Method and device for processing MSG A retransmission in two-step random access process in wireless communication system

Abstract

The present disclosure relates to a communication technology and a system thereof for integrating a fifth generation (5G) communication system for supporting a higher data transfer rate than a fourth generation (4G) system with IoT technology. The present disclosure can be applied to smart services based on 5G communication technology and IoT-related technologies, such as smart homes, smart buildings, smart cities, smart cars, connected cars, healthcare, digital education, smart retail, security and safety services. The present disclosure provides a method and apparatus for processing message A retransmission in a two-step random access process.

Description

Technical Field

[0001] The present disclosure relates generally to wireless communication systems, and more particularly, to an apparatus and method for processing message A (MSG A) retransmission in a two-step random access procedure in a wireless communication system. Background Art

[0002] In order to meet the requirements of increasing wireless data services since the deployment of 4G communication systems, efforts have been made to develop improved 5G or pre-5G communication systems. 5G or pre-5G communication systems are also referred to as "super 4G networks" or "post-LTE systems". Therefore, 5G communication systems are considered to be implemented in higher frequency (mmWave) bands, for example, 60GHz bands, to achieve higher data rates. In order to reduce the propagation loss of radio waves and increase the transmission distance, beamforming, massive multiple input multiple output (MIMO), full-dimensional MIMO (FD-MIMO), array antennas, analog beamforming, and large antenna technologies in 5G communication systems are discussed. In addition, in 5G communication systems, development of system network improvements based on advanced small cells, cloud radio access networks (RAN), ultra-dense networks, device-to-device (D2D) communications, wireless backhaul, mobile networks, collaborative communications, coordinated multi-point (CoMP), receiving-end interference elimination, etc. is underway. In 5G systems, hybrid FSK and FQAM modulation and sliding window superposition coding (SWSC) have been developed as advanced coded modulation (ACM), and filter bank multi-carrier (FBMC), non-orthogonal multiple access (NOMA) and sparse code multiple access (SCMA) as advanced access technologies.

[0003] The Internet is a human-centered connectivity network where humans generate and consume information, and is now evolving into the Internet of Things (IoT), in which distributed entities such as things exchange and process information without human intervention. The Internet of Everything, which combines IoT technology and big data processing technology through connection with cloud servers, has emerged. In order to realize the IoT, technical elements such as "sensing technology", "wired / wireless communication and network infrastructure", "service interface technology", and "security technology" are required, and sensor networks, machine-to-machine (M2M) communication, machine-type communication (MTC), etc. have been studied recently. This IoT environment can provide intelligent Internet technology services to create new value for human life by collecting and analyzing data generated between connected things. Through the integration and combination between existing information technology (IT) and various industrial applications, IoT can be applied to various fields, including smart homes, smart buildings, smart cities, smart cars or connected cars, smart grids, health care, smart appliances, and advanced medical services.

[0004] Therefore, various attempts have been made to apply 5G communication systems to IoT networks. For example, technologies such as sensor networks, MTC, and M2M communications can be implemented through beamforming, MIMO, and array antennas. The application of cloud RAN, which is the above-mentioned big data processing technology, can also be considered as an example of the fusion between 5G technology and IoT technology.

[0005] Recently, the next generation wireless communication systems require an enhanced two-step random access procedure. Summary of the invention

[0006] [Technical issues]

[0007] There is a need to enhance the two-step random access (or 2step RACH) procedure in next generation wireless communication systems.

[0008] [Technical solution]

[0009] One aspect of the present disclosure provides a communication method and system for integrating a 5G communication system supporting a higher data rate than a 4G communication system.

[0010] According to one aspect of the present disclosure, a method performed by a terminal in a wireless communication system is provided. The method includes: storing a media access control protocol data unit (MAC PDU) in a buffer, wherein the MAC PDU is obtained for message A (MSG A) transmission; sending MSG A including the MAC PDU to a base station; receiving a message B (MSG B) sent in response to MSG A from the base station, wherein MSG B includes fallback information; and sending an uplink transmission to the base station based on the fallback information, wherein the uplink transmission includes the MAC PDU.

[0011] According to another aspect of the present disclosure, a method for a base station in a wireless communication system is provided. The method includes: receiving MSG A including MAC PDU from a terminal, wherein the MAC PDU is obtained by the terminal for MSG A transmission and is stored in a buffer of the terminal; sending MSG B in response to MSG A to the terminal, wherein MSG B includes fallback information; and receiving uplink transmission from the terminal based on the fallback information, wherein the uplink transmission includes the MAC PDU.

[0012] According to another aspect of the present disclosure, a terminal is provided. The terminal includes: a transceiver; and a controller, the controller being configured to: store a MAC PDU in a buffer, wherein the MAC PDU is obtained for MSG A transmission; transmit MSG A including the MAC PDU to a base station via the transceiver; receive MSG B transmitted in response to MSG A from the base station via the transceiver, wherein MSG B includes fallback information; and transmit an uplink transmission to the base station via the transceiver based on the fallback information, wherein the uplink transmission includes the MAC PDU.

[0013] According to another aspect of the present disclosure, a base station is provided. The base station includes: a transceiver; and a controller, the controller being configured to: receive a MSG A transmission including a MAC PDU from a terminal via the transceiver, wherein the MAC PDU is obtained by the terminal for MSG A transmission and is stored in a buffer of the terminal; send a MSG B in response to MSG A to the terminal via the transceiver, wherein MSG B includes fallback information, and receive an uplink transmission from the terminal via the transceiver based on the fallback information, wherein the uplink transmission includes the MAC PDU.

[0014] [Beneficial Effects]

[0015] According to various embodiments of the present disclosure, a two-step random access (or two-step RACH) procedure in a next generation wireless communication system may be effectively enhanced. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The above and other aspects, features and advantages of certain embodiments of the present disclosure will become more apparent from the following description in conjunction with the accompanying drawings, in which:

[0017] Figure 1 is a flow chart of the two-step random access process;

[0018] Figure 2 is a diagram of the structure of MSG A in a two-step random access procedure according to an embodiment;

[0019] Figure 3 is a flow chart of a two-step random access process according to an embodiment;

[0020] Figure 4 is a flow chart of a two-step random access process according to an embodiment;

[0021] Figure 5a and Figure 5b is a flow chart of a two-step random access process according to an embodiment;

[0022] Figure 6a and Figure 6b is a flow chart of a two-step random access process according to an embodiment;

[0023] Figure 7a and Figure 7b is a flow chart of a two-step random access process according to an embodiment;

[0024] Figure 8a and Figure 8b is a flow chart of a two-step random access process according to an embodiment;

[0025] Figure 9a and Figure 9b is a flow chart of a two-step random access process according to an embodiment;

[0026] Fig.10a and Fig.10b is a flow chart of a two-step random access process according to an embodiment;

[0027] Fig.11 is a flow chart of a contention-based two-step random access procedure according to an embodiment;

[0028] Fig.12 is a flow chart of a contention-based two-step random access method according to an embodiment;

[0029] Fig.13 is a flow chart of a contention-based two-step random access method according to an embodiment;

[0030] Fig.14 is a diagram of message A (MSG A) reception timing according to an embodiment;

[0031] Fig.15 is a diagram of physical uplink shared channel (PUSCH) resource signaling according to an embodiment;

[0032] Fig.16 is a diagram of PUSCH resource signaling according to an embodiment;

[0033] Fig.17 is a flow chart of a method for handling fallback to random access during handover without random access channel (no RACH) according to an embodiment;

[0034] Fig.18 is a flow chart of a method for handling fallback to random access during RACH-free switching according to an embodiment;

[0035] Fig.19 is a diagram of processing fallback to random access during RACH-free handover according to an embodiment;

[0036] Fig. 20 is a block diagram of a terminal according to an embodiment; and

[0037] Fig.21 is a block diagram of a base station according to an embodiment. DETAILED DESCRIPTION

[0038] The following description with reference to the accompanying drawings is provided to assist in understanding the present disclosure as defined by the appended claims and their equivalents, including various specific details to assist in understanding, but these are intended to be considered as exemplary only. Therefore, one of ordinary skill in the art will recognize that various changes and modifications may be made to the present disclosure described herein without departing from the scope and spirit of the present disclosure. In addition, for the sake of clarity and brevity, descriptions of well-known functions and configurations are omitted.

[0039] The words used in the following description and claims are not intended to be limited to their dictionary meanings, but are only used to facilitate the understanding of the present disclosure. Therefore, it should be apparent to those skilled in the art that the following description of various embodiments of the present disclosure is provided for illustrative purposes only, and not for the purpose of limiting the present disclosure as defined by the attached claims and their equivalents.

[0040] It is to be understood that the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a component surface" includes reference to one or more of such surfaces.

[0041] The term “substantially” indicates that the recited characteristic, parameter or value need not be achieved precisely, but deviations or changes may occur within an amount that does not impair the effect that the characteristic is intended to provide, including, for example, tolerances, measurement errors, measurement accuracy limitations and other factors known to those skilled in the art.

[0042] The combination of the blocks and flow charts of the flowchart (or sequence diagram) can be represented and executed by computer program instructions. These computer program instructions can be loaded onto a processor of a general-purpose computer, a special-purpose computer, or a programmable data processing device. When the loaded program instructions are executed by the processor, they create a method for performing the functions described in the flowchart. Since the computer program instructions can be stored in a non-temporary computer-readable memory that can be used in a special-purpose computer or a programmable data processing device, an article of manufacture that performs the functions described in the flowchart can also be created. Since the computer program instructions can be loaded onto a computer or a programmable data processing device, when executed as a process, the computer program instructions can perform the operations of the functions described in the flowchart.

[0043] The blocks of the flow chart may correspond to modules, segments or codes including one or more executable instructions that implement one or more logical functions, or may correspond to parts thereof. In some cases, the functions described by the blocks may be performed in an order different from the order listed. For example, two blocks listed in sequence may be performed simultaneously or in reverse order.

[0044] In the present disclosure, the words "unit", "module", etc. may refer to software components or hardware components that can perform functions or operations, such as field programmable gate arrays (FPGAs) or application specific integrated circuits (ASICs). However, "units", etc. are not limited to hardware or software. Units, etc. may be configured to reside in addressable storage media or drive one or more processors. Units, etc. may refer to software components, object-oriented software components, category components, task components, processes, functions, attributes, programs, subroutines, program code fragments, drivers, firmware, microcodes, circuits, data, databases, data structures, tables, arrays, or variables. The functions provided by components and units may be a combination of smaller components and units, and may be combined with other components and units to form larger components and units. Components and units may be configured to drive devices or one or more processors in a secure multimedia card.

[0045] The following describes terms or definitions necessary for understanding the present disclosure. These terms are intended to be interpreted in a non-limiting manner.

[0046] A base station (BS) is an entity that communicates with a user equipment (UE) and may be referred to as a BS, base transceiver station (BTS), node B (NB), evolved NB (eNB), access point (AP), 5G NB (5GNB), or gNB.

[0047] The UE is an entity communicating with the BS and may be referred to as a UE, a device, a mobile station (MS), a mobile equipment (ME), or a terminal.

[0048] In recent years, a variety of broadband wireless technologies have been developed to meet the growing number of broadband subscribers and provide more and better applications and services. The 2nd generation wireless communication system has been developed to provide voice services while ensuring user mobility. The 3rd generation wireless communication system supports not only voice services but also data services. In recent years, 4G wireless communication systems have been developed to provide high-speed data services. However, the current 4G wireless communication system resources are insufficient to meet the growing demand for high-speed data services. Therefore, the 5G wireless communication system is being developed to meet the growing demand for high-speed data services and support ultra-reliability and low latency applications.

[0049] 5G wireless communication systems will be implemented not only in lower frequency bands, but also in higher frequency (e.g., millimeter wave) bands such as 10 GHz to 100 GHz bands to achieve higher data rates. In order to mitigate the propagation loss of radio waves and increase the transmission distance, beamforming, MIMO, FD-MIMO, array antennas, analog beamforming, and large antenna technologies are being considered in the design of 5G wireless communication systems. In addition, 5G wireless communication systems are expected to address different use cases with completely different requirements in terms of data rate, latency, reliability, mobility, etc. However, the design of the air interface of the 5G wireless communication system will be flexible enough to be expected to serve UEs with completely different capabilities, depending on the use cases and market segments of the end customers served by the UE. Several exemplary use cases that the 5G wireless communication system wireless system is expected to address are enhanced mobile broadband (eMBB), massive MTC (m-MTC), ultra-reliable low latency communication (URLLC), etc. eMBB requirements such as tens of Gbps data rates, low latency, high mobility, etc. address the market segments representing traditional wireless broadband users who need to connect to the Internet anytime, anywhere. m-MTC requirements such as very high connection density, infrequent data transmission, very long battery life, low mobility address the market segment representing IoT / IoE where billions of devices are envisioned to be connected. URLLC requirements such as very low latency, very high reliability and variable mobility address the market segment representing industrial automation applications and vehicle-to-vehicle / vehicle-to-infrastructure communications which is considered one of the enablers for autonomous vehicles.

[0050] In 5G wireless communication systems operating in higher frequency (millimeter wave) bands, UEs and gNBs communicate with each other using beamforming. Beamforming technology is used to mitigate propagation path loss and increase propagation distance for communication in higher frequency bands. Beamforming enhances the transmission and reception performance using high-gain antennas. Beamforming can be classified into transmit (TX) beamforming performed at the transmitting end, and receive (RX) beamforming performed at the receiving end.

[0051] Generally, TX beamforming allows the area of ​​signal propagation to be densely located in a specific direction to increase directivity by using multiple antennas. In this case, the aggregation of multiple antennas can be referred to as an antenna array, where each antenna included in the antenna array can be referred to as an array element. The antenna array can be configured in various forms, such as a linear array, a planar array, and the like. The use of TX beamforming increases the directivity of the signal, thereby increasing the propagation distance. In addition, since the signal may not be sent in a direction other than the directivity direction, the signal interference to the other receiving end is significantly reduced. The receiving end can perform beamforming on the RX signal by using an RX antenna array. RX beamforming increases the strength of the RX signal sent in a specific direction by allowing the propagation to be concentrated in a specific direction, and excludes signals sent in directions other than the specific direction from the RX signal, thereby providing an effect of blocking interference signals.

