Fallback from two-step random access channel (RACH) procedure to four-step RACH procedure

Abstract

Certain aspects of the present disclosure provide techniques for random access channel (RACH) communications.For example, certain aspects provide methods for falling back from a 2-step RACH procedure to a 4-step RACH procedure.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Application No. 16 / 597,672 filed on October 9, 2019, which claims priority to U.S. Provisional Patent Application No. 62 / 744,043 filed on October 10, 2018, both of which are assigned to the assignee of this application, are considered part of this patent application, and are incorporated by reference into this patent application.

[0003] introduction

[0004] Aspects of the present disclosure relate generally to wireless communications and, more particularly, to random access channel (RACH) procedures.

[0005] Wireless communication systems are widely deployed to provide various telecommunication services such as telephony, video, data, messaging, and broadcast. Typical wireless communication systems may employ multiple access technologies capable of supporting communication with multiple users by sharing available system resources (e.g., bandwidth, transmit power). Examples of such multiple access technologies include Long Term Evolution (LTE) systems, Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, Frequency Division Multiple Access (FDMA) systems, Orthogonal Frequency Division Multiple Access (OFDMA) systems, Single Carrier Frequency Division Multiple Access (SC-FDMA) systems, and Time Division Synchronous Code Division Multiple Access (TD-SCDMA) systems.

[0006] In some examples, a wireless multiple access communication system may include several base stations, each base station supporting communication of multiple communication devices (also referred to as user equipment (UE)) simultaneously. In an LTE or LTE-A network, a set of one or more base stations may define an evolved B node (eNB). In other examples (e.g., in a next generation or 5G network), a wireless multiple access communication system may include several distributed units (DUs) (e.g., edge units (EUs), edge nodes (ENs), radio heads (RHs), smart radio heads (SRHs), transmission reception points (TRPs), etc.) in communication with several central units (CUs) (e.g., central nodes (CNs), access node controllers (ANCs), etc.), wherein a set of one or more distributed units in communication with a central unit may define an access node (e.g., new radio base stations (NR BSs), new radio B nodes (NR NBs), network nodes, 5GNBs, gNBs, etc.). A base station or DU may communicate with a group of UEs on a downlink channel (e.g., for transmission from a base station or to a UE) and an uplink channel (e.g., for transmission from a UE to a base station or a distributed unit).

[0007] These multiple access technologies have been adopted in various telecommunication standards to provide a common protocol that enables different wireless devices to communicate at a city, country, region, and even global level. An example of an emerging telecommunication standard is called New Radio (NR), for example, 5G radio access. It is designed to better support mobile broadband Internet access by improving spectrum efficiency, reducing costs, improving services, utilizing new spectrum, and better integrating with other open standards using OFDMA with cyclic prefix (CP) on downlink (DL) and uplink (UL), and supports beamforming, multiple-input multiple-output (MIMO) antenna technology, and carrier aggregation.

[0008] However, as the demand for mobile broadband access continues to grow, there is a need for further improvements to NR technology. Preferably, these improvements should be applicable to other multiple access technologies and the telecommunication standards that employ these technologies.

[0009] Brief Overview

[0010] The systems, methods, and devices of the present disclosure each have several aspects, no single one of which is solely responsible for its desirable attributes. Without limiting the scope of the present disclosure as expressed in the appended claims, some features will now be discussed briefly. After considering this discussion, and particularly after reading the section entitled "Detailed Description," it will be understood how the features of the present disclosure provide advantages including improved communication between access points and stations in a wireless network.

[0011] Certain aspects of the present disclosure provide a method for wireless communication by a user equipment (UE). The method generally includes: initiating a two-step random access (RA) channel procedure (RACH) by sending a first message to a base station, the first message including at least a RACH preamble and a payload; receiving a random access response (RAR) message from the base station, wherein if the response message has a first format, the RAR message indicates that the base station successfully received both the RACH preamble and the payload; and if the RAR message has a second format, transitioning to a four-step RACH procedure before successfully completing the two-step RACH procedure.

[0012] Certain aspects of the present disclosure provide a method for wireless communication that may be performed by a network entity. The method generally includes sending a random access response (RAR) message to the UE, wherein a first format of the RAR message indicates successful receipt of both the RACH preamble and payload, and a second format of the RAR message signals to the UE to transition to a four-step RACH procedure prior to successful completion of the two-step RACH procedure.

[0013] Aspects generally include methods, apparatus (devices), systems, computer-readable media, and processing systems substantially as described herein with reference to and as illustrated by the accompanying figures.

[0014] To achieve the foregoing and related ends, the one or more aspects include the features fully described below and particularly pointed out in the claims. The following description and drawings set forth in detail certain illustrative features of the one or more aspects. However, these features are merely indicative of several of the various ways in which the principles of the various aspects may be employed, and this description is intended to cover all such aspects and their equivalents. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to understand in detail the manner in which the above-stated features of the present disclosure are used, a more particular description of the content briefly summarized above may be made with reference to various aspects, some of which are illustrated in the accompanying drawings. However, it should be noted that the accompanying drawings illustrate only certain typical aspects of the present disclosure and are not to be considered as limiting its scope, as the description may admit to other equally effective aspects.

[0017] Figure 1 is a block diagram conceptually illustrating an example telecommunications system in accordance with certain aspects of the present disclosure.

[0018] Figure 2 is a block diagram illustrating an example logical architecture of a distributed RAN in accordance with certain aspects of the present disclosure.

[0019] Figure 3 is a diagram illustrating an example physical architecture of a distributed RAN in accordance with certain aspects of the present disclosure.

[0020] Figure 4 is a block diagram conceptually illustrating a design of an example BS and user equipment (UE) in accordance with certain aspects of the present disclosure.

[0021] Figure 5 is a diagram illustrating an example for implementing a communication protocol stack in accordance with certain aspects of the present disclosure.

[0022] Figure 6 An example of a frame format for a New Radio (NR) system is illustrated in accordance with certain aspects of the present disclosure.

[0023] Figure 7 A call flow diagram illustrating an example four-step random access channel (RACH) procedure in accordance with certain aspects of the present disclosure.

[0024] Figure 8 A call flow diagram illustrating an example two-step RACH procedure in accordance with certain aspects of the present disclosure.

[0025] Fig. 9

[0013] Example operations for wireless communications by a user equipment (UE) are illustrated, in accordance with certain aspects of the present disclosure.

[0026] Fig.10

[0013] Example operations for wireless communications by a network entity are illustrated in accordance with certain aspects of the present disclosure.

[0027] Figure 11-13 A call flow diagram illustrating an example procedure for falling back from a two-step RACH procedure to a four-step RACH procedure, in accordance with certain aspects of the present disclosure.

[0028] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one aspect may be beneficially utilized in other aspects without specific recitation.

[0029] Detailed Description

[0030] Aspects of the present disclosure provide apparatus, methods, processing systems, and computer-readable media for random access channel (RACH) communications.

[0031] Certain aspects of the present disclosure may be applied to New Radio (NR) (new radio access technology or 5G technology). NR may support various wireless communication services, such as enhanced mobile broadband (eMBB) targeting wide bandwidth (e.g., more than 80 MHz), millimeter wave (mmW) targeting high carrier frequency (e.g., 60 GHz), massive MTC (mMTC) targeting non-backward compatible MTC technology, and / or mission critical services targeting ultra-reliable low latency communication (URLLC). These services may include latency and reliability requirements. These services may also have different transmission time intervals (TTIs) to meet corresponding quality of service (QoS) requirements. In addition, these services can coexist in the same subframe.

[0032] The following description provides examples rather than limiting the scope, applicability or examples set forth in the claims. Changes may be made to the functions and arrangements of the elements discussed without departing from the scope of the present disclosure. Various examples may appropriately omit, replace, or add various procedures or components. For example, the described method may be performed in an order different from the order described, and various steps may be added, omitted, or combined. Moreover, the features described with reference to some examples may be combined in some other examples. For example, any number of aspects set forth herein may be used to implement a device or practice method. In addition, the scope of the present disclosure is intended to cover such devices or methods practiced using other structures, functionality, or structures and functionality as supplements to the various aspects of the present disclosure set forth herein. It should be understood that any aspect of the present disclosure disclosed herein may be implemented by one or more elements of the claims. The wording "exemplary" is used herein to mean "used as an example, instance, or explanation". Any aspect described herein as "exemplary" need not be interpreted as being superior to or superior to other aspects.

[0033] The technology described herein can be used for various wireless communication networks, such as LTE, CDMA, TDMA, FDMA, OFDMA, SC-FDMA and other networks. The terms "network" and "system" are often used interchangeably. A CDMA network can implement radio technologies such as Universal Terrestrial Radio Access (UTRA), cdma2000, etc. UTRA includes other variants of Wideband CDMA (WCDMA) and CDMA. cdma2000 covers IS-2000, IS-95 and IS-856 standards. A TDMA network can implement radio technologies such as Global System for Mobile Communications (GSM). An OFDMA network can implement radio technologies such as NR (e.g., 5G RA), Evolved UTRA (E-UTRA), Ultra Mobile Broadband (UMB), IEEE802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, Flash-OFDMA, etc. UTRA and E-UTRA are part of Universal Mobile Telecommunications System (UMTS). NR is an emerging wireless communication technology being developed in collaboration with the 5G Technology Forum (5GTF). 3GPP Long Term Evolution (LTE) and Advanced LTE (LTE-A) are versions of UMTS that use E-UTRA. UTRA, E-UTRA, UMTS, LTE, LTE-A, and GSM are described in documents from an organization called the "3rd Generation Partnership Project" (3GPP). Cdma2000 and UMB are described in documents from an organization called the "3rd Generation Partnership Project 2" (3GPP2). The techniques described herein can be used for the wireless networks and radio technologies mentioned above as well as other wireless networks and radio technologies. For clarity, although various aspects may be described herein using terms commonly associated with 3G and / or 4G wireless technologies, various aspects of the present disclosure may be applied in communication systems based on other generations, such as 5G and later generations, including NR technologies.

