Patent Publication Number: US-11381296-B2

Title: System and method for beam management

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a Continuation of U.S. application Ser. No. 16/038,081, entitled “SYSTEM AND METHOD FOR BEAM MANAGEMENT” and filed on Jul. 17, 2018, which is a Continuation of U.S. application Ser. No. 15/474,802, entitled “SYSTEM AND METHOD FOR BEAM MANAGEMENT” and filed on Mar. 30, 2017, now U.S. Pat. No. 10,069,555 which Issued on Sep. 4, 2018, which claims the benefit of U.S. Provisional Application Ser. No. 62/322,168, entitled “TRANSMIT REQUEST FOR BEAM TRACKING” and filed on Apr. 13, 2016, U.S. Provisional Application Ser. No. 62/329,180, entitled “TRANSMIT REQUEST FOR BEAM TRACKING” and filed on Apr. 28, 2016, U.S. Provisional Application Ser. No. 62/333,120, entitled “TRANSMIT REQUEST FOR BEAM TRACKING” and filed on May 6, 2016, U.S. Provisional Application Ser. No. 62/337,829, entitled “TRANSMIT REQUEST FOR BEAM TRACKING” and filed on May 17, 2016, U.S. Provisional Application Ser. No. 62/338,484, entitled “TRANSMIT REQUEST FOR BEAM TRACKING” and filed on May 18, 2016, U.S. Provisional Application Ser. No. 62/341,051, entitled “TRANSMIT REQUEST FOR BEAM TRACKING” and filed on May 24, 2016 and U.S. Provisional Application Ser. No. 62/447,386, entitled “SYSTEM AND METHOD FOR BEAM INDEX and filed on Jan. 17, 2017. The disclosures of the aforementioned applications are expressly incorporated by reference herein in their entireties. 
    
    
     BACKGROUND 
     Field 
     The present disclosure relates generally to communication systems, and more particularly, to a user equipment and a base station that may communicate through one of more beams. 
     Background 
     Wireless communication systems are widely deployed to provide various telecommunication services such as telephony, video, data, messaging, and broadcasts. Typical wireless communication systems may employ multiple-access technologies capable of supporting communication with multiple users by sharing available system resources. Examples of such multiple-access technologies include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, orthogonal frequency division multiple access (OFDMA) systems, single-carrier frequency division multiple access (SC-FDMA) systems, and time division synchronous code division multiple access (TD-SCDMA) systems. 
     These multiple access technologies have been adopted in various telecommunication standards to provide a common protocol that enables different wireless devices to communicate on a municipal, national, regional, and even global level. An example telecommunication standard is Long Term Evolution (LTE). LTE is a set of enhancements to the Universal Mobile Telecommunications System (UMTS) mobile standard promulgated by Third Generation Partnership Project (3GPP). LTE is designed to support mobile broadband access through improved spectral efficiency, lowered costs, and improved services using OFDMA on the downlink, SC-FDMA on the uplink, and multiple-input multiple-output (MIMO) antenna technology. However, as the demand for mobile broadband access continues to increase, there exists a need for further improvements in LTE technology. These improvements may also be applicable to other multi-access technologies and the telecommunication standards that employ these technologies. 
     An example of an improvement to LTE may include fifth generation wireless systems and mobile networks (5G). 5G is a telecommunications standard that may extend beyond LTE and/or 4G standards. For example, 5G may offer higher capacity and, therefore, serve a larger number of users in an area. Further, 5G may improve data consumption and data rates. 
     SUMMARY 
     The following presents a simplified summary of one or more aspects in order to provide a basic understanding of such aspects. This summary is not an extensive overview of all contemplated aspects, and is intended to neither identify key or critical elements of all aspects nor delineate the scope of any or all aspects. Its sole purpose is to present some concepts of one or more aspects in a simplified form as a prelude to the more detailed description that is presented later. 
     Path loss may be relatively high in millimeter wave (mmW) systems. Transmission may be directional to mitigate path loss. A base station may transmit one or more beam reference signals by sweeping in all directions so that a user equipment (UE) may identify a best “coarse” beam. Further, the base station may transmit a beam refinement request signal so that the UE may track “fine” beams. If a “coarse” beam identified by the UE changes, the UE may need to inform the base station so that the base station may train one or more new “fine” beams for the UE. 
     In a first aspect, a first method, first apparatus, and first computer-readable medium are provided. The first apparatus may communicate with a UE through a first active beam. The first apparatus may determine that beam tracking is to be performed with the UE, including identifying a new beam for communication between the UE and the first apparatus. The first apparatus may perform beam tracking with the UE based on the determination that beam tracking is to be performed. The first apparatus may communicate with the UE through a second active beam based on the beam tracking. In an aspect, the determination that beam tracking is to be performed for the UE includes determining a time at which the UE is to transition from an inactive cycle of discontinuous reception (DRX) to an active cycle of DRX, and the performance of the beam tracking is based on the determined time. In an aspect, the performance of the beam tracking includes one or more of: transmitting at least one beam reference signal (BRS); receiving, from the UE, a first indication of a first beam index based on the at least one BRS; transmitting, based on the first indication of the first beam index, at least one beam refinement reference signal (BRRS); and receiving, based on the at least one BRRS, a second indication of a second beam index, the second beam index corresponding to the second active beam. In an aspect, the performance of the beam tracking includes one or more of: receiving, from the UE, a request for beam tracking; transmitting, based on the request for beam tracking, at least one BRRS; and receiving, based on the at least one BRRS, an indication of a beam index, the beam index corresponding to the second active beam. In an aspect, the communication with the UE through the first active beam includes sending a reference signal to the UE to determine if the first active beam is failing, and the determination that beam tracking is to be initiated for the UE includes receiving a response from the UE based on the reference signal, and detecting a radio link failure based on the received response. In an aspect, the communication with the UE through the first active beam is performed with a first radio access technology (RAT), and the response is received through a second RAT, the second RAT having a lower carrier frequency than the first RAT. In an aspect, the reference signal is one of a channel state information reference signal (CSI-RS), a cell-specific reference signal (CRS), a secondary synchronization signal (SSS), a mobility reference signal (MRS), a demodulation reference signal (DMRS), or a beam reference signal (BRS), and the response includes at least one of a channel quality information (CQI), a signal-to-interference-plus-noise ratio (SINR), a signal-to-noise radio (SNR), a received signal strength indicator (RSSI), a reference signal received power (RSRP), or a reference signal received quality (RSRQ). In an aspect, the performance of the beam tracking with the UE includes sending a message to the UE indicating that beam tracking is to be performed, wherein the message is sent on a physical downlink control channel (PDCCH) or a physical downlink shared channel (PDSCH). In an aspect, the message is sent through downlink control information (DCI) in the PDCCH. In an aspect, the determination that beam tracking is to be initiated for the UE includes determining an absence of communication with the UE through the first active beam. In an aspect, the determination of the absence of the communication with the UE through the first active beam is based on an absence of data carried on a physical uplink control channel (PUCCH), an absence of data carried on a physical uplink shared channel (PUSCH), or an absence of acknowledgement/negative acknowledgement (ACK/NACK) messages from the UE. 
     In a second aspect, a second method, second apparatus, and second computer-readable medium are provided. The second apparatus may communicate with a base station through a first active beam. The second apparatus may receive a signal from the base station associated with beam tracking, the beam tracking including identifying a new beam for communication between the second apparatus and the base station. The second apparatus may communicate with the base station through a second active beam based on the signal associated with beam tracking. In an aspect, the signal includes a BRRS, and the second apparatus may send, to the base station, a beam index corresponding to the second active beam based on the BRRS. In an aspect, the signal comprises a BRS, and the second apparatus may send, to the base station, a beam index corresponding to a coarse beam. In an aspect, the second apparatus may send, to the base station, a request to perform beam tracking based on the signal. In an aspect, the communication with the base station through the first active beam includes receiving a reference signal, and the second apparatus may detect a radio link failure based on the reception of the reference signal and send an indication to the base station based on the detected radio link failure. In an aspect, the reference signal is one of a CSI-RS, a CRS, an SSS, an MRS, a DMRS, or a BRS, and the indication includes at least one of a CQI, an SINR, an SNR, an RSSI, an RSRP, or a RSRQ. In an aspect, the communication with the base station through the first active beam is performed with a first RAT, and the indication is sent through a second RAT, the first RAT having a higher carrier frequency than the second RAT. In an aspect, the second apparatus may perform beam tracking with the base station. In an aspect, the performance of the beam tracking includes one or more of: receiving, from the base station, at least one BRS; transmitting, to the base station, a first indication of a first beam index based on the BRS; receiving at least one BRRS; and transmitting, based on the at least one BRRS, a second indication of a second beam index. In an aspect, the signal is received on a PDCCH or a PDSCH. In an aspect, the signal is received through DCI on the PDCCH. 
     In a third aspect, a third method, third apparatus, and third computer-readable medium are provided. The third apparatus may communicate with a UE through an active beam. The third apparatus may transmit, to the UE, information indicating a periodicity at which control information is to be communicated on a control channel through a control-information beam. The third apparatus may communicate, with the UE, the control information on the control channel through the control-information beam at the periodicity. In an aspect, the control channel includes a PUCCH, and the communication, with the UE, of the control information on the control channel includes receiving, from the UE, the control information carried on the PUCCH through the control-information beam based on the periodicity. In an aspect, the control-information beam includes at least one candidate beam, the at least one candidate beam corresponding to a beam index included in a set of candidate beam indexes maintained by the third apparatus. In an aspect, the control-information beam includes at least one wide beam, the at least one wide beam having an angle greater than that of the active beam. In an aspect, the information indicating the periodicity is transmitted through radio resource control (RRC) signaling. In an aspect, the information indicating the periodicity is transmitted on a PDCCH. In an aspect, the information indicating the periodicity includes DCI of the PDCCH. In an aspect, the third apparatus may receive a request to change the active beam, the request indicating a beam index corresponding to a second beam and change the active beam to the second beam corresponding to the beam index indicated by the request. In an aspect, the request indicates the beam index through at least one of a cyclic shift or spreading across symbols. In an aspect, the request indicates the beam index through at least one of a subcarrier region or a random access channel (RACH). 
     In a fourth aspect, a fourth method, fourth apparatus, and fourth computer-readable medium are provided. The fourth apparatus may communicate with a base station through an active beam. The fourth apparatus may receive, from the base station, information indicating a periodicity at which control information is to be communicated on a control channel through a control-information beam. The fourth apparatus may communicate, with the base station, the control information on the control channel through the control-information beam at the periodicity. In an aspect, the control channel includes a PUCCH, and the communication of the control information on the control channel includes sending, to the base station, the control information on the PUCCH through the control-information beam based on the periodicity. In an aspect, the control-information beam includes at least one candidate beam, the at least one candidate beam corresponding to a beam index included in a set of candidate beam indexes. In an aspect, the control-information beam includes at least one wide beam, the at least one wide beam having an angle greater than that of the active beam. In an aspect, the information indicating the periodicity is received using RRC signaling. In an aspect, the information indicating the periodicity is received on a PDCCH. In an aspect, the information indicating the periodicity is indicated by DCI of the PDCCH. In an aspect, the fourth apparatus may transmit, to the base station, a request to change the active beam, the request indicating a beam index corresponding to a second beam, and change the active beam to the second beam corresponding to the beam index indicated by the request. In an aspect, the request indicates the beam index through at least one of a cyclic shift or spreading across symbols. In an aspect, the request indicates the beam index through at least one of a subcarrier region or a RACH. 
     In a fifth aspect, a fifth method, fifth apparatus, and fifth computer-readable medium are provided. The fifth apparatus may transmit, to a UE, on a control channel, one or more indications of one or more beam indexes corresponding to one or more beams. The fifth apparatus may transmit, to the UE, one or more reference signals through the one or more beams corresponding to the one or more beam indexes. In an aspect, the control channel includes a PDCCH, and the one or more indications are included in one or more bits of a DCI message. In an aspect, the transmission of the one or more indications of the one or more beam indexes corresponding to the one or more beams includes transmission of one or more beam indexes associated with one or more BRSs, the one or more BRSs transmitted during a synchronization subframe. In an aspect, the transmission of the one or more indications of the one or more beam indexes corresponding to the one or beams includes reception, from the UE, of one or more beam indexes corresponding to the one or more beams, and transmission of the one or more beam indexes corresponding to the one or more beams based on the one or more beam indexes that are received most recently. In an aspect, the one or more beam indexes corresponding to the one or more beams are received on a PUSCH or a PUCCH. In an aspect, the one or more beam indexes corresponding to the one or more beams are transmitted based on the one or more beam indexes received through the PUSCH when more than two symbols are used for the reference signal transmission. In an aspect, the one or more beam indexes associated with one or more beams are transmitted based on the one or more beam indexes received through the PUCCH when two or fewer symbols are used for the reference signal transmission. In an aspect, the one or more reference signals include at least one of a CSI-RS or a BRRS. In an aspect, the transmission of the one or more indications of the one or more beam indexes associated with the one or more beams includes transmission of the one or more beam indexes associated with the one or more beams through which at least one of the CSI-RSs was previously transmitted. 
     To the accomplishment of the foregoing and related ends, the one or more aspects comprise the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative features of the one or more aspects. These features are indicative, however, of but a few of the various ways in which the principles of various aspects may be employed, and this description is intended to include all such aspects and their equivalents. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an example of a wireless communications system and an access network. 
         FIGS. 2A, 2B, 2C, and 2D  are diagrams illustrating LTE examples of a DL frame structure, DL channels within the DL frame structure, an UL frame structure, and UL channels within the UL frame structure, respectively. 
         FIG. 3  is a diagram illustrating an example of a base station and user equipment (UE) in an access network. 
         FIGS. 4A and 4B  are diagrams of a wireless communications system. 
         FIGS. 5A through 5G  are diagrams of a wireless communications system. 
         FIG. 6  is a diagram of a wireless communications system. 
         FIG. 7  is a diagram of a wireless communications system. 
         FIG. 8  is a diagram of a wireless communications system. 
         FIGS. 9A through 9E  are diagrams of a wireless communications system. 
         FIGS. 10A and 10B  is a flowchart of a method of wireless communication. 
         FIGS. 11A and 11B  is a flowchart of a method of wireless communication. 
         FIG. 12  is a flowchart of a method of wireless communication. 
         FIG. 13  is a flowchart of a method of wireless communication. 
         FIG. 14  is a flowchart of a method of wireless communication. 
         FIG. 15  is a flowchart of a method of wireless communication. 
         FIG. 16  is a flowchart of a method of wireless communication. 
         FIG. 17  is a conceptual data flow diagram illustrating the data flow between different means/components in an exemplary apparatus. 
         FIG. 18  is a diagram illustrating an example of a hardware implementation for an apparatus employing a processing system. 
         FIG. 19  is a conceptual data flow diagram illustrating the data flow between different means/components in an exemplary apparatus. 
         FIG. 20  is a diagram illustrating an example of a hardware implementation for an apparatus employing a processing system. 
         FIG. 21  is a conceptual data flow diagram illustrating the data flow between different means/components in an exemplary apparatus. 
         FIG. 22  is a diagram illustrating an example of a hardware implementation for an apparatus employing a processing system. 
         FIG. 23  is a conceptual data flow diagram illustrating the data flow between different means/components in an exemplary apparatus. 
         FIG. 24  is a diagram illustrating an example of a hardware implementation for an apparatus employing a processing system. 
         FIG. 25  is a conceptual data flow diagram illustrating the data flow between different means/components in an exemplary apparatus. 
         FIG. 26  is a diagram illustrating an example of a hardware implementation for an apparatus employing a processing system. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of various configurations and is not intended to represent the only configurations in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. In some instances, well known structures and components are shown in block diagram form in order to avoid obscuring such concepts. 
     Several aspects of telecommunication systems will now be presented with reference to various apparatus and methods. These apparatus and methods will be described in the following detailed description and illustrated in the accompanying drawings by various blocks, components, circuits, processes, algorithms, etc. (collectively referred to as “elements”). These elements may be implemented using electronic hardware, computer software, or any combination thereof. Whether such elements are implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. 
     By way of example, an element, or any portion of an element, or any combination of elements may be implemented as a “processing system” that includes one or more processors. Examples of processors include microprocessors, microcontrollers, graphics processing units (GPUs), central processing units (CPUs), application processors, digital signal processors (DSPs), reduced instruction set computing (RISC) processors, systems on a chip (SoC), baseband processors, field programmable gate arrays (FPGAs), programmable logic devices (PLDs), state machines, gated logic, discrete hardware circuits, and other suitable hardware configured to perform the various functionality described throughout this disclosure. One or more processors in the processing system may execute software. Software shall be construed broadly to mean instructions, instruction sets, code, code segments, program code, programs, subprograms, software components, applications, software applications, software packages, routines, subroutines, objects, executables, threads of execution, procedures, functions, etc., whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise. 
     Accordingly, in one or more example embodiments, the functions described may be implemented in hardware, software, or any combination thereof. If implemented in software, the functions may be stored on or encoded as one or more instructions or code on a computer-readable medium. Computer-readable media includes computer storage media. Storage media may be any available media that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can comprise a random-access memory (RAM), a read-only memory (ROM), an electrically erasable programmable ROM (EEPROM), optical disk storage, magnetic disk storage, other magnetic storage devices, combinations of the aforementioned types of computer-readable media, or any other medium that can be used to store computer executable code in the form of instructions or data structures that can be accessed by a computer. 
       FIG. 1  is a diagram illustrating an example of a wireless communications system and an access network  100 . The wireless communications system (also referred to as a wireless wide area network (WWAN)) includes base stations  102 , UEs  104 , and an Evolved Packet Core (EPC)  160 . The base stations  102  may include macro cells (high power cellular base station) and/or small cells (low power cellular base station). The macro cells include base stations. The small cells include femtocells, picocells, and microcells. 
     The base stations  102  (collectively referred to as Evolved Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (E-UTRAN)) interface with the EPC  160  through backhaul links  132  (e.g., Si interface). In addition to other functions, the base stations  102  may perform one or more of the following functions: transfer of user data, radio channel ciphering and deciphering, integrity protection, header compression, mobility control functions (e.g., handover, dual connectivity), inter-cell interference coordination, connection setup and release, load balancing, distribution for non-access stratum (NAS) messages, NAS node selection, synchronization, radio access network (RAN) sharing, multimedia broadcast multicast service (MBMS), subscriber and equipment trace, RAN information management (RIM), paging, positioning, and delivery of warning messages. The base stations  102  may communicate directly or indirectly (e.g., through the EPC  160 ) with each other over backhaul links  134  (e.g., X2 interface). The backhaul links  134  may be wired or wireless. 
     The base stations  102  may wirelessly communicate with the UEs  104 . Each of the base stations  102  may provide communication coverage for a respective geographic coverage area  110 . There may be overlapping geographic coverage areas  110 . For example, the small cell  102 ′ may have a coverage area  110 ′ that overlaps the coverage area  110  of one or more macro base stations  102 . A network that includes both small cell and macro cells may be known as a heterogeneous network. A heterogeneous network may also include Home Evolved Node Bs (eNBs) (HeNBs), which may provide service to a restricted group known as a closed subscriber group (CSG). The communication links  120  between the base stations  102  and the UEs  104  may include uplink (UL) (also referred to as reverse link) transmissions from a UE  104  to a base station  102  and/or downlink (DL) (also referred to as forward link) transmissions from a base station  102  to a UE  104 . The communication links  120  may use multiple-input and multiple-output (MIMO) antenna technology, including spatial multiplexing, beamforming, and/or transmit diversity. The communication links may be through one or more carriers. The base stations  102 /UEs  104  may use spectrum up to Y MHz (e.g., 5, 10, 15, 20, 100 MHz) bandwidth per carrier allocated in a carrier aggregation of up to a total of Yx MHz (x component carriers) used for transmission in each direction. The carriers may or may not be adjacent to each other. Allocation of carriers may be asymmetric with respect to DL and UL (e.g., more or less carriers may be allocated for DL than for UL). The component carriers may include a primary component carrier and one or more secondary component carriers. A primary component carrier may be referred to as a primary cell (PCell) and a secondary component carrier may be referred to as a secondary cell (SCell). 
     The wireless communications system may further include a Wi-Fi access point (AP)  150  in communication with Wi-Fi stations (STAs)  152  via communication links  154  in a 5 GHz unlicensed frequency spectrum. When communicating in an unlicensed frequency spectrum, the STAs  152 /AP  150  may perform a clear channel assessment (CCA) prior to communicating in order to determine whether the channel is available. 
     The small cell  102 ′ may operate in a licensed and/or an unlicensed frequency spectrum. When operating in an unlicensed frequency spectrum, the small cell  102 ′ may employ NR and use the same 5 GHz unlicensed frequency spectrum as used by the Wi-Fi AP  150 . The small cell  102 ′, employing NR in an unlicensed frequency spectrum, may boost coverage to and/or increase capacity of the access network. 
     The gNodeB (gNB)  180  may operate in millimeter wave (mmW) frequencies and/or near mmW frequencies in communication with the UE  182 . When the gNB  180  operates in mmW or near mmW frequencies, the gNB  180  may be referred to as an mmW base station. Extremely high frequency (EHF) is part of the RF in the electromagnetic spectrum. EHF has a range of 30 GHz to 300 GHz and a wavelength between 1 millimeter and 10 millimeters. Radio waves in the band may be referred to as a millimeter wave. Near mmW may extend down to a frequency of 3 GHz with a wavelength of 100 millimeters. The super high frequency (SHF) band extends between 3 GHz and 30 GHz, also referred to as centimeter wave. Communications using the mmW/near mmW radio frequency band has extremely high path loss and a short range. The mmW base station  180  may utilize beamforming  184  with the UE  182  to compensate for the extremely high path loss and short range. 
     The EPC  160  may include a Mobility Management Entity (MME)  162 , other MMEs  164 , a Serving Gateway  166 , a Multimedia Broadcast Multicast Service (MBMS) Gateway  168 , a Broadcast Multicast Service Center (BM-SC)  170 , and a Packet Data Network (PDN) Gateway  172 . The MME  162  may be in communication with a Home Subscriber Server (HSS)  174 . The MME  162  is the control node that processes the signaling between the UEs  104  and the EPC  160 . Generally, the MME  162  provides bearer and connection management. All user Internet protocol (IP) packets are transferred through the Serving Gateway  166 , which itself is connected to the PDN Gateway  172 . The PDN Gateway  172  provides UE IP address allocation as well as other functions. The PDN Gateway  172  and the BM-SC  170  are connected to the IP Services  176 . The IP Services  176  may include the Internet, an intranet, an IP Multimedia Subsystem (IMS), a PS Streaming Service (PSS), and/or other IP services. The BM-SC  170  may provide functions for MBMS user service provisioning and delivery. The BM-SC  170  may serve as an entry point for content provider MBMS transmission, may be used to authorize and initiate MBMS Bearer Services within a public land mobile network (PLMN), and may be used to schedule MBMS transmissions. The MBMS Gateway  168  may be used to distribute MBMS traffic to the base stations  102  belonging to a Multicast Broadcast Single Frequency Network (MBSFN) area broadcasting a particular service, and may be responsible for session management (start/stop) and for collecting eMBMS related charging information. 