[0052] By using beamforming, the transmitter can generate multiple transmit beam patterns in different directions. Each of these transmit beam patterns can be referred to as a TX beam. Wireless communication systems operating at high frequencies use multiple narrow TX beams to transmit signals in a cell, since each narrow TX beam provides coverage to a portion of the cell. The narrower the TX beam, the higher the antenna gain, and therefore the greater the propagation distance of the signal sent using beamforming. The receiver can generate multiple RX beam patterns in different directions. Each of these receive patterns can also be referred to as an RX beam.

[0053] The 5G wireless communication system supports independent operation mode as well as dual connection (DC). In DC, multiple Rx / Tx UEs can be configured to utilize resources provided by two different nodes (or NBs) connected via a non-ideal backhaul. One node acts as a master node (MN) and the other node acts as a secondary node (SN). The MN and the SN are connected via a network interface, and at least the MN is connected to the core network. The new radio (NR) supports multi-radio access technology (RAT) dual connection (MR-DC) operation, whereby a UE in a radio resource control connection state (or RRC_CONNECTED) is configured to utilize radio resources provided by two different schedulers, which are located in two different nodes connected via a non-ideal backhaul and provide universal mobile telecommunications system (UMTS) terrestrial radio access (E-UTRA) (for example, if the node is ng-eNB) or NR access (ie, if the node is gNB). In NR, for a UE in RRC_CONNECTED that is not configured with carrier aggregation (CA) / DC, there is only one serving cell including a primary cell (PCell). For a UE in RRC_CONNECTED configured with CA / DC, the term "serving cell" is used to refer to a set of cells including a specific cell and all secondary cells (SCells). In NR, the term master cell group (MCG) refers to a serving cell group associated with a master node, which includes a PCell and optionally one or more SCells. In NR, the term secondary cell group (SCG) refers to a serving cell group associated with a secondary node, which includes a PSCell and optionally one or more SCells. In NR, PCell refers to a serving cell in an MCG operating on a primary frequency, where the UE performs an initial connection establishment procedure or initiates a connection reestablishment procedure. In NR, for a UE configured with CA, an Scell ​​is a cell that provides additional radio resources on top of a specific cell. A primary SCG cell (PSCell) refers to a serving cell in an SCG, where the UE performs random access when performing a reconfiguration with a synchronization (sync) procedure. For DC operation, the term SpCell (i.e., specific cell) refers to a PCell of an MCG or a PSCell of an SCG, otherwise the term specific cell refers to a PCell.

[0054] In 5G wireless communication systems, the cell broadcast synchronization signal and physical broadcast channel (PBCH) blocks in the Node B (gNB) or base station consist of the primary synchronization signal (PSS) and the secondary synchronization signal (SSS) as well as system information. System information includes common parameters required for communication in the cell. In 5G wireless communication systems (also known as next generation radio or NR), system information (SI) is divided into a master information block (MIB) and multiple system information blocks (SIBs), where:

[0055] -MIB is always sent on PBCH with a period of 80ms, repeated within 80ms, and includes the parameters required to obtain SIB1 from the cell.

[0056] -SIB1 is sent on the downlink shared channel (DL-SCH) with a period of 160ms, with variable transmission repetition. The default transmission repetition period of SIB1 is 20ms, but the actual transmission repetition period is determined by the network implementation. SIB1 includes information about the availability and scheduling of other SIBs (e.g., mapping of SIBs to SI messages, periodicity, SI window size), and indicates whether one or more SIBs are provided only on demand, and in this case, the configuration required for the UE to perform the SI request. SIB1 is a cell-specific SIB; and

[0057] - SIBs other than SIB1 are carried in a System Information (SI) message, which is sent on the DL-SCH. Only SIBs with the same periodicity can be mapped to the same SI message.

[0058] In the 5G wireless communication system, the physical downlink control channel (PDCCH) is used to schedule downlink (DL) transmission on the physical downlink shared channel (PDSCH) and schedule uplink (UL) transmission on the PUSCH, wherein the downlink control information (DCI) on the PDCCH includes: downlink allocation, including at least the modulation and coding format, resource allocation and hybrid automatic repeat request (HARQ) information related to the DL-SCH; uplink scheduling authorization, including at least the modulation and coding format, resource allocation and hybrid ARQ information related to the UL-SCH. In addition to scheduling, PDCCH can also be used to activate and deactivate configured PUSCH transmissions with configured grants; activate and deactivate PDSCH semi-persistent transmissions; notify one or more UEs of the time slot format; notify one or more UEs of physical resource blocks (PRBs) and orthogonal frequency division multiplexing (OFDM) symbols, where the UE can assume that there is no transmission for the UE; send transmit power control (TPC) commands for physical uplink control channel (PUCCH) and PUSCH; send one or more TPC commands for sounding reference signal (SRS) transmissions for one or more UEs; and switch the active bandwidth part of the UE; initiate a random access procedure. The UE monitors a set of candidate PDCCHs according to the configured monitoring opportunities in one or more configured control resource sets (CORESETs) according to the corresponding search space configuration. A CORESET consists of a set of PRBs with a duration of 1 to 3 OFDM symbols. Resource element groups (REGs) and control channel elements (CCEs) of resource units are defined within a CORESET, and each CCE consists of a set of REGs. The control channel is formed by the aggregation of CCEs. Different coding rates for control channels are achieved by aggregating different numbers of CCEs. Interleaved and non-interleaved CCE to REG mapping is supported in CORESET. Polarity coding is used for PDCCH. Each resource element group carrying PDCCH carries its own demodulation reference signal (DMRS). Quadrature Phase Shift Keying (QPSK) modulation is used for PDCCH.

[0059] In a 5G wireless communication system, for each configured bandwidth part (BWP), a list of search space configurations is signaled by the gNB, where each search configuration is uniquely identified by an identifier. The identifiers of search space configurations for specific purposes (such as paging reception, SI reception, and random access response reception) are explicitly signaled by the gNB. The NR search space configuration includes the parameters monitoring-periodicity-PDCCH-slot, monitoring-offset-PDCCH-slot, monitoring-symbols-PDCCH-within-slot, and duration. The UE uses the parameters PDCCH monitoring period (monitoring-periodicity-PDCCH-slot), PDCCH monitoring offset (monitoring-offset-PDCCH-slot), and PDCCH monitoring mode (monitoring-symbols-PDCCH-within-slot) to determine the PDCCH monitoring timing within a timeslot. The PDCCH monitoring occasion is in time slots x to x+duration, where the time slot numbered X in the radio frame numbered Y satisfies the following equation (1):

[0060] (y*(number of slots in a radio frame)+x-monitoring-offset-PDCCH-slot)

[0061] modulo(monitoring-periodicity-PDCCH-slot)=0 (1)

[0062] The starting symbol of the PDCCH monitoring opportunity in each slot with a PDCCH monitoring opportunity is given by monitoring-symbols-PDCCH-within-slot. The length of the PDCCH monitoring opportunity (in symbols) is given in the CORESET associated with the search space. The search space configuration includes an identifier of the CORESET configuration associated with the search space. For each configured BWP, a list of CORESET configurations is signaled by the gNB, where each CORESET configuration is uniquely identified by an identifier. The frame duration of each radio is 10ms. A radio frame is identified by a radio frame number or a system frame number. Each radio frame includes several slots, where the number of slots and the slot duration in a radio frame depends on the subcarrier spacing. The number of slots and the slot duration in a radio frame depends on the radio frame for each SCS supported and are predefined in the NR. Each CORESET configuration is associated with a Transmit Configuration Indicator (TCI) state list. Each TCI state configures one DL RS ID (SSB or CSI RS). The TCI state list corresponding to the CORESET configuration is signaled by the gNB via RRC signaling. One of the TCI states in the TCI state list is activated by the gNB and indicated to the UE. The TCI state indicates the DL TX beam transmitted by the gNB in ​​the PDCCH monitoring occasion for search space (the DL TX beam is quasi-collocated (QCL) with the SSB / CSIRS of the TCI state).

[0063] In 5G wireless communication systems, bandwidth adaptation (BA) is supported. With BA, the UE's receiving and transmitting bandwidth does not need to be as large as the cell's bandwidth, and can be adjusted: the bandwidth can be commanded to change (for example, shrink during low activity to save power); the position can be moved in the frequency domain (for example, to increase scheduling flexibility); and the subcarrier spacing can be commanded to change (for example, to allow different services).

[0064] A subset of the total cell bandwidth of a cell is called a BWP. BA is achieved by configuring a BWP for an RRC-connected UE and notifying the UE which of the configured BWP is currently activated. When BA is configured, the UE only needs to monitor the PDCCH on one activated BWP, i.e., the UE does not have to monitor the PDCCH on the entire DL frequency of the serving cell. In the RRC connected state, the UE is configured with one or more DL and UL BWPs for each configured serving cell (i.e., PCell or SCell). For an activated serving cell, there is always one activated UL and DL BWP at any point in time. The serving BWP switching is used to activate inactivated BWPs and deactivate activated BWPs. BWP switching is controlled by a PDCCH indicating a downlink allocation or uplink grant, by a bwp-Inactivity timer, by RRC signaling, or by a medium access control (MAC) entity when a random access procedure is initiated. After adding a SpCell or activating an SCell, the DL BWP and UL BWP indicated by firstActiveDownlinkBWP-Id and BWP-Id (firstActiveUplinkBWP-Id), respectively, are activated without receiving a PDCCH indicating a downlink allocation or uplink grant. The activated BWP of the serving cell is indicated by RRC or PDCCH. For unpaired spectrum, the DL BWP is paired with the UL BWP, and BWP switching is common for both UL and DL. When the BWP inactivation timer expires, the UE switches the activated DL BWP to the default DL BWP or the initial DL BWP (if no default DL BWP is configured).

[0065] In 5G wireless communication systems (also known as NR), the random access (RA) procedure is used to achieve uplink time synchronization. The RA procedure is used by non-synchronized UEs in the RRC CONNECTED state during initial access, handover, RRC connection reestablishment procedure, scheduling request transmission, SCG addition / modification, and data or control information transmission in the uplink.

[0066] During the contention-based random access (CBRA) procedure (which consists of four steps), the UE first sends a random access preamble (also known as Msg1) and then waits for a random access response (RAR) or Msg2 in the RAR window corresponding to its random access preamble transmission. The gNB sends the RAR on the PDSCH addressed to the random access radio network temporary identifier (RA-RNTI).

[0067] RA-RNTI identifies the time-frequency resource (also known as physical random access channel (PRACH) opportunity or PRACH TX opportunity or RACH opportunity) in which the random access preamble is detected by the gNB. The maximum size of the RAR window is one radio frame, i.e., 10ms. The calculation of RA-RNTI is as follows:

[0068] RA-RNTI=1+s_id+14*t_id+14*80*f_id+14*80*8*ul_carrier_id (2)

[0069] In the above equation (2), s_id is the index of the first OFDM symbol of the PRACH opportunity in which the UE has sent Msg1 (ie, RA preamble); 0≤s_id<14,

[0070] t_id is the index of the first time slot of the PRACH opportunity (0≤t_id<80),

[0071] f_id is the index of the PRACH opportunity within the slot in the frequency domain (0≤f_id<8), and

[0072] ul_carrier_id is the UL carrier used for Msg1 transmission (0 for NUL (normal uplink carrier) and 1 for SUL (supplementary carrier)).

[0073] Several RARs for various random access preambles detected by the gNB may be multiplexed by the gNB in ​​the same RAR MAC Protocol Data Unit (PDU). If the RAR in the MAC PDU includes the Random Access Preamble Identifier (RAPID) of a Random Access Preamble sent by the UE, then the RAR corresponds to a Random Access Preamble transmission by the UE. If the RAR corresponding to its Random Access Preamble transmission is not received during the RAR window and the UE has not sent a Random Access Preamble for a configurable (configured by the gNB in ​​the RACH configuration) number of times, then the UE retransmits the Random Access Preamble. The RA procedure is considered successful if the RAR corresponding to its Random Access Preamble transmission is received and the UE has sent a dedicated Random Access Preamble.

[0074] If the UE has sent a non-dedicated (i.e., contention-based) random access preamble, upon successful reception of the RAR, the UE sends Msg3 in the UL grant received in the RAR. The UE generates a new MAC PDU using the data available in the logical channel. Msg3 includes messages such as RRC connection request, RRC connection reestablishment request, RRC handover confirmation, scheduling request, etc. Msg3 also includes a UE identity (i.e., cell RNTI (C-RNTI) or service temporary subscriber identity (S-TMSI) or a random number). After sending Msg3, the UE starts a contention resolution timer. When the contention resolution timer is running, if the UE receives a PDCCH addressed to the C-RNTI included in Msg3, the contention resolution is considered to be successful, the contention resolution timer is stopped, and the RA process is completed. When the contention resolution timer is running, if the UE receives a contention resolution MAC CE including the UE's contention resolution identity (the first X bits of the common control channel (CCCH) service data unit (SDU) sent in Msg3), the contention resolution is considered to be successful, the contention resolution timer is stopped, and the RA process is completed. If the contention resolution timer expires and the UE has not sent a random access preamble for a configurable number of times, the UE retransmits the random access preamble.

[0075] In 5G wireless communication systems, a contention-free random access (CFRA) procedure is also supported. CFRA is used for situations such as handovers requiring low latency, timing advance establishment of Scells, etc. The ENB or gNB allocates a non-contention RA preamble to the UE in dedicated signaling. The UE sends the allocated non-contention RA preamble. The ENB or gNB sends a RAR on the PDSCH addressed to the RA-RNTI. The RAR transmits the RA preamble identifier and timing alignment information. The RAR may also include a UL grant. The RAR is sent in a RAR window similar to the contention-based RA procedure. After receiving the RAR, the contention-free RA procedure terminates.

[0076] In order to reduce the waiting time of the four-step CBRA process, a two-step RACH process is being studied. Two-step RACH refers to a process that can complete CBRA in two steps.

[0077] Figure 1 is a flow chart of the two-step random access process.

[0078] refer to Figure 1In step 110, UE 101 transmits a random access preamble on PRACH and a payload (i.e., MAC PDU) on PUSCH. The random access preamble and payload transmission is also referred to as MSG A. In a second step 120, after MSG A transmission, UE 101 monitors a response from network 103 (i.e., gNB). The response is also referred to as MsgB. MsgB is addressed to MSGB-RNTI, where MSGB-RNTI is defined as follows in equation (3).