[0034] Example Wireless Communication System

[0035] Figure 1 An example wireless network 100 is illustrated in which aspects of the present disclosure may be performed. For example, the wireless network may be a new radio (NR) or 5G network. The NR wireless communication system may employ beams, where a BS and a UE communicate via an active beam. As described herein, the BS may monitor the active beam using measurements of reference signals (e.g., MRS, CSI-RS, and synch) transmitted via a reference beam.

[0036] The UE 120 may be configured to perform the operations 1000 and methods described herein for detecting a mobility event based at least in part on mobility parameters associated with a beam set. The BS 110 may include a transmit reception point (TRP), a B node (NB), a 5G NB, an access point (AP), a new radio (NR) BS, etc. The BS 110 may be configured to perform the operations 900 and methods described herein for configuring beam sets and mobility parameters associated with each of the beam sets. The BS may receive an indication of a detected mobility event based on the mobility parameters and may make a decision regarding mobility management of the UE based on the event trigger.

[0037] like Figure 1 As illustrated in , the wireless network 100 may include several BSs 110 and other network entities. A BS may be a station that communicates with a UE. Each BS 110 may provide communication coverage for a specific geographic area. In 3GPP, the term "cell" may refer to the coverage area of ​​a B node and / or a B node subsystem that serves the coverage area, depending on the context in which the term is used. In an NR system, the terms "cell" and gNB, B node, 5G NB, AP, NR BS, NR BS, or TRP may be interchangeable. In some examples, a cell may not necessarily be stationary, and the geographic area of ​​the cell may move according to the location of a mobile base station. In some examples, base stations may be interconnected to each other and / or to one or more other base stations or network nodes (not shown) in the wireless network 100 via various types of backhaul interfaces (such as direct physical connections, virtual networks, or analogs using any suitable transport networks).

[0038] In general, any number of wireless networks may be deployed in a given geographic area. Each wireless network may support a specific radio access technology (RAT) and may operate on one or more frequencies. RAT may also be referred to as radio technology, air interface, etc. Frequency may also be referred to as carrier, frequency channel, etc. Each frequency may support a single RAT in a given geographic area to avoid interference between wireless networks of different RATs. In some cases, NR or 5G RAT networks may be deployed.

[0039] A BS may provide communication coverage for macro cells, pico cells, femto cells, and / or other types of cells. A macro cell may cover a relatively large geographic area (e.g., several kilometers in radius) and may allow unrestricted access by UEs with service subscriptions. A pico cell may cover a relatively small geographic area and may allow unrestricted access by UEs with service subscriptions. A femto cell may cover a relatively small geographic area (e.g., a residence) and may allow restricted access by UEs associated with the femto cell (e.g., UEs in a closed subscriber group (CSG), UEs of users in a residence, etc.). A BS for a macro cell may be referred to as a macro BS. A BS for a pico cell may be referred to as a pico BS. A BS for a femto cell may be referred to as a femto BS or a home BS. In Figure 1 In the example shown in FIG. 1 , BSs 110a, 110b, and 110c may be macro BSs for macro cells 102a, 102b, and 102c, respectively. BS 110x may be a pico BS for pico cell 102x. BSs 110y and 110z may be femto BSs for femto cells 102y and 102z, respectively. A BS may support one or more (e.g., three) cells.

[0040] The wireless network 100 may also include a relay station. A relay station is a station that receives transmissions of data and / or other information from an upstream station (e.g., a BS or a UE) and sends transmissions of the data and / or other information to a downstream station (e.g., a UE or a BS). A relay station may also be a UE that relays transmissions for other UEs. Figure 1 In the example shown in , a relay station 110r may communicate with a BS 110a and a UE 120r to facilitate communication between the BS 110a and the UE 120r. A relay station may also be referred to as a relay BS, a relay, or the like.

[0041] The wireless network 100 may be a heterogeneous network including different types of BSs (e.g., macro BSs, pico BSs, femto BSs, relays, etc.). These different types of BSs may have different transmit power levels, different coverage areas, and different impacts on interference in the wireless network 100. For example, a macro BS may have a high transmit power level (e.g., 20 watts), while a pico BS, a femto BS, and a relay may have a lower transmit power level (e.g., 1 watt).

[0042] Wireless network 100 can support synchronous or asynchronous operation. For synchronous operation, each BS can have similar frame timing, and transmissions from different BSs can be roughly aligned in time. For asynchronous operation, each BS can have different frame timing, and transmissions from different BSs may not be aligned in time. The techniques described herein can be used for both synchronous and asynchronous operation.

[0043] A network controller 130 may couple to a set of BSs and provide coordination and control for these BSs. Network controller 130 may communicate with BSs 110 via a backhaul. BSs 110 may also communicate with each other, directly or indirectly, for example, via a wireless or wired backhaul.

[0044] UE 120 (e.g., 120x, 120y, etc.) may be dispersed throughout the wireless network 100, and each UE may be stationary or mobile. UE may also be referred to as a mobile station, terminal, access terminal, subscriber unit, station, customer premises equipment (CPE), cellular phone, smart phone, personal digital assistant (PDA), wireless modem, wireless communication device, handheld device, laptop computer, cordless phone, wireless local loop (WLL) station, tablet device, camera, gaming device, netbook, smartbook, ultrabook, medical device or medical equipment, biometric sensor / device, wearable device (such as smart watch, smart clothing, smart glasses, smart wristband, smart jewelry (e.g., smart ring, smart necklace, etc.)), entertainment device (e.g., music device, video device, satellite radio, etc.), transportation component or sensor, smart meter / sensor, industrial manufacturing equipment, global positioning system device, or any other suitable device configured to communicate via wireless or wired medium. Some UEs may be considered as evolved or machine type communication (MTC) devices or evolved MTC (eMTC) devices. MTC and eMTC UEs include, for example, robots, drones, remote devices, sensors, meters, monitors, location tags, etc., which can communicate with a BS, another device (e.g., a remote device), or some other entity. A wireless node can provide connectivity for or to a network (e.g., a wide area network such as the Internet or a cellular network), for example, via a wired or wireless communication link. Some UEs may be considered Internet of Things (IoT) devices.

[0045] exist Figure 1 In FIG. 1 , a solid line with double arrows indicates desired transmissions between a UE and a serving BS, which is a BS designated to serve the UE on a downlink and / or uplink. A dashed line with double arrows indicates interfering transmissions between a UE and a BS.

[0046] Some wireless networks (e.g., LTE) utilize orthogonal frequency division multiplexing (OFDM) on the downlink and single carrier frequency division multiplexing (SC-FDM) on the uplink. OFDM and SC-FDM divide the system bandwidth into multiple (K) orthogonal subcarriers, which are also often referred to as frequency tones, frequency bins, etc. Each subcarrier can be modulated with data. In general, modulation symbols are sent in the frequency domain for OFDM and in the time domain for SC-FDM. The spacing between adjacent subcarriers can be fixed, and the total number of subcarriers (K) can depend on the system bandwidth. For example, the spacing of the subcarriers can be 15kHz, and the minimum resource allocation (called a 'resource block') can be 12 subcarriers (or 180kHz). Therefore, for system bandwidths of 1.25, 2.5, 5, 10, or 20 megahertz (MHz), the nominal FFT size can be equal to 128, 256, 512, 1024, or 2048, respectively. The system bandwidth can also be divided into subbands. For example, a subband may cover 1.08 MHz (ie, 6 resource blocks), and there may be 1, 2, 4, 8, or 16 subbands for system bandwidths of 1.25, 2.5, 5, 10, or 20 MHz, respectively.

[0047] Although aspects of the examples described herein may be associated with LTE technology, aspects of the present disclosure may be applicable to other wireless communication systems, such as NR.

[0048] NR can utilize OFDM with CP on the uplink and downlink and includes support for half-duplex operation using TDD. A single component carrier bandwidth of 100 MHz can be supported. NR resource blocks can span 12 subcarriers with a subcarrier bandwidth of 75 kHz over a duration of 0.1 ms. Each radio frame may include 50 subframes with a length of 10 ms. Therefore, each subframe may have a length of 0.2 ms. Each subframe may indicate the link direction (i.e., DL or UL) used for data transmission, and the link direction of each subframe may be switched dynamically. Each subframe may include DL / UL data and DL / UL control data. The UL and DL subframes for NR may be as follows about Figure 6 and Figure 7 Described in more detail. Beamforming can be supported and the beam direction can be dynamically configured. MIMO transmission with precoding can also be supported. The MIMO configuration in the DL can support up to 8 transmit antennas (multi-layer DL transmission with up to 8 streams) and up to 2 streams per UE. Multi-layer transmission of up to 2 streams per UE can be supported. Up to 8 serving cells can be used to support the aggregation of multiple cells. Alternatively, in addition to being based on OFDM, NR can support different air interfaces. The NR network can include entities such as CU and / or DU.

