms described below, and other terms with equivalent technical meanings may be used.

[0038] 5G systems must support services that simultaneously satisfy diverse requirements, allowing them to freely reflect the diverse needs of users and service providers. Services being considered for 5G systems include enhanced mobile broadband (eMBB), massive machine type communication (mMTC), and ultra-reliable and low-latency communication (URLLC).

[0039] eMBB aims to provide data transmission rates that are significantly higher than those supported by existing LTE, LTE-A, or LTE-Pro. For example, in a 5G system, eMBB must be able to support a peak data rate of 20 Gbps in the downlink and a peak data rate of 10 Gbps in the uplink from the perspective of a single base station. Furthermore, 5G systems must provide both peak data rates and increased user-perceived data rates. To meet these requirements, improvements in various transmission and reception technologies, including improved multi-input, multi-output (MIMO) transmission technologies, may be required. Furthermore, while LTE systems transmit signals using a maximum 20 MHz transmission bandwidth in the 2 GHz band, 5G systems can utilize a wider bandwidth than 20 MHz in the 3 to 6 GHz or higher frequency bands, thereby meeting the data transmission rates required by 5G communication systems.

[0040] At the same time, mMTC is being considered to support application services such as the Internet of Things (IoT) in 5G systems. To efficiently provide the IoT, mMTC requires supporting large-scale terminal connections within a cell, improved terminal coverage, improved battery life, and reduced terminal costs. The IoT requires the ability to support a large number of terminals (e.g., 1,000,000 terminals / km2) within a cell, as it connects to various sensors and devices to provide communication functions. Furthermore, terminals supporting mMTC are likely to be located in shadow areas, such as basements, beyond cell coverage due to the nature of the service, requiring wider coverage than other services provided by 5G communication systems. Terminals supporting mMTC must be comprised of low-cost terminals, and since frequent battery replacement is difficult, they require extremely long battery lifespans, such as 10 to 16 years.

[0041] Finally, URLLC refers to cellular-based wireless communication services used for mission-critical purposes. Examples include remote control of robots or machinery, industrial automation, unmanaged aerial vehicles, remote health care, or emergency alerts. Therefore, URLLC communications must provide extremely low latency and high reliability. For example, services supporting URLLC must meet air interface latency requirements of less than 0.5 milliseconds and a packet error rate of less than 10-5. Therefore, for services supporting URLLC, 5G systems must provide shorter transmit time intervals (TTIs) than other services, while simultaneously allocating ample resources in the frequency band to ensure the reliability of the communication link.

[0042] Furthermore, data traffic from the three aforementioned services—eMBB, URLLC, and mMTC—can be multiplexed and transmitted within the 5G and / or 6G systems. To meet the varying requirements of each service, different transmission and reception techniques and parameters may be used across the services.

[0043] FIG. 1 illustrates a wireless environment network in a wireless communication system according to various embodiments of the present disclosure. FIG. 1 illustrates a base station (110), a first terminal (120), and a second terminal (130) as some of the nodes utilizing a wireless channel in the wireless communication system. While FIG. 1 illustrates only one base station, other base stations identical to or similar to the base station (110) may be included.

[0044] The base station (110) is a network infrastructure that provides wireless access to terminals (120, 130). The base station (110) has coverage defined as a certain geographical area based on the distance at which a signal can be transmitted. In addition to the base station, the base station (110) includes an 'access point (AP)', an 'eNodeB (eNB)', and a '5G node (5 th It may be referred to as 'next generation node (gNB)', 'wireless point', 'transmission / reception point (TRP)' or other terms having equivalent technical meaning.

[0045] Each of the first terminal (120) and the second terminal (130) is a device used by a user and communicates with the base station (110) via a wireless channel. In some cases, at least one of the first terminal (120) and the second terminal (130) may be operated without the involvement of the user. That is, at least one of the first terminal (120) and the second terminal (130) is a device that performs machine type communication (MTC) and may not be carried by the user. Each of the first terminal (120) and the second terminal (130) may be referred to as a 'user equipment (UE)', a 'mobile station', a 'subscriber station', a 'remote terminal', a 'wireless terminal', a 'user device', or other terms having an equivalent technical meaning thereto.

[0046] The base station (110), the first terminal (120), and the second terminal (130) can transmit and receive wireless signals in the millimeter wave (mmWave) band (e.g., 28 GHz, 30 GHz, 38 GHz, 60 GHz). At this time, in order to improve channel gain, the base station (110), the first terminal (120), and the second terminal (130) can perform beamforming. Here, the beamforming can include transmission beamforming and reception beamforming. That is, the base station (110), the first terminal (120), and the second terminal (130) can provide directionality to a transmission signal or a reception signal. To this end, the base station (110) and the terminals (120, 130) can select serving beams through a beam search or beam management procedure. After serving beams are selected, subsequent communications can be performed through resources that are in a quasi-co-located (QCL) relationship with the resource that transmitted the serving beams.

[0047] If large-scale characteristics of a channel carrying a symbol on a first antenna port can be inferred from a channel carrying a symbol on a second antenna port, the first antenna port and the second antenna port can be evaluated to have a QCL relationship. For example, the large-scale characteristics may include at least one of delay spread, Doppler spread, Doppler shift, average gain, average delay, and a spatial receiver parameter.

[0048] FIG. 2 illustrates the functional configuration of a base station in a wireless communication system according to various embodiments of the present disclosure. The configuration illustrated in FIG. 2 may be understood as the configuration of a base station (110). Terms such as "... unit" and "... unit" used hereinafter refer to a unit that processes at least one function or operation, which may be implemented using hardware, software, or a combination of hardware and software.

[0049] Referring to FIG. 2, the base station includes a wireless communication unit (210), a backhaul communication unit (220), a storage unit (230), and a control unit (240).

[0050] The wireless communication unit (210) performs functions for transmitting and receiving signals via a wireless channel. For example, the wireless communication unit (210) performs a conversion function between baseband signals and bit streams according to the physical layer specifications of the system. For example, when transmitting data, the wireless communication unit (210) encodes and modulates the transmitted bit stream to generate complex symbols. Additionally, when receiving data, the wireless communication unit (210) restores the received bit stream by demodulating and decoding the baseband signal.

[0051] In addition, the wireless communication unit (210) upconverts a baseband signal into an RF (radio frequency) band signal and transmits it through an antenna, and downconverts an RF band signal received through the antenna into a baseband signal. To this end, the wireless communication unit (210) may include a transmission filter, a reception filter, an amplifier, a mixer, an oscillator, a digital to analog convertor (DAC), an analog to digital convertor (ADC), etc. In addition, the wireless communication unit (210) may include a plurality of transmission and reception paths. Furthermore, the wireless communication unit (210) may include at least one antenna array composed of a plurality of antenna elements.

[0052] In terms of hardware, the wireless communication unit (210) may be composed of a digital unit and an analog unit, and the analog unit may be composed of a plurality of sub-units depending on operating power, operating frequency, etc. The digital unit may be implemented with at least one processor (e.g., a digital signal processor (DSP)).

[0053] The wireless communication unit (210) transmits and receives signals as described above. Accordingly, all or part of the wireless communication unit (210) may be referred to as a "transmitter," a "receiver," or a "transceiver." Furthermore, in the following description, transmission and reception performed via a wireless channel are used to mean that the wireless communication unit (210) performs the processing described above.

[0054] The backhaul communication unit (220) provides an interface for communicating with other nodes within the network. That is, the backhaul communication unit (220) converts a bit string transmitted from a base station to another node, such as another access node, another base station, an upper node, a core network, etc., into a physical signal, and converts a physical signal received from another node into a bit string.

[0055] The storage unit (230) stores data such as basic programs, application programs, and setting information for the operation of the base station. The storage unit (230) may be composed of volatile memory, non-volatile memory, or a combination of volatile and non-volatile memory. In addition, the storage unit (230) provides stored data upon request from the control unit (240).

