Patent Publication Number: US-2023139016-A1

Title: Time-averaged radio frequency (rf) exposure per exposure scenario

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present Application for Patent claims benefit of U.S. Provisional Application No. 63/274,660, filed Nov. 2, 2021, and U.S. Provisional Application No. 63/365,696, filed Jun. 1, 2022, each of which is hereby expressly incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     Field of the Disclosure 
     Aspects of the present disclosure relate to wireless communications, and more particularly, to radio frequency (RF) exposure across tissues and/or body locations. 
     Description of Related Art 
     Wireless communication systems are widely deployed to provide various telecommunication services such as telephony, video, data, messaging, broadcasts, etc. Modern wireless communication devices (such as cellular telephones) are generally mandated to meet radio frequency (RF) exposure limits set by domestic and international standards and regulations. To ensure compliance with the standards, such devices currently undergo an extensive certification process prior to being shipped to market. To ensure that a wireless communication device complies with an RF exposure limit, techniques have been developed to enable the wireless communication device to assess RF exposure from the wireless communication device and adjust the transmission power of the wireless communication device accordingly to comply with the RF exposure limit. 
     SUMMARY 
     The systems, methods, and devices of the disclosure each have several aspects, no single one of which is solely responsible for its desirable attributes. Without limiting the scope of this disclosure as expressed by the claims which follow, some features will now be discussed briefly. After considering this discussion, and particularly after reading the section entitled “Detailed Description,” one will understand how the features of this disclosure provide advantages that include evaluating radio frequency (RF) exposure per exposure scenario and/or category. 
     Aspects of the present disclosure provide a method of wireless communication, for example, by a wireless communication device. The method generally includes tracking sets of radio frequency (RF) exposures separately across a plurality of locations associated with a human body over time, and transmitting a first signal at a first transmit power determined based on a time-averaged RF exposure limit and at least one of the tracked sets of RF exposures. 
     Aspects of the present disclosure provide an apparatus for wireless communication. The apparatus generally includes a memory and a processor coupled to the memory. The processor is configured to track sets of RF exposures separately across a plurality of locations associated with a human body over time, and to transmit a first signal at a first transmit power determined based on a time-averaged RF exposure limit and at least one of the tracked sets of RF exposures. 
     Aspects of the present disclosure provide an apparatus for wireless communication. The apparatus generally includes means for tracking sets of RF exposures separately across a plurality of locations associated with a human body over time, and means for transmitting a first signal at a first transmit power determined based on a time-averaged RF exposure limit and at least one of the tracked sets of RF exposures. 
     Aspects of the present disclosure provide a computer-readable medium. The computer-readable medium generally includes instructions stored thereon for tracking sets of RF exposures separately across a plurality of locations associated with a human body over time; and for transmitting a first signal at a first transmit power determined based on a time-averaged RF exposure limit and at least one of the tracked sets of RF exposures. 
     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 appended 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above-recited features of the present disclosure can be understood in detail, a more particular description, briefly summarized above, may be had by reference to aspects, some of which are illustrated in the drawings. It is to be noted, however, that the appended drawings illustrate only certain typical aspects of this disclosure and are therefore not to be considered limiting of its scope, for the description may admit to other equally effective aspects. 
         FIG.  1    is a block diagram conceptually illustrating an example wireless communication network, in accordance with certain aspects of the present disclosure. 
         FIG.  2    is a block diagram conceptually illustrating a design of an example a base station (BS) and user equipment (UE), in accordance with certain aspects of the present disclosure. 
         FIG.  3    is a block diagram of an example radio frequency (RF) transceiver, in accordance with certain aspects of the present disclosure. 
         FIG.  4    is a diagram illustrating an example of a normalized specific absorption rate (SAR) distribution combined with a normalized power density (PD) distribution, in accordance with certain aspects of the present disclosure. 
         FIGS.  5 A and  5 B  are graphs illustrating examples of transmit powers over time in compliance with a time-averaged RF exposure limit, in accordance with certain aspects of the present disclosure. 
         FIG.  6    is a diagram illustrating a system for measuring RF exposure distributions, in accordance with certain aspects of the present disclosure. 