[0049] In some examples, access to the air interface may be scheduled, wherein a scheduling entity (e.g., a base station) allocates resources for communication between some or all devices and equipment within its service area or cell. In the present disclosure, as further discussed below, a scheduling entity may be responsible for scheduling, assigning, reconfiguring, and releasing resources for one or more subordinate entities. That is, for scheduled communications, a subordinate entity utilizes resources allocated by a scheduling entity. A base station is not the only entity that can be used as a scheduling entity. That is, in some examples, a UE may act as a scheduling entity, thereby scheduling resources for one or more subordinate entities (e.g., one or more other UEs). In this example, the UE is acting as a scheduling entity, and other UEs utilize resources scheduled by the UE for wireless communication. A UE may act as a scheduling entity in a peer-to-peer (P2P) network and / or in a mesh network. In a mesh network example, UEs may optionally communicate directly with each other in addition to communicating with a scheduling entity.

[0050] Thus, in a wireless communication network having scheduled access to time-frequency resources and having a cellular configuration, a P2P configuration, and a mesh configuration, a scheduling entity and one or more subordinate entities may communicate using the scheduled resources.

[0051] As mentioned above, the RAN may include a CU and a DU. A NR BS (e.g., a gNB, a 5G Node B, a Node B, a Transmit Receiving Point (TRP), an Access Point (AP)) may correspond to one or more BSs. An NR cell may be configured as an access cell (ACell) or a data-only cell (DCell). For example, a RAN (e.g., a central unit or a distributed unit) may configure these cells. A DCell may be a cell used for carrier aggregation or dual connectivity but not for initial access, cell selection / reselection, or switching. In some cases, a DCell may not transmit a synchronization signal—in some cases, a DCell may transmit an SS. The NR BS may transmit a downlink signal to the UE to indicate the cell type. Based on the cell type indication, the UE may communicate with the NR BS. For example, the UE may determine the NR BS to be considered for cell selection, access, switching, and / or measurement based on the indicated cell type.

[0052] Figure 2 An example logical architecture including a distributed radio access network (RAN) 200 is illustrated, which may be Figure 1206. The 5G access node 206 may include an access node controller (ANC) 202. The ANC may be a central unit (CU) of the distributed RAN 200. The backhaul interface to the next generation core network (NG-CN) 204 may be terminated at the ANC. The backhaul interface to the adjacent next generation access node (NG-AN) may be terminated at the ANC. The ANC may include one or more TRPs 208 (which may also be referred to as BS, NR BS, B node, 5G NB, AP or some other term). As described above, TRP may be used interchangeably with "cell".

[0053] The TRP 208 may be a DU. The TRP may be connected to one ANC (ANC 202) or more than one ANC (not illustrated). For example, for RAN sharing, Radio as a Service (RaaS), and service-specific AND deployments, the TRP may be connected to more than one ANC. The TRP may include one or more antenna ports. The TRP may be configured to serve traffic to the UE individually (e.g., dynamically selected) or jointly (e.g., jointly transmitted).

[0054] The local architecture 200 can be used to illustrate the fronthaul definition. The architecture can be defined to support fronthaul solutions across different deployment types. For example, the architecture can be based on transport network capabilities (e.g., bandwidth, latency, and / or jitter).

[0055] The architecture may share features and / or components with LTE. According to various aspects, the next generation AN (NG-AN) 210 may support dual connectivity with NR. For LTE and NR, the NG-AN may share a common fronthaul.

[0056] The architecture may enable collaboration between and among TRPs 208. For example, collaboration may exist within a TRP and / or across TRPs via ANC 202. According to various aspects, an inter-TRP interface may not be required / existent.

[0057] According to various aspects, dynamic configuration of split logic functions may exist within architecture 200. Figure 5 Described in more detail, a radio resource control (RRC) layer, a packet data convergence protocol (PDCP) layer, a radio link control (RLC) layer, a medium access control (MAC) layer, and a physical (PHY) layer may be adaptively placed at a DU or CU (e.g., at a TRP or ANC, respectively). According to certain aspects, a BS may include a central unit (CU) (e.g., ANC 202) and / or one or more distributed units (e.g., one or more TRPs 208).

[0058] Figure 3An example physical architecture of a distributed RAN 300 is illustrated in accordance with aspects of the present disclosure. A centralized core network unit (C-CU) 302 may host core network functions or a multi-RAT core network (MR-CN). The C-CU may be centrally deployed. C-CU functionality may be offloaded (e.g., to advanced wireless services (AWS)) in an effort to handle peak capacity.

[0059] The centralized RAN unit (C-RU) 304 may host one or more ANC functions of a multi-RAT access node (MR-AN) and / or a 5G AN. Optionally, the C-RU may host core network functions locally. The C-RU may have a distributed deployment. The C-RU may be closer to the network edge.

[0060] The DU 306 may host one or more TRPs (edge ​​node (EN), edge unit (EU), radio head (RH), smart radio head (SRH), etc.) The DU may be located at the edge of the network with radio frequency (RF) functionality.

[0061] Figure 4 Explained Figure 1 10 and UE 120, which may be used to implement various aspects of the present disclosure. The BS may include a TRP. One or more components of the BS 110 and UE 120 may be used to practice various aspects of the present disclosure. For example, the antenna 452, Tx / Rx 454, processors 466, 458, 464, and / or controller / processor 480 of the UE 120, and / or the antenna 434, processors 420, 430, 438, and / or controller / processor 440 of the BS 110 may be used to perform the operations described herein and with reference to Figure 9-10 Explanation of the operation.

[0062] Figure 4 A block diagram showing a design of a BS 110 and a UE 120, which may be Figure 1 For the scenario of constrained association, base station 110 may be Figure 1 1. The macro BS 110c in FIG. 1 may be a macro BS 110c, and the UE 120 may be a UE 120y. The base station 110 may also be some other type of base station. The base station 110 may be equipped with antennas 434a to 434t, and the UE 120 may be equipped with antennas 452a to 452r.

[0063] At the base station 110, the transmit processor 420 may receive data from the data source 412 and control information from the controller / processor 440. The control information may be for a physical broadcast channel (PBCH), a physical control format indicator channel (PCFICH), a physical hybrid ARQ indicator channel (PHICH), a physical downlink control channel (PDCCH), etc. The data may be for a physical downlink shared channel (PDSCH), etc. The processor 420 may process (e.g., encode and symbol map) the data and the control information to obtain data symbols and control symbols, respectively. The processor 420 may also generate reference symbols (e.g., for PSS, SSS, and cell-specific reference signals (CRS)). The transmit (TX) multiple-input multiple-output (MIMO) processor 430 may perform spatial processing (e.g., precoding) on ​​the data symbols, control symbols, and / or reference symbols, where applicable, and may provide output symbol streams to modulators (MODs) 432a to 432t. Each modulator 432 may process a respective output symbol stream (e.g., for OFDM, etc.) to obtain an output sample stream. Each modulator 432 may further process (e.g., convert to analog, amplify, filter, and upconvert) the output sample stream to obtain a downlink signal. The downlink signals from modulators 432a through 432t may be transmitted via antennas 434a through 434t, respectively.

[0064] At the UE 120, antennas 452a to 452r may receive downlink signals from the base station 110 and may provide received signals to demodulators (DEMODs) 454a to 454r, respectively. Each demodulator 454 may condition (e.g., filter, amplify, downconvert, and digitize) a respective received signal to obtain input samples. Each demodulator 454 may further process the input samples (e.g., for OFDM, etc.) to obtain received symbols. A MIMO detector 456 may obtain received symbols from all demodulators 454a to 454r, perform MIMO detection on the received symbols where applicable, and provide detected symbols. A receive processor 458 may process (e.g., demodulate, deinterleave, and decode) the detected symbols, provide decoded data for the UE 120 to a data sink 460, and provide decoded control information to a controller / processor 480.

[0065] On the uplink, at the UE 120, a transmit processor 464 may receive and process data from a data source 462 (e.g., for a physical uplink shared channel (PUSCH)) and control information from a controller / processor 480 (e.g., for a physical uplink control channel (PUCCH)). The transmit processor 464 may also generate reference symbols for a reference signal. The symbols from the transmit processor 464 may be precoded by a TX MIMO processor 466, if applicable, further processed by demodulators 454a through 454r (e.g., for SC-FDM, etc.), and transmitted to the base station 110. At the BS 110, the uplink signal from the UE 120 may be received by the antenna 434, processed by the modulator 432, detected by the MIMO detector 436, if applicable, and further processed by the receive processor 438 to obtain decoded data and control information sent by the UE 120. Receive processor 438 may provide decoded data to data sink 439 and decoded control information to controller / processor 440 .

[0066] Controllers / processors 440 and 480 may direct operations at base station 110 and UE 120, respectively. Processor 440 and / or other processors and modules at base station 110 may perform or direct the execution of, for example, the functional blocks illustrated in the various figures, and / or other processes for the techniques described herein. Processor 480 and / or other processors and modules at UE 120 may also perform or direct the execution of corresponding / supplementary processes for the techniques described herein and as illustrated in the various figures. Memories 442 and 482 may store data and program codes for BS 110 and UE 120, respectively. Scheduler 444 may schedule UEs for data transmission on the downlink and / or uplink.

[0067] Figure 5 A diagram 500 showing an example for implementing a communication protocol stack according to various aspects of the present disclosure is illustrated. The illustrated communication protocol stack may be implemented by a device operating in a 5G system. Diagram 500 illustrates a communication protocol stack including a radio resource control (RRC) layer 510, a packet data convergence protocol (PDCP) layer 515, a radio link control (RLC) layer 520, a media access control (MAC) layer 525, and a physical (PHY) layer 530. In various examples, these layers of the protocol stack may be implemented as separate software modules, parts of a processor or ASIC, parts of non-co-located devices connected by a communication link, or various combinations thereof. Co-located and non-co-located implementations may be used, for example, in a protocol stack for a network access device (e.g., AN, CU, and / or DU) or a UE.

