Patent Publication Number: US-11665694-B2

Title: Method and apparatus for enhanced contention based random access procedure

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation application of prior application Ser. No. 15/722,629, filed on Oct. 2, 2017, which has issued as U.S. Pat. No. 10,917,879 on Feb. 9, 2021 and is based on and claims priority under 35 U.S.C. § 119(e) of a U.S. provisional application filed on Oct. 7, 2016 in the U.S. Patent and Trademark Office and assigned Ser. No. 62/405,555, the entire disclosure of which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a method and an apparatus for a contention based random access (RA) procedure. More particularly, the present disclosure relates to a method and an apparatus for performing a two-step contention based RA procedure and fall back between two-step and four-step contention based RA procedures. 
     BACKGROUND 
     To meet the demand for wireless data traffic having increased since deployment of fourth generation (4G) communication systems, efforts have been made to develop an improved fifth generation (5G) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘beyond 4G network’ or a ‘post long term evolution (LTE) system’. The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, beamforming, massive multiple-input multiple-output (MIMO), full dimensional MIMO (FD-MIMO), array antenna, analog beam forming, and large scale antenna techniques are discussed in 5G communication systems. In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud radio access networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, coordinated multi-points (CoMP), reception-end interference cancellation and the like. In the 5G system, hybrid frequency shift keying (FSK) and quadrature amplitude modulation (QAM) modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed. 
     In the recent years several broadband wireless technologies have been developed to meet the growing number of broadband subscribers and to provide more and better applications and services. The second generation (2G) wireless communication system has been developed to provide voice services while ensuring the mobility of users. The third generation (3G) wireless communication system supports not only the voice service but also data service. The 4G wireless communication system has been developed to provide high-speed data service. However, the 4G wireless communication system currently suffers from lack of resources to meet the growing demand for high speed data services. Therefore, the 5G wireless communication system is being developed to meet the growing demand of various services with diverse requirements, e.g., high speed data services, ultra-reliability and low latency applications and massive machine type communication. 
     In existing wireless communication system, such as LTE, random access (RA) procedure is used to achieve uplink (UL) time synchronization. RA procedure is used in LTE during initial access, handover, radio resource control (RRC) connection re-establishment procedure, positioning purpose, scheduling request transmission, secondary cell group (SCG) addition/modification and data or control information transmission in UL by non-synchronized user equipment (UE) in RRC_CONNECTED state. In LTE two types of RA procedure are defined: contention-based and contention-free. 
     A procedure for idle to connected state transition using contention-based RA (CBRA) procedure leads to a total delay of 29.5 transmit time intervals (TTIs). However, for the next generation radio access technology the idle to connected transition delay requirement is 10 ms. Therefore, a need exits for enhancing the CBRA procedure. 
     The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure. 
     The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure. 
     SUMMARY 
     Aspects of the present disclosure are to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present disclosure is to provide a communication method and system for converging a fifth generation (5G) communication system for supporting higher data rates beyond a fourth generation (4G) system. 
     In accordance with an aspect of the present disclosure, a method of a user equipment (UE) for performing a random access (RA) procedure is provided. The method includes transmitting a first message including a RA preamble and a UE identifier (ID) to a base station (BS), and receiving a second message including a sequence index of the RA preamble from the BS. 
     In accordance with another aspect of the present disclosure, a UE for performing a RA procedure is provided. The UE includes a transceiver and a controller. The transceiver is configured to receive signals from a BS, and transmit signals to the BS. The controller is configured to control the transceiver to transmit a first message including a RA preamble and a UE ID to the BS, and control the transceiver to receive a second message including a sequence index of the RA preamble from the BS. 
     In accordance with another aspect of the present disclosure, a method of a BS for performing a RA procedure is provided. The method includes receiving a first message including a RA preamble and a UE ID from a UE, and transmitting a second message including a sequence index of the RA preamble to the UE. 
     In accordance with another aspect of the present disclosure, a BS for performing a RA procedure is provided. The BS includes a transceiver and a controller. The transceiver is configured to receive signals from a UE, and transmit signals to the UE. The controller is configured to control to the transceiver to receive a first message including a RA preamble and a UE ID from the UE, and control to the transceiver to transmit a second message including a sequence index of the RA preamble to the UE. 
     Other aspects, advantages, and salient features of the disclosure will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses various embodiments of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of certain embodiments of the present disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    illustrates a contention-based random access (CBRA) procedure; 
         FIG.  2    illustrates a contention-free random access (CFRA) procedure; 
         FIG.  3    illustrates a procedure for idle to connected state transition using a CBRA procedure; 
         FIG.  4    illustrates a two-step contention based random access (RA) procedure according to an embodiment of the present disclosure; 
         FIG.  5    illustrates an alternative two-step contention based RA procedure according to an embodiment of the present disclosure; 
         FIGS.  6 A and  6 B  illustrate enhanced physical random access channel (PRACH) slot/resource formats for performing a two-step RA procedure according to various embodiments of the present disclosure; 
         FIG.  6 C  illustrates an enhanced PRACH slot/resource format for performing a two-step RA procedure according to an embodiment of the present disclosure; 
         FIG.  6 D  illustrates an enhanced PRACH slot/resource format for performing a two-step RA procedure according to an embodiment of the present disclosure; 
         FIG.  6 E  illustrates an enhanced PRACH slot/resource format for performing a two-step RA procedure according to an embodiment of the present disclosure; 
         FIG.  7    illustrates a two-step to four-step fall back method for a RA according to an embodiment of the present disclosure; 
         FIG.  8    illustrates an two-step to four-step fall back method for a RA according to an embodiment of the present disclosure; 
         FIG.  9    illustrates an two-step to four-step fall back method for a random access according to an embodiment of the present disclosure; 
         FIG.  10    illustrates an two-step to four-step fall back method for a RA according to an embodiment of the present disclosure; 
         FIG.  11    illustrates an RA procedure when a user equipment (UE) does not receive an RA response (RAR) after transmitting RA preamble and additional information according to an embodiment of the present disclosure; 
         FIG.  12    is a block diagram of a UE for performing an RA procedure according to an embodiment of the present disclosure; and 
         FIG.  13    is a block diagram of a base station (BS) for performing an RA procedure according to an embodiment of the present disclosure. 
     
