Patent Publication Number: US-2023139625-A1

Title: Handover Random Access

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority to U.S. patent application Ser. No. 17/563,204, filed Dec. 28, 2021, which is a continuation of U.S. patent application Ser. No. 16/720,225, filed Dec. 19, 2019 (now U.S. Pat. No. 11,252,619), which is a continuation of U.S. patent application Ser. No. 15/933,301, filed Mar. 22, 2018 (now U.S. Pat. No. 10,568,007), which claims the benefit of U.S. Provisional Patent Application No. 62/475,033, filed Mar. 22, 2017, each of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     In wireless communications, such as between a wireless device and a base station, a random access (RA) procedure may be performed to initiate the communications. A wireless device may handover from a source base station to a target base station, e.g., if the wireless device moves from a location in a first cell served by the source base station to a location in a second cell served by the target base station. The source base station and the target base station may perform different RA procedures to initiate communications with wireless devices. Difficulties may arise in determining a type of RA procedure to perform when a handover occurs. 
     SUMMARY 
     The following summary presents a simplified summary of certain features. The summary is not an extensive overview and is not intended to identify key or critical elements. 
     Systems, apparatuses, and methods are described for determining a type of RA procedure to perform when a handover occurs. RA procedures may comprise different steps. For example, a two-step RA procedure or a four-step RA procedure may be performed. The type of RA procedure may be identified by one or more indicators. The one or more indicators may be included in messages sent to or from devices involved in a handover, such as a wireless device, a source base station, a target base station, and/or one or more core network devices. 
     These and other features and advantages are described in greater detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some features are shown by way of example, and not by limitation, in the accompanying drawings. In the drawings, like numerals reference similar elements. 
         FIG.  1    shows example sets of orthogonal frequency division multiplexing (OFDM) subcarriers. 
         FIG.  2    shows example transmission time and reception time for two carriers in a carrier group. 
         FIG.  3    shows example OFDM radio resources. 
         FIG.  4    shows hardware elements of a base station and a wireless device. 
         FIG.  5 A ,  FIG.  5 B ,  FIG.  5 C  and  FIG.  5 D  show examples for uplink and downlink signal transmission. 
         FIG.  6    shows an example protocol structure with multi-connectivity. 
         FIG.  7    shows an example protocol structure with carrier aggregation (CA) and dual connectivity (DC). 
         FIG.  8    shows example timing advance group (TAG) configurations. 
         FIG.  9    shows example message flow in a random access process in a secondary TAG. 
         FIG.  10 A  and  FIG.  10 B  show examples for interfaces between a 5G core network and base stations. 
         FIG.  11 A ,  FIG.  11 B ,  FIG.  11 C ,  FIG.  11 D ,  FIG.  11 E , and  FIG.  11 F  show examples for architectures of tight interworking between 5G RAN and long term evolution (LTE) radio access network (RAN). 
         FIG.  12 A ,  FIG.  12 B , and  FIG.  12 C  show examples for radio protocol structures of tight interworking bearers. 
         FIG.  13 A  and  FIG.  13 B  show examples for gNodeB (gNB) deployment scenarios. 
         FIG.  14    functional split option examples of a centralized gNB deployment scenario. 
         FIG.  15    shows examples of contention-based and contention free random access procedures. 
         FIG.  16    shows an example of a random access preamble selection procedure. 
         FIG.  17    shows an example media access control (MAC) packet data unit (PDU) format of an example of MAC PDU comprising a MAC header and MAC random access responses (RARs) for a four-step RA procedure. 
         FIG.  18    shows an example of shows an example of an uplink resource for a transmission in a first step of a two-step RA procedure. 
         FIG.  19    shows an example MAC RAR format of an example of MAC RAR comprising a timing advance command, Uplink (UL) Grant, and Temporary Cell-Radio Network Temporary Identifier for a four-step RA procedure. 
         FIG.  20    shows an example of a two-step RA procedure. 
         FIG.  21    shows an example of contention resolution for a two-step RA procedure. 
         FIG.  22    shows an example of a two-step RA procedure of an example failure of UL transmission for n times. 
         FIG.  23    shows example RARs with a fixed size 8 bytes for example RAR formats for two-step RA procedures and for a four-step RA procedure. 
         FIG.  24    shows an example RAR with a fixed size 12 bytes for example RAR formats for two-step and four-step RA procedures. 
         FIG.  25    shows an example for hybrid automatic repeat request (HARQ) retransmission, such as when a cell detects a random access preamble identifier but fails to decode data. 
         FIG.  26    shows an example of a two-step RA procedure failure as the number of HARQ retransmission reaches a threshold. 
         FIG.  27    shows an example of a two-step RA procedure when a base station decodes a RAP and UL data and responds with an RAR to a wireless device. 
         FIG.  28    shows an example of a handover of a wireless device from a source base station to a target base station. 
         FIG.  29    shows an example of a handover of a wireless device from a source base station to a target base station that is facilitated by a core network entity. 
         FIG.  30    shows an example of handover procedures for a wireless device. 
         FIG.  31    shows an example of handover procedures for a source base station. 
         FIG.  32    shows an example of handover procedures for a target base station. 
         FIG.  33    shows an example of handover procedures for a core network device. 
         FIG.  34    shows an example of handover procedures with different types of RA procedures for a wireless device. 
     
    
    
     DETAILED DESCRIPTION 
     The accompanying drawings, which form a part hereof, show examples of the disclosure. It is to be understood that the examples shown in the drawings and/or discussed herein are non-exclusive and that there are other examples of how the disclosure may be practiced. 
     Examples may enable operation of carrier aggregation and may be employed in the technical field of multicarrier communication systems. Examples may relate to signal timing in a multicarrier communication systems. 
     The following Acronyms are used throughout the present disclosure: 
     3GPP 3rd Generation Partnership Project 
     5G 5th generation wireless systems 
     5GC 5G Core Network 
     ACK Acknowledgement 
     AMF Access and Mobility Management Function 
     ASIC application-specific integrated circuit 
     BPSK binary phase shift keying 
     CA carrier aggregation 
     CC component carrier 
     CDMA code division multiple access 
     CP cyclic prefix 
     CPLD complex programmable logic devices 
     CSI channel state information 
     CSS common search space 
     CU central unit 
     DC dual connectivity 
     DCI downlink control information 
     DFTS-OFDM discrete fourier transform spreading OFDM 
     DL downlink 
     DU distributed unit 
     eLTE enhanced LTE 
     eMBB enhanced mobile broadband 
     eNB evolved Node B 
     EPC evolved packet core 
     E-UTRAN evolved-universal terrestrial radio access network 
     FDD frequency division multiplexing 
     FPGA field programmable gate arrays 
     Fs-C Fs-control plane 
     Fs-U Fs-user plane 
     gNB next generation node B 
     HARQ hybrid automatic repeat request 
     HDL hardware description languages 
     ID identifier 
     IE information element 
     LTE long term evolution 
     MAC media access control 
     MCG master cell group 
     MeNB master evolved node B 
     MIB master information block 
     MME mobility management entity 
     mMTC massive machine type communications 
     NACK Negative Acknowledgement 
     NAS non-access stratum 
     NG CP next generation control plane core 
     NGC next generation core 
     NG-C NG-control plane 
     NG-U NG-user plane 
     NR MAC new radio MAC 
     NR PDCP new radio PDCP 
     NR PHY new radio physical 
     NR RLC new radio RLC 
     NR RRC new radio RRC 
     NR new radio 
     NSSAI network slice selection assistance information 
     OFDM orthogonal frequency division multiplexing 
     PCC primary component carrier 
     PCell primary cell 
     PDCCH physical downlink control channel 
     PDCP packet data convergence protocol 
     PDU packet data unit 
     PHICH physical HARQ indicator channel 
     PHY physical 
     PLMN public land mobile network 
     PSCell primary secondary cell 
     pTAG primary timing advance group 
     PUCCH physical uplink control channel 
     PUSCH physical uplink shared channel 
     QAM quadrature amplitude modulation 
     QPSK quadrature phase shift keying 
     RA random access 
     RACH random access channel 
     RAN radio access network 
     RAP random access preamble 
     RAPID/RAP ID random access preamble identifier 
     RAR random access response 
     RB resource blocks 
     RBG resource block groups 
     RLC radio link control 
     RRC radio resource control 
     RRM radio resource management 
     RV redundancy version 
     SCC secondary component carrier 
     SCell secondary cell 
     SCG secondary cell group 
     SC-OFDM single carrier-OFDM 
     SDU service data unit 
     SeNB secondary evolved node B 
     SFN system frame number 
     S-GW serving gateway 
     SIB system information block 
     SC-OFDM single carrier orthogonal frequency division multiplexing 
     SRB signaling radio bearer 
     sTAG(s) secondary timing advance group(s) 
     TA timing advance 
     TAG timing advance group 
     TAI tracking area identifier 
     TAT time alignment timer 
     TDD time division duplexing 
     TDMA time division multiple access 
     TTI transmission time interval 
     TB transport block 
     UE user equipment 
     UL uplink 
     UPGW user plane gateway 
     URLLC ultra-reliable low-latency communications 
     VHDL VHSIC hardware description language 
     Xn-C Xn-control plane 
     Xn-U Xn-user plane 
     Xx-C Xx-control plane 
     Xx-U Xx-user plane 
     Examples may be implemented using various physical layer modulation and transmission mechanisms. Example transmission mechanisms may include, but are not limited to: CDMA, OFDM, TDMA, Wavelet technologies, and/or the like. Hybrid transmission mechanisms such as TDMA/CDMA, and OFDM/CDMA may also be employed. Various modulation schemes may be applied for signal transmission in the physical layer. Examples of modulation schemes include, but are not limited to: phase, amplitude, code, a combination of these, and/or the like. An example radio transmission method may implement QAM using BPSK, QPSK, 16-QAM, 64-QAM, 256-QAM, and/or the like. Physical radio transmission may be enhanced by dynamically or semi-dynamically changing the modulation and coding scheme depending on transmission requirements and radio conditions. 
       FIG.  1    shows example sets of OFDM subcarriers. As shown in this example, arrow(s) in the diagram may depict a subcarrier in a multicarrier OFDM system. The OFDM system may use technology such as OFDM technology, DFTS-OFDM, SC-OFDM technology, or the like. For example, arrow  101  shows a subcarrier transmitting information symbols.  FIG.  1    is shown as an example, and a typical multicarrier OFDM system may include more subcarriers in a carrier. For example, the number of subcarriers in a carrier may be in the range of 10 to 10,000 subcarriers.  FIG.  1    shows two guard bands  106  and  107  in a transmission band. As shown in  FIG.  1   , guard band  106  is between subcarriers  103  and subcarriers  104 . The example set of subcarriers A  102  includes subcarriers  103  and subcarriers  104 .  FIG.  1    also shows an example set of subcarriers B  105 . As shown, there is no guard band between any two subcarriers in the example set of subcarriers B  105 . Carriers in a multicarrier OFDM communication system may be contiguous carriers, non-contiguous carriers, or a combination of both contiguous and non-contiguous carriers. 
       FIG.  2    shows an example with transmission time and reception time for two carriers. A multicarrier OFDM communication system may include one or more carriers, for example, ranging from 1 to 10 carriers. Carrier A  204  and carrier B  205  may have the same or different timing structures. Although  FIG.  2    shows two synchronized carriers, carrier A  204  and carrier B  205  may or may not be synchronized with each other. Different radio frame structures may be supported for FDD and TDD duplex mechanisms.  FIG.  2    shows an example FDD frame timing. Downlink and uplink transmissions may be organized into radio frames  201 . In this example, radio frame duration is 10 milliseconds (msec). Other frame durations, for example, in the range of 1 to 100 msec may also be supported. In this example, each 10 msec radio frame  201  may be divided into ten equally sized subframes  202 . Other subframe durations such as including 0.5 msec, 1 msec, 2 msec, and 5 msec may also be supported. Subframe(s) may consist of two or more slots (e.g., slots  206  and  207 ). For the example of FDD, 10 subframes may be available for downlink transmission and 10 subframes may be available for uplink transmissions in each 10 msec interval. Uplink and downlink transmissions may be separated in the frequency domain. A slot may be 7 or 14 OFDM symbols for the same subcarrier spacing of up to 60 kHz with normal CP. A slot may be 14 OFDM symbols for the same subcarrier spacing higher than 60 kHz with normal CP. A slot may contain all downlink, all uplink, or a downlink part and an uplink part, and/or alike. Slot aggregation may be supported, e.g., data transmission may be scheduled to span one or multiple slots. In an example, a mini-slot may start at an OFDM symbol in a subframe. A mini-slot may have a duration of one or more OFDM symbols. Slot(s) may include a plurality of OFDM symbols  203 . The number of OFDM symbols  203  in a slot  206  may depend on the cyclic prefix length and subcarrier spacing. 
       FIG.  3    shows an example of OFDM radio resources. The resource grid structure in time  304  and frequency  305  is shown in  FIG.  3   . The quantity of downlink subcarriers or RBs may depend, at least in part, on the downlink transmission bandwidth  306  configured in the cell. The smallest radio resource unit may be called a resource element (e.g.,  301 ). Resource elements may be grouped into resource blocks (e.g.,  302 ). Resource blocks may be grouped into larger radio resources called Resource Block Groups (RBG) (e.g.,  303 ). The transmitted signal in slot  206  may be described by one or several resource grids of a plurality of subcarriers and a plurality of OFDM symbols. Resource blocks may be used to describe the mapping of certain physical channels to resource elements. Other pre-defined groupings of physical resource elements may be implemented in the system depending on the radio technology. For example, 24 subcarriers may be grouped as a radio block for a duration of 5 msec. A resource block may correspond to one slot in the time domain and 180 kHz in the frequency domain (for 15 kHz subcarrier bandwidth and 12 subcarriers). 
     Multiple numerologies may be supported. A numerology may be derived by scaling a basic subcarrier spacing by an integer N. Scalable numerology may allow at least from 15 kHz to 480 kHz subcarrier spacing. The numerology with 15 kHz and scaled numerology with different subcarrier spacing with the same CP overhead may align at a symbol boundary every 1 msec in a NR carrier. 
       FIG.  4    shows hardware elements of a base station  401  and a wireless device  406 . A communication network  400  may include at least one base station  401  and at least one wireless device  406 . The base station  401  may include at least one communication interface  402 , one or more processors  403 , and at least one set of program code instructions  405  stored in non-transitory memory  404  and executable by the one or more processors  403 . The wireless device  406  may include at least one communication interface  407 , one or more processors  408 , and at least one set of program code instructions  410  stored in non-transitory memory  409  and executable by the one or more processors  408 . A communication interface  402  in the base station  401  may be configured to engage in communication with a communication interface  407  in the wireless device  406 , such as via a communication path that includes at least one wireless link  411 . The wireless link  411  may be a bi-directional link. The communication interface  407  in the wireless device  406  may also be configured to engage in communication with the communication interface  402  in the base station  401 . The base station  401  and the wireless device  406  may be configured to send and receive data over the wireless link  411  using multiple frequency carriers. Base stations, wireless devices, and other communication devices may include structure and operations of transceiver(s). A transceiver is a device that includes both a transmitter and receiver. Transceivers may be employed in devices such as wireless devices, base stations, relay nodes, and/or the like. Examples for radio technology implemented in the communication interfaces  402 ,  407  and the wireless link  411  are shown in  FIG.  1   ,  FIG.  2   ,  FIG.  3   ,  FIG.  5   , and associated text. The communication network  400  may comprise any number and/or type of devices, such as, for example, wireless devices, mobile devices, handsets, tablets, laptops, internet of things (IoT) devices, hotspots, cellular repeaters, computing devices, and/or, more generally, user equipment (e.g., UE). Although one or more of the above types of devices may be referenced herein (e.g., UE, wireless device, etc.), it should be understood that any device herein may comprise any one or more of the above types of devices or similar devices. The communication network  400 , and any other network referenced herein, may comprise an LTE network, a 5G network, or any other network for wireless communications. Apparatuses, systems, and/or methods described herein may generally be described as implemented on one or more devices (e.g., wireless device, base station, eNB, gNB, computing device, etc.), in one or more networks, but it will be understood that one or more features and steps may be implemented on any device and/or in any network. As an example, any reference to a base station may comprise an eNB, a gNB, a computing device, or any other device, and any reference to a wireless device may comprise a UE, a handset, a mobile device, a computing device or any other device. 
     The communications network  400  may comprise Radio Access Network (RAN) architecture. The RAN architecture may comprise one or more RAN nodes that may be a next generation Node B (gNB) (e.g.,  401 ) providing New Radio (NR) user plane and control plane protocol terminations towards a first wireless device (e.g.  406 ). A RAN node may be a next generation evolved Node B (ng-eNB), providing Evolved UMTS Terrestrial Radio Access (E-UTRA) user plane and control plane protocol terminations towards a second wireless device. The first wireless device may communicate with a gNB over a Uu interface. The second wireless device may communicate with a ng-eNB over a Uu interface. Base station  401  may comprise at least one of a gNB, ng-eNB, and or the like. 
     A gNB or an ng-eNB may host functions such as: radio resource management and scheduling, IP header compression, encryption and integrity protection of data, selection of Access and Mobility Management Function (AMF) at User Equipment (UE) attachment, routing of user plane and control plane data, connection setup and release, scheduling and transmission of paging messages (originated from the AMF), scheduling and transmission of system broadcast information (originated from the AMF or Operation and Maintenance (O&amp;M)), measurement and measurement reporting configuration, transport level packet marking in the uplink, session management, support of network slicing, Quality of Service (QoS) flow management and mapping to data radio bearers, support of UEs in RRC_INACTIVE state, distribution function for Non-Access Stratum (NAS) messages, RAN sharing, and dual connectivity or tight interworking between NR and E-UTRA. 
     One or more gNBs and/or one or more ng-eNBs may be interconnected with each other by means of Xn interface. A gNB or an ng-eNB may be connected by means of NG interfaces to 5G Core Network (5GC). 5GC may comprise one or more AMF/User Plan Function (UPF) functions. A gNB or an ng-eNB may be connected to a UPF by means of an NG-User plane (NG-U) interface. The NG-U interface may provide delivery (e.g. non-guaranteed delivery) of user plane Protocol Data Units (PDUs) between a RAN node and the UPF. A gNB or an ng-eNB may be connected to an AMF by means of an NG-Control plane (NG-C) interface. The NG-C interface may provide functions such as NG interface management, UE context management, UE mobility management, transport of NAS messages, paging, PDU session management, configuration transfer or warning message transmission. 
     A UPF may host functions such as anchor point for intra-/inter-Radio Access Technology (RAT) mobility (when applicable), external PDU session point of interconnect to data network, packet routing and forwarding, packet inspection and user plane part of policy rule enforcement, traffic usage reporting, uplink classifier to support routing traffic flows to a data network, branching point to support multi-homed PDU session, QoS handling for user plane, e.g. packet filtering, gating, Uplink (UL)/Downlink (DL) rate enforcement, uplink traffic verification (e.g. Service Data Flow (SDF) to QoS flow mapping), downlink packet buffering and/or downlink data notification triggering. 
     An AMF may host functions such as NAS signaling termination, NAS signaling security, Access Stratum (AS) security control, inter Core Network (CN) node signaling for mobility between 3 rd  Generation Partnership Project (3GPP) access networks, idle mode UE reachability (e.g., control and execution of paging retransmission), registration area management, support of intra-system and inter-system mobility, access authentication, access authorization including check of roaming rights, mobility management control (subscription and policies), support of network slicing and/or Session Management Function (SMF) selection 
     An interface may be a hardware interface, a firmware interface, a software interface, and/or a combination thereof. The hardware interface may include connectors, wires, electronic devices such as drivers, amplifiers, and/or the like. A software interface may include code stored in a memory device to implement protocol(s), protocol layers, communication drivers, device drivers, combinations thereof, and/or the like. A firmware interface may include a combination of embedded hardware and code stored in and/or in communication with a memory device to implement connections, electronic device operations, protocol(s), protocol layers, communication drivers, device drivers, hardware operations, combinations thereof, and/or the like. 
     The term configured may relate to the capacity of a device whether the device is in an operational or a non-operational state. Configured may also refer to specific settings in a device that effect the operational characteristics of the device whether the device is in an operational or a non-operational state. In other words, the hardware, software, firmware, registers, memory values, and/or the like may be “configured” within a device, whether the device is in an operational or a nonoperational state, to provide the device with specific characteristics. Terms such as “a control message to cause in a device” may mean that a control message has parameters that may be used to configure specific characteristics in the device, whether the device is in an operational or a non-operational state. 
     A 5G network may include a multitude of base stations, providing a user plane NR PDCP/NR RLC/NR MAC/NR PHY and control plane (NR RRC) protocol terminations towards the wireless device. The base station(s) may be interconnected with other base station(s) (e.g., employing an Xn interface). The base stations may also be connected employing, for example, an NG interface to an NGC.  FIG.  10 A  and  FIG.  10 B  show examples for interfaces between a 5G core network (e.g., NGC) and base stations (e.g., gNB and eLTE eNB). For example, the base stations may be interconnected to the NGC control plane (e.g., NG CP) employing the NG-C interface and to the NGC user plane (e.g., UPGW) employing the NG-U interface. The NG interface may support a many-to-many relation between 5G core networks and base stations. 
     A base station may include many sectors, for example: 1, 2, 3, 4, or 6 sectors. A base station may include many cells, for example, ranging from 1 to 50 cells or more. A cell may be categorized, for example, as a primary cell or secondary cell. At RRC connection establishment/re-establishment/handover, one serving cell may provide the NAS (non-access stratum) mobility information (e.g., TAI), and at RRC connection re-establishment/handover, one serving cell may provide the security input. This cell may be referred to as the Primary Cell (PCell). In the downlink, the carrier corresponding to the PCell may be the Downlink Primary Component Carrier (DL PCC), while in the uplink, it may be the Uplink Primary Component Carrier (UL PCC). Depending on wireless device capabilities, Secondary Cells (SCells) may be configured to form together with the PCell a set of serving cells. In the downlink, the carrier corresponding to an SCell may be a Downlink Secondary Component Carrier (DL SCC), while in the uplink, it may be an Uplink Secondary Component Carrier (UL SCC). An SCell may or may not have an uplink carrier. 
     A cell, comprising a downlink carrier and optionally an uplink carrier, may be assigned a physical cell ID and a cell index. A carrier (downlink or uplink) may belong to only one cell. The cell ID or cell index may also identify the downlink carrier or uplink carrier of the cell (depending on the context it is used). The cell ID may be equally referred to a carrier ID, and cell index may be referred to carrier index. In implementation, the physical cell ID or cell index may be assigned to a cell. A cell ID may be determined using a synchronization signal transmitted on a downlink carrier. A cell index may be determined using RRC messages. For example, reference to a first physical cell ID for a first downlink carrier may indicate that the first physical cell ID is for a cell comprising the first downlink carrier. The same concept may apply to, for example, carrier activation. Reference to a first carrier that is activated may indicate that the cell comprising the first carrier is activated. 
     Examples may be configured to operate as needed. The disclosed mechanisms may be performed when certain criteria are met, for example, in a wireless device, a base station, a radio environment, a network, a combination of the above, and/or the like. Example criteria may be based, at least in part, on for example, traffic load, initial system set up, packet sizes, traffic characteristics, a combination of the above, and/or the like. When the one or more criteria are met, various examples may be applied. Therefore, it may be possible to implement examples that selectively implement disclosed protocols. 
     A base station may communicate with a mix of wireless devices. Wireless devices may support multiple technologies, and/or multiple releases of the same technology. Wireless devices may have some specific capability(ies) depending on its wireless device category and/or capability(ies). A base station may comprise multiple sectors. Reference to a base station communicating with a plurality of wireless devices may indicate that a subset of the total wireless devices in a coverage area. A plurality of wireless devices of a given LTE or 5G release, with a given capability and in a given sector of the base station, may be used. The plurality of wireless devices may refer to a selected plurality of wireless devices, and/or a subset of total wireless devices in a coverage area which perform according to disclosed methods, and/or the like. There may be a plurality of wireless devices in a coverage area that may not comply with the disclosed methods, for example, because those wireless devices perform based on older releases of LTE or 5G technology. 