[0056] The control unit (240) (or controller) controls the overall operations of the base station. For example, the control unit (240) transmits and receives signals through the wireless communication unit (210) or the backhaul communication unit (220). In addition, the control unit (240) records and reads data in the storage unit (230). In addition, the control unit (240) can perform the functions of the protocol stack required by the communication standard. According to another implementation example, the protocol stack can be included in the wireless communication unit (210). To this end, the control unit (240) can include at least one processor.

[0057] According to various embodiments, the control unit (240) can control the base station to perform operations according to various embodiments described below.

[0058] FIG. 3 illustrates the functional configuration of a terminal in a wireless communication system according to various embodiments of the present disclosure. The configuration illustrated in FIG. 3 may be understood as the configuration of terminals (120, 130). Terms such as "...unit" and "...unit" used hereinafter refer to a unit that processes at least one function or operation, which may be implemented using hardware, software, or a combination of hardware and software.

[0059] Referring to FIG. 3, the terminal includes a communication unit (310), a storage unit (320), and a control unit (330).

[0060] The communication unit (310) performs functions for transmitting and receiving signals via a wireless channel. For example, the communication unit (310) performs a conversion function between a baseband signal and a bit stream according to the physical layer specifications of the system. For example, when transmitting data, the communication unit (310) generates complex symbols by encoding and modulating a transmission bit stream. In addition, when receiving data, the communication unit (310) restores a reception bit stream by demodulating and decoding the baseband signal. In addition, the communication unit (310) upconverts a baseband signal to an RF band signal and transmits it through an antenna, and downconverts an RF band signal received through the antenna to a baseband signal. For example, the communication unit (310) may include a transmission filter, a reception filter, an amplifier, a mixer, an oscillator, a DAC, an ADC, etc.

[0061] In addition, the communication unit (310) may include a plurality of transmission and reception paths. Furthermore, the communication unit (310) may include at least one antenna array composed of a plurality of antenna elements. In terms of hardware, the communication unit (310) may be composed of digital circuits and analog circuits (e.g., radio frequency integrated circuits (RFIC)). Here, the digital circuits and analog circuits may be implemented in a single package. In addition, the communication unit (310) may include a plurality of RF chains. Furthermore, the communication unit (310) may perform beamforming.

[0062] The communication unit (310) transmits and receives signals as described above. Accordingly, all or part of the communication unit (310) may be referred to as a "transmitter," a "receiver," or a "transmitting and receiving unit." Furthermore, in the following description, transmission and reception performed via a wireless channel are used to mean processing performed by the communication unit (310) as described above.

[0063] The storage unit (320) stores data such as basic programs, application programs, and setting information for the operation of the terminal. The storage unit (320) may be composed of volatile memory, non-volatile memory, or a combination of volatile and non-volatile memory. In addition, the storage unit (320) provides stored data upon request from the control unit (330).

[0064] The control unit (330) (or controller) controls the overall operations of the terminal. For example, the control unit (330) transmits and receives signals through the communication unit (310). In addition, the control unit (330) records and reads data in the storage unit (320). In addition, the control unit (330) can perform the functions of the protocol stack required by the communication standard. To this end, the control unit (330) may include at least one processor or microprocessor, or may be a part of a processor. In addition, a part of the communication unit (310) and the control unit (330) may be referred to as a CP (communication processor).

[0065] According to various embodiments, the control unit (330) can control the terminal to perform operations according to various embodiments described below.

[0066] FIG. 4 illustrates an example of a wireless resource region in a wireless communication system according to embodiments of the present disclosure. In various embodiments of the present disclosure, the wireless resource region may include a structure in the time-frequency domain. In one embodiment, the wireless communication system may include an NR communication system.

[0067] Referring to Fig. 4, in the wireless resource domain, the horizontal axis represents the time domain, and the vertical axis represents the frequency domain. The length of a radio frame (404) is 10 ms. The radio frame (404) may be a time domain section composed of 10 subframes. The length of a subframe (403) is 1 ms. The unit of configuration in the time domain may be an OFDM (orthogonal frequency division multiplexing) and / or a DFT-s-OFDM (DFT (discrete Fourier transform)-spread-OFDM) symbol, and N symb OFDM and / or DFT-s-OFDM symbols (401) may be grouped to form one slot (402). According to various embodiments of the present disclosure, an OFDM symbol may include a symbol for transmitting and receiving a signal using an OFDM multiplexing scheme, and a DFT-s-OFDM symbol may include a symbol for transmitting and receiving a signal using a DFT-s-OFDM or SC-FDMA (single carrier frequency division multiple access) multiplexing scheme. The minimum transmission unit in the frequency domain is a subcarrier, and the carrier bandwidth constituting the resource grid is a total of N scBW It may be composed of subcarriers (405). In addition, in the present disclosure, an embodiment regarding downlink signal transmission and reception is described for convenience of explanation, but this is also applicable to an embodiment regarding uplink signal transmission and reception.

[0068] According to one embodiment, the number of slots (402) constituting one subframe (403) and the length of the slots (402) may vary depending on the subcarrier spacing. This subcarrier spacing may be referred to as a numerology (μ). For example, the subcarrier spacing, the number of slots included in a subframe, the length of the slots, and the length of the subframe may be configured variably. For example, in an NR communication system, when the subcarrier spacing (SCS) is 15 kHz, one slot (402) constitutes one subframe (403), and the lengths of the slot (402) and the subframe (403) may each be 1 ms. In addition, for example, when the subcarrier spacing is 30 kHz, two slots may constitute one subframe (403). In this case, the length of the slot is 0.5 ms and the length of the subframe is 1 ms.

[0069] In one embodiment, the subcarrier spacing, the number of slots included in a subframe, the length of the slot, and the length of the subframe may be variably applied depending on the communication system. For example, in the case of an LTE system, the subcarrier spacing may be 15 kHz, two slots may constitute one subframe, and in this case, the length of the slot may be 0.5 ms and the length of the subframe may be 1 ms. As another example, in the case of an NR system, the subcarrier spacing (μ) may be one of 15 kHz, 30 kHz, 60 kHz, 120 kHz, 240 kHz, 480 kHz, and 960 kHz, and the number of slots included in one subframe depending on the subcarrier spacing (μ) may be 1, 2, 4, 8, 16, 32, and 64.

[0070] The basic unit of resources in the time-frequency domain may be a resource element (RE) (406), and the resource element (406) may be expressed by an OFDM symbol index and a subcarrier index. A resource block may include a plurality of resource elements. In an NR system, a resource block (RB) (or physical resource block (PRB)) (407) may be N in the frequency domain. SC RB can be defined as a series of consecutive subcarriers. The number of subcarriers N SC RB =12 can be. The frequency domain can include common resource blocks (CRBs). Physical resource blocks (PRBs) can be defined in the bandwidth part (BWP) of the frequency domain. The CRB and PRB numbers can be determined differently depending on the subcarrier spacing. In the LTE system, RBs are N in the time domain. symb N consecutive OFDM symbols in the frequency domain SC RB can be defined as a series of consecutive subcarriers.

[0071] In NR and / or LTE systems, scheduling information for downlink data or uplink data may be transmitted from a base station (110) to a terminal (120) via downlink control information (DCI). According to various embodiments of the present disclosure, DCI may be defined according to various formats, and each format may indicate whether the DCI includes scheduling information for uplink data (e.g., UL grant), scheduling information for downlink data (DL resource allocation), whether it is compact DCI with small control information size, whether it is fall-back DCI, whether spatial multiplexing using multiple antennas is applied, and / or whether it is DCI for power control. For example, NR DCI format 1_0 or NR DCI format 1_1 may include scheduling for downlink data. Also, for example, NR DCI format 0_0 or NR DCI format 0_1 ​​may include scheduling for uplink data.