[0079] MSGB-RNTI=1+s_id+14*t_id+14*80*f_id+14*80*8*ul_carrier_id

[0080] +14*80*8*2(3)

[0081] In equation (3) above, s_id is the index of the first OFDM symbol of the PRACH opportunity in which UE 101 has sent the PRACH preamble; 0≤s_id<14,

[0082] t_id is the index of the first time slot of the PRACH opportunity (0≤t_id<80),

[0083] f_id is the index of the PRACH opportunity within the slot in the frequency domain (0≤f_id<8), and

[0084] ul_carrier_id is the UL carrier used for Msg1 transmission (0 for a normal uplink (NUL) carrier and 1 for a supplemental uplink (SUL) carrier).

[0085] If a CCCH SDU is sent in the MSG A payload, the UE 101 performs contention resolution using the contention resolution information in MsgB. If the contention resolution identifier received in MsgB matches the first 48 bits of the CCCH SDU sent in MSG A, contention resolution is successful. If a C-RNTI is sent in the MSG A payload, contention resolution is successful if the UE 101 receives a PDCCH addressed to the C-RNTI. If contention resolution is successful, the random access procedure is considered to have been successfully completed. Different from the contention resolution information corresponding to the sent MSG A, MsgB may include fallback information corresponding to the random access preamble sent in MSG A. If fallback information is received, the UE 101 sends Msg3 and performs contention using Msg4 as in the CBRA process. If contention resolution is successful, the random access procedure is considered to have been successfully completed. If contention resolution fails at the time of fallback (i.e., at the time of sending Msg3), UE 101 returns to the first step 110 for MSG A transmission, i.e., UE 101 transmits a random access preamble on PRACH and transmits a payload (i.e., MAC PDU) on PUSCH. If the window for UE 101 to monitor the network response expires after sending MSG A and UE 101 does not receive MsgB including contention resolution information or fallback information as described above, UE 101 returns to the first step 110 and transmits MSG A. If the random access procedure is not successfully completed even after sending MSG A for a configurable number of times, UE 101 falls back to the four-step RACH procedure, in which UE 103 only transmits the PRACH preamble (i.e., Msg1).

[0086] The MSG A payload may include one or more of the following: CCCH SDU, dedicated control channel (DCCH) SDU, dedicated traffic channel (DTCH) SDU, buffer status report (BSR) MAC control element (CE), power headroom report (PHR) MAC CE, synchronization signal block (SSB) information, C-RNTI MAC CE, or padding. In the first step 110, the MSGA may include a UE identifier (ID) (e.g., random ID, S-TMSI, C-RNTI, recovery ID, etc.) and a preamble. The UE ID may be included in the MAC PDU of MSG A. The UE ID (such as C-RNTI) may be carried in the MAC CE, where the MAC CE is included in the MAC PDU. Other UE IDs (such as random ID, S-TMSI, C-RNTI, recovery ID, etc.) may be carried in the CCCH SDU. The UE ID may be one of a random ID, S-TMSI, C-RNTI, recovery ID, IMSI, idle mode ID, inactive mode ID, etc. In different situations where UE 101 performs the RA procedure, the UE ID may be different.

[0087] When UE 101 performs RA after power-on (before UE 101 attaches to network 103), UE ID is a random ID. When UE 101 performs RA in an idle state after UE 101 attaches to network 103, UE ID is S-TMSI. If UE 101 has an assigned C-RNTI (for example, UE 101 is in a connected state), UE ID is C-RNTI. In the case where UE 101 is in an inactive state, UE ID is a recovery ID. In addition to UE ID, some additional control information may also be sent in MSG A. Control information may be included in the MAC PDU of MSG A. Control information may include one or more of the following: connection request indication, connection recovery request indication, SI request indication, buffer status indication, beam information (for example, one or more DL TX beam IDs or SSB IDs), beam failure recovery indication / information, data indicator, cell / BS / TRP (transmitting point and receiving point) switching indication, connection reconstruction indication, reconfiguration completion message or switching completion message, etc.

[0088] Figure 2 is a diagram of the structure of message A (or MSG A) in a two-step random access procedure according to an embodiment.

[0089] refer to Figure 2In two-step CFRA, the gNB allocates a dedicated random access preamble and PUSCH resources to the UE for MSG A transmission. The RACH opportunity (or RO) to be used for preamble transmission may also be indicated. The UE transmits a random access preamble 215 on a PRACH opportunity 210 and a payload 225 on a PUSCH resource 220 using contention-free random access resources (i.e., dedicated preamble / PUSCH resources / RO). After MSG A transmission, the UE monitors for a response from the network (i.e., gNB) within a configured window.

[0090] 2-step CFRACH: MSG A payload processing at the completion of the RA process

[0091] Figure 3 and Figure 4 is a flow chart of a two-step random access procedure according to an embodiment.

[0092] refer to Figure 3 and Figure 4 In the case of two-step CF RACH, NB 303 signals contention-free random access resources (i.e., dedicated preamble / RACH opportunity and PUSCH resources) to UE 301 in steps 310 and 410. UE 301 sends a PRACH preamble in a PRACH opportunity and sends a MSG A payload (or MSG A MAC PDU) in a PUSCH resource / opportunity using the allocated contention-free random access resources in steps 320, 330, 420, and 430, respectively.

[0093] If gNB 303 receives MSG A (i.e., both the PRACH preamble and the MSG A payload), then in step 340, gNB 303 sends msgB including RAPID, TA, and UL grant. If the RAPID in the received msgB matches the RAPID of the PRACH preamble sent by UE 301, the random access procedure is completed. This procedure is performed in Figure 3 As shown in . Optionally, if gNB303 receives MSG A (i.e., both the PRACH preamble and the MSG A payload), gNB 303 sends a PDCCH addressed to the C-RNTI. Contention-free random access is performed in the RRC connected state in which UE 301 has an assigned C-RNTI. gNB 303 can identify UE 301 based on the received PRACH preamble and therefore can send a PDCCH addressed to the C-RNTI. If UE 301 receives a PDCCH addressed to the C-RNTI, the random access procedure is completed. When the random access procedure is completed, the buffer and / or HARQ buffer for MSG A are discarded.

[0094] If the gNB 303 receives only the PRACH preamble, the gNB 303 sends a msgB including the RAPID, TA, and UL grant in step 440. If the RAPID in the received msgB matches the RAPID of the PRACH preamble sent by the UE 301, the random access procedure is completed. Figure 4 As shown in . Optionally, if the gNB 303 receives MSG A (i.e., both the PRACH preamble and the MSG A payload), the gNB 303 sends a PDCCH addressed to the C-RNTI. Contention-free random access is performed in the RRC connected state in which the UE 301 has an assigned C-RNTI. The gNB 303 can identify the UE 301 based on the received PRACH preamble, and therefore can also send a PDCCH addressed to the C-RNTI. If the UE 301 receives a PDCCH addressed to the C-RNTI, the random access procedure is completed. When the random access procedure is completed, the buffer and / or HARQ buffer for MSG A are discarded.

[0095] In this process, if the gNB 303 does not receive the MSG A payload, the MSG A payload is lost. To avoid the loss of the MSG A payload, the UE 301 may store the MSG A payload. When the random access process is completed, the UE 301 may retransmit the MSG A payload or discard the MSG A payload based on the information received in the msgB corresponding to the transmitted MSG A. In the present disclosure, several methods are provided to enable the UE 301 to identify whether to retransmit the MSG A payload or discard the MSG A payload after the two-step CF RACH process is completed.

[0096] Figure 5a and Figure 5b is a flow chart of a two-step random access procedure according to an embodiment.

[0097] refer to Figure 5a and Figure 5bIn the case of two-step CF RACH, the gNB 503 signals the contention-free random access resources (i.e., dedicated preamble / RACH opportunity and PUSCH resources) to the UE 501 in steps 510 and 550, respectively. The UE 501 transmits the PRACH preamble in the PRACH opportunity and transmits the MSG A payload (or MSG A MAC PDU) in the PUSCH resource / opportunity using the allocated contention-free random access resources in steps 520, 530, 560, and 570, respectively. The UE 501 stores the MSG A payload (i.e., MSG A MAC PDU) in the MSG A buffer. For the MSG A payload transmission, the HARQ entity in the UE 501 performs a new HARQ packet transmission, where the MSG A payload in the MSG A buffer is used to generate the HARQ packet for transmission. The UE 501 maintains multiple parallel HARQ processes. Each HARQ process supports one transport block (TB). Each HARQ process is associated with a HARQ process identifier. For UL transmission of MSG A payload, HARQ process identifier 0 is used.

[0098] After sending MSG A, UE 501 starts msgB-ResponseWindow and monitors the PDCCH for random access response (i.e., msgB) in msgB-ResponseWindow. When msgB-ResponseWindow is running, UE 501 monitors the PDCCH of the SpCell for the random access response identified by MSGB-RNTI. If C-RNTI MAC CE is included in the MSG A payload, UE 501 also monitors the PDCCH of the SpCell for the random access response identified by C-RNTI when msgB-ResponseWindow is running.

[0099] If gNB 503 successfully receives MSG A (i.e., both the PRACH preamble and the MSG A payload), gNB 503 includes the C-RNTI of UE 501 (which sent MSG A) in msgB, as shown in Figure 5a As shown in step 540 in msgB. gNB 503 can identify UE 501 based on the received PRACH preamble. The PRACH preamble is received in the contention-free random access resources allocated to UE 501. However, if gNB 503 only receives the PRACH preamble but fails to receive the MSG A payload, gNB 503 does not include the C-RNTI in msgB, as shown in step 540 in msgB. Figure 5bAs shown in step 580 in msgB. msgB includes the RAPID corresponding to the PRACH preamble, regardless of whether gNB 503 receives the MSG A payload.

[0100] If MSG A is transmitted using contention-free random access resources and if msgB received by UE 501 includes a RAPID corresponding to the PRACH preamble transmitted by UE 501, UE 501 considers that msgB is successfully received and the random access procedure is successfully completed.

[0101] If msgB includes C-RNTI, UE 501 discards MSG A payload, that is, UE 501 discards MSG A buffer and HARQ buffer used for MSG A payload transmission.

[0102] If msgB does not include C-RNTI, UE 501 retransmits MSG A payload (i.e., MSG A MAC PDU) in the UL grant received in msgB. For the transmission of MSG A payload in the UL grant received in msgB, the HARQ entity in UE 501 performs a new HARQ packet transmission, wherein the HARQ packet for transmission is generated using the MSG A payload in the MSG A buffer. For the UL transmission of the MSG A payload, the HARQ process identifier 0 is used. After transmitting the MSG A payload in the UL grant received in msgB, if UE 501 receives a PDCCH addressed to the C-RNTI for HARQ process zero in which the new data indicator (NDI) is not switched, UE 501 performs HARQ retransmission. If msgB does not include C-RNTI and no UL grant is received in msgB, UE 501 retransmits MSG A payload in a subsequent UL grant using HARQ process zero after receiving a PDCCH addressed to C-RNTI for HARQ process zero, wherein UE 501 performs HARQ retransmission.

[0103] Figure 6a and Figure 6b is a flow chart of a two-step random access procedure according to an embodiment.

[0104] refer to Figure 6a and Figure 6bIn the case of two-step CF RACH, the gNB 603 signals the contention-free random access resources (i.e., dedicated preamble / RACH opportunity and PUSCH resources) to the UE 601 in steps 610 and 650, respectively. The UE 601 transmits the PRACH preamble in the PRACH opportunity and transmits the MSG A payload (or MSG A MAC PDU) in the PUSCH resource / opportunity using the allocated contention-free random access resources in steps 620, 630, 660, and 670, respectively. The UE 601 stores the MSG A payload (i.e., MSG A MAC PDU) in the MSG A buffer. For the MSG A payload transmission, the HARQ entity in the UE 601 performs a new HARQ packet transmission, where the MSG A payload in the MSG A buffer is used to generate a HARQ packet for transmission. The UE 601 maintains multiple parallel HARQ processes. Each HARQ process supports one TB. Each HARQ process is associated with a HARQ process identifier. For UL transmission of MSG A payload, HARQ process identifier 0 is used.

[0105] After sending MSG A, UE 601 starts msgB-ResponseWindow and monitors the PDCCH for random access response (i.e., msgB) in msgB-ResponseWindow. When msgB-ResponseWindow is running, UE 601 monitors the PDCCH of the SpCell for the random access response identified by MSGB-RNTI. If C-RNTI MAC CE is included in the MSG A payload, UE 601 also monitors the PDCCH of the SpCell for the random access response identified by C-RNTI when msgB-ResponseWindow is running.

[0106] If gNB 603 successfully receives MSG A (i.e., both PRACH preamble and MSG A payload), gNB 603 does not include the UL grant in msgB, as shown in Figure 6a If gNB 603 receives only the PRACH preamble but fails to receive the MSG A payload, gNB 603 includes the UL grant in msgB, as shown in step 640 of FIG. Figure 6b As shown in step 680 in msgB. msgB includes the RAPID corresponding to the PRACH preamble, regardless of whether gNB 603 receives the MSG A payload.

[0107] If MSG A is transmitted using contention-free random access resources and if msgB received by UE 601 includes a RAPID corresponding to the PRACH preamble transmitted by UE 601, UE 601 considers that msgB is successfully received and the random access procedure is successfully completed.

[0108] If msgB does not include UL grant, UE 601 discards MSG A payload, that is, discards MSG A buffer and HARQ buffer used for MSG A payload transmission.

[0109] If msgB includes an UL grant, UE 601 retransmits the MSG A payload in the UL grant received in msgB. For the transmission of the MSG A payload in the UL grant received in msgB, the HARQ entity in UE 601 performs a new HARQ packet transmission, where the MSG A payload in the MSG A buffer is used to generate a HARQ packet for transmission. For the UL transmission of the MSG A payload, the HARQ process identifier 0 is used. After transmitting the MSG A payload in the UL grant received in msgB, if UE 601 receives a PDCCH addressed to the C-RNTI for the HARQ process zero whose NDI is not switched, UE 601 performs HARQ retransmission.

[0110] Figure 7a and Figure 7b is a flow chart of a two-step random access procedure according to an embodiment.

[0111] refer to Figure 7a and Figure 7bIn the case of two-step CF RACH, the gNB 703 signals the contention-free random access resources (i.e., dedicated preamble / RACH opportunity and PUSCH resources) to the UE 701 in steps 710 and 750, respectively. The UE 701 transmits the PRACH preamble in the PRACH opportunity and transmits the MSG A payload (or MSG A MAC PDU) in the PUSCH resource / opportunity using the allocated contention-free random access resources in steps 720, 730, 760, and 770, respectively. The UE 701 stores the MSG A payload (i.e., MSG A MAC PDU) in the MSG A buffer. For the MSG A payload transmission, the HARQ entity in the UE 701 performs a new HARQ packet transmission, where the MSG A payload in the MSG A buffer is used to generate a HARQ packet for transmission. The UE 701 maintains multiple parallel HARQ processes. Each HARQ process supports one TB. Each HARQ process is associated with a HARQ process identifier. For UL transmission of MSG A payload, HARQ process identifier 0 is used.