     A base station may transmit (e.g., to a wireless device) one or more messages (e.g. RRC messages) that may comprise a plurality of configuration parameters for one or more cells. One or more cells may comprise at least one primary cell and at least one secondary cell. In an example, an RRC message may be broadcasted or unicasted to the wireless device. In an example, configuration parameters may comprise common parameters and dedicated parameters. 
     Services and/or functions of an RRC sublayer may comprise at least one of: broadcast of system information related to AS and NAS; paging initiated by 5GC and/or NG-RAN; establishment, maintenance, and/or release of an RRC connection between a wireless device and NG-RAN, which may comprise at least one of addition, modification and release of carrier aggregation; or addition, modification, and/or release of dual connectivity in NR or between E-UTRA and NR. Services and/or functions of an RRC sublayer may further comprise at least one of security functions comprising key management; establishment, configuration, maintenance, and/or release of Signaling Radio Bearers (SRBs) and/or Data Radio Bearers (DRBs); mobility functions which may comprise at least one of a handover (e.g. intra NR mobility or inter-RAT mobility) and a context transfer; or a wireless device cell selection and reselection and control of cell selection and reselection. Services and/or functions of an RRC sublayer may further comprise at least one of QoS management functions; a wireless device measurement configuration/reporting; detection of and/or recovery from radio link failure; or NAS message transfer to/from a core network entity (e.g. AMF, Mobility Management Entity (MME)) from/to the wireless device. 
     An RRC sublayer may support an RRC_Idle state, an RRC_Inactive state and/or an RRC_Connected state for a wireless device. In an RRC_Idle state, a wireless device may perform at least one of: Public Land Mobile Network (PLMN) selection; receiving broadcasted system information; cell selection/re-selection; monitoring/receiving a paging for mobile terminated data initiated by 5GC; paging for mobile terminated data area managed by 5GC; or DRX for CN paging configured via NAS. In an RRC_Inactive state, a wireless device may perform at least one of: receiving broadcasted system information; cell selection/re-selection; monitoring/receiving a RAN/CN paging initiated by NG-RAN/5GC; RAN-based notification area (RNA) managed by NG-RAN; or DRX for RAN/CN paging configured by NG-RAN/NAS. In an RRC_Idle state of a wireless device, a base station (e.g. NG-RAN) may keep a 5GC-NG-RAN connection (both C/U-planes) for the wireless device; and/or store a UE AS context for the wireless device. In an RRC_Connected state of a wireless device, a base station (e.g. NG-RAN) may perform at least one of: establishment of 5GC-NG-RAN connection (both C/U-planes) for the wireless device; storing a UE AS context for the wireless device; transmit/receive of unicast data to/from the wireless device; or network-controlled mobility based on measurement results received from the wireless device. In an RRC_Connected state of a wireless device, an NG-RAN may know a cell that the wireless device belongs to. 
     System information (SI) may be divided into minimum SI and other SI. The minimum SI may be periodically broadcast. The minimum SI may comprise basic information required for initial access and information for acquiring any other SI broadcast periodically or provisioned on-demand, i.e. scheduling information. The other SI may either be broadcast, or be provisioned in a dedicated manner, either triggered by a network or upon request from a wireless device. A minimum SI may be transmitted via two different downlink channels using different messages (e.g. MasterInformationBlock and SystemInformationBlockType1). The other SI may be transmitted via SystemInformationBlockType2. For a wireless device in an RRC_Connected state, dedicated RRC signaling may be employed for the request and delivery of the other SI. For the wireless device in the RRC_Idle state and/or the RRC_Inactive state, the request may trigger a random-access procedure. 
     A wireless device may report its radio access capability information which may be static. A base station may request what capabilities for a wireless device to report based on band information. When allowed by a network, a temporary capability restriction request may be sent by the wireless device to signal the limited availability of some capabilities (e.g. due to hardware sharing, interference or overheating) to the base station. The base station may confirm or reject the request. The temporary capability restriction may be transparent to 5GC (e.g., static capabilities may be stored in 5GC). 
     When CA is configured, a wireless device may have an RRC connection with a network. At RRC connection establishment/re-establishment/handover procedure, one serving cell may provide NAS mobility information, and at RRC connection re-establishment/handover, one serving cell may provide a security input. This cell may be referred to as the PCell. Depending on the capabilities of the wireless device, SCells may be configured to form together with the PCell a set of serving cells. The configured set of serving cells for the wireless device may comprise one PCell and one or more SCells. 
     The reconfiguration, addition and removal of SCells may be performed by RRC. At intra-NR handover, RRC may also add, remove, or reconfigure SCells for usage with the target PCell. When adding a new SCell, dedicated RRC signaling may be employed to send all required system information of the SCell. While in connected mode, wireless devices may not need to acquire broadcasted system information directly from the SCells. 
     The purpose of an RRC connection reconfiguration procedure may be to modify an RRC connection, (e.g. to establish, modify and/or release RBs, to perform handover, to setup, modify, and/or release measurements, to add, modify, and/or release SCells and cell groups). As part of the RRC connection reconfiguration procedure, NAS dedicated information may be transferred from the network to the wireless device. The RRCConnectionReconfiguration message may be a command to modify an RRC connection. It may convey information for measurement configuration, mobility control, radio resource configuration (e.g. RBs, MAC main configuration and physical channel configuration) comprising any associated dedicated NAS information and security configuration. If the received RRC Connection Reconfiguration message includes the sCellToReleaseList, the wireless device may perform an SCell release. If the received RRC Connection Reconfiguration message includes the sCellToAddModList, the wireless device may perform SCell additions or modification. 
     An RRC connection establishment (or reestablishment, resume) procedure may be to establish (or reestablish, resume) an RRC connection. an RRC connection establishment procedure may comprise SRB1 establishment. The RRC connection establishment procedure may be used to transfer the initial NAS dedicated information message from a wireless device to E-UTRAN. The RRCConnectionReestablishment message may be used to re-establish SRB1. 
     A measurement report procedure may be to transfer measurement results from a wireless device to NG-RAN. The wireless device may initiate a measurement report procedure after successful security activation. A measurement report message may be employed to transmit measurement results. 
       FIG.  5 A ,  FIG.  5 B ,  FIG.  5 C , and  FIG.  5 D  show examples for uplink and downlink signal transmission.  FIG.  5 A  shows an example for an uplink physical channel The baseband signal representing the physical uplink shared channel may be processed according to the following processes, which may be performed by structures described below. While these structures and corresponding functions are shown as examples, it is anticipated that other structures and/or functions may be implemented in various examples. The structures and corresponding functions may comprise, e.g., one or more scrambling devices  501 A and  501 B configured to perform scrambling of coded bits in each of the codewords to be transmitted on a physical channel; one or more modulation mappers  502 A and  502 B configured to perform modulation of scrambled bits to generate complex-valued symbols; a layer mapper  503  configured to perform mapping of the complex-valued modulation symbols onto one or several transmission layers; one or more transform precoders  504 A and  504 B to generate complex-valued symbols; a precoding device  505  configured to perform precoding of the complex-valued symbols; one or more resource element mappers  506 A and  506 B configured to perform mapping of precoded complex-valued symbols to resource elements; one or more signal generators  507 A and  507 B configured to perform the generation of a complex-valued time-domain DFTS-OFDM/SC-FDMA signal for each antenna port; and/or the like. 
     Example modulation and up-conversion to the carrier frequency of the complex-valued DFTS-OFDM/SC-FDMA baseband signal for each antenna port and/or the complex-valued physical random access channel (PRACH) baseband signal is shown in  FIG.  5 B . For example, the baseband signal, represented as s 1 (t), may be split, by a signal splitter  510 , into real and imaginary components, Re{s 1 (t)} and Im{s 1 (t)}, respectively. The real component may be modulated by a modulator  511 A, and the imaginary component may be modulated by a modulator  511 B. The output signal of the modulator  511 A and the output signal of the modulator  511 B may be mixed by a mixer  512 . The output signal of the mixer  512  may be input to a filtering device  513 , and filtering may be employed by the filtering device  513  prior to transmission. 
     An example structure for downlink transmissions is shown in  FIG.  5 C . The baseband signal representing a downlink physical channel may be processed by the following processes, which may be performed by structures described below. While these structures and corresponding functions are shown as examples, it is anticipated that other structures and/or functions may be implemented in various examples. The structures and corresponding functions may comprise, e.g., one or more scrambling devices  531 A and  531 B configured to perform scrambling of coded bits in each of the codewords to be transmitted on a physical channel; one or more modulation mappers  532 A and  532 B configured to perform modulation of scrambled bits to generate complex-valued modulation symbols; a layer mapper  533  configured to perform mapping of the complex-valued modulation symbols onto one or several transmission layers; a precoding device  534  configured to perform precoding of the complex-valued modulation symbols on each layer for transmission on the antenna ports; one or more resource element mappers  535 A and  535 B configured to perform mapping of complex-valued modulation symbols for each antenna port to resource elements; one or more OFDM signal generators  536 A and  536 B configured to perform the generation of complex-valued time-domain OFDM signal for each antenna port; and/or the like. 
     Example modulation and up-conversion to the carrier frequency of the complex-valued OFDM baseband signal for each antenna port is shown in  FIG.  5 D . For example, the baseband signal, represented as s 1   (p) (t), may be split, by a signal splitter  520 , into real and imaginary components, Re{s 1   (p) (t)} and Im{s 1   (p) (t)}, respectively. The real component may be modulated by a modulator  521 A, and the imaginary component may be modulated by a modulator  521 B. The output signal of the modulator  521 A and the output signal of the modulator  521 B may be mixed by a mixer  522 . The output signal of the mixer  522  may be input to a filtering device  523 , and filtering may be employed by the filtering device  523  prior to transmission. 
       FIG.  6    and  FIG.  7    show examples for protocol structures with CA and multi-connectivity. In  FIG.  6   , NR may support multi-connectivity operation, whereby a multiple receiver/transmitter (RX/TX) UE in RRC_CONNECTED may be configured to utilize radio resources provided by multiple schedulers located in multiple gNBs connected via a non-ideal or ideal backhaul over the Xn interface. gNBs involved in multi-connectivity for a certain UE may assume two different roles: a gNB may either act as a master gNB (e.g.,  600 ) or as a secondary gNB (e.g.,  610  or  620 ). In multi-connectivity, a UE may be connected to one master gNB (e.g.,  600 ) and one or more secondary gNBs (e.g.,  610  and/or  620 ). Any one or more of the Master gNB  600  and/or the secondary gNBs  610  and  620  may be a Next Generation (NG) NodeB. The master gNB  600  may comprise protocol layers NR MAC  601 , NR RLC  602  and  603 , and NR PDCP  604  and  605 . The secondary gNB may comprise protocol layers NR MAC  611 , NR RLC  612  and  613 , and NR PDCP  614 . The secondary gNB may comprise protocol layers NR MAC  621 , NR RLC  622  and  623 , and NR PDCP  624 . The master gNB  600  may communicate via an interface  606  and/or via an interface  607 , the secondary gNB  610  may communicate via an interface  615 , and the secondary gNB  620  may communicate via an interface  625 . The master gNB  600  may also communicate with the secondary gNB  610  and the secondary gNB  621  via interfaces  608  and  609 , respectively, which may include Xn interfaces. For example, the master gNB  600  may communicate via the interface  608 , at layer NR PDCP  605 , with the secondary gNB  610  at layer NR RLC  612 . The master gNB  600  may communicate via the interface  609 , at layer NR PDCP  605 , with the secondary gNB  620  at layer NR RLC  622 . 
       FIG.  7    shows an example structure for the UE side MAC entities, e.g., if a Master Cell Group (MCG) and a Secondary Cell Group (SCG) are configured. Media Broadcast Multicast Service (MBMS) reception may be included but is not shown in this figure for simplicity. 
     In multi-connectivity, the radio protocol architecture that a particular bearer uses may depend on how the bearer is set up. As an example, three alternatives may exist, an MCG bearer, an SCG bearer, and a split bearer, such as shown in  FIG.  6   . NR RRC may be located in a master gNB and SRBs may be configured as a MCG bearer type and may use the radio resources of the master gNB. Multi-connectivity may have at least one bearer configured to use radio resources provided by the secondary gNB. Multi-connectivity may or may not be configured or implemented. 
     In the case of multi-connectivity, the UE may be configured with multiple NR MAC entities: e.g., one NR MAC entity for a master gNB, and other NR MAC entities for secondary gNBs. In multi-connectivity, the configured set of serving cells for a UE may comprise two subsets: e.g., the Master Cell Group (MCG) containing the serving cells of the master gNB, and the Secondary Cell Groups (SCGs) containing the serving cells of the secondary gNBs. 
     For an SCG, one or more of the following may be applied. At least one cell in the SCG may have a configured UL component carrier (CC) and one of the UL CCs, e.g., named PSCell (or PCell of SCG, or sometimes called PCell), may be configured with PUCCH resources. If the SCG is configured, there may be at least one SCG bearer or one split bearer. If a physical layer problem or a random access problem on a PSCell occurs or is detected, if the maximum number of NR RLC retransmissions has been reached associated with the SCG, or if an access problem on a PSCell during a SCG addition or a SCG change occurs or is detected, then an RRC connection re-establishment procedure may not be triggered, UL transmissions towards cells of the SCG may be stopped, a master gNB may be informed by the UE of a SCG failure type, and for a split bearer the DL data transfer over the master gNB may be maintained. The NR RLC Acknowledge Mode (AM) bearer may be configured for the split bearer. Like the PCell, a PSCell may not be de-activated. The PSCell may be changed with an SCG change (e.g., with a security key change and a RACH procedure). A direct bearer type may change between a split bearer and an SCG bearer, or a simultaneous configuration of an SCG and a split bearer may or may not be supported. 
     With respect to the interaction between a master gNB and secondary gNBs for multi-connectivity, one or more of the following principles may be applied. The master gNB may maintain the RRM measurement configuration of the UE, and the master gNB may, (e.g., based on received measurement reports, and/or based on traffic conditions and/or bearer types), decide to ask a secondary gNB to provide additional resources (e.g., serving cells) for a UE. If a request from the master gNB is received, a secondary gNB may create a container that may result in the configuration of additional serving cells for the UE (or the secondary gNB decide that it has no resource available to do so). For UE capability coordination, the master gNB may provide some or all of the Active Set (AS) configuration and the UE capabilities to the secondary gNB. The master gNB and the secondary gNB may exchange information about a UE configuration, such as by employing NR RRC containers (e.g., inter-node messages) carried in Xn messages. The secondary gNB may initiate a reconfiguration of its existing serving cells (e.g., PUCCH towards the secondary gNB). The secondary gNB may decide which cell is the PSCell within the SCG. The master gNB may or may not change the content of the NR RRC configuration provided by the secondary gNB. In the case of an SCG addition and an SCG SCell addition, the master gNB may provide the latest measurement results for the SCG cell(s). Both a master gNB and a secondary gNBs may know the system frame number (SFN) and subframe offset of each other by operations, administration, and maintenance (OAM) (e.g., for the purpose of discontinuous reception (DRX) alignment and identification of a measurement gap). In an example, when adding a new SCG SCell, dedicated NR RRC signaling may be used for sending required system information of the cell for CA, except, e.g., for the SFN acquired from an MIB of the PSCell of an SCG. 
       FIG.  7    shows an example of dual-connectivity (DC) for two MAC entities at a UE side. A first MAC entity may comprise a lower layer of an MCG  700 , an upper layer of an MCG  718 , and one or more intermediate layers of an MCG  719 . The lower layer of the MCG  700  may comprise, e.g., a paging channel (PCH)  701 , a broadcast channel (BCH)  702 , a downlink shared channel (DL-SCH)  703 , an uplink shared channel (UL-SCH)  704 , and a random access channel (RACH)  705 . The one or more intermediate layers of the MCG  719  may comprise, e.g., one or more hybrid automatic repeat request (HARQ) processes  706 , one or more random access control processes  707 , multiplexing and/or de-multiplexing processes  709 , logical channel prioritization on the uplink processes  710 , and a control processes  708  providing control for the above processes in the one or more intermediate layers of the MCG  719 . The upper layer of the MCG  718  may comprise, e.g., a paging control channel (PCCH)  711 , a broadcast control channel (BCCH)  712 , a common control channel (CCCH)  713 , a dedicated control channel (DCCH)  714 , a dedicated traffic channel (DTCH)  715 , and a MAC control  716 . 
     A second MAC entity may comprise a lower layer of an SCG  720 , an upper layer of an SCG  738 , and one or more intermediate layers of an SCG  739 . The lower layer of the SCG  720  may comprise, e.g., a BCH  722 , a DL-SCH  723 , an UL-SCH  724 , and a RACH  725 . The one or more intermediate layers of the SCG  739  may comprise, e.g., one or more HARQ processes  726 , one or more random access control processes  727 , multiplexing and/or de-multiplexing processes  729 , logical channel prioritization on the uplink processes  730 , and a control processes  728  providing control for the above processes in the one or more intermediate layers of the SCG  739 . The upper layer of the SCG  738  may comprise, e.g., a BCCH  732 , a DCCH  714 , a DTCH  735 , and a MAC control  736 . 
     Serving cells may be grouped in a TA group (TAG). Serving cells in one TAG may use the same timing reference. For a given TAG, user equipment (UE) may use at least one downlink carrier as a timing reference. For a given TAG, a UE may synchronize uplink subframe and frame transmission timing of uplink carriers belonging to the same TAG. In an example, serving cells having an uplink to which the same TA applies may correspond to serving cells hosted by the same receiver. A UE supporting multiple TAs may support two or more TA groups. One TA group may contain the PCell and may be called a primary TAG (pTAG). In a multiple TAG configuration, at least one TA group may not contain the PCell and may be called a secondary TAG (sTAG). In an example, carriers within the same TA group may use the same TA value and/or the same timing reference. When DC is configured, cells belonging to a cell group (e.g., MCG or SCG) may be grouped into multiple TAGs including a pTAG and one or more sTAGs. 
       FIG.  8    shows example TAG configurations. In Example 1, a pTAG comprises a PCell, and an sTAG comprises an SCell 1 . In Example 2, a pTAG comprises a PCell and an SCell 1 , and an sTAG comprises an SCell 2  and an SCell 3 . In Example 3, a pTAG comprises a PCell and an SCell 1 , and an sTAG 1  comprises an SCell 2  and an SCell 3 , and an sTAG 2  comprises a SCell 4 . Up to four TAGs may be supported in a cell group (MCG or SCG), and other example TAG configurations may also be provided. In various examples, structures and operations are described for use with a pTAG and an sTAG. Some of the examples may be applied to configurations with multiple sTAGs. 
     In an example, an eNB may initiate an RA procedure, via a PDCCH order, for an activated SCell. The PDCCH order may be sent on a scheduling cell of this SCell. When cross carrier scheduling is configured for a cell, the scheduling cell may be different than the cell that is employed for preamble transmission, and the PDCCH order may include an SCell index. At least a non-contention based RA procedure may be supported for SCell(s) assigned to sTAG(s). 
       FIG.  9    shows an example of random access processes, and a corresponding message flow, in a secondary TAG. A base station, such as an eNB, may transmit an activation command  900  to a wireless device, such as a UE. The activation command  900  may be transmitted to activate an SCell. The base station may also transmit a PDDCH order  901  to the wireless device, which may be transmitted after the activation command  900 . The wireless device may begin to perform a RACH process for the SCell, which may be initiated after receiving the PDDCH order  901 . The RACH process may include the wireless device transmitting to the base station a preamble  902  (e.g., Msg1), such as a random access preamble (RAP). The preamble  902  may be transmitted in response to the PDCCH order  901 . The wireless device may transmit the preamble  902  via an SCell belonging to an sTAG. In an example, preamble transmission for SCells may be controlled by a network using PDCCH format 1A. The base station may send a random access response (RAR)  903  (e.g., Msg2 message) to the wireless device. The RAR  903  may be in response to the preamble  902  transmission via the SCell. The RAR  903  may be addressed to a random access radio network temporary identifier (RA-RNTI) in a PCell common search space (CSS). If the wireless device receives the RAR  903 , the RACH process may conclude. The RACH process may conclude after or in response to the wireless device receiving the RAR  903  from the base station. After the RACH process, the wireless device may transmit an uplink transmission  904 . The uplink transmission  904  may comprise uplink packets transmitted via the same SCell used for the preamble  902  transmission. 
     Initial timing alignment for communications between the wireless device and the base station may be achieved through a random access procedure, such as described above regarding  FIG.  9   . The random access procedure may involve a wireless device, such as a UE, transmitting a random access preamble and a base station, such as an eNB, responding with an initial TA command NTA (amount of timing advance) within a random access response window. The start of the random access preamble may be aligned with the start of a corresponding uplink subframe at the UE assuming NTA=0. The eNB may estimate the uplink timing from the random access preamble transmitted by the UE. The TA command may be derived by the eNB based on the estimation of the difference between the desired UL timing and the actual UL timing. The UE may determine the initial uplink transmission timing relative to the corresponding downlink of the sTAG on which the preamble is transmitted. 
     The mapping of a serving cell to a TAG may be configured by a serving eNB with RRC signaling. The mechanism for TAG configuration and reconfiguration may be based on RRC signaling. If an eNB performs an SCell addition configuration, the related TAG configuration may be configured for the SCell. An eNB may modify the TAG configuration of an SCell by removing (e.g., releasing) the SCell and adding (e.g., configuring) a new SCell (with the same physical cell ID and frequency) with an updated TAG ID. The new SCell with the updated TAG ID may initially be inactive subsequent to being assigned the updated TAG ID. The eNB may activate the updated new SCell and start scheduling packets on the activated SCell. In some examples, it may not be possible to change the TAG associated with an SCell, but rather, the SCell may need to be removed and a new SCell may need to be added with another TAG. For example, if there is a need to move an SCell from an sTAG to a pTAG, at least one RRC message, such as at least one RRC reconfiguration message, may be sent to the UE. The at least one RRC message may be sent to the UE to reconfigure TAG configurations, e.g., by releasing the SCell and then configuring the SCell as a part of the pTAG. If, e.g., an SCell is added or configured without a TAG index, the SCell may be explicitly assigned to the pTAG. The PCell may not change its TA group and may be a member of the pTAG. 
     In LTE Release-10 and Release-11 CA, a PUCCH transmission is only transmitted on a PCell (e.g., a PSCell) to an eNB. In LTE-Release 12 and earlier, a UE may transmit PUCCH information on one cell (e.g., a PCell or a PSCell) to a given eNB. As the number of CA capable UEs increase, and as the number of aggregated carriers increase, the number of PUCCHs and the PUCCH payload size may increase. Accommodating the PUCCH transmissions on the PCell may lead to a high PUCCH load on the PCell. A PUCCH on an SCell may be introduced to offload the PUCCH resource from the PCell. More than one PUCCH may be configured. For example, a PUCCH on a PCell may be configured and another PUCCH on an SCell may be configured. One, two, or more cells may be configured with PUCCH resources for transmitting CSI, acknowledgment (ACK), and/or non-acknowledgment (NACK) to a base station. Cells may be grouped into multiple PUCCH groups, and one or more cell within a group may be configured with a PUCCH. In some examples, one SCell may belong to one PUCCH group. SCells with a configured PUCCH transmitted to a base station may be called a PUCCH SCell, and a cell group with a common PUCCH resource transmitted to the same base station may be called a PUCCH group. 