     The base station may also be referred to as a gNB, Node B, evolved Node B (eNB), an access point, a base transceiver station, a radio base station, a radio transceiver, a transceiver function, a basic service set (BSS), an extended service set (ESS), or some other suitable terminology. The base station  102  provides an access point to the EPC  160  for a UE  104 . Examples of UEs  104  include a cellular phone, a smart phone, a session initiation protocol (SIP) phone, a laptop, a personal digital assistant (PDA), a satellite radio, a global positioning system, a multimedia device, a video device, a digital audio player (e.g., MP3 player), a camera, a game console, a tablet, a smart device, a wearable device, or any other similar functioning device. The UE  104  may also be referred to as a station, a mobile station, a subscriber station, a mobile unit, a subscriber unit, a wireless unit, a remote unit, a mobile device, a wireless device, a wireless communications device, a remote device, a mobile subscriber station, an access terminal, a mobile terminal, a wireless terminal, a remote terminal, a handset, a user agent, a mobile client, a client, or some other suitable terminology. 
     Referring again to  FIG. 1 , in certain aspects, the base station  180  may be configured to determine that beam tracking  198  is to be performed with the UE  182 , including identifying a new beam for communication between the UE  182  and the base station  180 . The base station  180  may perform beam tracking  198  with the UE  182  based on the determination that beam tracking is to be performed. The base station  180  may communicate with the UE through a second active beam based on the beam tracking  198 . In an aspect, the determination that beam tracking  198  is to be performed for the UE  182  includes determining a time at which the UE  182  is to transition from an inactive cycle of discontinuous reception (DRX) to an active cycle of DRX, and the performance of the beam tracking  198  is based on the determined time. In an aspect, the performance of the beam tracking  198  includes one or more of: transmitting at least one beam reference signal (BRS); receiving, from the UE  182 , a first indication of a first beam index based on the at least one BRS; transmitting, based on the first indication of the first beam index, at least one beam refinement reference signal (BRRS); and receiving, based on the at least one BRRS, a second indication of a second beam index, the second beam index corresponding to the second active beam. In an aspect, the performance of the beam tracking  198  includes one or more of: receiving, from the UE  182 , a request for beam tracking  198 ; transmitting, based on the request for beam tracking  198 , at least one beam refinement reference signal; and receiving, based on the at least one beam refinement reference signal, an indication of a beam index, the beam index corresponding to the second active beam. In an aspect, the communication with the UE  182  through the first active beam includes sending a reference signal to the UE  182  to determine if the first active beam is failing, and the determination that beam tracking  198  is to be initiated for the UE  182  includes receiving a response from the UE  182  based on the reference signal, and detecting a radio link failure based on the received response. In an aspect, the communication with the UE  182  through the first active beam is performed with a first radio access technology (RAT), and the response is received through a second RAT, the second RAT having a lower carrier frequency than the first RAT. In an aspect, the reference signal is one of a channel state information reference signal (CSI-RS), a cell-specific reference signal (CRS), a secondary synchronization signal (SSS), a mobility reference signal (MRS), a demodulation reference signal (DMRS), or a beam reference signal (BRS), and the response includes at least one of a channel quality information (CQI), a signal-to-interference-plus-noise ratio (SINR), a signal-to-noise radio (SNR), a received signal strength indicator (RSSI), a reference signal received power (RSRP), or a reference signal received quality (RSRQ). In an aspect, the performance of the beam tracking  198  with the UE  182  includes sending a message to the UE  182  indicating that beam tracking  198  is to be performed, wherein the message is sent on a physical downlink control channel (PDCCH) or a physical downlink shared channel (PDSCH). In an aspect, the message is sent through downlink control information (DCI) in the PDCCH. In an aspect, the determination that beam tracking  198  is to be initiated for the UE  182  includes determining an absence of communication with the UE  182  through the first active beam. In an aspect, the determination of the absence of the communication with the UE  182  through the first active beam is based on an absence of data carried on a physical uplink control channel (PUCCH), an absence of data carried on a physical uplink shared channel (PUSCH), or an absence of acknowledgement/negative acknowledgement (ACK/NACK) messages from the UE  182 . 
     Correspondingly, the UE  182  may communicate with a base station  180  through a first active beam. The UE  182  may receive a signal from the base station  180  associated with beam tracking  198 , the beam tracking  198  including identifying a new beam for communication between the UE  182  and the base station  180 . The UE  182  may communicate with the base station  180  through a second active beam based on the signal associated with beam tracking  198 . In an aspect, the signal includes a BRRS, and the UE  182  may send, to the base station  180 , a beam index corresponding to the second active beam based on the BRRS. In an aspect, the signal comprises a BRS, and the UE  182  may send, to the base station  180 , a beam index corresponding to a coarse beam. In an aspect, the UE  182  may send, to the base station  180 , a request to perform beam tracking  198  based on the signal. In an aspect, the communication with the base station  180  through the first active beam includes receiving a reference signal, and the UE  182  may detect a radio link failure based on the reception of the reference signal and send an indication to the base station  180  based on the detected radio link failure. In an aspect, the reference signal is one of a CSI-RS, a CRS, an SSS, an MRS, a DMRS, or a BRS, and the indication includes at least one of a CQI, an SINR, an SNR, an RSSI, an RSRP, or a RSRQ. In an aspect, the communication with the base station  180  through the first active beam is performed with a first RAT, and the indication is sent through a second RAT, the first RAT having a higher carrier frequency than the second RAT. In an aspect, the UE  182  may perform beam tracking  198  with the base station  180 . In an aspect, the performance of the beam tracking  198  includes one or more of: receiving, from the base station  180 , at least one BRS; transmitting, to the base station  180 , a first indication of a first beam index based on the BRS; receiving at least one BRRS; and transmitting, based on the at least one BRRS, a second indication of a second beam index. In an aspect, the signal is received on a PDCCH or a PDSCH. In an aspect, the signal is received through DCI on the PDCCH. 
       FIG. 2A  is a diagram  200  illustrating an example of a DL frame structure.  FIG. 2B  is a diagram  230  illustrating an example of channels within the DL frame structure.  FIG. 2C  is a diagram  250  illustrating an example of an UL frame structure.  FIG. 2D  is a diagram  280  illustrating an example of channels within the UL frame structure. Other wireless communication technologies may have a different frame structure and/or different channels. A frame (10 ms) may be divided into 10 equally sized subframes. Each subframe may include two consecutive time slots. A resource grid may be used to represent the two time slots, each time slot including one or more time concurrent resource blocks (RBs) (also referred to as physical RBs (PRBs)). The resource grid is divided into multiple resource elements (REs). For a normal cyclic prefix, an RB contains 12 consecutive subcarriers in the frequency domain and 7 consecutive symbols (for DL, OFDM symbols; for UL, SC-FDMA symbols) in the time domain, for a total of 84 REs. For an extended cyclic prefix, an RB contains 12 consecutive subcarriers in the frequency domain and 6 consecutive symbols in the time domain, for a total of 72 REs. The number of bits carried by each RE depends on the modulation scheme. 
     As illustrated in  FIG. 2A , some of the REs carry DL reference (pilot) signals (DL-RS) for channel estimation at the UE. The DL-RS may include CRS (also sometimes called common RS), UE-specific reference signals (UE-RS), and CSI-RS.  FIG. 2A  illustrates CRS for antenna ports  0 ,  1 ,  2 , and  3  (indicated as R 0 , R 1 , R 2 , and R 3 , respectively), UE-RS for antenna port  5  (indicated as R 5 ), and CSI-RS for antenna port  15  (indicated as R).  FIG. 2B  illustrates an example of various channels within a DL subframe of a frame. The physical control format indicator channel (PCFICH) is within symbol  0  of slot  0 , and carries a control format indicator (CFI) that indicates whether the PDCCH occupies 1, 2, or 3 symbols ( FIG. 2B  illustrates a PDCCH that occupies 3 symbols). The PDCCH carries downlink control information (DCI) within one or more control channel elements (CCEs), each CCE including nine RE groups (REGs), each REG including four consecutive REs in an OFDM symbol. A UE may be configured with a UE-specific enhanced PDCCH (ePDCCH) that also carries DCI. The ePDCCH may have 2, 4, or 8 RB pairs ( FIG. 2B  shows two RB pairs, each subset including one RB pair). The physical hybrid automatic repeat request (ARQ) (HARQ) indicator channel (PHICH) is also within symbol  0  of slot  0  and carries the HARQ indicator (HI) that indicates HARQ ACK/NACK feedback based on the PUSCH. The primary synchronization channel (PSCH) may be within symbol  6  of slot  0  within subframes  0  and  5  of a frame. The PSCH carries a primary synchronization signal (PSS) that is used by a UE to determine subframe/symbol timing and a physical layer identity. The secondary synchronization channel (SSCH) may be within symbol  5  of slot  0  within subframes  0  and  5  of a frame. The SSCH carries a secondary synchronization signal (SSS) that is used by a UE to determine a physical layer cell identity group number and radio frame timing. Based on the physical layer identity and the physical layer cell identity group number, the UE can determine a physical cell identifier (PCI). Based on the PCI, the UE can determine the locations of the aforementioned DL-RS. The physical broadcast channel (PBCH), which carries a master information block (MIB), may be logically grouped with the PSCH and SSCH to form a synchronization signal (SS) block. The MIB provides a number of RBs in the DL system bandwidth, a PHICH configuration, and a system frame number (SFN). The PDSCH carries user data, broadcast system information not transmitted through the PBCH such as system information blocks (SIBs), and paging messages. 
     As illustrated in  FIG. 2C , some of the REs carry DMRS for channel estimation at the base station. The UE may additionally transmit sounding reference signals (SRS) in the last symbol of a subframe. The SRS may have a comb structure, and a UE may transmit SRS on one of the combs. The SRS may be used by a base station for channel quality estimation to enable frequency-dependent scheduling on the UL.  FIG. 2D  illustrates an example of various channels within an UL subframe of a frame. A physical random access channel (PRACH) may be within one or more subframes within a frame based on the PRACH configuration. The PRACH may include six consecutive RB pairs within a subframe. The PRACH allows the UE to perform initial system access and achieve UL synchronization. A physical uplink control channel (PUCCH) may be located on edges of the UL system bandwidth. The PUCCH carries uplink control information (UCI), such as scheduling requests, a channel quality indicator (CQI), a precoding matrix indicator (PMI), a rank indicator (RI), and HARQ ACK/NACK feedback. The PUSCH carries data, and may additionally be used to carry a buffer status report (BSR), a power headroom report (PHR), and/or UCI. 
       FIG. 3  is a block diagram of a base station  310  in communication with a UE  350  in an access network. In the DL, IP packets from the EPC  160  may be provided to a controller/processor  375 . The controller/processor  375  implements layer  3  and layer  2  functionality. Layer  3  includes a radio resource control (RRC) layer, and layer  2  includes a packet data convergence protocol (PDCP) layer, a radio link control (RLC) layer, and a medium access control (MAC) layer. The controller/processor  375  provides RRC layer functionality associated with broadcasting of system information (e.g., MIB, SIBs), RRC connection control (e.g., RRC connection paging, RRC connection establishment, RRC connection modification, and RRC connection release), inter radio access technology (RAT) mobility, and measurement configuration for UE measurement reporting; PDCP layer functionality associated with header compression/decompression, security (ciphering, deciphering, integrity protection, integrity verification), and handover support functions; RLC layer functionality associated with the transfer of upper layer packet data units (PDUs), error correction through ARQ, concatenation, segmentation, and reassembly of RLC service data units (SDUs), re-segmentation of RLC data PDUs, and reordering of RLC data PDUs; and MAC layer functionality associated with mapping between logical channels and transport channels, multiplexing of MAC SDUs onto transport blocks (TBs), demultiplexing of MAC SDUs from TBs, scheduling information reporting, error correction through HARQ, priority handling, and logical channel prioritization. 
     The transmit (TX) processor  316  and the receive (RX) processor  370  implement layer  1  functionality associated with various signal processing functions. Layer  1 , which includes a physical (PHY) layer, may include error detection on the transport channels, forward error correction (FEC) coding/decoding of the transport channels, interleaving, rate matching, mapping onto physical channels, modulation/demodulation of physical channels, and MIMO antenna processing. The TX processor  316  handles mapping to signal constellations based on various modulation schemes (e.g., binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), M-phase-shift keying (M-PSK), M-quadrature amplitude modulation (M-QAM)). The coded and modulated symbols may then be split into parallel streams. Each stream may then be mapped to an OFDM subcarrier, multiplexed with a reference signal (e.g., pilot) in the time and/or frequency domain, and then combined together using an Inverse Fast Fourier Transform (IFFT) to produce a physical channel carrying a time domain OFDM symbol stream. The OFDM stream is spatially precoded to produce multiple spatial streams. Channel estimates from a channel estimator  374  may be used to determine the coding and modulation scheme, as well as for spatial processing. The channel estimate may be derived from a reference signal and/or channel condition feedback transmitted by the UE  350 . Each spatial stream may then be provided to a different antenna  320  via a separate transmitter  318 TX. Each transmitter  318 TX may modulate an RF carrier with a respective spatial stream for transmission. 
     At the UE  350 , each receiver  354 RX receives a signal through its respective antenna  352 . Each receiver  354 RX recovers information modulated onto an RF carrier and provides the information to the receive (RX) processor  356 . The TX processor  368  and the RX processor  356  implement layer  1  functionality associated with various signal processing functions. The RX processor  356  may perform spatial processing on the information to recover any spatial streams destined for the UE  350 . If multiple spatial streams are destined for the UE  350 , they may be combined by the RX processor  356  into a single OFDM symbol stream. The RX processor  356  then converts the OFDM symbol stream from the time-domain to the frequency domain using a Fast Fourier Transform (FFT). The frequency domain signal comprises a separate OFDM symbol stream for each subcarrier of the OFDM signal. The symbols on each subcarrier, and the reference signal, are recovered and demodulated by determining the most likely signal constellation points transmitted by the base station  310 . These soft decisions may be based on channel estimates computed by the channel estimator  358 . The soft decisions are then decoded and deinterleaved to recover the data and control signals that were originally transmitted by the base station  310  on the physical channel. The data and control signals are then provided to the controller/processor  359 , which implements layer  3  and layer  2  functionality. 
     The controller/processor  359  can be associated with a memory  360  that stores program codes and data. The memory  360  may be referred to as a computer-readable medium. In the UL, the controller/processor  359  provides demultiplexing between transport and logical channels, packet reassembly, deciphering, header decompression, and control signal processing to recover IP packets from the EPC  160 . The controller/processor  359  is also responsible for error detection using an ACK and/or NACK protocol to support HARQ operations. 
     Similar to the functionality described in connection with the DL transmission by the base station  310 , the controller/processor  359  provides RRC layer functionality associated with system information (e.g., MIB, SIBs) acquisition, RRC connections, and measurement reporting; PDCP layer functionality associated with header compression/decompression, and security (ciphering, deciphering, integrity protection, integrity verification); RLC layer functionality associated with the transfer of upper layer PDUs, error correction through ARQ, concatenation, segmentation, and reassembly of RLC SDUs, re-segmentation of RLC data PDUs, and reordering of RLC data PDUs; and MAC layer functionality associated with mapping between logical channels and transport channels, multiplexing of MAC SDUs onto TBs, demultiplexing of MAC SDUs from TBs, scheduling information reporting, error correction through HARQ, priority handling, and logical channel prioritization. 
     Channel estimates derived by a channel estimator  358  from a reference signal or feedback transmitted by the base station  310  may be used by the TX processor  368  to select the appropriate coding and modulation schemes, and to facilitate spatial processing. The spatial streams generated by the TX processor  368  may be provided to different antenna  352  via separate transmitters  354 TX. Each transmitter  354 TX may modulate an RF carrier with a respective spatial stream for transmission. 
     The UL transmission is processed at the base station  310  in a manner similar to that described in connection with the receiver function at the UE  350 . Each receiver  318 RX receives a signal through its respective antenna  320 . Each receiver  318 RX recovers information modulated onto an RF carrier and provides the information to a RX processor  370 . 
     The controller/processor  375  can be associated with a memory  376  that stores program codes and data. The memory  376  may be referred to as a computer-readable medium. In the UL, the controller/processor  375  provides demultiplexing between transport and logical channels, packet reassembly, deciphering, header decompression, control signal processing to recover IP packets from the UE  350 . IP packets from the controller/processor  375  may be provided to the EPC  160 . The controller/processor  375  is also responsible for error detection using an ACK and/or NACK protocol to support HARQ operations. 
     Extremely high frequency (EHF) is part of the RF in the electromagnetic spectrum. EHF has a range of 30 GHz to 300 GHz and a wavelength between 1 millimeter and 10 millimeters. Radio waves in the band may be referred to as a millimeter wave. Near mmW may extend down to a frequency of 3 GHz with a wavelength of 100 millimeters (the super high frequency (SHF) band extends between 3 GHz and 30 GHz, also referred to as centimeter wave). While the disclosure herein refers to mmWs, it should be understood that the disclosure also applies to near mmWs. Further, while the disclosure herein refers to mmW base stations, it should be understood that the disclosure also applies to near-mmW base stations. 
     In order to build a useful communication network in the millimeter wavelength spectrum, a beamforming technique may be used to compensate for path loss. Beamforming technique focuses the RF energy into a narrow direction to allow the RF beam to propagate farther in that direction. Using the beamforming technique, non-line of sight (NLOS) RF communication in the millimeter wavelength spectrum may rely on reflection and/or diffraction of the beams to reach the UE. If the direction becomes blocked, either because of UE movement or changes in the environment (e.g., obstacles, humidity, rain, etc.), the beam may not be able to reach the UE. Thus, in order to ensure that the UE has continuous, seamless coverage, multiple beams in as many different direction as possible may be available. In an aspect, the beamforming technique may require that the mmW base stations and the UEs transmit and receive in a direction that allows the most RF energy to be collected. 
       FIGS. 4A and 4B  are diagrams illustrating an example of the transmission of beamformed signals between a base station  402  and a UE  404 . The base station  402  may be embodied as a base station in a mmW system (e.g., mmW base station). When the UE  404  turns on, the UE  404  searches for a nearby NR network. The UE  404  discovers the base station  402 , which belongs to an NR network. The base station  402  transmits an SS block including the PSS, SSS, and the PBCH (including the MIB) periodically in different transmit directions  402   a - 402   h . The UE  404  receives the transmission  402   e  including the PSS, SSS, and PBCH. Based on the received SS block, the UE  404  synchronizes to the NR network and camps on a cell associated with the base station  402 . 
     Referring to  FIG. 4A , diagram  400  illustrates a base station  402  of a mmW system transmitting beamformed signals  406  (e.g., a BRS) in different transmit directions (e.g., directions A, B, C, and D). In an example, the base station  402  may sweep through the transmit directions according to a sequence A-B-C-D. In another example, the base station  402  may sweep through the transmit directions according to the sequence B-D-A-C. Although only four transmit directions and two transmit sequences are described with respect to  FIG. 4A , any number of different transmit directions and transmit sequences are contemplated. 
     After transmitting the signals, the base station  402  may switch to a receive mode. In the receive mode, the base station  402  may sweep through different receive directions in a sequence or pattern corresponding (mapping) to a sequence or pattern in which the base station  402  previously transmitted the synchronization/discovery signals in the different transmit directions. For example, if the base station  402  previously transmitted the synchronization/discovery signals in transmit directions according to the sequence A-B-C-D, then the base station  402  may sweep through receive directions according to the sequence A-B-C-D in an attempt to receive an association signal from a UE  404 . In another example, if the base station  402  previously transmitted the synchronization/discovery signals in transmit directions according to the sequence B-D-A-C, then the base station  402  may sweep through receive directions according to the sequence B-D-A-C in an attempt to receive the association signal from the UE  404 . 
     A propagation delay on each beamformed signal allows a UE  404  to perform a receive (RX) sweep. The UE  404  in a receive mode may sweep through different receive directions in an attempt to detect a synchronization/discovery signal  406  (see  FIG. 4B ). One or more of the synchronization/discovery signals  406  may be detected by the UE  404 . When a strong synchronization/discovery signal  406  is detected, the UE  404  may determine an optimal transmit direction of the base station  402  and an optimal receive direction of the UE  404  corresponding to the strong synchronization/discovery signal. For example, the UE  404  may determine preliminary antenna weights/directions of the strong synchronization/discovery signal  406 , and may further determine a time and/or resource where the base station  402  is expected to optimally receive a beamformed signal. Thereafter, the UE  404  may attempt to associate with the base station  402  via a beamformed signal. 
     The base station  402  may sweep through a plurality of directions using a plurality of ports in a cell-specific manner in a first symbol of a synchronization subframe. For example, the base station  402  may sweep through different transmit directions (e.g., directions A, B, C, and D) using four ports in a cell-specific manner in a first symbol of a synchronization subframe. In an aspect, these different transmit directions (e.g., directions A, B, C, and D) may be considered “coarse” beam directions. In an aspect, a BRS may be transmitted in different transmit directions (e.g., directions A, B, C, and D). 
     In an aspect, the base station  402  may sweep the four different transmit directions (e.g., directions A, B, C, and D) in a cell-specific manner using four ports in a second symbol of a synchronization subframe. A synchronization beam may occur in a second symbol of the synchronization subframe. 
     Referring to diagram  420  of  FIG. 4B , the UE  404  may listen for beamformed discovery signals in different receive directions (e.g., directions E, F, G, and H). In an example, the UE  404  may sweep through the receive directions according to a sequence E-F-G-H. In another example, the UE  404  may sweep through the receive directions according to the sequence F-H-E-J. Although only four receive directions and two receive sequences are described with respect to  FIG. 4B , any number of different receive directions and receive sequences are contemplated. 
     The UE  404  may attempt the association by transmitting beamformed signals  426  (e.g., association signals or another indication of a best “coarse” beam or a best “fine” beam) in the different transmit directions (e.g., directions E, F, G, and H). In an aspect, the UE  404  may transmit an association signal  426  by transmitting along the optimal receive direction of the UE  404  at the time/resource where the base station  402  is expected to optimally receive the association signal. The base station  402  in the receive mode may sweep through different receive directions and detect the association signal  426  from the UE  404  during one or more timeslots corresponding to a receive direction. When a strong association signal  426  is detected, the base station  402  may determine an optimal transmit direction of the UE  404  and an optimal receive direction of the base station  402  corresponding to the strong association signal. For example, the base station  402  may determine preliminary antenna weights/directions of the strong association signal  426 , and may further determine a time and/or resource where the UE  404  is expected to optimally receive a beamformed signal. Any of the processes discussed above with respect to  FIGS. 4A and 4B  may be refined or repeated over time such that the UE  404  and base station  402  eventually learn the most optimal transmit and receive directions for establishing a link with each other. Such refinement and repetition may be referred to as beam training. 
     In an aspect, the base station  402  may choose a sequence or pattern for transmitting the synchronization/discovery signals according to a number of beamforming directions. The base station  402  may then transmit the signals for an amount of time long enough for the UE  404  to sweep through a number of beamforming directions in an attempt to detect a synchronization/discovery signal. For example, a BS beamforming direction may be denoted by n, where n is an integer from 0 to N, N being a maximum number of transmit directions. Moreover, a UE beamforming direction may be denoted by k, where k is an integer from 0 to K, K being a maximum number of receive directions. When the UE  404  detects a synchronization/discovery signal from the base station  402 , the UE  404  may discover that the strongest synchronization/discovery signal is received when the UE  404  beamforming direction is k=2 and the base station  402  beamforming direction is n=3. Accordingly, the UE  404  may use the same antenna weights/directions for responding (transmitting a beamformed signal) to the base station  402  in a corresponding response timeslot. That is, the UE  404  may send a signal to the base station  402  using UE  404  beamforming direction k=2 during a timeslot when the base station  402  is expected to perform a receive sweep at base station  402  beamforming direction n=3. 