[0112] After sending MSG A, UE 701 starts msgB-ResponseWindow and monitors the PDCCH for random access response (i.e., msgB) in msgB-ResponseWindow. When msgB-ResponseWindow is running, UE 701 monitors the PDCCH of the SpCell for the random access response identified by MSGB-RNTI. If C-RNTI MAC CE is included in the MSG A payload, UE 701 also monitors the PDCCH of the SpCell for the random access response identified by C-RNTI when msgB-ResponseWindow is running.

[0113] If gNB 703 successfully receives MSG A (i.e., both PRACH preamble and MSG A payload), gNB 703 sets the NEW_TX bit in msgB to 1, as shown in Figure 7a If gNB 701 receives only the PRACH preamble but fails to receive the MSG A payload, gNB 703 sets the NEW_TX bit in msgB to 0, as shown in step 740 of FIG. Figure 7b As shown in step 780 in msgB. msgB includes the RAPID corresponding to the PRACH preamble, regardless of whether gNB 703 receives the MSG A payload.

[0114] If MSG A is transmitted using contention-free random access resources and if msgB received by UE 701 includes a RAPID corresponding to the PRACH preamble transmitted by UE 701, UE 701 considers that msgB is successfully received and the random access procedure is successfully completed.

[0115] If msgB includes the NEW_TX bit in msgB set to 1, UE 701 discards the MSG A payload, ie, discards the MSG A buffer and the HARQ buffer used for MSG A payload transmission. UE 701 sends a new MAC PDU in the UL grant received in msgB.

[0116] If msgB includes the NEW_TX bit set to 0, UE 701 retransmits the MSG A payload in the UL grant received in msgB. For the transmission of the MSG A payload in the UL grant received in msgB, the HARQ entity in UE 701 performs a new HARQ packet transmission, where the MSG A payload in the MSG A buffer is used to generate the HARQ packet for transmission. For the UL transmission of the MSG A payload, the HARQ process identifier 0 is used. After transmitting the MSG A payload in the UL grant received in msgB, if UE 701 receives a PDCCH addressed to the C-RNTI for HARQ process zero whose NDI is not switched, UE 701 performs a HARQ retransmission. If msgB includes the NEW_TX bit set to 0 and no UL grant is received in msgB, UE 701, upon receiving a PDCCH addressed to the C-RNTI for HARQ process zero, retransmits the MSG A payload in a subsequent UL grant using HARQ process zero.

[0117] Figure 8a and Figure 8b is a flow chart of a two-step random access procedure according to an embodiment.

[0118] refer to Figure 8a and Figure 8bIn the case of two-step CF RACH, gNB 803 signals the contention-free random access resources (i.e., dedicated preamble / RACH opportunity and PUSCH resources) to UE 801 in steps 810 and 850, respectively. UE 801 transmits the PRACH preamble in the PRACH opportunity and transmits the MSG A payload (or MSG A MAC PDU) in the PUSCH resource / opportunity using the allocated contention-free random access resources in steps 820, 830, 860, and 870, respectively. UE 801 stores the MSG A payload (i.e., MSG A MAC PDU) in the MSG A buffer. For the MSG A payload transmission, the HARQ entity in UE 801 performs a new HARQ packet transmission, where the MSG A payload in the MSG A buffer is used to generate the HARQ packet for transmission. UE 801 maintains multiple parallel HARQ processes. Each HARQ process supports one TB. Each HARQ process is associated with a HARQ process identifier. For UL transmission of MSG A payload, HARQ process identifier 0 is used.

[0119] After sending MSG A, UE 801 starts msgB-ResponseWindow and monitors the PDCCH for random access response (i.e., msgB) in msgB-ResponseWindow. While msgB-ResponseWindow is running, UE 801 monitors the PDCCH of the SpCell for the random access response identified by MSGB-RNTI. If C-RNTIMAC CE is included in the MSG A payload, UE 801 also monitors the PDCCH of the SpCell for the random access response identified by C-RNTI when msgB-ResponseWindow is running.

[0120] If gNB 803 successfully receives MSG A (i.e., both PRACH preamble and MSG A payload), gNB 803 includes the RAPID, TA command, UL grant, C-RNTI of UE 801 (which sent MSG A) in msgB, as shown in Figure 8a The PDCCH for msgB is addressed to MSG B-RNTI. Upon receiving msgB in which the RAPID corresponds to the random access preamble sent by UE 801, the random access procedure is successfully completed. In this case, UE 801 discards the MSG A payload, i.e., discards the MSG A buffer and the HARQ buffer for the MSG A payload transmission.

[0121] gNB 803 may identify UE 801 based on the received PRACH preamble. The PRACH preamble is received in contention-free resources allocated to UE 801. If gNB 803 receives only the PRACH preamble and fails to receive the MSG A payload, gNB 80-3 includes the RAPID, TA command, UL grant, and temporary C-RNTI (TC-RNTI) in msgB, as shown in Figure 8b As shown in step 880 in .

[0122] If msgB includes RAPID, TA command, UL grant and TC-RNTI, where RAPID corresponds to the random access preamble sent by UE 801, the random access procedure is successfully completed. UE 801 retransmits the MSG A payload in the UL grant received in msgB. For the MSG A payload transmission in the UL grant received in msgB, the HARQ entity in UE 801 performs a new HARQ packet transmission, where the MSG A payload in the MSG A buffer is used to generate the HARQ packet for transmission. For the UL transmission of the MSG A payload, the HARQ process identifier 0 is used. After transmitting the MSG A payload in the UL grant received in msgB, if UE 801 receives a PDCCH addressed to the C-RNTI for the HARQ process zero whose NDI is not switched, UE 801 performs HARQ retransmission. If no UL grant is received in msgB, UE 801 retransmits the MSG A payload in a subsequent UL grant using HARQ process zero after receiving a PDCCH addressed to the C-RNTI for HARQ process zero.

[0123] If gNB 803 successfully receives MSG A (i.e., both PRACH preamble and MSG A payload), gNB 803 sends a PDCCH addressed to the C-RNTI of UE 801 (which sent MSG A), where the PDCCH may schedule an UL grant for a new transmission, or the PDCCH may include a DL assignment. Upon receiving the PDCCH, the random access procedure is successfully completed. In this case, UE 801 discards the MSG A payload, i.e., discards the MSG A buffer and the HARQ buffer used for MSG A payload transmission.

[0124] gNB 803 can identify UE 801 based on the received PRACH preamble. The PRACH preamble is received in the contention-free resources allocated to UE 801. If gNB 803 receives only the PRACH preamble and fails to receive the MSG A payload, gNB 803 includes the RAPID, TA command, UL grant, and TC-RNTI in msgB. The PDCCH for msgB is addressed to MSG B-RNTI.

[0125] If msgB includes RAPID, TA command, UL grant and TC-RNTI, where RAPID corresponds to the random access preamble sent by UE 801, the random access procedure is successfully completed. UE 801 retransmits the MSG A payload in the UL grant received in msgB. For the MSG A payload transmission in the UL grant received in msgB, the HARQ entity in UE 801 performs a new HARQ packet transmission, where the MSG A payload in the MSG A buffer is used to generate the HARQ packet for transmission. For the UL transmission of the MSG A payload, the HARQ process identifier 0 is used. After transmitting the MSG A payload in the UL grant received in msgB, if UE 801 receives a PDCCH addressed to the C-RNTI for the HARQ process zero whose NDI is not switched, UE 801 performs HARQ retransmission.

[0126] If gNB 803 successfully receives MSG A (i.e., both PRACH preamble and MSG A payload), gNB 803 sends a PDCCH addressed to the C-RNTI of UE 801 (which sent MSG A), where the PDCCH can schedule an UL grant or DL ​​allocation for a new transmission. Upon receiving the PDCCH, the random access procedure is successfully completed. In this case, UE 801 discards the MSG A payload, i.e., discards the MSG A buffer. However, the HARQ buffer used for the MSG A payload transmission is not discarded.

[0127] gNB 803 can identify UE 801 based on the received PRACH preamble. The PRACH preamble is received in the contention-free resources allocated to UE 801. If gNB 803 only receives the PRACH preamble and fails to receive the MSG A payload, gNB 803 sends a PDCCH addressed to the C-RNTI of UE 801, where the PDCCH can schedule UL grants or DL ​​allocations for new transmissions. Upon receiving the PDCCH, the random access procedure is successfully completed. In this case, UE 801 discards the MSG A payload, that is, discards the MSG A buffer. However, the HARQ buffer used for MSG A payload transmission is not discarded.

[0128] At the completion of the random access procedure, if gNB 803 did not receive the MSG A payload during the random access procedure, gNB 803 shall send a PDCCH addressed to the C-RNTI for HARQ process zero with NDI not switched. If UE 801 receives a PDCCH addressed to the C-RNTI for HARQ process zero with NDI not switched, UE 801 performs HARQ retransmission.

[0129] When the random access procedure is completed:

[0130] - If the random access procedure is completed when the Fallback RAR is received (the Fallback RAR is received in msgB and consists of RAPID, TA, TC-RNTI and UL Grant), then:

[0131] *UE does not refresh the HARQ buffer in the Msg3 buffer and the MSG A buffer for MAC PDU transmission;

[0132] - If the random access procedure is not completed when the fallback RAR is received,

[0133] * Flush the HARQ buffers in the msg3 buffer and the MSG A buffer for MAC PDU transmission.

[0134] In the proposed embodiment of the present invention, when the random access procedure is completed:

[0135] - If the last random access preamble sent was not selected from the contention-based preambles, then:

[0136] *UE does not refresh the HARQ buffer in the Msg3 buffer and the MSG A buffer for MAC PDU transmission;

[0137] -otherwise:

[0138] * Flush the HARQ buffers in the msg3 buffer and the MSG A buffer for MAC PDU transmission.

[0139] In the proposed embodiment of the present invention, when the random access procedure is completed:

[0140] - If the last random access preamble sent was not selected from the contention-based preambles, then:

[0141] *UE does not refresh the HARQ buffer in the Msg3 buffer and the MSG A buffer for MAC PDU transmission;

[0142] -otherwise:

[0143] * Flush the HARQ buffers in the msg3 buffer and the MSG A buffer for MAC PDU transmission

[0144] Figure 9a and Figure 9b is a flow chart of a two-step random access procedure according to an embodiment.

[0145] refer to Figure 9a and Figure 9b In the case of two-step CF RACH, the gNB 903 signals the contention-free random access resources (i.e., dedicated preamble / RACH opportunity and PUSCH resources) to the UE 901 in steps 910 and 960, respectively. The UE 901 transmits the PRACH preamble in the PRACH opportunity and transmits the MSG A payload (or MSG A MAC PDU) in the PUSCH resource / opportunity using the allocated contention-free random access resources in steps 920, 930, 970, and 980, respectively. The UE 901 stores the MSG A payload (i.e., MSG A MAC PDU) in the MSG A buffer. For the MSG A payload transmission, the HARQ entity in the UE 901 performs a new HARQ packet transmission, where the MSG A payload in the MSG A buffer is used to generate a HARQ packet for transmission. The UE 901 maintains multiple parallel HARQ processes. Each HARQ process supports one TB. Each HARQ process is associated with a HARQ process identifier. For UL transmission of MSG A payload, HARQ process identifier 0 is used.

[0146] After sending MSG A, UE 901 starts msgB-ResponseWindow and monitors the PDCCH for random access response (i.e., msgB) in msgB-ResponseWindow. When msgB-ResponseWindow is running, UE 901 monitors the PDCCH of the SpCell for the random access response identified by MSGB-RNTI. If C-RNTI MAC CE is included in the MSG A payload, UE 901 also monitors the PDCCH of the SpCell for the random access response identified by C-RNTI when msgB-ResponseWindow is running.

[0147] If gNB 901 successfully receives MSG A (i.e., both PRACH preamble and MSG A payload), gNB 901 sends RAPID and TA commands in msgB, as Figure 9a Subsequently, in step 950, gNB 903 sends a PDCCH addressed to the C-RNTI of UE 901 scheduling the UL grant and toggles the NDI bit in the DCI for the HARQ process ID reserved for MSG A (e.g., HARQ process zero). If gNB 903 receives only the PRACH preamble and fails to receive the MSG A payload, gNB 903 sends a RAPID and TA command in msgB, as shown in FIG. Figure 9b As shown in step 990 in . Subsequently, in step 995, gNB 903 sends a PDCCH addressed to the C-RNTI of UE 901 scheduling the UL grant, and for the HARQ process ID reserved for MSG A, the NDI bit in the DCI is not toggled. gNB 903 can identify UE 901 based on the received PRACH preamble. The PRACH preamble is received in the contention-free random access resources allocated to UE 901.

[0148] If MSG A is transmitted using contention-free resources and if msgB received by UE 901 includes a RAPID corresponding to the PRACH preamble transmitted by UE 901, UE 901 considers that msgB is successfully received and the random access procedure is successfully completed.

[0149] After the random access procedure is successfully completed, if UE 901 receives a PDCCH addressed to the C-RNTI that scheduled the UL grant and the NDI bit in the DCI is toggled for the HARQ process ID reserved for MSG A, then UE 901 discards the MSG A payload. UE 901 sends a new MAC PDU in the received UL grant.

[0150] After the random access procedure is successfully completed, if UE 901 receives a PDCCH addressed to the C-RNTI that scheduled the UL grant and the NDI bit in the DCI is not toggled for the HARQ process ID reserved for MSG A, UE 901 retransmits the MSG A payload in the received UL grant.

[0151] Fig.10a and Fig.10b is a flow chart of a two-step random access procedure according to an embodiment.

[0152] refer to Fig.10a and Fig.10b In the case of two-step CF RACH, the gNB 1003 signals the contention-free random access resources (i.e., dedicated preamble / RACH opportunity and PUSCH resources) to the UE 1001 in steps 1010 and 1050, respectively, and the UE 1001 sends the MSG A PRACH preamble and the MSG A payload in steps 1020, 1030, 1060, and 1070, respectively. The UE 1001 stores the MSG A payload (i.e., MAC PDU) in the MSG A buffer. For the MSG A payload transmission, the HARQ entity in the UE 1001 performs a new HARQ packet transmission, where the MSG A payload in the MSG A buffer is used to generate a HARQ packet for transmission. The UE 1001 maintains multiple parallel HARQ processes. Each HARQ process supports one TB. Each HARQ process is associated with a HARQ process identifier. For UL transmission of the MSG A payload, the HARQ process identifier 0 is used.