     A MAC entity may have a configurable timer, e.g., timeAlignmentTimer, per TAG. The timeAlignmentTimer may be used to control how long the MAC entity considers the serving cells belonging to the associated TAG to be uplink time aligned. If a Timing Advance Command MAC control element is received, the MAC entity may apply the Timing Advance Command for the indicated TAG; and/or the MAC entity may start or restart the timeAlignmentTimer associated with a TAG that may be indicated by the Timing Advance Command MAC control element. If a Timing Advance Command is received in a Random Access Response message for a serving cell belonging to a TAG, the MAC entity may apply the Timing Advance Command for this TAG and/or start or restart the timeAlignmentTimer associated with this TAG. Additionally or alternatively, if the Random Access Preamble is not selected by the MAC entity, the MAC entity may apply the Timing Advance Command for this TAG and/or start or restart the timeAlignmentTimer associated with this TAG. If the timeAlignmentTimer associated with this TAG is not running, the Timing Advance Command for this TAG may be applied, and the timeAlignmentTimer associated with this TAG may be started. If the contention resolution is not successful, a timeAlignmentTimer associated with this TAG may be stopped. If the contention resolution is successful, the MAC entity may ignore the received Timing Advance Command The MAC entity may determine whether the contention resolution is successful or whether the contention resolution is not successful. 
       FIG.  10 A  and  FIG.  10 B  show examples for interfaces between a 5G core network (e.g., NGC) and base stations (e.g., gNB and eLTE eNB). For example, in  FIG.  10 A , a base station, such as a gNB  1020 , may be interconnected to an NGC  1010  control plane employing an NG-C interface. The base station, e.g., the gNB  1020 , may also be interconnected to an NGC  1010  user plane (e.g., UPGW) employing an NG-U interface. As another example, in  FIG.  10 B , a base station, such as an eLTE eNB  1040 , may be interconnected to an NGC  1030  control plane employing an NG-C interface. The base station, e.g., the eLTE eNB  1040 , may also be interconnected to an NGC  1030  user plane (e.g., UPGW) employing an NG-U interface. An NG interface may support a many-to-many relation between 5G core networks and base stations. 
       FIG.  11 A ,  FIG.  11 B ,  FIG.  11 C ,  FIG.  11 D ,  FIG.  11 E , and  FIG.  11 F  are examples for architectures of tight interworking between a 5G RAN and an LTE RAN. The tight interworking may enable a multiple receiver/transmitter (RX/TX) UE in an RRC_CONNECTED state to be configured to utilize radio resources provided by two schedulers located in two base stations (e.g., an eLTE eNB and a gNB). The two base stations may be connected via a non-ideal or ideal backhaul over the Xx interface between an LTE eNB and a gNB, or over the Xn interface between an eLTE eNB and a gNB. Base stations involved in tight interworking for a certain UE may assume different roles. For example, a base station may act as a master base station or a base station may act as a secondary base station. In tight interworking, a UE may be connected to both a master base station and a secondary base station. Mechanisms implemented in tight interworking may be extended to cover more than two base stations. 
     In  FIG.  11 A  and  FIG.  11 B , a master base station may be an LTE eNB  1102 A or an LTE eNB  1102 B, which may be connected to EPC nodes  1101 A or  1101 B, respectively. This connection to EPC nodes may be, e.g., to an MME via the S1-C interface and/or to an S-GW via the S1-U interface. A secondary base station may be a gNB  1103 A or a gNB  1103 B, either or both of which may be a non-standalone node having a control plane connection via an Xx-C interface to an LTE eNB (e.g., the LTE eNB  1102 A or the LTE eNB  1102 B). In the tight interworking architecture of  FIG.  11 A , a user plane for a gNB (e.g., the gNB  1103 A) may be connected to an S-GW (e.g., the EPC  1101 A) through an LTE eNB (e.g., the LTE eNB  1102 A), via an Xx-U interface between the LTE eNB and the gNB, and via an S1-U interface between the LTE eNB and the S-GW. In the architecture of  FIG.  11 B , a user plane for a gNB (e.g., the gNB  1103 B) may be connected directly to an S-GW (e.g., the EPC  1101 B) via an S1-U interface between the gNB and the S-GW. 
     In  FIG.  11 C  and  FIG.  11 D , a master base station may be a gNB  1103 C or a gNB  1103 D, which may be connected to NGC nodes  1101 C or  1101 D, respectively. This connection to NGC nodes may be, e.g., to a control plane core node via the NG-C interface and/or to a user plane core node via the NG-U interface. A secondary base station may be an eLTE eNB  1102 C or an eLTE eNB  1102 D, either or both of which may be a non-standalone node having a control plane connection via an Xn-C interface to a gNB (e.g., the gNB  1103 C or the gNB  1103 D). In the tight interworking architecture of  FIG.  11 C , a user plane for an eLTE eNB (e.g., the eLTE eNB  1102 C) may be connected to a user plane core node (e.g., the NGC  1101 C) through a gNB (e.g., the gNB  1103 C), via an Xn-U interface between the eLTE eNB and the gNB, and via an NG-U interface between the gNB and the user plane core node. In the architecture of  FIG.  11 D , a user plane for an eLTE eNB (e.g., the eLTE eNB  1102 D) may be connected directly to a user plane core node (e.g., the NGC  1101 D) via an NG-U interface between the eLTE eNB and the user plane core node. 
     In  FIG.  11 E  and  FIG.  11 F , a master base station may be an eLTE eNB  1102 E or an eLTE eNB  1102 F, which may be connected to NGC nodes  1101 E or  1101 F, respectively. This connection to NGC nodes may be, e.g., to a control plane core node via the NG-C interface and/or to a user plane core node via the NG-U interface. A secondary base station may be a gNB  1103 E or a gNB  1103 F, either or both of which may be a non-standalone node having a control plane connection via an Xn-C interface to an eLTE eNB (e.g., the eLTE eNB  1102 E or the eLTE eNB  1102 F). In the tight interworking architecture of  FIG.  11 E , a user plane for a gNB (e.g., the gNB  1103 E) may be connected to a user plane core node (e.g., the NGC  1101 E) through an eLTE eNB (e.g., the eLTE eNB  1102 E), via an Xn-U interface between the eLTE eNB and the gNB, and via an NG-U interface between the eLTE eNB and the user plane core node. In the architecture of  FIG.  11 F , a user plane for a gNB (e.g., the gNB  1103 F) may be connected directly to a user plane core node (e.g., the NGC  1101 F) via an NG-U interface between the gNB and the user plane core node. 
       FIG.  12 A ,  FIG.  12 B , and  FIG.  12 C  are examples for radio protocol structures of tight interworking bearers. 
     In  FIG.  12 A , an LTE eNB  1201 A may be an S1 master base station, and a gNB  1210 A may be an S1 secondary base station. An example for a radio protocol architecture for a split bearer and an SCG bearer is shown. The LTE eNB  1201 A may be connected to an EPC with a non-standalone gNB  1210 A, via an Xx interface between the PDCP  1206 A and an NR RLC  1212 A. The LTE eNB  1201 A may include protocol layers MAC  1202 A, RLC  1203 A and RLC  1204 A, and PDCP  1205 A and PDCP  1206 A. An MCG bearer type may interface with the PDCP  1205 A, and a split bearer type may interface with the PDCP  1206 A. The gNB  1210 A may include protocol layers NR MAC  1211 A, NR RLC  1212 A and NR RLC  1213 A, and NR PDCP  1214 A. An SCG bearer type may interface with the NR PDCP  1214 A. 
     In  FIG.  12 B , a gNB  1201 B may be an NG master base station, and an eLTE eNB  1210 B may be an NG secondary base station. An example for a radio protocol architecture for a split bearer and an SCG bearer is shown. The gNB  1201 B may be connected to an NGC with a non-standalone eLTE eNB  1210 B, via an Xn interface between the NR PDCP  1206 B and an RLC  1212 B. The gNB  1201 B may include protocol layers NR MAC  1202 B, NR RLC  1203 B and NR RLC  1204 B, and NR PDCP  1205 B and NR PDCP  1206 B. An MCG bearer type may interface with the NR PDCP  1205 B, and a split bearer type may interface with the NR PDCP  1206 B. The eLTE eNB  1210 B may include protocol layers MAC  1211 B, RLC  1212 B and RLC  1213 B, and PDCP  1214 B. An SCG bearer type may interface with the PDCP  1214 B. 
     In  FIG.  12 C , an eLTE eNB  1201 C may be an NG master base station, and a gNB  1210 C may be an NG secondary base station. An example for a radio protocol architecture for a split bearer and an SCG bearer is shown. The eLTE eNB  1201 C may be connected to an NGC with a non-standalone gNB  1210 C, via an Xn interface between the PDCP  1206 C and an NR RLC  1212 C. The eLTE eNB  1201 C may include protocol layers MAC  1202 C, RLC  1203 C and RLC  1204 C, and PDCP  1205 C and PDCP  1206 C. An MCG bearer type may interface with the PDCP  1205 C, and a split bearer type may interface with the PDCP  1206 C. The gNB  1210 C may include protocol layers NR MAC  1211 C, NR RLC  1212 C and NR RLC  1213 C, and NR PDCP  1214 C. An SCG bearer type may interface with the NR PDCP  1214 C. 
     In a 5G network, the radio protocol architecture that a particular bearer uses may depend on how the bearer is setup. At least three alternatives may exist, e.g., an MCG bearer, an SCG bearer, and a split bearer, such as shown in  FIG.  12 A ,  FIG.  12 B , and FIG.  12 C. The NR RRC may be located in a master base station, and the SRBs may be configured as an MCG bearer type and may use the radio resources of the master base station. Tight interworking may have at least one bearer configured to use radio resources provided by the secondary base station. Tight interworking may or may not be configured or implemented. 
     In the case of tight interworking, the UE may be configured with two MAC entities: e.g., one MAC entity for a master base station, and one MAC entity for a secondary base station. In tight interworking, the configured set of serving cells for a UE may comprise of two subsets: e.g., the Master Cell Group (MCG) containing the serving cells of the master base station, and the Secondary Cell Group (SCG) containing the serving cells of the secondary base station. 
     For an SCG, one or more of the following may be applied. At least one cell in the SCG may have a configured UL CC and one of them, e.g., a PSCell (or the PCell of the SCG, which may also be called a PCell), is configured with PUCCH resources. If the SCG is configured, there may be at least one SCG bearer or one split bearer. If one or more of a physical layer problem or a random access problem is detected on a PSCell, if the maximum number of (NR) RLC retransmissions associated with the SCG has been reached, and/or if an access problem on a PSCell during an SCG addition or during an SCG change is detected, then: an RRC connection re-establishment procedure may not be triggered, UL transmissions towards cells of the SCG may be stopped, a master base station may be informed by the UE of a SCG failure type, and/or for a split bearer the DL data transfer over the master base station may be maintained. The RLC AM bearer may be configured for the split bearer. Like the PCell, a PSCell may not be de-activated. A PSCell may be changed with an SCG change, e.g., with security key change and a RACH procedure. A direct bearer type change, between a split bearer and an SCG bearer, may not be supported. Simultaneous configuration of an SCG and a split bearer may not be supported. 
     With respect to the interaction between a master base station and a secondary base station, one or more of the following principles may be applied. The master base station may maintain the RRM measurement configuration of the UE. The master base station may determine to ask a secondary base station to provide additional resources (e.g., serving cells) for a UE. This determination may be based on, e.g., received measurement reports, traffic conditions, and/or bearer types. If a request from the master base station is received, a secondary base station may create a container that may result in the configuration of additional serving cells for the UE, or the secondary base station may determine that it has no resource available to do so. The master base station may provide at least part of the AS configuration and the UE capabilities to the secondary base station, e.g., for UE capability coordination. The master base station and the secondary base station may exchange information about a UE configuration such as by using RRC containers (e.g., inter-node messages) carried in Xn or Xx messages. The secondary base station may initiate a reconfiguration of its existing serving cells (e.g., PUCCH towards the secondary base station). The secondary base station may determine which cell is the PSCell within the SCG. The master base station may not change the content of the RRC configuration provided by the secondary base station. If an SCG is added and/or an SCG SCell is added, the master base station may provide the latest measurement results for the SCG cell(s). Either or both of a master base station and a secondary base station may know the SFN and subframe offset of each other by OAM, (e.g., for the purpose of DRX alignment and identification of a measurement gap). If a new SCG SCell is added, dedicated RRC signaling may be used for sending required system information of the cell, such as for CA, except, e.g., for the SFN acquired from an MIB of the PSCell of an SCG. 
       FIG.  13 A  and  FIG.  13 B  show examples for gNB deployment scenarios. A core  1301  and a core  1310 , in  FIG.  13 A  and  FIG.  13 B , respectively, may interface with other nodes via RAN-CN interfaces. In a non-centralized deployment scenario in  FIG.  13 A , the full protocol stack (e.g., NR RRC, NR PDCP, NR RLC, NR MAC, and NR PHY) may be supported at one node, such as a gNB  1302 , a gNB  1303 , and/or an eLTE eNB or LTE eNB  1304 . These nodes (e.g., the gNB  1302 , the gNB  1303 , and the eLTE eNB or LTE eNB  1304 ) may interface with one of more of each other via a respective inter-BS interface. In the centralized deployment scenario in  FIG.  13 B , upper layers of a gNB may be located in a Central Unit (CU)  1311 , and lower layers of the gNB may be located in Distributed Units (DUs)  1312 ,  1313 , and  1314 . The CU-DU interface (e.g., Fs interface) connecting CU  1311  and DUs  1312 ,  1313 , and  1314  may be ideal or non-ideal. The Fs-C may provide a control plane connection over the Fs interface, and the Fs-U may provide a user plane connection over the Fs interface. In the centralized deployment, different functional split options between the CU  1311  and the DUs  1312 ,  1313 , and  1314  may be possible by locating different protocol layers (e.g., RAN functions) in the CU  1311  and in the DUs  1312 ,  1313 , and  1314 . The functional split may support flexibility to move the RAN functions between the CU  1311  and the DUs  1312 ,  1313 , and  1314  depending on service requirements and/or network environments. The functional split option may change during operation after the Fs interface setup procedure, or the functional split option may change only in the Fs setup procedure (e.g., the functional split option may be static during operation after Fs setup procedure). 
       FIG.  14    shows examples for different functional split options of a centralized gNB deployment scenario. Element numerals that are followed by “A” or “B” designations in  FIG.  14    may represent the same elements in different traffic flows, e.g., either receiving data (e.g., data  1402 A) or sending data (e.g.,  1402 B). In the split option example 1, an NR RRC  1401  may be in a CU, and an NR PDCP  1403 , an NR RLC (e.g., comprising a High NR RLC  1404  and/or a Low NR RLC  1405 ), an NR MAC (e.g., comprising a High NR MAC  1406  and/or a Low NR MAC  1407 ), an NR PHY (e.g., comprising a High NR PHY  1408  and/or a LOW NR PHY  1409 ), and an RF  1410  may be in a DU. In the split option example 2, the NR RRC  1401  and the NR PDCP  1403  may be in a CU, and the NR RLC, the NR MAC, the NR PHY, and the RF  1410  may be in a DU. In the split option example 3, the NR RRC  1401 , the NR PDCP  1403 , and a partial function of the NR RLC (e.g., the High NR RLC  1404 ) may be in a CU, and the other partial function of the NR RLC (e.g., the Low NR RLC  1405 ), the NR MAC, the NR PHY, and the RF  1410  may be in a DU. In the split option example 4, the NR RRC  1401 , the NR PDCP  1403 , and the NR RLC may be in a CU, and the NR MAC, the NR PHY, and the RF  1410  may be in a DU. In the split option example 5, the NR RRC  1401 , the NR PDCP  1403 , the NR RLC, and a partial function of the NR MAC (e.g., the High NR MAC  1406 ) may be in a CU, and the other partial function of the NR MAC (e.g., the Low NR MAC  1407 ), the NR PHY, and the RF  1410  may be in a DU. In the split option example 6, the NR RRC  1401 , the NR PDCP  1403 , the NR RLC, and the NR MAC may be in CU, and the NR PHY and the RF  1410  may be in a DU. In the split option example 7, the NR RRC  1401 , the NR PDCP  1403 , the NR RLC, the NR MAC, and a partial function of the NR PHY (e.g., the High NR PHY  1408 ) may be in a CU, and the other partial function of the NR PHY (e.g., the Low NR PHY  1409 ) and the RF  1410  may be in a DU. In the split option example 8, the NR RRC  1401 , the NR PDCP  1403 , the NR RLC, the NR MAC, and the NR PHY may be in a CU, and the RF  1410  may be in a DU. 
     The functional split may be configured per CU, per DU, per UE, per bearer, per slice, and/or with other granularities. In a per CU split, a CU may have a fixed split, and DUs may be configured to match the split option of the CU. In a per DU split, each DU may be configured with a different split, and a CU may provide different split options for different DUs. In a per UE split, a gNB (e.g., a CU and a DU) may provide different split options for different UEs. In a per bearer split, different split options may be utilized for different bearer types. In a per slice splice, different split options may be applied for different slices. 
     A new radio access network (new RAN) may support different network slices, which may allow differentiated treatment customized to support different service requirements with end to end scope. The new RAN may provide a differentiated handling of traffic for different network slices that may be pre-configured, and the new RAN may allow a single RAN node to support multiple slices. The new RAN may support selection of a RAN part for a given network slice, e.g., by one or more slice ID(s) or NSSAI(s) provided by a UE or provided by an NGC (e.g., an NG CP). The slice ID(s) or NSSAI(s) may identify one or more of pre-configured network slices in a PLMN. For an initial attach, a UE may provide a slice ID and/or an NSSAI, and a RAN node (e.g., a gNB) may use the slice ID or the NSSAI for routing an initial NAS signaling to an NGC control plane function (e.g., an NG CP). If a UE does not provide any slice ID or NSSAI, a RAN node may send a NAS signaling to a default NGC control plane function. For subsequent accesses, the UE may provide a temporary ID for a slice identification, which may be assigned by the NGC control plane function, to enable a RAN node to route the NAS message to a relevant NGC control plane function. The new RAN may support resource isolation between slices. When the RAN resource isolation is implemented, shortage of shared resources in one slice does not cause a break in a service level agreement for another slice. 
     The amount of data traffic carried over networks is expected to increase for many years to come. The number of users/devices is increasing and each user/device accesses an increasing number and variety of services, e.g., video delivery, large files, and images. This requires not only high capacity in the network, but also provisioning very high data rates to meet customers&#39; expectations on interactivity and responsiveness. More spectrum may be required for network operators to meet the increasing demand Considering user expectations of high data rates along with seamless mobility, it is beneficial that more spectrum be made available for deploying macro cells as well as small cells for communication systems. 
     Striving to meet the market demands, there has been increasing interest from operators in deploying some complementary access utilizing unlicensed spectrum to meet the traffic growth. This is exemplified by the large number of operator-deployed Wi-Fi networks and the 3GPP standardization of LTE/WLAN interworking solutions. This interest indicates that unlicensed spectrum, when present, may be an effective complement to licensed spectrum for network operators, e.g., to help address the traffic explosion in some scenarios, such as hotspot areas. Licensed Assisted Access (LAA) offers an alternative for operators to make use of unlicensed spectrum while managing one radio network, offering new possibilities for optimizing the network&#39;s efficiency. 
     Listen-before-talk (clear channel assessment) may be implemented for transmission in an LAA cell. In a listen-before-talk (LBT) procedure, equipment may apply a clear channel assessment (CCA) check before using the channel. For example, the CCA may utilize at least energy detection to determine the presence or absence of other signals on a channel in order to determine if a channel is occupied or clear, respectively. For example, European and Japanese regulations mandate the usage of LBT in the unlicensed bands. Apart from regulatory requirements, carrier sensing via LBT may be one way for fair sharing of the unlicensed spectrum. 
     Discontinuous transmission on an unlicensed carrier with limited maximum transmission duration may be enabled. Some of these functions may be supported by one or more signals to be transmitted from the beginning of a discontinuous LAA downlink transmission. Channel reservation may be enabled by the transmission of signals, by an LAA node, after gaining channel access, e.g., via a successful LBT operation, so that other nodes that receive the transmitted signal with energy above a certain threshold sense the channel to be occupied. Functions that may need to be supported by one or more signals for LAA operation with discontinuous downlink transmission may include one or more of the following: detection of the LAA downlink transmission (including cell identification) by UEs, time synchronization of UEs, and frequency synchronization of UEs. 
     DL LAA design may employ subframe boundary alignment according to LTE-A carrier aggregation timing relationships across serving cells aggregated by CA. This may not indicate that the eNB transmissions may start only at the subframe boundary. LAA may support transmitting PDSCH when not all OFDM symbols are available for transmission in a subframe according to LBT. Delivery of necessary control information for the PDSCH may also be supported. 
     LBT procedures may be employed for fair and friendly coexistence of LAA with other operators and technologies operating in unlicensed spectrum. LBT procedures on a node attempting to transmit on a carrier in unlicensed spectrum may require the node to perform a clear channel assessment to determine if the channel is free for use. An LBT procedure may involve at least energy detection to determine if the channel is being used. For example, regulatory requirements in some regions, e.g., in Europe, specify an energy detection threshold such that if a node receives energy greater than this threshold, the node assumes that the channel is not free. While nodes may follow such regulatory requirements, a node may optionally use a lower threshold for energy detection than that specified by regulatory requirements. In an example, LAA may employ a mechanism to adaptively change the energy detection threshold, e.g., LAA may employ a mechanism to adaptively lower the energy detection threshold from an upper bound. Adaptation mechanism may not preclude static or semi-static setting of the threshold. A Category 4 LBT mechanism or other type of LBT mechanisms may be implemented. 
     Various example LBT mechanisms may be implemented. For some signals, in some implementation scenarios, in some situations, and/or in some frequencies, no LBT procedure may performed by the transmitting entity. For example, Category 2 (e.g., LBT without random back-off) may be implemented. The duration of time that the channel is sensed to be idle before the transmitting entity transmits may be deterministic. For example, Category 3 (e.g., LBT with random back-off with a contention window of fixed size) may be implemented. The LBT procedure may have the following procedure as one of its components. The transmitting entity may draw a random number N within a contention window. The size of the contention window may be specified by the minimum and maximum value of N. The size of the contention window may be fixed. The random number N may be employed in the LBT procedure to determine the duration of time that the channel is sensed to be idle before the transmitting entity transmits on the channel In an example, Category 4 (e.g., LBT with random back-off with a contention window of variable size) may be implemented. The transmitting entity may draw a random number N within a contention window. The size of contention window may be specified by the minimum and maximum value of N. The transmitting entity may vary the size of the contention window when drawing the random number N. The random number N may be used in the LBT procedure to determine the duration of time that the channel is sensed to be idle before the transmitting entity transmits on the channel 
     LAA may employ uplink LBT at the UE. The UL LBT scheme may be different from the DL LBT scheme, e.g., by using different LBT mechanisms or parameters. These differences in schemes may be due to the LAA UL being based on scheduled access, which may affect a UE&#39;s channel contention opportunities. Other considerations motivating a different UL LBT scheme may include, but are not limited to, multiplexing of multiple UEs in a single subframe. 
     A DL transmission burst may be a continuous transmission from a DL transmitting node, e.g., with no transmission immediately before or after from the same node on the same CC. An UL transmission burst from a UE perspective may be a continuous transmission from a UE, e.g., with no transmission immediately before or after from the same UE on the same CC. A UL transmission burst may be defined from a UE perspective or from an eNB perspective. If an eNB is operating DL and UL LAA over the same unlicensed carrier, DL transmission burst(s) and UL transmission burst(s) on LAA may be scheduled in a TDM manner over the same unlicensed carrier. An instant in time may be part of a DL transmission burst or part of an UL transmission burst. 
