Abstract:
The present disclosure relates to a pre-5 th -Generation (5G) or 5G communication system to be provided for supporting higher data rates Beyond 4 th -Generation (4G) communication system such as long term evolution (LTE). According to the present disclosure, a method for providing a connection with a radio access network via a wireless backhaul comprises determining one of a first state in which a first node connected with the radio access network is operated using all of beams and a second state in which the first node is operated using an beam of the first node determined in the first state as an operation mode of the first node and providing, by the first node, the connection with the radio access network to a radio access node in the determined operation mode.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY 
       [0001]    The present application is related to and claims the benefit under 35 U.S.C. §119(a) of a Korean patent application filed in the Korean Intellectual Property Office on Aug. 13, 2015 and assigned Serial No. 10-2015-0114605, the entire disclosure of which is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present disclosure concerns technology for providing a connection with a radio access network through a backhaul network, and more specifically, to methods and apparatuses for wireless backhaul communication by backhaul nodes providing network connection to radio access nodes based on beamforming. 
       BACKGROUND 
       [0003]    To meet the demand for wireless data traffic having increased since deployment of 4G (4th-Generation) communication systems, efforts have been made to develop an improved 5G (5th-Generation) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘beyond 4G network’ or a ‘post LTE system’. 
         [0004]    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, the beamforming, massive multiple-input multiple-output (MIMO), full dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems. 
         [0005]    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. 
         [0006]    In the 5G system, hybrid FSK and QAM modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed. 
         [0007]    Meanwhile, wireless backhaul schemes may break down into point-to-point (PTP) wireless backhaul and point-to-multipoint (PMP) wireless backhaul. PTP wireless backhaul offers one-to-one communication between one transmitter and one receiver. In PTP wireless backhaul, accordingly, the respective antennas of the transmitter and the receiver are commonly installed in fixed directions that give the optimal performance with respect to each other while generating very narrow beams. Here, use of the higher frequency may further narrow beams generated by antennas with the same size while increasing antenna gain. Therefore, PTP wireless backhaul is more advantageous for high frequency and benefits in light of less interference and excellent communication performance through an increased antenna gain. However, the need of being installed with beam oriented accurately at the position where line-of-sight (LOS) is attained results in wireless backhaul being primarily intended for broad service coverage macro base stations, but not adequate for base stations with relatively smaller coverage (referred to, hereinafter, as small cell base stations). 
         [0008]    In PMP wireless backhaul, meanwhile, one hub node (HN) connected with a wired backhaul offers a network connection by wirelessly communicating with multiple remote backhaul nodes (RBN). Conventionally, PMP backhaul adopts antennas with a large beam width of about 60 degrees to about 90 degrees at 6 GHz or less. For the reasons, PMP backhaul, despite being capable of communication even under a non-line-of-sight (NLOS) environment, is vulnerable to interference and exhibits poor communication performance due to the backhaul&#39;s decreased antenna gain. Thus, PMP wireless backhaul gives more advantages to small cell base stations that have reduced communication capacity but require easy installation of multiple RBNs regardless of position. 
         [0009]    Recently, vigorous research and 3GPP long term evolution (LTE) standardization are underway for heterogeneous cell technology for adding small cells in macro cell service coverage for maximized service areal capacity. As a result, future mobile communication systems are expected to present significantly increased small cell capacity due to coexistence of a number of small cells in macro cells. However, conventional PMP wireless backhaul cannot afford to meet capacity requirements for future small cells due to tiny communication capacity. Conventional PTP wireless backhaul costs a lot for installation and operation, albeit with more communication capacity than that of PMP backhaul. Therefore, a need exists for schemes for increasing small cell backhaul capacity in future mobile communication systems. 
         [0010]    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 
       [0011]    The present disclosure discloses a scheme for operating a PMP wireless backhaul that is easy to install and operate and has a large communication capacity in hierarchical cell mobile communication system. 
         [0012]    The present disclosure may lead to minimized interference and maximized communication efficiency by generating transmission/reception beams with a very high antenna gain at a high frequency and communicating signals. 
         [0013]    To address the above-discussed deficiencies, it is a primary object to provide, a method for providing a connection with a radio access network via a wireless backhaul comprises determining one of a first state in which a first node connected with the radio access network is operated using all of beams and a second state in which the first node is operated using an beam of the first node determined in the first state as an operation mode of the first node and providing, by the first node, the connection with the radio access network to a radio access node in the determined operation mode. 
         [0014]    According to an embodiment of the present disclosure, a method for providing a connection with a radio access network via a wireless backhaul comprises when a second node connected with the radio access network meets a first state transition condition, operating in a first state where all of beams are used to determine an first node connected with the radio access network and an beam based on a signal received through all of the beams and accessing the first node to communicate with the first node. 
         [0015]    According to an embodiment of the present disclosure, an apparatus for providing a connection with a radio access network via a wireless backhaul comprises a transceiver communicating with a first node connected with the radio access network and a controller determining one of a first state in which the first node is operated using all of beams and a second state in which the first node is operated using an beam of the first node determined in the first state as an operation mode of the first node and controlling the transceiver so that the first node provides the connection with the radio access network to a radio access node in the determined operation mode. 
         [0016]    According to an embodiment of the present disclosure,  32 . A second node for providing a connection with a radio access network via a wireless backhaul comprises when the second node connected with the radio access network meets a first state transition condition, a controller operates in a first state where all of beams are used to determine an first node connected with the radio access network and an beam based on a signal received through all of the beams and a transceiver accessing the first node to communicate with the first node. 
         [0017]    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 exemplary embodiments of the disclosure. 
         [0018]    Before undertaking the DETAILED DESCRIPTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
           [0020]      FIG. 1  illustrates an example of a wireless backhaul configuration providing a network connection to a small cell base station (SBS) in a mobile communication system according to an embodiment of the present disclosure; 
           [0021]      FIG. 2  illustrates an operation state transition of an HN according to an embodiment of the present disclosure; 
           [0022]      FIG. 3  illustrates a state transition of a RBN according to an embodiment of the present disclosure; 
           [0023]      FIG. 4  illustrates an example of a frame structure according to an embodiment of the present disclosure, wherein according to an embodiment of the present disclosure, the exemplified frame structure is one used for cellular mobile communication, for ease of description; 
           [0024]      FIG. 5  illustrates an example of configuring an SS and BCH in a frame according to an embodiment of the present disclosure; 
           [0025]      FIGS. 6 a  and 6 b    illustrate examples of configuring a DL control slot and UL control slot in a frame according to an embodiment of the present disclosure; 
           [0026]      FIG. 7  illustrates an example of configuring a DL and UL data slot in a frame according to an embodiment of the present disclosure; 
           [0027]      FIG. 8  illustrates an example of configuring a BM slot in a frame according to an embodiment of the present disclosure; 
           [0028]      FIG. 9  illustrates an example of configuring a UL RACH slot in a frame according to an embodiment of the present disclosure; 
           [0029]      FIG. 10  illustrates a frame structure when an HN and RBN configuring a wireless backhaul are operated in a BUS according to an embodiment of the present disclosure; 
           [0030]      FIG. 11  illustrates a frame structure when an HN and RBN configuring a wireless backhaul are operated in a BOCS, and the communication mode is an FBM according to an embodiment of the present disclosure; 
           [0031]      FIG. 12  illustrates a frame structure when an HN and RBN configuring a wireless backhaul are operated in a BOCS, and the communication mode is an ABM according to an embodiment of the present disclosure; 
           [0032]      FIG. 13  illustrates an example of an applicable beacon interval (BI) structure according to an embodiment of the present disclosure; 
           [0033]      FIG. 14  illustrates beams used for transmission and reception of information and slots when HNs and RBNs configuring a wireless backhaul are operated in a BUS according to an embodiment of the present disclosure; 
           [0034]      FIG. 15  illustrates beams used when HNs and RBNs configuring a wireless backhaul are operated in an FBM of a BOCS according to an embodiment of the present disclosure; 
           [0035]      FIG. 16  illustrates a beam used when HNs and RBNs configuring a wireless backhaul are operated in an ABM of a BOCS according to an embodiment of the present disclosure; 
           [0036]      FIG. 17  illustrates a result obtained through a wireless backhaul operated in a BUS according to an embodiment of the present disclosure; 
           [0037]      FIG. 18  illustrates an installation structure for a RAN and backhaul node (BN) according to an embodiment of the present disclosure; 
           [0038]      FIG. 19  illustrates a structure of an antenna for radio access and an antenna for wireless backhaul according to an embodiment of the present disclosure; 
           [0039]      FIG. 20  illustrates an example of a radio access network where a wireless backhaul is installed according to an embodiment of the present disclosure; 
           [0040]      FIG. 21  illustrates an example of a S1 user plane protocol stack structure for supporting an operation of an HN and RBN configuring a wireless backhaul according to an embodiment of the present disclosure; 
           [0041]      FIG. 22  illustrates an example of an X2 user plane protocol stack structure for supporting an operation of an HN and RBN configuring a wireless backhaul according to an embodiment of the present disclosure; 
           [0042]      FIG. 23  illustrates an example of an S1 control plane protocol stack structure for supporting an operation of an HN and RBN configuring a wireless backhaul according to an embodiment of the present disclosure; 
           [0043]      FIG. 24  illustrates an example of an X2 control plane protocol stack structure for supporting an operation of an HN and RBN configuring a wireless backhaul according to an embodiment of the present disclosure; 
           [0044]      FIG. 25  illustrates an example of a wireless control plane protocol stack structure for supporting an operation of an HN and RBN configuring a wireless backhaul according to an embodiment of the present disclosure; 
           [0045]      FIG. 26  illustrates an example of a wireless user plane protocol stack structure for supporting an operation of an HN and RBN configuring a wireless backhaul according to an embodiment of the present disclosure; 
           [0046]      FIGS. 27 a  and 27 b    illustrate examples of an operation flow of a first case of BUS transition conditions according to an embodiment of the present disclosure; 
           [0047]      FIGS. 28 a , 28 b , and 28 c    illustrate examples of an operation flow of a second case of the BUS transition conditions according to an embodiment of the present disclosure; 
           [0048]      FIGS. 29 a , 29 b , 29 c , and 29 d    illustrate examples of an operation flow of a third case of the BUS transition conditions according to an embodiment of the present disclosure; 
           [0049]      FIG. 30  illustrates an example of an operation flow of a fourth case of the BUS transition conditions according to an embodiment of the present disclosure; 
           [0050]      FIG. 31  illustrates an operation flow of an RBN according to an embodiment of the present disclosure; 
           [0051]      FIG. 32  illustrates an operation flow of an RBN according to another embodiment of the present disclosure; 
           [0052]      FIGS. 33 a , 33 b , and 33 c    illustrate an example of an operation mode of an HN operated in a BOCS and a communication mode transition operation according to an embodiment of the present disclosure; and 
           [0053]      FIG. 34  illustrates an example of a configuration of a backhaul node and RAN according to an embodiment of the present disclosure. 
       
    
    