    
    
     Throughout the drawings, like reference numerals will be understood to refer to like parts, components, and structures. 
     DETAILED DESCRIPTION 
     The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of various embodiments of the present disclosure as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the various embodiments described herein can be made without departing from the scope and spirit of the present disclosure. In addition, descriptions of well-known functions and constructions may be omitted for clarity and conciseness. 
     The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the present disclosure. Accordingly, it should be apparent to those skilled in the art that the following description of various embodiments of the present disclosure is provided for illustration purpose only and not for the purpose of limiting the present disclosure as defined by the appended claims and their equivalents. 
     It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces. 
     By the term “substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide. 
     It is known to those skilled in the art that blocks of a flowchart (or sequence diagram) and a combination of flowcharts may be represented and executed by computer program instructions. These computer program instructions may be loaded on a processor of a general purpose computer, special purpose computer, or programmable data processing equipment. When the loaded program instructions are executed by the processor, they create a means for carrying out functions described in the flowchart. Because the computer program instructions may be stored in a computer readable memory that is usable in a specialized computer or a programmable data processing equipment, it is also possible to create articles of manufacture that carry out functions described in the flowchart. Because the computer program instructions may be loaded on a computer or a programmable data processing equipment, when executed as processes, they may carry out steps of functions described in the flowchart. 
     A block of a flowchart may correspond to a module, a segment, or a code containing one or more executable instructions implementing one or more logical functions, or may correspond to a part thereof. In some cases, functions described by blocks may be executed in an order different from the listed order. For example, two blocks listed in sequence may be executed at the same time or executed in reverse order. 
     In this description, the words “unit”, “module” or the like may refer to a software component or hardware component, such as, for example, a field-programmable gate array (FPGA) or an application-specific integrated circuit (ASIC) capable of carrying out a function or an operation. However, a “unit”, or the like, is not limited to hardware or software. A unit, or the like, may be configured so as to reside in an addressable storage medium or to drive one or more processors. Units, or the like, may refer to software components, object-oriented software components, class components, task components, processes, functions, attributes, procedures, subroutines, program code segments, drivers, firmware, microcode, circuits, data, databases, data structures, tables, arrays or variables. A function provided by a component and unit may be a combination of smaller components and units, and may be combined with others to compose larger components and units. Components and units may be configured to drive a device or one or more processors in a secure multimedia card. 
       FIG.  1    illustrates a contention-based random access (CBRA) procedure. 
     Referring to  FIG.  1   , a user equipment (UE) transmits a random access (RA) preamble to evolved node B (eNB) at operation  110 . 
     The UE selects one of the available 64-Ncf contention based RA preambles. Ncf is the number of RA preambles reserved for contention free access. The contention based RA preambles can be optionally partitioned into two groups. If two groups are configured, the UE selects the group based on size of a message (i.e., MSG3) that the UE can transmit at operation  130 . The initial RA preamble transmission power is set based on open loop estimation after compensating for path loss. 
     The ENB transmits a RA response (RAR) on physical downlink shared channel (PDSCH) addressed to RA-radio network temporary identifier (RA-RNTI) to the UE at operation  120 . 
     The RA-RNTI identifies the time-frequency slot in which RA preamble was detected by eNB. RAR conveys RA preamble identifier (ID), timing alignment information, temporary cell-radio network temporary identifier (C-RNTI) and uplink (UL) grant for a message (i.e., MSG3) to be transmitted at operation  130 . RAR may also include back off indicator to instruct the UE to back off for period of time before retrying RA attempt. RAR is transmitted in RAR window. RAR window starts at subframe ‘x+3’ for RA preamble transmitted in subframe ‘x’. RAR window size is configurable. 