[0072] As described above, FIG. 4 illustrates an example of a downlink and uplink slot structure in a wireless communication system. In particular, FIG. 4 illustrates the structure of a resource grid of a 3GPP NR system. Referring to FIG. 4, a slot may include a plurality of orthogonal frequency division multiplexing (OFDM) symbols in the time domain and a plurality of resource blocks (RBs) in the frequency domain. A signal may be composed of part or all of the resource grid. In addition, the number of OFDM symbols included in a slot may generally vary depending on the length of a cyclic prefix (CP). In FIG. 4, for convenience of explanation, a case in which a slot is composed of 14 OFDM symbols is illustrated, but the signal referred to in the present disclosure does not specify the symbol configuration. In addition, the modulation method of the generated signal is not limited to a specific value of QAM (Quadrature Amplitude Modulation), and can follow the modulation methods of various communication standards, such as BPSK (Binary phase-shift keying) and QPSK (Quadrature Phase Shift Keying).

[0073] According to various embodiments of the present disclosure, operations for controlling uplink retransmission for efficient signal transmission are described based on an LTE communication system or an NR communication system. However, the contents of the present disclosure are not limited thereto and can be applied to various wireless communication systems for transmitting downlink or uplink control information. Furthermore, it goes without saying that the contents of the present disclosure can be applied to unlicensed bands as well as licensed bands, as needed.

[0074] Hereinafter, in the present disclosure, higher layer signaling or higher signal may be a signal transmission method in which a base station (110) transmits a signal to a terminal (120) using a downlink data channel of a physical layer, or a signal transmission method in which a terminal (120) transmits a signal to a base station (110) using an uplink data channel of a physical layer. According to one embodiment, the higher layer signaling may include at least one of radio resource control (RRC) signaling, signaling according to an F1 interface between a centralized unit (CU) and a distributed unit (DU), or a signal transmission method in which a MAC control element (MAC CE) is transmitted. In addition, according to one embodiment, the higher layer signaling or higher signal may include system information that is commonly transmitted to a plurality of terminals (120), for example, a system information block (SIB).

[0075] In a 5G wireless communication system, a synchronization signal block (SSB) (also referred to as an SS block, SS / PBCH block, etc.) may be transmitted for initial access, and the synchronization signal block may be composed of a primary synchronization signal (PSS), a secondary synchronization signal (SSS), and a physical broadcast channel (PBCH). In addition, the SSB may include information about a beam that the base station uses to transmit a signal, and the SSB index or SSB described below may mean at least one beam. In the initial access phase when a terminal first accesses the system, the terminal may obtain downlink time and frequency domain synchronization and obtain a cell ID from a synchronization signal through a cell search procedure. The synchronization signal may include a PSS and an SSS. The terminal may receive a PBCH including a master information block (MIB) from the base station to obtain system information and basic parameter values ​​related to transmission and reception, such as system bandwidth or related control information. Based on the received PBCH, the terminal can decode the physical downlink control channel (PDCCH) and physical downlink shared channel (PDSCH) to obtain a system information block (SIB). Afterwards, the terminal can exchange identities with the base station through a random access phase and undergo registration, authentication, and other steps to initially access the network.

[0076] As described above, one slot may include 14 symbols, and according to various embodiments of the present disclosure, the uplink-downlink configuration of symbols and / or slots in a 5G communication system may be set in three stages.

[0077] In the first method, the uplink-downlink of a symbol and / or slot can be configured semi-statically through cell-specific configuration information via system information at the symbol level. More specifically, the cell-specific uplink-downlink configuration information via system information may include uplink-downlink pattern information and reference subcarrier information. The uplink-downlink pattern information may indicate a pattern periodicity, the number of consecutive downlink slots from the start of each pattern, the number of symbols in the next slot, the number of consecutive uplink slots from the end of the pattern, and the number of symbols in the next slot. Slots and symbols that are not indicated as uplink or downlink may be considered flexible slots / symbols.

[0078] In a second way, through user-specific configuration information via dedicated upper layer signaling, a flexible slot or a slot containing flexible symbols can be indicated by the number of consecutive downlink symbols from the start symbol of the slot and the number of consecutive uplink symbols from the end of the slot, or by the entire downlink or the entire uplink of the slot, respectively.

[0079] In a third method, in order to dynamically change the downlink signal transmission and uplink signal transmission sections, symbols indicated as flexible symbols in each slot (e.g., symbols not indicated as downlink or uplink) can be indicated as downlink symbols, uplink symbols, or flexible symbols, through a slot format indicator (SFI) included in a downlink control channel. The slot format indicator can select one index from a table in which an uplink-downlink configuration of 14 symbols in one slot is preset.

[0080] FIG. 5 is a diagram illustrating an example of a downtime occurring in an L3 handover procedure according to various embodiments of the present disclosure.

[0081] Referring to FIG. 5, the interruption time (540) that occurs during handover of a terminal can be defined as the time from the time when the terminal receives a cell change command including information about a target cell from a source cell in a terminal reconfiguration procedure (510), to the time when the terminal transmits the first uplink data after a downlink synchronization procedure (520) and a RACH procedure (UL synchronization (RACH procedure)) (530) for uplink synchronization are performed.

[0082] Conditional handover can be used to reduce the downtime required for the UE reconfiguration procedure, but since the procedures that account for a large portion of the downtime are the downlink synchronization procedure and the uplink synchronization procedure, not the UE reconfiguration procedure, it may be difficult to significantly reduce the downtime through methods such as conditional handover that focus on reducing the time required for the UE reconfiguration procedure.

[0083] Meanwhile, the LTM (L1(layer 1) / L2(layer 2) Triggered Mobility) procedure can be considered as a mobility management method that can significantly reduce the time required for the downlink synchronization procedure and uplink synchronization procedure, which account for a large proportion of the downtime. In the case of the L3 handover procedure, the downlink and uplink synchronization procedures with the target cell are performed after the terminal receives the handover command message, whereas in the case of the LTM procedure, the terminal can synchronize with the target cell in advance through the early synchronization procedure. The downtime that occurs in the procedure for handover from the source cell to the target cell can be significantly reduced through the early synchronization procedure performed in the LTM procedure.

[0084] In addition, in the case of the L3 handover procedure, a handover command message instructing a handover from a source cell to a target cell is generated in the RRC layer based on an L3 measurement report, so a terminal receiving the handover command message must perform L3 layer processing. On the other hand, in the LTM procedure, a handover command message instructing a handover from a source cell to a target cell is generated in the MAC layer based on an L1 measurement report, so a terminal receiving the handover command message does not need to perform L3 layer processing, which has an advantage over the L3 handover procedure in that the downtime can be further reduced.

[0085] Hereinafter, in order to distinguish from the L3 handover procedure in the present disclosure, the mobility management method that can significantly reduce the time required for the downlink synchronization procedure and the uplink synchronization procedure may be referred to as the LTM procedure, the L1 handover procedure, or various expressions that can be interpreted identically or similarly thereto. In addition, in order to distinguish from the L3 handover that changes the serving cell through the L3 handover procedure, the operation of changing the serving cell through the LTM procedure may be referred to as a handover based on the LTM procedure, or various expressions that can be interpreted identically or similarly thereto.

[0086]

[0087] Below, the LTM procedure is described in detail with reference to Fig. 6.

[0088] FIG. 6 is a diagram illustrating an example of an LTM procedure according to various embodiments of the present disclosure.

[0089] Referring to FIG. 6, an LTM preparation procedure may be performed (610). More specifically, the LTM preparation procedure (610) may be described as follows: a terminal in an RRC connection state may perform L3 measurements on SSB or CSI-RS for LTM candidate target cells(s) to a gNB (source cell) and transmit an L3 measurement result report message. At this time, the gNB (source cell) may configure an LTM procedure and determine the start of LTM preparation. Next, the gNB (source cell) may determine the target cell(s) reported through the L3 measurement results as LTM candidate cell(s) and transmit an RRC reconfiguration message including configuration(s) for the LTM candidate cell(s) to the terminal. The configuration(s) for LTM candidate cell(s) are not applied immediately when the UE receives the configuration(s), but are applied later when the UE receives a cell switch command from the gNB (source cell) to the LTM candidate cell(s), and this can be defined as an LTM candidate configuration. The configuration(s) for the LTM candidate cell(s) may include a CSI reporting configuration for L1 measurement reporting for the target cell. Here, the LTM candidate cell(s) may refer to cells other than the source cell that the UE can select as a new serving cell. The configurations for the LTM candidate cells that the gNB (source cell) transmits to the UE may be those received by the gNB (source cell) from the LTM candidate cells, but are not limited to the above example. The gNB (source cell) may select different candidate cells for each UE. The terminal may store the configuration(s) for the LTM candidate cell(s) and send an RRC reconfiguration complete message to the gNB (source cell).