[0153] After sending MSG A, UE 1001 starts msgB-ResponseWindow and monitors the PDCCH for random access response (i.e., msgB) in msgB-ResponseWindow. While msgB-ResponseWindow is running, UE 1001 monitors the PDCCH of the SpCell for the random access response identified by MSGB-RNTI. If C-RNTI MAC CE is included in the MSG A payload, UE 1001 also monitors the PDCCH of the SpCell for the random access response identified by C-RNTI while msgB-ResponseWindow is running.

[0154] If gNB 1003 successfully receives MSG A (i.e., both PRACH preamble and MSG A payload), gNB 1003 sends RAPID, TA command and UL grant in msgB, as shown in Fig.10aAs shown in step 1040 in msgB. A HARQ process ID other than zero is included in msgB. If gNB 1003 receives only the PRACH preamble but fails to receive the MSG A payload, gNB 1003 sends the RAPID, TA command, and UL grant in msgB, as shown in step 1040 in msgB. Fig.10b In this case, HARQ process ID zero is included in msgB.

[0155] If MSG A is transmitted using contention-free resources and if msgB received by UE 1001 includes a RAPID corresponding to the PRACH preamble transmitted by UE 1001, UE 1001 considers that msgB is successfully received and the random access procedure is successfully completed.

[0156] If msgB includes a HARQ process ID other than zero, UE 1001 discards the MSG A payload. UE 1001 sends a new MAC PDU in the UL grant received in msgB.

[0157] If msgB includes HARQ process ID zero, UE 1001 retransmits MSG A payload in the UL grant received in msgB. For the transmission of MSG A payload in the UL grant received in msgB, the HARQ entity in UE 1001 performs a new HARQ packet transmission, where the HARQ packet for transmission is generated using the MSG A payload in the MSG A buffer. For the UL transmission of the MSG A payload, HARQ process identifier 0 is used. After transmitting the MSG A payload in the UL grant received in msgB, if UE 1001 receives a PDCCH addressed to the C-RNTI for HARQ process zero whose NDI is not switched, UE 1001 performs HARQ retransmission. If msgB includes HARQ process ID zero and if no UL grant is received in msgB, UE 1001 retransmits MSG A payload in a subsequent UL grant using HARQ process zero after receiving a PDCCH addressed to C-RNTI for HARQ process zero with NDI not switched.

[0158] 2-step RACH: Msg3 MAC PDU generation during fallback

[0159] Fig.11 is a flow chart of a contention-based two-step random access procedure according to an embodiment.

[0160] refer to Fig.11In the case of contention-based two-step RACH, if in steps 1110 and 1120, gNB 1103 only receives the PRACH preamble but fails to receive the MSG A payload, then gNB 1103 may indicate to UE 1101 to fall back to four-step RACH. For indicating fallback, gNB 1103 sends RAPID, TA command, UL grant, and TC-RNTI in msgB, as shown in Fig.11 After receiving the fallback indication in msgB, UE 1101 generates msg3 and sends it in the UL grant received in msgB in step 1140. In step 1150, contention resolution is performed via msg4 as in the four-step RACH procedure.

[0161] In four-step RACH, after receiving the UL grant in msg2, a new MAC PDU for msg3 is generated using the available MAC CE and SDU in the logical channel buffer in the MAC. In case of two-step RACH, since the required information has already been sent in MSGA, it is not available in the logical channel buffer.

[0162] Fig.12 is a flow chart of a contention-based two-step random access procedure according to an embodiment.

[0163] refer to Fig.12 , the UE stores the MAC PDU sent in MSG A. If the RA procedure is successfully completed after receiving msgB, the UE discards the stored MSG A MAC PDU. If msgB indicates fallback, the UE uses the content of MSG A MAC PDU to prepare msg3 MAC PDU.

[0164] In step 1210, the UE sends a PRACH preamble in a PRACH opportunity and sends a MSG A payload (or MSG A MAC PDU) in a PUSCH resource / opportunity. The PRACH preamble is a contention-based preamble. In step 1220, the UE stores the MSG A payload (i.e., MSG A MAC PDU) in an MSG A buffer. For MSG A payload transmission, the HARQ entity in the UE performs a new HARQ packet transmission, where the MSG A payload in the MSG A buffer is used to generate a HARQ packet for transmission. The UE maintains multiple parallel HARQ processes. Each HARQ process supports one TB. Each HARQ process is associated with a HARQ process identifier. For UL transmission of MSG A payload, HARQ process identifier 0 is used.

[0165] After sending MSG A, the UE starts msgB-ResponseWindow and monitors the PDCCH for the random access response (i.e., msgB) in msgB-ResponseWindow. While msgB-ResponseWindow is running, the UE monitors the PDCCH of the SpCell for the random access response identified by MSGB-RNTI. If C-RNTI MAC CE is included in the MSG A payload, the UE also monitors the PDCCH of the SpCell for the random access response identified by C-RNTI when msgB-ResponseWindow is running.

[0166] If, in step 1230, the UE receives msgB and msgB includes fallback information (i.e., RAPID, TA, TC-RNTI, and UL grant), and in step 1240, the RAPID in the fallback information matches the random access preamble index of the PRACH preamble sent by the UE, then in step 1260, the UE prepares msg3 MAC PDU using the content of the MSG A MAC PDU and sends the msg3 MAC PDU in the UL grant received in msgB. Otherwise, if msgB does not include fallback information and the RA procedure is successfully completed after receiving msgB, then in steps 1240 and 1250, the UE discards the MSG A payload.

[0167] For the transmission of the msg3 payload in the UL grant received in msgB, the HARQ entity in the UE performs a new HARQ packet transmission, wherein the msg3 payload is used to generate the HARQ packet for transmission. For the UL transmission of the msg3 payload, the HARQ process identifier 0 is used. After transmitting the msg3 payload in the UL grant received in msgB, if the UE receives a PDCCH addressed to the TC-RNTI for HARQ process zero, the UE performs a HARQ retransmission.

[0168] Fig.13 is a flow chart of a contention-based two-step random access procedure according to an embodiment.

[0169] refer to Fig.13 When the received msgB indicates fallback in step 1310, the UE generates the msg3 MAC PDU using the content of the MSG A MAC PDU as follows.

[0170] If the UL grant size in msgB is the same as the size of the MSG A MAC PDU, then in steps 1320 and 1360, the UE uses the MSG A MAC PDU as the msg3 MAC PDU.

[0171] If the UL grant size in msgB is larger than the size of the MSG A MAC PDU in step 1330, the UE generates a new MAC PDU by adding padding at the end of the MSG A MAC PDU in step 1340. Optionally, in step 1340, the UE generates a new MAC PDU using the content of the MSG A MAC PDU and the available MAC CE and SDU in the logical channel buffer by applying Logical Channel Priority (LCP).

[0172] Optionally, the UE generates a new MAC PDU using the MAC subPDU carrying the MAC SDU from the MSG A MAC PDU and the available MAC CE and SDU in the logical channel buffer by applying the LCP. In other words, in step 1340, the UE indicates to the multiplexing and assembly entity to include the MAC subPDU carrying the MAC SDU from the obtained MSG A MAC PDU in subsequent uplink transmissions.

[0173] Optionally, the MAC of the UE may inform the RLC entity of the logical channels whose MAC SDUs are included in the MSG A MAC PDUs in which these MAC SDUs are lost. The lost MAC SDUs may also be provided by the MAC to the RLC entity. Then, in step 1340, the RLC entity may retransmit those MAC SDUs.

[0174] If the UL grant size in msgB is smaller than the size of the MSG A MAC PDU in step 1330, the UE generates a new MAC PDU using the content of the MSG A MAC PDU and the available MAC CE and SDU in the logical channel buffer by applying LCP. In step 1350, the UE discards the MSG A MAC PDU.

[0175] Optionally, the UE generates a new MAC PDU using the MAC subPDU carrying the MAC SDU from the MSG A MAC PDU and the available MAC CE and SDU in the logical channel buffer by applying the LCP. In other words, in step 1350, the UE indicates to the multiplexing and assembly entity to include the MAC subPDU carrying the MAC SDU from the obtained MSG A MAC PDU in subsequent uplink transmissions.

[0176] Optionally, the MAC of the UE may inform the RLC entity of the logical channels whose MAC SDUs are included in the MSG A MAC PDUs in which these MAC SDUs are lost. The lost MAC SDUs may also be provided by the MAC to the RLC entity. Then, in step 1350, the RLC entity may retransmit those MAC SDUs.

[0177] MsgB receiving timing

[0178] Fig.14 is a diagram of message A (or MSG A) reception timing according to an embodiment.

[0179] refer to Fig.14 In a two-step RACH, the channel structure of MSG A includes a PRACH preamble and PUSCH 1410 and 1420 carrying a payload. The PRACH preamble and PUSCH (or GAP 1425 in addition) in MSG A are time division multiplexed (TDM). Therefore, unlike starting the msgB-ResponseWindow from the end of the random access preamble transmission in MSG A, the msgB-ResponseWindow for msgB reception is started at the first PDCCH opportunity of msgB reception at least one symbol away from the end of the PUSCH transmission in MSG A. In other words, the msgB-ResponseWindow for msgB reception is started at the first PDCCH opportunity of msgB reception at least one symbol after the last symbol of the PUSCH opportunity corresponding to the PUSCH transmission. The PDCCH opportunity for msgB reception is configured by the ra-SearchSpace parameter in the PDCCH-ConfigCommon information element (IE) of the BWP for the RA procedure.

[0180] In the two-step random access procedure, if the UE sends only the PRACH preamble during the MSG A transmission, the msgB-ResponseWindow is started at the first PDCCH opportunity of the random access response (i.e., msgB) at least one symbol from the end of the random access preamble transmission. If there is no available PUSCH opportunity corresponding to the PRACH preamble and PRACH opportunity selected by the UE, the MSG A payload in the PUSCH opportunity is not sent. In the case of UL transmission in unlicensed spectrum, if the channel is not available during the PUSCH opportunity, the MSG A payload is not sent. If the UE sends both the PRACH preamble and the PUSCH, the msgB-ResponseWindow for the random access response (i.e., msgB) reception is started at the first PDCCH opportunity of the response reception at least one symbol from the end of the PUSCH.

[0181] PUSCH resource signaling for MSGA payload in 2-step RACH

[0182] Fig.15 is a diagram of PUSCH resource signaling according to an embodiment.

[0183] refer to Fig.15 , the gNB signals the parameters N2, N3, L, O and MSG APUSCH-FrequencyStart. These parameters are signaled for each BWP that supports two-step RACH. These parameters may be signaled as part of the two-step RACH configuration in system information or dedicated RRC signaling. Using these parameters, the UE can identify PUSCH resources relative to each set of frequency division multiplexing (FDM) PRACH opportunities, such as Fig.15 As shown. The PRACH opportunities of this group of FDM refer to the RACH opportunities of the PRACH time slot. The time slot with the PRACH opportunity is configured by the parameter PRACH configuration (config) index, as in the traditional four-step RACH. MSG APUSCH-FrequencyStart indicates the starting PRB of the PUSCH resource of FDM in the frequency domain. The starting PRB is indicated relative to PRB 0 of the BWP. N2 is the number of PUSCH resources for FDM. N3 is the number of PRBs in a PUSCH resource. L is the length of the symbol and / or time slot of the PUSCH resource. O is the offset of the PUSCH resource relative to the PRACH opportunity (i.e., relative to the start or end of the PRACH time slot carrying the PRACH opportunity). The offset can be in symbols or time slots. For N2>=N1, the number of PUSCH resources mapped to one PRACH opportunity is equal to N2 / N1, where N1 is the number of PRACH opportunities for FDM. For N2 less than N1, N1 / N2 PRACH opportunities are mapped to one PUSCH resource. In this case, the preambles used in the PRACH opportunities mapped to the same PUSCH resource should be different.

[0184] Fig.16 is a diagram of PUSCH resource signaling according to an embodiment.

[0185] refer to Fig.16 , the gNB signals the parameters N2, N3, L, O and MSG APUSCH-FrequencyStart. These parameters may be signaled as part of the two-step RACH configuration in system information or dedicated RRC signaling. These parameters are signaled for each BWP that supports two-step RACH. Using these parameters, the UE can identify PUSCH resources relative to each set of frequency division multiplexing (FDM) PRACH opportunities, such as Fig.16As shown. The PRACH opportunities of this group of FDM refer to the RACH opportunities of the PRACH time slot. MSG APUSCH-FrequencyStart indicates the starting PRB of the PUSCH resource of FDM in the frequency domain. The starting PRB is indicated relative to PRB 0 of the BWP. N2 is the number of PUSCH resources for FDM. N3 is the number of PRBs in one PUSCH resource. L is the length of the symbol and / or time slot of the PUSCH resource. O is the offset of the PUSCH resource relative to the PRACH opportunity (i.e., relative to the start or end of the PRACH time slot carrying the PRACH opportunity). The offset can be in symbols or time slots. For N2>=M, the number of PUSCH resources mapped to one PRACH opportunity is equal to N2 / M; M is equal to N1*N4, where N1 is the number of PRACH opportunities for FDM, and N4 is the number of PRACH opportunities in one PRACH time slot. For N2 less than M, M / N2 PRACH opportunities are mapped to one PUSCH resource. In this case, the preambles used in the PRACH opportunities mapped to the same PUSCH resource should be different.

[0186] RACH Less switching or reconfiguration

[0187] In NR TCI, the framework for beam indication / switching for PDCCH, PDSCH and channel state information reference signal (CSI-RS) is specified. One DL RS (SSB or CSI RS) is RRC configured per TCI state. The UE identifies its beam information based on the DL RS of the indicated TCI state. For PDCCH beam indication, the TCI state list is RRC configured per CORESET. One of these TCI states based on the UE determination of the PDCCH beam is indicated by the MAC CE.