         FIG.  7    is a flow diagram illustrating example operations for grouping antennas for RF exposure compliance by a UE, in accordance with certain aspects of the present disclosure. 
         FIG.  8    is a block diagram illustrating an example grouping of multiple antennas of a wireless communication device, in accordance with certain aspects of the present disclosure. 
         FIG.  9    is a flow diagram illustrating example operations for determining backoff factors for antenna groups, in accordance with certain aspects of the present disclosure. 
         FIG.  10    is a flow diagram illustrating example operations for assigning antennas to groups based on the backoff factors, in accordance with certain aspects of the present disclosure. 
         FIG.  11    is a flow diagram illustrating example operations for wireless communication by a UE, in accordance with certain aspects of the present disclosure. 
         FIG.  12    illustrates a diagram of example locations of a wireless device relative to a body of a user, in accordance with certain aspects of the present disclosure. 
         FIG.  13    illustrates example RF exposure time windows, in accordance with certain aspects of the present disclosure. 
         FIG.  14    illustrates example RF exposure settings for certain exposure scenarios and/or exposure categories, in accordance with certain aspects of the present disclosure. 
         FIG.  15    illustrates various antenna groupings for exposure categories, in accordance with certain aspects of the present disclosure. 
         FIG.  16    is a flow diagram illustrating example operations for wireless communication by a wireless device, in accordance with certain aspects of the present disclosure. 
         FIG.  17    illustrates a communications device (e.g., a UE) that may include various components configured to perform operations for the techniques disclosed herein, in accordance with certain aspects of the present disclosure. 
         FIG.  18    illustrates a communications device (e.g., a UE) that may include various components configured to perform operations for the techniques disclosed herein, in accordance with certain aspects of the present disclosure. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one aspect may be beneficially utilized on other aspects without specific recitation. 
     DETAILED DESCRIPTION 
     Aspects of the present disclosure provide apparatus, methods, processing systems, and computer-readable mediums for complying with radio frequency (RF) exposure. The RF exposure may be determined and/or tracked across different tissues and/or locations on a body of a user. Certain examples determine and/or operate in compliance with RF exposure limit per exposure scenario and/or category. 
     A wireless communication device may adjust certain RF exposure settings (e.g., backoff factor, antenna grouping, RF exposure limit, etc.) in response to a change in RF exposure scenario (e.g., head exposure, body exposure, extremity exposure, or hotspot exposure). A device state index (DSI) may be representative of or associated with a specific RF exposure scenario, such that a specific DSI may refer to a particular RF exposure scenario. During regulatory certification, the RF exposure may be characterized for different usage conditions or exposure scenarios (e.g., DSIs) including, for example, head, body-worn (trunk region), hotspot (trunk region), and extremity (hand or feet region) scenarios. 
     In certain cases, the RF exposure produced by a wireless device may be tracked over time independent of the RF exposure scenarios exhibited by the wireless device. For example, when an exposure scenario changes from head to body-worn or vice versa, the wireless device may consider the RF exposure produced in the head exposure scenario when evaluating the time-averaged RF exposure compliance for a transmission in the body-worn exposure scenario. In such cases, the wireless device may operate at a reduced transmit power, for example, due to RF exposure history from different exposure scenarios being combined in a time-averaged RF exposure evaluation. 
     Aspects of the present disclosure provide techniques and apparatus for evaluating the time-averaged RF exposure per RF exposure scenario and/or RF exposure category. When an exposure scenario changes from head to body-worn or vice versa, for example, the exposure from an active radio may not expose the same tissue to RF energy, and therefore, a wireless communication device may take into account or consider the change in exposure scenario when evaluating time-averaged RF exposure compliance. As an example, the time-averaged RF exposure encountered for a head exposure scenario may be evaluated separately from the time-averaged RF exposure encountered for an extremity exposure scenario in some configurations, such that the RF exposure history for each RF exposure scenario is tracked and/or assessed separately, as further described herein. The RF exposure time averaging may be performed per exposure scenario or exposure category, which may include one or more exposure scenarios, as further described herein. 