     Random access (RA) procedures may be used to establish communications between a wireless device and a base station in a cell. A four-step RA procedure may include, e.g., a first step comprising a wireless device sending, to a base station, a request to establish communications with the base station. A second step may include, e.g., the base station sending, to the wireless device, a response indicating that the wireless device may send additional information for establishing the requested communications. A third step may include, e.g., the wireless device sending, to the base station, the additional information for establishing the requested communications. A fourth step may include, e.g., the base station sending, to the wireless device, information confirming the establishing of the requested communications. A four-step RA procedure may have an associated latency, e.g., which may be a minimum of fourteen transmission time intervals (TTI). As an example, 3GPP TR 38.804 v14.0.0 indicates a minimum latency of fourteen TTIs comprising, e.g., 3 TTIs after a message from step 1 of a four-step RA procedure, 1 TTI for a message from step 2 of a four-step RA procedure, 5 TTIs after the message from step 2, 1 TTI for a message from step 3 of a four-step RA procedure, 3 TTIs after the message from step 3, and 1 TTI for a message from step 4 of a four-step procedure (e.g., 3+1+5+1+3+1=14). Reducing the number of steps in an RA procedure may reduce latency. By using parallel transmissions, a four-step RA procedure may be reduced to a two-step RA procedure. A two-step RA procedure may have an associated latency, e.g., which may be a minimum of four TTIs and which may be less than an associated latency for a four-step RA procedure. As an example, 3GPP TR 38.804 v14.0.0 indicates a minimum latency of four TTIs comprising, e.g., 3 TTIs after a message from step 1 of a two-step RA procedure and 1 TTI for a message from step 2 of a two-step RA procedure. 
       FIG.  15    shows examples of (a) a contention-based four-step RA procedure, (b) a contention free three-step RA procedure, (c) descriptions of a contention-based four-step RA procedure, and (d) a contention free two-step RA procedure. A four-step RA procedure may comprise a RAP transmission in a first step, an RAR transmission in a second step, a scheduled transmission of one or more transport blocks (TBs) in a third step, and contention resolution in a fourth step. 
     In step  1501 , a base station may transmit four-step RA configuration parameters to a wireless device (e.g., a UE). The base station may generate and transmit RA configuration parameters periodically, e.g., based on a timer. The base station may broadcast RA configuration parameters in one or more messages. The wireless device may perform a RAP selection process at step  1502 , e.g., after receiving the four-step RA configuration parameters. In a contention-based RA procedure, such as shown in part (a) of  FIG.  15   , the RA configuration parameters may comprise a root sequence that may be used by the wireless device to generate a RAP. The RAP may be randomly selected by the wireless device, among various RAP candidates generated by the root sequence, during the RAP selection process. The wireless device may perform the RAP selection using, e.g., the procedure described below regarding  FIG.  16   , and/or one or more RAP selections procedures described herein. 
     During a first step of the RA procedure, at step  1503 , a wireless device may transmit a RAP, e.g., using a configured RA preamble format with a single particular transmission (Tx) beam. A random access channel (RACH) resource may be defined as a time-frequency resource to transmit a RAP. Broadcast system information may indicate whether wireless device should transmit one preamble, or multiple or repeated preambles, within a subset of RACH resources. 
     In the second step of the four-step RA procedure, at step  1504 , a base station may transmit a random access response (RAR) to the wireless device. The base station may transmit the RAR in response to an RAP that the wireless device may transmit. A wireless device may monitor the physical-layer downlink control channel for RARs identified by the RA-RNTI in an RA response window. The RA response window may start at a subframe that contains the end of an RAP transmission, plus three subframes, and the RA response window may have the length ra-ResponseWindowSize. A wireless device may determine the RA-RNTI associated with the PRACH in which the wireless device transmits an RAP by the following operation: 
       RA-RNTI=1+ t_id+ 10* f_id    
     where t_id is the index of the first subframe of the specified PRACH (0≤t_id&lt;10), and f_id is the index of the specified PRACH within that subframe, in ascending order of frequency domain (0≤f_id&lt;6). Different types of UEs, e.g., narrow band-Internet of Things (NB-IoT), bandwidth limited (BL)-UE, and/or UE-Extended Coverage (UE-EC), may use different formulas or operations for determining RA-RNTI. A base station may configure an association between a DL signal or channel, a subset of RACH resources, and/or a subset of RAP indexes. Such an association may be for determining the DL transmission in the second step of the RA procedure, at step  1504  of  FIG.  15   . Based on the DL measurement and the corresponding association, a wireless device may select the subset of RACH resources and/or the subset of RAP indexes. 
     In the third step of the four-step RA procedure (e.g., step  1505  in  FIG.  15   ), a wireless device may adjust an UL time alignment by using the TA value corresponding to the TA command in the received RAR in the second step (e.g., step  1504  in  FIG.  15   ). A wireless device may transmit one or more TBs to a base station using the UL resources assigned in the UL grant in the received RAR. One or more TBs that a wireless device may transmit in the third step (e.g., step  1505  in  FIG.  15   ) may comprise RRC signaling, such as an RRC connection request, an RRC connection Re-establishment request, or an RRC connection resume request. The one or more TBs may also comprise a wireless device identity, e.g., which may be used as part of the contention-resolution mechanism in the fourth step (e.g., step  1506  in  FIG.  15   ). 
     The fourth step in the four-step RA procedure (e.g., step  1506  in  FIG.  15   ) may comprise a DL message for contention resolution. Based on the second step (e.g., step  1504  in  FIG.  15   ), one or more wireless devices may perform simultaneous RA attempts using the same RAP in the first step (e.g., step  1503  in  FIG.  15   ), and/or receive the same RAR with the same TC-RNTI in the second step (e.g., step  1504  in  FIG.  15   ). The contention resolution in the fourth step may be to ensure that a wireless device does not incorrectly use another wireless device identity. The contention resolution mechanism may be based on either a C-RNTI on a PDCCH, or a UE Contention Resolution Identity on a DL-SCH, e.g., depending on whether or not a wireless device has a C-RNTI. If a wireless device has a C-RNTI, e.g., if the wireless device detects the C-RNTI on the PDCCH, the wireless device may determine the success of RA procedure. If the wireless device does not have a C-RNTI (e.g., if a C-RNTI is not pre-assigned), the wireless device may monitor a DL-SCH associated with a TC-RNTI, e.g., that a base station may transmit in an RAR of the second step. In the fourth step (e.g., step  1506  in  FIG.  15   ), the wireless device may compare the identity in the data transmitted by the base station on the DL-SCH with the identity that the wireless device transmits in the third step (e.g., step  1505  in  FIG.  15   ). If the wireless determines that two identities are the same or satisfy a threshold similarity, the wireless device may determine that the RA procedure is successful. If the wireless device determines that the RA is successful, the wireless device may promote the TC-RNTI to the C-RNTI. A TC-RNTI may be an identifier initially assigned to a wireless device when the wireless device first attempts to access a base station. A TC-RNTI may be used for a wireless device in an idle state. After access is allowed by the base station, a C-RNTI may be used for identifying the wireless device. A C-RNTI may be used for a wireless device in an inactive or an active state. 
     The fourth step in the four-step RA procedure (e.g., step  1506  in  FIG.  15   ) may allow HARQ retransmission. A wireless device may start a mac-ContentionResolutionTimer when the wireless device transmits one or more TBs to a base station in the third step (e.g., step  1505  in  FIG.  15   ). The wireless may restart the mac-ContentionResolutionTimer at each HARQ retransmission. When a wireless device receives data on the DL resources identified by C-RNTI or TC-RNTI in the fourth step (e.g., step  1506  in  FIG.  15   ), the wireless device may stop the mac-ContentionResolutionTimer. If the wireless device does not detect the contention resolution identity that matches the identity transmitted by the wireless device in the third step (e.g., step  1505  in  FIG.  15   ), the wireless device may determine that the RA procedure has failed and the wireless device may discard the TC-RNTI. Additionally or alternatively, if the mac-ContentionResolutionTimer expires, the wireless device may determine that the RA procedure has failed and the wireless device may discard the TC-RNTI. If the wireless device determines that the contention resolution has failed, the wireless device may flush the HARQ buffer used for transmission of the MAC PDU and the wireless device may restart the four-step RA procedure from the first step (e.g., step  1503  in  FIG.  15   ). The wireless device may delay subsequent RAP transmission, e.g., by a backoff time. The backoff time may be randomly selected, e.g., according to a uniform distribution between 0 and the backoff parameter value corresponding to the BI in the MAC PDU for RAR. 
     In a four-step RA procedure, the usage of the first two steps may be, e.g., to obtain a UL time alignment for a wireless device and/or to obtain an uplink grant. The UL time alignment may not be necessary in one or more scenarios. For example, in small cells, or for stationary wireless devices, the process for acquiring the UL time alignment may not be necessary if either a TA equal to zero may be sufficient (e.g., for small cells), or if a stored TA value from the last RA may be able to serve for the current RA (e.g., a stationary wireless device). If a wireless device is in an RRC connected state, e.g., with a valid TA value and no resource configured for UL transmission, the UL time alignment may not be necessary when the wireless device attempts to obtain an UL grant. 
     Part (b) of  FIG.  15    shows a three-step contention free RA procedure. A base station may transmit RA configuration parameters to a wireless device, e.g., a UE, in step  1510 . In a contention-free RA procedure, such as shown in part (b) of  FIG.  15   , the configuration parameters may indicate to the wireless device what preamble to send to the base station and when to send the preamble. The base station may also transmit a control command to the wireless device at step  1511 . The control command may comprise, e.g., downlink control information. In a first step of the RA procedure, the wireless device may transmit a random access preamble transmission to the base station at step  1512 . The RAP transmission may be based on the RA configuration parameters and the control command. In a second step of the RA procedure, the base station may transmit to the wireless device a random access response at step  1513 . In a third step of the RA procedure, the wireless device may transmit scheduled transmissions at step  1514 . The scheduled transmissions may be based on the RAR. The contention free RA procedure may end with the third step. Thereafter, the base station may transmit a downlink transmission to the wireless device at step  1515 . This downlink transmission may comprise, e.g., an acknowledgement (ACK) indication, a non-acknowledgement (NACK) indication, data, or other information. Contention-free RA procedures such as described above may have reduced latency compared with contention-based RA procedures. Contention-based RA procedures may involve collisions, such as when more than one wireless device is attempting to communicate with the same base station at the same time. 
     Part (c) of  FIG.  15    shows an example of common language descriptions that may facilitate an understanding of some of the messaging involved in the contention-based four-step RA procedure described above regarding part (a) of  FIG.  15   . In step 1 of the RA procedure, a wireless device may send a communication to a base station similar to a request such as, “Hello, can I camp on?” (step  1520 ). If the base station can accommodate the wireless device request, the base station may respond to the wireless device with a message similar to an instruction such as “Send your info &amp; data here” (step  1521 ). Based on the base station&#39;s response, the wireless device may send a message similar to a response such as “Here you are” (step  1522 ). Based on the information received by the base station, the base station may respond with a message similar to a grant such as “You are now in” (step  1523 ). 
     Part (d) of  FIG.  15    shows an example of a two-step contention free random access procedure of a wireless device. At step  1530 , the wireless device may receive RA configuration parameters from a base station (e.g., from a handover source base station, and/or from a handover target base station via the handover source base station). The RA configuration parameters may comprise one or more parameters indicating a type of a random access process. The type of the random access process may indicate a two-step random access process. At step  1531 , the wireless device may transmit an RA preamble and one or more transport blocks as a first step of the procedure, e.g., overlapping in time with each other. In response to the RA preamble and/or the one or more transport blocks, at step  1532 , the wireless device may receive an RA response from a base station (e.g., a handover target base station). 
       FIG.  16    shows an example of a RAP selection procedure for preamble groups that may be used as the RAP selection process at step  1502  of  FIG.  15   . A base station may broadcast the RAP grouping information, along with one or more thresholds, in system information. At step  1601 , a wireless device may select a preamble group comprising preambles. Two or more RAP groups may be indicated by broadcast system information, and one or more of the RAP groups may be optional. At step  1602 , the wireless device may select a RAP group among a plurality of RAP groups, based on, e.g., a size of data that the wireless may have to transmit, a measured pathloss, and/or other information. At step  1603 , the wireless device may generate a RAP from the selected RAP group. If a base station configures two groups, e.g., in a four-step RA procedure, a wireless device may use the pathloss and a size of the message transmitted by the wireless device in the third step of the RA procedure, to determine from which group the wireless device selects an RAP. A base station may use a group type to which a RAP belongs as an indication of the message size in the third step and/or the radio conditions at a wireless device. The process may end at step  1603 . 
       FIG.  17    shows an example of a MAC PDU comprising a MAC header and MAC RARs. A four-step RA procedure may use the arrangement shown in  FIG.  17   . A two-step RA procedure may also use the arrangement shown in  FIG.  17   . Additionally or alternatively, a two-step RA procedure may use a variation of the arrangement shown in  FIG.  17   , e.g., with additional or fewer fields, and/or with longer or shorter fields. If an RAR comprises a RAPID corresponding to a RAP that a wireless device transmits, the wireless device may process the data in the RAR. The data in the RAR may comprise, e.g., one or more of a timing advance (TA) command, a UL grant, and/or a Temporary C-RNTI (TC-RNTI). The MAC header may comprise subheaders, such as an E/T/R/R/BI subheader (described further below) and up to n number of E/T/RAPID subheaders (described further below). The E/T/R/R/BI subheader may comprise an octet of bits comprising 1 bit each of E, T, R, and R, and four bits of BI. Each of n E/T/RAPID subheaders may comprise an octet comprising 1 bit each of E and T, and 6 bits of an RAPID. 
       FIG.  18    shows an example of an uplink resource  1801  that may be used, e.g., for a transmission of a random access preamble and data in a first step of a two-step RA procedure. A transmission may comprise a random access preamble  1803 , e.g., via a physical random access channel (PRACH), and data  1802 . The data  1802  may comprise one or more transport blocks, an identifier of a wireless device, and/or other information. The data may be included in, e.g., an RRC connection request. The RRC connection request may comprise one or more of, e.g., the data  1802 , an identifier of a wireless device, an indication of a type of data (e.g., emergency, high priority access, standard access, signaling, etc.), and/or other information. The RAP  1803  may comprise a preamble sequence, e.g., bits arranged in octets  1804 . A guard time and/or a cyclic prefix may be inserted, e.g., at either end of the preamble sequence. Additionally or alternatively, a guard band may be inserted above the preamble sequence and/or below the preamble sequence. 
       FIG.  19    shows examples of MAC RAR formats comprising a TA command, a UL Grant, and a TC-RNTI for a four-step RA procedure. Part (a) shows an example MAC RAR format, part (b) shows an example MAC RAR for a PRACH enhanced coverage level 2 or level 3, and part (c) shows an example MAC RAR for NB-IoT UEs. 
     In  FIG.  19    part (a), a first octet comprises 1 bit of R, and 7 bits of the TA command The second octet comprises an additional 4 bits of the TA command as well as 4 bits of the UL grant. The third and fourth octet each comprise 8 additional bits of the UL grant. And, the fifth and sixth octet each comprise 8 bits of the TC-RNTI. In  FIG.  19    part (b), the MAC RAR for PRACH enhanced coverage level 2 or level 3 comprises a MAC RAR similar to the MAC RAR example in  FIG.  19    part (a), except that in part (b) the UL grant comprises 8 bits in the third octet and the TC-RNTI is included in the fourth and fifth octets. In  FIG.  19    part (c), a MAC RAR example for NB-IoT UEs comprises a MAC RAR similar to the MAC RAR in  FIG.  19    part (a), except that the fourth octet in part (c) comprises 3 bits of the UL grant and 5 bits of R. As shown in  FIG.  19    parts (a), (b), and (c), a MAC RAR may comprise one or more reserved bits (R bits). One or more of the R bits in the MAC RAR may indicate whether a RA procedure is a 2-step RA procedure or a 4-step RA procedure. 
       FIG.  20    shows a two-step RA procedure that may comprise an uplink (UL) transmission of an RAP and data, followed by a downlink (DL) transmission of an RAR and contention resolution information. A two-step RA procedure may reduce RA latency compared with a four-step RA process, e.g., by integrating a process to obtain a timing advance (TA) value with a data transmission. In the UL transmission of a two-step RA procedure at step  2003 , a wireless device may transmit, via a cell and to a base station, a RAP for UL time alignment and/or an UL message. The UL message may comprise, e.g., an UL grant, a wireless device ID, one or more TBs, a C-RNTI, and/or other information. In the DL transmission at step  2004 , a base station may transmit an RAR and contention resolution information. The DL transmission may identify an assignment of dedicated resources for the wireless to transmit data, e.g., which may include an assignment of one or more transport blocks. The DL transmission may be in response to the UL transmission. The RAR may comprise an acknowledgement of a reception of the one or more transport blocks, and/or an indication of a successful decoding of the one or more transport blocks. The contention resolution may comprise, e.g., TA information, an UL grant, a C-RNTI, and/or a contention resolution identity. 
     In the UL transmission of a two-step RA procedure, a wireless device, may transmit, via a cell and to a base station, an RAP in parallel with one or more TBs. The wireless device may acquire one or more configuration parameters for the UL transmission before the wireless device starts a two-step RA procedure, e.g., at step  2001 . In a contention-based RA procedure such as shown in  FIG.  20   , the one or more configuration parameters may comprise a root sequence that may be used by the wireless device to generate an RAP. The wireless device may determine an RAP at step  2002 . An RAP selection by the wireless device may be based on the RA configuration parameters received at step  2001 , e.g., comprising one or more RAP selections procedures described herein. The wireless device may use the root sequence to generate one or more candidate preambles, and the wireless device may randomly select one of the candidate preambles as the RAP. The one or more candidate preambles may be organized into groups that may indicate an amount of data for transmission. For example a first group may comprise RAPs indicated for small data transmissions, and a second group may comprise RAPs indicated for larger data transmissions. By transmitting an RAP from a specific group of RAPs, the wireless device may be able to indicate a size of data it may have for transmission. The wireless device may transmit the RAP via a RACH resource. The wireless device may transmit the one or more TBs via an UL resource associated with the RAP. The UL transmissions may occur, e.g., in the same subframe, in consecutive subframes, or in the same burst. A two-step RA procedure may be on a contention basis. The contention may occur for the RAP and/or data transmission. 
     In the UL transmission, the RAP may be used to adjust UL time alignment for a cell and/or to aid in channel estimation for one or more TBs. A portion of the UL transmission for one or more TBs may comprise, e.g., a wireless device ID, a C-RNTI, a service request such as buffer state reporting (e.g., a buffer status report) (BSR), one or more user data packets, and/or other information. A wireless device in an RRC connected state may use a C-RNTI as an identifier of the wireless device (e.g., a wireless device ID). A wireless device in an RRC inactive state may use a C-RNTI (if available), a resume ID, or a short MAC-ID as an identifier of the wireless device. A wireless device in an RRC idle state may use a C-RNTI (if available), a resume ID, a short MAC-ID, an IMSI (International Mobile Subscriber Identifier), a T-IMSI (Temporary-IMSI), and/or a random number as an identifier of the wireless device. 
     The UL transmission may comprise one or more TBs that may be transmitted using a two-step RA procedure different ways. User data packet(s) may be multiplexed in the first step of a two-step RA procedure. A base station may configure one or more resources reserved for the UL transmission that may be indicated to a wireless device before the UL transmission. If the wireless device transmits one or more TBs in the first step of the two-step RA procedure, a base station may transmit in a DL transmission an RAR that may comprise a contention resolution message and/or an acknowledgement/non-acknowledgement message of the UL data transmission. The DL transmission may be in response to the UL transmission. A wireless device may transmit one or more TBs after the reception of an RAR. The wireless device may transmit an indicator, such as buffer state reporting, in the UL transmission. The indicator may indicate to a base station an amount of data the wireless device may attempt to transmit. The base station may assign a UL grant based on the indicator. The base station may transmit the UL grant to the wireless device via an RAR. If UL data transmission, based on the UL grant via an RAR, occurs after the reception of RAR, the UL data transmission may occur on a contention-based channel. The UL data transmission may occur after a wireless device receives the RAR, e.g., in a subframe x+5, or x+n, where x is a subframe in which the RAR is received by the wireless device and n is any whole number greater than zero. 
     A wireless device may provide, to a base station, an indication of a required UL grant size. The wireless device may provide this indication, e.g., by determining a RAP selection (e.g., at step  2002 ), as opposed to transmitting a BSR, e.g., comprising one or more RAP selections procedures described herein. A base station may partition RAPs available to the base station into one or more RAP groups such that each partition may indicate a particular UL grant size. A wireless device may indicate a request, to a base station, of a small or large grant by selecting a RAP from a designated group. The base station may determine the requested grant size based on a RAP that the base station receives. A base station may configure an association between RAP groups and a UL grant size. The base station may broadcast one or more parameters via system information to indicate the association between RAP groups and a UL grant size. 
     A wireless device may provide, to a base station, an indication of a required UL grant size by transmitting an RAP on a partitioned radio resource. A base station may partition radio resources used for RAP transmission into one or more groups such that one or more resources in a group carrying an RAP may indicate a UL grant size that a wireless device may request. The base station may determine the requested grant size based on a RAP received by the base station via resources in a group. When a high granularity is required, a base station may configure a large number of radio resources for the RAP transmission. A base station may configure an association between radio resource groups and a UL grant size. The base station may broadcast one or more parameters via system information to indicate the association between radio resource groups and a UL grant size. 
     In the second step of the two-step RA procedure (e.g., step  2004 ), a base station may transmit an RAR to a wireless device. The base station may transmit the RAR in response to receiving the RAP and data from the wireless device. The RAR may comprise TA information, a contention resolution identity, a UL grant, and/or a C-RNTI. A MAC PDU may comprise one or more of an RAR MAC subheader and a corresponding RAR. The TA may be used by the wireless device for a two-step RA procedure, e.g., when a TA timer has expired. 
     A base station may or may not transmit the contention resolution identity to a wireless device. If a wireless device transmits a C-RNTI (e.g., as a wireless device ID) in a UL transmission, the wireless device may complete contention resolution based on a C-RNTI in an RAR. If a wireless device transmits a shared RNTI, that may be monitored by more than one wireless device as a wireless device ID in a UL transmission, the wireless device may complete contention resolution based on a contention resolution identity in an RAR. Other identifiers for a wireless device, such as a random number, resume ID, T-IMSI, and/or IMSI may be used to complete the contention resolution. 
     The UL grant may be for a wireless device that may have subsequent UL data to transmit. BSR may be transmitted by a wireless device in the UL transmission. A base station may use the BSR for determining a UL grant. 
     A wireless device may not have a C-RNTI, such as a wireless device in an RRC inactive state. If a two-step RA procedure is used for state transition from inactive to connected, a base station may assign a C-RNTI to a wireless device that lacks a C-RNTI. 
     A wireless device may acquire one or more two-step RA configuration parameters (e.g., in step  2001  of  FIG.  20   ) from one or more messages broadcast and/or unicast by a cell. A base station may broadcast or multicast, via a cell, one or more two-step RA configuration parameters comprised in one or more system information blocks. The base station may transmit configuration parameters to a wireless device via dedicated resource(s) and signaling, such as via a unicast to a wireless device in an RRC connected state. 
     A base station may configure or restrict the usage of the two-step RA procedure to one or more case-based procedures, services, or radio conditions. If a cell is small such that there may be no need for a TA, a base station in the cell may use broadcast signaling to configure all wireless devices under its coverage to use a two-step RA procedure. A wireless device may acquire the configuration, via one or more system information blocks, and/or via L1 control signaling used to initiate a two-step RA procedure for downlink data arrival. 
     If a base station has macro coverage, a wireless device having a stored and/or persisted TA value, e.g., a stationary or near stationary wireless device such as a sensor-type wireless device, may perform a two-step RA procedure. A base station having macro coverage may use dedicated signaling to configure a two-step RA procedure with one or more wireless devices having stored and/or persisted TA values under the coverage. 
     A wireless device in an RRC connected state may perform a two-step RA procedure, e.g., when performing a network initiated handover, and/or when the wireless device requires or requests a UL grant within a required delay and there are no physical-layer uplink control channel resources available to transmit a scheduling request. A wireless device in an RRC inactive state may perform a two-step RA procedure, e.g., for a small data transmission while remaining in the inactive state or for resuming a connection. A wireless device may initiate a two-step RA procedure, for example, for initial access such as establishing a radio link, re-establishment of a radio link, handover, establishment of UL synchronization, and/or a scheduling request when there is no UL grant. 