[0188] In case of handover, the UE is configured with a list of TCI states for the target cell. The configured TCI states are initially deactivated at handover. If the UE receives an initial configuration of multiple TCI states for PDCCH reception via the higher layer parameter TCI-States, but has not yet received a MAC CE activation command for one of the TCI states, the UE assumes that the DM-RS antenna ports associated with PDCCH reception are quasi-collocated with the SS / PBCH blocks identified by the UE during the random access procedure of the target cell.

[0189] RACH-free handover is being discussed to reduce handover latency. In the case of RACH-free handover, since the UE does not perform random access in the target cell, the UE knows how to receive the PDCCH in the target cell when all TCI states received in the handover command are in the deactivated state.

[0190] If no RACH handover is indicated in the reconfiguration message, the gNB indicates a single TCI state for the target cell. In case of BWP operation, in the DL BWP indicated by the first activated DL BWP ID, if no RACH handover is indicated in the reconfiguration message, the gNB configures only a single TCI state. The UE uses the TCI state information provided by the TCI state to receive the PDCCH in the target cell at the time of handover. The UE assumes that the DM-RS antenna ports associated with PDCCH reception are quasi-collocated with one or more DL RS configured by the TCI state.

[0191] The gNB may configure a single TCI state based on the SSB ID / CSI RS ID of the target cell included in the last measurement report. Therefore, when the UE performs a handover to the target cell, the single TCI state provided in the reconfiguration message may be invalid. The following items may be considered at the time of handover:

[0192] 1) If RACH-less switching is configured and if the SSB ID / CSI RS ID indicated by the TCI status is not appropriate, the UE performs random access.

[0193] 2) If the UE fails to receive any PDCCH within a configurable duration, the UE performs random access.

[0194] If no RACH handover is indicated in the reconfiguration message, the gNB indicates an activated TCI state in the TCI state list configured for the target cell (i.e., the gNB signals the activated TCI state ID in the reconfiguration message). In case of BWP operation, in the DL BWP indicated by the first activated DL BWP ID, if no RACH handover is indicated in the reconfiguration message, the gNB indicates an activated TCI state in the TCI state list configured for the target cell (i.e., the gNB signals the activated TCI state ID in the reconfiguration message). If no RACH handover is indicated in the reconfiguration message, a predefined (e.g., first) TCI state in the default TCI state list is activated. The UE uses the TCI state information provided by the activated TCI state to receive the PDCCH in the target cell at the time of handover. The UE assumes that the DM-RS antenna ports associated with PDCCH reception are quasi-collocated with one or more DL RS configured by the TCI state.

[0195] The gNB may indicate the activated TCI state based on the SSB ID / CSI RS ID of the target cell included in the last measurement report. Therefore, when the UE performs a handover to the target cell, the activated TCI state provided in the reconfiguration message may be invalid. The following items may be considered at the time of handover:

[0196] 1) If RACH-less switching is configured and if the SSB ID / CSI RS ID indicated by the activated TCI state is not suitable, the UE performs random access.

[0197] 2) If the UE fails to receive any PDCCH based on the activated TCI state within a configurable duration, the UE performs random access.

[0198] If no RACH switching is indicated in the reconfiguration message and the gNB indicates a TCI state list for the target cell, the UE assumes that the DM-RS antenna port associated with PDCCH reception is quasi-collocated with the SS / PBCH block or CSI RS with the highest signal quality reported by the UE in the last measurement report for the target cell.

[0199] 1) If the SS / PBCH block or CSI RS with the highest signal quality reported by the UE in the last measurement report for the target cell is not suitable at the time of handover, the UE performs random access even if no RACH handover is indicated in the reconfiguration message.

[0200] 2) If the UE fails to receive any PDCCH in the target cell within a configurable duration, the UE performs random access.

[0201] Hereinafter, another embodiment of the present disclosure is described, which relates to a method and apparatus for falling back to random access during RACH-free switching.

[0202] In the 5G wireless communication system (also known as NR), a UE in the RRC CONNECTED state supports network-controlled cell-level mobility. The typical process of cell-level mobility is as follows:

[0203] 1. The source gNB configures the UE measurement procedure and the UE reports according to the measurement configuration.

[0204] 2. The source gNB decides to handover the UE based on the MeasurementReport and Radio Resource Management (RRM) information.

[0205] 3. The source gNB issues a handover request message to the target gNB to pass a transparent RRC container with the necessary information to prepare the handover on the target side. The handover request message includes at least the target cell ID, KgNB*, C-RNTI of the UE in the source gNB, RRM configuration including the UE inactive time, basic AS configuration including antenna information and DL carrier frequency, mapping rules of the current QoS flow to DRB applied to the UE, SIB1 from the source gNB, UE capabilities of different radio access technologies (RATs), information related to packet data unit (PDU) sessions; and measurement information including beam related information reported by the UE (if available). Information related to the PDU session includes slice information (if supported) and QoS profiles at the QoS flow level.

[0206] 4. Admission control may be performed by the target gNB. If slice information is sent to the target gNB, slice-aware admission control shall be performed. If a PDU Session is associated with an unsupported slice, the target gNB shall reject such PDU Session.

[0207] 5. The target gNB prepares for the L1 / L2 handover and sends a HANDOVER REQUEST ACKNOWLEDGE to the source gNB, which includes a transparent container to be sent as an RRC message to the UE to perform the handover.

[0208] 6. The source gNB triggers the UE handover by sending an RRCReconfiguration message to the UE. The RRCReconfiguration message includes the information required to access the target cell: at least the target cell ID, the new C-RNTI, and the target gNB security algorithm identifier for the selected security algorithm. The RRCReconfiguration message may also include a set of dedicated RACH resources, the association between RACH resources and SSBs, the association between RACH resources and UE-specific CSI-RS configurations, common RACH resources, and system information of the target cell.

[0209] 7. The source gNB sends an SN STATUS TRANSFER message to the target gNB.

[0210] 8. After DL synchronization, the UE synchronizes to the target cell and the UE performs a random access procedure for UL synchronization. The UE completes the RRC handover procedure by sending an RRCReconfigurationComplete message to the target gNB.

[0211] 9. The target gNB sends a PATH SWITCH REQUEST message to the Access Management Function (AMF) to trigger the 5G Core (5GC) to switch the DL data path towards the target gNB and establish an interface instance towards the target gNB NG-C.

[0212] 10.5GC switches the DL data path towards the target gNB. The UPF sends one or more "end marker" packets to the source gNB on the old path for each PDU session / channel, and then any U-Plane / channel (TNL) resources may be released towards the source gNB.

[0213] 11.AMF confirms the PATH SWITCH REQUEST message with a PATH SWITCH REQUEST ACKNOWLEDGE message.

[0214] 12.After receiving the PATH SWITCH REQUEST ACKNOWLEDGE message from the AMF, the target gNB sends UECONTEXT RELEASE to inform the source gNB that the handover is successful. The source gNB may then release the radio and C-plane related resources associated with the UE context. Any ongoing data forwarding may continue.

[0215] In order to reduce the handover waiting time, RACH-free handover is being studied. In the case of RACH-free handover, the UE is not required to perform random access with the target cell. RACH-free handover is supported in 4G wireless communication systems. In a 4G wireless communication system, a handover command (ie, RRCReconfiguration) indicates whether the UE skips random access with the target cell. For UL transmission in the target cell, the handover command also indicates whether the timing advance of the source cell is applied or whether the TA is equal to zero. Optionally, the handover command may provide a pre-allocated UL grant for sending RRCReconfigurationComplete and / or UL data. The same process can be applied to RACH-free handover in NR. NR supports high frequency bands (>6GHz), also known as FR2 bands, and lower frequency bands (<=6GHz), also known as FR1 bands. At high frequencies, beamforming is necessary. In the current handover process, the initial beam alignment between the UE and the target cell is performed via a random access process.

[0216] In the case of RACH-free switching, for beam alignment, the pre-allocated UL grant signaled in the switching command can be associated with one or more SSB / CSI RS. The UE can select the SSB / CSI RS and then transmit in the UL grant corresponding to the selected SSB / CSI RS. Methods are needed for selecting the SSB / CSI RS among the SSB / CSI RS associated with the pre-allocated UL grant. Methods are needed for handling scenarios where there is no SSB / CSI RS selectable among the SSB / CSI RS associated with the pre-allocated UL grant.

[0217] Based on the measurement of SS-RSRP / CSI-RS of SSB / CSI-RS associated with the pre-allocated UL grant, retreat

[0218] Various methods in the present disclosure for handling a non-SSB / CSI RS optional scenario in an SSB / CSI RS associated with a pre-allocated UL grant are described below.

[0219] Fig.17 is a flow chart of a method for handling fallback to random access during handover without random access channel (no RACH) according to an embodiment.

[0220] refer to Fig.17 In step 1710, UE 1701 receives an RRCReconfiguration message from a source cell 1703 (e.g., a gNB). The spCellConfig in the RRCReconfiguration message includes reconfigurationWithSync. The information in the reconfigurationWithSyncIE (e.g., an indication of skipping RACH) indicates that UE 1701 should skip RACH toward the target cell 1705. The RRCReconfiguration message includes a pre-allocated UL grant configuration (parameters indicating periodically occurring UL grants). The configuration is provided for at least the UL BWP indicated by firstActiveUplinkBWP-Id. The RRCReconfiguration message also includes information of the SSB / CSI-RS associated with the pre-allocated UL grant (i.e., SSB ID / CSI-RS ID).

[0221] In step 1720, UE 1701 measures SS-RSRP / CSI-RSRP of SSB / CSI-RS associated with the pre-allocated UL grant.

[0222] If there is at least one available SSB / CSI-RS in the SSB / CSI RS associated with the pre-allocated UL grant, whose SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-threshold, then in step 1730, the UE 1701 performs the following operations:

[0223] - The UE skips RACH towards the target cell.

[0224] * Selecting an SSB / CSI-RS having an SS-RSRP / CSI-RSRP higher than the configured ss-rsrp-threshold / csi-rsrp-threshold among the SSB / CSI RS associated with the pre-allocated UL grant (1740).

[0225] **If SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-

[0226] If multiple SSB / CSI-RSs of threshold / csi-rsrp-threshold are available, the UE selects any one or the UE selects the SSB / CSI RS that is first available for its associated UL grant (in the time domain).

[0227] * ReconfigurationComplete (1750) is sent in the UL grant corresponding to the selected SSB / CSI-RS (in the pre-allocated UL grant).

[0228] If there is no SSB / CSI-RS in the SSB / CSI RS associated with the pre-allocated UL grant whose SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-threshold, then in step 1730, UE 1701 performs the following operations:

[0229] - The UE falls back to the random access procedure, i.e., the UE initiates the random access procedure (1760).

[0230] * ReconfigurationComplete is sent in the UL grant received in RAR.

[0231] ss-rsrp-threshold / csi-rsrp-threshold is signaled by gNB 1703 in the RRCReconfiguration message. If the UL grant in the pre-allocated UL grant configuration is associated with SSB, then ss-rsrp-threshold is configured. If the UL grant in the pre-allocated UL grant configuration is associated with CSI-RS, then csi-rsrp-threshold is configured. ss-rsrp-threshold is configured in the RACH configuration.

[0232] In step 1710, UE 1701 receives an RRCReconfiguration message from gNB 1703. The spCellConfig in the RRCReconfiguration message includes reconfigurationWithSync. The information in the reconfigurationWithSyncIE (e.g., an indication of skipping RACH) instructs UE 1701 to skip RACH toward the target cell 1705. The RRCReconfiguration message includes a pre-allocated UL grant configuration (parameters indicating UL grants that occur periodically). The configuration is provided for at least the UL BWP indicated by firstActiveUplinkBWP-Id. The RRCReconfiguration message also includes information of the SSB / CSI-RS associated with the pre-allocated UL grant (i.e., SSBID / CSI-RS ID).

[0233] In step 1720, UE 1701 measures SS-RSRP / CSI-RSRP of SSB / CSI-RS associated with the pre-allocated UL grant.

[0234] If there is at least one available SSB / CSI-RS in the SSB / CSI RS associated with the pre-allocated UL grant, whose SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-threshold, then in step 1730, the UE 1701 performs the following operations:

[0235] -UE skips RACH towards the target cell

[0236] * Selecting an SSB / CSI-RS having an SS-RSRP / CSI-RSRP higher than the configured ss-rsrp-threshold / csi-rsrp-threshold among the SSB / CSI RS associated with the pre-allocated UL grant (1740).

[0237] **If SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-

[0238] If multiple SSB / CSI-RSs of threshold / csi-rsrp-threshold are available, the UE selects any one or the UE selects the SSB / CSI RS that is first available for its associated UL grant (in the time domain).

[0239] * ReconfigurationComplete (1750) is sent in the UL grant corresponding to the selected SSB / CSI-RS (in the pre-allocated UL grant).

[0240] If there is no SSB / CSI-RS in the SSB / CSI RS associated with the pre-allocated UL grant whose SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-threshold, then in step 1730, UE 1701 performs the following operations:

[0241] - If the fallback indication is set to true in the RRCReconfiguration message, the UE falls back to the random access procedure, that is, the UE initiates the random access procedure (1760).

[0242] * ReconfigurationComplete is sent in the UL grant received in RAR.

[0243] -otherwise:

[0244] * Select any of the SSB / CSI RS associated with the pre-allocated UL grant.

[0245] *ReconfigurationComplete is sent in the UL grant (in the pre-allocated UL grant) corresponding to the selected SSB / CSI-RS.

[0246] ss-rsrp-threshold / csi-rsrp-threshold is signaled by gNB 1703 in the RRCReconfiguration message. If the UL grant in the pre-allocated UL grant configuration is associated with SSB, then ss-rsrp-threshold is configured. If the UL grant in the pre-allocated UL grant configuration is associated with CSI-RS, then csi-rsrp-threshold is configured. ss-rsrp-threshold can be configured in the RACH configuration.

[0247] In step 1710, UE 1701 receives an RRCReconfiguration message from gNB 1703. The spCellConfig in the RRCReconfiguration message includes reconfigurationWithSync. The information in the reconfigurationWithSyncIE (e.g., an indication of skipping RACH) instructs UE 1701 to skip RACH toward the target cell 1705. The RRCReconfiguration message includes a pre-allocated UL grant configuration (parameters indicating UL grants that occur periodically). The configuration is provided for at least the UL BWP indicated by firstActiveUplinkBWP-Id. The RRCReconfiguration message also includes information of the SSB / CSI-RS associated with the pre-allocated UL grant (i.e., SSBID / CSI-RS ID).

[0248] In step 1720, UE 1701 measures SS-RSRP / CSI-RSRP of SSB / CSI-RS associated with the pre-allocated UL grant.