[0090] Next, an early synchronization procedure may be performed (620). More specifically, the early synchronization procedure (620) may be a procedure for performing downlink / uplink synchronization (DL sync / UL sync) with candidate cells prior to receiving a cell change command. More specifically, downlink synchronization may be obtained by the UE receiving the SSB of the target cell prior to receiving the cell change command (cell switch), and uplink synchronization may be obtained by the UE pre-acquiring the TA value from the target cell through PRACH transmission triggered by a PDCCH order prior to receiving the cell change command, or may be updated UE-based. In this case, the candidate target cell does not transmit an RAR for the PRACH transmitted for the early TA operation. The cell change command may mean a command that instructs the UE to select a specific LTM candidate cell as a new serving cell based on the LTM procedure and to perform a handover operation to connect to the new serving cell.

[0091] In other words, the UE can perform downlink synchronization with the LTM candidate cell(s) before the UE receives a cell switch command from the gNB, and can perform uplink synchronization with the LTM candidate cell(s). If UE-based TA (timing advance) measurement is configured, the UE can obtain the TA value of the LTM candidate cell(s) through the measurement. Before the UE receives the cell switch command from the gNB, the UE can perform early TA acquisition with the LTM candidate cell(s) upon request from the network. The early TA acquisition can be performed through contention-free random access (CFRA) triggered by a physical downlink control channel (PDCCH) order from a source cell, and then the UE can transmit a random access preamble for the indicated candidate cell. To minimize data interruption of the source cell due to CFRA for candidate cells, the UE does not receive a random access response (RAR) for the purpose of acquiring a TA value, and the TA value of the LTM candidate cell may be indicated by a cell change command. Furthermore, the UE does not maintain a TA timer for the candidate cell, and TA validity guarantee may depend on the network implementation.

[0092] Afterwards, the LTM execution procedure may be performed (630). More specifically, the LTM execution procedure (630) may include the terminal performing L1 measurements on the configured candidate cell(s) and transmitting L1 measurement report(s) to the gNB (source cell). At this time, the L1 measurement report(s) may be performed through periodic CSI reports. The L1 measurements may be performed while the RRC reconfiguration received by the terminal in the LTM preparation procedure (610) is applicable.

[0093] Afterwards, the CU (centralized unit) of the base station can decide to perform a cell switch / handover from the source cell to the target cell, and can transmit a MAC (medium access control) CE (control element) including a candidate configuration index of the target cell to trigger the cell switch to the terminal. In addition, the MAC CE may include a TA value acquired through an early sync procedure. The terminal can perform a switch from the source cell to the target cell and apply the configuration indicated by the candidate configuration index. At this time, if the terminal does not have a valid TA of the target cell, the terminal can perform a random access procedure for the target cell.

[0094] Next, an LTM completion procedure may be performed (640). More specifically, the LTM completion procedure (640) may be described as follows: the terminal may complete the LTM cell change procedure by transmitting an RRC reconfiguration completion message to the target cell. At this time, if the terminal performs a random access procedure for the target cell in the LTM execution procedure (630), when the random access procedure is successfully completed, the terminal may determine that the LTM cell change execution has been successfully completed. Alternatively, in the case of an LTM procedure in which a random access procedure is not performed (RACH-less LTM), when the terminal determines that the network has successfully received the first UL data to the terminal's target cell, the terminal may determine that the LTM cell change execution has been successfully completed.

[0095] As described in FIG. 6, through the Early sync operation, the UE does not need to perform a separate random access procedure with the target cell to obtain timing advance (TA) information for the target cell after receiving a cell change command from the source cell, and the UE can obtain TA information before receiving the cell change command. Methods for obtaining TA information in advance can be divided into gNB-based and UE-based methods.

[0096] Figure 7 is a flowchart illustrating an example of an operation for a terminal to acquire a TA for a target cell before receiving a cell change command. More specifically, Figure 7 relates to a gNB-based pre-TA acquisition method.

[0097] Although not shown in FIG. 7, first, the target cell (or target gNB, or target DU) (705) can transmit information for contention free PRACH transmission of the terminal (701) to the source cell (or source gNB, or source DU) (703).

[0098] Next, the source cell (or source gNB, or source DU) (703) may transmit a PDDCH command to the terminal (701) to trigger PRACH transmission of the terminal (701) (710). At this time, the PDCCH order (DCI 1_0) used to trigger PRACH transmission of the terminal (701) may be configured as shown in Table 2 below.

[0099]

[0100] Thereafter, the terminal (701) can perform PRACH transmission to the target cell (or target gNB, or target DU) (705) (720). At this time, the target cell (or target gNB, or target DU) (705) can obtain a TA (timing advance) for uplink synchronization based on the PRACH transmission received from the terminal (701).

[0101] Thereafter, the target cell (or target gNB, or target DU) (705) can transmit a DU-CU TA INFORMATION TRANSFER message containing information about the acquired TA to the CU (707) (730).

[0102] Next, the CU (707) can transmit information about the TA received from the target cell (or target gNB, or target DU) (705) to the source cell (or source gNB, or source DU) (703) via a CU-DU TA INFORMATION TRANSFER message (740). Here, the CU-DU TA INFORMATION TRANSFER message can be configured as shown in Table 3 below.

[0103]

[0104] There may be two problems with the aforementioned PRACH-based early synchronization operation.

[0105] First, since the PRACH transmission of a terminal triggered by a PDCCH command is performed in a broadcast manner, the PRACH transmission of the terminal may be received not only by a specific target candidate cell that is the target of PRACH-based early synchronization, but also by other candidate cells. At this time, the target cells that receive the PRACH transmission transmit a random access response (RAR) in response thereto, so not only is the RAR transmission performed by the specific target candidate cell that is the target of PRACH-based early synchronization, but other candidate cells may also unnecessarily perform RAR transmission.

[0106] FIG. 8 and FIG. 9 are diagrams showing examples of situations in which unnecessary RAR transmission occurs as PRACH transmission is performed in a broadcast manner.

[0107] First, referring to FIG. 8, the serving DU (801) (source DU) can trigger the terminal's PRACH transmission (821) for candidate target DU 1 (803), the terminal's PRACH transmission (823) for candidate target DU 2 (805), and the terminal's PRACH transmission (825) for candidate target DU 3 (807) through the PDCCH command (810). A terminal that receives a PDCCH command (810) from a serving DU (801) (source DU) performs PRACH transmissions (821, 823, 825) for each of candidate target DU 1 (803), candidate target DU 2 (805), and candidate target DU 3 (807). At this time, PRACH transmissions (821, 823, 825) for each of candidate target DU 1 (803), candidate target DU 2 (805), and candidate target DU 3 (807) may be performed in a broadcast manner.

[0108] Referring to FIG. 9, a problem is illustrated from the perspective of a PRACH transmission (921) transmitted by a terminal in a broadcast manner to candidate target DU 1 (903). Although the PRACH transmission is intended for candidate target DU 1 (903), because it is transmitted in a broadcast manner, the PRACH transmission is received not only by candidate target DU 1 (903) but also by candidate target DU 2 (905) (923) and can also be received by candidate target DU 3 (907) (925). At this time, for the PRACH transmission (921) with candidate target DU 1 (903) in mind, candidate target DU 1 (903) can recognize that the PRACH transmission is for UL early synchronization (hereinafter, may be referred to as early TA, etc.) according to the LTM standard, so candidate target DU 1 (903) does not transmit RAR for the received PRACH, but candidate target DUs 2 and 3 (905, 907) may transmit RAR according to the conventional random access procedure. At this time, since the PRACH transmission transmitted by the UE was a PRACH transmission for early TA operation, the UE does not need to receive RAR, and such unnecessary garbage RAR transmission causes waste of UL / DL resources in the candidate target cell.