     The UE performs scheduled UL transmission on UL shared channel (UL SCH) at operation  130 . 
     A message, such as RRC Connection Request, RRC Connection Re-establishment request, RRC handover confirm, scheduling request, and the like, may be transmitted at operation  130 . The message transmitted at operation  130  is commonly referred as MSG3. The message transmitted at operation  130  may include the UE identity, such as C-RNTI or system architecture evolution (SAE)-temporary mobile subscriber identity (S-TMSI) or a random number. Hybrid automatic repeat request (HARQ) is used for the scheduled UL transmission. 
     The ENB transmits a contention resolution message at operation  140 . 
     HARQ is used for transmission of a contention resolution message. A contention resolution message is addressed to C-RNTI (if C-RNTI is included in MSG3) or temporary C-RNTI (if the UE identity is included in MSG3). On successful decoding of the contention resolution message, HARQ feedback is only sent by the UE which detects its own UE ID (or C-RNTI). 
       FIG.  2    illustrates a contention-free RA (CFRA) procedure. 
     Referring to  FIG.  2   , contention free RA procedure is used for scenarios, such as handover where low latency is required, timing advance establishment for secondary cell (Scell), and the like. 
     The ENB assigns to the UE non-contention RA preamble in dedicated signaling at operation  210 . 
     The UE transmits the assigned non-contention RA preamble at operation  220 . 
     The ENB transmits the RAR on PDSCH addressed to RA-RNTI at operation  230 . RAR conveys RA preamble ID and timing alignment information. RAR may also include UL grant. RAR is transmitted in RAR window similar to contention based RA procedure. Contention free RA procedure terminates after receiving the RAR. 
       FIG.  3    illustrates a procedure for idle to connected state transition using a CBRA procedure. 
     Referring to  FIG.  3   , the UE transmits an RA preamble to the eNB at operation  310 , the eNB transmits a RAR on PDSCH addressed to RA-RNTI to the UE at operation  320 , and the UE performs scheduled UL transmission on UL SCH at operation  330 , similar to the CBRA procedure illustrated  FIG.  1   . The scheduled transmission in operation  330 , i.e., MSG3 may refer to a connection request or a connection resume request. 
     The ENB transmits a contention resolution message at operation  340 . The contention resolution message in operation  340  of  FIG.  3    may refer to a connection setup (if a connection request is received at operation  330 ) or a connection resume (if a connection resume request is received at operation  330 ). 
     The UE performs scheduled transmission at operation  350 . The scheduled transmission in operation  350  may refer to a connection setup complete (if a connection setup is received at operation  340 ) or a connection resume complete (if a connection resume is received at operation  340 ). 
     Meanwhile, the CBRA procedure leads to a total delay of 29.5 transmit time intervals (TTIs) as shown in Table 1 below. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Delay Calculation * 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Average delay due to RACH scheduling period (1 ms 
                 0.5 
               
               
                   
                 RACH cycle) 
               
               
                   
                 Transmission of RACH Preamble 
                 1 
               
               
                   
                 Preamble detection and transmission of RA response 
                 3 
               
               
                   
                 (Time between the end RACH transmission and UE&#39;s 
               
               
                   
                 reception of scheduling grant and timing adjustment) 
               
               
                   
                 UE Processing Delay (decoding of scheduling grant, 
                 5 
               
               
                   
                 timing alignment and C-RNTI assignment + 
               
               
                   
                 L1 encoding of RRC Connection Request) 
               
               
                   
                 Transmission of MSG 3 
                 1 
               
               
                   
                 Processing delay in eNB 
                 4 
               
               
                   
                 Transmission of MSG4 
                 1 
               
               
                   
                 Processing delay in UE 
                 12 
               
               
                   
                 Transmission of MSG 5 
                 1 
               
               
                   
                   
               
            
           
         
       