       [0054]    Throughout the drawings, like reference numerals will be understood to refer to like parts, components, and structures. 
       DETAILED DESCRIPTION 
       [0055]      FIGS. 1 through 34 , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged electronic device. 
         [0056]    Hereinafter, embodiments of the present disclosure are described in detail with reference to the accompanying drawings. The same reference numerals are used to refer to same elements throughout the drawings. When determined to make the subject matter of the present disclosure unclear, the detailed of the known functions or configurations may be skipped. The terms as used herein are defined considering the functions in the present disclosure and may be replaced with other terms according to the intention or practice of the user or operator. Therefore, the terms should be defined based on the overall disclosure. 
         [0057]    According to an embodiment of the present disclosure, a wireless backhaul includes at least one hub node (HN) and at least one remote backhaul node (RBN). Each HN is connected to a network through a wired backhaul or another wireless backhaul and performs wireless backhaul communication with one or more RBNs. According to an embodiment of the present disclosure, the HN uses a point-to-multipoint (PMP) wireless backhaul technology for providing a network connection to a radio access node (RAN) connected with at least one RBN through wireless backhaul communication. Here, the RAN corresponds to a base station (BS) in a mobile communication system and provides a network connection to a mobile station (MS) through mobile communication. According to an embodiment of the present disclosure, such an example is assumed and described that signal communication is conducted using a beamforming antenna generating very narrow beams at a high frequency, e.g., 6 GHz or more. 
         [0058]      FIG. 1  illustrates an example of a wireless backhaul configuration providing a network connection to a small cell base station (SBS) in a mobile communication system according to an embodiment of the present disclosure. 
         [0059]    Referring to  FIG. 1 , as an example, an HN  110   a  is installed together with a macro cell BS (MBS)  110 , a RBN 1   120   a  is installed along with a small cell base station  1  (SBS 1 )  120 , and a RBN 2   130   a  is installed along with an SBS 2   130 . The HN  110   a  may afford a network connection to the SBS 1   120  and SBS 2   130  by simultaneously providing wireless backhaul communication to the RBN 1   120   a  and RBN 2   130   a.    
         [0060]    The MBS  110  provides radio access for mobile communication to a MS 11   111 , and the SBS 1   120  and SBS 2   130 , respectively, provide radio access for mobile communication to terminals, MS 21   121  and MS 31   131 , located in their own service coverage. In  FIG. 1 , wireless backhaul communication between the HN  110   a  and the RBNs, i.e., the RBN 1   120   a  and RBN 2   130   a , may use the same or different frequency than that used for radio access communication between the MBS  110  and MS 11   111  or SBS 1   120  and SBS 2   130  and MS 21   121  and MS 31   131 . When the same frequency is in use, an antenna for wireless backhaul communication and an antenna for radio access communication are rendered to use beams differently oriented, thereby minimizing interference. Further, according to the present disclosure, the wireless backhaul may be implemented based on mobile communication radio access technology using beamforming antennas or based on wireless LAN technology using beamforming antennas. According to an embodiment of the present disclosure, described is an example in which BS installed in HN is a MBS, and base station installed in RBN is an SBS, but the present disclosure may also apply to scenarios in which HN and RBN both are installed in MBS or SBS. Further, the present disclosure may also be applicable likewise where HN is connected to the network via a wired backhaul, as well as were HN is connected to the network by way of another wireless backhaul means. 
         [0061]    According to an embodiment of the present disclosure, the wireless backhaul is operated to run in one of two operation states in order to maximize wireless backhaul communication capacity and efficiency while enabling RBN automated installation and operation. According to an embodiment of the present disclosure, the two operation states include a backhaul update state (BUS) and a backhaul optimum communication state (BOCS). The BUS and BOCS both support data communication of the wireless backhaul, but the BOCS offers a much higher communication efficiency. According to an embodiment of the present disclosure, an HN and RBNs configuring the wireless backhaul are operated in the same operation state. In other words, when the HN is operated in the BUS, all of the RBNs connected to the HN to communicate are also operated in the BUS, whereas when the HN is operated in the BOCS, all the RBNs are operated in the BOCS as well. 
         [0062]      FIG. 2  illustrates an operation state transition of an HN according to an embodiment of the present disclosure. 
         [0063]    Referring to  FIG. 2 , according to an embodiment of the present disclosure, an HN is operated in one of a BUS  220  and BOCS  240  according to predetermined operation transition conditions. 
         [0064]    According to an embodiment of the present disclosure, a first condition (hereinafter, referred to as a BUS transition condition) for the wireless backhaul to transit from BOCS to BUS includes when the network instructs the HN to transit to the BUS to install a new RBN in the wireless backhaul or when the communication environment of the wireless backhaul already installed is varied to result in a decrease in performance. The decrease in performance may include when the HN determines to transit to the BUS upon recognizing that a sharp performance drop or link failure between the HN and RBN occurs, when receiving a request from an adjacent HN due to a deterioration of performance of the RBN connected with the adjacent HN, or when receiving an instruction for transition to BUS from the network. Additionally, the performance decrease may also include when the network instructs a transition to BUS periodically or by the operator&#39;s instruction for a performance enhancement in the wireless backhaul. In the case of RBN, there are included the situations where power is on, the link to RBN fails, or the serving HN connected with the RBN makes a state change into BUS. The above-enumerated BUS state transition conditions are described below in further detail. 
         [0065]    The wireless backhaul meeting such BUS transition conditions transit into BUS and operate. 
         [0066]    A first assumption is made, among others, to the scenario where a new RBN is installed in the wireless backhaul. According to an embodiment of the present disclosure, the wireless backhaul discovers an HN offering the optimal network connection to the new RBN and connects them together. 
         [0067]    A next assumption comes into such a scenario that the communication environment of the wireless backhaul already installed varies and reveals a performance down among the BUS transition conditions. Here, when the communication environment of the already installed wireless backhaul undergoes a variation and the wireless backhaul&#39;s resultant performance drop encompasses scenarios in which backhaul communication environments of pre-installed RBNs are subjected to a change resulting in a link failure with the serving HN used to be in linkage or a quality deterioration of existing link with the serving HN. According to an embodiment of the present disclosure, the wireless backhaul may discover another HN giving better communication than the HN presently linked with the RBN does and alter serving HNs for the RBN. Or, the wireless backhaul may identify a communication path providing a better communication performance for the same serving HN and change into the identified communication path. In order to vary serving HNs or beams in serving HN, the HN repeatedly transmits a sync signal, common control information and system information twice or more in directions or to locations corresponding to all of the beams the HN provides. The HN finds an optimal beam by conducting beam measurement and training on all of the HN&#39;s beams on downlink (DL) and uplink (UL). The HN receives random access preamble or association request signal transmitted from the RBN through all the beams and supports a function of enabling a new RBN or RBN already in connection with other HN to link with the optimal HN. Further, the RBN conducts searching and scanning on all of the HN&#39;s beams while in BUS to detect signals transmitted from the HNs and does beam measurement and training on the HNs detected by all of the HNs&#39; own beams on UL and DL to discover the optimal HN and the optimal beam with the optimal HN. It sends a random access preamble or association request signal to the optimal HN and connects to the optimal HN. 
         [0068]    According to an embodiment of the present disclosure, a BOCS transition condition of wireless backhaul includes when a new RBN is complete to install or a link-failed RBN successfully links to the network within a predetermined time. The condition also includes when the link-failed RBN fails to link with the network within the predetermined time or when a RBN subjected to a drastic link performance down makes link performance better by varying beams or serving HNs within a predetermined time. Also included is the scenario that the RBN drastically deteriorated in link performance fails to enhance link performance within the predetermined time, and a transition is made as per an instruction from the network. For RBN, such context is also included that the linked serving HN transitions to BOCS. 
         [0069]    The wireless backhaul, which meets the BOCS transition condition and thus operates in the BOCS, communicates using the optimal transmission/reception beam and runs to maximize the efficiency and performance of radio resources. For that purpose, according to an embodiment of the present disclosure, the HN operating in the BOCS sends sync signals to the HN&#39;s connected RBNs using partially limited beams among all of the HNs&#39; own beams and communicates control information. According to an embodiment of the present disclosure, no operation is supported for the HN to discover the optimal beam while in the BOCS, and if necessary, beam measurement and training are restrictively carried out on only some beams positioned adjacent to the optimal beam discovered in the BUS, thereby updating and tracking the optimal beam. The HN does not allow random access for access to a new RBN or communication of association request signals while in BOCS and does not back up access to the HN of the new RBN or RBN connecting with an adjacent HN. 
         [0070]    According to an embodiment of the present disclosure, the RBN does not conduct search or scan for discovering a new serving HN while in the BOCS nor does the RBN performs beam measurement and training for discovering the optimal beam for all the RBN&#39;s beams. If necessary, beam measurement and training are limitedly done on some beams adjacent to the optimal beam discovered in the BUS, thereby updating and tracking beams. No procedure (i.e., random access or transmission of an association signal) for accessing the adjacent HN is performed. 
         [0071]      FIG. 3  illustrates a state transition of a RBN according to an embodiment of the present disclosure. 
         [0072]    Referring to  FIG. 3 , according to an embodiment of the present disclosure, the RBN, when powering on in operation  310 , transitions to the BUS in operation  320 . When meeting the BOCS transition condition while operating in the BUS, the RBN transitions to the BOCS in operation  340 . Likewise, upon identifying that the RBN operating in the BOCS meets the BUS transition condition, the RBN transitions back to the BUS. 
         [0073]    According to an embodiment of the present disclosure, the RBN may operate in three communication modes to support the optimal communication in the BOCS. Here, the three communication modes correspond to a fixed beam mode (FBM), an adaptive beam mode (ABM), or a low duty mode (LDM). As set forth above, according to an embodiment of the present disclosure, while the HN and RBNs connected with the HN, which configure the wireless backhaul, are operated in the same manner, no separate limitation is imposed on communication mode of RBN operating in the BOCS. Accordingly, a plurality of RBNs connected with the same HN may operate in the same or different communication modes while operating in the BOCS. 
         [0074]    According to an embodiment of the present disclosure, in the FBM, HN and RBN communicate using a fixed optimal transmission/reception beam. Thus, the FBM may reduce waste of radio resources for discovering the optimal beam, leading to a maximized use efficiency of radio resources. 
         [0075]    When HN and RBN communicate using a fixed beam as in the FBM, the antennas of HN or RBN may be vibrated by an influence from the external environment, such as strong wind gust, while in communication, resulting in performance deterioration. According to an embodiment of the present disclosure, HN or RBN in the ABM limitedly conducts beam measurement and training although using the same frame structure as that used in the FBM. As such, beam measurement and training are limitedly performed on some beams adjacent to the fixed optimal beam, thereby enabling use of the optimal beam updated considering external environment. 
         [0076]    However, wireless backhaul communication environments, mostly, are not affected a lot by external environment and requiring operating in the ABM is infrequent. Thus, according to an embodiment of the present disclosure, the wireless backhaul in the BOCS may have a default communication mode set to basically operate in the FBM  342  as shown in  FIG. 3 . Upon determining that a beam change is required due to, e.g., influence from external environment, the wireless backhaul transitions and operates in the ABM  344 . When it is identified that there is no influence from external environment, the wireless backhaul transitions and operates in the FBM  342 . Lastly, when there is no terminal linked to a BS installed in the RBN or an active terminal is among terminals linked to the BS, the BS may transition and operate in the LDM  346  where signal communication is minimized. In such case, the RBN may also transition to the LDM, minimizing the RBN&#39;s power consumption and interference with other wireless backhaul. When recognizing the case where normal communication needs to be performed for the BS in the LDM  346 , a switch may be made into the FBM  342  or AMB  346  and wireless backhaul communication may be carried out. Here, specific conditions for switching into each communication mode are described below in detail. The RBN operating in the LDM according to the present disclosure sends backhaul alive (BA) bits or information to the RBN&#39;s serving HN at a period previously agreed to, thereby periodically reminding the serving HN that the wireless backhaul link is not disconnected but alive. Accordingly, the HN identifies the reception of BA bits or information at each period for the RBN operating in the LDM. When the result of identification reveals that no BA bit or information is received from the RBN at the period, it may be identified that a link failure occurs on the RBN, and the wireless backhaul may be swiftly restored for the disconnected link. 
         [0077]      FIG. 4  illustrates an example of a frame structure according to an embodiment of the present disclosure, wherein according to an embodiment of the present disclosure, the exemplified frame structure is one used for cellular mobile communication, for ease of description. 
         [0078]    For example, it is assumed that according to an embodiment of the present disclosure, HN and RBN each send and receive signals through frames having a fixed size through beamforming. In such case, the frame includes multiple subframes with a fixed length. Each of the multiple subframes includes multiple slots with a fixed length, and one slot includes multiple symbols. Referring to  FIG. 4 , for example, the frame  400  includes five subframes, and one subframe  402  includes 20 slots. One slot may have a different number of symbols and a different structure depending on the slot&#39;s type. For example, assuming that the frame  400  is 5 ms long, one subframe  402  is 1 ms long, and one slot is 50 us long. The number and structure of symbols constituting one slot may be varied depending on the type of slot. In the embodiment shown in  FIG. 4 , a type 1 slot  404   a  includes ten 5 us-long symbols constituted of a 4 us-long information interval and a 1 us-long cyclic prefix (CP) or protection interval. A type 2 slot  404   b  includes eleven symbols each of which has the same length of information interval, 4 us. However, symbol 0 is 5 us long, and the symbol 0&#39;s CP is 1 us long. Symbols 1 to 10 all are 4.5 us long and their CP is 0.5 us long. A type 3 slot  404   c  includes 25 symbols, each being 2 us long, without a CP. A type 4 slot  404   d  includes 6.25 symbols, each being 8 us long, without a CP. When receiving symbols including CPs during downlink and uplink communication, HN and RBN receive only information intervals except the CPs. 
         [0079]    According to an embodiment of the present disclosure, a frame on DL where HN sends a signal to RBN as shown in  FIG. 4  may distinctively include an SS and BCH slot for transmitting a synchronization signal (SS) and shared control information (e.g., broadcast channel (BCH)), a beam measurement (BM) slot, a control slot, and a data slot. Further, the frame on UL where RBN sends a signal to HN may distinctively include a random access channel (RACH) slot, a BM slot, a control slot, and a data slot. Here, although the size of subframe and slot remains unchanged, the number and combination of slots selected to constitute one subframe may be varied by communication environments, such as the number of antennas of HN and RBN, the number of beams of the antennas, and the number of RBNs. It is now assumed that according to an embodiment of the present disclosure each frame includes at least one SS and BCH slot and at least one downlink control slot. 
         [0080]    According to an embodiment of the present disclosure, the frame structure may support frequency division duplex (FDD) schemes in which DL and UL communication is simultaneously performed at different frequencies, respectively, as well as time division duplex (TDD) schemes in which the same frequency but different time slots are used. Although FDD schemes have the same frame structure for both DL and UL, the FDD schemes may have different selections and combinations of slots constituting the frame. For example, TDD schemes may assign some slots in each subframe in the given frame structure to DL communication and the remaining slots to UL communication and may designate one or two slots, as a protection interval, during a transmission-reception switch between DL and UL. As a specific example, slots 0 through 11 may be assigned to DL communication, and slots 13 through 19 to UL communication, and slot 12 may be set aside as a protection interval. 
         [0081]      FIG. 5  illustrates an example of configuring an SS and BCH in a frame according to an embodiment of the present disclosure. 
         [0082]    Referring to  FIG. 5 , according to an embodiment of the present disclosure, the SS and BCH slot are positioned in areas designated as minimum units for transmitting SS and BCH in one slot  500  having a fixed length constituting a frame. Here, in order for the transmitting end to be able to receive SS and shared control information, the SS and shared control information are repeatedly transmitted per antenna beam in the SS and BCH slot. For example, when slot 1 in subframe 0 of  FIG. 4  is the SS and BCH slot, the transmitting end transmits SS and shared control information in a fixed way in slot 1 of subframe 0 included in each frame. Here, when the transmitting end comes up with a total of five beams, SS and shared control information in beam [0] using symbols 0 and 1, SS and shared control information in beam [1] using symbols 2 and 3, and SS and shared control information in beam [2] using symbols 4 and 5 are repeatedly sent. In order for the receiving end to identify what number of transmission repetition SS and shared control information is received at, information indicating the number of transmissions may be included and transmitted in either the SS or shared control information. Here, the number of SS and BCH slots may be configured to correspond with the number of transmission beams owned by the transmitting end. It is assumed that while the number of SS and BCH slots in a frame may be variably configured, the position of the first slot at which the SS starts to be sent is fixed in the frame, and the SS and BCH slots are consecutively present. For example, it is assumed that SS and shared control information are intended to be sent using ten transmission beams. In such scenario, two slots, i.e., slots 1 and 2, may be allotted as the SS and BCH slots in subframe 0. Further, when HN further narrows beam width and increases the number of beams in order to obtain a higher beam gain, the HN needs to repeatedly send SS and BCH slots corresponding to the increased number of beams. For example, when HN uses 60 transmission beams, SS and BCH slots need to be transmitted twelve times. 
         [0083]      FIGS. 6 a  and 6 b    illustrate examples of configuring a DL control slot and UL control slot in a frame according to an embodiment of the present disclosure. 
         [0084]    Referring to  FIG. 6 a   , according to an embodiment of the present disclosure, the DL control slot and UL control slot each are positioned in an area designated as a minimum unit for transmitting DL control information in one slot having a fixed length constituting a frame. For example, the DL control slot is positioned in an area designated as a minimum unit in a slot  600 , and the UL control slot is positioned in an area designated as a minimum area in a slot  610 . In the FDD scheme, slot 0 in each subframe may be designated as DL control slot and UL control slot, and in the TDD scheme, slot 0 in each subframe may be designated as DL control slot, and slot 13 as UL control slot. In the above DL/UL control slots, each symbol may be allotted for a different RBN than those of the other symbols in the control slot, and HN may use a different DL transmission beam or UL reception beam at each symbol. Specifically, in the embodiment shown in  FIG. 6 , ten symbols may be transmitted in one control slot  600 , thereby enabling transmission of control symbols with up to ten HN beams. In  FIG. 6 a   , HN sequentially sends the ten symbols constituting the DL control slot through up to ten transmission beams. Each RBN receives all the symbols of the DL control slot through the optimal reception beam and receives control information for each RBN among all the symbols of the DL control slot. Further, in  FIG. 6 b   , HN sequentially receives the ten symbols of the UL control slot through up to ten reception beams, and each RBN sends a UL control symbol to HN using each RBN&#39;s optimal transmission beam at a time corresponding to the symbol assigned to the RBN. 
         [0085]      FIG. 7  illustrates an example of configuring a DL and UL data slot in a frame according to an embodiment of the present disclosure. 
         [0086]    Referring to  FIG. 7 , according to an embodiment of the present disclosure, an example is shown in which eleven symbols are transmitted and received through one slot  700  having a fixed length constituting a frame in order to raise transmission efficiency in DL/UL data slots. According to another embodiment, ten symbols may also be used through the slot  700 . According to an embodiment of the present disclosure, HN and RBN do not change beams upon transmission or reception of symbols in their respective corresponding data slots and the HN and RBN may change beams only at the moment that a slot begins. HN and RBN, upon reception of a corresponding data slot, eliminate the CP from each symbol while receiving the information interval of signal, thereby receiving data. 
         [0087]      FIG. 8  illustrates an example of configuring a BM slot in a frame according to an embodiment of the present disclosure. 
         [0088]    Referring to  FIG. 8 , an embodiment of configuring a BM slot is shown in which symbols each being 2 us long and without CP are repeatedly transmitted in one slot  800  having a fixed length and constituting each frame. On DL, HN sends the BM slot of  FIG. 8  in fixed transmission beams, and RBN switches reception beams, e.g., in 2.5 us-long units. RBN assigns the foremost 0.5 us interval  802  as a protection interval for beam switching while receiving the remaining 2 us-long signal to measure the power of the transmission/reception beam combination. Further, the same HN may form a plurality of beams with a plurality of antennas to simultaneously transmit a plurality of BM signals, and the same HN&#39;s other neighbor HNs may also transmit BM slots in the BUS at the same time. In such case, RBN may be able to receive, without interference, the BM signals simultaneously transmitted from the plurality of HNs and the antennas of the HNs. In particular, when the multiple neighbor HNs are not temporally synced with each other or have different distances from RBN and resultantly different propagation times, the multiple BM slots received by RBN may fail to sync and cause interference. In order to prevent such interference, according to an embodiment of the present disclosure, RBN may exclude the head  802  and the 2.5 us end  804  of the BM slot from use in BM like in the embodiment shown in  FIG. 8 . As a result, in the  FIG. 8  embodiment, measurement may be made on one transmission beam and 18 reception beams. On DL, HN repeatedly sends the BL slot while sequentially switching the transmission beams for all the HN&#39;s transmission beams. Thus, when on DL HN has NHN transmission beams, and RBN has NRBN reception beams, completion of one-time BM for all transmission/reception beam combinations with the BL slot shown in  FIG. 8  requires NHN [NRBN/18]up BM slots. Here, [X]up denotes a minimum one among natural numbers equal or larger than X. For example, when HN and RBN each have 60 beams, transmission/reception of 60*[60/18]up=60*4=240 BM slots is required to perform one-time measurement on all transmission/reception beam combinations. 
         [0089]    In the case of UL BM, RBN sends BL slot in fixed beams, and HN receives signals while switching reception beams as shown in  FIG. 8 , thereby measuring receive power on transmission/reception beam combination. In such case, HN may be able to receive, without interference, the BM signals simultaneously transmitted from the plurality of RBNs and the antennas of the RBNs. The same principle detailed above for DL BM applies to UL BM, and no further description is given. Further, it may also be possible in UL BM to measure beams by receiving BM slot in fixed beams while switching transmission beams in the slot, rather than transmitting BM slot in fixed beams and switching reception beams in the slot. 
         [0090]      FIG. 9  illustrates an example of configuring a UL RACH slot in a frame according to an embodiment of the present disclosure. 
         [0091]      FIG. 9  shows an embodiment of configuring a RACH slot, in which RACH preamble symbols each being 8 us long and without CP are repeatedly transmitted 6.25 times in one slot  900  having a fixed length constituting a frame. On UL, RBN sends the RACH slot using fixed transmission beams. On UL, HN may repeatedly detect RACH frame signals while varying reception beams, e.g., four times, as shown in  FIG. 9 . HN receives the RACH slot in beam [0] and uses the 8 us+2.5 us=10.5 us head  902  of the RACH slot as a protection interval without detecting signal and detects the RACH preamble for the next 8 us-long signal. HN then switches the reception beam into beam [1] and receives an 8 us-long signal after the 2.5 us-long protection interval to detect the RACH preamble. Likewise, HN sequentially switches the reception beam into beam [2] and beam [3] and receives an 8 us-long signal subsequent to the 2.5 us-long protection interval in each beam to detect the RACH preamble. According to an embodiment of the present disclosure, HN may detect a round trip delay up to 8 us through the RACH slot structure and establish a UL frame sync for RBNs located within a distance up to 1.2 km. In the embodiment shown in  FIG. 9 , the RACH slot does not back up RACH signal detection for four HN reception beams in each slot. Thus, in order to detect a RACH signal with more reception beams, more RACH slots as much may be allocated. For example, when HN detects a RACH signal with 60 reception beams, 15 RACH slots may be allocated. 
         [0092]    According to an embodiment of the present disclosure, HN and RBN may conduct wireless backhaul communication using slots in a frame as configured above. According to an embodiment of the present disclosure, various combinations may be possible for the above-described slots depending on antennas and beam environments used by HN and RBN, number of RBNs, radio channel state or other various communication environments, or necessary communication functionality and capability. As such, in order to select different slot combinations according to the need in frame, the frame structure sends control and system information over several distinct stages. In order to communicate with HN, RBN may first receive SS and BCH slot to establish synchronization and receive shared control information. Here, the shared control information contains shared information that needs to be known to RBN to establish frame synchronization with HN and to receive signals. For example, the shared information may include information on the position or order of shared control information symbol in frame, information on physical identifier and global identifier of HN, system frame number (SFN), information differentiating operation states (i.e., BUS or BOCS) of HN, number of SS and BCH slots per frame, number of antennas used by HN, number of beams per antenna of HN, duplex information such as TDD or FDD, or signal bandwidth information. 
         [0093]    According to an embodiment of the present disclosure, the DL/UL control slot may contain physical layer control information and other short control information necessary for HN to conduct DL and UL wireless communication with RBN. For example, such information may include radio resource allocation-related information on data or data slots communicated between HN and each RBN, such as physical downlink control channel (PDCCH) set forth in 3GPP LTE standards, modulation and code scheme (MCS) information, information on antennas and antenna techniques used by HN, power control information, paging-related information, and RACH-related information. Further, as information corresponding to 3GPP LTE physical control format indicator channel (PCFICH), information on the number of DL/UL control slots allotted to each subframe in the frame may be included as well. Further, as UL physical control information necessary for communication between HN and RBN, such as 3GPP LTE physical uplink control channel (PUCCH), channel state information (CSI), hybrid automatic repeat request (ARQ) Ack/Nack information, and scheduling request (SR) information may also be included. Further, other additional short control information may be included, such as information on transmission/reception beam being used or which to be switched into by HN and RBN, or information (backhaul status information (BSI)) indicating the status of wireless backhaul of RBN operating in LDM of BOCS, or other short control information for HN to control wireless communication of RBN. 
         [0094]    According to an embodiment of the present disclosure, the DL/UL data slot includes data transferred between HN and RBN through DL and UL wireless communication. In other words, all user data and control information communicated between BS installed in RBN and network through HN are transmitted and received in data slots. Various higher layer control information between HN and RBN which are necessary for wireless communication between HN and RBN may also be included. Further, the data slot may include various system information (SI) that needs to be known for RBN to access and communicate with HN. For example, included are information corresponding to various SI defined in 3GPP LTE standards, detailed information on DL and UL BM and use of beams, and other system information necessary for operating in BUS or BOCS. 
         [0095]    The following Table 1 represents slots, signals, or information transmitted and received in the BUS and BOCS when HN and RBN constituting a wireless backhaul according to the present disclosure perform wireless backhaul communication using the frame architectures described above in connection with  FIGS. 4 to 9 . 
         [0096]    Table 1 shows signals and information communicated in communication mode and operation state of wireless backhaul according to an embodiment of the present disclosure 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Operation 
                 Operation mode (BOCS) 
               