     Path loss may be relatively high in mmW systems. Transmission may be directional to mitigate path loss. A base station may transmit one or more beam reference signals by sweeping in all directions so that a UE may identify a best “coarse” beam. Further, the base station may transmit a BRRS so that the UE may track “fine” beams. In various aspects, a CSI-RS may be used to track fine beams and, therefore, reference to BRRS may include reference to a CSI-RS. If a “coarse” beam identified by the UE changes, the UE may need to inform the base station so that the base station may train one or more new “fine” beams for the UE. 
     In various aspects, the UE may send an index of a best beam and corresponding beam refinement reference signal session request to the base station in a subframe reserved for a RACH. The UE may occupy one or more tones reserved for RACH. Further, the UE may occupy tones that are reserved for scheduling request but not for RACH transmission. 
       FIGS. 5A through 5G  are diagrams illustrating an example of the transmission of beamformed signals between a base station and a UE. The base station  502  may be embodied as a base station in a mmW system (mmW base station), such as the mmW base station  180 . In one aspect, the base station  502  may be collocated with another base station, such as an eNB, a cellular base station, or other base station (e.g., a base station configured to communicate in a sub-6 GHz band). While some beams are illustrated as adjacent to one another, such an arrangement may be different in different aspects (e.g., beams transmitted during a same symbol may not be adjacent to one another). Additionally, the number of illustrated beams is to be regarded as illustrative. 
     The base station  502  may include hardware for performing analog and/or digital beamforming. If the base station  502  is equipped with analog beamforming, at any one time, the base station  502  may transmit or receive a signal in only one direction. If the base station  502  is equipped with digital beamforming, the base station  502  may concurrently transmit multiple signals in multiple directions or may receive multiple signals concurrently in multiple directions. 
     Further, the UE  504 , for example, may include hardware for performing analog and/or digital beamforming. If the UE  504  is equipped with analog beamforming, at any one time, the UE  504  may transmit or receive a signal in only one direction. If the UE  504  is equipped with digital beamforming, the UE  504  may concurrently transmit multiple signals in multiple directions or may concurrently receive multiple signals in multiple directions. 
     In the mmW network, UEs may perform beam sweeps with mmW base stations within range. For example, the base station  502  may transmit m beams in a plurality of different spatial directions. The UE  504  may listen/scan for the beam transmissions from the base station  502  in n different receive spatial directions. When listening/scanning for the beam transmissions, the UE  504  may listen/scan for the beam sweep transmission from the base station  502   m  times in each of the n different receive spatial directions (a total of m*n scans). In another aspect, in a beam sweep, the UE  504  may transmit n beams in a plurality of different spatial directions. The base station  502  listens/scans for the beam transmissions from the UE  504  in m different receive spatial directions. When listening/scanning for the beam transmissions, the base station  502  may listen/scan for the beam sweep transmission from the UE  504   n  times in each of the m different receive spatial directions (a total of m*n scans). 
     Based on the performed beam sweeps, the UEs and/or the mmW base stations may determine a channel quality associated with the performed beam sweeps. For example, the UE  504  may determine the channel quality associated with the performed beam sweeps. Alternatively, the base station  502  may determine the channel quality associated with the performed beam sweeps. If the UE  504  determines a channel quality associated with the performed beam sweeps, the UE  504  may send the channel quality information (also referred to as beam sweep result information) to the base station  502 . The UE  504  may send the beam sweep result information to the base station  502 . If the base station  502  determines a channel quality associated with the performed beam sweeps, the base station  502  may send the beam sweep result information to the UE  504 . In an aspect, the channel quality may be affected by a variety of factors. The factors include movement of the UE  504  along a path or due to rotation (e.g., a user holding and/or rotating the UE  504 ), movement along a path behind obstacles, and/or movement within particular environmental conditions (e.g., obstacles, rain, humidity). The UE  504  and the base station  502  may also exchange other information, for example, associated with for beamforming (e.g., analog or digital beamforming capabilities, beamforming type, timing information, configuration information, etc.). 
     Based on the received information, the base station  502  and/or the UE  504  may determine various configuration information, such as mmW network access configuration information, information for adjusting beam sweeping periodicity, information regarding overlapping coverage for predicting a handoff to another base station, such as a mmW base station. 
     In an aspect, a beam set may contain eight different beams. For example,  FIG. 5A  illustrates eight beams  521 ,  522 ,  523 ,  524 ,  525 ,  526 ,  527 ,  528  for eight directions. In aspects, the base station  502  may be configured to beamform for transmission of at least one of the beams  521 ,  522 ,  523 ,  524 ,  525 ,  526 ,  527 ,  528  toward the UE  504 . In one aspect, the base station  502  can sweep/transmit directions using eight ports during a subframe (e.g., synchronization subframe). 
     In an aspect, a base station may transmit a signal, such as a BRS, in a plurality of directions, for example, during a synchronization subframe. In one aspect, this transmission may be cell-specific. Referring to  FIG. 5B , the base station  502  may transmit a first set of beams  521 ,  523 ,  525 ,  527  in four directions. For example, the base station  502  may transmit a BRS in a synchronization subframe of each of the transmit beams  521 ,  523 ,  525 ,  527 . 
     In an aspect, these beams  521 ,  523 ,  525 ,  527  transmitted in the four directions may be odd-indexed beams  521 ,  523 ,  525 ,  527  for the four directions out of a possible eight for the beam set. For example, the base station  502  may be capable of transmitting beams  521 ,  523 ,  525 ,  527  in directions adjacent to other beams  522 ,  524 ,  526 ,  528  that the base station  502  is configured to transmit. In an aspect, this configuration in which the base station  502  transmits beams  521 ,  523 ,  525 ,  527  for the four directions may be considered a “coarse” beam set. 
     The UE  504  may determine a respective beam index (sometimes abbreviated as “BI”) corresponding to a respective beam. In various aspects, the beam index may be indicate at least a direction for communicating through a corresponding beam toward the UE  504  (e.g., a beamforming direction). For example, the beam index may be a logical beam index associated with an antenna port, OFDM symbol index, and/or BRS transmission period, which may be indicated by one or more bits (e.g., 9 bits). For example, the UE  504  may be configured to determine a beam index corresponding to a beam based on a time at which a BRS is received—e.g., a symbol or slot during which a BRS is received may indicate a beam index corresponding to a beam. 
     In  FIG. 5C , the UE  504  may determine or select a beam index (sometimes abbreviated as “BI”) that is strongest or preferable. For example, the UE  504  may determine that the beam  525  carrying a BRS is strongest or preferable. The UE  504  may select a beam by measuring values for a received power or received quality associated with each of the first set of beams  521 ,  523 ,  525 ,  527 . In one aspect, the received power may be referred to as a BRS received power (BRSRP). 
     The UE  504  may compare respective values to one another. The UE  504  may select a “best” beam. In an aspect, the best beam may be a beam that corresponds to the greatest or highest value (e.g., the best beam may be a beam with the highest BRSRP). The selected beam may correspond to a beam index, which may be a beam index with respect to the base station  502 . For example, the UE  504  may determine that the BRSRP corresponding to the fifth beam  525  is the highest, and therefore the fifth beam  525  is the best beam as determined by the UE  504 . 
     The UE  504  may transmit a first indication  560  of the fifth beam  525  to the base station  502 . In an aspect, the first indication  560  may include a request to transmit a BRRS. The BRRS may be UE-specific. One of ordinary skill would appreciate that the BRRS may be referred to by different terminology without departing from the present disclosure, such as a beam refinement signal, a beam tracking signal, or another term. 
     In one aspect, the base station  502  may trigger transmission of the first indication  560 . For example, the base station  502  may trigger transmission of the first indication  560  by a DCI message. 
     The base station  502  may receive the first indication  560 . In one aspect, the first indication  560  may include a beam adjustment request (BAR) (e.g., a request for beam tracking, a request for a BRRS, a request for the base station to start transmitting on an indicated beam index without any further beam tracking, and the like). In one aspect, the BAR may be included in a MAC control element (CE). In one aspect, the first indication  560  may be indicated by a scheduling request. 
     In one aspect, the UE  504  may transmit a BAR once during a specific interval, which may be defined by a timer configured through RRC signaling (e.g., a “prohibit BAR timer”). In aspects, the UE  504  may trigger a BAR if the prohibit BAR timer is not running and at least one condition is satisfied. In one aspect, if the UE  504  has uplink resources allocated for new transmission for a transmission time interval (TTI), the UE  504  may generate and transmit a BAR MAC CE, and start or restart the prohibit BAR timer. In another aspect, the UE  504  may be configured with a dedicated scheduling request for BRRS request. In such an aspect, the UE  504  may signal the dedicated scheduling request for BRRS request in a scheduling request region of a RACH subframe (and the UE  504  may start or restart the prohibit BAR timer). In another aspect, the UE  504  may trigger a scheduling request (e.g., when a dedicated scheduling request for BRRS request is not configured for the UE  504  and/or when the UE  504  lacks uplink resources allocated for new transmission for this TTI). 
     In aspects, the first indication  560  may include a beam state information (BSI) report. The BSI report may include a beam index and a received power. For example, the UE  504  may measure a BRSRP corresponding to a beam through which a BRS is received, and the UE  504  may include the BRSRP and the beam index corresponding to the beam with a “best” (e.g., highest) BRSRP. The UE  504  may transmit the first indication  560  on PUCCH (e.g., xPUCCH) or an PUSCH (e.g., xPUSCH), for example, as a format of uplink control information where a BSI report consists of a beam index and a BRSRP. 
     Based on the first indication  560 , the base station  502  may determine the beam index corresponding to the fifth beam  525 . In  FIG. 5D , the base station  502  may transmit a second set of beams based on the first indication  560  (e.g., based on a beam index indicated by the first indication  560 ). For example, the UE  504  may indicate that a fifth beam  525  is the best beam and, in response, the base station  502  may transmit a second set of beams  524 ,  525 ,  526  to the UE  504  based on the indicated beam index. In an aspect, the beams  524 ,  525 ,  526  transmitted based on the first indication  560  may be closer (e.g., spatially and/or directionally) to the fifth beam  525  than those other beams  521 ,  523 ,  527  of the first set of beams. 
     In an aspect, the beams  524 ,  525 ,  526  transmitted based on the first indication  560  may be considered a “fine” beam set. In an aspect, the base station  502  may transmit a BRRS through each of the beams  524 ,  525 ,  526  of the fine beam set. In an aspect, the beams  524 ,  525 ,  526  of the fine beam set may be adjacent. In an aspect, BRRS transmission can span 1, 2, 5 or 10 OFDM symbols and may be associated with a BRRS resource allocation, BRRS process indication, and/or a beam refinement process configuration. 
     Based on the BRRS transmission through the beams  524 ,  525 ,  526  of the fine beam set, the UE  504  may transmit a second indication  565  to the base station  502  to indicate a “best” beam. In an aspect, the second indication  565  may use two (2) bits to indicate the selected beam. For example, the UE  504  may transmit the second indication  565  that indicates a beam index corresponding to the selected beam  525 . In one aspect, the second indication  565  may report beam refinement information (BRI). In one aspect, the second indication  565  may include a resource index (e.g., a BRRS-RI) and/or a reference power (RP) associated with the reception of the BRRS as measured by the UE  504  (e.g., a BRRS-RP). The base station  502  may then communicate with the UE  504  through the selected beam  525 . 
     Referring to  FIG. 5E , the base station  502  may transmit a BRS in a plurality of directions during a synchronization subframe. In an aspect, the base station  502  may transmit the BRS periodically and/or continuously, e.g., even after the UE  504  has communicated the second indication  565 . For example, the base station  502  may transmit beams  521 ,  523 ,  525 ,  527  that each include a BRS (e.g., a “coarse” beam set). 
     Referring to  FIG. 5F , the quality of a selected beam  525  may deteriorate so that the UE  504 . For example, when the base station  502  and the UE  504  are communicating through the selected beam  525 , the selected beam  525  may become occluded or otherwise unsatisfactory such that the base station  502  and the UE  504  may prefer to communicate through another beam. Based on the BRS (e.g., transmitted during a synchronization subframe), the UE  504  may determine a new beam  523  through which to communicate. For example, the UE  504  may determine that the third beam  523  through which a BRS is communicated may be the best beam. The UE  504  may select a beam based by measuring values for a received power (e.g., BRSRP) or received quality associated with each of the set of beams  521 ,  523 ,  525 ,  527 , comparing respective values to one another, and selecting the beam that corresponds to the highest value. The selected beam may correspond to a beam index at the base station  502 . The UE  504  may transmit a third indication  570  indicating this beam index to the base station  502 . In an aspect, the third indication  570  may include a request to transmit a BRRS. The BRRS may be UE-specific. In one aspect, a BAR may be used to request the base station  502  to transmit a BRRS. In one aspect, the third indication  570  may be triggered by the base station  502 , such as by a DCI message. Similar to the first indication  560 , the third indication  570  may be included in a scheduling request. 
     With respect to  FIG. 5G , the base station  502  may receive the third indication  570  from the UE  504 . The base station  502  may be configured to determine a beam index based on at least the third indication  570 . The base station  502  and the UE  504  may perform a beam refinement procedure, such as illustrated with respect to  FIG. 5E  (e.g., in order to select a new beam through which to communicate). 
     Referring to  FIG. 6 , a diagram of a wireless communications system  600  is illustrated. The base station  602  may be an aspect of the base station  502 , the base station  310 , the base station  102 , the mmW base station  180 , and/or another base station. The UE  604  may be an aspect of the UE  504 , the UE  350 , the UE  104 , the UE  182 , and/or another UE. 
     In the illustrated aspect, the base station  602  may include up to eight antenna ports for BRS transmission. In various aspects, the base station  602  may send, to the UE  604 , one or more BRSs  612   a - h  (e.g., as described with respect to  FIGS. 5A-5G ). Each BRS  612   a - h  may be communicated through a respective beam  620   a - h . For example, the base station  602  may send a first BRS  612   a  through the first beam  620   a  with which the first BRS  612   a  is associated. The UE  604  may track one or more beams  620   a - h  through periodically measuring a BRS  612   a - h  associated with a respective one of the beams  620   a - h . In an aspect, the transmission period of the BRSs may be configured by an indicator on a PBCH, such as an enhanced or evolved PBCH (ePBCH). The transmission period may be associated with the time to sweep the beams  620   a - h  through which the BRSs  612   a - h  are transmitted. 
     In aspects, the UE  604  may receive, through the set of beams  620   a - h , a set of BRSs  612   a - h . Each BRS  612  of the set of BRSs  612   a - h  may be associated with a beam index that corresponds to the beam  620   a - h  through which the BRS  612  is sent. The UE  604  may measure a signal quality of the BRSs  612   a - h , and each measured signal quality may correspond to a beam  620   a - h  of the set of beams. For example, the UE  604  may measure the signal qualities of the third BRS  612   c , the fourth BRS  612   d , the fifth BRS  612   e , and the sixth BRS  612   f , which respectively correspond to the third beam  620   c , the fourth beam  620   d , the fifth beam  620   e , and the sixth beam  620   f  In aspects, the UE  604  may not receive each of the BRSs  612   a - h.    
     In one aspect, the UE  604  may measure the signal quality as a received power. In one aspect, the signal quality may correspond to a BRSRP. For example, the UE  604  may measure the BRSRP in decibels (dB) and/or decibel-milliwatts (dBm). In other aspects, the UE  604  may measure the signal quality as another value, such as a received quality (RQ), an signal-to-interference ratio (SIR), an SINR, an SNR, an RSRP, an RSRQ, an RSSI, or another metric. 
     The UE  604  may transmit, to the base station  602 , a first indication (e.g., the first indication  560 ) that indicates a beam index corresponding to a beam with a “best” (e.g., highest) measured signal quality (e.g., BRSRP). The first indication may be a BSI report, which may include a beam index and a BRSRP corresponding to a beam of the set of beams  620   a - h . For example, the UE  604  may measure a best BRSRP for the fifth beam  620   e . The UE  604  may generate a BSI report that includes a BRSRP and a beam index corresponding to the fifth beam  620   e . The UE  604  may transmit the BSI report to the base station  602  as the first indication. 
     The base station  602  may transmit a BRRS  614 . In aspects, the BRRSs  614   c - f  may not be transmitted on each beam  620   a - h . For example, the base station  602  may transmit the BRRSs  614   c - f  on a subset of beams  620   c - f  of the set of beams  620   a - h . For example, the base station  602  may transmit the BRRSs  614   c - f  on the subset of beams  620   c - f  that is near or close to the beam  620   e  indicated by the first indication. In an aspect, the base station  602  may transmit the beams  620   c - f  as a fine set of beams for beam refinement with the UE  604 . 
     The UE  604  may select a beam of the subset of beams  620   c - f . For example, the UE  604  may measure a signal quality (e.g., received power) for each BRRS  614   c - f  of the set of BRRSs  614   c - f  sent through the subset of beams  620   c - f . The UE  604  may determine a best (e.g., highest) signal quality (e.g., received power) for a BRRS of the set of BRRSs  614   c - f  (e.g., the fifth BRRS  614   e ). The UE  604  may transmit a second indication (e.g., the second indication  565 ) to the base station  602 , which may include a beam index corresponding to the selected beam (e.g., a beam index corresponding to the fifth beam  620   e ). 
     The UE  604  and the base station  602  may then communicate  640  through the selected beam corresponding the beam index indicated by the UE  604  in the second indication. The communication  640  may be uplink and/or downlink communication. For example, the base station  602  and the UE  604  may communicate through the same or different beams for uplink communication and downlink communication. 
     In an aspect, at least one of the base station  602  and/or the UE  604  may maintain an set of beam indexes  630 . The set of beam indexes  630  may include beam indexes corresponding to beams from the base station  602  and/or another transmission point (e.g., another eNB or base station), which may allow dynamic point selection and/or joint transmission from multiple transmission points to the UE  604 . 
     Although the UE  604  is illustrated as maintaining the set of beam indexes  630 , the base station  602  may maintain a similar set of beam indexes. The base station  602  may maintain an set of beam indexes based on information from the UE  604 , such as one or more BSI reports. 
     The UE  604  may maintain a set of beam indexes  630  as an active set. In such an aspect, the UE  604  may synchronize with the base station  602  based on beams in the set of beam indexes  630  (e.g., soft handover from one beam of the beams  620   a - h  to another beam of the beams  620   a - h ). Beams corresponding to the beam indexes included in the set of beam indexes  630  may be used for PDCCH (e.g., xPDCCH, such as an enhanced PDCCH (ePDCCH)), PDSCH (e.g., xPDSCH, such as an enhanced PDSCH (ePDSCH)), PUCCH (e.g., xPUCCH, such as an enhanced PUCCH (ePUCCH)), and/or PUSCH (e.g., xPUSCH, such as an enhanced PUSCH (ePUSCH)) communication. The base station  602  and/or the UE  604  may periodically and/or continuously update the set of beam indexes  630 , for example, based on measurements of a BRS  612  and/or BRRS  614  (e.g. comparison of a signal quality to a threshold or relative to signal qualities corresponding to a beam included in the set of beam indexes  630 ). 
     In one aspect, the base station  602  and/or the UE  604  may maintain the set of beam indexes  630  as a candidate set. The base station  602  and/or the UE  604  may simultaneously maintain both an active set of beam indexes and a candidate set of beam indexes. For a candidate set of beam indexes, the beams corresponding to the beam indexes in the set of beam indexes  630  may be maintained for inclusion in an active set of beam indexes. Similar to the active set of beam indexes, the base station  602  and/or the UE  604  may update the candidate set of beam indexes periodically, for example, based on measurements of a BRS  612  or BRRS  614  (e.g. comparison of a signal quality to a threshold or relative to signal qualities corresponding to a beam included in the candidate set of beam indexes). However, the periodicity for updating the active set of beam indexes may be different than the periodicity for updating the candidate set of beam indexes. 
     The base station  602  and/or the UE  604  may update the set of beam indexes  630 , for example, by adding a beam index to or removing a beam index from a set of beam indexes  630 . Additionally, the base station  602  and/or the UE  604  may move a beam index between the active set of beam indexes and the candidate set of beam indexes. The criteria for updating a set of beam indexes  630  may be based on a signal quality measurement, such as a received power for a BRS  612  or BRRS  614 . For example, a signal quality may be compared to a threshold or to another signal quality corresponding to a beam index currently included in a set of beam indexes. 
     In various aspects, the base station  602  may communicate  640  with the UE  604  through the fifth beam  620   e , which may be a current active beam or current serving beam. The communication  640  may be performed with a first RAT, such as a mmW RAT, a 5G RAT (e.g., based on one or more technical standards promulgated by 3GPP), and the like. The communication  640  may be uplink communication, downlink communication, or both uplink and downlink communication. 
     In various aspects, the base station  602  may determine that beam tracking is to be performed with the UE  604 . Beam tracking may include one or more operations for selecting or identifying a beam of the set of beams  620   a - h  for the communication  640  between the base station  602  and the UE  604  (e.g., uplink and/or downlink communication). For example, the base station  602  may determine that the communication  640  through the fifth beam  620   e  is degraded (e.g., a radio link failure) and/or that another beam may provide better quality than the fifth beam  620   e . Thus, the base station  602  may determine that a new serving beam of the beams  620   a - h  may be used to communicate  640  with the UE  604 . 
     Based on the determination that beam tracking is to be performed, the base station  602  may perform beam tracking with the UE  604 . In various aspects, beam tracking may include one or more operations associated with identifying or selecting a new active or new serving beam for communication between the base station  602  and the UE  604 . 
     According to one aspect of beam tracking, the base station  602  may transmit at least one of the BRSs  612   a - h . The UE  604  may receive one or more of the BRSs  612   a - h , each of which may correspond to a respective beam of the beams  620   a - h . The UE  604  may select or identify at least one beam index corresponding to one of the beams  620   a - h  based on the reception of the one or more BRSs  612   a - h  (e.g., the UE  604  may identify one or more BRSs  612   a - h  having a highest signal quality). The UE  604  may transmit, to the base station  602 , a first indication of one or more beam indexes corresponding to one or more of the beams  620   a - h  having a best (e.g., highest) signal quality measured for a respective received BRS of the BRSs  612   a - h  (e.g., a beam index corresponding to the sixth beam  620   f ). Based on the first indication of the one or more beam indexes, the base station  602  may transmit one or more BRRSs  614   c - f . For example, the base station  602  may transmit the BRRSs  614   c - f  through the respective beams  620   c - f  that are close or proximate to the one or more beams corresponding to the one or more beam indexes indicated by the first indication. The UE  604  may receive one or more of the BRRSs  614   c - f , each of which may correspond to a respective beam of the beams  620   c - f . The UE  604  may select or identify at least one beam index corresponding to one of the beams  620   c - f  based on the reception of the one or more BRRSs  614   c - f  (e.g., the UE  604  may identify one or more BRRSs  614   c - f  having a highest signal quality). The UE  604  may transmit, to the base station  602 , a second indication of one or more beam indexes corresponding to one or more of the beams  620   c - f  having a best (e.g., highest) signal quality measured for a respective received BRRS of the BRRSs  614   c - f  (e.g., a beam index corresponding to the sixth beam  620   f ). The base station  602  and the UE  604  may communicate through a new active beam, which may correspond to the beam index indicated by the second indication, such as the sixth beam  620   f.    