[0068] The first option 505-a illustrates a split implementation of the protocol stack, where the implementation of the protocol stack is performed on a centralized network access device (e.g., Figure 2 ANC 202 in the system) and distributed network access equipment (e.g., Figure 2 In the first option 505-a, the RRC layer 510 and the PDCP layer 515 may be implemented by a central unit, while the RLC layer 520, the MAC layer 525, and the PHY layer 530 may be implemented by the DU. In various examples, the CU and the DU may be co-located or non-co-located. The first option 505-a may be useful in a macro cell, a micro cell, or a pico cell deployment.

[0069] The second option 505-b shows a unified implementation of the protocol stack, where the protocol stack is implemented in a single network access device (e.g., access node (AN), new radio base station (NR BS), new radio node B (NR NB), network node (NN), etc.). In the second option, the RRC layer 510, the PDCP layer 515, the RLC layer 520, the MAC layer 525, and the PHY layer 530 can each be implemented by the AN. The second option 505-b can be useful in a femtocell deployment.

[0070] Regardless of whether the network access device implements part or all of the protocol stack, the UE may implement the entire protocol stack (eg, the RRC layer 510, the PDCP layer 515, the RLC layer 520, the MAC layer 525, and the PHY layer 530).

[0071] Figure 6 6 is a diagram showing an example of a frame format 600 for NR. The transmission timeline of each of the downlink and uplink may be divided into units of radio frames. Each radio frame may have a predetermined duration (e.g., 10 ms) and may be divided into 10 subframes with indices 0 to 9, each subframe being 1 ms. Each subframe may include a variable number of slots, depending on the subcarrier spacing. Each slot may include a variable number of symbol periods (e.g., 7 or 14 symbols), depending on the subcarrier spacing. An index may be assigned to the symbol period in each slot. A mini slot, which may be referred to as a subslot structure, refers to a transmission time interval having a duration less than a slot (e.g., 2, 3, or 4 symbols).

[0072] Each symbol in a slot may indicate a link direction (e.g., DL, UL, or flexible) for data transmission, and the link direction for each subframe may be switched dynamically. The link direction may be based on the slot format. Each slot may include DL / UL data and DL / UL control information.

[0073] In NR, a synchronization signal (SS) block is transmitted. The SS block includes PSS, SSS, and two-symbol PBCH. The SS block can be in a fixed time slot position (such as Figure 6 The PSS and SSS can be used by the UE for cell search and acquisition. The PSS can provide half-frame timing, and the SS can provide CP length and frame timing. The PSS and SSS can provide cell identity. The PBCH carries some basic system information, such as downlink system bandwidth, timing information within a radio frame, SS burst set periodicity, system frame number, etc. SS blocks can be organized into SS bursts to support beam sweeping. Further system information (such as remaining minimum system information (RMSI), system information block (SIB), other system information (OSI)) can be transmitted on the physical downlink shared channel (PDSCH) in certain subframes.

[0074] The UE may operate in various radio resource configurations, including a configuration associated with transmitting a pilot using a dedicated resource set (e.g., a radio resource control (RRC) dedicated state, etc.), or a configuration associated with transmitting a pilot using a shared resource set (e.g., an RRC shared state, etc.). When operating in the RRC dedicated state, the UE may select a dedicated resource set for transmitting a pilot signal to the network. When operating in the RRC shared state, the UE may select a shared resource set for transmitting a pilot signal to the network. In either case, the pilot signal transmitted by the UE may be received by one or more network access devices (such as an AN, or a DU, or parts thereof). Each receiving network access device may be configured to receive and measure a pilot signal transmitted on a shared resource set, and also receive and measure a pilot signal transmitted on a dedicated resource set allocated to a UE, wherein the network access device is a member of a monitoring network access device set for the UE. One or more receiving network access devices or a CU to which the receiving network access device transmits pilot signal measurements may use these measurements to identify a serving cell of the UE or initiate a change in a serving cell for one or more UEs.

[0075] Example Random Access Channel (RACH) Procedure

[0076] The random access channel (RACH) is so named because it refers to a wireless channel (medium) that can be shared by multiple UEs and used by these UEs to (randomly) access the network for communication. For example, RACH can be used for call setup and access network for data transmission. In some cases, RACH can be used for initial access to the network when the UE switches from radio resource control (RRC) connected idle mode to active mode, or when switching in RRC connected mode. In addition, when the UE is in RRC idle or RRC inactive mode, and when reestablishing a connection with the network, RACH can be used for downlink (DL) and / or uplink (UL) data arrival. Certain aspects of the present disclosure provide multiple RACH procedures and techniques for selecting a RACH procedure for communication.

[0077] Figure 7 7 is a timing diagram (or "call flow diagram") 700 illustrating an example four-step RACH procedure in accordance with certain aspects of the present disclosure. A first message (MSG1) may be sent from UE 120 to eNB 110 on a physical random access channel (PRACH). In this case, MSG1 may include only a RACH preamble. eNB 110 may respond with a random access response (RAR) message (MSG2), which may include an identifier (ID) of the RACH preamble, a timing advance (TA), an uplink grant, a cell radio network temporary identifier (C-RNTI), and a backoff indicator. As illustrated, MSG2 may include a PDCCH communication including control information regarding subsequent communications on the PDSCH. In response to MSG2, MSG3 is transmitted from UE 120 to eNB 110 on the PUSCH. MSG3 may include an RRC connection request, a tracking area update, and a scheduling request. The eNB 110 then responds with MSG4, which may include a contention resolution message.

[0078] In some cases, to speed up access, a two-step RACH procedure may be supported. As the name implies, the two-step RACH procedure effectively "collapses" the four messages of the four-step RACH procedure into two messages.

[0079] Figure 88 is a timing diagram 800 illustrating an example two-step RACH procedure in accordance with certain aspects of the present disclosure. A first enhanced message (eMSG1) may be sent from UE 120 to eNB 110. In this case, eMSG1 may include a RACH preamble for random access and a payload. For example, the eMSG1 payload may include a UE-ID and other signaling information (e.g., a buffer status report (BSR)) or a scheduling request (SR). eNB 110 may respond with a random access response (RAR) message (eMSG2), which may effectively combine MSG2 and MSG4 described above. For example, eMSG2 may include an ID of a RACH preamble, a timing advance (TA), a backoff indicator, a contention resolution message, an UL / DL grant, and a transmit power control (TPC) command.

[0080] Example fallback from 2-step to 4-step RACH procedure

[0081] Certain aspects of the present disclosure generally relate to procedures for transitioning ("falling back") from implementing a two-step RACH procedure to a four-step RACH procedure.

[0082] In several different scenarios, the UE may fall back from the 2-step RACH procedure to the 4-step RACH procedure. For example, the 2-step RACH procedure may not succeed due to path loss and large timing advance (TA). As another example, when the UE starts in the 2-step RACH procedure, the gNB may only detect the preamble of enhanced MSG1 and not the payload portion. In this case, the UE may continue with the 4-step RACH procedure.

[0083] Fig. 9 1000 illustrates example operations 900 that may be performed by a UE as part of a RACH procedure in accordance with certain aspects of the present disclosure. Operations 1000 may be performed, for example, by a UE such as UE 120 (or Figure 4 Executed by one or more of the processors shown in .

[0084] Operations 900 begin, at 902, by initiating a two-step random access channel procedure (RACH) by sending a first message to a base station, the first message including at least a RACH preamble and a payload. At 904, the UE receives a random access response (RAR) message from the base station, wherein, if the response message has a first format, the RAR message indicates that the base station successfully received both the RACH preamble and the payload. At 906, if the RAR message has a second format, the UE transitions to a four-step RACH procedure before successfully completing the two-step RACH procedure.

[0085] Fig.101000 as part of a RACH procedure that may be performed by a network entity in accordance with certain aspects of the present disclosure. Operations 1000 may be performed by an eNB (such as eNB 110) (or Figure 4 Executed by one or more of the processors shown in .

[0086] Operations 1000 begin by receiving at least a random access channel (RACH) preamble in a first message from a user equipment (UE) as part of a two-step RACH procedure, at 1002. At 1004, the network entity sends a random access response (RAR) message to the UE, wherein a first format of the RAR message indicates successful receipt of both the RACH preamble and payload, and a second format of the RAR message signals to the UE to transition to a four-step RACH procedure prior to successful completion of the two-step RACH procedure.

[0087] Various alternatives for falling back from a 2-step RACH procedure to a 4-step RACH procedure are provided herein. The first alternative (Alternative 1) may be referred to as a hard fallback, where the UE starts with a 2-step RACH procedure and completes the entire procedure (including all power ramps), but still fails to access communication resources. After the failure, the UE moves to a 4-step RACH procedure starting from the initial 4-step RACH power level.

[0088] Fig.11 The hard fallback illustrated in FIG. 4 can be contrasted with other alternative fallback procedures in which the UE starts with a 2-step RACH procedure and somewhere in the middle of the 2-step RACH procedure (i.e., the RACH procedure has not yet completed), the UE moves to a 4-step RACH procedure. As will be described in more detail below, the decision to transition to a 4-step RACH procedure can be made autonomously by the UE or can be triggered by signaling from the gNB (e.g., with the new MSG2 format).

[0089] Hard fallback can be viewed as a relatively straightforward solution since the two-step and four-step RACH procedures are not integrated. In this case, if a new MSG1 preamble is detected, but no UE-ID is detected, the gNB will simply not respond. Since the two-step RACH procedure is allowed to complete, the performance of the hard fallback alternative may not be optimal (e.g. due to access delays, additional interference, etc.), but the procedure can be relatively simple to implement with minimal gNB involvement.