             
          
           
               
                 Slot/signal/information 
                 mode 
                 Communication 
                 Communication 
                 Communication 
               
               
                 types 
                 (BUS) 
                 mode (FBM) 
                 mode (ABM) 
                 mode (LDM) 
               
               
                   
               
               
                 DL SS/BCH slot 
                 full 
                 Optimal beam 
                 Optimal beam 
                 Optimal beam &amp; 
               
               
                   
                 coverage 
                   
                   
                 scheduling 
               
               
                 UL RACH slot 
                 full 
                 X 
                 X 
                 X 
               
               
                   
                 coverage 
               
               
                 DL/UL BM slot 
                 Full 
                 X 
                 Limited 
                 X 
               
               
                 DL/UL control slot 
                 ◯ 
                 ◯ 
                 ◯ 
                 Scheduling 
               
               
                 DL/UL data slot 
                 ◯ 
                 ◯ 
                 ◯ 
                 X 
               
               
                   
               
             
          
         
       
     
         [0097]    Table 2 below represents various operations and functions supported by wireless backhaul in the BUS and BOCS according to an embodiment of the present disclosure. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Operation mode (BOCS) 
               
             
          
           
               
                 Operations and 
                 Operation mode 
                 Communication 
                 Communication 
                 Communication 
               
               
                 functions 
                 (BUS) 
                 mode (FBM) 
                 mode (ABM) 
                 mode (LDM) 
               
               
                   
               
               
                 HN searching 
                 ◯ 
                 X 
                 X 
                 X 
               
               
                 Network 
                 ◯ 
                 X 
                 X 
                 X 
               
               
                 entry/handover 
               
               
                 Beam training 
                 Full 
                 X 
                 Limited 
                 X 
               
               
                 control information 
                 Broadcast/dedicated 
                 Dedicated 
                 Dedicated 
                 Scheduling &amp; 
               
               
                   
                   
                   
                   
                 dedicated 
               
               
                 Data 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                   
               
             
          
         
       
     
         [0098]    Referring to Table 1 above, according to an embodiment of the present disclosure, HN and RBN constituting a wireless backhaul, when operating in the BUS, uses all of the frame including an SS and BCH slot, UL RACH slot, DL/UL BM slot, DL/UL control slot, and data slot and supports all functions for mobile communication between a BS and MS on a mobile communication system. HN operating in the BUS sends the DL SS and BCH slot for the HN&#39;s full coverage for RBNs to sync with the HN at all positions in the HN&#39;s coverage and receives HN&#39;s shared control information and system information. Here, HN includes and transmits, in the shared control information, information indicating that the HN&#39;s operation mode is the BUS. Since HN uses a narrow beam, the HN repeatedly sends the signal while varying transmission beams in order to send the signal for the HN&#39;s full communication coverage. RBN operating in the BUS receives neighbor HNs&#39; SS and shared control information as well as that of the RBN&#39;s serving HN and performs searching and scanning on HN to discover an HN providing the optimal communication quality. 
         [0099]    Further, HN operating in the BUS receives the UL RACH slot assigned for all of the RBNs positioned in the HN&#39;s coverage to establish UL synchronization with the HN and perform UL communication. HN repeatedly assigns RACH slots while switching reception beams and receives signals from a corresponding RBN in order to receive RACH slot regardless of the position of RBN located in the HN&#39;s coverage. Referring to Table 2, HN receives, in the BUS, RACH signals from a RBN positioned in a neighbor HN and a new RBN as well as a RBN positioned in the HN&#39;s coverage to support UL sync, network entry (attach), and handover. RBN operating in the BUS may send a RACH signal to the serving HN to establish a UL sync and sends a RACH signal to a neighbor HN, rather than the RBN serving HN, to link or hand over to the neighbor HN. 
         [0100]    As shown in Table 1 above, HN and RBN operating in the BUS conduct BM and training on all transmission/reception beam combinations on DL and UL to select the optimal transmission/reception beam combination on DL and UL each. Based on the BM result of selecting the optimal transmission/reception beam combination, each RBN may determine the optimal HN offering the optimal service quality to the RBN. For example, assuming that HN and RBN each uses 60 beams, one-time BM on DL and UL for all transmission/reception beam combinations requires 60*[60/18]up=60*4=240 BM slots. Here, since wireless backhaul communication is done when HN and RBN both are located stationary, the time restriction for completing BM is not that strict unlike on mobile communication. Thus, e.g., when one slot is allocated per subframe constituting a frame for BM on DL and UL, one time of BM may be done at a time of 240 subframes, i.e., 240 ms. When HN sends SS and BCH slot with 60 transmission beams, 12 DLs lots need to be assigned for SS and BCH slot per frame. From above, it can be seen that significant waste of radio resources occurs. Here, waste of radio resources may be reduced by decreasing the number of beams used for the SS and BCH slots, but RBN may fail to communicate with HN due to a sync failure. Further, reception of 60 beams for RACH slot requires 15 uplink RACH slots to be allocated. 
         [0101]    SI of a new HN is needed for RBN operating in the BUS to access and communicate with the new HN. Such SI is transmitted through the DL/UL data slot. Accordingly, a corresponding RBN may be able to receive the SI in the full coverage regardless of the position of the RBN located in the HN coverage as the HN does on shared control information. Thus, although data is transmitted using a fixed transmission beam in the DL/UL data slot, when SI is included as shown in Table 1, transmission beams provided to be sent in the full coverage are switched and repeatedly transmitted. Likewise, since radio resource allocation information on the SI maybe transmitted in the full coverage of HN, the DL/UL control slot containing the radio resource allocation information switches and repeatedly transmits transmission beams provided to be transmitted in the full coverage. 
         [0102]    Referring to Table 1 above, according to an embodiment of the present disclosure, when the wireless backhaul is operated in the BOCS, HN and RBN supports only some of the functions supported by the mobile communication system. Accordingly, HN and RBN operating in the BOCS each use the optimal beam for transmission/reception of UL/DL control slot and data slot. Further, upon transmission of DL SS and BCH slot, HN does not broadcast in the HN&#39;s full coverage but sends them to only RBNs currently linked thereto. As possible, the HN sends signals with a minimum number of beams. Further, HN operating in the BOCS sends the HN&#39;s SI to a corresponding RBN only once or when required. Referring to Table 2 above, RBN operating in the BOCS receives the sync signal from only serving HN but not from neighbor HN. Nor does the RBN perform searching and scanning on neighbor HN. 
         [0103]    Referring to Table 1, according to an embodiment of the present disclosure, when the wireless backhaul operates in the BOCS, the wireless backhaul does not support UL RACH transmission/reception and thus does not assign a UL RACH slot. Referring to Table 2, HN operating in the BOCS does not support handover or access for new RBNs or RBNs belonging to a neighbor HN nor does the HN support RACH transmission/reception for RBNs linked thereto. Likewise, RBN operating in the BOCS does not send a RACH signal for updating UL sync with the serving HN and does not send RACH signals to neighbor HNs. Further, the RBN does not support DL and UL measurement and training on all of the beams of HN and RBN. Nor does the RBN support BM and training on neighbor HNs. As shown in Table 1, no BM slot is assigned to RBNs operating in the FBM and LDM of the BOCS, and BM and training are not supported. However, RBN operating in the ABM conducts limited BM and adaptation using some candidate beams adjacent to the optimal transmission/reception beam discovered in the BUS for the serving HN. HN and RBN operating in the FBM and ARM of the BOCS communicate data and control information using the optimal beam to provide wireless backhaul communication, and HN and RBN operating in the LDM communicate control information at a time previously agreed on. Further, HN sends a sync signal to RBN operating in the LDM only at times previously agreed on, and RBN operating in the LDM abstains from transmission/reception of data slot. 
         [0104]      FIGS. 10 to 12  illustrate examples of frame structures according to operation modes of wireless backhaul according to embodiments of the present disclosure. For ease of description, it is assumed that, based on a TDD scheme, a total of 12 slots, i.e., slot 0 through slot 11, are allocated to DL communication in each subframe, and a total of seven slots, i.e., slot 13 through slot 19, are allocated to UL communication. Slot 12 is assigned as a protection interval. Thus, a total of 60 slots and a total of 35 slots are assigned to DL and UL, respectively, per frame through the five subframes. 
         [0105]      FIG. 10  illustrates a frame structure when an HN and RBN configuring a wireless backhaul are operated as BUSs according to an embodiment of the present disclosure. 
         [0106]    Referring to  FIG. 10 , HN operating in the BUS assigns slot 0 and slot 13 to DL and UL control slot, respectively, per subframe, as an example. For example, assuming that HN transmits SS and BCH symbols using 60 beams, a total of 12 slots are needed. To that end, in the embodiment shown in  FIG. 10 , a total of 12 slots are assigned as SS and BCH slots through slots 1 to 3 of each of four subframes, i.e., subframes 0 to 3. Further, slot 4 in each subframe is assigned as a BM slot. HN sends BM slots while switching the HN&#39;s beams at a period of 12 frames each constituted of five subframes or 60 subframes corresponding to the number of the HN&#39;s beams. Here, when RBN performs BM measurement using 60 reception beams, one time of BM requires 240 ms for all transmission/reception beam combinations on DL. 
         [0107]    Meanwhile, in the embodiment shown in  FIG. 10 , slot 18 in each subframe is assigned as a UL BM slot. Further, slot 19 in each subframe is assigned as a UL RACH slot. Here, assuming that HN detects RACH preamble with four reception beams for one RACH slot, the HN may detect RACH preamble using 20 reception beams per frame. Thus, HN needs a total of three frames to detect RACH preamble using 60 beams. As described above in connection with Table 1, HN operating in the BUS, when sending SI through the DL data slot, repeatedly sends the DL data slot while switching the beams. Upon sending radio resource allocation information on SI through DL control slot, the HN may repeatedly send the DL control slot while switching the beams in the same way. The amount of SI to be sent by HN may vary depending on system implementations. When estimated based on the embodiment shown in  FIG. 10 , it may be regarded for SI of HN that data slot transmitted through slots 1 to 3 of subframe 4 are assigned for transmission of SI. Then, data slots used only for data communication in the embodiment of frame structure shown in  FIG. 10  include 35 slots on downlink and 20 slots on uplink for each frame. 
         [0108]      FIG. 11  illustrates a frame structure when an HN and RBN configuring a wireless backhaul are operated as BOCSs, and the communication mode is an FBM according to an embodiment of the present disclosure. 
         [0109]    Referring to  FIG. 11 , HN and RBN operating in the FBM, both, are located at fixed positions, and communication channel remains substantially unchanged, and such environment thus requires a very small amount of control information. Accordingly, HN operating in FBM assigns, e.g., slots 0 and 13 of subframe 0, as DL and UL uplink control slots, respectively, but does not assign control slots to the other subframes in the frame. 
         [0110]    Further, since the FBM uses only fixed optimal beam between HN and RBN upon transmission of SS and BCH symbols, the number of slots requires is also very small. Accordingly, in the embodiment shown in  FIG. 11 , SS and BCH slot are assigned to only slot 1 of subframe 0. 
         [0111]    Further, the FBM makes no allocation of BM slot and UL RACH. Thus, in the frame structure according to the embodiment shown in  FIG. 11 , as data slots used purely for data transmission, a total of 58 slots on DL and a total of 34 slots on UL may be used in each frame. As compared with the embodiment shown in  FIG. 10 , it can be identified that performance of data transmission is enhanced by about 65% and about 70% for DL and UL, respectively. 
         [0112]      FIG. 12  illustrates a frame structure when an HN and RBN configuring a wireless backhaul are operated as BOCSs, and the communication mode is an ABM according to an embodiment of the present disclosure. 
         [0113]    Referring to  FIG. 12 , the ABM performs DL/UL BM that is not done in the FBM. DL/UL BM performed in the AMB is limited for some adjacent beams of the optimal beam determined in the BUS, and thus, it may be conducted more rapidly as compared with the BM period of BUS. For example, it is assumed that HN operating in the ABM conducts BM on five candidate beams with the existing optimal beam selected for each of four RBNs. In such case, HN performs BM on a total of 4×5=20 candidate beams. Further, it is assumed for RBN that BM is measured on nine candidate beams with the existing optimal beam selected, for example. In such case, DL beam measurement period for HN is reduced to 20*[9/18] up =20*1=20 ms. Assuming that BM is simultaneously performed on UL for each of the four RBNs, BM period is down to 9*[20/18] up =9*2=18 ms, and when BM is sequentially measured on the four RBNs, the period is down to 4*9*[5/18] up =4*9*1=36 ms. When the BM measurement period is unnecessarily short as compared with the antenna vibration speed, resource waste may be reduced by decreasing the number of BM slots and increasing the BM period. For example, in the embodiment shown in  FIG. 12 , only one slot, as BM slot, is assigned per subframe. Accordingly, four more slots may be utilized as data slots per frame, leading to increased efficiency. Although BM period is increased to 100 ms, 90 ms, and 180 ms, which are five times as much as that when four slots come in use, a much shorter time may be taken as compared with the BM period in the BUS, i.e., 240 ms. According to another embodiment, when HN operating in the BOCS conducts wireless backhaul communication with multiple RBNs, and BM is required only for HN and some RBNs, HN sends or receives BM slot for the RBNs requiring BM in fixed beams, and only RBNs which are intended to do BM, do so. 
         [0114]    According to another embodiment, when HN operating in the BOCS conducts wireless backhaul communication with multiple RBNs, and HN alone requires BM but the RBNs don&#39;t, HN sends or receives BM slot to the RBNs to perform BM, and the RBNs receive or send the BM slot in fixed beams to support HN for BM. 
         [0115]    According to another embodiment, when HN operating in the BOCS carries out wireless backhaul communication with multiple RBNs, and at least one of the RBNs is in the LDM communication mode, HN sends SS and BCH symbol for the at least one RBN at a time previously agreed on with the at least one RBN and sends and receives DU/UL control slot symbols. HN communicates signals with RBNs, which are not in the LDM communication mode, using a frame structure as illustrated in  FIG. 11 or 12 , according to the communication mode of the RBNs. HN does not communicate data slot and BM slot with the at least one RBN operating in the LDM. Further, the at least one RBN operating in the LDM sends BA bit or information to the serving HN in UL control slot at a previously agreed period to periodically remind the serving HN that the wireless backhaul link is not disconnected but remains alive. 
         [0116]      FIG. 13  illustrates an example of an applicable beacon interval (BI) structure according to an embodiment of the present disclosure. 
         [0117]    BI in  FIG. 13  shows an example of a BI structure as used in IEEE 802.11ad. The IEEE 802.11ad standard defines wireless LAN communication between directional multi-gigabit (DMB) stations (STAs) communicating signals using beamforming antenna at a high frequency, e.g., 60 GHz, and channel access in BI units. Referring to  FIG. 13 , a BI includes a beacon header interval (BHI) and a data transfer interval (DTI). The BTI includes a beacon transmission interval (BTI), association beamforming training (A-BFT) interval, and announcement transmission interval (ATI). The DTI includes one or more contention-based access periods (CBAPs) and scheduled service period (SP). Personal basic service set control point (PCP)/access point (AP) STA assigns a BI and provides BI allocation information to non-PCP/non-AP STA. BTI is an access interval during which PCP/AP STA sends at least one DMG beacon frame to non-PCP/non-AP STA. Here, beacon frames not only provide a network sync to STA but also transfers BI allocation information, DMG capability information, and information corresponding to the shared control information and SI of mobile communication. A-BFT is an interval during which STAs select the optimal beam by performing beamforming training. ATI exchanges a request frame and response frame for access management on a one-to-one basis with PCP/AP STA or non-PCP/non-AP STA to offer association and disassociation of STA, send a request for service period, or transfer scheduling information. In CBAP of DTI, all STAs have a chance of random data transmission in a carrier sense multiple access with collision avoidance (CSMA/CA) scheme, and in SP, STAs transmits data in a time division multiple access (TDMA) scheme according to PCP or AP&#39;s scheduling. 
         [0118]    The above-described IEEE 802.11ad standard may be used to implement a wireless backhaul according to an embodiment of the present disclosure. In this case, the following Table 3 represents access intervals supported in the BUS and BOCS when HN and RBN constituting a wireless backhaul according to the present disclosure conduct wireless backhaul communication based on the IEEE 802.11ad standard, and Table 4 represents operations and functions supported by the wireless backhaul according to the present disclosure. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 Operation mode 
               