     According to one aspect of beam tracking, the UE  604  may transmit a request for beam tracking  648  (e.g., a BAR). The request for beam tracking  648  may request the base station  602  to transmit the BRRSs  614   c - f . Based on the request for beam tracking  648 , the base station  602  may transmit the one or more BRRSs  614   c - f . For example, the base station  602  may transmit the BRRSs  614   c - f  through the respective beams  620   c - f  that are close or proximate to the first active beam (e.g., the fifth beam  620   e ). The UE  604  may receive the one or more of the BRRSs  614   c - f , each of which may correspond to a respective beam of the beams  620   c - f . The UE  604  may select or identify at least one beam index corresponding to one of the beams  620   c - f  based on the reception of the one or more BRRSs  614   c - f  (e.g., the UE  604  may identify one or more BRRSs  614   c - f  having a highest signal quality). The UE  604  may transmit, to the base station  602 , an indication of one or more beam indexes corresponding to one or more of the beams  620   c - f  having a best (e.g., highest) signal quality measured for a respective received BRRS of the BRRSs  614   c - f  (e.g., a beam index corresponding to the sixth beam  620   f . The base station  602  and the UE  604  may communicate through a new active beam, which may correspond to the beam index indicated by the indication, such as the sixth beam  620   f.    
     In one aspect, the base station  602  may determine that beam tracking is to be performed with the UE  604  based on the communication  640  with the UE  604 . For example, the base station  602  may transmit, to the UE  604 , a message  642  indicating that beam tracking is to be performed (e.g., based on the determination that beam tracking is to be performed). In an aspect, the base station  602  may send the message  642  on a PDCCH (e.g., xPDCCH) or a PDSCH (e.g., xPDSCH). In an aspect, the base station  602  may send the message  642  as DCI on the PDCCH. For example, one or more bits associated with a DCI may be reserved to indicate the message  642 , and the base station  602  may use the one or more reserved bits of the DCI to indicate the message to perform beam tracking. The reserved bits may vary according to DCI formats. 
     In one aspect, the base station  602  may transmit the message through the fifth beam  620   e , which may be a current active beam or current serving beam. In other words, the base station  602  may transmit the message  642  to the UE  604  using a first RAT (e.g., a mmW RAT). In another aspect, the base station  602  may transmit the message  642  to the UE through a second RAT  610 , which may have a lower carrier frequency than the first RAT (e.g., a sub-6 GHz or LTE RAT). 
     In an aspect, the UE  604  may acknowledge the message  642 , such as by transmitting an ACK message. The UE  604  may transmit an acknowledgment message through the first RAT or the second RAT  610 . The base station  602  and the UE  604  may then perform beam tracking using the first RAT. In aspect, the base station  602  may perform beam tracking (such as be transmitting the BRSs  612   a - h ) when an acknowledgement is not received in response to the message  642  (e.g., within a predetermined period of time). 
     In an aspect, the base station  602  may communicate with the UE  604  using the current active beam (e.g., fifth beam  620   e ) by sending a reference signal  644  to the UE  604 . For example, the base station  602  may send the reference signal  644  to determine if the current active beam (e.g., fifth beam  620   e ) is failing. According to various aspects, the reference signal  644  may be a CSI-RS, a CRS, an SSS, an MRS, a DMRS, or a BRS (e.g., the BRS  612   e ). 
     In an aspect, if the base station  602  does not receive a response to the reference signal  644 , then the base station  602  may determine that the current active beam (e.g., fifth beam  620   e ) is failing. In another aspect, the UE  604  may transmit a response  646 , for example, based on the reference signal  644 . The base station  602  may detect a radio link failure (e.g., determine that the current active beam is failing) based on the response  646 . For example, the base station  602  may detect a radio link failure by determining a value based on the response  646  and comparing the value to a predetermined threshold. 
     In an aspect, the UE  604  may send the response  646  through the second RAT  610  (e.g., a sub-6 GHz or LTE RAT). For example, the UE  604  may measure a value (e.g., a SINR, SNR, RSSI, RSRP, RSRQ, etc.) and determine that the current active beam is failing, and therefore, send the response  646  using the second RAT  610  in order to increase the probability that the base station  602  receives the response  646 . In another aspect, the UE  604  may send the response  646  through the first RAT (e.g., the mmW RAT). The response  646  may include at least one of a CQI, an SINR, an SNR, an RSSI, an RSRP, an RSRQ, and the like. 
     In an aspect, the base station  602  may select and transmit reference signals used for radio link failure for specific transmission points so that reference signals are restricted to specific transmission points. A transmission point may be, for example, different sectors of a same base station and/or coordinated multipoint transmission from different base stations, such as where a controller indicates transmission properties to a base station. A transmission point may also be an antenna port of one base station. For example, the base station  602  may select reference signals (similar to the reference signal  644 ) to be transmitted in one sector that includes the UE  604 . The base station  602  may allow separate radio link failure procedures for separate transmission ports. 
     The UE  604  may receive at least the first reference signal  644  and may monitor this first reference signal  644  to determine whether there is an event, which may be a Qin event or a Qout event. For a Qin event, the UE  604  may measure one or more metrics (e.g., signal quality, received power, RSSI, SNR, etc.) associated with the first reference signal  644  to determine whether the one or more metrics are greater than or equal to a threshold. For a Qout event, the UE  604  may measure one or more metrics (e.g., signal quality, received power, RSSI, SNR, etc.) associated with the first reference signal  644  to determine whether the one or more metrics are less than or equal to a threshold, which may indicate radio link failure. In an aspect, the one or more metrics may include at least an SNR or SINR. 
     In an aspect, the UE  604  may use the first reference signal  644  to detect radio link failure. In another aspect, the UE  604  may use a plurality of reference signals (including the reference signal  644 ) to detect radio link failure. For example, the UE  604  may use a best SNR estimate or another metric among all reference signals (including the reference signal  644 ) associated with a plurality of beams (e.g., the fourth beam  620   d , the fifth beam  620   e , the sixth beam  620   f ). In another example, the UE  604  may combine one or more metrics measured from the reference signals (including the reference signal  644 ) to detect radio link failure. 
     In an aspect, the UE  604  may maintain separate radio link failure processes for different receiving beams or subarrays, for example, using the reference signals associated with different receiving beams or subarrays. For example, the UE  604  may maintain one process for the current active beam (e.g., the fifth beam  620   e ), which may be based on monitoring the first reference signal  644 . Further, the UE  604  may maintain another process for a first candidate beam (e.g., the sixth beam  620   f ), which may be based on monitoring a second reference signal (e.g., the sixth BRS  612   f ) associated with the first candidate beam (e.g., the sixth beam  620   f ). 
     In an aspect, when the UE  604  detects a radio link failure, the UE  604  and the base station  602  may perform beam tracking. In an aspect, the UE  604  may request a beam tracking procedure with the base station  602  when a radio link failure is detected. For example, the UE  604  may transmit a BAR. In an aspect, the UE  604  may transmit an indication of the detected radio link failure, for example, so that the base station  602  may determine to perform beam tracking with the UE  604 . 
     In an aspect, base station  602  may determine that beam tracking is to be performed with the UE  604  based on an absence of the communication  640 . For example, the base station  602  may determine that beam tracking is to be performed with the UE  604  when base station  602  expects the UE  604  to transmit one or more messages to the base station  602 , but the one or more messages are absent (e.g., for a predetermined period of time). For example, the base station  602  may schedule communication  640  with the UE  604  on a PUCCH and/or a PUSCH. If the base station  602  determines that the communication  640  is absent on the PUCCH and/or PUSCH (e.g., when the UE  604  is scheduled), then the base station  602  may determine that beam tracking is to be performed with the UE  604 . Similarly, if the base station  602  is expecting an ACK/NACK message (e.g., to the message  642 , to the reference signal  644 , and the like), but the ACK/NACK message is absent (e.g., for a predetermined period of time), then the base station  602  may determine that beam tracking is to be performed with the UE  604 . Similarly, if the base station  602  receives a plurality of NACK messages (e.g., a threshold amount of NACK messages before receiving an ACK message), then the base station  602  may determine that beam tracking is to be performed with the UE  604 . 
     In an aspect, the UE  604  may operate according to DRX and, therefore, may alternate between continuous reception cycles (e.g., during which the UE  604  is active and receiving) and DRX cycles (e.g., during which the UE  604  is inactive and not receiving). The DRX may affect the current active beam (e.g., the fifth beam  620   e ), such as when the UE  604  moves during DRX. Therefore, when the UE  604  transitions from DRX to continuous reception cycle, the UE  604  may use a synchronization subframe or a tracking subframe to find a beam. In an aspect, the base station  602  may transmit a signal (e.g., a BRS  612  in a synchronization subframe or a BRRS  614  in a tracking subframe). In an aspect, the tracking signal may serve many UEs sharing a same DRX cycle. The UE  604  may monitor a subset of tracking signals in a synchronization subframe based on one or more active beam indexes from a previous DRX cycle. In another aspect, the base station  602  may use the active beam of the previous continuous reception cycle of the UE  604  to determine the beam indexes that the base station  602  should transmit during the beam tracking session. 
     In an aspect, the base station  602  may consider semi-persistent scheduling as a type of triggering for beam tracking, which may be based on DRX durations (e.g., one PDCCH triggers multiple tracking signal transmissions over multiple subframes). That is, the base station  602  may determine that beam tracking is to be performed based on DRX durations. In an aspect, the base station  602  may determine that beam tracking is to be performed with the UE  604  by determining a time at which the UE  604  is to transition from an inactive cycle of DRX to an active cycle of DRX. For example, the current active beam (e.g., fifth beam  620   e ) may fail while the UE  604  is in an inactive DRX cycle. The base station  602  may be aware of DRX cycles for the UE  604  and may determine to perform beam tracking that corresponds to a time at which the UE  604  is to transition out of the inactive cycle and into an active cycle for reception. 
     In an aspect, the base station  602  may perform beam tracking based on the first active beam. For example, the UE  604  may transition into an inactive DRX cycle when the current active beam (e.g., fifth beam  620   e ) is being used. The UE  604  may not drift too far during the inactive DRX cycle, and so the base station  602  may use beams that are close to the current active beam (e.g., fifth beam  620   e ) when performing beam tracking after the UE  604  is in the inactive DRX cycle. The base station  602  may transmit the message  642  indicating that beam tracking is to be performed, for example, based on a time at which the UE  604  is to transition to an active DRX cycle from an inactive DRX cycle. 
     Referring to  FIG. 7 , a diagram of a wireless communications system  700  is illustrated. The base station  702  may be an aspect of the base station  602 , the base station  502 , the base station  310 , the base station  102 , the mmW base station  180 , and/or another base station. The UE  704  may be an aspect of the UE  604 , the UE  504 , the UE  350 , the UE  104 , the UE  182 , and/or another UE. 
     In various aspects, the base station  702  and the UE  704  may communicate through a first active beam, such as the fifth beam  720   e . In aspects, the UE  704  may need to indicate a beam of the beams  720   a - h  to the base station  702 , e.g., when the communication through the fifth beam  720   e  deteriorates, when the UE  704  transitions from an inactive DRX cycle to an active DRX cycle, etc. However, the base station  702  may detect transmission from the UE  704  in the direction of the fifth beam  720   e.    
     According to various aspects, the UE  704  may send, to the base station  702 , a request  750  to change the active beam. The request  750  may indicate a beam index corresponding to a new beam. In an aspect, the UE  704  may use a Zadoff-Chu sequence to indicate the beam change request. 
     In aspects, the UE  704  and the base station  702  may communicate through the fifth beam  720   e , which may be a current active beam. The UE  704  may identify or select a new beam, such as the sixth beam  720   f , for communication with the base station  702 . For example, the UE  704  may determine that communication through the fifth beam  720   e  is deteriorating or unsatisfactory. The UE  704  may select or identify the sixth beam  720   f  as the new beam, for example, based on measuring a signal quality of a BRS and/or BRRS transmitted through the sixth beam  720   f . The UE  704  may generate a request  750  to change the active beam from the fifth beam  720   e  to the sixth beam  720   f . The UE  704  may generate the request  750  to indicate a beam index corresponding to the sixth beam  720   f . The UE  704  may send the request  750  to the base station  702 . 
     In one aspect, the UE  704  may generate the request  750  to indicate the beam index corresponding to the new beam (e.g., the sixth beam  720   f ) based on a cyclic shift. For example, the UE  704  and/or the base station  702  may maintain a mapping of one or more cyclic shifts to one or more respective beam indexes. In a further example, one or more bits of the request  750  may correspond to a beam index of the new beam. The UE  704  may identify a cyclic shift corresponding to the sixth beam  720   f  and may send the request  750  with the identified cyclic shift corresponding to the sixth beam  720   f . Accordingly, the base station  702  may receive the request  750  with the cyclic shift and identify the beam index associated with the cyclic shift in order to determine the sixth beam  720   f  corresponding to the beam index. 
     In one aspect, the UE  704  may generate the request  750  to indicate the beam index corresponding to the new beam (e.g., the sixth beam  720   f ) based on a spreading across symbols. For example, the UE  704  and/or the base station  702  may maintain a mapping of one or more spreading across symbols to one or more respective beam indexes. The UE  704  may identify a spread across symbols corresponding to the sixth beam  720   f  and may send the request  750  with the identified spread across symbols corresponding to the sixth beam  720   f . Accordingly, the base station  702  may receive the request  750  with the spread across symbols and identify the beam index associated with the spread across symbols in order to determine the sixth beam  720   f  corresponding to the beam index. 
     In one aspect, the UE  704  may generate the request  750  to indicate the beam index corresponding to the new beam (e.g., the sixth beam  720   f ) based on one or more subcarriers that carries the request  750 . For example, the UE  704  and/or the base station  702  may maintain a mapping of one or more subcarriers to one or more respective beam indexes. The UE  704  may identify one or more subcarriers corresponding to the sixth beam  720   f  and may send the request  750  on the identified one or more subcarriers corresponding to the sixth beam  720   f . Accordingly, the base station  702  may receive the request  750  one the one or more subcarriers and identify the beam index associated with the one or more subcarriers in order to determine the sixth beam  720   f  corresponding to the beam index. 
     Based on the beam index indicated by the request  750 , the base station  702  may change the active beam from the fifth beam  720   e  to the sixth beam  720   f . The base station  702  and the UE  704  may then communicate through the sixth beam  720   f . The communication may be uplink and/or downlink communication. 
     In an aspect, the UE  704  may use a subframe  740  in order to indicate the new beam (e.g., because beamforming may not be required for RACH in a cell provided by the base station  702 ). In aspects, the subframe  740  may be a RACH subframe. The subframe  740  may include a plurality of resources—e.g., 10 time resources (e.g., slots) and  12  frequency resources (e.g., carriers). The subframe  740  may include resources  742  for scheduling request (SR) (e.g., SR collection or region) and resources  744  for RACH (e.g., RACH collection or region). Each resource may include six physical resource blocks (PRBs), and each PRB may include twelve tones. 
     In one aspect, at least one of the base station  702  and/or the UE  704  may maintain a mapping between beams (e.g., beams  720   a - h ) associated with a synchronization (e.g., BRS) session and/or RACH session. That is, the UE  704  may be configured to indicate a beam index using one or more resources of a subframe  740 , such as by transmitting a request  750  on at least one resource corresponding to the beam index selected by the UE  704 . 
     In one aspect, the subframe  740  may include SR resources  742  (e.g., an SR collection). The UE  704  may indicate a selected beam on resources that are not included in the SR resources  742 . The UE  704  may be configured to select a beam of the beams  720   a - h , and each beam of the beams  720   a - h  may correspond to a beam index. The UE  704  may select a beam of the beams  720   a - h , for example, based on measuring a signal quality of one or more BRSs or BRRSs received through one or more of the beams  720   a - h . For example, the UE  704  may select a new beam (e.g., the second beam  720   b ), and the UE  704  may be configured to transmit the request  750  as a RACH sequence in symbols  0  and/or  1  of the resources not included in the SR resources  742  if the selected beam index corresponding to the new beam (e.g., the second beam  720   b ) corresponds to one of beams  720   a - d . Similarly, the UE  704  may be configured to transmit the request  750  as a RACH sequence in a symbols  2  and/or  3  of the resources not included in the SR resources  742  if the selected beam index corresponds to one of beams  720   e - h.    
     In one aspect, the UE  704  may indicate a specific beam within the range using at least one subcarrier. For example, the UE  704  may indicate a beam within the range of beams  720   a - d  by using at least one of a pair of subcarriers  770 ,  772 ,  774 ,  776 . Similarly, the UE  704  may indicate a beam within the range of beams  720   e - h  by using at least one of a pair of subcarriers  770 ,  772 ,  774 ,  776 . For example, subcarriers  770  may indicate a first beam of a range and, therefore, when the UE  704  transmits a RACH sequence on symbols  0  and  1  and subcarriers  770  (corresponding to a first beam within a range of beams), the UE  704  is indicating a selected first beam  720   a . By way of another example, the UE  704  may indicate a selected seventh beam  720   g  by transmitting a RACH sequence on subcarriers  774  (corresponding to a third beam within a range of the beams) on symbols  2  and  3  (corresponding to the range of beams  720   e - h ). By way of another example, the UE  704  may indicate a selected sixth beam  720   f  by transmitting a RACH sequence on subcarriers  772  (corresponding to a second beam within a range of beams) on symbols  2  and  3  (corresponding to the range of beams  720   e - h ). The base station  702  may therefore determine a selected beam index based on the at least the resources on which the RACH sequence is transmitted, which are resources not included in the SR resources  742 . 
     In another aspect, the UE  704  may use resources not included in the RACH resources  744  to indicate a selected beam. In an aspect, a BAR procedure may be configured in the UE  704 . For example, if a dedicated SR for BRRS request is configured to the UE  704 , a PHY layer of the UE  704  may signal a dedicated SR for BRRS request in the resources not included in the RACH resources  744 . 
     In an aspect, the UE  704  may only transmit on the resources not included in the RACH resources  744  (including SR resources  742 ) when the UE  704  is timing aligned with the base station  702 . The number of available cyclic shifts associated with the resources not included in the RACH resources  744  may be higher than those available in the RACH resources  744 . Accordingly, there may be a higher degree of freedom associated with the resources not included in the RACH resources  744  (e.g., 192 degrees of freedom) compared to the RACH resources  744  (e.g., 48 degrees of freedom). For example, a plurality of UEs may be able to transmit requests (e.g., requests for beam tracking and/or BRRS, a BAR, etc.) on the resources not included in the RACH resources  744 . That is, different cyclic shifts and/or different spreading across symbols may be used to distinguish between different UEs at the base station  702 . For example, the UE  704  may transmit a Zadoff-Chu sequence in two symbols, whereas another UE may multiply a sequence with [+1, −1] and transmit that product. 
     In an aspect, the UE  704  may select a transmission time for SR based on symbol index of the strongest beam (e.g., a beam in which a strongest BRS is received during a synchronization subframe). In an aspect, the UE  704  may transmit an SR during a subframe  740  if instructed by a higher layer. For example, a PHY layer of the UE  704  may be provided with a plurality of parameters, including a band number N SR , cyclic shift ν, a root u, a parameter ƒ′, a system frame number (SFN), a BRS transmission period N BRS , a number of symbols N RACH  during the subframe  740  for which the base station  702  may apply different beams (e.g., different receive beams), a number of subframes M (e.g., number of RACH subframe) in each frame, an index of the current subframe m (e.g., current RACH subframe), a symbol with the strongest synchronization beam S Sync   BestBeam . The root u may be cell specific. The UE  704  may calculate a symbol index  1  based on the SFN, N BRS , N RACH , M, m, and S Sync   BestBeam . For example,
 
 l =(( S   Sync   BestBeam −(SFN· M·N   RACH   +m·N   RACH ) %  N   BRS ) %  N   BRS )· N   rep ,
 
     Where N rep  may denote the number of symbols dedicated to a single RACH transmission (e.g., N rep =2). 
     In one aspect, at least one of the base station  702  and/or the UE  704  maintains a mapping between beams (e.g., beams  720   a - h  associated with a synchronization (or BRS) session and the resources not included in the RACH resources  744 . That is, the UE  704  may be configured to indicate a beam index using one or more resources of a subframe  740 , such as by transmitting a request  750  on at least one resource corresponding to the beam index selected by the UE  704 . 
     For example, the UE  704  may be configured to transmit the request  750  in a symbol  0  and  1  of the subframe  740  if the selected beam index (e.g., the second beam  720   b ) corresponds to one of beams  720   a - d . Similarly, the UE  704  may be configured to transmit the request  750  in a symbol  2  and  3  of the subframe  740  if the selected beam index corresponds to one of beams  720   e - h.    
     In one aspect, UE  704  may indicate a specific beam within the range using at least one subcarrier. For example, the UE  704  may indicate a beam within the range of beams  720   a - d  by using at least one of a pair of subcarriers  760 ,  762 ,  764 ,  766 . Similarly, the UE  704  may indicate a beam within the range of beams  720   e - h  by using at least one of a pair of subcarriers  760 ,  762 ,  764 ,  766 . For example, subcarriers  760  may indicate a first beam of a range and, therefore, when the UE  704  transmits a request  750  on symbols  0  and  1  and subcarriers  760 , the UE  704  is indicating a selected first beam  720   a . By way of another example, the UE  704  may indicate a selected seventh beam  720   g  by transmitting a request on subcarriers  764  (corresponding to a third beam within a range) on symbols  2  and  3  (corresponding to the range of beams  720   e - h ). The base station  702  may therefore determine a selected beam index based on the at least one resource on which the request is transmitted. 
     Referring to  FIG. 8 , a diagram of a wireless communications system  800  is illustrated. The base station  802  may be an aspect of the base station  702 , the base station  602 , the base station  502 , the base station  310 , the base station  102 , the mmW base station  180 , and/or another base station. The UE  804  may be an aspect of the UE  704 , the UE  604 , the UE  504 , the UE  350 , the UE  104 , the UE  182 , and/or another UE. 
     In  FIG. 8 , the UE  804  may communicate with the base station  802  using an active beam  820   e  (e.g., the active beam  820   e  may be an aspect of one of the beams  720   a - h  described in  FIG. 7 , such as the fifth beam  720   e ). The UE  804  may communicate with the base station  802  through a set of beams  840   d - e  at the UE  804 . In an aspect, the active beam  802   e  may be paired with a first beam  840   d  at the UE  804 , which may form an active beam pair. Similarly, the control-information beam  820   f  may be paired with a second beam at the UE  804 , which may form a control-information beam pair. The UE  804  may identify beam pairs for communication (e.g., uplink and/or downlink communication) with the base station  802 . In one aspect, the UE  804  may be configured to monitor a control channel (e.g., a PDCCH, an ePDCCH, a new radio (NR) PDCCH, etc.) on one or more beam pair links simultaneously and/or the UE  804  may monitor the control channel on different beam pair link(s) in different OFDM symbols. 