[0090] Even with hard fallback, the gNB may have some control by configuring the maximum number of attempts the UE can make in 2-step PRACH before falling back to 4-step RACH procedure.

[0091] In the case of a backoff from 2-step to 4-step (whether a hard backoff or the "soft" alternative described below), there are various options for selecting the transmit (Tx) power level for the UE to start a 4-step RACH.

[0092] For example, according to the first option for selecting the Tx power level, the UE starts with a Tx power level based on a transmit power control parameter configured for 4-step RACH (e.g., preambleReceivedTargetPower), the value of which may be configured differently than a similar parameter for 2-step RACH. In other words, the UE may have separate open-loop power control parameters for 2-step and 4-step RACH procedures. In some cases, the power ramp step size may also be configured differently between 2-step and 4-step RACH procedures.

[0093] According to the second option for selecting the Tx power level, the UE may recover from the current Tx power level (from 2-step RACH) when starting the 4-step RACH procedure. According to the third option for selecting the Tx power level, the UE may recover from the current Tx power level when starting the 4-step RACH procedure and also apply an additional power ramp up.

[0094] The second alternative for fallback operation may be referred to as integrated fallback operation because the procedure effectively integrates the 2-step and 4-step RACH procedures. In this alternative, the UE makes the decision to fall back (autonomously) in the middle of the 2-step RACH procedure (e.g., the gNB may still not participate). In some cases, the UE may alternate between 2-step RACH and 4-step RACH for the same power level and then move to the next power level (and alternate between 2-step RACH and 4-step RACH again). In other cases, the UE may try 2-step RACH several times at different power levels before switching to 4-step RACH at different power levels. These approaches may be beneficial because the UE may have different 2-step RACH or 4-step RACH experiences depending on the Tx power, so it may be beneficial to have the UE try these different RACH procedures for each power level. As described above, other options for selecting the Tx power level may be used. For example, the UE may recover from Tx power levels between 2-step RACH and 4-step RACH without additional power ramping; or even use additional power ramping when changing from 2-step to 4-step RACH (or vice versa).

[0095] A third alternative for fallback operation may be referred to as a gNB-assisted fallback procedure. In this case, the gNB may assist in fallback when the UE sends a new MSG1, and the gNB detects the preamble but not the payload (UE-ID). In the case where a preamble has been detected, falling back to a 4-step RACH procedure may be desirable since the UE does not need to send the preamble again. Thus, the UE-ID transmission (which has been included in the payload of the 2-step MSG1) may be moved out of the PRACH resources to avoid collisions.

[0096] In some cases, the gNB may trigger a fallback to a 4-step RACH procedure via a new format for MSG2. For example, the first (conventional 2-step RACH) format of MSG2 may be used to confirm the reception of the new MSG1. The format may include RA preamble identifier (RAPID), TA, C-RNTI, UE ID.

[0097] like Fig.12 As explained in , the second (new) format "MSG2" may be designed to trigger fallback and may include at least the same fields as the old MSG2, including RAPID, TA, TC-RNTI, and UL grant. Various options for the new format MSG2 are described in more detail below. In either case, the UE may identify the format of the new MSG2 and may check the RAPID / UE-ID (to the preamble it sent in MSG1). If the new MSG2 is the fallback format and has the correct RAPID, the UE may assume that the gNG detected the preamble portion of the 2-step MSG1, and may thereby start the 4-step RACH procedure by transmitting MSG3 with UE-ID.

[0098] In some cases, if MSG2 is sent via a PDCCH containing a hash of the preamble and the UE-ID, there may be challenges in triggering the fallback if the gNB fails to decode the UE-ID in the new MSG1. To address this, a default UE-ID (or a special UE-ID / reserved UE-ID such as all '0') may be used for MSG2. In this case, if the UE decodes the PDCCH with the RA-RNTI and compares the hashed ID field to the field generated by the preamble + UE-ID and the preamble + default UE-ID, the UE may know to attempt PDSCH decoding if either of the two fields matches the received hashed ID field. Accordingly, there may still be a hash collision, and the probability of a collision increases as both candidates are checked. In either case, after successfully decoding the PDSCH, the UE will identify the format of the new MSG2. If MSG2 is a new format, the UE will check the RAPID to see if it is for itself and not a collision, and proceed to fallback to 4-step RACH.

[0099] like Fig.13 As explained in , in some cases, the gNB may utilize soft combining of UE-ID decoding (from 2-step MSG1) for MSG3 transmission decoding in fallback. As mentioned above, in fallback, MSG3 transmission may be triggered by the new MSG2 format with UL grant. The gNB may support some degree of soft combining of the payload / data portion of the new (2-step RACH) MSG1 with the granted transmission of MSG3 during fallback to 4-step RACH procedure.

[0100] When the payload of MSG1 is sent on PUSCH, it is possible for the gNB to combine MSG3 with the earlier transmitted payload (using LLR combining). In some cases, both transmissions in (2-step) MSG1 and fallback MSG3 may be encoded via LDPC codes. In some cases, when MSG3 transmission is triggered, special rules may be used for TBS calculation (e.g., the payload may be fixed to the UE-ID length instead of zero padding the TBS calculated from the grant).

[0101] When the payload of MSG1 is sent on PUCCH, soft combining may not be possible due to the different coding of PUCCH and MSG3 (i.e., PUCCH may be encoded using polar codes, while MSG3 may be encoded using LDPC codes). Alternatively, the UL grant may allow the PUCCH transmission to enable possible combining (e.g., so that the MSG3 transmission also uses polar codes). As an alternative, the PUSCH may be specially formulated for triggered MSG3 transmissions (e.g., and using polar codes instead of LDPC).

[0102] Various information, such as RACH configuration information, is often communicated via Remaining Minimum System Information (RMSI) broadcast transmissions. In some cases, RMSI configuration information may be provided to support fallback from a two-step to a four-step RACH procedure. For example, RMSI may convey various parameters, such as 2-step and 4-step thresholds, power ramp-up step size, maximum number of attempts, and / or different PRACH received powers (strict TDM or interleaved, how to calculate timers, how to ramp power when the UE switches between 2-step and 4-step RACH, how to count the number of attempts).

[0103] In addition to the preamble and payload resource association indication, the gNB may also indicate additional information in the RMSI. For example, the RMSI may configure different RSRP thresholds for 2-step and 4-step RACH procedures, different preambleReceivedTargetPower for 2-step and 4-step RACH procedures, different preambleTransMax (preamble maximum number of transmissions) for 2-step and 4-step RACH procedures respectively, or preambleTransMax includes the total number of attempts in the case of both 2-step and 4-step RACH, different powerRampingStep (power ramp step size) for 2-step and 4-step RACH, and / or different backoff indicators for 2-step and 4-step RACH.

[0104] In some cases, the UE may be allowed to select between 2-step and 4-step RACH procedures based on its listen-before-talk (LBT) load / channel occupancy measurements. For example, if the LBT load is high and the UE transmission may have a higher probability of being gated by LBT, the UE may select 2-step even if the RSRP threshold would indicate that the UE performs a 4-step RACH procedure.

[0105] There are various options for applying a transmit power offset (e.g., delta (Δ) msg3) for a 2-step and / or 4-step RACH procedure. According to a first option, a power offset (delta msg3) is applied to the new MSG1 payload in a 2-step procedure. This may include a new MSG1 payload with or without an embedded DMRS. When the new MSG1 payload uses a preamble as a DMRS, delta msg3 may be applied to the payload transmission. When the new MSG1 payload uses its own embedded MDRS, delta msg3 may be applied to the payload as well as the embedded DMRS transmission.

[0106] According to another option, when 2 steps fall back to 4 steps, incremental msg3 may be applied only to MSG3. The same Tx power spectrum density (PSD) may be used between the preamble and payload transmission in the new MSG1. In other words, the new MSG1 may apply incremental msg3 on top of the payload (+ potential embedded DMRS). In addition, when the UE falls back to MSG3 transmission under the 4-step RACH procedure, an additional incremental msg3 may be applied on top of the payload sent in the new MSG1. These two options for applying a transmit power offset may also be combined, and an additional transmit power control (TPC) field may be a supplement to either of these two options.

[0107] In NR, for contention-based random access (CBRA), the subcarrier spacing (SCS) for MSG3 is configured in the RACH configuration (RMSI) separately from the SCS for MSG1 (e.g., via 1 bit). In addition, the network signals the waveform for MSG3 as 1 bit in the RMSI (e.g., the waveform for MSG3 can be DFT-S-OFDM or CP-OFDM).

[0108] In some cases, the UE may be allowed to use different SCS for the preamble and payload in the case of a new MSG1. According to a first option, the UE may be allowed to use different SCS for the preamble and payload, and may follow the current SCS for 4-step indication. According to a second option, the same SCS may be used for the preamble and payload of the new MSG1. With this option, the SCS may follow the current MSG1 or MSG3 in the 4-step procedure, or it may be indicated separately for the new MSG1 under the 2-step procedure.

[0109] In some cases, the new 2-step RACH Msg1 may be used for contention-free random access (CFRA). In some cases, the new Msg1 may be used at least for CFRA-based handover, where the UE needs to send an UL Msg. Conventionally, the UL msg is sent based on the UL grant issued in Msg2. In this case, it may be beneficial to attach the payload in the new Msg1 even for CFRA. In some cases, the gNB may instruct the UE whether to send only the preamble or the new Msg1 for CFRA. As mentioned above, there are various options for the new MSG2 design that allows the gNB to trigger a fallback from a 2-step to a 4-step RACH procedure. In Release 15NR 4-step RACH, the RA-RNTI associated with the PRACH in which the random access preamble is transmitted is calculated as follows:

[0110] RA-RNTI=1+s_id+14×t_id+14×80×f_id+14×80×8×ul_carrier_id

[0111] where s_id is the index of the first OFDM symbol of the designated PRACH (0≤s_id<14), t_id is the index of the first time slot of the designated PRACH in the system frame (0≤t_id<80), f_id is the index of the designated PRACH in the frequency domain (0≤f_id<8), and ul_carrier_id is the UL carrier used for MSG1 transmission (0 for the normal uplink (NUL) carrier and 1 for the supplementary uplink (SUL) carrier).