               
                   
                 (BOCS) 
               
             
          
           
               
                   
                 Operation mode 
                 Communication 
                 Communication 
                 Communication 
               
               
                 BI 
                 (BUS) 
                 mode (FBM) 
                 mode (ABM) 
                 mode (LBM) 
               
               
                   
               
               
                 BTI 
                 full coverage 
                 Optimal beam 
                 Optimal beam 
                 Optimal beam 
               
               
                 A-BFT 
                 ◯ 
                 X 
                 X 
                 X 
               
               
                 ATI 
                 ◯ 
                 Optimal beam 
                 Optimal beam 
                 Scheduling 
               
               
                 DTI CSAB 
                 ◯ 
                 X 
                 X 
                 X 
               
               
                 DTI SP 
                 ◯ 
                 ◯ 
                 ◯ 
                 X 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
             
             
               
                   
                   
               
               
                   
                 Operation mode 
               
               
                   
                 (BOCS) 
               
             
          
           
               
                 Operations and 
                 Operation mode 
                 Communication 
                 Communication 
                 Communication 
               
               
                 functions 
                 (BUS) 
                 mode (FBM) 
                 mode (ABM) 
                 mode (LBM) 
               
               
                   
               
               
                 Scan 
                 ◯ 
                 X 
                 X 
                 X 
               
               
                 Association/change 
                 ◯ 
                 X 
                 X 
                 X 
               
               
                 HN 
               
               
                 Beam training 
                 Full 
                 X 
                 Limited 
                 X 
               
               
                 Control 
                 Broadcast/dedicated 
                 Dedicated 
                 Dedicated 
                 Scheduling &amp; 
               
               
                   
                   
                   
                   
                 dedicated 
               
               
                 Data 
                 CSMA/TDMA 
                 TDMA 
                 TDMA 
                 X 
               
               
                   
               
             
          
         
       
     