     
     For the next generation radio access technology the idle to connected transition delay requirement is 10 ms. So there is need to enhance the CBRA procedure. 
       FIG.  4    illustrates a two-step contention based RA procedure according to an embodiment of the present disclosure. In the procedure illustrated in  FIG.  4   , contention resolution is performed in two steps. 
     Referring to  FIG.  4   , the UE transmits a RA channel (RACH) preamble and a UE ID to a base station (BS) (e.g., node B (NB), eNB, or gNB) at operation  410 . RACH preamble and UE ID can be transmitted in a single physical layer protocol data unit (PDU) or separate physical layer PDUs. The RACH preamble and the UE ID at operation  410  can be sent by UE in same time slot or different time slots. The operation  410  of transmitting RACH preamble and UE ID can be referred as MSG1 transmission. The UE ID can be at least one of Random ID, S-TMSI, C-RNTI, Resume ID, IMSI, idle mode ID, Inactive Mode ID, and the like. The UE ID can be different in different scenarios in which the UE performs the RA procedure. When the UE performs RA after power on (i.e., before the UE is attached to the network), the UE ID can be a random ID. Meanwhile, the UE may have a previously assigned or predetermined UE ID. For example, when the UE perform RA in IDLE state after the UE is attached the network the UE can use a previously assigned S-TMSI as the UE ID. If UE has an assigned C-RNTI (e.g., in connected state) the UE can use the C-RNTI as the UE ID. In case the UE is in INACTIVE state, the UE can use a resume ID as the UE ID. 
     NB determines whether NB has received the RACH preamble and the UE ID at operation  415 . 
     If the NB has received both the RACH preamble and the UE ID, NB sends a RAR (also referred as MSG2) to the UE at operation  420 . The RAR include at least one of timing advanced (TA), RACH preamble ID (RAPID) or the UE ID. The RAPID may refer to information on the received RACH preamble, e.g., a sequence index of the received RACH preamble. UE ID included in RAR is the UE ID received from UE. C-RNTI (or temporary C-RNTI) can be additionally included in RAR if the UE ID received from UE is not C-RNTI. Grant for UL transmission may also be included. 
     If physical downlink control channel (PDCCH) scheduling RAR is addressed to RA-RNTI corresponding to UE&#39;s RACH preamble transmitted at operation  410 , RAPID received in RAR at operation  420  corresponds to UE&#39;s RACH preamble transmitted in operation  410 , and the UE ID included in RAR is the UE ID transmitted by UE at operation  410 , then the UE considers contention resolution as successful. Note that RA-RNTI is used to mask cyclic redundancy check (CRC) of PDCCH which schedules the RAR message. The RA-RNTI is specific to time and/or frequency resource in which RACH preamble is transmitted. The RAR message may include a RAPID. 
       FIG.  5    illustrates an alternative two-step contention based RA procedure according to an embodiment of the present disclosure 
     Referring to  FIG.  5   , the two-step contention based RA procedure illustrated in  FIG.  5    is similar to that illustrated in  FIG.  4    except that in addition to the UE ID some additional control information can be sent in operation  510 . In other words, the UE transmits a RACH preamble, a UE ID, and control information to NB at operation  510 . RACH preamble, UE ID and control information can be transmitted in a single physical layer PDU or separate physical layer PDUs. The RACH preamble, the UE ID and the control information at operation  510  can be sent by UE in same time slot or different time slots. The operation  510  of transmitting RACH preamble, UE ID and control information can also be referred as MSG1 transmission. The control information may include one or more of connection request indication, connection resume request indication, system information (SI) request indication, bitmap wherein each bit is mapped to SI message or system information block (SIB) requested by UE, buffer status indication, beam information (e.g., one or more DL TX beam ID(s)), data indicator, cell/BS/transmission reception point (TRP) switching indication, or connection re-establishment indication, and the like. Note that any other control information is not precluded. 
     NB determines whether NB has received the RACH preamble, the UE ID and control information at operation  515 . 
     If RACH preamble, UE ID and control information is received, NB sends a RAR to the UE at operation  520 . In addition, NB may transmit a control message based on the control information received at operation  510 . In other words, NB may respond with a control message in addition to RAR message at operation  525 . The control message may depend on control information received at operation  510 . The RAR at operation  520  and the control message at operation  525  can be sent by NB in same time slot or different time slot. 
     Network can signal in dedicated or broadcast signaling the events or services for which UE should use two-step contention based random access procedure. 
       FIGS.  6 A and  6 B  illustrate enhanced PRACH slot/resource formats for performing a two-step RA procedure according to various embodiments of the present disclosure. 