     The RF exposure compliance (e.g., per exposure scenario/category) described herein may enable desirable transmit power for specific radios, antennas, and/or antenna groups, for example, due to differing exposure encountered by each. The desirable transmit power may provide desirable uplink performance, such as higher uplink data rates, reduced latencies, enhanced uplink carrier aggregation, and/or enhanced communication range (e.g., an uplink connection at the edge of a cell). 
     The following description provides examples of RF exposure compliance in communication systems, and is not limiting of the scope, applicability, or examples set forth in the claims. Changes may be made in the function and arrangement of elements discussed without departing from the scope of the disclosure. Various examples may omit, substitute, or add various procedures or components as appropriate. For instance, the methods described may be performed in an order different from that described, and various steps may be added, omitted, or combined. Also, features described with respect to some examples may be combined in some other examples. For example, an apparatus may be implemented or a method may be practiced using any number of the aspects set forth herein. In addition, the scope of the disclosure is intended to cover such an apparatus or method which is practiced using other structure, functionality, or structure and functionality in addition to, or other than, the various aspects of the disclosure set forth herein. It should be understood that any aspect of the disclosure disclosed herein may be embodied by one or more elements of a claim. 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. 
     In general, any number of wireless networks may be deployed in a given geographic area. Each wireless network may support a particular radio access technology (RAT) and may operate on one or more frequencies. A RAT may also be referred to as a radio technology, an air interface, etc. A frequency may also be referred to as a carrier, a subcarrier, a frequency channel, a tone, a subband, etc. Each frequency may support a single RAT in a given geographic area in order to avoid interference between wireless networks of different RATs, or may support multiple RATs. 
     The techniques described herein may be used for various wireless networks and radio technologies. While aspects may be described herein using terminology commonly associated with 3G, 4G, and/or new radio (e.g., 5G NR) wireless technologies, aspects of the present disclosure can be applied in other generation-based communication systems and/or to wireless technologies such as 802.11, 802.15, etc. 
     NR access may support various wireless communication services, such as enhanced mobile broadband (eMBB) targeting wide bandwidth (e.g., 80 MHz or beyond), millimeter wave (mmWave) targeting high carrier frequency (e.g., 24 GHz to 53 GHz or beyond), massive machine type communications MTC (mMTC) targeting non-backward compatible MTC techniques, and/or mission critical targeting ultra-reliable low-latency communications (URLLC). These services may include latency and reliability requirements. These services may also have different transmission time intervals (TTI) to meet respective quality of service (QoS) requirements. In addition, these services may co-exist in the same subframe. NR supports beamforming, and beam direction may be dynamically configured. Multiple-input, multiple-output (MIMO) transmissions with precoding may also be supported, as may multi-layer transmissions. Aggregation of multiple cells may be supported. 
     Example Wireless Communication Network and Devices 
       FIG.  1    illustrates an example wireless communication network  100  in which aspects of the present disclosure may be performed. For example, the wireless communication network  100  may be an NR system (e.g., a 5G NR network), an Evolved Universal Terrestrial Radio Access (E-UTRA) system (e.g., a 4G network), a Universal Mobile Telecommunications System (UMTS) (e.g., a 2G/3G network), or a code division multiple access (CDMA) system (e.g., a 2G/3G network), or may be configured for communications according to an IEEE standard such as one or more of the 802.11 standards, etc. As shown in  FIG.  1   , the UE  120   a  includes an RF exposure manager  122  that enforces RF exposure compliance (e.g., per exposure scenario/category), as further described herein. In some examples, RF exposure is managed among mutually exclusive exposure scenarios and/or categories, in accordance with aspects of the present disclosure. 
     As illustrated in  FIG.  1   , the wireless communication network  100  may include a number of BSs  110   a - z  (each also individually referred to herein as BS  110  or collectively as BSs  110 ) and other network entities. A BS  110  may provide communication coverage for a particular geographic area, sometimes referred to as a “cell,” which may be stationary or may move according to the location of a mobile BS  110 . In some examples, the BSs  110  may be interconnected to one another and/or to one or more other BSs or network nodes (not shown) in wireless communication network  100  through various types of backhaul interfaces (e.g., a direct physical connection, a wireless connection, a virtual network, or the like) using any suitable transport network. In the example shown in  FIG.  1   , the BSs  110   a ,  110   b , and  110   c  may be macro BSs for the macro cells  102   a ,  102   b , and  102   c , respectively. The BS  110   x  may be a pico BS for a pico cell  102   x . The BSs  110   y  and  110   z  may be femto BSs for the femto cells  102   y  and  102   z , respectively. ABS may support one or multiple cells. 