         [0119]    Referring to Table 3 above, according to an embodiment of the present disclosure, when wireless backhaul operates in the BUS, the wireless backhaul may support all access intervals as per the IEEE 802.11ad standard and all wireless LAN functionality as per the IEEE 802.11ad standard. According to an embodiment of the present disclosure, wireless backhaul may additionally provide operation mode indication information indicating that the operation mode of wireless backhaul is the BUS in the DMB beacon frame. HN operating in the BUS sends DMG beacon frame using the beams of all the HN&#39;s antennas during BTI and provides synchronization to RBNs intended to access the HN. Further, RBN operating in the BUS receives DMB beacon frame and performs HN scanning based on the DMB beacon frame. Further, the RBN may support sector level sweep (SLS) beam training between HN and RBN using BTI and A-BFT intervals to select the optimal sector beam. Additionally, the RBN may perform beam refinement protocol (BRP) to select the optimal reception sector beam or may select a more accurate optimal transmission/reception beam through more precise beam training. BRP setup for performing BRP may be done during ATI or DTI, and BRP beam training may be done during DTI. During ATI, HN and RBN exchange a request frame and response frame for access management on a one-to-one basis to offer association and disassociation of STA, send a request for service period, or transfer scheduling information. During CBAP of DTI, all the RBNs have a change of data transmission in a CSMA/CA scheme. Further, during SP of DTI, HN sends data to RBN in a TDMA scheme or each RBN sends data to HN during a designated SP interval in a TDMA scheme in response to HN&#39;s instruction. DTI scheduling information is transmitted from HN to RBNs in the announce frame of ATI or DMG beacon frame of BTI. As a result, in the wireless backhaul according to an embodiment of the present disclosure, RBN operating in the BUS may scan HNs, search for a new HN offering the optimal service quality to the RBN, connect with the searched new HN or change the serving HN into the new HN. All the beams may be used to support beam training to discover and select the optimal transmission/reception sector beam and detailed beam for wireless backhaul communication between HN and RBN, and control information and data of HN and RBN may be communicated in the CSMA/CA scheme and TDMA scheme. 
         [0120]    Referring to Table 3 above, when the wireless backhaul according to an embodiment of the present disclosure operates in the BOCS, only some of the functions provided in the IEEE 802.11ad wireless LAN standard are supported. Also in such case, HN may additionally send operation mode indication information indicating that the HN&#39;s operation mode is the BOCS through DMB beacon frame. According to an embodiment of the present disclosure, HN sends DMG beacon frame of BTI to only RBNs already connected in the BOCS, and in this case, limits the number of times of transmission to the minimum using the optimal transmission beam determined in the BUS. Referring to Table 3 above, according to an embodiment of the present disclosure, when the wireless backhaul operates in the BOCS, no A-BFT interval is allocated, and ATI interval is limited to the minimum. Further, in the BOCS, no DTI CBAP is assigned while only SP is assigned. Referring to Table 4 above, the BOCS does neither support SLS beam training nor BRP beam training. However, upon operating in the ABM of BOCS, beam tracking is supported of conducting beam training on some candidate beams adjacent to the optimal beam already selected during the DTI interval. Further, according to an embodiment of the present disclosure, such a LDM is supported where a power save mode is operated in which RBN operating in the BOCS wakes up at a pre-agreed time and does not communicate signals at other times. As set forth above in Table 2, RBN operating in the BOCS does not scan HN and does not support connection to a new HN or change of serving HN. Further, the ABM provides limited beam training support and thus supports beam tracking, a type of beam training for varying or re-discovering the sector beams of HN and RBN and detailed beam of the sector beam. Also in use of the IEEE 802.11ad standard, according to an embodiment of the present disclosure, the wireless backhaul operating in the BOCS uses the optimal HN and optimal beam discovered in the BUS to communicate control information and data to minimize waste of access interval. Here, rather than the control information being broadcast in all the beams, information necessary for each RBN is communicated in the optimal beam, and the data is communicated in a TDMA scheme by scheduling without support of CSMA/CA. 
         [0121]      FIGS. 14 to 17  illustrate examples of beams used for communication of slots, symbols, signals, or information according to the operation mode and communication mode which HN and RBN constituting a wireless backhaul according to the present disclosure have currently switched into. 
         [0122]      FIG. 14  illustrates beams used for transmission and reception of information and slots when HNs and RBNs configuring a wireless backhaul are operated as BUSs according to an embodiment of the present disclosure. Here, HN and RBN operating in the BUS use their own respective transmission/reception beams. 
         [0123]      FIG. 14  illustrates an example in which HN 3  uses all the beams upon communication with each of RBN  31  to RBN  34  located in the HN 3 &#39;s coverage, and RBN  31  to RBN  34  also use all their beams for communication with HN 3 . Likewise, the remaining HNs and RBNs also use all their transmission beams to perform communication in the instant embodiment. 
         [0124]      FIG. 15  illustrates beams used when HNs and RBNs configuring a wireless backhaul are operated in an FBM of a BOCS according to an embodiment of the present disclosure. 
         [0125]    Referring to  FIG. 15 , RBN communicates with the RBN&#39;s optimal HN using a fixed optimal beam. For example, RBN  44  communicates with the optimal HN, HN  4 , using a fixed optimal transmission/reception beam. HN  4  operating in the FBM of BOCS communicates with each of RBN  41  to RBN  43  permitted to access HN  4  using the optimal transmission/reception beam. Likewise, the remaining HNs, i.e., HN  1  to HN  3 , operating in the FBM of BOCS also communicate with the RBNs permitted to access them using the optimal transmission/reception beam. Accordingly, the backhaul operating in the FBM of BOCS may increase communication capability by minimizing interference and waste of radio resources. 
         [0126]      FIG. 16  illustrates beams used when HNs and RBNs configuring a wireless backhaul are operated in an ABM of a BOCS according to an embodiment of the present disclosure. 
         [0127]    According to an embodiment of the present disclosure, HN and RBN operating in the ABM of BOCS, upon recognizing that beam training is required, sends or receives additional BM slot to discover the optimal transmission or reception beam. Referring to  FIG. 16 , HN 1  and RBN 11  to RBN 14  all perform beam training to discover the optimal beam. On the contrary, while RBN 21  to RBN 23  conduct beam training, their serving HN, HN 2 , remains in the state of having beam fixed. In contrast, HN 3  discovers the optimal beam by performing beam training on each of RBN 31  to RBN 33  currently connected while RBN 31  to RBN 33  remain with their beams fixed. While HN 4  discovers the optimal beam by performing beam training on RBN 41  and RBN 43  among RBNs connected to HN 4 , RBN 42  and RBN 44  are in the state of their beams being fixed. RBN 41  and RBN 42  discover the optimal beam by performing beam training whereas RBN 43  and RBN 44  are in the state where beams remain fixed. 
         [0128]      FIG. 17  illustrates a result obtained through a wireless backhaul operated as a BUS according to an embodiment of the present disclosure. 
         [0129]    Referring to  FIG. 17 , there is shown a state in which new RBNs, RBN 24  and RBN 34 , are installed in the existing wireless backhaul, as denoted in reference number  1700 , and RBN  24  and RBN 34  are connected with HN 2  and HN 3 , respectively. Reference number  1702  denotes an example in which RBN 12  recognizes that HN 3  offers a higher communication quality than serving HN 1  linked and communicating with RBN 12 , disconnects communication with HN 1 , and changes serving HNs connecting to HN 3 . Likewise, reference number  1704  denotes an example in which RBN 31  recognizes that HN 2  provides a higher communication quality than serving HN 3  used to connect and communicated with RBN 31  does, disconnects communication with HN 3 , and changes serving HNs connected with HN 2 . Regarding reference number  1706 , it is assumed that there is a reflecting object  1 , e.g., a building or object, which present those larger than the LOS communication path and beam used by HN 1  and RBN 14  upon communication. In such case, such a circumstance is shown where HN 1  and RBN 14  each change the optimal beam into a beam corresponding to the direction of reflecting object  1  by recognizing that the communication quality of NLOS path where radio waves are reflected and received by reflecting object  1  is higher. Regarding reference number  1708 , it is likewise assumed that there is another reflecting object  2 , e.g., a building or object presenting those larger than the LOS communication path and beam used by HN 4  and RBN 44  upon communication. In such case, such a circumstance is shown where HN 4  and RBN 44  each change the optimal beam into a beam corresponding to the direction of reflecting object  2  by recognizing that the communication quality of path where radio waves are reflected and received by reflecting object  2  is higher. 
         [0130]    According to an embodiment of the present disclosure, HN and RBN operating in the BUS perform beam training on all their transmission/reception beam combinations to select the optimal HN and the optimal transmission/reception beam combination to be used for communication with the optimal HN for each RBN. Since beam training is performed on all the transmission/reception beam combinations for such purpose, the time for beam training is very long, and waste of radio resources is increased. In contrast, according to an embodiment of the present disclosure, HN and RBN operating in the ABM of BOCS conduct limited beam training on only some adjacent beams using the optimal beam already discovered during the BUS, so that the time required for beam training is reduced, and the amount of radio resources used is shortened. As a specific example, assuming that HN and RBN each uses a total of 60 beams, HN and RBN operating in the BUS may perform beam training on 60×60=3600 beam combinations at each measurement period. In comparison, under the assumption that HN and RBN operating in the ABM performs beam training only on nine candidate beams, the total number of beam combinations for beam training is significantly reduced to 9×9=81. Beam training in the ABM may quickly discover the optimal beam when there is an influence from external environment. However, since beam training and selection of optimal beam in the ABM are limited to the existing optimal communication path already discovered in the BUS, no change in the beam direction corresponding to the change in the communication path occurs as in reference numbers  1706  and  17087  of  FIG. 17 . 
         [0131]      FIG. 18  illustrates an installation structure for a RAN and backhaul node (BN) according to an embodiment of the present disclosure. Here, BN corresponds to HN or RBN, and RAN corresponds to MBS or SBS. 
         [0132]    Referring to  FIG. 18 , in the structure denoted in reference number  1800 A, a BN  1820 A and a RAN  1840 A are implemented as devices independent from each other and are respectively installed in different cases, boxes, housings, or racks. BN  1820 A and RAN  1840 A are connected with each other via a connecting line or cable  1860 A. Although in the structure denoted in reference number  1800 B a BN  1820 A and a RAN  1840 B are implemented as devices independent from each other, the BN  1820 A and the RAN  1840 B may be installed in a single shared case  1800 B. In the structure denoted in reference number  1800 B, e.g., HN and BS respectively are implemented as boards independent from each other, installed in the same rack, and connected together by way of a backplane. In the structure denoted in reference number  1800 C, BN and RAN are implemented as a single device. For example, in the structure denoted in reference number  1800 C, RBN and BS are implemented on the same board and are connected together via a bus. 
         [0133]      FIG. 19  illustrates a structure of an antenna for radio access and an antenna for wireless backhaul according to an embodiment of the present disclosure. 
         [0134]    Referring to  FIG. 19 , for example, in the antenna  1  structure  1900 A, an antenna  1920 A for wireless backhaul and an antenna  1940 A for radio access are independently implemented and separately installed from each other. The antenna  1  structure  1900 A may be advantageous when some characteristics among antenna-related information including use frequency, number of antennas and beams, antenna pattern or beam pattern, and vertical or horizontal direction of antenna differ for the antenna  1920 A for wireless backhaul and antenna  1940 A for radio access. An antenna structure  2   1900 B is a structure in which an antenna array  1920 B for wireless backhaul and an antenna array  1940 B for radio access are implemented on the same antenna hardware. In such case, antenna arrays  1920 B and  1940 B, despite being implemented on the same antenna hardware, have different radio frequency (RF) elements connected with the antenna arrays. In particular, in such case, the antenna arrays  1920 B and  1940 B may have some characteristics (use frequency or number and direction of antennas) designated to be the same and other characteristics (number of beams, beam pattern, and beam direction) designated to be different. An antenna structure  3   1900 C is a structure in which beams  1922 C and  1924 C for wireless backhaul and beams  1942 C and  1944 C for radio access are created through the same antenna hardware and array. The antenna structure  3   1900 C, albeit having the same antenna hardware and array, have RF elements connected thereto implemented to be independent from each other to generate different beams. In such case, substantially all of the characteristics of antennas and beams for wireless backhaul and radio access are the same while beams for wireless backhaul and beams for radio access may be selected differently from each other. 
         [0135]      FIG. 20  illustrates an example of a radio access network where a wireless backhaul is installed according to an embodiment of the present disclosure. Although the radio access network shown in  FIG. 20  illustrates an LTE RAN structure as an example, the present disclosure may also be applicable to other communication networks to which beamforming may apply. 
         [0136]    Referring to  FIG. 20 , according to an embodiment of the present disclosure, in the random access network  2000  where wireless backhaul is installed, a RBN  2002  is wirelessly connected to an HN  2004  to provide network connection to a RBN-BS which is a random access node of RBN.  FIG. 20  illustrates an example in which the BSs respectively installed in the RBN and the HN are shown as single devices RBN-BS  2002  and HN-BS  2004  without being separated from the RBN and HN. Thus, RBN and HN respectively may hereinafter be denoted as RBN-BS  2002  and HN-BS  2004 . 
         [0137]    RBN-BS  2002  and HN-BS  2004  are connected together via a wireless interface Ub. 
         [0138]    According to an embodiment of the present disclosure, wireless interface Ub for wireless backhaul may use beamforming-based cellular mobile communication random access technology as described above in connection with  FIGS. 4 to 12  or beamforming wireless LAN random access technology defined in IEEE 802.11ad. Further, RBN-BS  2002  terminates S1 and X2 interface for operation of BS connected thereto. HN-BS  2004  provides an S1 and X2 proxy function to RBN-BS  2002  and other network nodes, i.e., another BS  2006 , mobility management entity (MME)/serving gateway (S-GW)  2008  and  2010 . The S1 and X2 proxy function includes a function for delivering S1 and X2 signaling messages for MS and a function for transferring GTP data packets between the S1 and X2 interface related to RBN and S1 and X2 interface related to other network nodes. The proxy function enables HN-BS  2004  to play a role as a MME (with respect to S1-MME), adjacent BS (with respect to X2), or S-GW (with respect to S1-U). Further, HN-BS  2004  embeds and provides S-GW/pdn-gateway (P-GW) functions necessary for RBN operation on RBN-BS  2002 . Specifically, The HN-BS  2004  generates a session for RBN, manages a radio bearer for RBN, and terminates the S1 interface towards the MME serving RBN. Further, HN-BS  2004  performs data packet mapping and signaling to radio bearers set for RBN operation. Signaling and data packet mapping are conducted on radio bearers. HN-BS  2004  embeds the P-GW functionality of assigning an internet protocol (IP) address other than the S1 IP address of RBN-BS  2002  to RBN-BS  2002  for operation and maintenance (OAM). 
         [0139]    According to an embodiment of the present disclosure, the wireless backhaul provides a GTP tunnel in HN and RBN to deliver radio bearers of MS located in the service coverage of BS installed in RBN. For the GTP tunnel, HN and RBN according to an embodiment of the present disclosure supports an S1 and X2 user plane interface. 
         [0140]      FIG. 22  illustrates an example of a S1 user plane protocol stack structure for supporting an operation of an HN and RBN configuring a wireless backhaul according to an embodiment of the present disclosure.  FIG. 22  is a view illustrating an example of an X2 user plane protocol stack structure for supporting an operation of an HN and RBN configuring a wireless backhaul according to an embodiment of the present disclosure. 
         [0141]    Referring to  FIGS. 21 and 22 , S1 and X2 user plane packets are mapped to radio bearers through a Ub interface. 
         [0142]    The protocol stack of RBN includes, e.g., GTP, user datagram protocol (UDP), IP and packet data convergence protocol (PCDP)/radio link control (RLC)/medium access control (MAC)/physical layer (PHY). The protocol stack of S-GW includes GTP, UDP, IP, layer2 (L2), and L1. According to an embodiment of the present disclosure, HN has a stack structure where the protocol stack of RBN and the protocol stack of S-GW are mapped to provide a connection with S-GW to RBN. 
         [0143]    Further, embodiment of the present disclosure supports an S1 and X2 control plane interface for controlling wireless backhaul.  FIG. 23  is a view illustrating an example of an S1 control plane protocol stack structure for supporting an operation of an HN and RBN configuring a wireless backhaul according to an embodiment of the present disclosure. 
         [0144]    Referring to  FIG. 23 , one S1 interface relation exists between each RBN and HN, and an S1 interface relation is present between each MME included in MME pool and HN. HN processes and delivers all S1 messages between RBN and MME for all UE-dedicated procedures. S1-AP message processing includes varying S1-AP UE IDs, transport layer addresses, and GTP tunnel endpoint identifiers (TEIDs) and does not change other portions in S1-AP message. Non-UE-dedicated S1-AP procedures are terminated in HN and are internally processed between RBN and HN and between HN and MME. Upon receiving non-UE-dedicated S1 message from MME, HN may trigger corresponding S1 non-UE-dedicated procedures for RBN. 
         [0145]    Therefore, an example protocol stack of RBN includes S1-AP, stream control transmission protocol (SCTP), IP, and PDCP/RLC/MAC/PHY. MME includes S1-AP, STCP, IP, L2, and L1. Likewise, HN providing connection between MME and RBN has a stack structure in which the protocol stacks of RBN and MME are mapped. 
         [0146]      FIG. 24  illustrates an example of an X2 control plane protocol stack structure for supporting an operation of an HN and RBN configuring a wireless backhaul according to an embodiment of the present disclosure. 
         [0147]    Referring to  FIG. 24 , according to an embodiment of the present disclosure, there is one X2 interface relation between each RBN and HN. Further, HN may have an X2 interface relation with adjacent BSs. Accordingly, the respective protocol stacks of RBN, HN, and BS have the same stack structure except for X2-AP for X2 interface that is used instead of S1-AP of  FIG. 23 . HN processes and delivers all X2 messages between RBN and other BSs for all UE-dedicated procedures. X2-AP message processing includes varying S1/X2-AP UE IDs, transport layer addresses, and GTP TEIDs and does not change other portions in the X2 message. All non-UE-dedicated X2-AP procedures are terminated in HN and are internally processed between RBN and HN and between HN and other BSs. Upon receiving X2 non-UE-dedicated message irrelevant to cell from RBN or neighbor BS, HN may trigger corresponding non-UE-dedicated X2-AP procedures for neighbor BS or RBN(s). When receiving cell-related X2 non-UE-dedicated message from RBN or neighbor BS, HN may deliver relevant information to the neighbor BS or RBN(s) based on cell information included in the message. 
         [0148]    S1 and X2 interface signaling packets are mapped to radio bearers through a Ub interface. 
         [0149]      FIG. 25  illustrates an example of a wireless control plane protocol stack structure for supporting an operation of an HN and RBN configuring a wireless backhaul according to an embodiment of the present disclosure.  FIG. 26  illustrates an example of a wireless user plane protocol stack structure for supporting an operation of an HN and RBN configuring a wireless backhaul according to an embodiment of the present disclosure. 
         [0150]    Referring to  FIGS. 25 and 26 , RBN is connected with HN through a Ub interface that uses the same procedure as wireless protocol by which a terminal connects to a base station. An example of wireless control plane protocol stack of RBN includes non-access stratum (NAS) providing access to MME, RRC, PDCP, RLC, MAC, and PHY, and MME includes NAS layer alone, and HN includes the protocols of RBN except for NAS. The user plane protocol stack of RBN and HN include PDCP, RLC, MAC, and PHY. 
         [0151]    Further, the following functions are supported for communication of HN and RBN based on the above-described protocol stack structure, according to an embodiment of the present disclosure. 
         [0152]    RRC layer using a Ub interface performs such function as configuring or reconfiguring a RBN frame for communication between RBN and HN through a RBN reconfiguration procedure. In such case, RBN may request HN to configure frame upon RRC connection establishment, and receiving the request, HN may initiate RRC signaling to configure RBN frame. Through the frame configuration function, HN may transfer configuration information for switch of operation mode or communication mode of wireless backhaul to RBN, or RBN may send a request for the configuration information to HN and receive the same from HN. When receiving frame configuration information through RRC signaling, RBN applies the frame configuration information to conduct communication with HN in the corresponding operation mode or communication mode according to an embodiment of the present disclosure 
         [0153]    Further, RRC layer using the Ub interface has a function of including and delivering newly changed SI in a dedicated message to RBN. By the above function, HN may transfer, to RBN, SI necessary to switch operation modes or communication modes of wireless backhaul according to an embodiment of the present disclosure, and RBN may receive and apply the SI to operate in the corresponding mode. 
         [0154]    The above-described BUS transition conditions according to an embodiment of the present disclosure may be embodied in the following four cases, and HN and RBN operation may be specified for each case. 
         [0155]    The first case is to install new RBN in the wireless backhaul. In this case, network OAM transfers a command to transition the HN operation mode into BUS to HN. RBN, when BUS powers on, automatically transitions the RBN&#39;s operation mode into BUS and operates. 
         [0156]    The second case is occurrence of a wireless backhaul link failure. A link failure of wireless backhaul may occur, e.g., when a building or obstacle newly stands on the wireless backhaul path to cut off communication path. HN may set, e.g., a failure to receive, without errors and within a predetermined time, a UL response to data or control information transmitted to RBN from RBN, as a reference for determining that a failure of link to RBN occurs (hereinafter, “link failure determination reference”). According to an embodiment of the present disclosure, the link failure determination reference may include when HN fails to receive, without error, UL data at a minimum transmission speed. The link failure determination reference may also include when HN fails to consecutively receive BA bits or information a predetermined number of times from RBN operating in the LDM of BOCS or fails to receive them within a predetermined time. 
         [0157]    RBN may set a link failure determination reference, e.g., including a failure to detect a sync of the RBN&#39;s serving HN, failure to receive shared control information without error, failure to receive, without error, control information of the RBN&#39;s serving HN, and failure to receive without error on DL at a minimum data transmission speed. In the second case, when HN identifies that the link failure determination reference is met, the HN determines to transition the HN&#39;s operation mode into BUS and delivers a command for transitioning the operation mode into BUS to the network OAM. Upon meeting the link failure determination reference, RBN directly transitions the RBN&#39;s operation mode into BUS. 
         [0158]    The third case is when link capability deterioration occurs in the wireless backhaul. The link capability deterioration in the wireless backhaul may include, e.g., such a circumstance where trees or woods on the communication path of wireless backhaul flourish to degrade link capability or where an object is newly installed to attenuate radio waves. According to an embodiment of the present disclosure, e.g., when wireless link capability or quality is continuously left lower than a threshold for a predetermined time may be set as a condition for determining the link capability deterioration (hereinafter, “link capability deterioration determination condition”). Here, the wireless link capability or quality may be determined by, e.g., mean power of received signal per unit radio resource, channel quality indicator (CQI) value, signal-to-noise ratio (SNR), data transmission speed per unit radio resource, and modulation and coding index (MCI) value. As specific examples, when a maximum value Pmax (t, t−Ts) of a predetermined capability index at a predetermined time interval (e.g., Ts=1 min.) remains worse than a reference capability value (e.g., P(t0)=capability value achieved by HN-RBN at the latest BUS-to-BOCS switch) by a threshold (e.g., ΔPs=6 dB) or more for a predetermined time (e.g., Tm=10□), it may be determined that a switch into BUS is needed. 
         [0159]    The above example may be represented in Equation 1 as follows: Decide S3, if, for t=t1 to t1−Tm, 
         [0000]        P max( t, t−Ts )&lt; P ( t 0)−: Δ Ps   Equation (1)
 