     Referring to  FIGS.  6 A and  6 B , in a PRACH slot, the UE performs a PRACH transmission in a PRACH resource (e.g., R 1  of  FIG.  6 A ). In other words, the UE transmits a RA preamble in a PRACH resource on PRACH at first step of a two-step RA procedure (e.g., operation  410  of  FIG.  4   , operation  510  of  FIG.  5   ). The PRACH format (i.e., RA preamble) for performing the PRACH transmission according to an embodiment of the present disclosure may include cyclic prefix (CP), PRACH preamble sequence, additional information (i.e., the UE ID, data, control information, CRC, and the like) and guard time (GT). In the system, there can be several PRACH formats wherein all PRACH format may not include additional information (i.e., the UE ID, data, control information, CRC, and the like) The PRACH format indicator in PRACH configuration may indicate the PRACH format to be used for PRACH configuration. In an embodiment of the present disclosure, NB may configure multiple PRACH configurations having PRACH formats which include additional information (i.e., the UE ID, data, control information, CRC, and the like) and which do not include additional information. The UE may choose PRACH configuration based on service/slice in which the UE is interested. For example, for ultra-reliable low latency (URLL) slice/service, the UE may use PRACH configuration which allows transmitting additional information. NB may also indicate which PRACH format to be used for which service/slice. 
       FIG.  6 C  illustrates an enhanced PRACH slot/resource format for performing a two-step RA procedure according to an embodiment of the present disclosure. 
     Referring to  FIG.  6 C , PRACH sequence and additional information (i.e., the UE ID, data, control information, CRC, and the like) are frequency division multiplexed in PRACH resources. PRACH format of  FIG.  6 C  requires more resources because of additional CP and GT compared to PRACH format of  FIG.  6 A . However, duration of a slot of PRACH format of  FIG.  6 C  is shorter, compared to that of  FIG.  6 B , which is beneficial in reducing latency. 
       FIG.  6 D  illustrates an enhanced PRACH slot/resource format for performing a two-step RA procedure according to an embodiment of the present disclosure. 
     Referring to  FIG.  6 D , PRACH sequence and additional information (i.e., the UE ID, data, control information, CRC, and the like) are frequency division multiplexed in PRACH resource, similar to a PRACH format illustrated  FIG.  6 C . Furthermore, resource R 1  for a PRACH sequence and resource R 3  for additional information are separated. The PRACH configuration indicates resources for a PRACH sequence (i.e., PRACH resources) as well as resources for additional information. Mapping between resources for a PRACH sequence and resources for additional information can be implicit (e.g., first resource for a PRACH sequence maps to first resource for additional information, and so on) or can be explicitly indicated. For example, first resource is for a PRACH preamble and then next N (N can be 1) resource(s) is for additional information, and so on. Since all UEs performing RA may not have to transmit additional information, separating resources for a PRACH preamble and resources for additional information can reduce contention on the resources for the additional information. There can be one to one mapping or one to many mapping between resources for a PRACH preamble and resource for additional information. In an embodiment of the present disclosure, only resources for a PRACH preamble are indicated, and resources for additional information can be pre-defined. 
       FIG.  6 E  illustrates an enhanced PRACH slot/resource format for performing a two-step RA procedure according to an embodiment of the present disclosure. 
     Referring to  FIG.  6 E , PRACH sequence and additional information (i.e., the UE ID, data, control information, CRC, and the like) are time division multiplexed. In other words, a slot for PRACH sequence (i.e., PRACH slot) and a slot for additional information (i.e., data slot) are different. A PRACH slot and a data slot may or may not be consecutive. 
     Meanwhile, it is possible that NB can only receive PRACH preamble and cannot decode additional information (i.e., the UE ID, data, control information, and the like) successfully. In an embodiment of the present disclosure, NB may not send RAR if only PRACH preamble is received and additional information is not received. But this is not efficient as the UE will again send RA preamble which leads to more delay. Therefore, in case only PRACH preamble is received and additional information is not successfully decoded, a two-step RA procedure may fall back to a four-step RA procedure according to an embodiment of the present disclosure. In the two-step to four-step fall back method, if only PRACH preamble is received and additional information cannot be successfully decoded, NB sends RAR including TA and RAPID. Temporary C-RNTI may also be included in the RAR. In addition, the RAR may include an indication that additional information is not decoded. NB may indicate that additional information is not decoded implicitly. For example, NB may transmit RAR without the UE ID and the absence of the UE ID in RAR may implicitly indicate that NB has only received RACH preamble. Alternatively, NB may indicate that additional information is not decoded explicitly. For example, NB may transmit one bit indication in RAR to indicate that only RACH preamble is received and additional information (e.g. UE ID) is not received. 