     Determining a type of RA procedure may comprise determining whether to do a 2-step or a 4-step RA procedure. An indicator may be provided in a MAC PDU subheader. Such an indicator may be included in a RAP identifier (RAPID), such as shown in  FIG.  17   . A RAPID may identify a specific RAP, where up to 2 n  unique RAPs are possible for an n-bit wide RAPID. Multiple wireless devices may transmit their own RAPs to a base station. A base station may receive RAPs from a plurality of wireless devices. Each RAPID may indicate one of the plurality of RAPs transmitted to a base station. A wireless device may determine a RAP. This determination may be performed by the wireless device using a random process. The wireless device may transmit the RAP. If a base station detects the RAP from the wireless device, the base station may use bits in a RAPID field to identify that RAP. The base station may transmit the RAPID in a MAC subheader within a MAC header of a MAC PDU, such as shown in  FIG.  17   . The wireless device that transmits the RAP may receive a MAC PDU comprising the RAPID in a MAC subheader. The wireless device may determine, based on the RAPID, that the RAP was successfully received by the base station. 
     The number of n bits in a RAPID may be 6 bits, such as in the RAPID shown in  FIG.  17   , or any number of bits greater or smaller than 6. The base station may determine an estimate of a number of wireless devices supported in a cell. The base station may also determine, based on the estimated number of wireless devices in a cell, a total number of RAPs that may be used for the cell. The base station may also determine, based on the number of RAPs that may be used for the cell, the number of bits to be used in the RAPID for identifying a RAP. The base station may reduce the number of bits used in the RAPID for identifying unique RAPs, which in turn, may allow one or more bits of the RAPID shown in  FIG.  17   , to be used for other information, such as a type of RA procedure to be used. Using fewer than n bits for identifying RAPs, may allow use of one or more unused RAPID bits as an indicator for other information such as whether a random access procedure uses a 2-step procedure or a 4-step procedure. The indication of whether a RA procedure is 2-steps or 4-steps may be in the form of a single bit, where “0” indicates one of the two possible RA procedures, such as a 2-step procedure, and “1” indicates the other, such as a 4-step procedure. Additionally or alternatively, more than one bit of a RAPID may be used, e.g., 2 or 3 bits, to indicate additional information, such as information specific to one or more steps in a RA procedure. 
     One or more bits of the E and/or T fields, such as in a MAC subheader comprising a RAPID shown in  FIG.  17   , may be used as an indication of a type of RA procedure, such as a 2-step RA procedure or a 4-step RA procedure. The E field may be an extension field. A MAC subheader with an E field set to 1 may indicate the presence of a backoff indicator (BI) in the subheader and the presence of an additional MAC subheader. A MAC subheader with an E field set to 0 may indicate the presence of a RAPID in the subheader. The T field may be an indication of a presence of additional MAC subheaders. A MAC subheader with a T field set to 1 may indicate the presence of an additional MAC subheader, and a MAC subheader with a T field set to 0 may indicate that the MAC subheader is the last MAC subheader in the MAC header. One or more of these E and/or T fields may be used to indicate a type of RA procedure, such as a 2-step RA procedure or a 4-step RA procedure. 
     A wireless device may perform a method for determining whether to perform a 2-step RA procedure or a 4-step RA procedure. The wireless device may transmit, to a base station, a RAP comprising a RAP identifier (e.g., RAPID/RAP ID) and one or more transport blocks. Additionally or alternatively, the wireless device may transmit, to a base station, one or more preambles in parallel with one or more transport blocks. Any of the above transmissions by the wireless device may be based on a determination, by the wireless device, that the type of the RA procedure is a two-step RA procedure. The wireless device may receive, from the base station, a MAC PDU comprising one or more MAC subheaders and one or more MAC RARs, such as shown in  FIG.  17   . This transmission by base station may be based on the first transmission by the wireless device. Each of the RARs may be associated with one of the one or more MAC subheaders. The wireless device may determine, based on a first field in a particular MAC subheader of the one or more MAC subheaders, a type of an RA procedure. Additionally or alternatively, based on the type of RA procedure (e.g., based on the first field in the particular MAC subheader), the wireless device may determine a size of an RAR that is associated with the particular MAC subheader comprising the field (e.g., a first MAC subheader, a second MAC subheader, etc.). The type of RA procedure may be, e.g., either a 2-step RA procedure or a 4-step RA procedure. Additionally or alternatively, the RA procedure may be a 3-step RA procedure, or some other number of steps. This particular MAC subheader may comprise a second field comprising the first field and/or one or more bits of an RAP identifier (e.g., a RAPID/RAP ID). This particular MAC subheader may also comprise an extension field, a type field, and/or a reserved bit. The wireless device may determine, based on a comparison of the field in the particular MAC subheader with the RAPID of the RAP transmitted by the wireless device, whether the RAP in the first transmission by the wireless device is successfully received by the base station. The wireless device may determine the type of RA procedure (e.g., a two-step RA procedure) based on whether an initial attempt of the particular RA procedure is successful. If the wireless device determines that the first attempt at the RA procedure is not successful, the wireless device may retransmit the first transmission and/or the wireless device may determine to perform a four-step RA procedure, such as shown in  FIG.  15    part (a) and described above. The wireless device may determine whether a random process has completed. This determining whether a random access has completed may be based on, e.g., the first MAC subheader comprising a random access preamble identifier of the one or more preambles, and/or a second field in the first RAR that indicates that the one or more transport blocks have been successfully received by the base station. Additionally or alternatively, the determining that the random access procedure has completed may be based on the type of the random access procedure of the first RAR being a two-step random access procedure. The wireless device may transmit, based on a second field in the first RAR that indicates that at least one of the one or more transport blocks have not been successfully received by the base station, one or more messages using a four-step random access procedure. The wireless device may receive, from a base station, one or more messages comprising configuration parameters of a two-step random access procedure and/or configuration parameters of a four-step random access procedure. The wireless device may receive, from the base station, an RAR that may comprise an uplink grant. The wireless device may retransmit, in response to a field of the RAR (e.g., a first field, a second field, etc.) indicating that one or more transport blocks have not been successfully received by the base station, the one or more transport blocks. A wireless device may perform any combination of one or more of the above steps. A base station, or any other device, may perform any combination of a step, or a complementary step, of one or more of the above steps. 
     A base station may perform a method for determining whether to perform a two-step RA procedure or a four-step RA procedure. The base station may receive, from a wireless device, an RAP. The base station may also receive, in this first transmission from the wireless device, one or more transport blocks. The base station may determine, based on the RAP, a type of RA procedure. The type of RA procedure may be, e.g., either a two-step RA procedure or a four-step RA procedure. Additionally or alternatively, the RA procedure may be a 3-step RA procedure, or some other number of steps. The base station may transmit, to the wireless device and based on the RAP, a MAC PDU comprising one or more MAC subheaders and one or more MAC RARs, such as shown in  FIG.  17   . A particular MAC subheader may comprise a field indicating the type of the RA procedure. This particular MAC subheader may comprise one or more bits of an RAP identifier (e.g., a RAPID/RAP ID). This particular subheader may also comprise an extension field, a type field, and/or a reserved bit. Each of the one or more RARs may be associated with one of the one or more MAC subheaders. The RAR may comprise the same size or one of a plurality of different sizes. An indication of the type of the RA procedure may be based on a RAPID in the RAP. Additionally or alternatively, an indication of the type of the RA procedure may be based on one or more of the extension field, the type field, and/or a reserved bit. A base station may perform any combination of one or more of the above steps. A wireless device, or any other device, may perform any combination of a step, or a complementary step, of one or more of the above steps. 
     A base station may determine a first MAC PDU for a two-step RA procedure comprising RARs only for two-step RA procedures, and a base station may determine a second and different MAC PDU for a 4-step RA procedure comprising RARs only for 4-step RA procedures, wherein each RAR in a MAC PDU is for the same type of RA procedure. The base station may provide an indicator as to which type of RA procedure the MAC PDU corresponds. Such an indicator may be included in any location of the MAC PDU, e.g., within a MAC header or within MAC RARs (e.g., within an R field comprising reserved bits). By including the indicator in the MAC header, a wireless device may be able to determine the type of RA procedure, as well as the size of the MAC RARs, prior to receiving or decoding the MAC RARs of the MAC PDU. 
     A base station may multiplex different type of RA responses (RARs) into one MAC PDU. A wireless device may require or request resources for both two-step RA and four-step RA procedures, and these resources for difference types of RA procedures may be independent of each other. A base station may require or request to use additional resources to accommodate a MAC PDU for 2-step RA procedures that may be different from a MAC PDU for 4-step procedures. If a base station multiplexes two types of RARs (e.g., RARs for 2-step RA procedures and RARs for 4-step RA procedures) into one MAC PDU, the base station may only be able to assign common RACH resources, where a UE can transmit a 2-step RAP or a 4-step RAP. Resources in the uplink and the downlink may be conserved by identifying an RA type without allocating separate resources for different MAC PDUs. If a base station multiplexes two types of RARs in the same MAC PDU, e.g., if the length of a two-step RAR may be different than the length of a four-step RAR, one or more indicators may be required to indicate one or more RAR boundaries in the same MAC PDU. 
     A base station may determine a type of RA procedure, such as a two-step RA procedure or a four-step RA procedure, for communications with one or more wireless devices. A base station may monitor RACH resources to determine whether one or more RAPs are received. If a RAP is received, the base station may determine a type of RA procedure for the RAP, such as a two-step RA procedure or a four-step RA procedure. The base station may determine, based on the type of the RA procedure for the RAP, a corresponding type of RA procedure for communications with the wireless device that transmitted the RAP. The base station may make the above determinations for a plurality of RAPs, and each of the plurality of RAPs may be associated with one of a plurality of wireless devices. The base station may multiplex a plurality of MAC PDUs. Each of the MAC PDUs may comprise a one or more MAC subheaders. At least one of the one or more MAC subheaders may comprise an indication of a type of an RA procedure, such as a two-step RA procedure or a four-step RA procedure. The base station may transmit the multiplexed plurality of MAC PDUs. One or more of the plurality of wireless devices may receive the multiplexed MAC PDUs and determine whether to perform one or more steps of an RA procedure with the base station. The one or more of the plurality of wireless devices may determine, based on one or more indications in a MAC subheader of at least one of the MAC PDUs, a type of the RA procedure for communications with the base station. 
     A base station and/or a wireless device may determine whether an attempt for an RA procedure is successful. If an attempt for an RA procedure is successful, the base station and the wireless device may communicate using the type of RA procedure of the successful attempt. If the attempt for an RA procedure is not successful, the base station and/or the wireless device may make another attempt for an RA procedure of the same type as the prior attempt. If one or more attempts (e.g., up to a threshold number) for an RA procedure of a particular type of RA procedure (e.g., a two-step RA procedure) are not successful, the base station and/or the wireless device may attempt an RA procedure of a different type (e.g., a four-step RA procedure). A base station may perform any combination of one or more of the above steps. A wireless device, or any other device, may perform any combination of a step, or a complementary step, of one or more of the above steps. 
     A two-step RA procedure may be attempted by a wireless device. The wireless device may transmit, to a base station, an RAP in parallel with data. The data may be an uplink message that may comprise, e.g., an identifier of the wireless device and other data such as one or more transport blocks. A base station may receive the transmission from the wireless device and the base station may decode one or more transport blocks received from the wireless device. The base station may transmit, to the wireless device, a random access response (RAR). The base station may include the RAR in a MAC PDU. The base station may also include one or more additional RARs in the MAC PDU. The base station may multiplex the MAC PDU with other MAC PDUs. The base station may transmit the multiplexed MAC PDUs. The base station may include in the RAR, which may be responsive to the first transmission of the wireless device, one or more indications of whether the one or more transport blocks were successfully received by the base station. The one or more indications may be included in one or more R fields of one or more reserved bits of the RAR. Examples of R fields are shown, e.g., as “R” in  FIG.  19    parts (a), (b), and (c). The wireless device may receive the multiplexed MAC PDUs. The wireless device may demultiplex the multiplexed MAC PDUs. The wireless device may determine, based on a RAPID or other indication in a MAC subheader of a MAC PDU, that a particular RAR associated with that MAC subheader is intended for the wireless device. For example, if a RAPID in a MAC subheader of a MAC PDU corresponds to an RAP that a wireless device transmitted to a base station, the wireless device may determine that a particular RAR associated with that MAC subheader comprising the RAPID is intended for the wireless device. The RAPID or other indication in the MAC subheader may include an indication of the identifier of the wireless device that was previously included in the uplink message of the transmission by the wireless device. The wireless device may determine, based on the indication in the RAR associated with the MAC subheader, whether the one or more transport blocks were successfully received by the base station. As described above, the indication may comprise one or more bits in one or more R fields in the RAR. R fields may comprise remainder bits of an octet comprising fields designated for other purposes, and these remainder bits may be reserved for future use or for one or more indications such as described above. An indication whether one or more transport blocks were successfully received by the base station may comprise a single bit. The indication may also comprise one or more additional bits to provide additional information to the wireless device. The additional information provided by one or more additional bits may comprise, e.g., a redundancy version for a retransmission of one or more transport blocks by the wireless device. 
     A wireless device may perform a method for determining whether a 2-step RA procedure is successful. A wireless device may receive, from a base station, one or more messages comprising configuration parameters for a RACH of a cell. The configuration parameters may comprise one or more RA parameters. The wireless device may transmit, to the base station, an RAP in parallel with: one or more first TBs with a first redundancy version (RV) associated with a HARQ process. This first transmission by the wireless device may be based on the one or more RA parameters. The one or more first TBs may comprise an identifier associated with the wireless device (e.g., UE ID). The wireless device may receive, from the base station, an RAR MAC PDU comprising, e.g., a preamble identifier (e.g., RAPID), an uplink grant, a field indicating whether the one or more first TBs are successfully received by a base station, and/or an RNTI. The RNTI may comprise, e.g., one or more of a C-RNTI or a TC-RNTI. The uplink grant may comprise an indication of uplink resources. The field indicating whether the one or more first TBs are successfully received by the base station may comprise: one or more of the identifier associated with the wireless device or a C-RNTI, if the one or more first TBs are received successfully by the base station; or one or more of a fixed value or a TC-RNTI, if the one or more first TBs are not received successfully by the base station. One or more transport blocks may be considered successfully received by the base station if the base station is able to decode information contained in the one or more transport blocks. Additionally or alternatively, the RAR MAC PDU may comprise at least one MAC subheader that comprises one or more of a TC-RNTI or an indication of a decoding failure. The wireless device may transmit, to the base station, one or more first TBs with a second RV associated with the HARQ process, wherein the second RV may be the same as /or different from the first RV. The wireless device may receive, from the base station, a downlink packet comprising the identifier associated with the wireless device, if the one or more first TBs with the first RV or the second RV are received successfully by the base station. The wireless device may receive, from the base station, an indication of a NACK, if TBs are not received successfully. The wireless device may transmit, based on the indication of the NACK, a HARQ comprising retransmitted TBs with another RV different from the prior first and second RVs. The wireless device may receive, from the base station, an indication of an ACK, if TBs are successfully received. If the one or more first TBs are not received successfully by the base station, the wireless device may determine that the RA procedure has failed and/or the wireless device may fall back to a four-step RA procedure, such as described above regarding  FIG.  15    part (a). A wireless device may perform any combination of one or more of the above steps. A base station, or any other device, may perform any combination of a step, or a complementary step, of one or more of the above steps. 
     A base station may perform a method for determining whether a two-step RA procedure is successful. A base station may transmit, to a wireless device, one or more messages comprising configuration parameters for a RACH of a cell. The configuration parameters may comprise one or more RA parameters. The base station may receive, from the wireless device, an RAP in parallel with: one or more first TBs with a first redundancy version (RV) associated with a HARQ process. This first transmission by the wireless device may be based on the one or more RA parameters. The one or more first TBs may comprise an identifier associated with the wireless device (e.g., UE ID). The base station may transmit, to the wireless device, an RAR MAC PDU comprising, e.g., a preamble identifier (e.g., RAPID), an uplink grant, a field indicating whether the one or more first TBs are received successfully, and/or an RNTI. The RNTI may comprise, e.g., one or more of a C-RNTI or a TC-RNTI. The uplink grant may comprise an indication of uplink resources. The field indicating whether the one or more first TBs are received successfully may comprise: one or more of the identifier associated with the wireless device or a C-RNTI, if the one or more first TBs are received successfully by the base station; or one or more of a fixed value or a TC-RNTI, if the one or more first TBs are not received successfully by the base station. One or more transport blocks may be considered successfully received by the base station if the base station is able to decode information contained in the one or more transport blocks. Additionally or alternatively, the RAR MAC PDU may comprise at least one MAC subheader that comprises one or more of a TC-RNTI or an indication of a decoding failure. The base station may receive, from the wireless device, one or more first TBs with a second RV associated with the HARQ process, wherein the second RV may be the same as or different from the first RV. The base station may transmit, to the wireless device, a downlink packet comprising the identifier associated with the wireless device, if the one or more first TBs with the first RV or the second RV are received successfully by the base station. The base station may transmit, to the wireless device, an indication of a NACK, if TBs are not received successfully. The base station may receive, based on the indication of the NACK, a HARQ comprising retransmitted TBs with another RV different from the prior first and second RVs. The base station may transmit, to the wireless device, an indication of an ACK, if TBs are successfully received. If the one or more first TBs are not received successfully by the base station, the base station may determine that the RA procedure has failed and/or that a four-step RA procedure, such as described above regarding  FIG.  15    part (a), should be attempted. The base station may transmit, to the wireless device, an indication that the RA procedure has failed and/or an indication to fall back to a four-step RA procedure. A base station may perform any combination of one or more of the above steps. A wireless device, or any other device, may perform any combination of a step, or a complementary step, of one or more of the above steps. 
     Because a two-step RA procedure may reduce latency of UL data transfer compared with a four-step RA procedure, two-step RA procedures may be advantageous for UL data transfer such as UL data arrival for a wireless device in an RRC connected state or UL data arrival for a wireless device in an RRC inactive state. If a wireless device is in an RRC connected state, using two-step RA procedure may improve the latency of receiving an uplink grant for UL data arrival. The two-step RA procedure may be used, e.g., if a TA timer expires or a physical-layer uplink control channel resource for the SR is not configured for a wireless device. If a wireless device is in an RRC inactive state, the wireless device may transmit a UL data arrival using a two-step RA procedure without a state transition to the RRC connected state. 
     If a base station configures four-step and two-step RA procedures, the base station may use separate preamble signature groups and/or separate time-frequency resources for each of four-step and two-step RA preamble transmissions. Using separate preamble signature groups and/or separate time-frequency resources for different types of RA procedures may help the base station determine whether a wireless device attempts to initiate a two-step RA procedure or a four-step RA procedure. A base station may broadcast and/or unicast one or more configuration parameters that indicate separate preamble signature groups, and/or that use separate time-frequency resources, for four-step and two-step RA preamble transmissions. 
     One or more RAP groups may be configured for a two-step RA procedure via broadcast system information. If a base station configures one or more groups in the two-step RA procedure, a wireless device may use a size of the message transmitted by the wireless device in the third step, and/or the pathloss, to determine for which group the wireless device selects an RAP. A base station may use a group type to which an RAP belongs as an indication of the message size in the third step and/or as an indication of the radio conditions at a wireless device. A base station may broadcast the RAP grouping information along with one or more thresholds on system information. 
     A process for generating a RAP may be predetermined, or may be determined, e.g., using two-step RA configuration parameters. A type of sequence for RAP generation, e.g., a Zadoff-Chu sequence, the number of samples in a sequence, a sub-carrier spacing for a RAP transmission, and a format of a RAP transmission in a subframe (e.g., guard time/frequency, cyclic prefix length for a RAP transmission, and/or a resource block size allocated for an RAP and data transmission) may be predetermined. A cell may broadcast one or more parameters, such as a root sequence index and cyclic shift interval (e.g., rootSequenceindex, highspeedflag, and/or zeroCorrelationZoneConfig in LTE), required for a wireless device to generate a set of RAPs. 
     The resources used for a RAP and data transmission may be pre-determined or indicated by RA configuration parameters. A table or other form of memory may indicate possible pairs of system frame number (SFN) and subframe number. A wireless device may attempt the first step of a two-step RA procedure, e.g., transmission of an RAP and data, based on such a table. A base station may broadcast one or more pairs of SFN and subframe used in the cell for the RAP and data transmission of a two-step RA procedure. A frequency offset with which a wireless device transmits an RAP and data within a subframe may be configured by two-step RA configuration parameters. A resource via which a data part is transmitted during the UL transmission may be pre-determined or configured by two-step RA configuration parameters. Such a resource may be associated with a selected RAP ID, such that a wireless devices that select different RAPs transmit data via different resources in the UL transmission. 
     One or more wireless devices may perform the first step of a two-step RA procedure using the same cell in the same subframe. The cell may respond to one or more wireless devices&#39; UL transmissions by multiplexing one or more RARs into a single MAC PDU as shown in  FIG.  17   .  FIG.  17    is an example MAC PDU format. Other fields may be added, e.g., to the subheader and/or to the RARs. A MAC PDU may comprise a MAC header and MAC RARs. The MAC header may comprise one or more MAC subheaders, at least one of which may comprise a BI. Other MAC subheaders may comprise a RAPID that may indicate an index number of one of available RAPs in a cell. Each MAC RAR may comprise a wireless device ID, a C-RNTI, a TA command, an UL grant, and/or other parameters. A wireless device may identify an RAR corresponding to the wireless device in a MAC PDU by first identifying a subheader having a RAPID that matches the RAP that the wireless device transmitted during the UL transmission. The wireless device may decode an RAR that is paired with the identified subheader. 
     The MAC PDU may comprise a subheader that may comprise a bit string, e.g., including a special bit string comprising zeros, that may be pre-defined to indicate the failure of RAP detection but success of data decoding at a cell. The bit string may be indicated by a field of the RAPID or by a dedicated field for the bit string in the subheader. An RAR that is a pair of the subheader having the special bit string may include the wireless device ID (and/or other IDs) that a wireless device transmits in the UL transmission. The wireless device may decode the RAR having a corresponding subheader that has the special bit string, to determine whether the RAR is intended for the wireless device. The MAC subheader may comprise a field that may be a RAP identifier associated with the RAP if the RAP is detected. If the RAP is not detected, the field may comprise a pre-defined format. Additionally or alternatively, a second field in the MAC PDU subheader may indicate that a RAP is not detected. 
     An RAR response timer may be configured using two-step RA configuration parameters. A wireless device may reset and/or start the RAR response timer in response to the wireless device transmitting a UL RAP and data transmission. The wireless device may monitor a downlink channel for an RAR on a cell until the RAR response timer is expired. A base station may transmit a MAC PDU that comprises one or more RARs, one or multiple times, in a DL transmission before the RAR response timer expires. The presence of an RAR may be indicated via a specific channel (e.g., a PDCCH in LTE) using an identity (e.g., RA-RNTI in LTE) created based on UL transmission time (e.g., as a combination of SFN and/or subframe number) and/or frequency offset. A wireless device may stop an RAR response timer when at least one of the following conditions are satisfied: the wireless device detects a MAC PDU that comprises a RAPID matching the RAP that the wireless device transmitted (and/or satisfying a threshold of similarity), the wireless device determines an RAR has a wireless device ID that the wireless device transmitted, and/or the RAR response timer is expired. 