         [0160]    In the third case, when detecting wireless backhaul link capability deterioration based on the above-described link capability deterioration determination condition, HN may report the wireless backhaul link capability deterioration to the network OAM. Or, upon detecting the wireless backhaul link capability deterioration, HN may determine to transition the HN&#39;s operation mode into BUS and deliver a request for transitioning operation mode into BUS to the network OAM. Then, HN may transition into BUS by receiving an operation mode transition instruction from the OAM. 
         [0161]    In the third case, when receiving a command to transition the operation mode into BUS from the serving HN, RBN transitions into the corresponding mode. 
         [0162]    In the fourth and last case, a transition into BUS may be made at a predetermined period for enhancing performance or in response to an instruction from the operator. In such case, when receiving a command to transition the operation mode into BUS from the network OAM, HN transitions into BUS. When receiving an instruction to transition into BUS from the serving HN, RBN transitions into BUS. 
         [0163]      FIGS. 27 a  and 27 b    illustrates examples of an operation flow of a first case of BUS transition conditions according to an embodiment of the present disclosure. In the embodiment shown in  FIGS. 27 a  and 27 b   , it is assumed that an OAM of network  2710  is requested or instructed to install a new RBN  2700  to install the new RBN  2700 . Here, information on where the new RBN  2700  is installed or information on HN preferred by the new RBN  2700  or predicted to be linked by the new RBN  2700  may also be sent to OAM  2718 . 
         [0164]    Referring to  FIGS. 27 a  and 27 b   , when RBN  2700  newly installed powers on in operation  2720   a , the RBN  2700  transitions the operation mode into BUS in operation  2722 . 
         [0165]    OAM  2718  having installed the new RBN  2700  in operation  2720   b  determines HNs having a chance of being linked by the new RBN  2700  and bundles the determined HNs in an HN group operating in the BUS (hereinafter, denoted as “BUS HN group”) in operation  2723 . According to an embodiment of the present disclosure, the BUS HN group may be selected using the information on the location of the new RBN  2700  or information on HN preferred or predicted to be linked by the new RBN  2700  which is included in the request or instruction to install the new RBN  2700 . In operations  2724   a  and  2724   b , OAM  2718  transfers a command to transition operation mode into BUS to each HN included in the BUS HN group. For ease of description, it is assumed that HN 1   27702  and HNn  2704  are determined to be in the BUS HN group. In operation  2726 , when receiving a command to transition operation mode into BUS, HN 1   2702  and HNn  2704  each transition into BUS and operate. Here, HN 1   2702  and HNn  2704  each mark “BUS” on the operation mode in the BCH or beacon frame information and broadcast SI necessary for RBNs connected thereto to operate in the BUS. Further, HN 1   2702  and HNn  2704  each transmit SS and BCH slot through all their beams on DL and receive an access or association request signal from a corresponding RBN through all their beams on UL. They each perform beam training on all the beams to select the optimal beam on each of DL and UL. 
         [0166]    In operation  2728 , the new RBN  2700  operating in the BUS searches or scans HNs while conducting beam training on all reception beams. As a result, when receiving the SS and BCH or beacon frame information from HN 1   2702  and HNn  2704 , the new RBN  2700  detects HN 1   2702  and HNn  2704  operating in the BUS. In operation  2730 , the new RBN  2700  conducts beam training on each of the detected HN 1   2702  and HNn  2704  to determine the optimal transmission/reception beam presenting the highest receive power for each HN and determine the HN presenting the maximum beam power as the optimal HN. As an example, it is assumed that HN 1   2702  is determined as the optimal HN. 
         [0167]    In operation  2740 , the new RBN  2700  then performs a procedure  2740  for initial access (attach or associate) to the network  2710  through HN 1   2702 . First, in operation  2741 , the new RBN  2700  sends a RACH preamble or association request signal to HN 1   2702  determined as the optimal HN. Here, the new RBN  2700  also transfers a result of measurement made on HN 1   2702  in operation  2730  to HN 1   2702 . When receiving the RACH preamble, HN 1   2702  operates as a serving HN of the new RBN  2700  to provide a network connection. Accordingly, in operation  2742 , HN 1   2702  delivers time advance (TA) value and scheduling grant to the new RBN  2700 , and in operation  2743 , the HN 1   2702  conducts the above-described RRC connection setup procedure. Through the RRC connection setup procedure, HN 1   2702  and the new RBN  2700  may configure frames for transitioning the operation mode or communication mode of wireless backhaul. In operation  2744 , RBN  2700  performs NAS access, authentication, and security procedures with network  2710 , and in operation  2745 , MME reports a result of the authentication and security procedure performed with the new RBN  2700  to a home subscriber server (HSS) to register the location information on the new RBN  2700 . Thereafter, io 2746 , HN 1   2702  and MME generate a session for GTP tunneling described above. According to an embodiment of the present disclosure, in operation  2747 , the new RBN  2700  then reconfigures a frame for operation in the BUS through RRC connection for HN 1   2702 , and in operation  2748 , HN 1   2702  performs S1 context setting with MME. 
         [0168]    In operation  2750 , the new RBN  2700  sets a configuration for wireless backhaul connection. Accordingly, in operation  2751 , the new RBN  2700  initializes parameters for wireless backhaul connection through OAM  2718  and sets up a configuration for the wireless backhaul connection. In operation  2752 , the new RBN  2700  configures an S1 interface with HN 1   2702 , and in operation  2753 , HN 1   2702  updates the configuration of S1 interface of RBN 1   2700 . In operation  2754 , the new RBN  2700  configures an X2 interface with HN 1   2702 , and in operation  2755 , HN 1   2702  updates the configuration of X2 interface of RBN 1   2700 . According to an embodiment of the present disclosure, operations and functions for user plane and control plane through S1 and X2 interface overlap those described above in connection with  FIGS. 20 to 26 , and thus, no detailed description thereof is given. 
         [0169]    Thereafter, in operation  2760   a , HN 1   2702 , a predetermined time after completing network access and configuration setup of the new RBN  2700 , sends a backhaul update status report to OAM  2718 . Likewise, when a predetermined time elapses, HNn  2704  in the BUS HN group also delivers a backhaul update status report to OAM  2718  in operation  2760   b.    
         [0170]    Upon identifying the reception of the backhaul update status reports from HN 1   2702  and HNn  2704  to which operation mode transition commands have been sent in operations  2724   a  and  2724   b , OAM  2718  determines whether to transition the operation mode of HN 1   2702  and HNn  274  into BOCS in operation  2762 . In this case, when the network access procedure of the new RBN  2700  is complete within a predetermined time or fails within a predetermined time, OAM  2718  may determine to transition the operation mode of HN 1   2702  and HNn  2704  into BOCS. Assuming that the operation mode of HN 1   2702  and HNn  2704  has been determined to transition into BOCS, OAM  2718  delivers commands to transition operation mode into BOCS to HN 1   2702  and HNn  2704 , respectively, in operations  2764   a  and  2764   b . Then, in operation  2766 , HN 1   2702  and HNn  2704  each transition into BOCS. HN 1   2702  transfers the HN 1 &#39;s transition into BOCS to the new RBN  2700 . When receiving a BOCS transition command from serving HN 1   2702 , new RBN  2700  in operation  2768  transitions into BOCS. 
         [0171]      FIGS. 28 a , 29 b , and 29 c    illustrate examples of an operation flow of a second case of the BUS transition conditions according to an embodiment of the present disclosure. 
         [0172]    Referring to  FIGS. 28 a , 28 b , and 28 c   , in operation  2822   a , a RBN  2800  identifies that one of the above-described link failure determination references is met and detects the occurrence of a link failure. RBN  2800  may recognize that one of the link failure determination references, e.g., when RBN  2800  fails to receive sync signal, control information, or user data of a serving HN (e.g., HN 2   2804 ) at the RBN  2800 &#39;s lowest data speed within a predetermined time as in operation  2821 , is met and thus a link failure occurs. In operation  2823 , RBN  2800  transitions the RBN  2800 &#39;s operation mode into BUS. 
         [0173]    Meanwhile, in operation  2822   b , a HN 2   2804  identifies that one of the above-described link failure determination references is met and detects the occurrence of a link failure. As the link failure determination reference, HN 2   2804  may recognize that a link failure occurs, e.g., when there is no UL response from RBN  2800  during a predetermined time as in operation  2821  or a link failure occurs when the HN 2   2804  fails to continuously receive BA bits or information a predetermined number of times from a RBN operating in the LDM. Then, in operation  2824 , HN 2   2804  delivers a backhaul update request containing a link failure report to an OAM  2818  of network  2810 . Here, HN 2   2804  may include a measurement result previously reported by RBN  2800  in the backhaul update request or determine candidate HNs having a high chance of being linked by RBN  2800  and include information on the determined candidate HNs in the backhaul update request. Here, regarding the candidate HNs highly likely to be linked by RBN  2800 , it may be determined that the candidate HN having a maximum received power of BM signal in the measurement result has a higher chance of being linked by RBN  2800 . Further, HN 2   2804  of RBN  2800  may additionally perform disassociation from network  2810  on RBN  2800 . 
         [0174]    In operation  2825 , OAM  2818  determines HNs likely to be accessed by RBN  2800  using information on the candidate HNs or measurement result of RBN  2800  obtained from the backhaul update request and puts the determined HNs in one BUS HN group. Hereinafter, operations  2826   a  to  2850  are substantially the same as operations  2724   a  to  2760   b  of  FIG. 27 , and thus, no repetitive description thereof is given. 
         [0175]    When receiving, in operation  2850 , backhaul update status reports from HNs (e.g., HN 1   2802  and HN 2   2804  are assumed) having sent commands to transition operation mode into BUS in operations  2826   a  and  2826   b , OAM  2818  determines whether to transition the operation mode of HN 1   2802  and HN 2   2804  into BOCS in operation  2851 . It is assumed that the network access procedure by RBN  2800  is complete within a predetermined time or fails within a predetermined time. In such case, OAM  2818  determines to transition the respective operation modes of HN 1   2802  and HN 2   2804  into BOCS. In operations  2852   a  and  2852   b , OAM  2818  delivers commands to transition operation mode into BOCS to HN 1   2802  and HN 2   2804 , respectively. Then, in operation  2853 , HN 1   2802  and HN 2   2804  each transition into BOCS. HN 2   2804  transfers the HN 2804 &#39;s transition into BOCS to RBN  2800 . When receiving a BOCS transition command from serving HN 2   2804 , RBN  2800  in operation  2854  transitions into BOCS. 
         [0176]    In another embodiment of the second case among the BUS transition conditions, the same operations as those described above in connection with  FIGS. 28 a  to 28 c    are performed except for the following: 
         [0177]    Specifically, in another exercise of the second case, HN 2 , which is a serving HN of the RAN having detected a link failure, instead of OAM in the embodiment of  FIG. 28 , determines HNs having a chance of being accessed by the RBN linked to HN 2  and puts the determined HNs in one BUS HN group. Here, among the determined HNs, ones highly likely to be accessed by the RBN may be selected and used based on the measurement result or utilizing the measurement result upon detection of the link failure from the RBN having detected the link failure. 
         [0178]    According to the other embodiment, the serving HN 2 , instead of OAM, sends a request for transitioning operation mode into BUS or the operation mode transition command to each HN included in the BUS HN group and informs the OAM in the network that HN 2  determines to transition operation mode into BUS for the HNs. 
         [0179]    According to the other embodiment, the serving HN 2  reports the transition into BUS to the network OAM after a predetermined time elapses. Further, after a predetermined time passes, each HN included in the BUS HN group also reports the transition into BUS to the serving HN 2  and network OAM. When receiving the report for transition into BUS from the HNs included in the BUS HN group, the serving HN 2  may determine whether to transition the operation mode of the HNs into BOCS. For example, when the network access of RBN having detected the link failure is complete within a predetermined time or fails within a predetermined time, the serving HN 2  may determine to transition the operation mode of each HN included in the BUS HN group into BOCS. Then, the serving HN 2  sends a request or command for transitioning operation mode into BOCS to each HN included in the BUS HN group, and the serving HN 2  reports to the network OAM that operation mode transition into BOCS of the HNs has been determined. 
         [0180]      FIGS. 29 a , 29 b , 29 c , and 29 d    illustrate examples of an operation flow of a third case of the BUS transition conditions according to an embodiment of the present disclosure. 
         [0181]    Referring to  FIGS. 29 a  to 29 d   , it is assumed that a deterioration of link capability of a source HN, which is a serving HN, is detected to determine a transition of operation mode into BUS in operation  2918 . Then, in operation  2920 , source HN  2902  sends a backhaul update request to an OAM  2916 . Here, the backhaul update request may include a measurement result received by the source HN from a RBN  2900  connected thereto or information on candidate HNs highly likely to be accessed by the RBN determined by the source HN. 
         [0182]    When receiving the backhaul update request, OAM  2916  in operation  2921  determines HNs likely to be accessed by RBN  2900  based on the measurement result of RBN  2900  or information on the candidate HNs highly likely to be accessed by RBN  2900  obtained from the backhaul update request and puts the determined HNs in one BUS HN group. An example is assumed in which the BUS HN group includes a source HN  2902  and a target HN  2904 . Then, in operations  2922   a  and  2922   b , OAM  2916  delivers commands to transition operation mode into BUS to source HN  2902  and target HN  2904 , respectively. In operation  2923 , source HN  2902  and target HN  2904  transit the HN  2902 &#39;s operation mode and the target HN  2904 &#39;s operation mode into BUS and operate, respectively. Here, source HN  2902  and target HN  2904  mark “BUS” on their operation modes in BCH or beacon frame information and broadcast SI necessary for RBNs to operate in the BUS. Further, source HN  2902  and target HN  2904  each send SS and BCH slot through all their beams on DL and receive RACH signal or association request signal for access by RBN through all their beams on UL to discover the optimal beam among all their beams on each of DL and UL. 
         [0183]    When receiving, in operation  2924 , operation mode indication information indicating that the operation mode of source HN  2902  is the BUS from the serving HN of RBN  2900 , i.e., source HN  2902 , RBN  2900  transitions the RBN  2900 &#39;s operation mode into BUS in operation  2924 . In operation  2925 , source HN  2902  instructs RBN  2900  to do BM. Here, in operations  2926   a  and  2926   b , communication of packet data is possible between source HN  2902  and RBN  2900 , and packet data communication is possible between source HN  2902  and GW  2914  of network  2910 . 
         [0184]    In operation  2926   c , source HN  2902  assigns a UL resource of RBN  2900 . Then, in operation  2927 , RBN  2900  performs BM measurement based on the UL resource to detect HNs operating in the BUS as well as source HN  2902  and performs BM measurement on the detected HNs and sends a result of the measurement to source HN  2902 . When receiving the BM measurement result from RBN  2900 , source HN  2902 , in operation  2928 , identifies whether there is HN capable of providing higher-quality communication to RBN  2900  and determines a handover of RBN  2900 . For example, when the HN having the maximum beam receive power known from the BM measurement result of RBN is not source HN  2902  but another HN, the other HN (e.g., it is assumed to be target HN  2904 ) is selected as a target HN for handover. 
         [0185]    Then, in the embodiment shown in  FIGS. 29 a  to 29 d   , a handover preparation procedure A is performed through operations  2929  to  2933 . First, operation  2929 , source HN  2902  sends a handover request to target HN  2904 . In operation  2930 , the target HN identifies whether access by RBN is acceptable, and when acceptable, the target HN sends a handover request acknowledgement to source HN  2902  to accept access by RBN  2900  in operation  2931 . Then, in operation  2932 , source HN  2902  assign DL resource of RBN  2900  and in operation  2933 , sends a RRC message based on the DL resource to instruct a handover to target HN  2904  and sends handover-related information. 