       FIG.  7    illustrates a two-step to four-step fall back method for a RA according to an embodiment of the present disclosure. 
     If the UE ID transmitted by UE to NB in MSG1 is not included in RAR (i.e., implicitly indicated) or there is an explicit indication in RAR that additional information (i.e., the UE ID, data, control information, and the like) is not decoded by NB, the UE falls back to a four-step RA procedure as shown in  FIG.  7   . 
     Referring to  FIG.  7   , the UE transmits MSG1 including RA preamble and additional information (i.e., the UE ID) to NB at operation  710 . Additional information may also include data and control information. 
     If MSG1 is received, NB tries decoding additional information. If the additional information is successfully decoded, NB sends MSG2 (i.e., RAR) including the UE ID, as illustrated in  FIGS.  4  and  5   . If the additional information is not successfully decoded, NB sends RAR including UL grant. RAR also includes TA and RAPID. Temporary C-RNTI may also be included in the RAR. 
     If RAR corresponding to the UE&#39;s transmitted RA preamble is received, the UE determines whether RAR includes the UE ID at operation  720 . Alternatively, the UE may determine whether RAR includes an indication that additional information (e.g., the UE ID) is received by NB. If RAR includes the UE ID or indicates that additional information is received, contention resolution is successful at operation  730  (i.e., two-step RA procedure END). Otherwise, the UE falls back to four-step procedure, i.e., the UE transmits MSG3 including at least the UE ID using the allocated grant at operation  740 . Other information such as connection request or connection resume request or SI request and the like can also be transmitted in MSG3. 
     After transmitting MSG3, the UE waits for MSG4 for pre-defined time at operation  750 . If MSG4 is received, the UE determines whether MSG4 includes the UE ID transmitted in MSG3 at operation  760 . If MSG4 includes the UE ID, contention resolution is successful at operation  770  (i.e., four-step fall back RA procedure END). If MSG4 including UE ID transmitted in MSG3 is not received for pre-defined time UE retransmits MSG1. 
       FIG.  8    illustrates a two-step to four-step fall back method for a RA according to an embodiment of the present disclosure. 
     Referring to  FIG.  8   , similar to a two-step to four-step fall back method of  FIG.  7   , if the UE ID is not included in RAR (i.e., implicitly indicated) or there is an explicit indication in RAR that additional information (i.e., the UE ID, data, control information, and the like) is not decoded by NB, the UE falls back to a four-step RA procedure as shown in  FIG.  8   . 
     Specifically, similar to the method of  FIG.  7   , the UE transmits MSG1 including RA preamble and additional information (i.e., the UE ID, data, control information, and the like) to NB at operation  810 . If MSG1 is received, NB tries decoding additional information. If the additional information is successfully decoded, NB sends MSG2 (i.e., RAR) including the UE ID, as illustrated in  FIGS.  4  and  5   . If the additional information is not successfully decoded, NB may determine whether to include grant in RAR. In other words, contrary to the method of  FIG.  7   , grant is not always included in RAR. If NB wants the UE to fall back to four-step procedure, NB includes grant in MSG2, otherwise NB does not include grant in RAR. RAR also includes TA and RAPID. Temporary C-RNTI may also be included in the RAR. 
     If RAR corresponding to the UE&#39;s transmitted RA preamble is received, the UE determines whether RAR includes the UE ID at operation  820 . Alternatively, the UE may determine whether RAR includes an indication that additional information (e.g., the UE ID) is received by NB. If RAR includes the UE ID or indicates that additional information is received, contention resolution is successful at operation  830  (i.e., two-step RA procedure END). 
     If the UE ID is not included in RAR or there is an explicit indication in RAR that additional information (i.e., the UE ID, data, control information, and the like) is not decoded by NB, the UE determines whether grant is received in RAR at operation  835 . If grant is not received in RAR, the UE retransmits RA preamble and the UE ID. If grant is received in RAR, the UE falls back to four-step procedure i.e., the UE transmits MSG3 including the UE ID using the allocated grant at operation  840 . Other information such as connection request or connection resume request or SI request and the like can also be transmitted in MSG3. 
     After transmitting MSG3, the UE waits for MSG4 for pre-defined time at operation  850 . If MSG4 is received, the UE determines whether MSG4 includes the UE ID transmitted in MSG3 at operation  860 . If MSG4 includes the UE ID, contention resolution is successful at operation  870  (i.e., four-step fall back RA procedure END). If MSG4 including UE ID transmitted in MSG3 is not received for pre-defined time UE retransmits MSG1. 
       FIG.  9    illustrates a two-step to four-step fall back method for an RA according to an embodiment of the present disclosure. 
     Referring to  FIG.  9   , similar to a two-step to four-step fall back methods of  FIGS.  7  and  8   , if the UE ID is not included in RAR (i.e., implicitly indicated) or there is an explicit indication in RAR that additional information (i.e., the UE ID, data, control information, and the like) is not decoded by NB, the UE falls back to a four-step RA procedure as shown in  FIG.  9   . 