     The UE  804  and/or the base station  802  may each maintain a set of candidate beam indexes  830 . In an aspect, the set of candidate beam indexes  830  includes at least a beam index corresponding to a control-information beam  820   f  (e.g., the control-information beam  820   f  may be an aspect of one of the beams  720   a - h  described in  FIG. 7 , such as the sixth beam  720   f ). The set of candidate beam indexes  830  may include additional beams  720   d ,  720   g ,  720   h  (e.g., as described with respect to  FIG. 7 ). The base station  802  and/or the UE  804  may maintain to the set of candidate beam indexes  830  to include additional beams  720   d ,  720   g ,  720   h  that are not currently used for communication with the UE  804  (e.g., the base station  802  may generate one or more of the additional beams  720   d ,  720   g ,  720   h  based on a request  750  to change the active beam). In an aspect, the set of candidate beam indexes  830  may include a set of beam pairs, such as the beam pair  820   f ,  840   e.    
     The UE  804  and/or the base station  802  may each maintain a set of active beam indexes, which may be similar to the set of candidate beam indexes  830 . The set of active beam indexes may include one or more beam indexes to be used for communication between the base station  802  and the UE  804  (including the active beam  820   e ). The base station  802  and/or the UE  804  may maintain to the set of active beam indexes to include additional beams (e.g., another of beams  720   a - h ), which may be used for uplink or downlink communication between the base station  802  and the UE  804 . In an aspect, the set of active beam indexes may include a set of beam pairs, such as the beam pair  820   e ,  840   d.    
     The base station  802  and the UE  804  may maintain two sets of beams: an active set and a candidate set. The active set may include the active beam  820   e . The candidate set may include at least one beam that may be a candidate for communication between the base station  802  and the UE  804  (e.g., if the active beam  820   e  fails). In one aspect, the candidate beam set may include the control-information beam  820   f.    
     In one aspect, the UE  804  transmits control information on a control channel so that it is received at the base station  802  using the active beam  820   e . In an aspect, the control channel may be a PUCCH. However, the base station  802  may periodically receive control information  844  carried on the control channel through a second beam. For example, the UE  804  may send control information  844  to the base station  802  using a control-information beam  820   f . In an aspect, the control-information beam  820   f  may correspond with a second beam  840   e  at the UE  802 . Therefore, the UE  804  may identify or select the second beam  840   e  through which to send the control information  844  based on usage of the control-information beam  820   f  instead of the active beam  820   e.    
     In an aspect, the UE  804  and the base station  802  may be further configured to communicate using a wide beam  822 . For example, the base station  802  and the UE  804  may communicate using a mmW RAT or near-mmW RAT in which beamforming weights for the active beam  820   e  and control-information beam  820   f  may have a first beam weight, whereas the wide beam  822  may have a second beam weight that is greater than the first beam weight. In an aspect, the base station  802  may sample antenna weights of the base station  802  such that a wider area of a sector associated with the base station  802  is covered. In an aspect, the wide beam  822  may have an angle that is greater than the angle of at least one of the active beam  820   e  and/or the control-information beam  820   f . For example, the active beam  820   e  and/or the control-information beam  820   f  may have an angle of approximately five or six degrees, whereas the wide beam  822  may have an angle of approximately twenty or thirty degrees. 
     In one aspect, the wide beam  822  may be directed toward at least one beam at the UE  804 . For example, the wide beam  822  may be directed toward the second beam  840   e  at the UE  804  and, therefore, when the UE  804  monitors the antenna port or subarray associated with the second beam  840   e , the UE  804  may receive information transmitted by the base station  802  through the wide beam  822 . 
     In an aspect, the UE  804  may periodically send information  844  (e.g., control information) to the base station  802  through the wide beam  822 . The UE  804  may measure a signal quality or channel estimate associated with communication with the base station  802 . For example, the UE  804  may measure a signal quality (e.g., SNR, SINR, received power, received quality, etc.) for a signal (e.g., a BRS, a CSI-RS, a reference signal, etc.), and the UE  804  and may send the signal quality or channel estimate to the base station  802 . In various aspects, the information may include UCI, CQI, SR, and/or other control information. The base station  802  may receive the signal quality or channel estimate through the wide beam  822 , for example, if the active beam  820   e  and/or the control-information beam  820   f  fail and/or become unsatisfactorily degraded. 
     In an aspect, the UE  804  may send the information (e.g., control information  844  on a control channel, such as a PUCCH) through the wide beam  822  without explicit signaling from the base station  802 . For example, the UE  804  may determine that the UE  804  is to send information through the wide beam  822  after an absence of signaling from the base station  802  on the active beam  820   e  and/or the control-information beam  820   f.    
     Accordingly, the base station  802  may receive information (e.g., the control information  844 ) from the UE  804  through the wide beam  822 . In an aspect, the base station  802  may determine that at least one of the active beam  820   e  and/or the control-information beam  820   f  have failed and/or become unsatisfactorily degraded based on reception of the information through the wide beam  822 . 
     In an aspect, the UE  804  may periodically send the control information  844  on the control channel through the control-information beam  820   f . The base station  802  may estimate uplink channel quality for the control-information beam  820   f  based on the periodic reception of the control information  844  from the UE  804 . In one aspect, the UE  804  may measure a quality (e.g., channel quality) associated with the active beam  820   e  and may generate the control information  844  to include an indication of the quality associated with the active beam  820   e.    
     The base station  802  receive the information  844 . Based on information  844 , the base station  802  may determine whether the active beam  820   e  is failing and/or is unsatisfactory (e.g., the quality does not satisfy a threshold). 
     While  FIG. 8  illustrates the communication of the information  844  through the control-information beam  820   f , the wide beam  822  may serve as the control-information beam, as described. For example, the UE  804  may send the information  844  through a beam corresponding to a beam index included in a set of candidate beams and/or the UE  804  may send the information  844  through the wide beam  822 . 
     The UE  804  and the base station  802  may switch to a beam corresponding to a beam index included in a the set of candidate beam indexes for communication based on the control information  844 . For example, the UE  804  and the base station  802  may switch to the control-information beam  820   f  for communication (in which case the control-information beam  820   f  may become the active beam and may carry other data instead or in addition to the control information  844 ). 
     In one aspect, the UE  804  and the base station  802  may change the active beam to the control-information beam  820   f  without explicit signaling (e.g., the UE  804  does not need to transmit through RACH or SR to indicate beam index, as described in  FIG. 7 ). For example, the UE  804  may determine that the active beam is to be changed based on the information  844  (e.g., based on a signal quality included in the information  844 ). Similarly, the base station  802  may determine that the active beam is to be changed based on the information  844 . 
     In an aspect, the base station  802  may transmit, to the UE  804 , information  842  indicating the periodicity at which control information is to transmitted on a control channel from the UE  804  to the base station  802 . In one aspect, the base station  802  may transmit the information  842  through the active beam  820   e . In one aspect, the base station  802  may transmit the information  842  to the UE  804  using RRC signaling. In another aspect, the base station  802  may transmit the information  842  to the UE  804  on a PDCCH. For example, the base station  802  the information  842  may indicate the periodicity to the UE  804  using one or more bits of DCI (e.g., one or more bits reserved for information  842  indicating the periodicity) and/or via a DCI format. 
     The UE  804  may receive the information  842  and may determine the periodicity based on the information  842 . The UE  804  may periodically transmit the control information  844  to the base station  802  at the determined periodicity. 
       FIGS. 9A, 9B, 9C, 9D, 9E  illustrate an example wireless communication environment including at least a base station  902  and UE  904 . The base station  902  may be an aspect of the base station  802 , the base station  702 , the base station  602 , the base station  502 , the base station  310 , the base station  102 , the mmW base station  180 , and/or another base station. The UE  904  may be an aspect of the UE  804 , the UE  704 , UE  604 , the UE  504 , the UE  350 , the UE  104 , the UE  182 , and/or another UE. 
     The UE  904  may include one or more antenna arrays and/or one or more subarrays. The UE  904  may receive, from the base station  902 , one or more reference signals (e.g., PSS/BRS, a discrete prolate spheroidal sequence (DPSS), a CSI-RS, a BRRS, etc.) through one or more beams, which may be received at multiple antennas of the UE  904  in accordance with multiple receive angle ranges, which may be detected at the UE  904  at receive combiner(s). Data communication between the base station  902  and the UE  904  may require a combination of one or more of: a best beam at the base station  902  (corresponding to a beam index), a best subarray at the UE  904  (e.g., a subarray at which a highest received power of one or more reference signals is detected), and/or a best receive combiner at the best array or subarray at the UE  904  (e.g., a receive combiner at which a highest received power of one or more reference signals is detected). 
     In an example, the base station  902  may sweep through the transmit directions according to a sequence A-B-C-D in order to transmit one or more reference signals (e.g., reference signals  950 ). In another example, the base station  902  may sweep through the transmit directions according to the sequence B-D-A-C. Although only four transmit directions and two transmit sequences are described with respect to  FIG. 9A , any number of different transmit directions and transmit sequences are contemplated. 
     In aspects, the base station  902  may send out a reference signal (e.g., the reference signals  950 ) on a plurality of successive symbols, although in different directions A-B-C-D. The base station  902  may sweep through the entire sector. 
     Referring to diagram  920 , the UE  904  may listen or detect for reference signals in different receive directions (e.g., directions E, F, G, and H). In an example, the UE  904  may sweep through the receive directions according to a sequence E-F-G-H. In another example, the UE  904  may sweep through the receive directions according to the sequence F-H-E-J. Although only four receive directions and two receive sequences are described, any number of different receive directions and receive sequences are contemplated. 
     In aspects, the UE  904  may try out different receive subarrays at different periods, such as four reference signal periods. The UE  904  may find the best subarray after the reference signal period(s). For example, the UE  904  may measure a signal quality for each reference signal received at a respective subarray during a respective period. 
     The UE  904  may determine the best subarray as the subarray having a highest signal quality measured for a reference signal, the subarray having a highest average signal quality for the set of reference signals received during the period in which that subarray is used, or another method of selecting a best subarray. 
       FIG. 9C  illustrates an aspect of a reference signal  950 . The reference signal  950  may be repeated in successive subframes, slots, or subslots. Fourteen symbols may be reserved for the reference signal  950 ; however, the reference signal  950  may not be transmitted during each symbol. For example, a reference signal  950  may be transmitted through a first beam on one symbol of the fourteen symbols, and another reference signal may be transmitted through a second beam on a different symbol of the fourteen symbols. The reference signal may be one or more of a PSS, SSS, BRS, BRRS, CSI-RS, DPSS, and the like. 
       FIG. 9D  illustrates a wireless communication system  960  in which a slot  962  includes fourteen symbols. One or more of the fourteen symbols may be used for a reference signal, such as the reference signal  950 . In the illustrated aspect, the base station  902  may transmit three beams  970 ,  972 ,  974  (e.g., a fine beam set). The base station  902  may transmit a reference signal through the set of beams  970 ,  972 ,  974  in the last three symbols  964 ,  966 ,  968  of the slot  962 . 
     The base station  902  may cause the reference signal to occupy one out of every fourth tone in each symbol of the three symbols  964 ,  966 ,  968 . The base station  902  may repeat the reference signal four times in each symbol of the three symbols  964 ,  966 ,  968 . 
     In aspects, the UE  904  may try out different receive combiners at different periods, such as four reference signal periods for each of the three symbols  964 ,  966 ,  968 . The UE  904  may find the best receive combiner after the reference signal period(s). For example, the UE  904  may measure a signal quality for each reference signal received at a respective receive combiner during a respective period. The UE  904  may determine a best receive combiner by determining the receive combiner at which a highest signal quality is measured for a reference signal, the receive combiner at which the highest average signal quality is measured for the set of reference signals received during the period in which the receive combiner is used, or another method of selecting a best receive combiner. 
     Additionally, the UE  904  may select a best beam corresponding to a beam index at the base station  902  based on the reference signals received during the three symbols  964 ,  966 ,  968 . For example, the UE  904  may determine a highest signal quality corresponding to reference signal received during one of the three symbols  964 ,  966 ,  968 . The UE  904  may determine a beam index corresponding to a beam at the base station  902 , for example, because each set of symbols of the three symbols  964 ,  966 ,  968  may correspond to a different beam. The UE  904  may communicate with the base station  902  through the best beam corresponding to the beam index (e.g., the UE  904  may send information indicating the beam index to the base station  902 ). 
       FIG. 9E  illustrates an example wireless communication environment  980  including at least a base station  902  and UE  904 . In aspects, the base station  902  may need to convey one or more beam indexes to the UE  904 , for example, to initiate reference signals transmission. In an aspect, the reference signal may include one or more of a MRS, BRRS, BRS, CSI-RS, PSS, SSS, DPSS, and the like. Based on the reference signal transmissions, the UE  904  may determine a best subarray and/or receive combiner at the UE  904 . 
     The base station  902  may determine one or more indexes associated with one or more beams  972 ,  974 ,  974 . According to one aspect, the base station  902  may receive one or more beam indexes corresponding to the one or more beams  972 ,  974 ,  976 , for example, as BSI. The base station  902  may determine that one or more reference signals are to be transmitted through the one or more beams corresponding to the one or more beam indexes most recently indicated to the base station  902  by the UE  904 . 
     For example, during reference signal transmission, the base station  902  may transmit different types of beams. In one aspect, the base station  902  may transmit BRS beams (e.g., coarse beams). The base station  902  may need to convey one or more indexes associated with one or more BRS beams to the UE  904 . In such an aspect, nine bits may be required if the BRS duration period is four slots or subframes—e.g., the number of symbols per subframe is fourteen with eight antenna ports. The UE  904  may send, to the base station  902 , information indicating one or more beam indexes corresponding to the one or more beams  972 ,  974 ,  976  through which one or more BRSs are received. 
     According to another aspect, the base station  902  may determine the one or more indexes associated with one or more beams  972 ,  974 ,  974  based on previously reported beams from the UE  904 . For example, the base station  902  may transmit the most recent beams that the UE  904  may have reported. For example, the UE  904  may report one or more beam indexes based on one or more BRSs received through one or more beams (e.g., the beams  972 ,  974 ,  976 ), which may be transmitted by the base station  902  during a synchronization slot or subframe. 
     In aspects, the UE  904  may transmit information indicating the beam indexes (e.g., BSI) to the base station  902  through PUCCH or PUSCH (e.g., the UE  904  may be able to transmit more information through the PUSCH than the PUCCH). The set of beams used to train the UE  904  receive beams may be implicitly derived based on the latest reported information and/or channel types used to convey that information. 
     In one aspect, if the base station  902  schedules the reference signal in one or two symbols, the base station  902  may reuse beam information that the base station received on the PUCCH from the UE  904 . In another aspect, if the base station  902  schedules the reference signal in more than two symbols, the base station  902  may reuse beam information that the base station  902  received during the PUSCH from the UE  904 . The base station  902  may utilize a few bits (e.g., less than nine) to send, to the UE  904 , information  990  indicating the one or more beam indexes of the one or more previously reported beams. 
     In another aspect, the base station  902  may determine the one or more beam indexes based on finer beams  972 ,  974 ,  974  previously transmitted to the UE  904 . The finer beams  972 ,  974 ,  974  may have been previously used for transmission of one or more reference signals (e.g., MRSs, CSI-RSs, and/or BRRSs). The base station  902  may only need to transmit a few bits (e.g., less than nine) to send, to the UE  904 , information  990  indicating the fine beams  972 ,  974 ,  974 . 
     Based on the determined one or more beams indexes, the base station  902  may transmit information  990  indicating the one or more beams indexes to the UE  904 . The base station  902  may transmit the information  990  on a control channel. In an aspect, the control channel may include a PDCCH. For example, the information  990  may be included in one or more bits of a DCI message of the PDCCH. 
     Based on the information  990 , the UE  904  may determine the one or more beam indexes used for reference signals indicated by the information  990 . Thereafter, the base station  902  may transmit one or more reference signals to the UE  904  through the one or more beams  972 ,  974 ,  976  corresponding to the one or more beam indexes. 
     The UE  904  may use the best subarray and/or receive combiner to detect the reference signal transmission. For example, the UE  904  may determine the best subarray and/or receive combiner for a symbol in which the reference signal is scheduled to be transmitted. The UE  904  may then detect for the reference signal using the determined subarray and/or receive combiner. 
       FIGS. 10A and 10B  are flowcharts of method  1000 ,  1020  of wireless communication. The methods  1000 ,  1020  may be performed by a base station (e.g., the base station  602 ). In one aspect, the methods  1000 ,  1020  may be performed by an apparatus. One of ordinary skill would understand that one or more operations may be omitted, transposed, and or performed contemporaneously. 
     Beginning with  FIG. 10A  at operation  1002 , the base station may transmit at least one BRS. For example, the base station may generate a set of BRSs and the base station may transmit the set of BRSs through a set of beams to be used for communication with a UE. In the context of  FIG. 6 , the base station  602  may transmit the set of BRSs  612   a - h  through the set of beams  620   a - h , and each BRS of the BRSs  612   a - h  may correspond to a beam of the beams  620   a - h.    
     At operation  1004 , the base station may receive, from the UE, an indication of a first beam index based on the BRS. The first beam index may be a coarse beam index to be used for beam refinement. In the context of  FIG. 6 , the base station  602  may receive, from the UE  604 , an indication of a beam index corresponding a coarse beam, such as the sixth beam  620   f . The indication may be the first indication  560 , as described in  FIG. 5 . 
     At operation  1006 , the base station may transmit, based on the first indication of the first beam index, at least one BRRS. In the context of  FIG. 6 , the first indication may indicate a beam index corresponding to the sixth beam  620   f , and the base station may transmit a fine beam set, such as by transmitting the BRRS  614   c - f  on beams  620   c - f  that are close to the beam indicated by the coarse beam index. 
     At operation  1008 , the base station may receive, based on the at least one BRRS, a second indication of a second beam index corresponding to a second beam. In the context of  FIG. 6 , the base station  602  may receive, from the UE  604 , an indication of a beam index corresponding a fine beam, such as the sixth beam  620   f . The indication may be the second indication  565 , as described in  FIG. 5 . 
     The base station may communicate with the UE through the beam corresponding to the index indicated in the second indication. The communication may be uplink communication and/or downlink communication. 
     Turning to  FIG. 10B , another method  1020  is illustrated for a base station performing beam tracking. At operation  1022 , the base station may receive, from a UE, a request for beam tracking. For example, the base station may receive a BAR. In the context of  FIG. 6 , the base station  602  may receive, from the UE  604 , a request for beam tracking. 
     At operation  1024 , the base station may transmit, based on the request for beam tracking, at least one BRRS. In the context of  FIG. 6 , the base station may transmit a fine beam set, such as by transmitting the BRRSs  614   c - f  on beams  620   c - f  that are close to a beam through which the base station  602  and the UE  604  previous communication (e.g., the fifth beam  620   e ). 
     At operation  1026 , the base station may receive, based on the at least one BRRS, a second indication of a second beam index corresponding to a second beam. In the context of  FIG. 6 , the base station  602  may receive, from the UE  604 , an indication of a beam index corresponding a fine beam, such as the sixth beam  620   f . The indication may be the second indication  565 , as described in  FIG. 5 . 
     The base station may communicate with the UE through the beam corresponding to the index indicated in the second indication. The communication may be uplink communication and/or downlink communication. 
       FIGS. 11A and 11B  are flowcharts of method  1100 ,  1120  of wireless communication. The methods  1100 ,  1120  may be performed by a UE (e.g., the UE  604 ). In one aspect, the methods  1100 ,  1120  may be performed by an apparatus. One of ordinary skill would understand that one or more operations may be omitted, transposed, and or performed contemporaneously. 
     Beginning with  FIG. 11A  at operation  1102 , the UE may receive, from a base station, at least one BRS. In the context of  FIG. 6 , the UE  604  may receive, from the base station  602 , the set of BRSs  612   a - h  through the set of beams  620   a - h , and each BRS of the BRSs  612   a - h  may correspond to a beam of the beams  620   a - h.    
     At operation  1104 , the UE may transmit, to the base station, an indication of a first beam index based on the BRS. The first beam index may be a coarse beam index to be used for beam refinement. For example, the UE may measure a signal quality or power for one or more of the received BRSs and select a beam index corresponding to a beam through which the BRS having a best or highest value is received. In the context of  FIG. 6 , the UE  604  may transmit, to the base station  602 , an indication of a beam index corresponding a coarse beam, such as the sixth beam  620   f . The indication may be the first indication  560 , as described in  FIG. 5 . 
     At operation  1106 , the UE may receive, from the base station (e.g., based on the first indication of the first beam index), at least one BRRS. In the context of  FIG. 6 , the UE  604  may receive a fine beam set, such as by receiving the BRRSs  614   c - f  on beams  620   c - f  that are close to the beam indicated by the coarse beam index. 
     At operation  1108 , the UE may transmit, based on the at least one BRRS, a second indication of a second beam index corresponding to a second beam. The second beam index may be a fine beam index to be used for communication. For example, the UE may measure a signal quality or power for one or more of the received BRRSs and select a beam index corresponding to a beam through which the BRRS having a best or highest value is received. In the context of  FIG. 6 , the UE  604  may transmit, to the base station  602 , an indication of a beam index corresponding a fine beam, such as the sixth beam  620   f . The indication may be the second indication  565 , as described in  FIG. 5 . 
     The UE may communicate with the UE through the beam corresponding to the index indicated in the second indication. The communication may be uplink communication and/or downlink communication. 
     Turning to  FIG. 11B , another method  1120  is illustrated for a base station performing beam tracking. At operation  1122 , the UE may transmit, to the base station, a request for beam tracking. For example, the UE may transmit a BAR. In the context of FIG.  6 , the UE  604  may transmit, to the base station  602 , a request for beam tracking  648  (e.g., a BAR). 
     At operation  1124 , the UE may receive, from the base station based on the request for beam tracking, at least one BRRS. In the context of  FIG. 6 , the UE  604  may receive a fine beam set, such as by receiving the BRRS  614   c - f  on beams  620   c - f  that are close to a beam through which the base station  602  and the UE  604  previous communication (e.g., the fifth beam  620   e ). 
     At operation  1126 , the UE may transmit, to the base station and based on the at least one BRRS, an indication of a beam index corresponding to a beam. For example, the UE may measure a signal quality or power for one or more of the received BRRSs and select a beam index corresponding to a beam through which the BRRS having a best or highest value is received. In the context of  FIG. 6 , the UE  604  may transmit, to the base station  602 , an indication of a beam index corresponding a fine beam, such as the sixth beam  620   f . The indication may be the second indication  565 , as described in  FIG. 5 . 
     The base station may communicate with the UE through the beam corresponding to the index indicated in the indication. The communication may be uplink communication and/or downlink communication. 
       FIG. 12  is a flowchart of method  1200  of wireless communication. The method  1200  may be performed by a base station (e.g., the base station  602 ). In one aspect, the method  1200  may be performed by an apparatus. One of ordinary skill would understand that one or more operations may be omitted, transposed, and or performed contemporaneously. 
     Beginning with operation  1202 , the base station may communicate with a UE through a first active beam. In an aspect, the communication may be downlink communication. For example, the base station may determine data to be transmitted to the UE and then the base station may send the determined data through the first active beam. In an aspect, the communication may be uplink communication. For example, the base station may determine a time (e.g., slot) at which the UE is scheduled to send data to the base station, and then the base station may receive the data through the first active beam at the scheduled time. In the context of  FIG. 6 , the base station  602  may communicate with the UE  604  through a first active beam, such as the fifth beam  620   e.    
     In an aspect, operation  1202  may include operation  1220 . At operation  1220 , the base station may send, to the UE, a reference signal to determine if the first active beam is failing. In an aspect, the reference signal may be a CSI-RS, a CRS, an SSS, an MRS, a DMRS, or a BRS. In the context of  FIG. 6 , the base station  602  may send the reference signal  644  through the first active beam (e.g., the fifth beam  620   e ). 