         [0186]    When the handover preparation procedure A is complete, a handover running procedure B is performed through operations  2934  to  2940 . In operation  2934 , the RBN releases connection with source HN  2902 , and in operation  2935 , source HN  2902  determines to deliver packet data supposed to be transmitted to RBN  2900  and stored in the buffer to target HN  2904 . In operation  2936 , source HN  2902  sends a packet sequence number (SN) status transmission message to target HN  2904 , and in operation  2936 , forwards the packet data supposed to be transmitted to RBN  2900  and stored in the RBN  2900 &#39;s buffer to target HN  2904 . Accordingly, in operation  2938 , target HN  2904  stores the data packets forwarded from source HN  2902  in the buffer. 
         [0187]    In operation  2939 , RBN  2900  sends a RACH signal or association request to access target HN  2904 . Then, in operation  2940 , target HN  2904  delivers UL resource allocation information of RBN  2900  and TA value for UL. Target HN  2904  also sends a response to the association request. When RBN  2900  successfully access target HN  2904 , the target HN  2904  sends a RRC message to target RBN  2904  to report that the handover is successfully done and also sends relevant information in operation  2941 . 
         [0188]    When the handover running procedure B is complete, target HN  2904 , like in operation  2942   a , may send packet data to RBN  2900 . A handover complete procedure C is performed through operations  2943  to  2950 . 
         [0189]    In operation  2943 , target HN  2904  sends a path switch request to a MME  2912  to report that the serving HN linked to RBN  2900  has been changed into target HN  2904 . Then, in operation  2944 , MME  2912  delivers a modify bearer request to an S-GW  2910 . Then, in operation  2945 , S-GW  2910  switches the path of DL data for RBN  2900  into target HN  2904 . Accordingly, when DL packet data occurs, the DL packet data is transferred to target HN  2904 , rather than source HN  2902 , as in operation  2946 . 
         [0190]    Thereafter, in operation  2947 , S-GW  2914  sends a modify bearer response message to MME  2912 . Then, in operation  2948 , MME  2912  sends a path switch request acknowledgement message to target HN  2904  in response to the path switch request. In operation  2949 , target HN  2904  sends a RBN context release message to source HN  2902  to inform that the handover of RBN  2900  to target HN  2904  has been successfully done and to trigger resource release of source HN  2902 . When receiving the RBN context release message, source HN  2902  releases wireless and control-related resources related to RBN  2900  in operation  2950 . In operation  2951 , source HN  2902  delivers a backhaul update status report including information on the handover result of RBN  2900  to OAM  2916  after a predetermined time elapses. Further, when a predetermined time elapses, the HNs included in the BUS HN group also deliver backhaul update status reports to source HN  2902  and OAM  2916 . 
         [0191]    Upon identifying the reception of the backhaul update status reports from source HN  2902  and target HN  2904  to which operation mode transition commands have been sent in operations  2922   a  and  2922   b , OAM  2916  determines whether to transition the operation mode of source HN  2902  and target HN  2904  into BOCS in operation  2952 . In this case, when a link capability enhancement of RBN  2900  is complete within a predetermined time or fails within a predetermined time, OAM  2916  may determine to transition the operation mode of source HN  2902  and target HN  2904  into BOCS. Assuming that the operation mode of source HN  2902  and target HN  2904  has been determined to transition into BOCS, OAM  2916  delivers commands to transition operation mode into BOCS to source HN  2902  and target HN  2904 , respectively, in operations  2753   a  and  2753   b . Then, in operation  2954 , source HN  2902  and target HN  2904  each transition into BOCS. Target HN  2904  transfers the target HN  2904 &#39;s transition into BOCS to RBN  2900 . When receiving the same, RBN  2900  transitions into BOCS in operation  2955 . 
         [0192]    In another embodiment of the third case among the BUS transition conditions, the same operations as those described above in connection with  FIGS. 29 a  to 29 d    are performed except for the following: 
         [0193]    Specifically, in another embodiment of the third case, the source HN, instead of OAM, determines HNs having a chance of being accessed by the RBN linked to source HN and puts the determined HNs in one BUS HN group. The BUS HN group may be selected using the measurement result of the RBN or information on the candidate HNs highly likely to be accessed by the RBN. The source HN delivers a request or command for transition operation mode into BUS to the HNs included in the BUS HN group and reports to the OAM that an operation mode transition into BUS of the HNs in the BUS HN group has been determined. According to another embodiment, the source HN, instead of the OAM, determines whether to transition the operation mode of the HNs in the BUS HN group into BOCS. Here, when a link capability enhancement of the RBN is complete within a predetermined time or fails within a predetermined time, the HNs may be determined to transition into BOCS. The source HN, instead of the OAM, delivers a request or command for transitioning operation mode into BOCS to the HNs in the BUS HN group, and the source HN further reports the determination of transition of the operation mode of the HNs into then BOCS to the OAM. 
         [0194]      FIG. 30  illustrates an example of an operation flow of a fourth case of the BUS transition conditions according to an embodiment of the present disclosure. 
         [0195]    Referring to  FIG. 30 , it is assumed that in operation  3020 , a serving HN  3002  of a RBN  3000  detects a deterioration of link capability and determines to request to update backhaul. Accordingly, in operation  3022 , serving HN  3002  delivers a backhaul update request to an OAM  3012  of a network  3010 . Here, the backhaul update request may include a measurement result received from RBN  3000  or information on candidate HNs highly likely to be accessed by RBN  3000  determined by source HN  3002  based on the BM result. In operation  3012 , OAM  3012  having received the backhaul update request determines HNs likely to be accessed by RBN  3000  using information on the candidate HNs or measurement result obtained from the backhaul update request and puts the determined HNs in one BUS HN group. Here, assuming that serving HN  3002  and an adjacent HN  3004  are in the BUS HN group, OAM  3012 , in operations  3026   a  and  3026   b , delivers a command to transition operation mode into BUS to each of the HNs, i.e., serving HN  3002  and adjacent HN  3004 , of the BUS HN group. 
         [0196]    When receiving the operation mode transition command, serving HN  3002  and adjacent HN  3004  each transition into BUS and operate in operation  3027 . Here, serving HN  3002  and adjacent HN  3004  each puts a mark indicating that the serving HN  3002 &#39;s operation mode and the adjacent HN  3004 &#39;s operation mode, respectively, into the BUS on the BCH and broadcasts SI necessary for RBNs linked thereto to operate in the BUS. Further, serving HN  3002  and adjacent HN  3004  transmit SS and BCH for all the beams on DL, receive RACH through all the RACH&#39;s beams on UL, and select the optimal beam by conducting BM on DL and UL. 
         [0197]    In operation  3028 , when receiving BCH broadcast from serving HN  3002  and identifying that the operation mode of serving HN  3002  is the BUS, RBN  3000  transitions into BUS and operates. 
         [0198]    In operation  3029 , serving HN  3002  instructs RBN  3000  to conduct BM, and in operation  3030 , serving HN  3002  delivers UL resource allocation information for RBN  3000  to RBN  3000 . In operation  3031 , RBN  3000  detects adjacent HNs, e.g., adjacent HN  3004 , operating in the BUS, as well as serving HN  3002  using a UL resource corresponding to the UL resource allocation information, conducts BM measurement on the detected adjacent HNs, and reports a result to serving HN  3002 . 
         [0199]    In operation  3032 , upon identifying that there is a communication path in a different direction which provides higher-quality communication to RBN  3000  based on the BM result, serving HN  3002  determines to change beams for RBN  3000 , and in operation  3040 , serving HN  3002  conducts a beam changing procedure on RBN  3000 . Specifically, in operation  3041 , serving HN  3002  instructs RBN  3000  to send a RACH signal in a new beam. Pursuant to the instruction, in operation  3042 , RBN  3000  sends a RACH signal in the new beam. When receiving the RACH signal, serving HN  3002  in operation  3043  gives a UL TA value to RBN  3000  and instructs RBN  3000  to change and use the new beam. A wireless backhaul adopting the IEEE 802.11ad wireless LAN standard changes optimal beams without RACH signal transmission/reception in operation  3042  and TA value transfer process in operation  3043  among the operations constituting the beam changing procedure  3040 . 
         [0200]    Thereafter, in operation  3050   a , serving HN  3002  delivers a backhaul update status report containing information on the beam change of RBN  300  to OAM  3012 . Further, in operation  3050   b , the HNs, i.e., adjacent HN  3004 , in the BUS HN group also deliver backhaul update status reports to serving HN  3002  and OAM  3012  when a predetermined time elapses. 
         [0201]    Upon identifying the reception of the backhaul update status reports from source HN  3002  and adjacent HN  3004  to which operation mode transition commands have been sent in operations  3026   a  and  3026   b , OAM  3012  determines whether to transition the operation mode of source HN  3002  and neighbor HN  3004  into BOCS in operation  3051 . In this case, when a link capability enhancement of RBN  3000  is complete within a predetermined time or fails within a predetermined time, OAM  3012  may determine to transition the operation mode of source HN  3002  and adjacent HN  3004  into BOCS. Assuming that the operation mode of source HN  3002  and adjacent HN  3004  has been determined to transition into BOCS, OAM  3000  delivers commands to transition operation mode into BOCS to source HN  3002  and adjacent HN  3004 , respectively, in operations  3052   a  and  3052   b . Then, in operation  3053 , source HN  3002  and adjacent HN  3004  each transition into BOCS. Adjacent HN  3004  transfers the HN  3004 &#39;s transition into BOCS to RBN  3000 . When receiving the same, RBN  3000  transitions into BOCS in operation  3054 . 
         [0202]    In another embodiment of the fourth case among the BUS transition conditions, the same operations as those described above in connection with  FIG. 30  are performed except for the following: 
         [0203]    Specifically, in another embodiment of the fourth case, the serving HN, instead of OAM, determines HNs having a chance of being accessed by the RBN linked to source HN and puts the determined HNs in one BUS HN group. The BUS HN group may be selected using the measurement result of the RBN or information on the candidate HNs highly likely to be accessed by the RBN. The serving HN delivers a request or command for transition operation mode into BUS to the HNs included in the BUS HN group and additionally reports to the OAM that an operation mode transition into BUS of the HNs in the BUS HN group has been determined. According to another embodiment, the serving HN determines whether to transition the operation mode of the HNs in the BUS HN group into BOCS. Here, when a link capability enhancement of the RBN is complete within a predetermined time or fails within a predetermined time, the HNs may be determined to transition into BOCS. The serving HN, instead of the OAM, delivers a request or command for transitioning operation mode into BOCS to the HNs in the BUS HN group, and the serving HN further reports the determination of transition of the operation mode of the HNs into then BOCS to the OAM. 
         [0204]    Further, according to another embodiment, the procedures shown in  FIGS. 29 and 30  may be performed at predetermined periods or by the operator&#39;s instruction to enhance capability. 
         [0205]      FIG. 31  illustrates an operation flow of a RBN according to an embodiment of the present disclosure. In connection with the embodiment shown in  FIG. 31 , RBN is assumed to be a RBN that attempts initial access to a network in a wireless backhaul or reaccesses the network after a link failure occurs. 
         [0206]    When powering on or detecting a link failure in operation  3102 , RBN transitions into BUS in operation  3104 . Through operations  3106  to  3110 , RBN stands by for detecting at least one HN that has transitioned into BUS. Specifically, RBN in operation  3106  searches for HNs using all the RBN&#39;s beams, and in operation  3108 , receives the BCH of a searched HN to obtain shared control information of the HN. In operation  3110 , RBN identifies whether the operation mode of the detected HN is the BUS. Unless the identification result shows that the operation mode of the detected HN is the BUS, RBN returns to operation  3106  to search for HNs. 
         [0207]    When the operation mode of the detected HN is identified as the BUS, RBN in operation  3112  measures the receive power of all transmission/reception beam combinations on the detected HN. In operation  3114 , RBN discovers the optimal transmission/reception beam combination giving the maximum receive power for each detected HN and further discovers the optimal HN (HNopt) having the maximum receive power. In operation  3116 , RBN sends a RACH preamble to HNopt, accesses and generates a wireless backhaul link, initializes parameters for the link and sets up a configuration, and reports the BM result to HNopt. In operation  3118 , RBN identifies whether the operation mode of HNopt, which is the serving HN, is the BOCS. When the operation mode of HNopt is identified to be the BOCS, RBN in operation  3120  transitions into BOCS and operates. When the operation mode of HNopt is identified to be the BUS, RBN in operation  3122  maintains the current operation mode. 
         [0208]      FIG. 32  is a view illustrating an operation flow of a RBN according to another embodiment of the present disclosure. RBN in the embodiment shown in  FIG. 32  is assumed to be a RBN detecting a link capability deterioration of wireless backhaul to change serving HNs or beams. 
         [0209]    When identifying that the operation mode of serving HN is the BUS through shared control information of BCH received from serving HN in operation  3202 , RBN transitions into BUS and operates in operation  3204 . In operation  3206 , RBN searches for HNs with all the RBN&#39;s beams, and in operation  3208 , RBN performs BM for measuring the receive power of all transmission/reception beam combinations on the detected HNs. In operation  3210 , RBN reports the measurement result to serving HN. 
         [0210]    In operation  3212 , RBN identifies whether an instruction to hand over to the target HN is received from the serving HN. When the handover instruction is identified to be received, RBN in operation  3216  hands over to the target HN and goes to operation  3220 . Unless the handover instruction is received, RBN proceeds with operation  3214 . In operation  3214 , RAN identifies whether to receive an instruction to change into a new beam from the serving HN. When the instruction to change is identified to be received, RBN in operation  3218  sends a RACH preamble to serving HN using the new beam as instructed by serving HN, receives a TA for the new beam from serving HN, and communicates with serving HN using the new beam. 
         [0211]    In operation  3220 , RBN identifies whether the operation mode of serving HN is the BOCS. When the operation mode of serving HN is identified to be the BOCS, RBN in operation  3222  transitions into BOCS and operates. When the operation mode of serving HN is identified to be the BUS, RBN in operation  3224  maintains the current operation mode. 
         [0212]    Meanwhile, according to an embodiment of the present disclosure, he wireless backhaul may optimize capability through an operation mode transition. As shown in  FIG. 3 , a transition between communication modes is possible while RBN is operating in the BOCS, thereby leading to further enhanced capability. Among the communication modes, ABM may support a mitigation of capability deterioration that occurs due to an influence from external environment. Transition condition and operations between communication modes according to an embodiment of the present disclosure may be specified as follows: 
         [0213]    Referring to  FIG. 3 , a transition (a) from FBM  342  to ABM  344  may be made under the environment that the antenna of HN or RBN is swayed by an external influence, e.g., a strong wind gust, resulting in the magnitude of received signal fluctuating. 
         [0214]    According to an embodiment of the present disclosure, as a reference of transition (a), FBM-to-ABM transition (a) may be determined to be made when, in order to detect such a phenomenon that wireless channel characteristics of serving HN and RBN are varied over time, and among link capabilities, the receive power of signal per unit radio resource for RBN or AGC of receiver is varied over time, the difference between the maxium value Pmax(t, t−Ta) and minimum value Pmin(t, t−Ta) of link capability index within a predetermined time interval (e.g., Ta=one min.) is larger than a threshold (e.g., :ΔPa=6 dB), and the time during which the difference is larger than the threshold lasts a predetermined time (e.g., Tm=10 min.) or longer. 
         [0215]    Transition (a) may be represented in Equation 2 below: 
         [0216]    Decide M1, if, for t=t1 ˜t1−Tm, 
         [0000]        P max( t,t−Ta )− P min( t,t−Ta )&gt;:Δ Pa.   Equation (2)
 