[0112] In some cases, the new MSG2 may use a similar RA-RNTI design. For example, there may be one new MSG2 per PDSCH, or multiplexing of MSG2s of different UEs in the PDSCH may be supported. Retransmission of the new MSG2 may be supported, and similarly, ACK / NAK for reception of the new MSG2 may also be supported.

[0113] According to the first option, the new MSG2 may be broadcast to a group of UEs with the same RA-RNTI. In this case, the new MSG2s of multiple UEs are multiplexed together into one PDSCH (e.g., with a DCI whose CRC is scrambled by the RA-RNTI). For the PDCCH, the MSG2 format may be based on the structure of a normal DL grant. Due to the multiplexing properties of the payload, HARQ feedback may not be supported because the UE does not know whether there is a new MSG2 for itself before decoding. Due to the lack of retransmissions, there may be a need for TB scaling of the old MSG2 grant. HARQ process ID, NDI, RVID are not required because there are no retransmissions and the space can be used for TB scaling.

[0114] For PDSCH, MAC layer multiplexing of multiple new MSG2s for different UEs can be used. The gNB can include RAPID, TA, C-RNTI, UE ID, etc. in the new MSG2. However, without feedback, the gNB may not know whether the UE has correctly received MSG2. Therefore, the gNB can opportunistically include UL grant in the new MSG2 for the UE to send Msg 5 (RRC configuration). Detection of a granted UL transmission can also serve as a confirmation that the UE has received the new MSG2.

[0115] According to the second option, the new MSG2 is broadcast to a group of UEs with the same RA-RNTI, but with ACK / NAK feedback and retransmissions. The PDCCH may still be CRC-scrambled by the RA-RNTI, but may require NDI and RVID fields for retransmission support. An ARI may also be included to combine with each new MSG2 to give a combined ARI for ACK transmission. A HARQ process ID may be included to allow multiple PDSCHs with different new MSG2 content: one group of new MSG2 is retransmitting, while another group of new MSG2 has been initiated. However, this may require the UE to decode multiple broadcast (RA-RNTI) PDSCHs.

[0116] For PDSCH under this option, the gNB may include RAPID, TA, C-RNTI, UE ID, etc. in the new MSG2. In addition, the gNB may include each new MSG2 ARI to indicate separate ACK / NAK feedback resources for each individual RAPID. In this case, the UE for which contention resolution is successful may send an ACK on the corresponding resource. No NACK is transmitted. Only when the UE decodes the PDSCH will it know that there is a MSG2 for itself. In this way, contention resolution can be determined without UL Msg5 transmission. For retransmissions, the gNB may choose to send the same or different PDSCH new MSG2 content based on UE feedback (via NDI or different HARQ process IDs). Even if a component of the new MSG2 is acknowledged to allow soft combining, the gNB scheduler may still decide to retransmit the entire PDSCH. As an alternative, the gNB may transmit a new PDSCH with a different mix of new MSG2 with NDI flipped.

[0117] There is also a third option to send the new MSG2 in unicast. In other words, the new MSG2 for different UEs is not multiplexed to support per-UE ACK / NAK and retransmission.

[0118] In this case, the PDCCH can be a normal DL grant with its CRC scrambled by the RA-RNTI. The PDCCH payload may include an UL indication (which points to the actual UE ID) and a RAPID. A hash function may be defined to map the UE ID to the UE indication field in the PDCCH. In this case, HARQ feedback can be supported naturally from the NDI / RVID / ARI fields in the DCI. The HARQ process ID may not be required (and may be replaced by the hashed ID field). Upon decoding the PDCCH with the correct UE indication, the UE may try to decode the new MSG2. In this case, there may be a collision problem (different UEs with different UE-ID and RAPID combinations hash to the same UE indication). However, the gNB may be aware of the collision and may withhold the transmission of the new MSG2 of a later UE to avoid confusion. By checking the UE indication field, the UE may only need to decode one more PDSCH after decoding the PDCCH.

[0119] For PDSCH, due to the limited space with UE indication in PDCCH, MSG2 can still include the actual UE ID to resolve hash collisions. In this case, only the UE with successful contention resolution sends an ACK on the resources indicated in the PDCCH. In the case of a collision, the UE still cannot send a NAK, so the UE does not know if MSG2 is for itself without decoding. In this case, if the gNB has not detected an ACK from the UE, it can retransmit a new MSG2.

[0120] In some cases, the new 2-step RACH MSG1 may be used for contention-free random access (CFRA). In some cases, the new MSG1 may be used at least for CFRA-based handovers, where the UE needs to send UL Msgs. Conventionally, the UL MSG is sent based on the UL grant issued in MSG2. In this case, it may be beneficial to attach the payload in the new MSG1 even for CFRA. In some cases, the gNB may instruct the UE whether to send only the preamble or the new MSG1 for CFRA.

[0121] Example Embodiments

[0122] Embodiment 1: A method for wireless communication by a user equipment (UE), comprising: initiating a two-step random access (RA) channel procedure (RACH) by sending a first message to a base station, the first message comprising at least a RACH preamble and a payload; receiving a random access response (RAR) message from the base station, wherein, in a case where the response message has a first format, the RAR message indicates that the base station successfully received both the RACH preamble and the payload; and, in a case where the RAR message has a second format, transitioning to a four-step RACH procedure before successfully completing the two-step RACH procedure.

[0123] Embodiment 2: The method as in Embodiment 1, wherein the RAR message triggering the transition has a RA preamble identifier (RAPID) field, and the RAPID field indicates that the base station has successfully detected at least the RACH preamble part of the first message.

[0124] Embodiment 3: A method as in Embodiment 2, wherein the UE starts the four-step RACH procedure by sending a message with an indication of an identifier (ID) of the UE.

[0125] Embodiment 4: The method as in any one of embodiments 1-3, wherein the RAR message is broadcast to multiple UEs having the same random access radio network temporary identity (RA-RNTI).

[0126] Embodiment 5: A method as in any one of embodiments 1-4, wherein the RAR message is sent to the UE as a unicast message.

[0127] Embodiment 6: A method as in any one of embodiments 1-5, wherein the UE is configured with separate transmit power control parameters for the two-step RACH procedure and the four-step RACH procedure, and these power control parameters include at least a preamble received target power and a power ramp-up step size.

[0128] Embodiment 7: The method of any one of embodiments 1-6, wherein the UE initiates the four-step RACH procedure at a transmit power level that depends on a current transmit power level of the two-step RACH procedure before the transition.

[0129] Embodiment 8: The method of Embodiment 7, wherein the UE initiates the four-step RACH procedure at a transmit power level with an additional power ramp relative to the current transmit power level of the two-step RACH procedure before the transition.

[0130] Embodiment 9: The method of any one of embodiments 1-8 further comprises: receiving remaining minimum system information (RMSI) that configures the UE with one or more parameters for supporting transition from the two-step RACH procedure and the four-step RACH procedure.

[0131] Embodiment 10: The method of any one of Embodiments 1-9, wherein the UE is configured to select between the two-step RACH procedure and the four-step RACH procedure based on a measurement of channel load or occupancy.

[0132] Embodiment 11: The method of any one of embodiments 1-10, wherein the UE applies a power offset to the payload portion of the first message of the two-step RACH procedure.

[0133] Embodiment 12: The method of embodiment 11, wherein the UE applies an additional power offset to the payload portion of the first message when retransmitting in the four-step RACH procedure.

[0134] Embodiment 13: The method of any one of Embodiments 1-12, wherein subcarrier spacing (SCS) of the preamble portion and the payload portion of the first message for the two-step RACH procedure are configured separately.

[0135] Embodiment 14: A method as in any one of Embodiments 1-13, wherein the UE uses the same subcarrier spacing (SCS) for the preamble part and the payload part of the first message of the two-step RACH procedure, and the SCS for the preamble part and the payload part of the first message of the two-step RACH procedure is indicated separately from the SCS used by the UE for message transmission of the four-step RACH procedure.

[0136] Embodiment 15: The method of any one of embodiments 1-14 further comprises: receiving signaling instructing the UE to include both the payload and the RACH preamble only when sending the first message of the two-step RACH procedure for contention-free random access (CFRA).

[0137] Embodiment 16: A method for wireless communication by a network entity, comprising: receiving at least a random access channel (RACH) preamble in a first message from a user equipment (UE) as part of a two-step RACH procedure; and sending a random access response (RAR) message to the UE, wherein a first format of the RAR indicates successful reception of both the RACH preamble and payload in the first message, and a second format of the RAR message signals to the UE to transition to a four-step RACH procedure before successfully completing the two-step RACH procedure.

[0138] Embodiment 17: A method as in Embodiment 16, wherein the RAR message of the second format that triggers the transition has a RA preamble identifier (RAPID) field, and the RAPID field indicates that the base station has successfully detected at least the RACH preamble portion of the first message.

[0139] Embodiment 18: The method of Embodiment 17, wherein the network entity starts the four-step RACH procedure by detecting a message with an indication of an identifier (ID) of the UE.