     The BSs  110  communicate with UEs  120   a - y  (each also individually referred to herein as UE  120  or collectively as UEs  120 ) in the wireless communication network  100 . The UEs  120  (e.g.,  120   x ,  120   y , etc.) may be dispersed throughout the wireless communication network  100 , and each UE  120  may be stationary or mobile. Wireless communication network  100  may also include relay stations (e.g., relay station  110   r ), also referred to as relays or the like, that receive a transmission of data and/or other information from an upstream station (e.g., a BS  110   a  or a UE  120   r ) and sends a transmission of the data and/or other information to a downstream station (e.g., a UE  120  or a BS  110 ), or that relays transmissions between UEs  120 , to facilitate communication between devices. 
     A network controller  130  may be in communication with a set of BSs  110  and provide coordination and control for these BSs  110  (e.g., via a backhaul). In certain cases, the network controller  130  may include a centralized unit (CU) and/or a distributed unit (DU), for example, in a 5G NR system. In aspects, the network controller  130  may be in communication with a core network  132  (e.g., a 5G Core Network (5GC)), which provides various network functions such as Access and Mobility Management, Session Management, User Plane Function, Policy Control Function, Authentication Server Function, Unified Data Management, Application Function, Network Exposure Function, Network Repository Function, Network Slice Selection Function, etc. 
       FIG.  2    illustrates example components of BS  110   a  and UE  120   a  (e.g., the wireless communication network  100  of  FIG.  1   ), which may be used to implement aspects of the present disclosure. 
     At the BS  110   a , a transmit processor  220  may receive data from a data source  212  and control information from a controller/processor  240 . The control information may be for the physical broadcast channel (PBCH), physical control format indicator channel (PCFICH), physical hybrid ARQ indicator channel (PHICH), physical downlink control channel (PDCCH), group common PDCCH (GC PDCCH), etc. The data may be for the physical downlink shared channel (PDSCH), etc. A medium access control (MAC)-control element (MAC-CE) is a MAC layer communication structure that may be used for control command exchange between wireless nodes. The MAC-CE may be carried in a shared channel such as a PDSCH, a physical uplink shared channel (PUSCH), or a physical sidelink shared channel (PSSCH). 
     The processor  220  may process (e.g., encode and symbol map) the data and control information to obtain data symbols and control symbols, respectively. The transmit processor  220  may also generate reference symbols, such as for the primary synchronization signal (PSS), secondary synchronization signal (SSS), PBCH demodulation reference signal (DMRS), and channel state information reference signal (CSI-RS). A transmit (TX) multiple-input multiple-output (MIMO) processor  230  may perform spatial processing (e.g., precoding) on the data symbols, the control symbols, and/or the reference symbols, if applicable, and may provide output symbol streams to the modulators (MODs) in transceivers  232   a - 232   t . Each modulator in transceivers  232   a - 232   t  may process a respective output symbol stream (e.g., for OFDM, etc.) to obtain an output sample stream. Each of the transceivers  232   a - 232   t  may further process (e.g., convert to analog, amplify, filter, and upconvert) the output sample stream to obtain a downlink signal. Downlink signals from the transceivers  232   a - 232   t  may be transmitted via the antennas  234   a - 234   t , respectively. 