         [0217]    Upon meeting the transition (a) reference, serving HN determines to transition the communication mode of RBN from FBM to ABM, assigns DL BM slot or UL BM slot, or DL and UL BM slot, and instructs RBN to perform DL BM or UL BM, or DL and UL BM. Then, HN and RBN conduct beam measurement according to an embodiment of the present disclosure. The DL optimal beam is determined by RBN, and the UL optimal beam is determined by HN. Or, according to an embodiment of the present disclosure, BM is performed only on DL, and UL may use a UL beam in the same direction as that of the DL optimal beam. Alternatively, BM may be performed only on UL, and DL may use a DL beam in the same direction as that of the UL optimal beam. According to an embodiment of the present disclosure, HN and RBN may perform adjustment so that the BM range encompasses the optimal beam while monitoring the variation in the optimal beam in a three-dimensional (3D) beam direction and adaptively varying the beam range to be measured. Further, according to an embodiment of the present disclosure, when each of a plurality of RBNs linked to one serving HN measures BM, the number of DL or UL beams may be different per RBN. 
         [0218]    Next, referring to  FIG. 3 , as a reference for a transition (b) from ABM  344  to FBM  342 , a transition into FBM may be made upon meeting a transition (b) reference where the optimal beam is not varied for a predetermined time in ABM. For example, when the optimal beams of serving HN and RBN operating in ABM for 10 or 30 minutes are not varied, transition (b) into FBM may be determined to be made. Then, according to an embodiment of the present disclosure, serving HN determines a communication mode transition of RBN and delivers a command to transition communication mode into FBM to RBN without assigning DL and UL BM slot to RBN. 
         [0219]    Referring to  FIG. 3 , in a case where a transition (f) is made from FBM to LDM, e.g., upon meeting such a transition (f) reference in which BS installed in RBN operates in LDM, RBN may send a request for transitioning communication mode into LDM to serving HN. 
         [0220]    When receiving the request for transitioning communication mode into LDM from RBN, serving HN determines DL paging period, UL scheduling request (SR) period, UL BA bit or information transmission period which are to be used when RBN operates in LDM and informs RBN. Then, RBN receives DL control information at each DL paging period to determine whether paging is present, has a chance of transmitting an SR bit at the uplink scheduling request period, and sends a BA bit at each UL BA bit transmission period. RBN sends BA bits for all the RBN&#39;s BA bit transmission times while operating in LDM. 
         [0221]    Referring to  FIG. 3 , in a transition (e) from LDM  346  to FBM  342  or a transition (d) to ABM  344 , e.g., when paging is received on DL by a UE located in the coverage of RBN-BS may be set as a mode transition condition in LDM. As another example, when RBN sends a UL SR bit or information to transmit UL information or data on UL may be determined as a mode transition condition in LDM. 
         [0222]    When the above-described mode transition condition in LDM is met, a communication mode prior to transition into LDM is identified, and a transition is made corresponding to the prior communication mode. Specifically, when the prior communication mode is FBM, a transition into FBM is made, and when the prior communication mode is ABM, a transition into ABM is made. Or, according to an embodiment of the present disclosure, FBM may be set as default, so that a communication mode transition in LDM always is made to FBM or ABM selectively. When meeting the mode transition condition in LDM as set forth above, RBN transitions into the prior communication mode or default communication mode and may use the optimal beam used to be used in the prior communication mode. 
         [0223]    Referring to  FIG. 3 , in a transition (c) from ABM  344  to LDM, like transition (f), when BS of RBN is operated in LDM may be set as a transition (c) condition. When the transition (c) condition is met, RBN sends a request for switching communication mode into LDM to serving HN. When receiving the communication mode switching request, serving HN determines DL paging period, UL SR period, UL BA bit or information transmission period which are to be used when RBN operates in LDM and informs RBN. RBN operating in LDM receives DL control information at each DL paging period to determine whether there is paging, has a chance of sending an SR bit at UL SR period, and sends a BA bit at each UL BA bit period. Here, RBN sends BA bits for all the RBN&#39;s BA bit transmission times while operating in LDM. 
         [0224]    Meanwhile, according to an embodiment of the present disclosure, when HN and RBN transition their operation mode from BUS to BOCS, FBM or ABM may be selected as a default communication mode of RBN. 
         [0225]      FIG. 33  is a view illustrating an example of an operation mode of an HN operated as a BOCS and a communication mode transition operation according to an embodiment of the present disclosure. 
         [0226]    Referring to  FIG. 33 , in operation  3300 , HN identifies whether to receive a command to switch operation mode into BUS from an OAM of network or adjacent HN. When the command of switching operation mode into BUS is identified to be received, HN in operation  3338  transitions operation mode into BUS. In such case, according to an embodiment of the present disclosure, HN may perform the operations described above in connection with  FIGS. 27 to 30 . 
         [0227]    Unless the command of switching operation mode into BUS is identified to be received, HN in operation  3302  identifies whether the communication mode of RBN linked to HN is LDM. When the communication mode is identified to be LDM, HN in operation  3304  identifies whether BA bit or information is received from RBN at least once without error during a predetermined BA period. When no BS bit or information is identified to be received during the BS period, HN in operation  3306  detects a wireless backhaul link failure between HN and RBN and transitions into BUS. In such case, HN having transitioned into BUS may operate according to the embodiment shown in  FIG. 28 . In this case, HN may correspond to HN 2  of  FIG. 28 . 
         [0228]    When BA is identified in operation  3304  to be received at least once or more, HN in operation  3308  determines that the wireless backhaul link with RBN is maintained and identifies whether SR bit or information is received from RBN to send UL data or control information, whether there is data to be sent on DL from HN to RBN, or whether paging is received from a MS linked to BS of RBN. As a result of the identification, upon meeting one of when SR bit or information is received, when there is data to be sent on DL, and when paging is received from MS, HN in operation  3310  switches the communication mode of RBN from FBM to ABM and conduct wireless backhaul communication. Here, the communication mode switch of RBN is operated corresponding to the operations related to transition (e) and transition (d) described above in connection with  FIG. 3 . As a result of the identification, when none of when SR bit or information is received, when there is data to be sent on DL, and when paging is received from MS are met, HN and RBN in operation  3312  maintain their current communication modes BOCS and LDM. 
         [0229]    When the communication mode of RBN is identified in operation  3302  to be not LDM, HN in operation  3312  identifies whether a UL response to DL control information or data is received from RBN within a predetermined time without error. As a result of the identification, when a UL response to DL control information or data is not received within the predetermined time or received information has an error, HN determines that a wireless backhaul link failure occurs between HN and RBN and HN in operation  3336  transitions the HN&#39;s operation mode into BUS. In this case, HN performs an operation corresponding to HN 2  of  FIG. 28 . 
         [0230]    When a UL response to DL control information or data is identified in operation  3312  to be received within the predetermined time, HN in operation  3314  identifies whether to receive a request for transitioning communication mode into LDM from RBN. When the request for transitioning communication mode into LDM is identified to be received, HN in operation  3316  delivers the request for transitioning communication mode into LDM to RBN. In this case, a transition operation is performed corresponding to transition (c) and transition (f) of  FIG. 3 . Unless the request for transitioning communication mode into LDM is identified to be received, HN in operation  3318  identifies whether the communication mode of RBN is FBM. 
         [0231]    When the communication mode of RBN is FBM, HN in operation  3320  identifies whether link capability for RBN is poor during a time determined as in Equation 1 described above. When the link capability is identified to be poor, HN in operation  3322  determines that a wireless backhaul link capability deterioration for RBN occurs and transitions the HN&#39;s operation mode into BUS. In this case, HN may perform operations corresponding to those shown in  FIG. 29 or 30 . Here, HN may operate as the source HN of  FIG. 29  or serving HN of  FIG. 30 . 
         [0232]    When the link capability of RBN is identified in operation  3320  to be not poor during the time, HN in operation  3324  identifies whether a variation in the wireless backhaul link capability for RBN is larger than a threshold, e.g., during a time determined as in Equation 2. When the variation in the link capability is identified to be larger than the threshold, HN in operation  3326  transitions the communication mode of RBN into ABM. Here, HN operates corresponding to the transition (a) operation described above in connection with  FIG. 3 . When the variation in the link capability is identified to be equal or smaller than the threshold, HN and RBN in operation  3328  maintain the current operation modes and communication modes, i.e., BOCS and FBM. 
         [0233]    When the communication mode of RBN is identified in operation  3318  to be not FBM, HN in operation  3330  identifies whether the optimal beam for RBN is changed during a predetermined time. When the optimal beam is changed, HN and RBN in operation  3332  maintain the current operation modes and communication modes, i.e., BOCS and ABM. When the optimal beam is identified to be not changed, HN in operation  3334  transitions the communication mode of RBN into FBM. In this case, HN operates corresponding to the transition (b) operation described above in connection with  FIG. 3 . 
         [0234]      FIG. 34  is a view illustrating an example of a configuration of a backhaul node and RAN according to an embodiment of the present disclosure. The configuration of the backhaul node and RAN of  FIG. 34  is merely an example for illustrative purposes, and according to an embodiment of the present disclosure, the configuration may be broken down into more detailed functional units, modules, components, devices, or the backhaul node and RAN components may be integrated into a single unit or varied to fit the business provider&#39;s intention. 
         [0235]    Referring to  FIG. 34 , backhaul node  3410  corresponds to an HN or RBN according to an embodiment of the present disclosure, and RAN corresponds to a MBS or SBS. 
         [0236]    Backhaul node  3410  includes, e.g., a transceiver unit  3418 , a radio frequency (RF) unit  3416 , a processor  3412 , and a memory  3414 , and RAN  3430  includes a processor  3432 , a memory  3434 , a RF unit  3436 , and a transceiver unit  3438 . Backhaul node  3410  is connected with a network via element  3440  and is connected with RAN  3430  via element  3420 . According to an embodiment of the present disclosure, processor  3412  performs wireless backhaul communication functions including transmission and reception of wireless backhaul data, wireless backhaul control information, and various wireless backhaul signals and a control function for wireless backhaul operation, and memory  3414  stores various wireless backhaul data, control information, and signals. Processor  3412  controls RF unit  3416  and connects with RF unit  3416  to communicate signals. RF unit  3416  is connected with transceiver unit  3418  to communicate RF signals. Here, processor  3412  controls transceiver unit  3418  and RF unit  3416  to select and control transmission/reception beams generated by transceiver unit  3418  and RF unit  3416 . Processor  3412  may be implemented in hardware, a control processing unit (CPU) and software, or in both hardware and software. As described above in connection with the structure shown in  FIG. 18 , backhaul node  3410  and RAN  3430  may be implemented in various structures, and reference number  3420  may be a communication cable connecting the devices, a backplane connecting boards, or a bus connecting chips on the same board. The network connection via element  3440  may be implemented wiredly or wirelessly through another backhaul node. When backhaul node  3410  is an HN, and element  3440  is connected to a RBN which is another backhaul node, a multi-hop or wireless mesh network may be established. When backhaul node  3410  is a RBN, element  3440  may be omitted. 
         [0237]    In sum, as described above, according to an embodiment of the present disclosure, a wireless backhaul may be operated in only one of two operation modes, i.e., BUS and BOCS, leading to optimized wireless backhaul capability and minimized costs necessary for installation and operation of wireless backhaul device. Further, according to an embodiment of the present disclosure, a wireless backhaul provides operation mode transition references and methods. According to an embodiment of the present disclosure, a wireless backhaul provides such functionality as to transition operation mode into BUS when a new RBN is installed to automatically discover the optimal HN and optimal transmission/reception beam for the new RBN, link to the optimal HN, provide network connection, and establish the optimal wireless backhaul link. As a result, costs for installation of a new RBN may be minimized. 
         [0238]    Further, according to an embodiment of the present disclosure, a wireless backhaul provides such functionality as to transition operation mode into BUS upon detecting a link failure between HN and RBN or capability deterioration due to a variation in communication environment of a wireless backhaul already established to discover a new optimal HN and optimal transmission/reception beam providing higher-quality communication to a RBN for which a capability deterioration has been detected, link to the new optimal HN, provide network connection, and restore the wireless backhaul link. Resultantly, costs (OPEX) incurred during the course of wireless backhaul operation may be minimized. 
         [0239]    Further, according to an embodiment of the present disclosure, a wireless backhaul provides such functionality as to transition operation mode into BUS when a variation in communication environment of a wireless backhaul already established is varied to cause a deterioration in communication capability of HN and RBN, and the network determines an operation mode switch due to the link capability deterioration to discover a transmission/reception beam corresponding to a new optimal communication path providing higher-quality communication to a RBN suffering from a communication capability deterioration, allow the same to change beams, and optimize wireless backhaul link capability. Also in this case, costs incurred during the course of wireless backhaul operation may be minimized. 
         [0240]    According to an embodiment of the present disclosure, a wireless backhaul, upon determining that the wireless backhaul is operated in BUS to achieve a target value, transitions into BOCS to conduct optimized wireless communication using only the optimal transmission/reception beam. In order to maximize efficiency while minimizing waste of radio resources in BOCS, according to an embodiment of the present disclosure, HN sends SS and BCH on DL using some limited beams among all the HN&#39;s beams that are able to be transmitted or received by the HN&#39;s antenna for only RBNs linked to HN, thus minimizing interference and waste of radio resources on DL and optimizing capability. Further, BM for selecting the optimal beam among all DL and UL beams is not supported, but rather, limited beam tracking is supported only on some candidate beams adjacent to the optimal beam, thereby leading to minimized waste of radio resources for beam measurement, along with increased beam measurement efficiency and performance. No RACH preamble or association request signal for access to HN of RBN is transmitted or received on UL, thus allowing for minimized interference and waste of UL radio resources and optimized capability. 
         [0241]    According to an embodiment of the present disclosure, a wireless backhaul may be implemented using a cellular mobile communication standard adopting beamforming technology or using the IEEE 802.11ad beamforming wireless LAN standard. However, according to an embodiment of the present disclosure, in BOCS, a wireless backhaul supports only some functions, but not all of the functions of mobile communication or wireless LAN communication as per the standard. 
         [0242]    According to an embodiment of the present disclosure, when operated in BOCS, RBN operates in only one communication mode of FBM, ABM, and LDM, leading to a further enhancement in efficiency and performance of wireless backhaul. According to an embodiment of the present disclosure, in a wireless backhaul, HN and RBN communicate in FBM of BOCS without changing the optimal transmission/reception beam discovered in BUS and does not communicate beam measurement signals for beam measurement and beam change, thus increasing radio resource use efficiency. FBM presents the optimal communication performance in most of fixed wireless backhaul communication environments where communication environment is not subject to change. According to an embodiment of the present disclosure, a wireless backhaul provides such functionality as to minimize a deterioration of performance due to external environments since HN or RBN additionally conducts level-limited BM in ABM of BOCS. Further, in ABM, limitation is imposed on the beam range measured, thus allowing for reduced waste of radio resources for beam measurement, increased communication efficiency, and reduced beam measurement period and the wireless backhaul&#39;s resultant enhancement in beam adaptation capability. 
         [0243]    Further, according to an embodiment of the present disclosure, a wireless backhaul may provide such functionality as to allow RBN to transition into LDM of BOCS to minimize power consumption and interference with other wireless backhaul. Use of LDM may maximize the power use efficiency of wireless backhaul while allowing other wireless backhaul enhanced communication performance. However, when the wireless backhaul needs to conduct normal communication, the wireless backhaul may transition into another communication mode of BOCS at HN&#39;s instruction or RBN&#39;s request and conduct wireless backhaul communication. Here, upon requesting to transition into the communication mode, RBN sends SR bit or information rather than UL RACH signal. Thus, waste of UL radio resources may be reduced, and communication efficiency may be increased. According to an embodiment of the present disclosure, in a wireless backhaul, RBN operating in LDM sends BA bit or information to serving HN at each predetermined period, thereby periodically reminding serving HN that the wireless backhaul link is not disconnected but alive. As a result, for RBN operating in LDM, HN may simply determine whether the wireless backhaul link faces a problem, and upon detecting a link failure, the HN enables a quick restoration of wireless backhaul. 
         [0244]    As is apparent from the foregoing description, according to an embodiment of the present disclosure, the wireless backhaul is distinctively operated in a state of being operated using all the beams or in a state where communication is carried out using only the optimal beam, wherein the state where communication is carried out using only the optimal beam is operated distinctively in a mode where a fixed beam is used, a mode where limited beam measurement is additionally conducted on beams adjacent to the optimal beam, and a power save mode in which signal transmission is made only at predetermined periods. Thus, the capability of wireless backhaul may be optimized, and costs for installation and operation of wireless backhaul apparatus may be minimized. 
         [0245]    Although the present disclosure has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.