[0109] Among the two problems in the aforementioned PRACH-based early synchronization operation, the second problem is that there may be a shortage of available preamble resources due to the operation that requires a random access preamble for a certain period of time for contention-free purposes in order to perform the PRACH-based early synchronization operation. This shortage of available preamble resources may result in a limitation on the number of LTM UEs that can simultaneously support early TA operation.

[0110] FIG. 10 is a diagram illustrating a problem in which the total available preamble resources become insufficient as non-contention-purpose preambles for PRACH-based early synchronization operations are occupied.

[0111] Referring to FIG. 10, the total number of preambles available for random access procedures for various purposes is 64, but excluding the preamble (1010) that can be used in the contention-based random access procedure for initial access and the preamble (1020) that can be used in the non-contention-based random access procedure for legacy handover operations, the number of available preambles for early TA becomes very small. In addition, in an actual system environment, since the PRACH transmission-related parameters and SSB resource operation methods are often the same in all cells, the available preamble resources may become even more insufficient.

[0112] Below, before describing methods for solving two problems in the aforementioned PRACH-based early synchronization operation, to help understand the methods described in the present disclosure, the LTM procedure from the L3 measurement report operation for the intra CU & inter DU structure to the final cell change command transmission process from the F1AP perspective is first described with reference to FIG. 11. The F1AP can provide signaling services between the gNB-CU and the gNB-DU.

[0113] Figure 11 is a flowchart showing an example of how the LTM procedure is performed from the F1AP perspective.

[0114] First, the UE (1101) transmits an L3 measurement report message including measurement results of neighboring cells to the source gNB-DU (1103), and the source gNB-DU (1103) can transmit an UL RRC MESSAGE TRANSFER message conveying the received L3 measurement report message to the gNB-CU (1107) (1110).

[0115] Next, the gNB-CU (1107) can determine configuration information for LTM operation (1115).

[0116] Thereafter, the gNB-CU (1107) transmits (1120) a UE CONTEXT SETUP REQUEST message to the candidate gNB-DU(s) (1105), and the UE CONTEXT SETUP REQUEST message may include one target candidate cell ID, an LTM configuration ID of the target candidate cell, an LTM configuration ID mapping list, and a CSI resource configuration. The gNB-CU (1107) may indicate a source gNB-DU ID and request PRACH resources from the candidate gNB-DU (1105). In addition, the gNB-CU (1107) may request the candidate gNB-DU (1105) to provide lower layer configuration for the purpose of generating a reference configuration.

[0117] Next, if the candidate gNB-DU (1105) accepts the LTM setup request, the candidate gNB-DU (1105) may respond with a UE CONTEXT SETUP RESPONSE message including lower layer RRC settings (e.g., TCI state settings, RACH settings, and CSI reporting settings, etc.) generated for the accepted target candidate cell (1125).

[0118] Thereafter, the gNB-CU (1107) may transmit a UE CONTEXT MODIFICATION REQUEST message including the collected CSI reporting settings, RACH settings, and TCI state settings for the accepted target candidate cell(s) to the source gNB-DU (1103) (1130).

[0119] Next, the source gNB-DU (1103) may respond to the gNB-CU (1107) with a UE CONTEXT MODIFICATION RESPONSE message containing updated lower layer settings, e.g., CSI reporting settings (1135).

[0120] Next, the gNB-CU (1107) may transmit a UE CONTEXT MODIFICATION REQUEST message to the candidate gNB-DU (s) (1140) that includes the CSI reporting configuration, TCI state information, RACH configuration, and the candidate cell LTM configuration ID of other candidate gNB-DU (s). In addition, the gNB-CU (1107) may provide the lower layer portion of the reference configuration to the candidate gNB-DU (s) and may provide the updated CSI resource configuration to the candidate gNB-DU (s).

[0121] Next, the candidate gNB-DU (1105) may respond to the gNB-CU (1107) with a UE CONTEXT MODIFICATION RESPONSE message containing the updated lower layer configuration (1145). Additionally, the candidate gNB-DU (1105) may also respond to the gNB-CU (1107) with the updated CSI reporting configuration.

[0122] Thereafter, the gNB-CU (1107) transmits (1150) a DL RRC MESSAGE TRANSFER message to the source gNB-DU (1103), and the DL RRC MESSAGE TRANSFER message may include an RRC reconfiguration message generated together with the LTM configuration.

[0123] Next, the source gNB-DU (1103) forwards the received RRC reconfiguration message to the UE (1101) (1155).

[0124] Afterwards, the UE (1101) responds to the source gNB-DU (1103) with an RRC Reconfiguration Complete message (1160).

[0125] Thereafter, the source gNB-DU (1103) transmits the RRCReconfiguration Complete message received from the UE (1101) to the gNB-CU (1107) via the UL RRC MESSAGE TRANSFER message (1165).

[0126] Afterwards, an early synchronization procedure between the UE (1101) and the candidate gNB-DU (1105) may be performed (1170).

[0127] Next, the candidate gNB-DU (1105) transmits the TA value, related CFRA resource information, candidate cell ID, and source gNB-DU ID to the source gNB-DU (1103) via the DU-CU TA INFORMATION TRANSFER and CU-DU TA INFORMATION TRANSFER messages (1175, 1180). At this time, the source gNB-DU ID may be omitted from the CU-DU TA INFORMATION TRANSFER message.

[0128] Thereafter, the UE (1101) may transmit an L1 measurement report for the candidate gNB-DU (1105) to the source gNB-DU (1103) (1185).

[0129] Next, the source gNB-DU (1103) may decide to execute LTM for a candidate target cell (1190).

[0130] Finally, if LTM execution for a candidate target cell is decided, the source gNB-DU (1103) may transmit a cell change command to the UE (1101) (1195).

[0131] Below, we describe solutions to two problems that may arise during PRACH-based early synchronization operations.

[0132] First, in order to solve the problem that unnecessary RAR transmission occurs and RAR resources are wasted because the PRACH transmission of the terminal is received not only by a specific target candidate cell that is the target of PRACH-based early synchronization but also by other candidate cells, the CU can transmit to the candidate target DU information about preamble index(es) for which RAR transmission should not be performed even if the candidate target DU receives a preamble. The information about the preamble index(es) for which RAR transmission should not be performed even if the candidate target DU receives a preamble can be configured in the form of a preamble index set including at least one preamble index. The candidate target DU that has received the information about the preamble index(es) for which RAR transmission should not be performed even if the candidate target DU receives a preamble can determine, after receiving the preamble, whether the received preamble matches any one of the preamble index(es) included in the information about the preamble index(es). At this time, if the received preamble matches any one of the preamble index(es) included in the information about the preamble index(es), RAR may not be scheduled for the received preamble.

[0133] Information about the preamble index(es) for which the candidate target DU receiving the preamble does not transmit the RAR for the preamble can be constructed based on the following methods.

[0134] First, as a method of setting individual preamble indices, information about preamble indices(s) for which a candidate target DU that has received a preamble does not transmit an RAR for the preamble can be configured according to a method of mapping an SSB index and a preamble index 1:1, or according to a method of mapping a CSI-RS resource index and a preamble index 1:1.

[0135] If the information composition method according to the first method is expressed as an information element (IE) that can be used in the F1AP standard, it can be composed as shown in Tables 4 and 5 below.

[0136] More specifically, Table 4 shows an example of how to map a preamble index and an SSB index, and Table 4 shows an example of how to map a preamble index and a CSI-RS resource index.

[0137]

[0138]

[0139] At this time, the total available number of CSI-RS can be defined by the 3GPP TS 38.331 specification.

[0140] As a second method of configuring information about preamble index(es) for which a candidate target DU that has received a preamble does not transmit an RAR for the preamble, a preamble index range can be set, and at this time, information can be configured in a manner of mapping each set range 1:1 to an ID (identifier) ​​for distinguishing the range.