     Specifically, similar to the methods of  FIGS.  7  and  8   , the UE transmits MSG1 including RA preamble and additional information (i.e., the UE ID, data, control information, and the like) to NB at operation  910 . If MSG1 is received, NB tries decoding additional information. If the additional information is successfully decoded, NB sends MSG2 (i.e., RAR) including the UE ID, as illustrated in  FIGS.  4  and  5   . If the additional information is not successfully decoded, NB may determine whether to include grant in RAR. In other words, contrary to the method of  FIG.  7   , grant is not always included in RAR. Similar to the method of  FIG.  8   , if NB wants the UE to fall back to four-step procedure, NB includes grant in MSG2, otherwise NB does not include grant in RAR. In addition, NB may determine whether to include an indication to transmit the UE ID in RAR. 
     If RAR corresponding to the UE&#39;s transmitted RA preamble is received, the UE determines whether RAR includes the UE ID at operation  920 . Alternatively, the UE may determine whether RAR includes an indication that additional information (e.g., the UE ID) is received by NB. If RAR includes the UE ID or indicates that additional information is received, contention resolution is successful at operation  930  (i.e., two-step RA procedure END). If the UE ID is not included in RAR or there is an explicit indication in RAR that additional information (i.e., the UE ID, data, control information, and the like) is not decoded by NB, the UE determines whether grant is received in RAR at operation  935 . If grant is received in RAR, the UE falls backs to 4 step procedure i.e., the UE transmits MSG3 including the UE ID using the allocated grant at operation  940 . Other information such as connection request or connection resume request or SI request and the like can also be transmitted in MSG3. After transmitting MSG3, the UE waits for MSG4 for pre-defined time at operation  950 . If MSG4 is received, the UE determines whether MSG4 includes the UE ID transmitted in MSG3 at operation  960 . If MSG4 includes the UE ID, contention resolution is successful at operation  970  (i.e., four-step fall back RA procedure END). If MSG4 including UE ID transmitted in MSG3 is not received for pre-defined time, the UE retransmits MSG1. 
     If grant is not received in RAR, the UE retransmits RA preamble. In addition, the UE may retransmit additional information (i.e., the UE ID, data, control information, and the like) while transmitting RA preamble based on indication from NB. Specifically, the UE may determine whether RAR includes indication to transmit additional information at operation  937 . If RAR includes the indication, the UE retransmits MSG1 including both RA preamble and additional information, otherwise the UE retransmits MSG1 only including RA preamble at operation  939 . 
       FIG.  10    illustrates a two-step to four-step fall back method for a RA according to an embodiment of the present disclosure. 
     Referring to  FIG.  10   , similar to a two-step to four-step fall back methods of  FIGS.  7 ,  8  and  9   , if the UE ID is not included in RAR (i.e., implicitly indicated) or there is an explicit indication in RAR that additional information (i.e., the UE ID, data, control information, and the like) is not decoded by NB, the UE falls back to a four-step RA procedure as shown in  FIG.  10   . 
     Specifically, similar to the methods of  FIGS.  7 ,  8 , and  9   , the UE transmits MSG1 including RA preamble and additional information (i.e., the UE ID, data, control information, and the like) to NB at operation  1010 . If MSG1 is received, the NB tries decoding additional information. If the additional information is successfully decoded, NB sends MSG2 (i.e., RAR) including the UE ID, as illustrated in  FIGS.  4  and  5   . If the additional information is not successfully decoded, the NB may determine whether to include grant in RAR. In other words, contrary to the method of  FIG.  7   , grant is not always included in RAR. Similar to the methods of  FIGS.  8  and  9   , if NB wants the UE to fall back to four-step procedure, NB includes grant in MSG2, otherwise NB does not include grant in RAR. 
     If RAR corresponding to UE&#39;s transmitted PA preamble is received, the UE determines whether RAR includes the UE ID at operation  1020 . Alternatively, the UE may determine whether RAR includes an indication that additional information (e.g., the UE ID) is received by NB. If RAR includes the UE ID or indicates that additional information is received, contention resolution is successful at operation  1030  (i.e., two-step RA procedure END). If the UE ID is not included in RAR or there is an explicit indication in RAR that additional information (i.e., the UE ID, data, control information, and the like) is not decoded by NB, the UE determines whether grant is received in RAR at operation  1035 . If grant is received in RAR, the UE falls backs to 4 step procedure i.e., the UE transmits MSG3 including the UE ID using the allocated grant at operation  1040 . Other information such as connection request or connection resume request or SI request and the like can also be transmitted in MSG3. After transmitting MSG3, the UE waits for MSG4 for pre-defined time at operation  1050 . If MSG4 is received, the UE determines whether MSG4 includes the UE ID transmitted in MSG3 at operation  1060 . If MSG4 includes the UE ID, contention resolution is successful at operation  1070  (i.e., four-step fall back RA procedure END). If MSG4 including UE ID transmitted in MSG3 is not received for pre-defined time UE retransmits MSG1. 