[0249] If there is at least one available SSB / CSI-RS in the SSB / CSI RS associated with the pre-allocated UL grant, whose SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-threshold, then in step 1730, the UE 1701 performs the following operations:

[0250] -UE skips RACH towards the target cell

[0251] * Selecting an SSB / CSI-RS having an SS-RSRP / CSI-RSRP higher than the configured ss-rsrp-threshold / csi-rsrp-threshold among the SSB / CSI RS associated with the pre-allocated UL grant (1740).

[0252] **If SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-

[0253] If multiple SSB / CSI-RSs of threshold / csi-rsrp-threshold are available, the UE selects any one or the UE selects the SSB / CSI RS that is first available for its associated UL grant (in the time domain).

[0254] * ReconfigurationComplete (1750) is sent in the UL grant corresponding to the selected SSB / CSI-RS (in the pre-allocated UL grant).

[0255] If there is no SSB / CSI-RS in the SSB / CSI RS associated with the pre-allocated UL grant whose SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-threshold, then in step 1730, UE 1701 performs the following operations:

[0256] - If the number of SSB / CSI-RS associated with the pre-allocated UL grant is less than the maximum SSB / CSIRS supported in the target cell, the UE falls back to the random access procedure, i.e., the UE initiates the random access procedure.

[0257] * ReconfigurationComplete is sent in the UL grant received in RAR.

[0258] -otherwise:

[0259] * Select any of the SSB / CSI RS associated with the pre-allocated UL grant.

[0260] * ReconfigurationComplete (1760) is sent in the UL grant corresponding to the selected SSB / CSI-RS (in the pre-allocated UL grant).

[0261] ss-rsrp-threshold / csi-rsrp-threshold is signaled by gNB 1703 in the RRCReconfiguration message. If the UL grant in the pre-allocated UL grant configuration is associated with SSB, then ss-rsrp-threshold is configured. If the UL grant in the pre-allocated UL grant configuration is associated with CSI-RS, then csi-rsrp-threshold is configured. ss-rsrp-threshold can be configured in the RACH configuration.

[0262] Fig.18 is a flow chart of a method for handling fallback to random access during handover without random access channel (no RACH) according to an embodiment.

[0263] refer to Fig.18In step 1810, UE 1801 receives an RRCReconfiguration message from a source cell 1803 (e.g., a gNB). The spCellConfig in the RRCReconfiguration message includes reconfigurationWithSync. The information in the reconfigurationWithSyncIE (e.g., an indication of skipping RACH) indicates that UE 1801 should skip RACH toward the target cell 1805. The RRCReconfiguration message includes a pre-allocated UL grant configuration (parameters indicating periodically occurring UL grants). The configuration is provided for at least the UL BWP indicated by firstActiveUplinkBWP-Id. In step 1820, the RRCReconfiguration message also includes information of the SSB / CSI-RS associated with the pre-allocated UL grant (i.e., SSB ID / CSI-RS ID).

[0264] In step 1830, UE 1801 starts a back-off timer.

[0265] UE 1801 measures the SS-RSRP / CSI-RSRP of the SSB / CSI-RS associated with the pre-allocated UL grant.

[0266] - If the back-off timer is running and there is at least one available SSB / CSI-RS in the SSB / CSI RS associated with the pre-allocated UL grant whose SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-threshold, then in step 1840, the UE 1801 performs the following operations:

[0267] -The UE stops the backoff timer (1860).

[0268] - The UE skips RACH towards the target cell.

[0269] *Select SS in SSB / CSI RS associated with pre-allocated UL grant-

[0270] RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-

[0271] SSB / CSI-RS(1870) of threshold.

[0272] **If SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-

[0273] If multiple SSB / CSI-RSs of threshold / csi-rsrp-threshold are available, the UE selects any one or the UE selects its associated UL

[0274] Grant the first available SSB / CSI RS (in the time domain).

[0275] * ReconfigurationComplete (1880) is sent in the UL grant corresponding to the selected SSB / CSI-RS (in the pre-allocated UL grant).

[0276] The measurement and comparison of the measured value with the threshold may be performed periodically until the backoff timer expires or the UE 1801 is able to select an SSB / CSI-RS above the threshold.

[0277] If the back-off timer expires, the UE 1801 falls back to the random access procedure, i.e., the UE 1801 initiates the random access procedure. In step 1850, ReconfigurationComplete is sent in the UL grant received in the RAR.

[0278] ss-rsrp-threshold / csi-rsrp-threshold is signaled by gNB 1803 in the RRCReconfiguration message. If the UL grant in the pre-allocated UL grant configuration is associated with SSB, then ss-rsrp-threshold is configured. If the UL grant in the pre-allocated UL grant configuration is associated with CSI-RS, then csi-rsrp-threshold is configured. ss-rsrp-threshold can be configured in the RACH configuration.

[0279] The duration of the back-off timer is signaled by gNB 1803 in the RRCReconfiguration message. The duration of the back-off timer may be predefined.

[0280] In step 1810, UE 1801 receives an RRCReconfiguration message from gNB 1803. The spCellConfig in the RRCReconfiguration message includes reconfigurationWithSync. The information in the reconfigurationWithSync IE (e.g., an indication of skipping RACH) indicates that UE 1801 should skip RACH toward the target cell 1805. The RRCReconfiguration message includes a pre-allocated UL grant configuration (parameters indicating UL grants that occur periodically). The configuration is provided for at least the UL BWP indicated by firstActiveUplinkBWP-Id. In step 1820, the RRCReconfiguration message also includes information of the SSB / CSI-RS associated with the pre-allocated UL grant (i.e., SSB ID / CSI-RS ID).

[0281] If the back-off timer is configured in the RRCReconfiguration message, in step 1830, UE 1801 starts the back-off timer.

[0282] UE 1801 measures the SS-RSRP / CSI-RSRP of the SSB / CSI-RS associated with the pre-allocated UL grant.

[0283] - If the back-off timer is not configured in the RRCReconfiguration message, and if there is at least one available SSB / CSI-RS in the SSB / CSI RS associated with the pre-allocated available UL grant, whose SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-threshold, then in step 1840, UE 1801 performs the following operations:

[0284] *UE skips RACH towards the target cell.

[0285] **Select SS-

[0286] RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-

[0287] SSB / CSI-RS(1870) of threshold.

[0288] ***If SS-RSRP / CSI-RSRP is higher than the configured ss-

[0289] If multiple SSB / CSI-RSs of rsrp-threshold / csi-rsrp-threshold are available, the UE selects any one or the UE selects the SSB / CSI RS that is first available for its associated UL grant (in the time domain).

[0290] **ReconfigurationComplete (1880) is sent in the UL grant corresponding to the selected SSB / CSI-RS (in the pre-allocated UL grant).

[0291] If there is no SSB / CSI-RS in the SSB / CSI RS associated with the pre-allocated UL grant whose SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-threshold, the UE performs the following operations:

[0292] *UE falls back to the random access procedure, that is, the UE initiates the random access procedure (1850).

[0293] **ReconfigurationComplete is sent in the UL grant received in RAR.

[0294] Otherwise, if the backoff timer is configured in the RRCReconfiguration message:

[0295] If the back-off timer is running and if there is at least one available SSB / CSI-RS in the SSB / CSI RS associated with the pre-allocated UL grant whose SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-threshold, the UE performs the following operations (1840):

[0296] *UE stops the backoff timer (1860).

[0297] *UE skips RACH towards the target cell.

[0298] **Select SS-

[0299] RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-

[0300] SSB / CSI-RS(1870) of threshold.

[0301] ***If SS-RSRP / CSI-RSRP is higher than the configured ss-

[0302] If multiple SSB / CSI-RSs of rsrp-threshold / csi-rsrp-threshold are available, the UE selects any one or the UE selects the SSB / CSI RS that is first available for its associated UL grant (in the time domain).

[0303] **ReconfigurationComplete (1880) is sent in the UL grant corresponding to the selected SSB / CSI-RS (in the pre-allocated UL grant).

[0304] - Measurements and comparison of the measured values ​​with the threshold may be performed periodically until the backoff timer expires or the UE is able to select a SSB / CSI-RS above the threshold.

[0305] - If the backoff timer expires, the UE falls back to the random access procedure, ie, the UE initiates the random access procedure. ReconfigurationComplete is sent in the UL grant received in the RAR (1850).

[0306] ss-rsrp-threshold / csi-rsrp-threshold is configured to be signaled by gNB 1803 in the RRCReconfiguration message. If the UL grant in the pre-allocated UL grant configuration is associated with SSB, then ss-rsrp-threshold is configured. If the UL grant in the pre-allocated UL grant configuration is associated with CSI-RS, then csi-rsrp-threshold is configured. ss-rsrp-threshold can be configured in the RACH configuration.

[0307] UE 1801 receives an RRCReconfiguration message from gNB 1803. The spCellConfig in the RRCReconfiguration message includes reconfigurationWithSync. The information in the reconfigurationWithSyncIE (e.g., an indication of skipping RACH) instructs UE 1801 to skip RACH toward the target cell 1805. The RRCReconfiguration message includes a pre-allocated UL grant configuration (parameters indicating UL grants that occur periodically). The configuration is provided for at least the UL BWP indicated by firstActiveUplinkBWP-Id. The RRCReconfiguration message also includes information of the SSB / CSI-RS associated with the pre-allocated UL grant (i.e., SSB ID / CSI-RS ID).

[0308] If a back-off timer is configured in the RRCReconfiguration message, UE 1801 starts the back-off timer.

[0309] UE 1801 measures the SS-RSRP / CSI-RSRP of the SSB / CSI-RS associated with the pre-allocated UL grant.

[0310] If there is at least one available SSB / CSI-RS among the SSB / CSI RS associated with the pre-allocated UL grant whose SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-threshold, the UE performs the following operations:

[0311] *UE skips RACH towards target cell

[0312] **Select SS-

[0313] RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-

[0314] threshold for SSB / CSI-RS.

[0315] ***If SS-RSRP / CSI-RSRP is higher than the configured ss-

[0316] If multiple SSB / CSI-RSs of rsrp-threshold / csi-rsrp-threshold are available, the UE selects any one or the UE selects the SSB / CSI RS that is first available for its associated UL grant (in the time domain).

[0317] **ReconfigurationComplete is sent in the UL grant (in the pre-allocated UL grant) corresponding to the selected SSB / CSI-RS.

[0318] If there is no SSB / CSI-RS in the SSB / CSI RS associated with the pre-allocated UL grant whose SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-threshold, the UE performs the following operations:

[0319] *If the fallback indication is set to true in the RRCReconfiguration message, the UE falls back to the random access procedure, i.e., the UE initiates the random access procedure.

[0320] **ReconfigurationComplete is sent in the UL grant received in RAR.

[0321] -otherwise:

[0322] * Select any of the SSB / CSI RS associated with the pre-allocated UL grant.

[0323] *ReconfigurationComplete is sent in the UL grant (in the pre-allocated UL grant) corresponding to the selected SSB / CSI-RS.

[0324] Otherwise, if the backoff timer is configured in the RRCReconfiguration message:

[0325] - If the back-off timer is running and if there is at least one available SSB / CSI-RS in the SSB / CSI RS associated with the pre-allocated UL grant whose SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-threshold, the UE performs the following operations:

[0326] *UE stops the backoff timer

[0327] *UE skips RACH towards target cell

[0328] **Select SS-

[0329] RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-

[0330] threshold for SSB / CSI-RS.

[0331] ***If SS-RSRP / CSI-RSRP is higher than the configured ss-

[0332] If multiple SSB / CSI-RSs of rsrp-threshold / csi-rsrp-threshold are available, the UE selects any one or the UE selects the SSB / CSI RS that is first available for its associated UL grant (in the time domain).

[0333] **ReconfigurationComplete is sent in the UL grant (in the pre-allocated UL grant) corresponding to the selected SSB / CSI-RS.

[0334] - Measurements and comparison of the measured values ​​with the threshold may be performed periodically until the backoff timer expires or the UE is able to select a SSB / CSI-RS above the threshold.

[0335] - If the fallback timer expires, the UE falls back to the random access procedure, i.e., the UE initiates the random access procedure. ReconfigurationComplete is sent in the UL grant received in the RAR.

[0336] ss-rsrp-threshold / csi-rsrp-threshold is configured to be signaled by gNB 1803 in the RRCReconfiguration message. If the UL grant in the pre-allocated UL grant configuration is associated with SSB, then ss-rsrp-threshold is configured. If the UL grant in the pre-allocated UL grant configuration is associated with CSI-RS, then csi-rsrp-threshold is configured. ss-rsrp-threshold can be configured in the RACH configuration.

[0337] UE 1801 receives an RRCReconfiguration message from gNB 1803. The spCellConfig in the RRCReconfiguration message includes reconfigurationWithSync. The information in the reconfigurationWithSyncIE (e.g., an indication of skipping RACH) instructs UE 1801 to skip RACH toward the target cell 1805. The RRCReconfiguration message includes a pre-allocated UL grant configuration (parameters indicating UL grants that occur periodically). The configuration is provided for at least the UL BWP indicated by firstActiveUplinkBWP-Id. The RRCReconfiguration message also includes information of the SSB / CSI-RS associated with the pre-allocated UL grant (i.e., SSB ID / CSI-RS ID).

[0338] If a back-off timer is configured in the RRCReconfiguration message, UE 1801 starts the back-off timer.

[0339] UE 1801 measures the SS-RSRP / CSI-RSRP of the SSB / CSI-RS associated with the pre-allocated UL grant.

[0340] If the backoff timer is not configured in the RRCReconfiguration message:

[0341] If there is at least one available SSB / CSI-RS among the SSB / CSI RS associated with the pre-allocated UL grant whose SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-threshold, the UE performs the following operations:

[0342] *UE skips RACH towards target cell

[0343] **Select SS-

[0344] RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-

[0345] threshold for SSB / CSI-RS.

[0346] ***If SS-RSRP / CSI-RSRP is higher than the configured ss-

[0347] If multiple SSB / CSI-RSs of rsrp-threshold / csi-rsrp-threshold are available, the UE selects any one or the UE selects the SSB / CSI RS whose associated UL grant (in the time domain) is available first.

[0348] **ReconfigurationComplete is sent in the UL grant (in the pre-allocated UL grant) corresponding to the selected SSB / CSI-RS.