[0141] The information configuration method according to the second method can be expressed in IE that can be used in the F1AP standard as shown in Table 6 below.

[0142]

[0143] A third way to configure information about preamble index(es) for which a candidate target DU that has received a preamble does not transmit RAR for the preamble is to configure available or unavailable preamble indexes, and in this case, information in the form of a bitmap can be configured to indicate available or unavailable preamble indexes.

[0144] The information configuration method according to the second method can be expressed in IE that can be used in the F1AP standard as shown in Table 7 below.

[0145]

[0146] More specifically, in the bitmap, the MSB or LSB can indicate the lowest preamble index or the highest preamble index, and the value of each bit position in the bitmap can indicate an available preamble index or an unavailable preamble index according to a predefined definition. For example, the bitmap can be configured to be composed of a 64-bit bit string, and the MSB can correspond to the lowest preamble index, and the LSB can correspond to sequentially higher preamble indices, respectively. At this time, if the value of each bit position in the bitmap represents 1, it can be predefined to indicate an available preamble index. That is, a candidate target DU that has received a preamble based on an available preamble index can transmit an RAR for the preamble. Conversely, if the value of each bit position in the bitmap represents 1, it can be predefined to indicate an unavailable preamble index. That is, based on an unavailable preamble index, A candidate target DU that has received a preamble can transmit an RAR for that preamble.

[0147] As another example, the bitmap may be configured as a 64-bit bit string, with the MSB corresponding to the highest preamble index and the LSB corresponding to sequentially lower preamble indices. In this case, if the value of each bit position in the bitmap represents 1, it may be predefined as indicating an available preamble index. That is, a candidate target DU that has received a preamble based on an available preamble index can transmit an RAR for the corresponding preamble. Conversely, if the value of each bit position in the bitmap represents 1, it may be predefined as indicating an unavailable preamble index. That is, a candidate target DU that has received a preamble based on an unavailable preamble index can transmit an RAR for the corresponding preamble.

[0148] A message for transmitting information about preamble index(es) for which a candidate target DU that has received the preamble described above does not transmit an RAR for the preamble may be structured as follows.

[0149] A cell-specific configuration message may be utilized as the first type of message to transmit information about the preamble index(es) for which the candidate target DU receiving the preamble does not transmit an RAR for the corresponding preamble. The GNB-CU CONFIGURATION UPDATE message defined in the specification may be used and may be transmitted from the gNB-CU to the gNB-DU.

[0150] As a second form of a message for transmitting information about preamble index(es) for which a candidate target DU that has received a preamble does not transmit an RAR for the preamble, a UE specific configuration message may be utilized. For example, a UE CONTEXT SETUP REQUEST message (1120 of FIG. 11), a UE CONTEXT MODIFICATION REQUEST message (1140 of FIG. 11), etc. may be utilized, and may be transmitted from a gNB-CU to a gNB-DU.

[0151] A third type of message, not defined in the specification, may be defined and utilized to transmit information about preamble index(es) for which the candidate target DU receiving the preamble does not transmit an RAR for that preamble. The message may be named, for example, "CU-DU NO RAR PREAMBLE INFORMATION TRANSFER" and may be transmitted from a gNB-CU to a gNB-DU.

[0152] A fourth type of message, intended to convey information about preamble indices for which a candidate target DU receiving a preamble does not transmit an RAR for that preamble, may utilize the E2 interface rather than the F1AP. The E2 interface may be an interface formed between the DU and a nearby real-time RAN intelligent controller (RT RIC).

[0153] Next, we describe a method for solving the problem of insufficient available preamble resources due to the requirement that a random access preamble for contention-free purposes be occupied for a certain period of time for PRACH-based early synchronization operation.

[0154] In order to efficiently use limited preamble resources, the same preamble index can be assigned to multiple UEs, and the same preamble index can be reused by multiple UEs. Methods for reusing limited radio resources can be broadly categorized into methods for reusing in the time domain, frequency domain, code domain, and spatial domain. However, in the case of the system model according to the embodiment of the present disclosure, since it is assumed that the PRACH transmission-related parameters and the SSB resource operation method are the same in all cells, the radio resource reuse methods that can be considered in the present disclosure can only include a method for reusing radio resources in the time domain.

[0155] FIG. 12 is a diagram illustrating an example of a method for efficiently using limited preamble resources according to one embodiment of the present disclosure.

[0156] More specifically, FIG. 12 relates to a case where a preamble index is reused on the time axis. Referring to FIG. 12, the PRACH transmission period set to all UEs is the same, but the DU can set the offset that triggers the PDCCH command to different values ​​for UEs using the same resource (same preamble index). According to FIG. 12, since the PRACH transmission of each of the UEs (1210, 1220, 1230) using the same preamble index is performed at a period of 10 ms and the same preamble index is reused by four UEs, the preamble reuse period can be 40 ms.

[0157] UE Group 1 (1210) = UE {A / B / C / D} uses the same preamble index 10, UE Group 2 (1220) = UE {E / F / G / H} uses the same preamble index 11, and UE Group 3 (1230) = UE {I / J / K / L} uses the same preamble index 12. UEs within the same group do not trigger PDCCH commands at the same time. On the other hand, PDCCH commands can be triggered at the same time between UEs belonging to different UE Groups. Since the preamble indices used for preamble transmission are different between UEs belonging to different UE Groups, the target cell can distinguish and receive PRACH signals transmitted by each UE included in each different UE Group.

[0158] In Fig. 12, a case is exemplified where four UEs are included in one UE Group that reuses the same preamble index, but the number of UEs included in one UE Group may be various different numbers, and of course, the preamble transmission cycle and reuse cycle may be set to various values ​​depending on the number of UEs included in the UE Group.

[0159] The TA value obtained by the target cell using the preamble index reused by UEs belonging to the same UE Group is transferred (740) to the source cell (DU / CU) through the procedure illustrated in FIG. 7. The DU-CU TA INFORMATION TRANSFER (730) or CU-DU TA INFORMATION TRANSFER (740) message used at this time can be configured as shown in Table 8 below.

[0160]

[0161] The only way to distinguish whether the TA value received by the source cell (of the DU / CU) is the TA value obtained from the PRACH transmitted by a UE within the UE group where the same preamble index is reused is through the message of Table 8 for transmitting the TA obtained from the target cell to the source cell (of the DU / CU). However, according to the way the RA-RNTI is calculated, if the same preamble index is reused between different UEs, it may be impossible for the source cell (of the DU / CU) to distinguish between different UEs that transmitted preambles using the same preamble index.

[0162] RA-RNTI can be calculated according to the mathematical formula below.

[0163] [Mathematical Formula 1]

[0164] 1 + s_id + 14 Х t_id + 14 Х 80 Х f_id + 14 Х 80 Х 8 Х ul_carrier_id

[0165] s_id: the index of the first OFDM symbol of the specified PRACH (0 <= s_id < 14)

[0166] t_id: the index of the first slot symbol of the specified PRACH in a system frame (0 <= t_id < 80)

[0167] s_id: the index of the specified PRACH in the frequency domain (0 <= s_id < 8)

[0168] ul_carrier_id: UL carrier used for Msg1 transmission (0 = normal carrier, 1 = SUL carrier)

[0169] In the above mathematical formula, s_id, t_id, and ul_carrier_id are parameters that are set to the same values ​​for all UEs. Meanwhile, in the case of t_id, t_id represents a slot index within a system frame, and if the set PRACH period is set to a multiple of the system frame unit (= 10 ms), the same value of t_id is used for all UEs. Therefore, according to the RA-RNTI calculation method as above, there may be cases where it is impossible to distinguish UEs that reuse the same preamble index.

[0170] Accordingly, according to one embodiment of the present disclosure, an information element (IE) may be added to a DU-CU TA INFORMATION TRANSFER or CU-DU TA INFORMATION TRANSFER message to enable distinguishing preamble resources reused by multiple UEs on the time axis.