[0140] Embodiment 19: The method of any one of Embodiments 16-18, wherein the RAR message of the second format that triggers the transition is broadcast to multiple UEs having the same random access radio network temporary identity (RA-RNTI).

[0141] Embodiment 20: The method of any one of Embodiments 16-19, wherein the RAR message in the second format that triggers the transition is sent to the UE as a unicast message.

[0142] Embodiment 21: The method of any one of Embodiments 16-20, wherein the network entity is configured to perform soft combining of a payload portion of the first message of the two-step RACH procedure to assist in decoding at least a portion of a message of the four-step RACH procedure.

[0143] Embodiment 22: A device for wireless communication by a user equipment (UE), comprising: a device for initiating a two-step random access (RA) channel procedure (RACH) by sending a first message to a base station, the first message comprising at least a RACH preamble and a payload; a device for receiving a random access response (RAR) message from the base station, wherein when the response message has a first format, the RAR message indicates that the base station has successfully received both the RACH preamble and the payload; and a device for transitioning to a four-step RACH procedure before successfully completing the two-step RACH procedure when the RAR message has a second format.

[0144] Embodiment 23: A device for wireless communication by a network entity, comprising: a device for receiving at least a random access channel (RACH) preamble in a first message from a user equipment (UE) as part of a two-step RACH procedure; and a device for sending a random access response (RAR) message to the UE, wherein a first format of the RAR indicates successful reception of both the RACH preamble and payload in the first message, and a second format of the RAR message signals to the UE to transition to a four-step RACH procedure before successfully completing the two-step RACH procedure.

[0145] Embodiment 24: A computer-readable medium having stored thereon instructions for the following operations: initiating a two-step random access (RA) channel procedure (RACH) by sending a first message to a base station, the first message comprising at least a RACH preamble and a payload; receiving a random access response (RAR) message from the base station, wherein, when the response message has a first format, the RAR message indicates that the base station successfully received both the RACH preamble and the payload; and, when the RAR message has a second format, transitioning to a four-step RACH procedure before successfully completing the two-step RACH procedure.

[0146] Embodiment 25: A computer-readable medium having stored thereon instructions for: receiving at least a random access channel (RACH) preamble in a first message from a user equipment (UE) as part of a two-step RACH procedure; and sending a random access response (RAR) message to the UE, wherein a first format of the RAR indicates successful reception of both the RACH preamble and payload in the first message, and a second format of the RAR message signals to the UE to transition to a four-step RACH procedure prior to successfully completing the two-step RACH procedure.

[0147] The methods disclosed herein include one or more steps or actions for implementing the described methods. These method steps and / or actions may be interchangeable with each other without departing from the scope of the claims. In other words, unless a specific order of steps or actions is specified, the order and / or use of specific steps and / or actions may be changed without departing from the scope of the claims.

[0148] As used herein, a phrase referring to "at least one of" a list of items refers to any combination of those items, including single members. As an example, "at least one of a, b, or c" is intended to cover: a, b, c, ab, ac, bc, and abc, as well as any combination with multiple of the same elements (e.g., aa, aaa, aab, aac, abb, acc, bb, bbb, bbc, cc, and ccc, or any other ordering of a, b, and c).

[0149] As used herein, the term "determining" encompasses a wide variety of actions. For example, "determining" may include calculating, computing, processing, deriving, investigating, looking up (e.g., looking up in a table, a database, or another data structure), ascertaining, and the like. Also, "determining" may include receiving (e.g., receiving information), accessing (e.g., accessing data in a memory), and the like. Also, "determining" may include resolving, selecting, choosing, establishing, and the like.

[0150] The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the universal principles defined herein may be applied to other aspects. Thus, the claims are not intended to be limited to the various aspects shown herein, but should be granted the full scope consistent with the language of the claims, wherein the singular reference to the element is not intended to mean "there is and only one" (unless specifically stated) but "one or more". Unless specifically stated otherwise, the term "some / some" refers to one or more. The elements of the various aspects described throughout this disclosure are all structural and functional equivalents currently or hereafter known to those of ordinary skill in the art and are expressly incorporated herein by reference, and are intended to be covered by the claims. In addition, nothing disclosed herein is intended to be donated to the public, regardless of whether such disclosure is explicitly recorded in the claims. Any element of a claim should not be interpreted under the provisions of 35 USC § 112, paragraph 6, unless the element is explicitly stated using the wording "device for..." or in the case of a method claim, the element is stated using the wording "step for...".

[0151] The various operations of the methods described above may be performed by any suitable device capable of performing the corresponding functions. These devices may include various hardware and / or software components and / or modules, including but not limited to circuits, application specific integrated circuits (ASICs), or processors. Generally, where there are operations illustrated in the accompanying drawings, these operations may have corresponding paired device-plus-function components with similar numbers.

[0152] The various illustrative logical blocks, modules, and circuits described in conjunction with the present disclosure may be implemented or executed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device (PLD), discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any commercially available processor, controller, microcontroller, or state machine. The processor may also be implemented as a combination of computing devices, for example, a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.

[0153] If implemented in hardware, an example hardware configuration may include a processing system in a wireless node. The processing system may be implemented using a bus architecture. Depending on the specific application and overall design constraints of the processing system, the bus may include any number of interconnecting buses and bridges. The bus may link together various circuits including a processor, a machine-readable medium, and a bus interface. The bus interface may be used to connect a network adapter, etc., to the processing system via the bus. The network adapter may be used to implement the signal processing functions of the PHY layer. In the user terminal 120 (see Figure 1 ), a user interface (e.g., a keypad, display, mouse, joystick, etc.) may also be connected to the bus. The bus may also link various other circuits, such as timing sources, peripherals, voltage regulators, power management circuits, and the like, which are well known in the art and will not be described further. The processor may be implemented with one or more general and / or special purpose processors. Examples include microprocessors, microcontrollers, DSP processors, and other circuit systems capable of executing software. Those skilled in the art will recognize how to best implement the functionality described with respect to the processing system, depending on the specific application and the overall design constraints imposed on the overall system.

[0154] If implemented in software, each function may be stored on or transmitted by a computer-readable medium as one or more instructions or codes. Software should be broadly interpreted to mean instructions, data, or any combination thereof, whether referred to as software, firmware, middleware, microcode, hardware description language, or other. Computer-readable media include both computer storage media and communication media, which include any media that facilitate the transfer of computer programs from one place to another. The processor may be responsible for managing the bus and general processing, including executing software modules stored on a machine-readable storage medium. A computer-readable storage medium may be coupled to a processor so that the processor can read and write information from / to the storage medium. In an alternative, a storage medium may be integrated into a processor. As an example, a machine-readable medium may include a transmission line, a carrier modulated by data, and / or a computer-readable storage medium having instructions stored thereon that is separated from a wireless node, all of which may be accessed by a processor through a bus interface. Alternatively or in addition, a machine-readable medium or any part thereof may be integrated into a processor, such as a cache and / or a general register file, which may be the case. As an example, examples of machine-readable storage media may include RAM (random access memory), flash memory, ROM (read only memory), PROM (programmable read only memory), EPROM (erasable programmable read only memory), EEPROM (electrically erasable programmable read only memory), registers, magnetic disks, optical disks, hard drives, or any other suitable storage media, or any combination thereof. Machine-readable media may be implemented in a computer program product.

[0155] A software module may include a single instruction, or many instructions, and may be distributed over several different code segments, between different programs, and across multiple storage media. A computer-readable medium may include several software modules. These software modules include instructions that cause a processing system to perform various functions when executed by a device (such as a processor). These software modules may include a transmission module and a receiving module. Each software module may reside in a single storage device or may be distributed across multiple storage devices. As an example, when a triggering event occurs, a software module may be loaded into a RAM from a hard drive. During the execution of the software module, the processor may load some instructions into a cache to increase access speed. One or more cache lines may then be loaded into a general register file for execution by the processor. When describing the functionality of a software module as described below, it will be understood that such functionality is implemented by the processor when the processor executes instructions from the software module.

[0156] Any connection is also properly referred to as a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technology (such as infrared (IR), radio, and microwave), then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technology (such as infrared, radio, and microwave) is included in the definition of medium. Disk and disc, as used herein, include compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk, and Disks, where disks often reproduce data magnetically, and discs reproduce data optically with lasers. Thus, in some aspects, computer-readable media may include non-transitory computer-readable media (e.g., tangible media). Additionally, for other aspects, computer-readable media may include transient computer-readable media (e.g., signals). Combinations of the above should also be included within the scope of computer-readable media.

[0157] Thus, some aspects may include a computer program product for performing the operations presented herein. For example, such a computer program product may include a computer-readable medium having instructions stored (and / or encoded) thereon, which instructions can be executed by one or more processors to perform the operations described herein.

[0158] In addition, it should be appreciated that modules and / or other appropriate means for performing the methods and techniques described herein may be downloaded and / or otherwise obtained by a user terminal and / or base station where applicable. For example, such a device can be coupled to a server to facilitate the transfer of means for performing the methods described herein. Alternatively, the various methods described herein can be provided via a storage device (e.g., RAM, ROM, a physical storage medium such as a compact disc (CD) or a floppy disk, etc.) so that once the storage device is coupled to or provided to a user terminal and / or base station, the device can obtain the various methods. In addition, any other suitable technology suitable for providing the methods and techniques described herein to a device may be utilized.

[0159] It will be understood that the claims are not limited to the precise configuration and components illustrated above. Various changes, substitutions and variations may be made in the arrangement, operation and details of the methods and apparatus described above without departing from the scope of the claims.