[0349] If there is no SSB / CSI-RS in the SSB / CSI RS associated with the pre-allocated UL grant whose SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-threshold, the UE performs the following operations:

[0350] *If the number of SSB / CSIRS associated with the pre-allocated UL grant is less than the maximum SSB / CSIRS supported in the target cell, the UE falls back to the random access procedure, i.e., the UE initiates the random access procedure.

[0351] **ReconfigurationComplete is sent in the UL grant received in RAR.

[0352] -otherwise:

[0353] * Select any of the SSB / CSI RS associated with the pre-allocated UL grant.

[0354] *ReconfigurationComplete is sent in the UL grant (in the pre-allocated UL grant) corresponding to the selected SSB / CSI-RS.

[0355] Otherwise, if the backoff timer is configured in the RRCReconfiguration message:

[0356] - If the back-off timer is running and if there is at least one available SSB / CSI-RS in the SSB / CSI RS associated with the pre-allocated UL grant whose SS-RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-threshold, the UE performs the following operations:

[0357] *UE stops the backoff timer

[0358] *UE skips RACH towards target cell

[0359] **Select SS-

[0360] RSRP / CSI-RSRP is higher than the configured ss-rsrp-threshold / csi-rsrp-

[0361] threshold for SSB / CSI-RS.

[0362] ***If SS-RSRP / CSI-RSRP is higher than the configured ss-

[0363] If multiple SSB / CSI-RSs of rsrp-threshold / csi-rsrp-threshold are available, the UE selects any one or the UE selects the SSB / CSI RS that is first available for its associated UL grant (in the time domain).

[0364] **ReconfigurationComplete is sent in the UL grant (in the pre-allocated UL grant) corresponding to the selected SSB / CSI-RS.

[0365] - Measurements and comparison of the measured values ​​with the threshold may be performed periodically until the backoff timer expires or the UE is able to select a SSB / CSI-RS above the threshold.

[0366] -If the fallback timer expires, the UE falls back to the random access procedure, i.e., the UE initiates the random access procedure. ReconfigurationComplete is sent in the UL grant received in the RAR. ss-rsrp-threshold / csi-rsrp-threshold is configured to be signaled by gNB 1803 in the RRCReconfiguration message. If the UL grant in the pre-allocated UL grant configuration is associated with SSB, ss-rsrp-threshold is configured. If the UL grant in the pre-allocated UL grant configuration is associated with CSI-RS, csi-rsrp-threshold is configured. ss-rsrp-threshold can be configured in the RACH configuration.

[0367] Back off based on TAT timer

[0368] Fig.19 is a diagram of handling fallback to random access during handover without random access channel (no RACH) according to an embodiment.

[0369] refer to Fig.19, the handover command indicates a RACH-free handover. The handover command indicates to use the TA of the source cell for UL transmission in the target cell. When the handover command is received, the TAT timer has X ms (e.g., 10 ms) remaining in the source cell. If the RACH-free handover is not completed within X ms, the TA of the source cell may become invalid during the handover execution. If T is greater than X, the TA value will become invalid.

[0370] Various methods to overcome the above problems are described below:

[0371] The UE receives an RRCReconfiguration message from the gNB. The spCellConfig in the RRCReconfiguration message includes reconfigurationWithSync. The information in the reconfigurationWithSync IE (e.g., an indication to skip RACH) indicates that the UE should skip RACH towards the target cell. The information also indicates the target timing advance (i.e., targetTA) (i.e., the TA of the primary timing advance group (PTAG) or the secondary timing advance group (STAG) in the source gNB) that the UE should use for UL transmissions in the PTAG at the target gNB. The RRCReconfiguration message includes a pre-allocated UL grant configuration (parameters indicating that UL grants occur periodically). This configuration is provided at least for the ULBWP indicated by firstActiveUplinkBWP-Id.

[0372] The UE initializes the TAT timer of PTAG to the remaining value of the TAT timer of targetTA.

[0373] The UE starts using the targetTA (i.e., the TA of the PTAG or STAG in the source gNB) for UL transmissions in the target cell.

[0374] If the TAT timer expires and if the RACH-less handover is not completed (i.e., the UE has not received a MAC CE indicating confirmation of the RACH-less handover completion):

[0375] -UE falls back to the random access procedure, i.e., the UE initiates the random access procedure.

[0376] Optionally, if the TAT timer expires and if the transmission on the pre-allocated UL grant has not been successful,

[0377] -UE falls back to the random access procedure, i.e., initiates the random access procedure.

[0378] The UE receives an RRCReconfiguration message from the gNB. The spCellConfig in the RRCReconfiguration message includes reconfigurationWithSync. The information in the reconfigurationWithSync IE (e.g., an indication to skip RACH) indicates that the UE should skip RACH towards the target cell. The information also indicates the targetTA (i.e., the TA of the PTAG or STAG in the source gNB) that the UE should use for UL transmissions in the PTAG at the target gNB. The RRCReconfiguration message includes a pre-allocated UL grant configuration (parameters indicating that UL grants occur periodically). This configuration is provided for at least the UL BWP indicated by firstActiveUplinkBWP-Id.

[0379] The UE starts the backoff timer and sets the backoff timer to the remaining value of the TAT timer of the targetTA.

[0380] The UE starts the timeAlignmentTimer associated with the PTAG.

[0381] The UE starts using the targetTA (i.e., the TA of the PTAG or STAG in the source gNB) for UL transmissions in the target cell.

[0382] If the back-off timer expires and if the RACH-less handover is not completed (i.e., the UE has not received a MAC CE indicating confirmation of the RACH-less handover completion):

[0383] -UE falls back to the random access procedure, i.e., initiates the random access procedure.

[0384] Optionally, if the back-off timer expires and if the transmission on the pre-allocated UL grant has not been successful,

[0385] -UE falls back to the random access procedure, i.e., the UE initiates the random access procedure.

[0386] The UL grant may be pre-configured in the RRCReconfiguration message, or provided on the PDCCH in the target cell if not pre-configured.

[0387] Regardless of the option used by the network to provide the UL grant, the validity of the UL grant may be based on a timer (specified in number of radio frames) provided in the RRCReconfiguration message.

[0388] - If such a timer is configured, the timer at the UE starts at the System Frame Number (SFN) at which the RRCReconfiguration message is received.

[0389] The UE shall fall back to random access on the target gNB in ​​the following cases:

[0390] a. If the transmission on the pre-configured UL grant is unsuccessful until the TAT (if running) of any serving cell of the reference source gNB expires or the validity timer of the UL grant (if configured) expires, whichever comes first, or

[0391] b. If none of the gNB beams (SSB / CSI RS) associated with the pre-allocated UL grant are suitable.

[0392] If the UE falls back to random access on the target gNB, CBRA on the target gNB continues until T304 expires.

[0393] If the validity timer is not configured in the RRCReconfiguration message, the pre-configured UL grant is valid until T304 expires.

[0394] Fig. 20 is a block diagram of a terminal 2000 according to an embodiment.

[0395] refer to Fig. 20 , the terminal 2000 includes a transceiver 2010, a controller 2020, and a memory 2030. The controller 2020 may refer to a circuit, an ASIC, or at least one processor. The transceiver 2010, the controller 2020, and the memory 2030 are configured to execute Figures 1 to 19 The operation of the UE shown or described above. Although the transceiver 2010, the controller 2020, and the memory 2030 are shown as separate entities, they can be implemented as a single entity such as a single integrated circuit or chip. The transceiver 2010, the controller 2020, and the memory 2030 can be electrically connected or coupled to each other.

[0396] The transceiver 2010 may transmit signals to other network entities (eg, a base station), and receive signals from the other network entities.

[0397] The controller 2020 may control the UE to perform functions according to one of the above-mentioned embodiments. The operation of the terminal 2000 may be implemented using a memory 2030 storing corresponding program codes. Specifically, the terminal 2000 may be equipped with a memory 2030 storing program codes for implementing desired operations. In order to perform the desired operation, the controller 2020 may read and execute the program code stored in the memory 2030 by using a processor or a central processing unit (CPU). Optionally, the controller 2020 may be implemented as at least one processor.

[0398] Fig.21 is a block diagram of a base station 2100 according to an embodiment.

[0399] refer to Fig.21 , the base station 2100 includes a transceiver 2110, a controller 2120 and a memory 2130. The transceiver 2110, the controller 2120 and the memory 2130 are configured to execute Figures 1 to 19 21. Although the transceiver 2110, the controller 2120, and the memory 2130 are shown as separate entities, they may be implemented as a single entity such as a single chip. The transceiver 2110, the controller 2120, and the memory 2130 may be electrically connected or coupled to each other.

[0400] The transceiver 2110 may send signals to other network entities (e.g., terminals) and receive signals from the other network entities. The controller 2120 may control the base station 2100 to perform functions according to one of the above-mentioned embodiments. The controller 2120 may refer to a circuit, an ASIC, or at least one processor. The operation of the base station 2100 may be implemented using a memory 2130 storing corresponding program codes. Specifically, the base station 2100 may be equipped with a memory 2130 storing program codes for implementing desired operations. In order to perform the desired operation, the controller 2120 may read and execute the program code stored in the memory 2130 by using a processor or a CPU. Optionally, the controller 2120 may be implemented as at least one processor.

[0401] While the present disclosure has been shown and described with reference to various embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the present disclosure as defined by the appended claims and their equivalents.

Claims

1. A method performed by a terminal in a wireless communication system, the method comprising: storing a media access control protocol data unit MAC PDU for a message MSG A; Sending a preamble for the MSG A to the base station on a physical random access channel PRACH; sending the MAC PDU for the MSG A to the base station on a physical uplink shared channel PUSCH; receiving a message MSG B comprising an uplink grant from said base station; In case the preamble is associated with a contention-free random access, determining that the random access procedure is successfully completed, and sending the MAC PDU to the base station based on the uplink grant, and The MSG B is received by monitoring a window started at a first physical downlink control channel PDCCH timing, wherein the first PDCCH timing is at least one symbol after the end of the PUSCH.

2. The method according to claim 1, in, In a case where a random access preamble identifier RAPID included in the MSG B matches an index of the preamble for the MSG A, the MAC PDU is transmitted.

3. The method according to claim 1, further comprising: receiving information configuring resources for the PUSCH from the base station, The information includes at least one of the following: information about a starting resource block RB, information about the number of RBs, information about the number of frequency division multiplexed PUSCH opportunities, information about a time slot offset, or information about a symbol length.

4. The method according to claim 1, in, In case the random access preamble identifier RAPID included in the MSG B matches the index of the preamble used for the MSG A, the random access procedure is deemed to be successfully completed.

5. A method performed by a base station in a wireless communication system, the method comprising: receiving a preamble for a message MSG A from a terminal on a physical random access channel PRACH; In case that no medium access control protocol data unit MAC PDU for the MSG A is received on the physical uplink shared channel PUSCH, sending a message MSG B including an uplink grant to the terminal; as well as receiving the MAC PDU from the terminal based on the uplink grant, wherein the received MAC PDU is stored in the terminal, wherein the random access procedure is deemed to be successfully completed if the preamble is associated with a contention-free random access, and The window for the MSG B starts at a first physical downlink control channel PDCCH opportunity, and the first PDCCH opportunity is at least one symbol after the end of the PUSCH.

6. The method according to claim 5, in, In a case where a random access preamble identifier RAPID included in the MSG B matches an index of the preamble for the MSG A, the MAC PDU is received.

7. The method according to claim 5, further comprising: sending information configuring resources for the PUSCH to the terminal, The information includes at least one of the following: information about a starting resource block RB, information about the number of RBs, information about the number of frequency division multiplexed PUSCH opportunities, information about a time slot offset, or information about a symbol length, and In which, when the random access preamble identifier RAPID included in the MSG B matches the index of the preamble used for the MSG A, the random access procedure is considered to be successfully completed.

8. A terminal in a wireless communication system, the terminal comprising: Transceiver; as well as Controller, configured as: storing a media access control protocol data unit MAC PDU for a message MSG A; sending, via the transceiver, a preamble for the MSG A to a base station on a physical random access channel PRACH, sending, via the transceiver, the MAC PDU for the MSG A to the base station on a physical uplink shared channel PUSCH, receiving, via the transceiver, from the base station a message MSG B comprising an uplink grant, In case the preamble is associated with a contention-free random access, determining that the random access procedure is successfully completed, and transmitting the MAC PDU to the base station via the transceiver based on the uplink grant, and The MSG B is received by monitoring a window started at a first physical downlink control channel PDCCH timing, wherein the first PDCCH timing is at least one symbol after the end of the PUSCH.

9. The terminal according to claim 8, in, In a case where a random access preamble identifier RAPID included in the MSG B matches an index of the preamble for the MSG A, the MAC PDU is transmitted.

10. The terminal according to claim 8, wherein: The controller is also configured to: receiving information configuring resources for the PUSCH from the base station, The information includes at least one of the following: information about a starting resource block RB, information about the number of RBs, information about the number of frequency division multiplexed PUSCH opportunities, information about a time slot offset, or information about a symbol length.

11. The terminal according to claim 8, in, In case the random access preamble identifier RAPID included in the MSG B matches the index of the preamble used for the MSG A, the random access procedure is deemed to be successfully completed.

12. A base station in a wireless communication system, the base station comprising: Transceiver; as well as Controller, configured as: receiving, via the transceiver, a preamble for a message MSG A from a terminal on a physical random access channel PRACH, sending, via the transceiver, a message MSG B including an uplink grant to the terminal in case a medium access control protocol data unit MAC PDU for the MSG A is not received on a physical uplink shared channel PUSCH, and receiving the MAC PDU from the terminal via the transceiver based on the uplink grant, wherein the received MAC PDU is stored in the terminal, Wherein, in the case where the preamble is associated with contention-free random access, The random access procedure is considered to be completed successfully, and The window for the MSG B starts at a first physical downlink control channel PDCCH opportunity, and the first PDCCH opportunity is at least one symbol after the end of the PUSCH.

13. The base station according to claim 12, in, In a case where a random access preamble identifier RAPID included in the MSG B matches an index of the preamble for the MSG A, the MAC PDU is received.

14. The base station according to claim 12, wherein: The controller is also configured to: sending information configuring resources for the PUSCH to the terminal, and The information includes at least one of the following: information about a starting resource block RB, information about the number of RBs, information about the number of frequency division multiplexed PUSCH opportunities, information about a time slot offset, or information about a symbol length.

15. The base station according to claim 12, in, In case the random access preamble identifier RAPID included in the MSG B matches the index of the preamble used for the MSG A, the random access procedure is deemed to be successfully completed.

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