[0171] First, as an IE to enable distinguishing preamble resources reused by multiple UEs on the time axis in the DU-CU TA INFORMATION TRANSFER or CU-DU TA INFORMATION TRANSFER message, a system frame number (SFN) in ms is added so that preamble resources reused by multiple UEs can be distinguished. That is, even if a source cell (of a DU / CU) receives a TA value acquired based on the same preamble, it can determine which UE in the same UE Group using the same preamble transmitted the received TA value. For example, if the SFN has a length of 10ms and the PRACH (preamble) transmission period of UEs within the same UE Group that use the same preamble is 10ms, if the SFN when a specific UE first performs a PRACH transmission within the UE Group is #n, the SFN when the next UE that performs a PRACH transmission performs a PRACH transmission will be #n+1, the SFN when the next UE that performs a PRACH transmission performs a PRACH transmission will be #n+2, etc., so that since the SFN related to the time point of PRACH transmission is included together with the TA value in the DU-CU TA INFORMATION TRANSFER or CU-DU TA INFORMATION TRANSFER message, even if the source cell (of the DU / CU) receives the TA value acquired based on the same preamble, the source cell (of the DU / CU) receives the TA value acquired based on the preamble transmitted by any UE within the same UE Group that uses the same preamble. You can determine whether it is a value or not

[0172] Second, as an IE to enable distinguishing preamble resources reused by multiple UEs on the time axis in the DU-CU TA INFORMATION TRANSFER or CU-DU TA INFORMATION TRANSFER message, information on the result value (SFN mod periodicity) obtained by modulating the SFN (system frame number) in ms by the preamble reuse period value is added, thereby enabling distinguishing preamble resources reused by multiple UEs. That is, even if a source cell (of a DU / CU) receives a TA value acquired based on the same preamble, it can determine which UE in the same UE Group using the same preamble transmitted the received TA value.For example, if the SFN has a length of 10ms and the PRACH (preamble) reuse period of UEs in the same UE Group that use the same preamble is 40ms, and if the SFN when a specific UE in the UE Group first performs a PRACH transmission is #0, the value obtained by modulating the SFN #0 with a reuse period of 40ms is 0, the SFN when the UE performing the next PRACH transmission performs a PRACH transmission will be #1, and the value obtained by modulating the SFN #1 with a reuse period of 40ms is 1, the SFN when the UE performing the next PRACH transmission performs a PRACH transmission will be #2, and the value obtained by modulating the SFN #2 with a reuse period of 40ms is 2, etc., therefore, the value obtained by modulating the SFN related to the time point of PRACH transmission in the DU-CU TA INFORMATION TRANSFER or CU-DU TA INFORMATION TRANSFER message with a reuse period value is TA. By including the TA value with the value, even if the source cell (of the DU / CU) receives the TA value obtained based on the same preamble, the source cell (of the DU / CU) can determine which UE within the same UE Group using the same preamble transmitted the TA value obtained based on the preamble.

[0173] Thirdly, a C-RNTI value assigned to a UE may be added as an IE to enable distinguishing preamble resources reused by multiple UEs on the time axis in a DU-CU TA INFORMATION TRANSFER or CU-DU TA INFORMATION TRANSFER message. Since the C-RNTI value assigned to the UE is included together with the TA value in the DU-CU TA INFORMATION TRANSFER or CU-DU TA INFORMATION TRANSFER message, even if the source cell (of the DU / CU) receives a TA value obtained based on the same preamble, the source cell (of the DU / CU) can determine which UE in the same UE Group that uses the same preamble transmitted the TA value. In particular, a PRACH setting for each C-RNTI may be determined in the UE CONTEXT SETUP REQUEST / RESPONSE process (1120, 1125) of FIG. 11.

[0174] Fourth, as an IE for distinguishing preamble resources reused by multiple UEs on the time axis in the DU-CU TA INFORMATION TRANSFER or CU-DU TA INFORMATION TRANSFER message, information (msg1-FrequencyStart) indicating a starting position on the frequency domain of frequency resources occupied by the UE to transmit the preamble may be added. Since the information indicating the starting position on the frequency domain of frequency resources occupied by the UE to transmit the preamble is included together with the TA value in the DU-CU TA INFORMATION TRANSFER or CU-DU TA INFORMATION TRANSFER message, even if the source cell (DU / CU of the source cell) receives a TA value acquired based on the same preamble, the source cell (DU / CU of the source cell) can determine which UE in the same UE Group that uses the same preamble transmitted the TA value. In particular, this IE can be conditionally used in cases where different msg1-FrequencyStarts are used for each SFN mod periodicity (e.g., where the PRACH transmission RB positions on the frequency axis are different).

[0175] FIG. 13 is a diagram for explaining the effect of performing an LTM procedure based on a preamble resource reuse method according to one embodiment of the present disclosure.

[0176] Referring to FIG. 13, from a C-plan perspective, the time (1310) during which a preamble index (resource) is reserved for a specific UE in a conventional LTM procedure is relatively long compared to the time (1320) during which the preamble index (resource) is actually used, so even if the LTM procedure is performed based on a preamble resource reuse method, the available preamble resources may not increase significantly.

[0177] Considering that even if a method of reusing preamble resources such as the above is used, the available preamble resources may not be increased significantly, another method of the present disclosure is described to solve the problem of insufficient available preamble resources due to the random access preamble for contention free purposes having to be occupied for a certain period of time for PRACH-based early synchronization operation.

[0178] If a source DU does not simultaneously trigger a PDCCH command to two or more UEs having the same preamble index at a specific RACH Occasion (RO), the source DU may transmit a message to the target DU and the CU, indicating whether to transmit a preamble (PRACH transmission). Here, the message indicating whether to transmit a preamble from the perspective of the target DU may be a message that allows the target DU to identify the UE that transmitted the preamble when the target DU receives a preamble for early sync from the UE. In addition, the message indicating whether to transmit a preamble from the perspective of the CU may be a message that allows the CU to identify the UE that transmitted the preamble used to acquire the TA value when the CU receives information about the TA value acquired by the target DU from the target DU.

[0179] Therefore, by defining a new message of a notification nature so that the source DU can transmit a message to the target DU and CU to inform them of whether to transmit a preamble (PRACH transmission), the limited preamble resources can be used more efficiently.

[0180] FIG. 14 is a flowchart illustrating an example of an operation in which a source DU transmits a message that can inform a target DU and CU of whether to transmit a preamble (PRACH transmission) according to one embodiment of the present disclosure.

[0181] Although not shown in FIG. 14, first, the target cell (or target gNB, or target DU) (1405) may transmit information for contention free PRACH transmission of the terminal (1401) to the source cell (or source gNB, or source DU) (1403).

[0182] Next, the source cell (or source gNB, or source DU) (1403) may transmit a PDDCH command to the terminal (1401) to trigger PRACH transmission of the terminal (1401) (1410).

[0183] Thereafter, the source cell (or source gNB, or source DU) (1403) can immediately notify the CU and target DU that the PDCCH command for early synchronization has been triggered by transmitting a message indicating whether to transmit a preamble (PRACH) to the UE to perform early synchronization (1415). At this time, the message indicating whether to transmit a preamble (PRACH) can be configured as shown in Table 9 below.

[0184]

[0185] As shown in Table 9 above, the structure of this message is basically similar to the DU-CU TA INFORMATION TRANSFER or CU-DU TA INFORMATION TRANSFER exchanged in steps 1425 and 1435, but an identifier IE may be added to this message to distinguish UEs using the same preamble index. The identifier ID added to this newly defined message may be the same IE as the IE added to the DU-CU TA INFORMATION TRANSFER or CU-DU TA INFORMATION TRANSFER message to distinguish preamble resources reused by multiple UEs on the time axis as described above.

[0186] Thereafter, the terminal (1401) can perform PRACH transmission to the target cell (or target gNB, or target DU) (1405) (1420). At this time, the target cell (or target gNB, or target DU) (1405) can obtain a TA (timing advance) for uplink synchronization based on the PRACH transmission received from the terminal (1401).