     A wireless device may monitor RARs for unique identifiers associated with the wireless device to determine whether information transmitted by the base station is intended for the wireless device. A base station may assign a unique identifier to each wireless device of a plurality of wireless devices in a cell. The unique identifier may be associated with a radio network temporary identifier (RNTI). For a particular cell, the RNTI may be a cell radio network temporary identifier (C-RNTI). A wireless device may have a plurality of RNTIs associated with it, each of which may be for a different purpose. A wireless device may monitor resources in a time and frequency resource map by identifying a C-RNTI corresponding to the wireless device. If a base station transmits information in these resources, the information may be scrambled by the unique identifier of the wireless device, e.g., C-RNTI, to which the information is intended. If the wireless device receives a transmission from a base station, the wireless device may attempt to decode packets with the unique identifier assigned to the wireless device by the base station. The wireless device may determine, using its unique identifier, whether there is any data for the particular wireless device. The wireless device may monitor one or more channels, such as downlink control channels, with its unique identifier, e.g., at least one of RNTI assigned to the wireless device, to determine whether information is intended for that particular wireless device. If the wireless device uses its unique identifier to attempt to decode information but is not successful, then the wireless device may determine that the information is intended for a different wireless device. The wireless device that was unsuccessful in its attempt to decode information may continue to monitor the one or more channels. 
     A two-step RA procedure may comprise two pairs of transmission, e.g., a first pair for a preamble transmission and a second pair for data transmission. Four overall outcomes may be possible from these pairs of transmission: both pairs succeed, both pairs fail, the first pair succeeds but the second pair fails, or the second pair succeeds but the first pair fails. A wireless device may determine, based on an indication in a MAC subheader such as described above, whether one or more preamble transmissions are successfully received by a base station. A wireless may determine, based on an indication in a MAC RAR such as described above, whether one or more data transmissions are successfully received by a base station. If either one or more preamble transmissions are not successfully received by a base station, and/or if either one or more data transmissions are not successfully received by a base station, a wireless device may determine whether to perform a retransmission of at least one of the first pair and/or the second pair of transmissions. If a wireless device determines to perform a retransmission, the wireless device may determine when to perform the retransmission. 
     A contention resolution may be completed based on, e.g., either a C-RNTI or a wireless device contention resolution identity in an RAR. If a base station detects an RAP and decodes one or more TBs or portion thereof that a wireless device transmits, the base station may respond with an RAR that comprises the C-RNTI and/or other wireless device identifiers that the wireless device transmits in the first step of a two-step RA procedure. By detecting the C-RNTI and/or other wireless device identifiers in the received RAR, the wireless device may determine the success of the two-step RA procedure. The wireless device may start monitoring the downlink control channel associated with the C-RNTI (or Temporary C-RNTI) from the time the wireless device detects the C-RNTI (or Temporary C-RNTI) in the RAR such as shown in  FIG.  21   . 
     If a base station detects an RAP but fails to decode one or more TBs or portion thereof that a wireless device transmits in the UL transmission of the two-step RA procedure, the base station may indicate such a failure. The base may transmit a MAC PDU that comprises a TC-RNTI, and/or one or more indicators in a MAC subheader and/or in an RAR, that may indicate a decoding failure to the wireless device of the RAP that the base station detected but failed to decode. A wireless device may determine, based on the one or more indicators, that the RAP was not successfully received or decoded by the base station. The wireless device may re-transmit the one or more transport blocks, e.g., by performing HARQ retransmission. The wireless device may start a mac-ContentionResolutionTimer when the wireless device retransmits, based on uplink grant in the RAR, the one or more transport blocks. The wireless device may not start a mac-ContentionResolutionTimer when the wireless device transmits one or more transport blocks based on uplink grant in the RAR, if the RAR indicates that one or more transport blocks are received and decoded successfully by the base station. 
     A wireless device may restart the mac-ContentionResolutionTimer at a HARQ retransmission. If a wireless device starts or restarts the mac-ContentionResolutionTimer, the wireless device may start monitoring a downlink control channel using the C-RNTI or TC-RNTI. The wireless device may start this monitoring at a subframe and/or at a time offset from a start or restart of the mac-ContentionResolutionTimer. If an RAR indicates that one or more transport blocks are received successfully by a base station, the wireless device may monitor the C-RNTI and/or TC-RNTI. The wireless device may start this monitoring at a subframe and/or at a time offset from receiving the RAR. 
     If a wireless device transmits a C-RNTI in the first step of a two-step RA procedure, the wireless device may monitor a downlink control channel using the C-RNTI. If a wireless device does not transmit a C-RNTI in the first step of a two-step RA procedure, the wireless device may monitor a downlink control channel using the TC-RNTI. If the mac-ContentionResolutionTimer expires, a wireless device may determine that the two-step RA procedure has failed. 
       FIG.  21    shows an example of contention resolution for a two-step RA procedure. At step  2101 , a wireless device may transmit, to a base station and via a cell, a random access preamble (RAP) and data. The base station associated with the cell may receive the RAP and the data. The RAP may include an identifier (e.g., RAPID=xxx). The data may comprise UL data and a redundancy version (e.g., RV=aa). The data may comprise an identifier of the wireless device (e.g., UE ID=yyy). The wireless device may start an RAR response timer (e.g., mac-ContentionResolutionTimer) at or near the time the wireless device transmits the RAP and data. At step  2102 , the base station may transmit, and the wireless device may receive, a MAC PDU comprising a subheader that includes a RAP identifier of the RAP (e.g., RAPID=xxx). The MAC PDU may also comprise an RAR that may correspond to the subheader comprising an uplink grant. The RAR may include one or more indications of a decoding failure by the base station of the UL data. If an indication of a decoding failure is turned off, the wireless device may start monitoring a downlink control channel with a C-RNTI of the wireless device. At step  2103 , if an indication of a decoding failure is turned on, the wireless device may perform a HARQ retransmission of the RAP and data, the wireless device may start monitoring the downlink control channel with the C-RNTI of the wireless device, and/or the wireless device may start and/or restart a timer (e.g., the mac-ContentionResolutionTimer). 
     The wireless device may transmit one or more transport blocks in a first subframe and via radio resources indicated in an uplink grant. The wireless device may start a contention resolution timer in the first subframe depending on whether the RAR comprises the identifier of the wireless device. The wireless device may stop monitoring for RAR(s), e.g., after decoding a MAC packet data unit for an RAR and determining that the RAP identifier matches the RAP transmitted by the wireless device. The MAC PDU may comprise one or more MAC RARs and a MAC header. The MAC header may comprise a subheader having a backoff indicator and one or more subheaders that comprises RAPIDs. 
     If one or more data transmissions are successfully received by a base station, the base station may transmit a MAC PDU comprising one or more corresponding RARs that each comprise an uplink grant. The uplink grant may indicate a particular subframe for a wireless device to transmit uplink data. The wireless device may start monitoring a downlink control channel from a second subframe. The wireless device may determine the second subframe based on, e.g.: a third subframe in which an RAR is received, if the RAR comprises the wireless device identifier; and/or the first subframe in which the wireless device transmits uplink resources based on the uplink grant. The wireless device may monitor the data for a C-RNTI, if the data comprises a C-RNTI; and/or the wireless device may monitor the RAR for a Temporary C-RNTI (TC-RNTI), if the data does not comprise a C-RNTI. 
     If a wireless device does not receive any MAC PDU that comprises the RAPID and/or the wireless device identifier associated with the RAP, and if an RAR response timer has expired, the wireless device may retry the first step of a two-step RA procedure. The wireless device may retransmit the RAP and the data on the same cell. 
     If the wireless device receives a MAC PDU that comprises a BI, the wireless device may select a backoff time. The backoff time may be random, and it may be determined according to a uniform distribution, e.g., between 0 and a BI value. The wireless device may delay the subsequent re-transmission of the RAP and the data by the selected backoff time. If the wireless device receives a MAC PDU that does not comprise any backoff indicator until an RAR response timer has expired, the backoff time may be set to zero. The wireless device may have a counter for counting the number of retransmissions of RAP and data. The wireless device may set the counter to zero (or 1) in the initial RAP transmission, and the wireless device may increase the counter by one whenever the wireless device re-tries the first step of a two-step RA procedure. The wireless device may reset the counter to zero (or 1) when the wireless device receives any MAC PDU that comprises the RAP ID or the wireless device ID, and/or when an RAR response timer expires. Two-step RA configuration parameters may have a parameter limiting an allowed maximum number of the retransmissions of RAP and data. If the counter reaches the maximum number, the wireless device may stop retransmission. The wireless device may perform a new RA on another cell with two-step or four-step RA procedure depending on two-step RA configuration parameters of a cell associated with the another cell. 
       FIG.  22    shows an example of a two-step RA procedure and the failure of UL transmission for n times. At step  2201 , a first base station (e.g., base station A) may transmit, via a first cell (e.g., cell A_a), two-step RA configuration parameters to a wireless device. The wireless device may determine a RAP selection, at step  2202 , e.g., comprising one or more RAP selections procedures described herein. At step  2203 , the wireless device may transmit a RAP and data (e.g. one or more transport blocks) of a two-step RA procedure to a base station. The RAP may comprise an identifier (RAP ID=xxx), and the data may comprise a wireless device identifier (UE ID=yyy). At or near the time the wireless device transmits the RAP and the data, the wireless device may start an RAR response timer. The base station may successfully decode and identify a RAP ID associated with the RAP, but the base station may fail to decode the data. Failure to decode the data may result from, e.g., collision or low signal quality. The base station may not transmit an RAR to the wireless in response to the RAP and data, or the base station may transmit an RAR comprising an indication of a decoding failure of the data. If the RAR response timer expires, and if the wireless device has not received an RAR indicating that the RAP and the data were successfully received by the base station, at step  2204 , the wireless device may retransmit the RAP and the data after a time period corresponding to a backoff time. The wireless device may perform the above steps  2203  through  2204 , to retransmit the RAP and the data, n number of times, where n may be any whole number. After the nth retransmission at step  2205 , and after the RAR response timer expires with the wireless device not receiving an RAR indicating that the RAP and the data were successfully received by the base station, the wireless device may receive RA configuration parameters from a second base station (e.g., base station B) via a second cell (e.g., cell B_a), at step  2206 . The RA configuration parameters from the second base station may be for a two-step RA procedure or for a four-step RA procedure. At step  2207 , the wireless device may determine, based on the RA configuration parameters from the second base station, a RAP selection, e.g., comprising one or more RAP selections procedures described herein. The wireless device may transmit, to the second base station, a new RAP (e.g., comprising RAP ID=zzz) and the data (e.g., comprising the UE ID=yyy). The wireless device may repeat any of the above steps until the wireless device determines that a RA procedure is successful. 
     The wireless device may receive a MAC PDU comprising a subheader that includes a RAP ID that the wireless device transmitted, but that also includes a decoding failure indicator in the subheader or in the RAR associated with the subheader. The decoding failure indicator may be implemented in different ways depending on a MAC PDU format. If RARs for data decoding failures and successes have the same size, a MAC PDU may have a dedicated field inserted in a subheader or in an RAR to indicate the data decoding success or failure. This field may comprise one bit, such that either zero or one may indicate data decoding success or failure. A special bit string may be also used in an existing field in an RAR to indicate data decoding success or failure. This special bit string may comprise, e.g., all zeros or a detectable pattern of ones and zeros in the field of a wireless device ID in an RAR to indicate data decoding failure, or to indicate data decoding success. If RARs for data decoding failure and success have the same size, a wireless device may determine, based on a pre-determined RAR size information, the boundary of an RAR in a MAC PDU. If RARs for data decoding failure and success have different sizes, the base station may insert a field, to indicate RAR size information, in a MAC subheader or in an RAR. A wireless device may determine the boundary of an RAR in a MAC PDU based on the field. If RARs for data decoding failure and successes have different sizes, the RARs may have different formats. For example, an RAR for a data decoding failure may comprise a field of Temporary Cell Radio Network Temporary Identity instead of a field of a contention resolution wireless device ID, and an RAR for a data decoding success may comprise a contention resolution wireless device ID instead of a TC-RNTI. 
     A wireless device may transmit, to a base station and as a part of a two-step RA process, a random access preamble and one or more transport blocks. The wireless device may receive a MAC PDU comprising: one or more MAC PDU subheaders, wherein a subheader comprises an RAP identifier; and one or more RARs, wherein each RAR corresponds to a MAC PDU subheader in the one or more MAC PDU sub-headers. The wireless device may determine whether the one or more transport blocks are received successfully based on one or more of: a first field in the subheader (e.g., a bit in the subheader indicating a fall back to a four-step RA procedure); a second field in an RAR associated with a first sub-header comprising an RAR identifier associated with the RAR (e.g., a bit in the RAR indicating a fall back to a four-step RA procedure). The wireless device may retransmit one or more transport blocks, if the one or more transport blocks are not received successfully by a base station. The wireless device may determine a size of the RAR based on one or more indications in the first field and/or in the second field. The wireless device may determine whether to fall back to a four-step RA procedure based on one or more indications in the first field and/or in the second field. 
     A wireless device may perform a method for determining whether a two-step RA procedure has failed based on one or more timers. A wireless device may receive, from a base station, RA configuration parameters. The wireless device may transmit, to a base station, an RAP and one or more first transport blocks. The one or more first transport blocks may comprise an identifier of the wireless device (e.g., UE ID). The wireless device may receive, from the base station, a MAC PDU comprising one or more MAC subheaders and one or more RARs. At least one of the one or more MAC subheaders may comprise a RAPID of the RAP transmitted by the wireless device. Each RAR of the one or more RARs may be associated with a MAC subheader of the one or more MAC subheaders. A first RAR may be associated with a first MAC header comprising an uplink grant for a first subframe. The wireless device may transmit, to the base station, in a first subframe, and via radio resources indicated by the uplink grant, one or more transport blocks. The wireless device may start, in response to transmitting one or more transport blocks, and based on whether the RAR comprises the identifier of the wireless device, a contention resolution timer. The contention resolution timer may be for determining whether a random access procedure is successful. The wireless device may stop, based on a determination that the RAPID of the MAC subheader matches the identifier of the wireless device, an RAR response timer that started in response to transmitting the RAP. The wireless device may stop, based on a determination that a downlink control information detected from a downlink control channel comprises the identifier of the wireless device, the contention resolution timer. The wireless device may perform one or more operations regarding the contention timer one or more times, and/or using one or more timers. The wireless device may restart, based on a retransmission of the one or more TBs, the contention resolution timer. The wireless device may determine that, based on an expiration of the contention resolution timer, a first RA procedure has failed. The wireless device may initiate, at a time period after determining that the first RA procedure has failed, a second RA procedure. The wireless device may determine, based on a backoff indicator in the RAR, the time period. The wireless device may monitor a downlink control channel associated with an RNTI. The wireless device may start the monitoring from a second subframe. The monitoring by the wireless device may comprise monitoring the downlink control channel for a downlink control information associated with the RNTI. The wireless device may determine the second subframe based on one or more of: a subframe in which an additional RAR is received that comprises the identifier of the wireless device; and/or a subframe in which the wireless device transmits uplink resources based on the uplink grant. The monitoring may be performed after detecting the RAR comprising the RNTI. The RNTI may comprise one or more of a C-RNTI or a TC-RNTI. The wireless device may determine that an RAR associated with the RAP has not been received. The wireless device may perform the following one or more times: retransmitting the RAP and the one or more transport blocks; starting, based on the retransmitting the RAP and the one or more transport blocks, a second timer; and determining that an RAR associated with the retransmitted RAP has not been received. This determining may occur at a second time period after the starting of the second timer. The wireless device may determine, based on a determination that an RAR associated with one or more retransmissions of the RAP has not been received, that an RA procedure with the base station has failed. The wireless device may receive, from a second base station, RA parameters. The wireless device may transmit, to the second base station, the RAP and the one or more transport blocks. A wireless device may perform any combination of one or more of the above steps. A base station, or any other device, may perform any combination of a step, or a complementary step, of one or more of the above steps. 
     A base station may multiplex, in a MAC PDU, RARs for two-step and four-step RA procedures. If RARs for two-step and four-step RA procedure have the same size, a wireless device may not require an RAR length indicator field and/or the wireless device may determine the boundary of each RAR in the MAC PDU based on pre-determined RAR size information. The RAR may have a field to indicate a type of RAR (e.g., comprising one or bits in a reserved “R” field as shown in  FIG.  23   ). An RAR may comprise different formats for two-step and four-step RARs with a fixed size. Some examples for different RAR formats are shown in  FIG.  23    parts (a), (b), and (c). By using RARs with different formats, the size of the sub-header may be reduced and/or additional bits may be available for other fields. Encoding RARs with an indication of an RAR type may reduce downlink signaling overhead. 
       FIG.  24    shows an example RAR format that may or may not comprise a field to indicate a type of RAR. The RAR shown in  FIG.  24    may comprise a fixed size using the same format for two-step and four-step RA. If RARs for two-step and four-step RA procedures have different sizes, a field for indicating an RAR type may be included in a subheader (such as a MAC subheader) or in an RAR. An RAR may comprise different types of fields that may correspond with an indicator in a subheader or in an RAR. A wireless device may determine the boundary of one or more RARs in a MAC PDU based on one or more indicators. 
     Different message formats may be used based on the type of device for communication. If a wireless device comprises, e.g., an IoT device, such as a smart appliance or other electronics equipment that may not require a large amount of data transmission, the format may include a small message space for transmissions by the device instead of including an uplink grant. If the wireless device comprises, e.g., a phone, tablet, or other device that requires a large amount of data transmissions, the format may include an uplink grant for larger transmissions. 
     A wireless device may determine whether an RAR is a two-step RAR or a four-step RAR, at least based on the RAP identifier in the corresponding MAC PDU sub-header. Two-step and four-step RA preamble identifiers may be selected from two different preamble groups. The wireless device may determine whether an RAR is a two-step RAR or a four-step RAR, at least based on a field indicating an RAR type. The MAC PDU subheader may comprise the field indicating an RAR type. The field may comprise one bit indicating a two-step or a four-step RAR type. The RAR length may be predetermined for each RAR type. A wireless device may determine a size of an RAR based on a determination of whether the RAR is a two-step RAR or a four-step RAR. 
     A wireless device may transmit, to a base station, a random access preamble via a random access channel in a subframe and using a frequency offset. A wireless device may determine an RA-RNTI based on one or more of a subframe number and/or a frequency index. A wireless device may monitor a control channel for a control packet associated with an RA-RNTI. A wireless device may receive a MAC PDU, associated with RA-RNTI, comprising: one or more MAC PDU subheaders, wherein a subheader may comprise an RAP identifier; one or more RARs, wherein each RAR of the one or more RARs may correspond to a MAC PDU subheader of the one or more MAC PDU sub-headers; and an uplink grant. The wireless device may transmit one or more transport blocks based on the uplink grant. 
     Example RAR formats are shown in  FIGS.  19 ,  23 , and  24   . An RAR may include one or more fields, such as a timing advance command, an uplink grant, a TC-RNTI, a C-RNTI, a wireless device (e.g., a UE) contention resolution identity, and/or other parameters. An RAR format may be determined based on the fields that are needed in the RAR. A present bit may be used for a field to indicate whether the field is included in the RAR. For example, a presence field may indicate whether or not an RAR includes an uplink grant. Other fields may be associated with a presence field. Multiple RAR types may include different fields that may be pre-defined. A field in the MAC subheader or in an RAR may determine the RAR type and a corresponding RAR length. A two bit field may indicate which of four or three RAR types are transmitted. Other fields, comprising any number of its, may be included in the MAC subheader and/or in an RAR to indicate information about an RA procedure. 
       FIG.  19    shows example RAR formats with a fixed size (e.g., 6 bytes) for various RA procedures (e.g., two-step and four-step RA procedures).  FIG.  23    parts (c) and (a) show example RAR formats with a fixed size (e.g., 6 bytes) for two-step and four-step RA procedures, respectively.  FIG.  23    part (b) shows an example RAR format with a fixed size (e.g., 8 bytes) for a two-step RA procedure.  FIG.  24    shows an example RAR format with a fixed size (e.g., 12 bytes) for two-step and four-step RA procedures. As shown in  FIG.  23    parts (a)-(c) and  FIG.  24   , an RAR format may comprise, e.g., one or more: reserved fields (e.g., “R”), timing advance command, uplink grant, temporary C-RNTI (TC-RNTI), and/or wireless device (e.g., UE) contention resolution identity. 
     A two-step RA procedure may comprise a hybrid automatic repeat request (HARQ), including, e.g., HARQ with soft combining, if a data decoding failure occurs. If a wireless device receives a MAC PDU that comprises a subheader with a RAP ID that matches or indicates the RAP that was transmitted by the wireless device, but a decoding failure indicator indicates a failure has occurred, the wireless device may perform HARQ. The wireless device may perform HARQ, e.g., by transmitting another redundancy version to the cell from which the wireless device received the MAC PDU. The HARQ transmission may occur at an a priori known subframe or time period, such as every eight subframes, after a prior HARQ transmission in the same HARQ process. The HARQ may predetermine a sequence of redundancy version numbers that the wireless device may transmit in a HARQ transmission in the same process. An RV number may start, e.g., from zero or one in an initial UL data transmission, and the next RV in the sequence may be transmitted if a wireless device determines that an RAR comprises an indicator requesting a next RV. 
       FIG.  25    shows an example of an RA procedure using HARQ retransmission. A HARQ retransmission may occur if a wireless device detects a RAPID in a cell but determines that the base station failed to decode data transmitted by the wireless device. At step  2501 , a wireless device may receive, from a base station and via a cell, RA configuration parameters. At step  2502 , the wireless device may determine an RAP selection, e.g., comprising one or more RAP selections procedures described herein. At step  2503 , the wireless device may transmit an RAP (e.g., comprising an RAP ID=xxx) and UL data (e.g., comprising a first RV=aa, and UE ID=yyy). At or near step  2503 , the wireless device may start or restart an RAR response timer. At step  2504 , the base station may transmit, and the wireless device may receive, a MAC PDU comprising a subheader (e.g., comprising RAP ID=xxx), such as a MAC subheader, with a TC-RNTI and a decoding failure indicator. Based on receiving the decoding failure indicator, the wireless device may stop the RAR response timer. At step  2505 , the wireless device may transmit a HARQ retransmission of the RAP and the UL data. The HARQ retransmission may comprise a second RV (e.g., RV=bb). The wireless device may transmit the HARQ retransmission based on a determination that a decoding failure has occurred (e.g., which may be indicated by the decoding failure indicator). At or near step  2505 , the wireless device may start a contention resolution timer. At step  2506 , the base station may transmit, to the wireless device, a NACK. The base station may transmit the NACK transmission based on receiving the HARQ retransmission. Based on receiving the NACK, the wireless device may stop the contention resolution timer. At step  2507 , the wireless device may transmit a second HARQ retransmission of the RAP and the UL data. The second HARQ retransmission may comprise a third RV (e.g., RV=cc). The wireless device may transmit the second HARQ retransmission based on the wireless device receiving the NACK. At or near step  2507 , the wireless device may start or restart a contention resolution timer. At step  2508 , the base station may transmit, to the wireless device, an ACK. The wireless device may receive the ACK. Based on receiving the ACK, the wireless device may determine that the second HARQ retransmission was successful. Based on receiving the ACK, the wireless device may stop the contention resolution timer. 
     For RA procedures using HARQ retransmission, each RV may be transmitted in an adaptive or in a non-adaptive manner. A base station may transmit, to a wireless device and via a cell, one or more indicators of a HARQ transmission type. For example, the base station may transmit, to the wireless device and via a downlink control channel, a new data indicator (NDI) with downlink control information (DCI). Additionally or alternatively, the base station may transmit, to the wireless device and via a downlink HARQ indicator channel, a one-bit HARQ acknowledgement (ACK) or non-acknowledgement (NACK). The wireless device may determine, based on the ACK or NACK, whether to transmit, to the base station and via the cell, another RV. If the wireless device detects an NDI that differs from a previously received NDI (e.g., an NDI bit has changed), the wireless device may, regardless of a HARQ ACK or NACK, transmit another RV specified in the DCI. The wireless device may transmit this RV using a resource and modulation and coding scheme (MCS) specified in the same DCI. If the wireless device detects a NDI non-toggled but receives a HARQ NACK message, the wireless device may transmit a predefined RV with the same resource and MCS as the previous HARQ transmission. 