     At operation  1204 , the base station may determine that beam tracking is to be performed with the UE. For example, the base station may determine that the first active beam is failing, and then the base station may initiate beam tracking in order to select a new active beam. In the context of  FIG. 6 , the base station  602  may determine that beam tracking is to be performed with the UE  604 . 
     In an aspect, operation  1204  may include operation  1222  and operation  1224 . At operation  1222 , the base station may receive a response from the UE based on a reference signal transmitted to the UE (e.g., the reference signal described at operation  1220 ). In an aspect, the response may include at least one of a CQI, an SINR, an SNR, an RSSI, an RSRP, or an RSRQ. In the context of  FIG. 6 , the base station  602  may receive, from the UE  604 , the response  646 . 
     At operation  1224 , the base station may detect a radio link failure based on the received response. For example, the base station may determine that a value (e.g., a CQI, SINR, SNR, RSRP, RSRQ, etc.) may be below a threshold, and the base station may determine that the value being below the threshold indicates a radio link failure. In another example, the base station may determine that the response indicates a NACK or that the response is absent, and the base station may determine a radio link failure has occurred based on the NACK or the absence of the response. In the context of  FIG. 6 , the base station  602  may detect the radio link failure through the first active beam (e.g., the fifth beam  620   e ) based on the response  646  or based on the absence of the response  646 . 
     In another aspect, operation  1204  may include operation  1226 . At operation  1226 , the base station may determine a time at which the UE is to transition from an inactive cycle of DRX to an active cycle of DRX. Because the base station and the UE may not communicate while the UE is in an inactive DRX cycle, the base station may determine that beam tracking is to be performed with the UE when the UE transition to an active cycle of DRX (e.g., because the UE may have shifted during the inactive DRX cycle). In the context of  FIG. 6 , the base station  602  may determine DRX cycles for the UE  604  (e.g., the base station  602  may configure DRX cycles for the UE  604 ), and when the UE  604  is to transition from an inactive DRX cycle to an active DRX cycle, the base station  602  may determine that the beam tracking is to be performed with the UE  604 . 
     In another aspect, operation  1204  may include operation  1228 . At operation  1228 , the base station may determine an absence of communication with the UE using the first active beam. For example, the base station may determine that the base station is not receiving data from the UE when the UE is scheduled to communicate with the base station. In another example, the base station may receive one or more NACK messages that indicate an absence of communication with the UE. In the context of  FIG. 6 , the base station  602  may determine an absence of the communication  640  with the UE  604  using the first active beam (e.g., the fifth beam  620   e ). 
     At operation  1206 , the base station may perform beam tracking with the UE based on the determination that the beam tracking is to be performed. For example, the base station may perform one or more operations described with respect to  FIGS. 10A and/or 10B . In the context of  FIG. 6 , the base station  602  may perform beam tracking with the UE  604 . 
     In an aspect, operation  1206  includes operation  1230 . At operation  1230 , the base station may send, to the UE, a message indicating that beam tracking is to be performed. The base station may send the message on a PDCCH or a PDSCH. In the context of  FIG. 6 , the base station  602  may send, to the UE  604 , the message  642  indicating that beam tracking is to be performed. The base station  602  may then perform one or more operations described with respect to  FIGS. 10A and/or 10B . 
     At operation  1208 , the base station may communicate with the UE through the second active beam based on the beam tracking. For example, the base station may receive, from the UE, an indication of a beam index corresponding to a beam at the base station. The base station may select the beam corresponding to the indicated beam index for at least one of uplink or downlink communication with the UE. In the context of  FIG. 6 , the base station  602  may communicate with the UE  604  through the second active beam (e.g., the sixth beam  620   f ). 
       FIG. 13  is a flowchart of method  1300  of wireless communication. The method  1300  may be performed by a UE (e.g., the UE  604 ). In one aspect, the method  1300  may be performed by an apparatus. One of ordinary skill would understand that one or more operations may be omitted, transposed, and or performed contemporaneously. 
     Beginning with operation  1302 , the UE may communicate with a base station through a first active beam. In an aspect, the communication may be downlink communication. For example, the UE may receive data to from the base station. In an aspect, the communication may be uplink communication. For example, the UE may determine a time (e.g., slot) at which the UE is scheduled to send data to the base station, and then the UE may transmit the data through the first active beam at the scheduled time. In the context of  FIG. 6 , the UE  604  may communicate with the base station  602  through a first active beam, such as the fifth beam  620   e.    
     In an aspect, operation  1302  may include operation  1320 . At operation  1320 , the UE may receive, from the base station, a reference signal. In an aspect, the reference signal may be a CSI-RS, a CRS, an SSS, an MRS, a DMRS, or a BRS. In the context of  FIG. 6 , the UE  604  may receive, from the base station  602 , the reference signal  644  through the first active beam (e.g., the fifth beam  620   e ). 
     At operation  1304 , the UE may receive, from the base station, a signal associated with beam tracking. In an aspect, the signal may be the reference signal. In another aspect, the signal may be a BRS. In another aspect, the signal may be a BRRS. In another aspect, the signal may be a message indicating that beam tracking is to be performed between the UE and the base station. In the context of  FIG. 6 , the UE  604  may receive, from the base station  602 , a signal associated with beam tracking. The signal may be a BRS of the BRSs  612   a - h , a BRRS of the BRRSs  614   c - f , the message  642 , the reference signal  644 , or another signal. 
     In an aspect, operation  1304  may include operation  1322  and operation  1324 . At operation  1322 , the UE may detect a radio link failure based on reception of the reference signal (as described at operation  1320 ). For example, the UE may measure a signal quality (e.g., an SINR, an SNR, a BRSRP, an RSRP, an RSRQ, or another signal quality) and compare the measured signal quality to a threshold. Based on the comparison, the UE may determine that communicate through the first active beam is failing or degraded. In the context of  FIG. 6 , the UE  604  may detect a radio link failure associated with the first active beam (e.g., the fifth beam  620   e ) based on reception of the reference signal  644 . 
     At operation  1324 , the UE may send an indication of the detected radio link failure to the base station based on the detected radio link failure. For example, the UE may send a measured signal quality associated with the reference signal (e.g., an SINR, an SNR, a BRSRP, an RSRP, an RSRQ, etc.), a CQI, or a BAR, or another indication of the detected radio link failure to the base station. In the context of  FIG. 6 , the UE  604  may send the response  646  to the base station  602 . 
     In an aspect, operation  1304  may include operation  1326 . At operation  1326 , the UE may send, to the base station, a beam index based on a BRS. For example, the UE may measure signal qualities for a plurality of BRSs and may determine a best (e.g., highest) signal quality corresponding to a best BRS. The UE may determine a beam index corresponding to the beam through which the best BRS is received and may send the determined beam index to the base station. In an aspect, this beam index may be a coarse beam index. In the context of  FIG. 6 , the UE  604  may send, to the base station  602 , a beam index corresponding to a beam of the beams  620   a - h  through which a best BRS of the BRSs  612   a - h  is received. For example, the UE  604  may send the first indication  560 , as described in  FIG. 5 . 
     In an aspect, operation  1304  may include operation  1328 . At operation  1328 , the UE may send, to the base station, a beam index based on a BRRS. For example, the UE may measure signal qualities for a plurality of BRRSs and may determine a best (e.g., highest) signal quality corresponding to a best BRRS. The UE may determine a beam index corresponding to the beam through which the best BRRS is received and may send the determined beam index to the base station. In an aspect, this beam index may be a fine beam index. In the context of  FIG. 6 , the UE  604  may send, to the base station  602 , a beam index corresponding to a beam of the beams  620   c - f  through which a best BRRS of the BRRSs  614   c - f  is received. For example, the UE  604  may send the second indication  565 , as described in  FIG. 5 . 
     At operation  1306 , the UE may perform beam tracking with the base station, for example, based on the signal associated with beam tracking. For example, the UE may perform one or more operations described with respect to  FIGS. 11A and/or 11B . For example, the UE  604  may send the request for beam tracking  648 . Based on the beam tracking, the UE may determine a second beam corresponding to a second beam index for communication with the base station. In the context of  FIG. 6 , the UE  604  may perform beam tracking with the base station  602 . 
     At operation  1308 , the UE may communicate with the base station through the second active beam based on the beam tracking. For example, the UE may transmit, to the base station, an indication of a beam index corresponding to a beam at the base station. The UE may select second active beam for at least one of uplink or downlink communication with the UE. In the context of  FIG. 6 , the UE  604  may communicate with the base station  602  through the second active beam (e.g., the sixth beam  620   f ). 
       FIG. 14  is a flowchart of method  1400  of wireless communication. The method  1400  may be performed by a base station (e.g., the base station  702  and/or the base station  802 ). In one aspect, the method  1400  may be performed by an apparatus. One of ordinary skill would understand that one or more operations may be omitted, transposed, and or performed contemporaneously. 
     Beginning with operation  1402 , the base station may communicate with a UE through a first active beam. In an aspect, the communication may be downlink communication. For example, the base station may determine data to be transmitted to the UE and then the base station may send the determined data through the first active beam. In an aspect, the communication may be uplink communication. For example, the base station may determine a time (e.g., slot) at which the UE is scheduled to send data to the base station, and then the base station may receive the data through the first active beam at the scheduled time. In the context of  FIG. 7 , the base station  702  may communicate with the UE  704  through a first active beam, such as the fifth beam  720   e . In the context of  FIG. 8 , the base station  802  may communicate with the UE  804  through the active beam  820   e.    
     At operation  1404 , the base station may transmit, to the UE, information indicating a periodicity at which control information is to be communicated on a control channel through a control-information beam. For example, the base station may determine a periodicity at which control information is to be sent to the base station by the UE (e.g., based on one or more standards promulgated by 3GPP). The base station may generate a message that indicates the periodicity and may transmit the generated message to the UE, for example, through the active beam. In the context of  FIG. 7 , the base station  702  may transmit, to the UE  704 , information indicating the periodicity through a current active beam, such as the fifth beam  720   e . In the context of  FIG. 8 , the base station  802  may transmit, to the UE  804 , the information  842  indicating the periodicity through the active beam  820   e.    
     In one aspect, the base station may transmit the information indicating the periodicity to the UE through RRC signaling. In another aspect, the base station may transmit the information indicating the periodicity to the UE on a PDCCH. For example, the base station may indicate the information in DCI of the PDCCH, such as one or more bits of one or more DCI formats that are reserved for information indicating the periodicity. 
     At operation  1406 , the base station may communicate, with the UE, the control information on the control channel through the control-information beam at the periodicity. For example, the base station may receive (e.g., attempt to receive, detect for, monitor for, etc.) the control information at each period corresponding to the indicated periodicity. The control information may include UCI, CQI, and the like. The base station may determine a quality (e.g., channel quality) associated with the active beam based on the control information. Accordingly, the base station may determine whether the active beam is satisfactory based on the control information. Alternatively, the base station may determine that the active beam is failing and/or unsatisfactory based on the control information, and the base station may determine that the active beam is to be changed. In the context of  FIG. 7 , the base station  702  may communicate, with the UE  704 , control information on a control channel through a beam of the beams  720   a - h , such as the sixth beam  720   f  when the fifth beam  720   e  is the active beam for communication between the base station  702  and the UE  704 . In the context of  FIG. 8 , the base station  802  may communicate, with the UE  804 , the information  844  on a control channel. 
     In one aspect, the base station may receive the control information through the control-information beam, which may be a beam corresponding to a beam index included in a set of a candidate beam indexes. In the context of  FIG. 8 , the base station  802  may communicate, with the UE  804 , the information  844  on a control channel through the control-information beam  820   f , and the beam index corresponding to the control-information beam  820   f  may be included in the set of candidate beam indexes  830 . 
     In another aspect, the base station may receive the control information through the control-information beam, which may be a wide beam having an angle greater than that of the active beam. In the context of  FIG. 8 , the base station  802  may communicate, with the UE  804 , the information  844  on a control channel through the wide beam  822  (e.g., the wide beam  822  may serve as the control-information beam). 
     In an aspect, operation  1406  may include operation  1420 . In such an aspect, the control channel may be a PUCCH. At operation  1420 , the base station may receive, from the UE, the control information carried on the PUCCH through the control-information beam. In the context of  FIG. 7 , the base station  702  may receive, from the UE  704 , control information on a PUCCH through a beam of the beams  720   a - h , such as the sixth beam  720   f  when the fifth beam  720   e  is the active beam for communication between the base station  702  and the UE  704 . In the context of  FIG. 8 , the base station  802  may receive, from the UE  804 , the information  844  on a PUCCH, for example, through the control-information beam  820   f  and/or the wide beam  822 . 
     At operation  1408 , the base station may receive a request to change the active beam. The request may indicate a beam index corresponding to a second beam. In an aspect, the request indicates the beam index through at least one of a cyclic shift and/or spreading across symbols. In an aspect, the request indicates the beam index through at least one of a subcarrier region (e.g., SR resources (e.g., region) of a subframe) and/or through RACH (e.g., RACH resources (e.g., region) of a subframe). 
     In the context of  FIG. 7 , the base station  702  may receive, from the UE  704 , the request  750 , which may indicate a beam index corresponding to a beam of the beams  720   a - h . In the context of  FIG. 8 , the base station  802 , may receive, from the UE  804 , a request, which may indicate a beam index corresponding to another beam other than the current active beam  820   e —e.g., the request may indicate the beam index corresponding to the control-information beam  820   f , which may be a candidate beam corresponding to a beam index included in a set of candidate beam indexes (e.g., the set of candidate beam indexes  830 ). 
     At operation  1410 , the base station may change the active beam to the second beam that corresponds to the beam index indicated by the request. The base station may then communicate with the UE through the second beam, which may be the new active beam. In the context of  FIG. 7 , the base station  702  may change the active beam to a beam of the beams  720   a - h  that corresponds to the beam index indicated by the request. In the context of  FIG. 8 , the base station  802 , may change the active beam from the current active beam  820   e  to a second beam corresponding to the beam index indicated by the request—e.g., the base station  802  may change the active beam to the control-information beam  820   f , which may be a candidate beam corresponding to a beam index included in a set of candidate beam indexes (e.g., the set of candidate beam indexes  830 ). 
       FIG. 15  is a flowchart of method  1500  of wireless communication. The method  1500  may be performed by a UE (e.g., the UE  704  and/or the UE  804 ). In one aspect, the method  1500  may be performed by an apparatus. One of ordinary skill would understand that one or more operations may be omitted, transposed, and or performed contemporaneously. 
     Beginning with operation  1502 , the UE may communicate with a base station through a first active beam. In an aspect, the communication may be downlink communication. For example, the UE may receive data to from the base station. In an aspect, the communication may be uplink communication. For example, the UE may determine a time (e.g., slot) at which the UE is scheduled to send data to the base station, and then the UE may transmit the data through the first active beam at the scheduled time. In the context of  FIG. 7 , the UE  704  may communicate with the base station  702  through a first active beam, such as the fifth beam  720   e . In the context of  FIG. 8 , the UE  804  may communicate with the base station  802  through the active beam  820   e.    
     At operation  1504 , the UE may receive, from the base station, information indicating a periodicity at which control information is to be communicated on a control channel through a control-information beam. For example, the UE may receive, from the base station, information that indicates the periodicity through the active beam. In the context of  FIG. 7 , the UE  704  may receive, from the base station  702 , information indicating the periodicity through a current active beam, such as the fifth beam  720   e . In the context of  FIG. 8 , the base station  802  may transmit, to the UE  804 , the information  842  indicating the periodicity through the active beam  820   e.    
     In one aspect, the UE may receive the information indicating the periodicity to the UE through RRC signaling. In another aspect, the UE may receive the information indicating the periodicity on a PDCCH. For example, the information may be indicated by DCI of the PDCCH, such as one or more bits of one or more DCI formats that are reserved for information indicating the periodicity. Accordingly, the UE may determine the information based on the DCI of the PDCCH. 
     At operation  1506 , the UE may communicate, with the base station, the control information on the control channel through the control-information beam at the periodicity. For example, the UE may transmit the control information at each period corresponding to the indicated periodicity. In various aspects, the UE may measure a signal quality or channel estimate associated with communication with the base station, e.g., through the active beam. For example, the UE may measure a signal quality (e.g., SNR, SINR, received power, received quality, etc.) for a signal (e.g., a BRS, a CSI-RS, a reference signal, etc.). The UE may generate a message to indicate the signal quality or channel estimate to the base station, and the UE may send the message to the base station. In various aspects, the information may include UCI, CQI, SR, and/or other control information. In the context of  FIG. 7 , the UE  704  may communicate, with the base station  702 , control information on a control channel through a beam of the beams  720   a - h , such as the sixth beam  720   f  when the fifth beam  720   e  is the active beam for communication between the base station  702  and the UE  704 . In the context of  FIG. 8 , the base station  802  may communicate, with the UE  804 , the information  844  on a control channel. 
     In one aspect, the UE may send the control information through the control-information beam, which may be a beam corresponding to a beam index included in a set of a candidate beam indexes. In the context of  FIG. 8 , the UE  804  may send, to the base station  802 , the information  844  on a control channel through the control-information beam  820   f , and the beam index corresponding to the control-information beam  820   f.    
     In another aspect, the UE may send the control information through the control-information beam, which may be a wide beam having an angle greater than that of the active beam. In the context of  FIG. 8 , the UE  804  may send, to the base station  802 , the information  844  on a control channel through the wide beam  822  (e.g., the wide beam  822  may serve as the control-information beam). 
     In an aspect, operation  1506  may include operation  1520 . In such an aspect, the control channel may be a PUCCH. At operation  1520 , the UE may send, to the base station, the control information on the PUCCH through the control-information beam. In the context of  FIG. 7 , the UE  704  may send, to the base station  702 , control information on a PUCCH through a beam of the beams  720   a - h , such as the sixth beam  720   f  when the fifth beam  720   e  is the active beam for communication between the base station  702  and the UE  704 . In the context of  FIG. 8 , the UE  804  may send, to the base station  802 , the information  844  on a PUCCH, for example, through the control-information beam  820   f  and/or the wide beam  822 . 
     At operation  1508 , the UE may send, to the base station, a request to change the active beam. The request may indicate a beam index corresponding to a second beam. For example, the UE may determine that the active beam is failing and/or unsatisfactory based on the measured signal quality (e.g., based on comparison of the measured signal quality to a threshold). Accordingly, the UE may request the base station to change the active beam because the current active beam is failing and/or unsatisfactory. In an aspect, the request indicates the beam index through at least one of a cyclic shift and/or spreading across symbols. In an aspect, the request indicates the beam index through at least one of a subcarrier region (e.g., SR resources (e.g., region) of a subframe) and/or through RACH (e.g., RACH resources (e.g., region) of a subframe). 
     In the context of  FIG. 7 , the UE  704  may transmit, to the base station  702 , the request  750 , which may indicate a beam index corresponding to a beam of the beams  720   a - h . In the context of  FIG. 8 , the UE  804  may transmit, to the base station  802 , a request, which may indicate a beam index corresponding to another beam other than the current active beam  820   e —e.g., the request may indicate the beam index corresponding to the control-information beam  820   f , which may be a candidate beam corresponding to a beam index included in a set of candidate beam indexes (e.g., the set of candidate beam indexes  830 ). 
     At operation  1510 , the UE may change the active beam to the second beam that corresponds to the beam index indicated by the request. The UE may then communicate with the base station through the second beam, which may be the new active beam. In the context of  FIG. 7 , the UE  704  may change the active beam to a beam of the beams  720   a - h  that corresponds to the beam index indicated by the request. In the context of  FIG. 8 , the UE  804 , may change the active beam from the current active beam  820   e  to a second beam corresponding to the beam index indicated by the request—e.g., the UE  804  may change the active beam to the control-information beam  820   f , which may be a candidate beam corresponding to a beam index included in a set of candidate beam indexes (e.g., the set of candidate beam indexes  830 ). 
       FIG. 16  illustrates a flowchart  1600  of a method of wireless communication. The method may be performed by a base station (e.g., the base station  902 ) communicating with a UE (e.g., the UE  904 ). One of ordinary skill would understand that one or more operations may be omitted, transposed, and or performed contemporaneously. 
     At operation  1602 , the base station may determine one or more indexes associated with one or more beams. For example, the base station may receive BSI from the UE that includes one or more beam indexes corresponding to one or more beams of the base station. The base station may then identify a fine beam set based on BSI received from the UE. In the context of  FIG. 9A-E , the base station  902  may determine one or more indexes corresponding to the one or more beams  970 ,  972 ,  974 . 
     At operation  1604 , the base station may transmit, to a UE, one or more indications of the one or more beam indexes. In various aspects, the one or more indications may be carried on a control channel, such as a PDCCH. For example, the one or more indications may be indicated through a DCI message of the PDCCH. In the context of  FIG. 9A-E , the base station  902  may transmit the information  990  to the UE  904 . 
     In an aspect, operation  1604  includes operations  1620  and  1622 . At operation  1620 , the base station may receive, from the UE, information indicating one or more indexes corresponding to one or more beams. In one aspect, the one or more indexes may be received from the UE through a PUSCH or a PUCCH. In one aspect, the information indicating the one or more beam indexes may be one or more BSI reports. In the context of  FIG. 9A-E , the base station  902  may receive, from the UE  904 , one or more indexes associated with one or more beams. 
     At operation  1622 , the base station may transmit one or more indexes based on the one or more indexes that are most recently received from the UE. In one aspect, the base station may transmit one or more indexes based on one or more indexes received from the UE through the PUSCH when more than two symbols are reserved for reference signal transmission. In another aspect, the base station may transmit one or more indexes based on one or more indexes received from the UE through the PUCCH if one or two symbols are reserved for reference signal transmission. In the context of  FIG. 9A-E , the base station  902  may transmit the information  990  indicating the one or more indexes to the UE  904  based on the one or more indexes that are most recently received from the UE  904 . 
     In one aspect, operation  1604  may include operation  1624 . At operation  1624 , the base station may transmit one or more indexes associated with one or more beam reference signals (e.g., one or more BRSs). The one or more beam reference signals may be transmitted during a synchronization subframe. In the context of  FIG. 9A-E , the base station  902  may transmit one or more indexes associated with one or more beam reference signals, which may be used for coarse beam training with the UE  904 . 
     In one aspect, operation  1604  may include operation  1628 . At operation  1628 , the base station may transmit one or more indexes corresponding to one or more beams that were transmitted during a previous reference signal transmission (e.g., CSI-RS or BRRS transmission). In the context of  FIG. 9A-E , the base station  902  may transmit the information  990  indicating the one or more beam indexes associated with one or more beams that were transmitted during a previous reference signal transmission to the UE  904 . 
     At operation  1606 , the base station may transmit, to the UE, a reference signal based on the one or more Dis associated with the one or more beams (e.g., as determined at operation  1602 ). In various aspects, the reference signal may be a CSI-RS, a BRRS, an MRS, or another reference signal described herein. In the context of  FIG. 9A-E , the base station  902  may transmit, to the UE  904 , one or more reference signals based on the one or more indexes associated with the one or more beams. 
       FIG. 17  is a conceptual data flow diagram  1700  illustrating the data flow between different means/components in an exemplary apparatus  1702 . The apparatus may be a base station (e.g., the base station  602 ). The data flow illustrated in the diagram  1700  is to be regarded as illustrative. Therefore, one or more additional means/components may be present, and one or more illustrated means/components may be absent, according to various aspects. Further, various data flow may occur between means/components in addition to and/or instead of the illustrated data flow. 