[0187] Next, the target cell (or target gNB, or target DU) (1405) can transmit a DU-CU TA INFORMATION TRANSFER message containing information about the acquired TA to the CU (1407) (1425). At this time, the CU can distinguish which UE the TA value received from the target DU belongs to through the identifier IE received in operation 1415, map the TA value and PRACH information (1430), and transfer the TA value to the source DU (1403) through the CU-DU TA TRANSFER message (1435).

[0188] Finally, the source DU (1403) may transmit an LTM cell change command (MAC CE) to the UE (1401) (1440).

[0189] In the case of the present disclosure, which distinguishes UEs through IEs added to existing DU-CU TA INFORMATION TRANSFER or CU-DU TA INFORMATION TRANSFER messages, the preamble can be reused only as many times as the number of rounded down quotients of the preamble reuse period divided by the actual PRACH transmission period. On the other hand, in the case of the method, since identifier information for distinguishing UEs can be transmitted through a newly defined message immediately after the PDCCH command, there can be no limitation on the number of UEs that can reuse the preamble index.

[0190] FIG. 15 is a flowchart illustrating an example of a method of operating a DU (distributed unit) according to various embodiments of the present disclosure.

[0191] First, the DU can receive information about a preamble index for which a random access response message is not transmitted from a CU (centralized unit) (1510).

[0192] Next, the DU can receive a preamble related to early synchronization with a candidate target DU of the terminal from the terminal (1520).

[0193] Next, the DU can determine whether the preamble index of the received preamble matches a preamble index to which the random access response message is not transmitted (1530).

[0194] Here, if the preamble index of the received preamble matches a preamble index for which the random access response message is not transmitted, the random access response message for the received preamble may not be transmitted.

[0195] FIG. 16 is a flowchart illustrating an example of a method of operating a CU (centralized unit) according to various embodiments of the present disclosure.

[0196] First, the CU can transmit information about a preamble index to which a random access response message is not transmitted as a DU (distributed unit) (1610).

[0197] Next, the CU can receive, from the DU, information about a TA (timing advance) obtained based on a preamble based on a preamble index that does not match the preamble index to which the random access response message is not transmitted (1620).

[0198] The methods according to the embodiments described in the claims or detailed description of the present disclosure may be implemented in the form of hardware, software, or a combination of hardware and software.

[0199] When implemented in software, a computer-readable storage medium storing one or more programs (software modules) may be provided. The one or more programs stored in the computer-readable storage medium are configured for execution by one or more processors within an electronic device. The one or more programs include instructions that cause the electronic device to execute methods according to embodiments described in the claims or specification of the present disclosure.

[0200] These programs (software modules, software) may be stored in random access memory, non-volatile memory including flash memory, read only memory (ROM), electrically erasable programmable read only memory (EEPROM), magnetic disc storage devices, compact disc-ROMs (CD-ROMs), digital versatile discs (DVDs) or other forms of optical storage devices, magnetic cassettes, or may be stored in memories formed by a combination of some or all of these. In addition, each configuration memory may include multiple copies.

[0201] Additionally, the program may be stored on an attachable storage device that is accessible via a communication network, such as the Internet, an intranet, a local area network (LAN), a wide area network (WAN), a storage area network (SAN), or a combination thereof. Such a storage device may be connected to a device implementing an embodiment of the present disclosure via an external port. Additionally, a separate storage device on the communication network may be connected to a device implementing an embodiment of the present disclosure.

[0202] In the specific embodiments of the present disclosure described above, components included in the disclosure are expressed singularly or plurally, depending on the specific embodiment presented. However, the singular or plural expressions are selected to suit the presented situation for convenience of explanation, and the present disclosure is not limited to singular or plural components. Components expressed in plural may be composed of singular elements, or components expressed in singular may be composed of plural elements.

[0203] While the detailed description of this disclosure has described specific embodiments, it should be understood that various modifications are possible without departing from the scope of this disclosure. Therefore, the scope of this disclosure should not be limited to the described embodiments, but should be defined not only by the scope of the claims described below, but also by equivalents thereof.

Claims

1. In a method performed by a DU (distributed unit) in a wireless communication system, the method comprises: A step of receiving information about a preamble index to which a random access response message is not transmitted from a CU (centralized unit); A step of receiving a preamble related to early synchronization with a candidate target DU of the terminal from the terminal; and Including a step of determining whether the preamble index of the received preamble matches the preamble index to which the random access response message is not transmitted, A method in which the random access response message for the received preamble is not transmitted when the preamble index of the received preamble matches a preamble index for which the random access response message is not transmitted.

2. In paragraph 1, A method in which information about the above preamble index is configured based on a one-to-one mapping relationship between the preamble index and a SSB (Synchronization Signal Block) or CSI-RS (channel state information reference signal) resource.

3. In paragraph 1, Information about the above preamble index is composed of a bitmap with a length equal to the total number of preamble indexes, A method in which each bit of the above bitmap is mapped to a different preamble index.

4. In paragraph 1, A method wherein the above preamble index is reused for preamble transmission at different times of at least one different terminal including the terminal.

5. In the fourth paragraph, if the preamble index of the received preamble does not match the preamble index to which the random access response message is not transmitted, the method, A step of receiving an identification message from a source DU, the identification message including information for identifying at least one different terminal; The above identification message is received before the preamble transmission of the terminal is performed after the preamble transmission of the terminal is triggered by the source DU; and A method further comprising the step of transmitting information about a timing advance (TA) obtained based on the preamble to the CU.

6. In a method performed by a CU (centralized unit) in a wireless communication system, the method comprises: A step of transmitting information about a preamble index to which a random access response message is not transmitted, as a DU (distributed unit); and A method comprising the step of receiving, from the DU, information about a TA (timing advance) obtained based on a preamble based on a preamble index that does not match the preamble index to which the random access response message is not transmitted.

7. In paragraph 6, A method in which information about the above preamble index is configured based on a one-to-one mapping relationship between the preamble index and a SSB (Synchronization Signal Block) or CSI-RS (channel state information reference signal) resource.

8. In paragraph 6, Information about the above preamble index is composed of a bitmap with a length equal to the total number of preamble indexes, A method in which each bit of the above bitmap is mapped to a different preamble index.

9. In paragraph 6, A method wherein the above preamble index is reused for preamble transmission at different points in time of at least one different terminal.

10. In the 9th paragraph, the method, Further comprising the step of receiving an identification message from a source DU, wherein the identification message includes information for identifying at least one different terminal, A method wherein the identification message is received before the preamble transmission of a specific terminal among the at least one different terminal is performed after the preamble transmission of the specific terminal is triggered by the source DU.

11. In a wireless communication system, in a DU (distributed unit), the DU is: transceiver; and comprising at least one processor coupled with the transceiver; At least one processor, Receive information about the preamble index for which a random access response message is not transmitted from the CU (centralized unit), Receive a preamble related to early synchronization with a candidate target DU of the terminal from the terminal, It is configured to determine whether the preamble index of the received preamble matches the preamble index to which the random access response message is not transmitted. If the preamble index of the received preamble matches the preamble index for which the random access response message is not transmitted, the random access response message for the received preamble is not transmitted, DU.

12. In paragraph 11, Information about the above preamble index is configured based on a one-to-one mapping relationship between the preamble index and SSB (Synchronization Signal Block) or CSI-RS (channel state information reference signal) resources.

13. In paragraph 11, Information about the above preamble index is composed of a bitmap with a length equal to the total number of preamble indexes, Each bit of the above bitmap is mapped to a different preamble index, DU.

14. In paragraph 11, The above preamble index is reused for preamble transmission at different points in time of at least one different terminal including the terminal.

15. In a wireless communication system, there is a CU (centralized unit), wherein the CU, transceiver; and comprising at least one processor coupled with the transceiver; At least one processor, As a DU (distributed unit), it transmits information about the preamble index to which the random access response message is not transmitted, A CU configured to receive, from the DU, information about a TA (timing advance) obtained based on a preamble based on a preamble index that does not match the preamble index to which the random access response message is not transmitted.

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