     The maximum number of HARQ transmissions may be determined for a two-step RA procedure, e.g., by maxHARQ-Msg3Tx in LTE. A wireless device may have a counter for counting the number of HARQ transmissions. A wireless device may set the counter to one based on transmitting the first RV. The wireless device may increase the counter by one based on transmitting a next RV in the cell. If the counter reaches the maximum number of HARQ transmissions configured in a cell (or a retransmission threshold and/or a HARQ transmission threshold), a wireless device may determine that the two-step RA procedure has failed. If the wireless determines that the two-step RA procedure has failed, the wireless device may perform a new RA procedure on a different cell, the same RA procedure but on a different cell, or a new RA procedure on the same cell. The new RA procedure may comprise a two-step or a four-step RA procedure. 
       FIG.  26    shows an example of a two-step RA procedure failure as the number of HARQ retransmission reaches a threshold. Each of  FIG.  26    steps  2601  through  2607  correspond to  FIG.  25    steps  2501  through  2607 , respectively, the descriptions of which are incorporated by reference here for  FIG.  26    steps  2601  through  2607 . At step  2608 , the base station may transmit, and the wireless device may receive, a second NACK. Steps  2607  and  2608  may be repeated any number of times, wherein each HARQ retransmission may include a different RV, up to a final HARQ retransmission and NACK at steps  2609  and  2610 , respectively. The final HARQ retransmission may be based on the total number of HARQ retransmissions reaching a threshold value, such as indicated by maxHARQ-Msg3Tx. At step  2611 , if the threshold value for HARQ retransmissions is reached, and if a wireless device receives a NACK (e.g., at step  2610 ) and/or the contention resolution timer expires without the wireless device receiving an ACK, then the wireless device may determine that the RA procedure has failed. A base station may determine that an RA procedure has failed, e.g., if the base station is unsuccessful in decoding a threshold number of HARQ retransmissions, and/or if the base station does not receive a HARQ retransmission from the wireless device after a threshold period of time. 
     A wireless device may determine that a two-step RA procedure is successful, e.g., if, prior to the expiration of an RAR response timer, the wireless device receives a MAC PDU that comprises the same RAP ID and wireless device ID that a wireless device transmitted in the UL transmission. An RA procedure may be successful if a base station identifies the wireless device&#39;s transmitted RAP, decodes the wireless device&#39;s transmitted data, and transmits, to the wireless device and before the wireless device&#39;s RAR timer expires, a MAC PDU comprising the RAP ID and wireless device ID. A base station may identify an RAP ID based on a peak detector. The peak detector may detect a peak from correlation outputs between a received signal and a set of RAPs available to a cell. If the resource block, over which the data or portion thereof is transmitted during the UL transmission, is associated with an RAP, an RAP ID may also be detectable based on an energy detector. The energy detector may measure an energy level of the resource block for a UL data transmission. 
       FIG.  27    shows an example of a successful two-step RA procedure, e.g., wherein a base station decodes an RAP and UL data, and a base station responds by transmitting an RAR to a wireless device. Each of  FIG.  27    steps  2701  through  2703  correspond to  FIG.  20    steps  2001  through  2003 , respectively, the descriptions of which are incorporated by reference here for  FIG.  27    steps  2701  through  2703 . At or near step  2703 , the wireless device may start or restart an RAR response timer. At step  2704 , the base station may transmit, to the wireless device, a MAC PDU comprising one or more MAC subheaders and one of more RARs. At least one of the MAC subheaders in the MAC PDU may comprise a RAP ID (e.g., RAP ID=xxx) that may correspond to the RAP ID included in the RAP that was transmitted by the wireless device in step  2703 . At least one of the RARs in the MAC PDU may comprise an identifier associated with the wireless device (e.g., UE ID=yyy) that may correspond to the identifier transmitted by the wireless device in step  2703 . If the wireless device determines that the RAP ID included in at least one of the MAC subheaders corresponds to the wireless device&#39;s RAP IP; if the wireless device determines that an RAP, corresponding to the MAC subheader with the wireless device&#39;s RAP ID, comprises the wireless device&#39;s identifier; and if the RAR response time has not expired; then the wireless device may determine that the two-step RA procedure was successful. 
     A wireless device may transmit in parallel, to a base station and via a first cell, a random access preamble, and one or more transport blocks with a first RV associated with a HARQ process, wherein the one or more TBs comprise the wireless device ID. The wireless device may receive an RAR MAC PDU comprising one or more of: a preamble identifier; an uplink grant; a field indicating whether the one or more TBs are received successfully; and/or an RNTI. The wireless device may transmit, using uplink resources, the one or more TBs with a second RV different from the first RV associated with the HARQ process. The uplink resources may be identified in the uplink grant. The wireless device may receive a downlink packet comprising the wireless device ID, if the one or more TBs are decoded successfully. The wireless device may receive one or more messages comprising configuration parameters of a RACH of a first cell. 
     A wireless device may handover from a source base station to a target base station. A handover may occur, e.g., if the wireless device moves from a location in a first cell served by the source base station to a location in a second cell served by the target base station, if the source base station is unable to accommodate communications with the wireless device, if it is determined that a different base station is a better candidate for accommodating communications with the wireless device than the source base station, or for various other reasons. 
     A random access procedure may be optimized for a handover of a wireless device from a source base station to a target base station. Such optimization may provide a faster handover, with reduced latency, and may provide a handover without suffering from collisions with other transmissions. A wireless device may perform, with a target base station, a contention-based four-step random access procedure that may take, e.g., 20 ms, a contention-free three-step random access procedure that may take, e.g., 14 ms, and/or a contention-based two-step random access procedure that may take, e.g., 4 ms. By communicating random access information from a target base station to a wireless device (e.g., via a source base station), prior to performing random access, the wireless device may employ a contention-free two-step random access procedure that may take, e.g., 1 ms. A contention-free, two-step random access procedure for handover with the above advantages may be achieved by communicating random access information with handover messaging described below. 
     A source base station and a target base station for a possible handover may perform different operations for RA procedures to initiate communications with the wireless device. Additionally or alternatively, a wireless device that is a candidate for a handover may be capable or incapable of one or more types of RA procedures, and/or conditions in a cell of the source base station may differ from conditions in a cell of a target base station. For one or more reasons, prior to initiating a handover of a wireless device from a source base station (e.g., an eNB, a gNB, etc.) to a target base station (e.g., an eNB, a gNB, etc.), a device may determine a type of random access procedure to use to perform the handover. The type of RA procedure may include, e.g., a two-step RA procedure or a 4-step RA procedure. Additionally or alternative, the type of RA procedure may include, e.g., whether the RA procedure is contention-based (such as a contention-based four-step RA procedure described above regarding  FIG.  15    part (a)) or contention-free (such as a three-step contention-free RA procedure described above regarding  FIG.  15    part (b), or a two-step contention-free RA procedure available that may be used for handover). The device that may determine the type of the RA procedure may be a target base station, a source base station, and/or a core network device (e.g., located at a Next Generation Core (NGC) network entity). Additionally or alternatively, a wireless device may determine the type of the RA procedure, e.g., based on random access information received in one or more messages. As an example, a source base station may send to a target base station information about the type of RA procedure (e.g., a contention-free two-step RA procedure) to perform for a wireless device that may be a candidate for a handover from the source base station to a target base station. Information about the type of RA procedure may also be sent by the target base station to the source base station, and by the source base station to the wireless device. 
     A wireless device may receive random access information from a source base station for accessing a target cell. The random access information may comprise, e.g., an indication of a type of random access procedure, such as a two-step random access procedure or a four-step random access procedure, for the wireless device. The random access information may also comprise timing advance information for uplink transmissions and/or downlink transmissions for the wireless device in the target cell, such as for a random access preamble, a random access response, and/or one or more transport blocks. The timing advance information may indicate, e.g., whether the timing advance value is zero or not, and/or an estimated time advance value. Receiving the random access information from a source base station, as opposed to waiting to receive system information comprising random access information for a target base station, may enable a wireless device to reduce handover delay. The wireless device may perform a random access procedure as part of a handover to a target cell based on one or more elements of received random access information. 
       FIG.  28    shows an example of a random access procedure for handover, such as via a direct interface. At step  2810 , a wireless device  2801  (e.g., a UE, or any device capable of communicating with a network) may send, to a source base station  2802  (e.g., a source gNB), a first message comprising a measurement report for one or more cells. The measurement report may be based on one or more attributes of the one or more cells. The measurement report may comprise, e.g., at least one of a list of cells, an RSRP (Reference Signal Received Power) for a cell or for each of a plurality of cells included in a list of cells, an RSRQ (Reference Signal Received Quality) for a cell or for each of a plurality of cells in a list of cells, cell priority of one or more base stations including a cell priority of the target base station  2803 , or other information that may be used in a handover decision. The wireless device  2801  may generate the measurement report from measurements of neighboring cells, and the measurement report may indicate one or more base stations, of a plurality of neighboring base station, that may be candidates for a handover of the wireless device  2801  from the source base station  2802  based on power measurements for each base station. The wireless device  2801  may generate the measurement report using the procedures described below regarding  FIG.  30   . 
     The source base station  2802  may determine a handover decision at step  2820 . The handover decision may be based on the measurement report. The source base station  2802  may determine a target base station  2803 , from among a plurality of potential target base stations. The source base station  2802  may determine, based on one or more elements of the measurement report, whether or not to initiate a handover procedure for the wireless device  2801 . If the base station  2802  determines that a handover should be performed, the base station  2802  may send a second message, at step  2830 , to a target base station  2803  (e.g., a target gNB). The second message may comprise a handover request message. The second message may also comprise one or more of the measurement report from the first message, an identifier of the wireless device  2801 , a wireless device context (e.g., one or more bearers, one or more slices, subscriber information, security information, etc.), capability information of the wireless device  2801  (e.g., one or more types of RA procedures the wireless device  2801  is capable of performing), one or more cell identifiers of a cell or of a plurality of cells served by the target base station  2803 , one or more elements of the measurement report received from the wireless device  2801 , or other information that may be used for performing a handover. The source base station  2802  may perform the handover decision in step  2820 , and the source base station  2802  may receive and send the first and second messages in steps  2810  and  2830 , respectively, using the procedures described below regarding  FIG.  31   . 
     The target base station  2803  may receive the second message, at step  2830 . The target base station  2803  may process the handover request. The target base station  2803  may determine, based on the second message, whether or not to accept the handover request for the wireless device  2801 . The target base station  2803  may provide an acknowledgement of the handover request, e.g., by sending, at step  2840 , a third message to the source base station  2802 . The target base station  2803  may process the handover request, and the target base station  2803  may receive and send the second and third messages in steps  2830  and  2840 , respectively, using the procedures described below regarding  FIG.  32   . 
     The third message, that may be sent by the target base station  2803  to the source base station  2802 , may comprise a handover request acknowledgement and random access information, including, e.g., an indication of a type of a random access procedure. The handover request acknowledgement may comprise an ACK that may be in response to the handover request. The target base station  2803  may determine random access information for inclusion in the handover request acknowledgement. The target base station  2803  may send the third message based on determining to accept the handover request. The target base station  2803  may include in the third message a handover request acknowledge message as well as random access information. 
     In a handover procedure, the handover related information may be transmitted by an upper layer message, such as an RRC message. A type of a random access procedure, such as a two-step random access procedure or a four-step random access procedure, may be indicated, e.g., by one or more RACH type indication parameters in an RRC connection reconfiguration message. The RRC reconfiguration message may be included with a handover command. Additionally or alternatively, one or more RACH type indication parameters may comprise one or more indications of a capability (e.g., configured for a contention-free two-step RA procedure) or an incapability (e.g., not configured for a contention-free two-step RA procedure). With respect to one or more indications of incapability, one or more default RA procedures may be indicated in the random access information of the third message (e.g., instructing a wireless device to default to a contention-based four-step RA procedure). Mobility control information (e.g., mobilityControlInfo) in an RRC connection reconfiguration message (e.g., RRCConnectionReconfiguration) may comprise one or more of the RACH type indication parameters. A target NG-RAN node to a source NG-RAN node transparent container may comprise RRC parameters comprising a handover command and/or one or more of the RACH type indication parameters. The handover command may comprise, e.g., an RRC E-UTRA handover command message, such as handoverCommandMessage. Additionally or alternatively, a type of a random access procedure may be indicated by one or more parameters in a mobility control information. The mobility control information may further indicate one or more of: mobilityControlInfoV2X, handoverWithoutWT-Change, makeBeforeBreak, rach-Skip, or sameSFN-Indication. 
     The random access information may comprise, e.g., at least one of a first indication indicating that a random access procedure for handover execution should be performed with one of a two-step random access procedure or a four-step random access procedure, a second indication for performing a two-step random access process, a preamble identifier for a two-step random access procedure, and/or a preamble identifier for a four-step random access procedure. The preamble identifier for a two-step random access procedure and/or a four-step random access procedure may be used to determine radio resources, e.g., for channel access by the wireless device  2801 . The preamble identifier may comprise one or more of a preamble index, PRACH mask index, root sequence, and/or the preamble, each of which may be used by the wireless device  2801  for a random access procedure. The random access information may further comprise a random access channel configuration information for a two-step random access procedure and/or radio resource configuration parameters for a two-step random access procedure. Additionally or alternatively, the third message may comprise, e.g., one or more of: a handover request acknowledgment, one or more bearers admitted, one or more slices admitted, one or more cells served by the target base station  2803 , radio resource configuration information (e.g., RACH configuration, channel configuration information, etc.) for the wireless device  2801 , one or more types of random access procedures, and/or mobility control information for the wireless device  2801 . The third message may further comprise timing advance information, e.g., for transmission of uplink data. The timing advance information may comprise at least one of an indication of a zero timing advance and/or a first value for the timing advance. The first value for the timing advance may be estimated by, e.g., the source base station  2802 , the second base station  2803 , and/or another device, such as a core network device described further below regarding  FIG.  29   . 
     The second message and/or the third message may be exchanged directly, e.g., via a direct interface such as an Xn interface, between the source base station  2802  and the target base station  2803 . Additionally or alternatively, one or more elements of the second message and/or one or more elements of the third message may be exchanged indirectly, e.g., via one or more core network entities. 
     At step,  2840 , the source base station  2802  may receive the third message. The source base station  2802  may send, to the wireless device  2801  and based on the third message, a fourth message, at step  2850 . The fourth message may comprise, e.g., a handover command and random access information. The handover command may comprise a command for the wireless device  2801  to execute a handover towards a cell of the target base station  2803 . The handover command may be based on one or more elements of the third message. The fourth message may comprise some or all of the information in the third message. The fourth message may be an RRC connection reconfiguration message. The fourth message may comprise, e.g., information associated with one or more cells served by a respective one or more base stations of a plurality of base stations that may comprise the target base station. Additionally or alternatively, the fourth message may comprise radio resource configuration information (e.g., RACH configuration, channel configuration information, etc.) for the wireless device  2801 , and/or a mobility control information for the wireless device  2801 . One or more elements of the fourth message may be based on an acknowledgement received by the source base station from the target base station for the handover request. The fourth message may comprise an indication indicating that a random access for a handover execution is performed with one of a two-step random access procedure and/or a four-step random access procedure. The wireless device  2801  may receive and process the fourth message using the procedures described below regarding  FIG.  34   . 
     The wireless device  2801  may send, based on the random access information in the fourth message, a fifth message that may be part of a random access procedure with the target base station  2803 , at step  2860 . The wireless device  2801  may send a fifth message, e.g., comprising a random access preamble, to the target base station  2803 , e.g., via a cell associated with the target base station  2803 . The message comprising the random access preamble may be part of a random access procedure that may be performed with, e.g., two messages (e.g., for two-step random access) or four messages (e.g., for four-step random access). The random access preamble may be transmitted in a first step of one of a two-step random access procedure and/or a four-step random access procedure. Timing advance information may be included in the fourth message. Timing advance information may comprise at least one of an indication of a zero timing advance and/or a first value for a timing advance. The first value of the timing advance may be estimated by the source base station and/or the target base station. The fifth message may comprise, e.g., one or more transport blocks that may comprise at least one of a handover complete message (e.g., an RRC connection reconfiguration complete message), uplink data, a buffer status report, and/or an identifier of the wireless device (e.g., C-RNTI). The fourth message may comprise, e.g., a preamble identifier. Radio resources for transmissions may be determined by the preamble identifier. The fourth message may comprise a random access channel configuration for a two-step random access procedure and/or radio resource configuration parameters for a two-step random access procedure. The random access preamble may be transmitted in a first step of one of a two-step random access procedure or a four-step random access procedure. The transmitting of the random access preamble message may be based on one or more elements of the random access information included in the third message and/or the fourth message from steps  2840  and/or  2850 , respectively. The type of random access procedure may be, e.g., a two-step random access procedure or a four-step random access procedure described further herein. The type of random access procedure may be indicated by the random access information in the fourth message, including, e.g., in a handover command. The random access information in the fourth message may be the same as the random access information in the third message. Additionally or alternatively, the random access information in the fourth message may comprise additional information provided by the source base station  2802  and/or less than all of the random access information included in the third message. 
     In a two-step random access procedure, a first random access message may be transmitted by the wireless device  2801  to the target base station  2803 . The first random access message may comprise at least one of a random access preamble, a radio resource control message, and/or one or more uplink packets. A second random access message of a two-step random access procedure may be transmitted to the wireless device  2801  by the target base station  2803 , e.g., if the target base station  2803  receives the first random access message. The second random access message may comprise, e.g., an acknowledgement of a reception of the first random access message. The second random access message of the two-step random access procedure may further comprise a resource grant for uplink message transmission. 
     In a four-step random access procedure, a first random access message may be transmitted by the wireless device  2801  to the target base station  2803 , and the first random access message may comprise a random access preamble. A second random access message of a four-step random access procedure may be transmitted to the wireless device  2801  by the target base station  2803 , e.g., if the target base station  2803  receives the first message. The second random access message may comprise a resource grant for uplink message transmission. A third random access message of the four-step random access procedure may be transmitted by the wireless device  2801  to the target base station  2803 , at least based on the resource grant in the second random access message. The third random access message may comprise a radio resource control message and/or one or more uplink packets. A fourth random access message may be transmitted by the target base station  2803  to the wireless device  2801 , and the fourth random access message may comprise an acknowledgement of a reception of the third random access message. The fourth random access message of the four-step random access procedure may further comprise a resource grant for uplink message transmission. 
     After completing the random access procedure initiated by the wireless device  2801 , the target base station  2803  may send a sixth message to network device. The network device may comprise a core network entity. The sixth message may be transmitted to the network device, e.g., to update downlink packet destination information such as a tunnel endpoint identifier and/or an IP address and/or another address. The sixth message may comprise a path switch request message and/or a downlink packet destination information for one or more bearers for the wireless device  2801 . The network device may send a seventh message to the target base station  2803  that may be in response to the sixth message. The seventh message may comprise a path switch request acknowledge message. Additionally or alternatively, the seventh message may comprise an uplink packet destination information, such as a tunnel endpoint identifier and/or an IP address and/or another address, for one or more bearers for the wireless device  2801 , and/or a list of bearers to be released. 
     After completing a random access procedure initiated by the wireless device  2801 , the target base station  2803  may notify the source base station  2802  that the handover has completed. The target base station  2803  may notify the source base station  2802 , e.g., by sending an eighth message to the source base station  2802 . The eighth message may comprise a UE context release message and/or an identifier of the wireless device  2801 . The eighth message may be sent, e.g., via a direct interface between the target base station  2803  and the source base station  2802 . Additionally or alternatively, one or more elements of the eighth message may be sent to the source base station  2802  indirectly, e.g., via one or more core network entities. In response to receiving the eighth message and/or the one or more elements of the eighth message, the source base station  2802  may release a wireless device context for the wireless device  2801 . Any one or more of the above described messages may be exchanged among one or more communication nodes between one or more of the wireless device  2801 , the source base station  2802 , and/or the target base station  2803 . 
       FIG.  29    shows an example of a random access procedure for handover, e.g., using a core network device  2904 . The core network device  2904  may be located at a Next Generation Core (NGC) network entity that may be configured to communicate, e.g., via an NG interface, with one or both of the source base station  2902  and the target base station  2903 . One or more of a wireless device  2901 , a source base station  2902  (e.g., an eNB, a gNB, etc.), and/or a target base station  2903  (e.g., an eNB, a gNB, etc.), and operations thereof, may correspond to the wireless device  2801 , the source base station  2802 , and/or the target base station  2803 , respectively, described above regarding  FIG.  28   , descriptions of which are incorporated herein with respect to  FIG.  29   . The messages sent in steps  2810 ,  2830 ,  2840 ,  2850 , and  2860  of  FIG.  28    may correspond to messages sent in similarly numbered steps  2910 ,  2930 ,  2940 ,  2950 , and  2960 , respectively, of  FIG.  29   . The handover decision in step  2820  of  FIG.  28    may also correspond to the handover decision in step  2920  of  FIG.  29   . 
     The core network device  2904  may communicate with the source base station  2902  and the target base station  2903 . The core network device  2904  may comprise one or more devices located at a core network entity, such as an NGC, one or more devices at an access and mobility management function (AMF) network entity, one or more devices at a mobility management entity (MME), or one or more devices at any other location. The core network device  2904  may determine an availability of base stations, including a determining whether a target base station, such as the target base station  2903 , is available. The core network device  2904  may communicate with the source base station  2902  and the target base station  2903  via an NG interface and/or via an S1 interface (e.g., for an MME), and the core network device  2904  may operate as an intermediary for communications between the source base station  2902  and the target base station  2903 . Processing in the core network device  2904 , and potential latency in communicating information to and from the core network device  2904 , may not delay a handover when, e.g., some or all of such processes and communications may occur prior to a handover. 
     At step  2910  the wireless device  2901  may send, to a source base station  2902 , a first message comprising a measurement report for one or more cells, including, e.g., a cell for the target base station  2903 . The first message in step  2910  may comprise some or all of the information in the first message in step  2810  described above. The wireless device may generate the measurement report using the procedures described below regarding  FIG.  30   . 
     The source base station  2902  may receive the first message at step  2910 . At step  2920 , the source base station  2902  may make a handover decision. The handover decision may be based on the measurement report in the first message. The handover decision may also be based on a determination whether to use a core network device  2904  for a handover. As an example, if an Xn connection is established between the source base station  2902  and the target base station  2903 , the source base station  2902  may determine to send a second message, e.g., comprising a handover request, to the target base station  2903  without involving the core network device  2904 ; and if an Xn connection is not established between the source base station  2902  and the target base station  2903 , then the source base station  2902  may send a second message, e.g., comprising a handover required message, to the core network device  2904  at step  2925 . The second message may be based on the measurement report. The second message (e.g., a handover request message and/or a handover required message) may further comprise one or more measurements results of a measurement report (e.g., that may be provided by the wireless device step  2810 ) and/or one or more of: timing advance information for transmission of uplink data, a random access preamble, and/or random access information. The second message in step  2925  may comprise some or all of the information in the second message in step  2830  described above. The source base station  2902  may perform the handover decision in step  2920 , and receive and send the first and second messages in steps  2910  and  2925 , respectively, using the procedures described below regarding  FIG.  31   . 
     The core network device  2904  may receive the second message at step  2925 . The core network device  2904  may process the second message, including, e.g., to determine whether the wireless device may require latency sensitive service. The core network device  2904  may determine whether the wireless device may require latency sensitive service based on, e.g., packet data unit session information of the wireless device. At step  2930 , the core network device  2904  may send, to the target base station  2903 , a third message comprising a handover request. The third message in step  2930  may comprise some or all of the information in the second message in step  2830  described above. The core network device  2904  may receive and process the second message in step  2925 , and determine and send the third message in step  2930 , using the procedures described below regarding  FIG.  33   . 