Claims

1. A method for wireless communication by a user equipment UE, comprising: Initiating a two-step random access channel (RACH) procedure by sending a first message to a base station, the first message comprising at least a preamble portion and a payload portion, the preamble portion comprising a RACH preamble, the payload portion comprising a payload, wherein sending the first message comprises applying a power offset to the payload portion of the first message; receiving a random access response (RAR) message from the base station, wherein, if the RAR message has a first format, the RAR message indicates that the base station successfully received both the RACH preamble and the payload; as well as In case the RAR message has the second format, transitioning to a four-step RACH procedure is performed before successfully completing the two-step RACH procedure.

2. The method of claim 1, wherein: The RAR message triggering the transition has a random access RA preamble identifier RAPID field, and the RAPID field indicates that the base station successfully detects at least a RACH preamble part of the first message.

3. The method of claim 2, wherein: The UE starts the four-step RACH procedure by sending a message with an indication of an identifier ID of the UE.

4. The method of claim 1, wherein the RAR message is broadcast to a plurality of UEs having the same Random Access Radio Network Temporary Identity (RA-RNTI).

5. The method of claim 1, wherein the RAR message is sent to the UE as a unicast message.

6. The method of claim 1, wherein: The UE is configured with separate transmit power control parameters for the two-step RACH procedure and the four-step RACH procedure; and The power control parameters include at least the target power received by the preamble and the power ramp-up step size.

7. The method of claim 1, wherein the UE initiates the four-step RACH procedure at a transmit power level that depends on a current transmit power level of the two-step RACH procedure before the transition.

8. The method of claim 7, wherein the UE initiates the four-step RACH procedure at a transmit power level with an additional power ramp-up relative to the current transmit power level of the two-step RACH procedure prior to the transition.

9. The method of claim 1, further comprising: Remaining minimum system information (RMSI) is received to configure the UE with one or more parameters for supporting a transition from the two-step RACH procedure and the four-step RACH procedure.

10. The method of claim 1, wherein the UE is configured to select between the two-step RACH procedure and the four-step RACH procedure based on a measurement of channel load or occupancy.

11. The method of claim 1, wherein the UE applies an additional power offset to the payload portion of the first message when retransmitting in the four-step RACH procedure.

12. The method of claim 1, wherein a subcarrier spacing (SCS) of a preamble part and a payload part of the first message for the two-step RACH procedure are configured separately.

13. The method of claim 1, wherein: The UE uses the same subcarrier spacing SCS for the preamble part and the payload part of the first message of the two-step RACH procedure; and The SCS of the preamble part and the payload part of the first message used for the two-step RACH procedure is indicated separately from the SCS used by the UE for message transmission of the four-step RACH procedure.

14. The method of claim 1, further comprising: Signaling is received, the signaling instructing the UE to include both the payload and the RACH preamble only when sending the first message of the two-step RACH procedure for contention-free random access CFRA.

15. A method for wireless communication by a network entity, comprising: receiving a first message as part of a two-step random access channel (RACH) procedure from a user equipment (UE), the first message comprising a preamble portion, the preamble portion comprising at least a RACH preamble; as well as A random access response (RAR) message is sent to the UE, wherein a first format of the RAR message indicates successful reception of both the RACH preamble and the payload included in a payload portion in the first message, and a second format of the RAR message signals to the UE to transition to a four-step RACH procedure prior to successful completion of the two-step RACH procedure, wherein a power offset is applied to the payload portion of the first message.

16. The method of claim 15, wherein: The RAR message of the second format triggering the transition has a random access RA preamble identifier RAPID field, and the RAPID field indicates that the network entity successfully detected at least a RACH preamble part of the first message.

17. The method of claim 16, wherein: The network entity starts the four-step RACH procedure by detecting a message with an indication of an identifier ID of the UE.

18. The method of claim 15, wherein the RAR message of the second format triggering the transition is broadcast to a plurality of UEs having the same Random Access Radio Network Temporary Identity (RA-RNTI).

19. The method of claim 15, wherein the RAR message in the second format that triggers the transition is sent to the UE as a unicast message.

20. The method of claim 15, wherein the network entity is configured to perform soft combining of a payload portion of the first message of the two-step RACH procedure to assist in decoding at least a portion of a message of the four-step RACH procedure.

21. A device for wireless communication by a user equipment UE, comprising: means for initiating a two-step random access channel (RACH) procedure by sending a first message to a base station, the first message comprising at least a preamble portion and a payload portion, the preamble portion comprising a RACH preamble, the payload portion comprising a payload, wherein sending the first message comprises applying a power offset to the payload portion of the first message; means for receiving a random access response (RAR) message from the base station, wherein, if the RAR message has a first format, the RAR message indicates that the base station successfully received both the RACH preamble and the payload; as well as Means for transitioning to a four-step RACH procedure prior to successful completion of the two-step RACH procedure if the RAR message has a second format.

22. The apparatus of claim 21, wherein: The RAR message triggering the transition has a random access RA preamble identifier RAPID field, and the RAPID field indicates that the base station successfully detects at least a RACH preamble part of the first message.

23. The apparatus of claim 22, wherein: The UE starts the four-step RACH procedure by sending a message with an indication of an identifier ID of the UE.

24. The apparatus of claim 21, wherein the RAR message is broadcast to a plurality of UEs having the same Random Access Radio Network Temporary Identity (RA-RNTI).

25. The apparatus of claim 21, wherein the RAR message is sent to the UE as a unicast message.

26. The apparatus of claim 21, wherein: The UE is configured with separate transmit power control parameters for the two-step RACH procedure and the four-step RACH procedure; and The power control parameters include at least the target power received by the preamble and the power ramp-up step size.

27. The apparatus of claim 21, wherein the UE initiates the four-step RACH procedure at a transmit power level that depends on a current transmit power level of the two-step RACH procedure prior to the transition.

28. The apparatus of claim 27, wherein the UE initiates the four-step RACH procedure at a transmit power level with an additional power ramp relative to the current transmit power level of the two-step RACH procedure prior to the transition.

29. The apparatus of claim 21, further comprising: Means for receiving remaining minimum system information (RMSI) for configuring the UE with one or more parameters for supporting a transition from the two-step RACH procedure and the four-step RACH procedure.

30. The apparatus of claim 21, wherein the UE is configured to select between the two-step RACH procedure and the four-step RACH procedure based on a measurement of channel load or occupancy.

31. The apparatus of claim 21, wherein the UE applies an additional power offset to the payload portion of the first message when retransmitting in the four-step RACH procedure.

32. The apparatus of claim 21, wherein a subcarrier spacing (SCS) of a preamble portion and a payload portion of the first message for the two-step RACH procedure are configured separately.

33. The apparatus of claim 21, wherein: The UE uses the same subcarrier spacing SCS for the preamble part and the payload part of the first message of the two-step RACH procedure; and The SCS of the preamble part and the payload part of the first message used for the two-step RACH procedure is indicated separately from the SCS used by the UE for message transmission of the four-step RACH procedure.

34. The apparatus of claim 21, further comprising: Means for receiving signaling instructing the UE to include both the payload and the RACH preamble only when sending the first message of the two-step RACH procedure for contention-free random access CFRA.

35. An apparatus for wireless communication by a network entity, comprising: means for receiving a first message as part of a two-step random access channel, RACH, procedure from a user equipment, UE, the first message comprising a preamble portion, the preamble portion comprising at least a RACH preamble; as well as means for sending a random access response (RAR) message to the UE, wherein a first format of the RAR message indicates successful reception of both the RACH preamble and a payload included in a payload portion in the first message, and a second format of the RAR message signals to the UE to transition to a four-step RACH procedure prior to successful completion of the two-step RACH procedure, wherein a power offset is applied to the payload portion of the first message.

36. The apparatus of claim 35, wherein: The RAR message of the second format triggering the transition has a random access RA preamble identifier RAPID field, and the RAPID field indicates that the network entity successfully detected at least a RACH preamble part of the first message.

37. The apparatus of claim 36, wherein: The network entity starts the four-step RACH procedure by detecting a message with an indication of an identifier ID of the UE.

38. The apparatus of claim 35, wherein the RAR message of the second format that triggers the transition is broadcast to a plurality of UEs having the same Random Access Radio Network Temporary Identity (RA-RNTI).

39. The apparatus of claim 35, wherein the RAR message in the second format that triggers the transition is sent to the UE as a unicast message.

40. The apparatus of claim 35, wherein the network entity is configured to perform soft combining of a payload portion of the first message of the two-step RACH procedure to facilitate decoding of at least a portion of a message of the four-step RACH procedure.

41. A computer readable medium having stored thereon instructions for: Initiating a two-step random access channel (RACH) procedure by sending a first message to a base station, the first message comprising at least a preamble portion and a payload portion, the preamble portion comprising a RACH preamble, the payload portion comprising a payload, wherein sending the first message comprises applying a power offset to the payload portion of the first message; receiving a random access response (RAR) message from the base station, wherein, if the RAR message has a first format, the RAR message indicates that the base station successfully received both the RACH preamble and the payload; as well as In case the RAR message has the second format, transitioning to a four-step RACH procedure is performed before successfully completing the two-step RACH procedure.

42. A computer readable medium having stored thereon instructions for: receiving a first message as part of a two-step random access channel (RACH) procedure from a user equipment (UE), the first message comprising a preamble part, the preamble part comprising at least a RACH preamble; and A random access response (RAR) message is sent to the UE, wherein a first format of the RAR message indicates successful reception of both the RACH preamble and the payload included in a payload portion in the first message, and a second format of the RAR message signals to the UE to transition to a four-step RACH procedure prior to successful completion of the two-step RACH procedure, wherein a power offset is applied to the payload portion of the first message.

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