     The apparatus  1702  may include a reception component  1704  configured to receive signals from a UE (e.g., the UE  1750 , a mmW UE, etc.). The apparatus  1702  may further include a transmission component  1710  configured to transmit signals to a UE (e.g., the UE  1750 , a mmW UE, etc.). 
     The apparatus  1702  may include a communication component  1708 . The communication component  1708  may be configured to determine a beam corresponding to a beam index. The communication component  1708  may provide an indication of the beam index to the reception component  1704  so that the reception component  1704  may receive data from the UE  1750  through the beam corresponding to the beam index. The communication component  1708  may provide an indication of the beam index to the transmission component  1710  so that the transmission component  1710  may transmit data to the UE  1750  through the beam corresponding to the beam index. The beam through which the apparatus  1702  communicates with the UE  1750  may be an active beam. 
     In an aspect, the communication component  1708  may generate a reference signal. The reference signal may be a CSI-RS, a CRS, an SSS, an MRS, a DMRS, or a BRS. The communication component  1708  may cause the transmission component  1710  to transmit the reference signal to the UE  1750  through a first active beam. In aspect, the communication component  1708  may cause the reference signal to be transmitted through a first RAT having a first carrier frequency (e.g., a 5G RAT, a mmW RAT, and/or a near-mmW RAT). 
     The apparatus  1702  may include a determination component  1712 . The determination component  1712  may be configured to determine that beam tracking is to be performed with the UE  1750 . Beam tracking may include selection or identification of an active beam through which the apparatus  1702  is to communicate with the UE  1750 . The determination component  1712  may provide an indication that beam tracking is to be performed to the beam tracking component  1706  in order to perform beam tracking. 
     In one aspect, the determination component  1712  may be configured to determine a time at which the UE  1750  is to transition from an inactive cycle of DRX to an active cycle of DRX. The determination component  1712  may determine that beam tracking is to be performed when the UE  1750  transition to the active cycle of DRX. The determination component  1712  may indicate, to the beam tracking component  1706 , that beam tracking is to be performed with the UE  1750  at the time at which the UE  1750  transition from the inactive cycle of DRX to the active cycle of DRX. 
     In one aspect, the determination component  1712  may be configured to determine that beam tracking is to be performed with the UE  1750  based on a response to the reference signal received from the UE  1750 . In an aspect, the response may be received through a second RAT having a different carrier frequency than the first RAT—e.g., the second RAT may have a lower carrier frequency than the first RAT, and the second RAT may be a sub-6 GHz RAT and/or LTE RAT). In an aspect, the response may include at least one of a CQI, an SINR, an SNR, an RSSI, a BRSRP, an RSRP, or an RSRQ. Thus, the response may indicate, to the determination component  1712 , a signal quality. The determination component  1712  may compare the signal quality to a threshold and determine that beam tracking is to be performed based on the comparison of the signal quality to the threshold. 
     In one aspect, the determination component  1712  may detect a radio link failure, for example, based on the response, an absence of the response, or at least one NACK message. The determination component  1712  may determine that beam tracking is to be performed based on the detected radio link failure. 
     In one aspect, the determination component  1712  may determine an absence of communication with the UE  1750  through the current active beam. The determination component  1712  may determine that beam tracking is to be performed based on the absence of communication. For example, the determination component  1712  may determine an absence of communication using a current active beam based on an absence of data carried on a PUCCH (e.g., when the UE  1750  is scheduled to communicate uplink data on the PUCCH), an absence of data carried on a PUSCH (e.g., when the UE  1750  is scheduled to communicate uplink data on the PUSCH), and/or an absence of ACK/NACK messages from the UE  1750  (e.g., in response to downlink data communicated to the UE  1750 ). 
     In one aspect, the beam tracking component  1706  may perform beam tracking with the UE  1750 . Beam tracking may allow the apparatus  1702  to select or identify an active beam (e.g., a new beam) for communication with the UE  1750 . In an aspect, the beam tracking component  1706  may cause the transmission component  1710  to transmit one or more BRSs (e.g., a coarse beam set). The beam tracking component  1706  may receive, through the reception component  1704 , a first indication of a first beam index based on one or more of the BRSs. The first beam index may be a coarse beam index. Based on the first beam index, the beam tracking component  1706  may select or identify a fine set of beams through which one or more BRRSs are to be transmitted, for example, for beam refinement. The beam tracking component  1706  may select or identify the fine beam set as one or more beams that are proximate or close to the beam corresponding to the first beam index. The beam tracking component  1706  may then transmit one or more BRRSs through the fine set of beams. The beam tracking component  1706  may receive, through the reception component  1704 , a second indication of a second beam index based on one or more of the BRRSs. The second beam index may be a fine beam index. Based on the second beam index, the beam tracking component  1706  may select or identify a fine beam through the communication component  1708  is to communicate with the UE  1750 . The beam tracking component  1706  may indicate, to the communication component  1708 , the second beam index, which may be a new active beam for communication (e.g., uplink and/or downlink communication) with the UE  1750 . 
     In one aspect, the beam tracking component  1706  may receive, through the reception component  1704 , a request to perform beam tracking (e.g., a BAR) from the UE  1750 . Based on the request to perform beam tracking, the beam tracking component  1706  may select or identify a fine set of beams through which one or more BRRSs are to be transmitted, for example, for beam refinement. The beam tracking component  1706  may select or identify the fine beam set as one or more beams that are proximate or close to a beam used for communication with the UE  1750  (e.g., a most recent beam through which the apparatus  1702  communicated with the UE  1750 ). The beam tracking component  1706  may then transmit one or more BRRSs through the fine set of beams. The beam tracking component  1706  may receive, through the reception component  1704 , an indication of a beam index based on one or more of the BRRSs. The beam index may be a fine beam index. Based on the beam index, the beam tracking component  1706  may select or identify a fine beam through the communication component  1708  is to communicate with the UE  1750 . The beam tracking component  1706  may indicate, to the communication component  1708 , the beam index, which may be a new active beam for communication (e.g., uplink and/or downlink communication) with the UE  1750 . 
     In an aspect, the beam tracking component  1706  may indicate, to the UE  1750 , that beam tracking is to be performed by causing transmission of a message. For example, the beam tracking component  1706  may cause transmission of a message indicating that beam tracking is to be performed, and the message may be carried on a PDCCH or a PDSCH. In an aspect, the message may be sent through DCI in the PDCCH. 
     The apparatus  1702  may include additional components that perform each of the blocks of the algorithm in the aforementioned flowcharts of  FIGS. 10A, 10B , and/or  12 . As such, each block in the aforementioned flowcharts of  FIGS. 10A, 10B , and/or  12  may be performed by a component and the apparatus  1702  may include one or more of those components. The components may be one or more hardware components specifically configured to carry out the stated processes/algorithm, implemented by a processor configured to perform the stated processes/algorithm, stored within a computer-readable medium for implementation by a processor, or some combination thereof. 
       FIG. 18  is a diagram  1800  illustrating an example of a hardware implementation for an apparatus  1702 ′ employing a processing system  1814 . The processing system  1814  may be implemented with a bus architecture, represented generally by the bus  1824 . The bus  1824  may include any number of interconnecting buses and bridges depending on the specific application of the processing system  1814  and the overall design constraints. The bus  1824  links together various circuits including one or more processors and/or hardware components, represented by the processor  1804 , the components  1704 ,  1706 ,  1708 ,  1710 ,  1712  and the computer-readable medium/memory  1806 . The bus  1824  may also link various other circuits such as timing sources, peripherals, voltage regulators, and power management circuits, which are well known in the art, and therefore, will not be described any further. 
     The processing system  1814  may be coupled to a transceiver  1810 . The transceiver  1810  is coupled to one or more antennas  1820 . The transceiver  1810  provides a means for communicating with various other apparatus over a transmission medium. The transceiver  1810  receives a signal from the one or more antennas  1820 , extracts information from the received signal, and provides the extracted information to the processing system  1814 , specifically the reception component  1704 . In addition, the transceiver  1810  receives information from the processing system  1814 , specifically the transmission component  1710 , and based on the received information, generates a signal to be applied to the one or more antennas  1820 . The processing system  1814  includes a processor  1804  coupled to a computer-readable medium/memory  1806 . The processor  1804  is responsible for general processing, including the execution of software stored on the computer-readable medium/memory  1806 . The software, when executed by the processor  1804 , causes the processing system  1814  to perform the various functions described supra for any particular apparatus. The computer-readable medium/memory  1806  may also be used for storing data that is manipulated by the processor  1804  when executing software. The processing system  1814  further includes at least one of the components  1704 ,  1706 ,  1708 ,  1710 ,  1712 . The components may be software components running in the processor  1804 , resident/stored in the computer readable medium/memory  1806 , one or more hardware components coupled to the processor  1804 , or some combination thereof. The processing system  1814  may be a component of the base station  310  and may include the memory  376  and/or at least one of the TX processor  316 , the RX processor  370 , and the controller/processor  375 . 
     In one configuration, the apparatus  1702 / 1702 ′ for wireless communication includes means for communicating with a UE through a first active beam. The apparatus  1702 / 1702 ′ may further include means for determining that beam tracking is to be performed with the UE—the beam tracking including identifying a new beam for communication between the UE and the base station. The apparatus  1702 / 1702 ′ may further include means for performing beam tracking with the UE based on the determination that beam tracking is to be performed. The apparatus  1702 / 1702 ′ may further include means for communicating with the UE through a second active beam based on the beam tracking. 
     In an aspect, the means for determining that beam tracking is to be performed for the UE is configured to determine a time at which the UE is to transition from an inactive cycle of DRX to an active cycle of DRX, and the means for performing the beam tracking is configured to perform the beam tracking based on the determined time. 
     In an aspect, the means for performing the beam tracking is configured for one or more of: transmission of at least one beam reference signal; reception, from the UE, of a first indication of a first beam index based on the beam reference signal; transmission, based on the first indication of the first beam index, of at least one beam refinement reference signal; and reception, based on the at least one beam refinement reference signal, of a second indication of a second beam index, the second beam index corresponding to the second active beam. 
     In an aspect, the means for performing the beam tracking is configured for one or more of: reception, from the UE, of a request for beam tracking; transmission, based on the request for beam tracking, of at least one beam refinement reference signal; and reception, based on the at least one beam refinement reference signal, of an indication of a beam index, the beam index corresponding to the second active beam. 
     In an aspect, the means for communicating with the UE through the first active beam is configured to send a reference signal to the UE to determine if the first active beam is failing, and the means for determining that beam tracking is to be initiated for the UE is configured to receive a response from the UE based on the reference signal; and detect a radio link failure based on the received response. 
     In an aspect, the communication with the UE through the first active beam is performed with a first RAT, and the response is received through a second RAT, the second RAT having a lower carrier frequency than the first RAT. In an aspect, the reference signal is one of a CSI-RS, a CRS, an SSS, an MRS, a DMRS, or a BRS, and the response includes at least one of a CQI, an SINR, an SNR, an RSSI, an RSRP, or a RSRQ. 
     In an aspect, the means for performing beam tracking with the UE is configured to send a message to the UE indicating that beam tracking is to be performed, and the message is sent on a PDCCH or a PDSCH. In an aspect, the message is sent through DCI in the PDCCH. 
     In an aspect, the means for determining that beam tracking is to be initiated for the UE is configured to determine an absence of communication with the UE through the first active beam. In an aspect, the determination of the absence of the communication with the UE through the first active beam is based on an absence of data carried on a PUCCH, an absence of data carried on a PUSCH, or an absence of ACK/NACK messages from the UE. 
     The aforementioned means may be one or more of the aforementioned components of the apparatus  1702  and/or the processing system  1814  of the apparatus  1702 ′ configured to perform the functions recited by the aforementioned means. As described supra, the processing system  1814  may include the TX Processor  316 , the RX Processor  370 , and the controller/processor  375 . As such, in one configuration, the aforementioned means may be the TX Processor  316 , the RX Processor  370 , and the controller/processor  375  configured to perform the functions recited by the aforementioned means. 
       FIG. 19  is a conceptual data flow diagram  1900  illustrating the data flow between different means/components in an exemplary apparatus  1902 . The apparatus may be a UE (e.g., the UE  604 ). The data flow illustrated in the diagram  1900  is to be regarded as illustrative. Therefore, one or more additional means/components may be present, and one or more illustrated means/components may be absent, according to various aspects. Further, various data flow may occur between means/components in addition to the illustrated data flow. 
     The apparatus  1902  may include a reception component  1904  configured to receive signals from a base station (e.g., the base station  1950 , a mmW base station, an eNB, etc.). The apparatus  1902  may further include a transmission component  1910  configured to transmit signals to a base station (e.g., the base station  1950 , a mmW base station, an eNB, etc.). 
     In an aspect, The apparatus  1902  may include a communication component  1908 . The communication component  1908  may be configured to determine a beam corresponding to a beam index, which may be a beam and corresponding beam index at the base station  1950 . The communication component  1908  may provide an indication of the beam index to the reception component  1904  so that the reception component  1904  may receive data from the base station  1950  through the beam corresponding to the beam index. The communication component  1908  may provide an indication of the beam index to the transmission component  1910  so that the transmission component  1910  may transmit data to the base station  1950  through the beam corresponding to the beam index. The beam through which the apparatus  1902  communicates with the base station  1950  may be an active beam. 
     In one aspect, the beam tracking component  1906  may perform beam tracking with the base station  1950 . Beam tracking may allow the apparatus  1902  to select or identify an active beam (e.g., a new beam) for communication with the base station  1950 . In an aspect, the beam tracking component  1906  may receive, through the reception component  1904 , one or more BRSs (e.g., a coarse beam set). The beam tracking component  1906  may measure respective signal qualities for one or more received BRSs and may select a best beam corresponding to a best (e.g., highest) signal quality for a received BRS. The beam tracking component  1906  may transmit, through the transmission component  1910 , a first indication of a first beam index corresponding to the selected best beam. The first beam index may be a coarse beam index. Based on the first beam index, the beam tracking component  1906  may receive one or more BRRSs, e.g., through a fine set of beams. The beam tracking component  1906  may select a fine beam based on the one or more BRRSs (e.g., the beam tracking component may select a best beam based on a best or highest signal quality for a BRRS). The beam tracking component  1906  may transmit, through the transmission component  1910 , a second indication of a second beam index corresponding to a BRRS having a best or highest signal quality. The second beam index may be a fine beam index. The beam tracking component  1906  may indicate, to the communication component  1908 , the second beam index, which may be a new active beam for communication (e.g., uplink and/or downlink communication) with the base station  1950 . 
     In one aspect, the beam tracking component  1906  may transmit, through the transmission component  1910 , a request to perform beam tracking (e.g., a BAR). Based on the request for beam tracking, the beam tracking component  1906  may receive one or more BRRSs, e.g., through a fine set of beams. The beam tracking component  1906  may select a fine beam based on the one or more BRRSs (e.g., the beam tracking component may select a best beam based on a best or highest signal quality for a BRRS). The beam tracking component  1906  may transmit, through the transmission component  1910 , an indication of a beam index corresponding to a BRRS having a best or highest signal quality. The beam index may be a fine beam index. The beam tracking component  1906  may indicate, to the communication component  1908 , the second beam index, which may be a new active beam for communication (e.g., uplink and/or downlink communication) with the base station  1950 . 
     In an aspect, the beam tracking component  1906  may receive, from the base station  1950 , a message (e.g., signal) indicating that beam tracking is to be performed. For example, the beam tracking component  1906  may receive, through the reception component  1904 , a message indicating that beam tracking is to be performed, and the message may be carried on a PDCCH or a PDSCH. In an aspect, the message may be received through DCI in the PDCCH. 
     The apparatus  1902  may include a determination component  1912 . The determination component  1912  may be configured to determine that beam tracking is to be performed with the base station  1950 . Beam tracking may include selection or identification of an active beam through which the apparatus  1902  is to communicate with the base station  1950 . The determination component  1912  may provide an indication that beam tracking is to be performed to the beam tracking component  1906  in order to perform beam tracking. 
     In one aspect, the determination component  1912  may be configured to receive, through the reception component  1904 , a signal associated with beam tracking. In one aspect, the signal may be a BRS or BRRS. In another aspect, the signal may be a reference signal. The reference signal may be one of a CSI-RS, a CRS, an SSS, an MRS, a DMRS, or the like. 
     In one aspect, the determination component  1912  may be configured to cause the transmission component  1910  to transmit a response to the reference signal. In various aspects, the response may include one of a CQI, an SINR, an SNR, an RSSI, a BRSRP, an RSRP, or an RSRQ. For example, the determination component  1912  may measure a signal quality (e.g., an SINR, an SNR, an RSSI, a BRSRP, an RSRP, an RSRQ, etc.) based on the reference signal. The determination component  1912  may generate a response that includes the measured signal quality. 
     In an aspect, the determination component  1912  may receive the signal through a first RAT (e.g., a 5G RAT, a mmW RAT, a near-mmW RAT, etc.). However, the determination component  1912  may cause transmission of the response through a second RAT having a different carrier frequency than the first RAT—e.g., the second RAT may have a lower carrier frequency than the first RAT, and the second RAT may be a sub-6 GHz RAT and/or LTE RAT. 
     In one aspect, the determination component  1912  may detect a radio link failure, for example, based on the signal (e.g., reference signal). For example, the determination component  1912  may measure a signal quality of the reference signal. The determination component  1912  may compare the signal quality to a threshold and detect a radio link failure based on comparison of the measured signal quality to the threshold. The determination component  1912  may determine that beam tracking is to be performed based on the detected radio link failure. The determination component  1912  may cause the transmission component  1910  to transmit, to the base station  1950 , an indication of the detected radio link failure and/or the determination component  1912  may indicate to the beam tracking component  1906  that beam tracking is to be performed, for example, to recover an active beam for communication. 
     The apparatus may include additional components that perform each of the blocks of the algorithm in the aforementioned flowcharts of  FIGS. 11A, 11B , and/or  13 . As such, each block in the aforementioned flowcharts of  FIGS. 11A, 11B , and/or  13  may be performed by a component and the apparatus  1902  may include one or more of those components. The components may be one or more hardware components specifically configured to carry out the stated processes/algorithm, implemented by a processor configured to perform the stated processes/algorithm, stored within a computer-readable medium for implementation by a processor, or some combination thereof. 
       FIG. 20  is a diagram  2000  illustrating an example of a hardware implementation for an apparatus  1902 ′ employing a processing system  2014 . The processing system  2014  may be implemented with a bus architecture, represented generally by the bus  2024 . The bus  2024  may include any number of interconnecting buses and bridges depending on the specific application of the processing system  2014  and the overall design constraints. The bus  2024  links together various circuits including one or more processors and/or hardware components, represented by the processor  2004 , the components  1904 ,  1906 ,  1908 ,  1910 ,  1912 , and the computer-readable medium/memory  2006 . The bus  2024  may also link various other circuits such as timing sources, peripherals, voltage regulators, and power management circuits, which are well known in the art, and therefore, will not be described any further. 
     The processing system  2014  may be coupled to a transceiver  2010 . The transceiver  2010  is coupled to one or more antennas  2020 . The transceiver  2010  provides a means for communicating with various other apparatus over a transmission medium. The transceiver  2010  receives a signal from the one or more antennas  2020 , extracts information from the received signal, and provides the extracted information to the processing system  2014 , specifically the reception component  1904 . In addition, the transceiver  2010  receives information from the processing system  2014 , specifically the transmission component  1910 , and based on the received information, generates a signal to be applied to the one or more antennas  2020 . The processing system  2014  includes a processor  2004  coupled to a computer-readable medium/memory  2006 . The processor  2004  is responsible for general processing, including the execution of software stored on the computer-readable medium/memory  2006 . The software, when executed by the processor  2004 , causes the processing system  2014  to perform the various functions described supra for any particular apparatus. The computer-readable medium/memory  2006  may also be used for storing data that is manipulated by the processor  2004  when executing software. The processing system  2014  further includes at least one of the components  1904 ,  1906 ,  1908 ,  1910 ,  1912 . The components may be software components running in the processor  2004 , resident/stored in the computer readable medium/memory  2006 , one or more hardware components coupled to the processor  2004 , or some combination thereof. The processing system  2014  may be a component of the UE  350  and may include the memory  360  and/or at least one of the TX processor  368 , the RX processor  356 , and the controller/processor  359 . 
     In one configuration, the apparatus  1902 / 1902 ′ for wireless communication includes means for communicating with a base station through a first active beam. The apparatus  1902 / 1902 ′ may further include means for receiving a signal from the base station associated with beam tracking, the beam tracking including identifying a new beam for communication between the apparatus  1902 / 1902 ′ and the base station. The apparatus  1902 / 1902 ′ may further include means for communicating with the base station through a second active beam based on the signal associated with beam tracking. 
     In an aspect, the signal includes a BRRS, and the apparatus  1902 / 1902 ′ further includes means for sending, to the base station, a beam index corresponding to the second active beam based on the BRRS. In an aspect, the signal includes a BRS, and the apparatus  1902 / 1902 ′ further includes means for sending, to the base station, a beam index corresponding to a coarse beam. In an aspect, the apparatus  1902 / 1902 ′ may further includes means for sending, to the base station, a request to perform beam tracking based on the signal. 
     In an aspect, the means for communicating with the base station through the first active beam is configured to receive a reference signal, and the apparatus  1902 / 1902 ′ further includes means for detecting a radio link failure based on the reception of the reference signal and means for sending an indication to the base station based on the detected radio link failure. In an aspect, the reference signal is one of a CSI-RS, a CRS, an SSS, an MRS, a DMRS, or a BRS, and the indication includes at least one of a CQI, an SINR, an SNR, an RSSI, an RSRP, or an RSRQ. In an aspect, the means for communicating with the base station through the first active beam is performed with a first RAT, and the means for sending the indication is configured to send the indication through a second RAT, the first RAT having a higher carrier frequency than the second RAT. 
     In an aspect, the apparatus  1902 / 1902 ′ further includes means for performing beam tracking with the base station. In an aspect, the means for performing the beam tracking is configured for one or more of: reception, from the base station, of at least one BRS; transmission, to the base station, of a first indication of a first beam index based on the BRS; reception of at least one BRRS; and transmission, based on the at least one BRRS, of a second indication of a second beam index. 
     In an aspect, the signal is received on a PDCCH or a PDSCH. In an aspect, the signal is received through DCI on the PDCCH. 
     The aforementioned means may be one or more of the aforementioned components of the apparatus  1902  and/or the processing system  2014  of the apparatus  1902 ′ configured to perform the functions recited by the aforementioned means. As described supra, the processing system  2014  may include the TX Processor  368 , the RX Processor  356 , and the controller/processor  359 . As such, in one configuration, the aforementioned means may be the TX Processor  368 , the RX Processor  356 , and the controller/processor  359  configured to perform the functions recited by the aforementioned means. 
       FIG. 21  is a conceptual data flow diagram  2100  illustrating the data flow between different means/components in an exemplary apparatus  2102 . The apparatus may be a base station (e.g., the base station  702 , the base station  802 , etc.). The data flow illustrated in the diagram  2100  is to be regarded as illustrative. Therefore, one or more additional means/components may be present, and one or more illustrated means/components may be absent, according to various aspects. Further, various data flow may occur between means/components in addition to and/or instead of the illustrated data flow. 