     The target base station  2903  may receive the third message at step  2930 . The target base station  2903  may determine whether the handover request is acceptable. The target base station  2903  may process a handover request in a direct handover from a source base station  2902 , e.g., via an Xn interface, using one or more of the same procedures as the target base station  2903  uses to process a handover request from a core network device  2904 , e.g., via an NG interface. The target base station  2903  may also determine characteristics that may be relevant for a handover, such as power, latency sensitive service type, cell size, and/or availability of one or more types of random access procedures. Based on one or more determinations, at step  2940 , the target base station  2903  may send, to the core network device  2904 , a fourth message comprising a handover request acknowledgement and random access information. The fourth message in step  2940  may comprise some or all of the information in the third message in step  2840  described above. The target base station  2903  may receive and process the third message in step  2930 , and determine and send the fourth message in step  2940 , using the procedures described below regarding  FIG.  32   . 
     The core network device  2904  may receive the fourth message at step  2940 . The core network device  2904  may process the fourth message to generate a fifth message comprising a handover request acknowledgement and random access information. At step  2945 , the core network device  2904  may send the fourth message to the source base station  2902 . The fifth message in step  2945  may comprise some or all of the information in the third message in step  2840  described above. The core network device  2904  may receive and process the fourth message in step  2940 , and determine and send the fifth message in step  2945 , using the procedures described below regarding  FIG.  33   . 
     The source base station  2902  may receive the fifth message at step  2945 . The source base station  2902  may process the fifth message to determine a sixth message, e.g., comprising a handover command and random access information. At step  2950 , the source base station may send, to the wireless device  2901 , the sixth message. The sixth message in step  2950  may comprise some or all of the information in the fourth message in step  2850  described above. The source base station  2902  may receive and process the fifth message in step  2945 , and determine and send the sixth message in step  2950 , using procedures described below regarding  FIG.  31   . 
     The wireless device  2901  may receive, from the source base station  2902 , the sixth message at step  2950 . The sixth message may comprise a handover command and random access information. The handover command may comprise a command for a handover a cell of the source base station  2902  to a cell of the target base station  2903 . At step  2960 , the wireless device  2901  may obtain channel access by proceeding with a random access procedure with the target base station  2903 . The wireless device  2901  may initiate the random access procedure by sending, to the target base station  2903 , a seventh message comprising a random access preamble and data. The wireless device may transmit, to the target base station  2903 , one or more transport blocks via radio resources of the cell of the target base station  2903 , wherein the radio resources may be determined at least based on one or more elements of the sixth message. The transmitting, by the wireless device  2901 , may further comprise transmitting a random access preamble overlapping in time with transmission of the one or more transport blocks. The sixth message may comprise, e.g., an indication for performing a two-step random access procedure, and/or a preamble identifier for a two-step random access procedure. The seventh message may comprise some or all of the information in step  2003  of the two-step random access procedure described above regarding  FIG.  20   . The wireless device  2901  may receive and process the sixth message in step  2950 , and determine and send the seventh message in step  2960 , using the procedures described below regarding  FIG.  34   . 
       FIG.  30    shows an example of handover procedures  3000  for a wireless device. The procedures may start with step  3001  during which a wireless device, such as the wireless device  2801  and/or the wireless device  2901 , measures target cell quality and serving cell quality. The wireless device may also measure cell quality of one or more neighboring cells that may become a target cell. The wireless device may make measurements to determine power of transmissions from one or more base stations in a respective one or more neighboring cells. The wireless device may also measure the power of transmissions by a source base station, such as the source base station  2802  and/or the source base station  2902 , that may be serving the wireless device. At step  3002 , the wireless device may determine whether one or more measurement events have occurred. A measurement event may be an indication that a target cell quality may exceed a serving cell quality and/or may exceed a serving cell quality by an offset value. A measurement event may comprise, e.g., one or more of a determination that an RSRP and/or an RSRQ of a target cell (e.g., a cell of a target base station) is greater than a first threshold; a determination that an RSRP and/or an RSRQ of a source cell (e.g., a cell of a source base station) is less than a second threshold; a determination that an RSRP and/or an RSRQ of a target cell exceeds a an RSRP and/or RSRQ of a source cell and/or exceeds an RSRP and/or RSRQ of a source cell by an offset power value; and/or a determination that a time duration of maintaining a condition, such as one or more of the above conditions, is greater than or equal to a time threshold value. If the wireless device determines that one or more measurement events have not occurred, the wireless device may return to step  3001 . If the wireless device determines that one or more measurement events have occurred, then the wireless device may send a measurement report to the source base station at step  3003 , such as described above regarding step  2810  in  FIG.  28    and/or step  2910  in  FIG.  29   . The wireless device may receive a handover command and RA information from a source base station at step  3004 , such as described above regarding step  2850  in  FIG.  28    and/or step  2950  in  FIG.  29   . The handover command and/or RA information may comprise an indication of a type of RA procedure that the wireless device is to perform for a handover to a target base station. The wireless device may proceed with a RA procedure for channel access with the target base station. The RA procedure may be based on the handover command and RA information received from the source base station. The process  3000  may end after step  3005 , or the process  3000  may repeat after a time period or after an event that may trigger a measurement of target cell quality and/or serving cell quality, such as one or more indications of a transmission failure and/or one or more indications of a neighbor cell transmission. 
     One or more additional steps (not shown) may also be performed by the wireless device, in between, before, or after, any of steps  3001 - 3005 , e.g., to determine whether to initiate a handover, and/or to determine a type of RA procedure to use. Such additional steps may include power measurements, distance measurements, traffic measurements, moving speed of the wireless devices, determination of other cells available to the wireless device, and any other network condition that may be used to determine a handover and/or a type of RA procedure. 
       FIG.  31    shows an example of handover procedures  3100  for a source base station. The procedures may start with step  3101  during which a source base station, such as the source base station  2802  and/or the source base station  2902 , receives a measurement report from a wireless device. Based on the measurement report, the source base station may determine, at step  3102 , whether one or more handover triggering condition have been satisfied. The one or more handover triggering conditions may comprise some or all of the measurement events described above in step  3002  of  FIG.  30   . Additionally or alternatively, the one or more handover triggering conditions may comprise a determination that an RSRP and/or RSRQ of a target cell indicates better performance for a wireless device than an RSRP and/or RSRQ of other candidate target cells; a determination that a moving speed of a wireless device is slower than a threshold speed value; and/or a cell identifier of a target cell is, or is not, among one or more cells in a handover restriction. If the source base station determines that one or more handover triggering conditions have not occurred, the source base station may end the process  3100 . If the source base station determines that one or more handover triggering conditions have occurred, then the source base station may proceed to step  3103 . 
     The source base station may determine, at step  3103 , whether to proceed with handover procedures via direct communications (e.g., employing an Xn interface) with a target base station, or via indirect communications with a target base station, e.g., using a core network device such as  2904  as an intermediary. The source base station may base its determination of direct or indirect communications with the target base station on the type of connection it may have with the target base station. If the source base station and the target base station are capable of sufficiently timely direct communications with each other, such as via an Xn interface, then the source base station may determine to proceed with direct communications with the target base station, e.g., via steps  3104  and  3105 , and/or via the process described above regarding  FIG.  28   . If the source base station and the target base station are not capable of sufficiently timely direct communications with each other, such as via an Xn interface (e.g., if these two base stations only communicate with each other via an X2 interface or if an X2 interface between these two base stations is not operating sufficiently to accommodate sufficiently timely communications), then the source base station may determine to proceed with indirect communications with the target base station, e.g., via steps  3106  and  3107 , and/or via the process described above regarding  FIG.  29   . 
     At step  3104 , a source base station may send a handover request message to a target base station, as described above regarding step  2830 . At step  3105 , the source base station may receive a handover request acknowledgement and RA information, as described above regarding step  2840 . At step  3106 , a source base station may send a handover required message to a core network device, such as core network device  2904 , as described above regarding step  2925 . At step  3107 , the source base station may receive a handover required acknowledgement and RA information, as described above regarding step  2945 . After step  3105  and/or after step  3107 , the process may continue to step  3108 . At step  3108 , the source base station may process the handover request acknowledgement and/or RA information, and the source base station may send a handover command and RA information to the wireless device for handover to the target base station, as described above regarding step  2850 . The process  3100  may end after step  3108 . 
     One or more of steps  3102 - 3103  may be omitted, or performed before or after other steps. Additional steps (not shown) may also be performed by the source base station, in between, before, or after, any of steps  3102 - 3103 , e.g., to determine whether to initiate a handover of the wireless device, to determine whether to send one or more handover-related messages to a target base station and/or to a core network device, and/or to determine a type of RA procedure. Such additional steps may include power measurements, distance measurements, service type measurements (e.g., slice type, packet data unit session type, QoS flow type, bearer type, and/or QoS requirement), traffic measurements, moving speed of the wireless devices, determination of other cells available to the wireless device, and any other network condition that may be used to determine a handover and/or a type of RA procedure. 
       FIG.  32    shows an example of handover procedures  3200  for a target base station. The procedures may start with step  3201  during which a target base station, such as the target base station  2803  and/or the target base station  2903 , receives a handover request message for a wireless device. Based on the handover request message, the target base station may determine, at step  3202 , whether the handover request is acceptable. The determination of whether the handover request is acceptable may be based on one or more of: whether radio resources are available for at least a default packet data unit session of the wireless device; whether a particular one or more service type are supported at the target cell; whether an RSRP and/or an RSRQ satisfies a threshold; whether a moving speed of the wireless device is below a speed threshold; and/or whether a cause for the handover is reasonable, such as to provide emergency service or to reduce traffic load of a source cell (e.g., if an RSRP and/or RSRQ does not satisfy a threshold). If a determination is made that the handover request is not acceptable, then the process may continue to step  3203  and the target base station may send a handover failure message, after which the process  3200  may end. If a determination is made that the handover request is acceptable, then the process may continue to step  3204 . 
     At step  3204 , the target base station may determine whether RSRP exceeds a power threshold. If RSRP exceeds a power threshold, the process may continue to step  3205 , in which case the target base station may send a handover request acknowledgement message indicating a four-step random access procedure for handover. A four-step random access procedure may use less power per transmission than a two-step random access procedure, e.g., because a two-step random access procedure may transmit more information in each step relative to a four-step random access procedure. If RSRP does not exceed a power threshold, the process may continue to step  3206 . 
     At step  3206 , the target base station may determine whether the wireless device requires latency sensitive service, such as for emergency or other time-sensitive messages. If the target base station determines that the wireless device does not require latency sensitive service, the target base station may determine, at step  3207 , whether the size of the handover target cell is small, or below a threshold size (e.g., a time alignment may less (or not) affect communication between the target base station and the wireless device). If the target base station determines that the size of the target cell is below a threshold size, the target base station may determine, at step  3208 , whether the wireless device supports a two-step RA procedure (e.g., based on UE capability information of the wireless device in the handover request message). If the target base station determines that the wireless device supports a two-step RA procedure, the target base station may determine, at step  3209 , whether one or more random access resources for two-step RA are available at the target cell. If the target base station determines that each condition in steps  3204 - 3209  is satisfied, the target base station may send, at step  3210 , a handover request acknowledgement message (e.g., to the source base station  2802  in step  2840   FIG.  28    and/or to the core network device  2904  in step  2940   FIG.  29   ) indicating a two-step RA procedure for the handover. If the target base station determines any one or more of the conditions in steps  3204 - 3209  are not satisfied, the target base station may send, at step  3205 , a handover request acknowledgement (e.g., to the source base station  2802  in step  2840   FIG.  28    and/or to the core network device  2904  in step  2940   FIG.  29   ) indicating a four-step RA procedure for the handover. After sending a handover request acknowledgement message, in step  3205  or in step  3210 , the target base station may proceed with a RA procedure (e.g., as in step  2860  in  FIG.  28    and/or as in step  2960  in FIG.  29 ) with the wireless device for channel access, in step  3211  (e.g., by receiving a corresponding RA preamble from the wireless device). The process  3200  may end after step  3211 . 
     One or more of steps  3204 - 3209  may be omitted, or performed before or after any of the other steps  3204 - 3209 . Additional steps (not shown) may also be performed by the target base station, in between, before, or after, any of steps  3204 - 3209 , to determine whether to indicate, in a handover request acknowledgement, a four-step RA procedure or a two-step RA procedure. Such additional steps may include power measurements, distance measurements, traffic measurements, moving speed of the wireless devices, determination of other cells available to the wireless device, and any other network condition that may be used to determine a handover and/or a type of RA procedure. 
       FIG.  33    shows an example of handover procedures  3300  for a core network device. The procedures may start with step  3301  during which a core network device, such as the core network device  2904  in  FIG.  29   , receives a handover required message from a source base station (e.g., a message in step  2925  in  FIG.  29   ). At step  3302 , the core network device may determine whether the wireless device requires latency sensitive service, such as for emergency or other time-sensitive messages. The core network device  2904  may determine whether the wireless device requires latency sensitive service based on, e.g., packet data unit session information of the wireless device. If latency services are not required, the process may continue to step  3304 . If latency services are required by the wireless device, then at step  3303 , the core network device may include a latency sensitive service indication in a handover request message, before continuing to step  3304 . At step  3304 , the core network device may send a handover request message to a target base station (e.g., as in step  2930  in  FIG.  29   ). At step  3305 , the core network device may receive a handover request acknowledgement and random access information from the target base station (e.g., as in step  2940  in  FIG.  29   ). At step  3306 , the core network device may send a handover required acknowledgement and random access information to the source base station (e.g., as in step  2945 ). The process  3300  may end after step  3306 . 
     One or more additional steps (not shown) may also be performed by the core network device, in between, before, or after, any of steps  3301 - 3305 , e.g., to determine whether to send one or more handover messages, to determine what to include in one or more handover messages, and/or to determine a type of RA procedure to indicate in one or more handover messages. Such additional steps may include power measurements, distance measurements, traffic measurements, moving speed of the wireless devices, determination of other cells available to the wireless device, and any other network condition that may be used to determine a handover and/or a type of RA procedure. 
       FIG.  34    shows an example of handover procedures  3400  for a wireless device. The procedures may start with step  3401  during which a wireless device, such as the wireless device  2801  and or the wireless device  2901 , receives a handover command message from a source base station (e.g., as in step  2850  in  FIG.  28    and/or as in step  2950  in  FIG.  29   ). The handover command message may comprise random access information. At step  3402 , the wireless device may determine, based on the random access information, whether the handover command message indicates a two-step random access procedure. If the wireless device determines that a two-step random access procedure is indicated, the wireless device, at step  3404 , may send to a target base station a random access preamble for a two-step RA procedure. If the wireless device determines that a two-step random access procedure is not indicated in the handover command message, then the wireless device, at step  3403 , may send to the target base station a random access preamble for a four-step RA procedure. After sending a random access preamble, the wireless device may continue with the particular random access procedure (e.g., a two-step RA procedure or a four-step RA procedure) at step  3405  (e.g., as in step  2860  in  FIG.  28    and/or as in step  2960  in  FIG.  29   ). The process  3400  may end after step  3405 . 
     One or more additional steps (not shown) may also be performed by the wireless device, in between, before, or after, any of steps  3401 - 3405 , e.g., to determine whether to proceed with a handover, to determine one or more RA transmissions, and/or to determine a type of RA procedure to use. Such additional steps may include power measurements, distance measurements, traffic measurements, moving speed of the wireless devices, determination of other cells available to the wireless device, and any other network condition that may be used to determine a handover and/or a type of RA procedure. 
     A wireless device may perform a method for determining a type of random access procedure to use for a handover from a source base station to a target base station. The wireless device may receive, from one or more cells, a plurality of messages comprising one or more of an indication of a power associated with a cell, an indication of a quality associated with a cell, and/or an indication of a cell priority. The wireless device may determine, based on the plurality of messages, a measurement report. The wireless device may transmit, to a first base station, a first message comprising a measurement report based on one or more second cells. The wireless device may receive, from the first base station, a second message comprising a command for a handover from the first cell to a second cell of the one or more second cells. The first cell may be a source cell for the wireless device, and the second cell may be a target cell. The second message may comprise one or more parameters indicating a type of a random access procedure for the handover. Additionally or alternatively, the second message may comprise timing advance information, e.g., for transmission of one or more uplink data and/or a random access preamble. The timing advance information may comprise one or more of an indication of a zero timing advance, or a value for a timing advance. The value for the timing advance may be estimated by the first base station and/or the second base station. One or more elements of the second message may be based on an acknowledgement received by the first base station, from the second base station, for the handover request. For a two-step random access procedure, the second message may comprise one or more of a preamble identifier for the RA procedure, random access channel configuration information for the RA procedure, random access resource information for the RA procedure, or radio resource configuration parameters. The wireless device may determine, based on the preamble identifier, radio resources to use for the RA procedure. For a four-step random access procedure, the second message may comprise a root sequence that the wireless may use to generate a random access preamble that the wireless device may transmit in a first step of the four-step RA procedure. The type of the random access procedure may comprise a two-step random access procedure or a four-step random access procedure. The wireless device may transmit, to the second base station, a random access preamble employing the type of random access procedure indicated in the second message. The wireless device may transmit, to the second base station, one or more transport blocks. The one or more transport blocks may comprise one or more of a handover complete message, uplink data, a buffer status report, or an identifier of the wireless device. The wireless device may transmit, to the second base station, a random access preamble overlapping in time with the one or more transport blocks. The wireless device may transmit, in a first step of a four-step random access procedure, a second random access preamble. A wireless device may perform any combination of one or more of the above steps. A base station, or any other device, may perform any combination of a step, or a complementary step, of one or more of the above steps. 
     A first base station may perform a method for indicating a type of random access procedure in a handover from the first base station (e.g., a source base station) to a second base station (e.g., a target base station). The first base station may receive, from a wireless device, a first message comprising a measurement report based on one or more second cells. The first base station may be associated with a first cell. The first base station may determine, based on the measurement report, to handover the wireless device from the first cell to a second cell of the one or more second cells. The first cell may be a source cell for the wireless device, and the second cell may be a target cell. The first base station may transmit, to a third device, a second message comprising an indication for the handover. The third device may comprise the second base station that may be associated with the second cell. Additionally or alternatively, the third device may comprise a core network device. The third device may determine the second cell from a plurality of cells. The second message may comprise a handover request and/or an indication that a handover is requested. The first base station may receive, from the third device, a third message indicating a type of a random access procedure for the handover. The type of the random access procedure may comprise a two-step random access procedure or a four-step random access procedure. The first base station may transmit, to the wireless device, a fourth message. The fourth message may comprise one or more parameters indicating the type of the random access procedure for the handover and/or the fourth message may comprise a command to proceed with the handover. The fourth message may comprise timing advance information, e.g., for transmission of one or more uplink data or a random access preamble. The timing advance information may comprise one or more of an indication of a zero timing advance, or a value for a timing advance. The value for the timing advance may be estimated by the first base station and/or the third device. For a two-step random access procedure, the fourth message may comprise a preamble identifier for the RA procedure. The third device may transmit, to a second base station and based on the second message, a fifth message comprising a handover request. The third device may receive, from the second base station, a sixth message indicating the type of the random access procedure for the handover and comprising an acknowledgement of the handover request. One or more base stations (e.g., a source base station, a target base station, etc.) may perform any combination of one or more of the above steps. A wireless device, a core network device, or any other device, may perform any combination of a step, or a complementary step, of one or more of the above steps. 
     A network device may perform a method for indicating a type of random access procedure in a handover from a first base station (e.g., a source base station  2902 ) to a second base station (e.g., a target base station  2903 ). The network device may comprise a core network device (e.g., core network device  2904 ). Additionally or alternatively, the network device may comprise the second base station. The network device may receive, from a first base station, a first message comprising an indication for a handover of a wireless device from a first cell associated with the first base station to a second cell of one or more second cells. The second base station may be associated with the second cell. The first cell may be a source cell for the wireless device, and the second cell may be a target cell. The network device may determine the second cell from a plurality of cells. The network device may determine, based on the first message, to proceed with the handover. The network device may transmit, to the first base station a second message. The second message may comprise one or more of an acknowledgement for the handover, a handover request, an indication that a handover is required, or an indication of a type of a random access procedure for the handover. The type of the random access procedure may comprise a two-step random access procedure or a four-step random access procedure. The network device may transmit, to a second base station and based on the first message, a third message comprising a handover request. The network device may receive, from the second base station, a fourth message. The fourth message may indicate the type of the random access procedure for the handover. The fourth message may comprise an acknowledgement of the handover request. One or more network devices may perform any combination of one or more of the above steps. A wireless device, a base station, or any other device, may perform any combination of a step, or a complementary step, of one or more of the above steps. 
     One or more of the messages described herein may be exchanged among one or more devices at a communication node in a network. One or more of the processes described herein may be performed in one or more devices in a network. For example, if an interface between a target base station (e.g., the target base station  2903 ) and a source base station (e.g., the source base station  2902 ) cannot accommodate sufficiently timely communications, such as if the interface between these base stations is not an Xn interface of if an X2 interface between them is not operative, a core network device (e.g., the core network device  2904 ) may be used to perform handover processes and send and receive handover related messages described herein with respect to a target base station and/or a source base station. As another example, if a wireless device (e.g., the wireless device  2801  and/or the wireless device  2901 ) is a fast moving device, a source base station (e.g., the source base station  2802  and/or the source base station  2902 ) may be used to perform handover processes and send and receive handover related messages described herein with respect to a target base station and/or a core network device. As another example, if a wireless device (e.g., the wireless device  2801  and/or the wireless device  2901 ) has latency sensitive information, such as emergency information, and if a source base station cannot accommodate the wireless device transmissions and cannot perform a handover decision in a sufficiently timely manner, the wireless device may perform handover processes and send and receive handover related messages described herein with respect to a source base station, a target base station, and/or a core network device. 
     Many of the elements in examples may be implemented as modules. A module may be an isolatable element that performs a defined function and has a defined interface to other elements. The modules may be implemented in hardware, software in combination with hardware, firmware, wetware (i.e., hardware with a biological element) or a combination thereof, all of which may be behaviorally equivalent. For example, modules may be implemented as a software routine written in a computer language configured to be executed by a hardware machine (such as C, C++, Fortran, Java, Basic, Matlab or the like) or a modeling/simulation program such as Simulink, Stateflow, GNU Octave, or LabVIEWMathScript. Additionally or alternatively, it may be possible to implement modules using physical hardware that incorporates discrete or programmable analog, digital and/or quantum hardware. Examples of programmable hardware may comprise: computers, microcontrollers, microprocessors, application-specific integrated circuits (ASICs); field programmable gate arrays (FPGAs); and complex programmable logic devices (CPLDs). Computers, microcontrollers, and microprocessors may be programmed using languages such as assembly, C, C++ or the like. FPGAs, ASICs, and CPLDs may be programmed using hardware description languages (HDL), such as VHSIC hardware description language (VHDL) or Verilog, that may configure connections between internal hardware modules with lesser functionality on a programmable device. The above mentioned technologies may be used in combination to achieve the result of a functional module. 
     Systems, apparatuses, and methods may perform operations of multi-carrier communications described herein. Additionally or alternatively, a non-transitory tangible computer readable media may comprise instructions executable by one or more processors configured to cause operations of multi-carrier communications described herein. An article of manufacture may comprise a non-transitory tangible computer readable machine-accessible medium having instructions encoded thereon for enabling programmable hardware to cause a device (e.g., a wireless communicator, a UE, a base station, and the like) to enable operation of multi-carrier communications described herein. The device, or one or more devices such as in a system, may include one or more processors, memory, interfaces, and/or the like. Other examples may comprise communication networks comprising devices such as base stations, wireless devices or user equipment (UE), servers, switches, antennas, and/or the like. Any device (e.g., a wireless device, a base station, or any other device) or combination of devices may be used to perform any combination of one or more of steps described herein, including, e.g., any complementary step or steps of one or more of the above steps. 
     Although examples are described above, features and/or steps of those examples may be combined, divided, omitted, rearranged, revised, and/or augmented in any desired manner Various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this description, though not expressly stated herein, and are intended to be within the spirit and scope of the disclosure. Accordingly, the foregoing description is by way of example only, and is not limiting.