     At the UE  120   a , the antennas  252   a - 252   r  may receive the downlink signals from the BS  110   a  and may provide received signals to the transceivers  254   a - 254   r , respectively. The transceivers  254   a - 254   r  may condition (e.g., filter, amplify, downconvert, and digitize) a respective received signal to obtain input samples. Each demodulator (DEMOD) in the transceivers  232   a - 232   t  may further process the input samples (e.g., for OFDM, etc.) to obtain received symbols. A MIMO detector  256  may obtain received symbols from all the demodulators in transceivers  254   a - 254   r , perform MIMO detection on the received symbols if applicable, and provide detected symbols. A receive processor  258  may process (e.g., demodulate, deinterleave, and decode) the detected symbols, provide decoded data for the UE  120   a  to a data sink  260 , and provide decoded control information to a controller/processor  280 . 
     On the uplink, at UE  120   a , a transmit processor  264  may receive and process data (e.g., for the physical uplink shared channel (PUSCH)) from a data source  262  and control information (e.g., for the physical uplink control channel (PUCCH)) from the controller/processor  280 . The transmit processor  264  may also generate reference symbols for a reference signal (e.g., for the sounding reference signal (SRS)). The symbols from the transmit processor  264  may be precoded by a TX MIMO processor  266  if applicable, further processed by the modulators (MODs) in transceivers  254   a - 254   r  (e.g., for SC-FDM, etc.), and transmitted to the BS  110   a . At the BS  110   a , the uplink signals from the UE  120   a  may be received by the antennas  234 , processed by the demodulators in transceivers  232   a - 232   t , detected by a MIMO detector  236  if applicable, and further processed by a receive processor  238  to obtain decoded data and control information sent by the UE  120   a . The receive processor  238  may provide the decoded data to a data sink  239  and the decoded control information to the controller/processor  240 . 
     The memories  242  and  282  may store data and program codes for BS  110   a  and UE  120   a , respectively. A scheduler  244  may schedule UEs for data transmission on the downlink and/or uplink. 
     Antennas  252 , processors  266 ,  258 ,  264 , and/or controller/processor  280  of the UE  120   a  and/or antennas  234 , processors  220 ,  230 ,  238 , and/or controller/processor  240  of the BS  110   a  may be used to perform the various techniques and methods described herein. As shown in  FIG.  2   , the controller/processor  280  of the UE  120   a  has an RF exposure manager  281  that enforces RF exposure compliance and is representative of the RF exposure manager  122 . In some examples, RF exposure is effected among mutually exclusive antenna groups assigned to a particular exposure scenario/category, according to aspects described herein. Although shown at the controller/processor, other components of the UE  120   a  and BS  110   a  may be used to perform the operations described herein. 
     NR may utilize orthogonal frequency division multiplexing (OFDM) with a cyclic prefix (CP) on the uplink and downlink. NR may support half-duplex operation using time division duplexing (TDD). OFDM and single-carrier frequency division multiplexing (SC-FDM) partition the system bandwidth into multiple orthogonal subcarriers, which are also commonly referred to as tones, bins, etc. Each subcarrier may be modulated with data. Modulation symbols may be sent in the frequency domain with OFDM and in the time domain with SC-FDM. The spacing between adjacent subcarriers may be fixed, and the total number of subcarriers may be dependent on the system bandwidth. The system bandwidth may also be partitioned into subbands. For example, a subband may cover multiple resource blocks (RBs). 
     While the UE  120   a  is described with respect to  FIGS.  1  and  2    as communicating with a BS and/or within a network, the UE  120   a  may be configured to communicate directly with/transmit directly to another UE  120 , or with/to another wireless device without relaying communications through a network. In some aspects, the BS  110   a  illustrated in  FIG.  2    and described above is an example of another UE  120 . 
     Example RF Transceiver 
       FIG.  3    is a block diagram of an example RF transceiver circuit  300 , in accordance with certain aspects of the present disclosure. The RF transceiver circuit  300  includes at least one transmit (TX) path  302  (also known as a transmit chain) for transmitting signals via one or more antennas  306  and at least one receive (RX) path  304  (also known as a receive chain) for receiving signals via the antennas  306 . When the TX path  302  and the RX path  304  share an antenna  306 , the paths may be connected with the antenna via an interface  308 , which may include any of various suitable RF devices, such as a switch, a duplexer, a diplexer, a multiplexer, and the like. 