     If grant is not received in RAR, the UE retransmits RA preamble. In addition, the UE may retransmit additional information (i.e., the UE ID, data, control information, and the like) while transmitting RA preamble based on parameter ‘N,’ where parameter ‘N’ indicates the number of times the UE can retransmit additional information (i.e., the UE ID, data, control information, and the like) along with RA preamble. Specifically, the UE may determine whether the UE has transmitted RA preamble with the UE ID ‘N’ times at operation  1037 . If the UE has transmitted RA preamble with the UE ID more than ‘N’ times, the UE retransmits MSG1 only including RA preamble at operation  1039 . Parameter ‘N’ can be pre-defined or indicated by network in broadcast or dedicated signaling. Alternatively, parameter ‘N’ can be signaled along with PRACH configuration. 
       FIG.  11    illustrates an RA procedure when a UE does not receive a RAR after transmitting an RA preamble and additional information according to an embodiment of the present disclosure. 
     Referring to  FIG.  11   , if the UE does not receive RAR after transmitting MSG1 including RA preamble and additional information (i.e., the UE ID, data, control information, and the like), then in an embodiment of the present disclosure, the UE may retransmit MSG1 including RA preamble and additional information. In an alternate embodiment of the present disclosure, the UE may determine whether to retransmit additional information along with RA preamble or not. The UE can determine whether to retransmit additional information along with RA preamble based on parameter ‘N,’ where parameter ‘N’ indicates the number of times the UE can retransmit additional information along with RACH preamble. Parameter ‘N’ can be pre-defined or indicated by network in broadcast or dedicated signaling. Alternatively, parameter ‘N’ can be signaled along with PRACH configuration. 
     Referring to  FIG.  11   , the UE transmits MSG1 including RA preamble and additional information (i.e., the UE ID, data, control information, and the like) to NB at operation  1110 . If nothing is received by NB, the NB does not transmit RAR to UE, and the UE cannot receive RAR. Therefore, the UE retransmits MSG1 including RA preamble and additional information to NB at operation  1120 . Meanwhile, if the UE retransmits MSG1 including RA preamble and additional information several times but RAR is not received, the UE may determine whether to retransmit additional information along with RA preamble to NB at operation  1130 . For example, the UE may determine whether to retransmit additional information along with RA preamble based on parameter ‘N,’ where parameter ‘N’ indicates the number of times the UE can retransmit additional information. Based on the determination, the UE may retransmit only RACH preamble, or both RACH preamble and additional information to the NB at operation  1140 . 
       FIG.  12    is a block diagram of a UE for performing an RA procedure according to an embodiment of the present disclosure. 
     Referring to  FIG.  12   , the UE include a transceiver ( 1210 ), a controller ( 1220 ) and a memory ( 1230 ). The controller ( 1220 ) may refer to a circuitry, an application-specific integrated circuit (ASIC), or at least one processor. The transceiver ( 1210 ), the controller ( 1220 ) and the memory ( 1230 ) are configured to perform the operations of the UE in the RA procedure illustrated in  FIGS.  4 ,  5 , and  7  to  11    or described above. For example, the transceiver ( 1210 ) is configured to receive signals from a BS and transmit signals to the BS. The controller ( 1220 ) may be configured to control the transceiver ( 1210 ) to transmit a first message including a RA preamble and a UE ID to the BS, and control the transceiver ( 1210 ) to receive a second message including a sequence index of the RA preamble from the BS. 
       FIG.  13    is a block diagram of a BS for performing an RA procedure according to an embodiment of the present disclosure. 
     Referring to  FIG.  13   , the BS includes a transceiver ( 1310 ), a controller ( 1320 ) and a memory ( 1330 ). The controller ( 1320 ) may refer to a circuitry, an ASIC, or at least one processor. The transceiver ( 1310 ), the controller ( 1320 ) and the memory ( 1330 ) are configured to perform the operations of NB in the RA procedure illustrated in  FIGS.  4 ,  5 , and  7  to  11    or described above. For example, the transceiver ( 1310 ) is configured to receive signals from a UE, and transmit signals to the UE. The controller ( 1320 ) may be configured to control the transceiver ( 1310 ) to receive a first message including a RA preamble and a UE ID from the UE, and control the transceiver ( 1310 ) to transmit a second message including a sequence index of the RA preamble to the UE. 
     While the present disclosure has been shown and described with reference to various embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present disclosure as defined by the appended claims and their equivalents.