     The apparatus  2102  may include a reception component  2104  configured to receive signals from a UE (e.g., the UE  2150 , a mmW UE, etc.). The apparatus  2102  may further include a transmission component  2110  configured to transmit signals to a UE (e.g., the UE  2150 , a mmW UE, etc.). 
     The apparatus  2102  may include a communication component  2108 . The communication component  2108  may be configured to determine a beam corresponding to a beam index. The communication component  2108  may provide an indication of the beam index to the reception component  2104  so that the reception component  2104  may receive data from the UE  2150  through the beam corresponding to the beam index. The communication component  2108  may provide an indication of the beam index to the transmission component  2110  so that the transmission component  2110  may transmit data to the UE  2150  through the beam corresponding to the beam index. The beam through which the apparatus  2102  communicates with the UE  2150  may be an active beam. 
     In an aspect, the apparatus  2102  may include a control component  2112 . The control component  2112  may be configured to determine a periodicity at which control information is to be communicated on a control channel through a control-information beam. In an aspect, the control component  2112  may cause the transmission component to transmit information indicating the periodicity to the UE. In one aspect, the control component  2112  may cause transmission of the information indicating the periodicity through RRC signaling. In another aspect, the control component  2112  may cause transmission of the information indicating the periodicity on a PDDCH. For example, the control component  2112  may cause transmission of the information indicating the periodicity as DCI of the PDCCH. 
     In an aspect, the control component  2112  may communicate, with the UE, the control information on the control channel through a control-information beam at the periodicity. The control-information beam may be different than the active beam. In an aspect, the control-information beam includes at least one candidate beam corresponding to a beam index included in a set of candidate beam indexes. In an aspect, the control-information beam includes at least one wide beam, which may have an angle greater than that of the active beam. 
     In an aspect, the control channel may include a PUCCH. The control component  2112  may be configured to receive, through the reception component, the control information carried on a PUCCH through the control-information beam based on the periodicity. 
     The apparatus  2102  may include a beam switching component  2106  for switching or changing the active beam. The beam switching component  2106  may receive, through the reception component  2104 , a request to change the active beam. The request may indicate a beam index corresponding to a second beam. For example, the request may indicate the beam index through at least one of a cyclic shift or spreading across symbols. In another example, the request may indicate the beam index through at least one of a subcarrier region or a RACH. The beam switching component  2106  may determine the second beam based on the request and provide the beam index to the communication component  2108  for communication through the new active beam. 
     The apparatus may include additional components that perform each of the blocks of the algorithm in the aforementioned flowcharts of  FIG. 14 . As such, each block in the aforementioned flowcharts of  FIG. 14  may be performed by a component and the apparatus may include one or more of those components. The components may be one or more hardware components specifically configured to carry out the stated processes/algorithm, implemented by a processor configured to perform the stated processes/algorithm, stored within a computer-readable medium for implementation by a processor, or some combination thereof. 
       FIG. 22  is a diagram  2200  illustrating an example of a hardware implementation for an apparatus  2102 ′ employing a processing system  2214 . The processing system  2214  may be implemented with a bus architecture, represented generally by the bus  2224 . The bus  2224  may include any number of interconnecting buses and bridges depending on the specific application of the processing system  2214  and the overall design constraints. The bus  2224  links together various circuits including one or more processors and/or hardware components, represented by the processor  2204 , the components  2104 ,  2106 ,  2108 ,  2110 ,  2112  and the computer-readable medium/memory  2206 . The bus  2224  may also link various other circuits such as timing sources, peripherals, voltage regulators, and power management circuits, which are well known in the art, and therefore, will not be described any further. 
     The processing system  2214  may be coupled to a transceiver  2210 . The transceiver  2210  is coupled to one or more antennas  2220 . The transceiver  2210  provides a means for communicating with various other apparatus over a transmission medium. The transceiver  2210  receives a signal from the one or more antennas  2220 , extracts information from the received signal, and provides the extracted information to the processing system  2214 , specifically the reception component  2104 . In addition, the transceiver  2210  receives information from the processing system  2214 , specifically the transmission component  2110 , and based on the received information, generates a signal to be applied to the one or more antennas  2220 . The processing system  2214  includes a processor  2204  coupled to a computer-readable medium/memory  2206 . The processor  2204  is responsible for general processing, including the execution of software stored on the computer-readable medium/memory  2206 . The software, when executed by the processor  2204 , causes the processing system  2214  to perform the various functions described supra for any particular apparatus. The computer-readable medium/memory  2206  may also be used for storing data that is manipulated by the processor  2204  when executing software. The processing system  2214  further includes at least one of the components  2104 ,  2106 ,  2108 ,  2110 ,  2112 . The components may be software components running in the processor  2204 , resident/stored in the computer readable medium/memory  2206 , one or more hardware components coupled to the processor  2204 , or some combination thereof. The processing system  2214  may be a component of the base station  310  and may include the memory  376  and/or at least one of the TX processor  316 , the RX processor  370 , and the controller/processor  375 . 
     In one configuration, the apparatus  2102 / 2102 ′ for wireless communication includes means for communicating with a UE through an active beam. The apparatus  2102 / 2102 ′ further includes means for transmitting, to the UE, information indicating a periodicity at which control information is to be communicated on a control channel through a control-information beam. The apparatus  2102 / 2102 ′ further includes means for communicating, with the UE, the control information on the control channel through the control-information beam at the periodicity. 
     In an aspect, the control channel includes a PUCCH, and the means for communicating, with the UE, the control information on the control channel is configured to receive, from the UE, the control information carried on the PUCCH through the control-information beam based on the periodicity. 
     In an aspect, the control-information beam includes at least one candidate beam, the at least one candidate beam corresponding to a beam index included in a set of candidate beam indexes maintained by the base station. In an aspect, the control-information beam includes at least one wide beam, the at least one wide beam having an angle greater than that of the active beam. 
     In an aspect, the information indicating the periodicity is transmitted through RRC signaling. In an aspect, the information indicating the periodicity is transmitted on a PDCCH. In an aspect, the information indicating the periodicity includes DCI of the PDCCH. 
     In an aspect, the apparatus  2102 / 2102 ′ may further include means for receiving a request to change the active beam, the request indicating a beam index corresponding to a second beam. In an aspect, the apparatus  2102 / 2102 ′ may further include means for changing the active beam to the second beam corresponding to the beam index indicated by the request. In an aspect, the request indicates the beam index through at least one of a cyclic shift or spreading across symbols. In an aspect, the request indicates the beam index through at least one of a subcarrier region or a RACH. 
     The aforementioned means may be one or more of the aforementioned components of the apparatus  2102  and/or the processing system  2214  of the apparatus  2102 ′ configured to perform the functions recited by the aforementioned means. As described supra, the processing system  2214  may include the TX Processor  316 , the RX Processor  370 , and the controller/processor  375 . As such, in one configuration, the aforementioned means may be the TX Processor  316 , the RX Processor  370 , and the controller/processor  375  configured to perform the functions recited by the aforementioned means. 
       FIG. 23  is a conceptual data flow diagram  2300  illustrating the data flow between different means/components in an exemplary apparatus  2302 . The apparatus may be a UE (e.g., the UE  704 , the UE  804 , etc.). The data flow illustrated in the diagram  2300  is to be regarded as illustrative. Therefore, one or more additional means/components may be present, and one or more illustrated means/components may be absent, according to various aspects. Further, various data flow may occur between means/components in addition to the illustrated data flow. 
     The apparatus  2302  may include a reception component  2304  configured to receive signals from a base station (e.g., the base station  2350 , a mmW base station, an eNB, etc.). The apparatus  2302  may further include a transmission component  2310  configured to transmit signals to a base station (e.g., the base station  2350 , a mmW base station, an eNB, etc.). 
     In an aspect, The apparatus  2302  may include a communication component  2308 . The communication component  2308  may be configured to determine a beam corresponding to a beam index, which may be a beam and corresponding beam index at the base station  2350 . The communication component  2308  may provide an indication of the beam index to the reception component  2304  so that the reception component  2304  may receive data from the base station  2350  through the beam corresponding to the beam index. The communication component  2308  may provide an indication of the beam index to the transmission component  2310  so that the transmission component  2310  may transmit data to the base station  2350  through the beam corresponding to the beam index. The beam through which the apparatus  2302  communicates with the base station  2350  may be an active beam. 
     In an aspect, the apparatus  2302  may include a control component  2312 . The control component  2312  may be configured to determine a periodicity at which control information is to be communicated on a control channel through a control-information beam. In an aspect, the control component  2312  may receive, through the reception component  2304 , the transmission component to transmit information indicating the periodicity from the base station  2350 . The control component  2312  may determine the periodicity based on the information indicating the periodicity. In one aspect, the control component  2312  may receive the information indicating the periodicity through RRC signaling. In another aspect, the control component  2312  may receive the information indicating the periodicity on a PDDCH. For example, the control component  2312  may receive the information indicating the periodicity as DCI of the PDCCH. 
     In an aspect, the control component  2312  may communicate, with the base station  2350 , the control information on the control channel through a control-information beam at the periodicity. The control-information beam may be different than the active beam. In an aspect, the control-information beam includes at least one candidate beam corresponding to a beam index included in a set of candidate beam indexes. In an aspect, the control-information beam includes at least one wide beam, which may have an angle greater than that of the active beam. 
     In an aspect, the control channel may include a PUCCH. The control component  2312  may be configured to send, to the base station  2350 , the control information through the control-information beam based on the periodicity. 
     The apparatus  2302  may include a beam switching component  2306  for switching or changing the active beam. The beam switching component  2306  may send, through the transmission component  2310 , a request to change the active beam. The request may indicate a beam index corresponding to a second beam. For example, the request may indicate the beam index through at least one of a cyclic shift or spreading across symbols. In another example, the request may indicate the beam index through at least one of a subcarrier region or a RACH. The beam switching component  2306  may provide the beam index to the communication component  2308  for communication through the new active beam, which may match the active beam at the base station  2350  after transmission of the request. 
     The apparatus may include additional components that perform each of the blocks of the algorithm in the aforementioned flowcharts of  FIG. 15 . As such, each block in the aforementioned flowcharts of  FIG. 15  may be performed by a component and the apparatus may include one or more of those components. The components may be one or more hardware components specifically configured to carry out the stated processes/algorithm, implemented by a processor configured to perform the stated processes/algorithm, stored within a computer-readable medium for implementation by a processor, or some combination thereof. 
       FIG. 24  is a diagram  2400  illustrating an example of a hardware implementation for an apparatus  2302 ′ employing a processing system  2414 . The processing system  2414  may be implemented with a bus architecture, represented generally by the bus  2424 . The bus  2424  may include any number of interconnecting buses and bridges depending on the specific application of the processing system  2414  and the overall design constraints. The bus  2424  links together various circuits including one or more processors and/or hardware components, represented by the processor  2404 , the components  2304 ,  2306 ,  2308 ,  2310 ,  2312  and the computer-readable medium/memory  2406 . The bus  2424  may also link various other circuits such as timing sources, peripherals, voltage regulators, and power management circuits, which are well known in the art, and therefore, will not be described any further. 
     The processing system  2414  may be coupled to a transceiver  2410 . The transceiver  2410  is coupled to one or more antennas  2420 . The transceiver  2410  provides a means for communicating with various other apparatus over a transmission medium. The transceiver  2410  receives a signal from the one or more antennas  2420 , extracts information from the received signal, and provides the extracted information to the processing system  2414 , specifically the reception component  2304 . In addition, the transceiver  2410  receives information from the processing system  2414 , specifically the transmission component  2310 , and based on the received information, generates a signal to be applied to the one or more antennas  2420 . The processing system  2414  includes a processor  2404  coupled to a computer-readable medium/memory  2406 . The processor  2404  is responsible for general processing, including the execution of software stored on the computer-readable medium/memory  2406 . The software, when executed by the processor  2404 , causes the processing system  2414  to perform the various functions described supra for any particular apparatus. The computer-readable medium/memory  2406  may also be used for storing data that is manipulated by the processor  2404  when executing software. The processing system  2414  further includes at least one of the components  2304 ,  2306 ,  2308 ,  2310 ,  2312 . The components may be software components running in the processor  2404 , resident/stored in the computer readable medium/memory  2406 , one or more hardware components coupled to the processor  2404 , or some combination thereof. The processing system  2414  may be a component of the UE  350  and may include the memory  360  and/or at least one of the TX processor  368 , the RX processor  356 , and the controller/processor  359 . 
     In one configuration, the apparatus  2302 / 2302 ′ for wireless communication includes means for communicating with a base station through an active beam. In an aspect, the apparatus  2302 / 2302 ′ further includes means for receiving, from the base station, information indicating a periodicity at which control information is to be communicated on a control channel through a control-information beam. In an aspect, the apparatus  2302 / 2302 ′ further includes means for communicating, with the base station, the control information on the control channel through the control-information beam at the periodicity. 
     In an aspect, the control channel includes a PUCCH, and the means for communicating the control information on the control channel is configured to send, to the base station, the control information on the PUCCH through the control-information beam based on the periodicity. 
     In an aspect, the control-information beam includes at least one candidate beam, the at least one candidate beam corresponding to a beam index included in a set of candidate beam indexes. In an aspect, the control-information beam includes at least one wide beam, the at least one wide beam having an angle greater than that of the active beam. In an aspect, the information indicating the periodicity is received using RRC signaling. In an aspect, the information indicating the periodicity is received on a PDCCH. In an aspect, the information indicating the periodicity is indicated by DCI of the PDCCH. 
     In an aspect, the apparatus  2302 / 2302 ′ further includes means for transmitting, to the base station, a request to change the active beam, the request indicating a beam index corresponding to a second beam. In an aspect, the apparatus  2302 / 2302 ′ further includes means for changing the active beam to the second beam corresponding to the beam index indicated by the request. In an aspect, the request indicates the beam index through at least one of a cyclic shift or spreading across symbols. In an aspect, the request indicates the beam index through at least one of a subcarrier region or a RACH. 
     The aforementioned means may be one or more of the aforementioned components of the apparatus  2302  and/or the processing system  2414  of the apparatus  2302 ′ configured to perform the functions recited by the aforementioned means. As described supra, the processing system  2414  may include the TX Processor  368 , the RX Processor  356 , and the controller/processor  359 . As such, in one configuration, the aforementioned means may be the TX Processor  368 , the RX Processor  356 , and the controller/processor  359  configured to perform the functions recited by the aforementioned means. 
       FIG. 25  is a conceptual data flow diagram  2500  illustrating the data flow between different means/components in an exemplary apparatus  2502 . The apparatus may be a base station (e.g., the base station  902 , etc.). The data flow illustrated in the diagram  2500  is to be regarded as illustrative. Therefore, one or more additional means/components may be present, and one or more illustrated means/components may be absent, according to various aspects. Further, various data flow may occur between means/components in addition to and/or instead of the illustrated data flow. 
     The apparatus  2502  may include a reception component  2504  configured to receive signals from a UE (e.g., the UE  2550 , a mmW UE, etc.). The apparatus  2502  may further include a transmission component  2510  configured to transmit signals to a UE (e.g., the UE  2550 , a mmW UE, etc.). 
     In aspects, the apparatus  2502  may include a determination component  2512 . The determination component  2512  may be configured to determine one or more beam indexes corresponding to one or more beams of the apparatus  2502  through which one or more reference signals are to be transmitted. For example, the determination component  2512  may receive, through the reception component  2504 , one or more beam indexes from the UE  2550 . The beam indexes may be based on one or more reference signals previously sent be the apparatus  2502 , such as one or more BRSs, BRRSs, CSI-RSs, or another reference signal. 
     In one aspect, the one or more beam indexes are received from the UE  2550  in one or more BSI reports. In one aspect, the one or more beam indexes corresponding to the one or more beams are received on a PUSCH or a PUCCH. The one or more beam indexes corresponding to the one or more beams may be received on a PUSCH when more than two symbols are reserved for reference signal transmission. The one or more beam indexes corresponding to the one or more beams may be received on a PUCCH when two or fewer symbols are reserved for reference signal transmission. 
     In one aspect, the determination component  2512  may determine the one or more beam indexes based on one more beam indexes that are most recently received form the UE  2550 . In one aspect, the determination component  2512  may determine the one or more beam indexes based on transmitting one or more BRSs during a synchronization subframe. The determination component  2512  may reuse the beams one which the one or more BRSs are transmitted, for example, based on feedback from the UE  2550  indicating the best beam indexes corresponding to the best beams (e.g., the beam(s) through an associated BRS is transmitted, and the BRS has a highest measured signal quality). 
     The determination component  2512  may provide an indication of the one or more beam indexes corresponding to the one or more beams to an indication component  2506 . The indication component  2506  may be configured to generate one or more indications of the one or more beam indexes corresponding to the one or more beams. The indication component  2506  may then cause the transmission component  2510  to transmit the one or more indications to the UE  2550 . In one aspect, the one or more indications may be carried on a PDCCH. For example, the one or more indications may be included in one or more bits of a DCI message of a PDCCH. 
     The determination component  2512  may provide an indication of the one or more beam indexes corresponding to the one or more beams to a reference signal component  2508 . The reference signal component  2508  may be configured to generate one or more reference signals. A generated reference signal may be at least one of a BRRS or a CSI-RS. The reference signal component  2508  may then cause the transmission component  2510  to transmit the one or more reference signals through the one or more beams corresponding to the one or more beam indexes, which were send to the UE  2550  by the indication component  2506 . 
     The apparatus may include additional components that perform each of the blocks of the algorithm in the aforementioned flowcharts of  FIG. 16 . As such, each block in the aforementioned flowcharts of  FIG. 16  may be performed by a component and the apparatus may include one or more of those components. The components may be one or more hardware components specifically configured to carry out the stated processes/algorithm, implemented by a processor configured to perform the stated processes/algorithm, stored within a computer-readable medium for implementation by a processor, or some combination thereof. 
       FIG. 26  is a diagram  2600  illustrating an example of a hardware implementation for an apparatus  2502 ′ employing a processing system  2614 . The processing system  2614  may be implemented with a bus architecture, represented generally by the bus  2624 . The bus  2624  may include any number of interconnecting buses and bridges depending on the specific application of the processing system  2614  and the overall design constraints. The bus  2624  links together various circuits including one or more processors and/or hardware components, represented by the processor  2604 , the components  2504 ,  2506 ,  2508 ,  2510 ,  2512  and the computer-readable medium/memory  2606 . The bus  2624  may also link various other circuits such as timing sources, peripherals, voltage regulators, and power management circuits, which are well known in the art, and therefore, will not be described any further. 
     The processing system  2614  may be coupled to a transceiver  2610 . The transceiver  2610  is coupled to one or more antennas  2620 . The transceiver  2610  provides a means for communicating with various other apparatus over a transmission medium. The transceiver  2610  receives a signal from the one or more antennas  2620 , extracts information from the received signal, and provides the extracted information to the processing system  2614 , specifically the reception component  2504 . In addition, the transceiver  2610  receives information from the processing system  2614 , specifically the transmission component  2510 , and based on the received information, generates a signal to be applied to the one or more antennas  2620 . The processing system  2614  includes a processor  2604  coupled to a computer-readable medium/memory  2606 . The processor  2604  is responsible for general processing, including the execution of software stored on the computer-readable medium/memory  2606 . The software, when executed by the processor  2604 , causes the processing system  2614  to perform the various functions described supra for any particular apparatus. The computer-readable medium/memory  2606  may also be used for storing data that is manipulated by the processor  2604  when executing software. The processing system  2614  further includes at least one of the components  2504 ,  2506 ,  2508 ,  2510 ,  2512 . The components may be software components running in the processor  2604 , resident/stored in the computer readable medium/memory  2606 , one or more hardware components coupled to the processor  2604 , or some combination thereof. The processing system  2614  may be a component of the base station  310  and may include the memory  376  and/or at least one of the TX processor  316 , the RX processor  370 , and the controller/processor  375 . 
     In one configuration, the apparatus  2502 / 2502 ′ for wireless communication includes means for transmitting, to a UE on a control channel, one or more indications of one or more beam indexes corresponding to one or more beams. The apparatus  2502 / 2502 ′ may further include means for transmitting, to the UE, one or more reference signals through the one or more beams corresponding to the one or more beam indexes. In an aspect, the control channel comprises a PDCCH, and the one or more indications are included in one or more bits of a DCI message. 
     In an aspect, the means for transmitting the one or more indications of the one or more beam indexes corresponding to the one or more beams is configured to transmit one or more beam indexes associated with one or more BRSs, the one or more BRSs transmitted during a synchronization subframe. 
     In an aspect, the means for transmitting the one or more indications of the one or more beam indexes corresponding to the one or beams is configured to: receive, from the UE, one or more beam indexes corresponding to the one or more beams; and transmit the one or more beam indexes corresponding to the one or more beams based on the one or more beam indexes that are received most recently. 
     In an aspect, the one or more beam indexes corresponding to the one or more beams are received on a PUSCH or a PUCCH. In an aspect, the one or more beam indexes corresponding to the one or more beams are transmitted based on the one or more beam indexes received through the PUSCH when more than two symbols are used for the reference signal transmission. In an aspect, the one or more beam indexes associated with one or more beams are transmitted based on the one or more beam indexes received through the PUCCH when two or fewer symbols are used for the reference signal transmission. 
     In an aspect, the one or more reference signals include at least one of a CSI-RS or a BRRS. In an aspect, the means for transmitting the one or more indications of the one or more beam indexes associated with the one or more beams is configured to transmit the one or more beam indexes associated with the one or more beams through which at least one of the CSI-RSs was previously transmitted. 
     The aforementioned means may be one or more of the aforementioned components of the apparatus  2502  and/or the processing system  2614  of the apparatus  2502 ′ configured to perform the functions recited by the aforementioned means. As described supra, the processing system  2614  may include the TX Processor  316 , the RX Processor  370 , and the controller/processor  375 . As such, in one configuration, the aforementioned means may be the TX Processor  316 , the RX Processor  370 , and the controller/processor  375  configured to perform the functions recited by the aforementioned means. 
     It is understood that the specific order or hierarchy of blocks in the processes/flowcharts disclosed is an illustration of exemplary approaches. Based upon design preferences, it is understood that the specific order or hierarchy of blocks in the processes/flowcharts may be rearranged. Further, some blocks may be combined or omitted. The accompanying method claims present elements of the various blocks in a sample order, and are not meant to be limited to the specific order or hierarchy presented. 
     The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects. Thus, the claims are not intended to be limited to the aspects shown herein, but is to be accorded the full scope consistent with the language claims, wherein reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more.” The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any aspect described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects. Unless specifically stated otherwise, the term “some” refers to one or more. Combinations such as “at least one of A, B, or C,” “one or more of A, B, or C,” “at least one of A, B, and C,” “one or more of A, B, and C,” and “A, B, C, or any combination thereof” include any combination of A, B, and/or C, and may include multiples of A, multiples of B, or multiples of C. Specifically, combinations such as “at least one of A, B, or C,” “one or more of A, B, or C,” “at least one of A, B, and C,” “one or more of A, B, and C,” and “A, B, C, or any combination thereof” may be A only, B only, C only, A and B, A and C, B and C, or A and B and C, where any such combinations may contain one or more member or members of A, B, or C. All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. The words “module,” “mechanism,” “element,” “device,” and the like may not be a substitute for the word “means.” As such, no claim element is to be construed as a means plus function unless the element is expressly recited using the phrase “means for.”