     Receiving in-phase (I) or quadrature (Q) baseband analog signals from a digital-to-analog converter (DAC)  310 , the TX path  302  may include a baseband filter (BBF)  312 , a mixer  314 , a driver amplifier (DA)  316 , and a power amplifier (PA)  318 . The BBF  312 , the mixer  314 , and the DA  316  may be included in one or more radio frequency integrated circuits (RFICs). The PA  318  may be external to the RFIC(s) for some implementations. 
     The BBF  312  filters the baseband signals received from the DAC  310 , and the mixer  314  mixes the filtered baseband signals with a transmit local oscillator (LO) signal to convert the baseband signal of interest to a different frequency (e.g., upconvert from baseband to a radio frequency). This frequency conversion process produces the sum and difference frequencies between the LO frequency and the frequencies of the baseband signal of interest. The sum and difference frequencies are referred to as the beat frequencies. The beat frequencies are typically in the RF range, such that the signals output by the mixer  314  are typically RF signals, which may be amplified by the DA  316  and/or by the PA  318  before transmission by the antenna  306 . While one mixer  314  is illustrated, several mixers may be used to upconvert the filtered baseband signals to one or more intermediate frequencies and to thereafter upconvert the intermediate frequency signals to a frequency for transmission. 
     The RX path  304  may include a low noise amplifier (LNA)  324 , a mixer  326 , and a baseband filter (BBF)  328 . The LNA  324 , the mixer  326 , and the BBF  328  may be included in one or more RFICs, which may or may not be the same RFIC that includes the TX path components. RF signals received via the antenna  306  may be amplified by the LNA  324 , and the mixer  326  mixes the amplified RF signals with a receive local oscillator (LO) signal to convert the RF signal of interest to a different baseband frequency (e.g., downconvert). The baseband signals output by the mixer  326  may be filtered by the BBF  328  before being converted by an analog-to-digital converter (ADC)  330  to digital I or Q signals for digital signal processing. 
     Certain transceivers may employ frequency synthesizers with a voltage-controlled oscillator (VCO) to generate a stable, tunable LO with a particular tuning range. Thus, the transmit LO may be produced by a TX frequency synthesizer  320 , which may be buffered or amplified by amplifier  322  before being mixed with the baseband signals in the mixer  314 . Similarly, the receive LO may be produced by an RX frequency synthesizer  332 , which may be buffered or amplified by amplifier  334  before being mixed with the RF signals in the mixer  326 . 
     A controller  336  may direct the operation of the RF transceiver circuit  300 , such as transmitting signals via the TX path  302  and/or receiving signals via the RX path  304 . The controller  336  may be a processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device (PLD), discrete gate or transistor logic, discrete hardware components, or any combination thereof. The memory  338  may store data and program codes for operating the RF transceiver circuit  300 . The controller  336  and/or memory  338  may include control logic. In certain cases, the controller  336  may determine a time-averaged RF exposure based on transmission power levels applied to the TX path  302  (e.g., certain levels of gain applied to the BBF  312 , the DA  316 , and/or the PA  318 ) to set a transmission power level that complies with an RF exposure limit set by domestic regulations and international standards as further described herein. 
     Example RF Exposures 
     RF exposure may be expressed in terms of a specific absorption rate (SAR), which measures energy absorption by human tissue per unit mass and may have units of watts per kilogram (W/kg). RF exposure may also be expressed in terms of power density (PD), which measures energy absorption per unit area and may have units of mW/cm 2 . In certain cases, a maximum permissible exposure (MPE) limit in terms of PD may be imposed for wireless communication devices using transmission frequencies above 6 GHz. The MPE limit is a regulatory metric for exposure based on area, e.g., an energy density limit defined as a number, X, watts per square meter (W/m 2 ) averaged over a defined area and time-averaged over a frequency-dependent time window in order to prevent a human exposure hazard represented by a tissue temperature change. 
     SAR may be used to assess RF exposure for transmission frequencies less than 6 GHz, which cover wireless communication technologies such as 2G/3G (e.g., CDMA), 4G (e.g., LTE), 5G (e.g., NR in 6 GHz bands), IEEE 802.11ac, etc. PD may be used to assess RF exposure for transmission frequencies higher than 6 GHz, which cover wireless communication technologies such as IEEE 802.11ad, 802.11ay, 5G in mmWave bands, etc. Thus, different metrics may be used to assess RF exposure for different wireless communication technologies. 
     A wireless communication device (e.g., UE  120 ) may simultaneously transmit signals using multiple wireless communication technologies. For example, the wireless communication device may simultaneously transmit signals using a first wireless communication technology operating at or below 6 GHz (e.g., 3G, 4G, 5G, etc.) and a second wireless communication technology operating above 6 GHz (e.g., mmWave 5G in 24 to 60 GHz bands, IEEE 802.11ad or 802.11ay). In certain aspects, the wireless communication device may simultaneously transmit signals using the first wireless communication technology (e.g., 3G, 4G, 5G in sub-6 GHz bands, IEEE 802.11ac, etc.) in which RF exposure is measured in terms of SAR, and the second wireless communication technology (e.g., 5G in 24 to 60 GHz bands, IEEE 802.11ad, 802.11ay, etc.) in which RF exposure is measured in terms of PD. As used herein, sub-6 GHz bands may include frequency bands of 300 MHz to 6,000 MHz in some examples, and may include bands in the 6,000 MHz and/or 7,000 MHz range in some examples. 
     To assess RF exposure from transmissions using the first technology (e.g., 3G, 4G, 5G in sub-6 GHz bands, IEEE 802.11ac, etc.), the wireless communication device may include multiple SAR values and/or distributions for the first technology stored in memory (e.g., memory  282  of  FIG.  2    or memory  338  of  FIG.  3   ). Each of the SAR values and/or distributions may correspond to a respective one of multiple transmit scenarios supported by the wireless communication device for the first technology. The transmit scenarios may correspond to various combinations of antennas (e.g., antennas  252   a  through  252   r  of  FIG.  2    or antenna  306  of  FIG.  3   ), frequency bands, channels, and/or body positions, as discussed further below. In some examples, the stored SAR includes a single value (e.g., a peak value determined based on the description below, or a sum of peak values). 
     The SAR values and/or distribution (also referred to as a SAR map) for each transmit scenario may be generated based on measurements (e.g., E-field measurements) performed in a test laboratory using a model of a human body. After generation, they are stored in the memory to enable a processor (e.g., processor  280  of  FIG.  2    or controller  336  of  FIG.  3   ) to assess RF exposure in real time, as discussed further below. Each SAR distribution may include a set of SAR values, where each SAR value may correspond to a different location (e.g., on the model of the human body). Each SAR value may comprise a SAR value averaged over a mass of 1 g or 10 g at the respective location. 
     The SAR values in each SAR distribution correspond to a particular transmission power level (e.g., the transmission power level at which the SAR values were measured in the test laboratory). Since SAR scales with transmission power level, the processor may scale a SAR value or distribution for any transmission power level by multiplying each SAR value (e.g., in the SAR distribution) by the following transmission power scaler: 
     
       
         
           
             
               
                 
                   
                     Tx 
                     c 
                   
                   
                     Tx 
                     SAR 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where Tx c  is a current transmission power level for the respective transmit scenario, and Tx SAR  is the transmission power level corresponding to the SAR values (e.g., the transmission power level at which the SAR values were measured in the test laboratory). 
     As discussed above, the wireless communication device may support multiple transmit scenarios for the first technology. In certain aspects, the transmit scenarios may be specified by a set of parameters. The set of parameters may include one or more of the following: an antenna parameter indicating one or more antennas used for transmission (i.e., active antennas), a frequency band parameter indicating one or more frequency bands used for transmission (i.e., active frequency bands), a channel parameter indicating one or more channels used for transmission (i.e., active channels), a body position parameter (e.g., a device state index (DSI)) indicating the location of the wireless communication device relative to the user&#39;s body location (head, trunk, away from the body, etc.), exposure category, and/or other parameters. In cases where the wireless communication device supports a large number of transmit scenarios, it may be very time-consuming and expensive to perform measurements for each transmit scenario in a test setting (e.g., test laboratory). To reduce test time, measurements may be performed for a subset of the transmit scenarios to generate SAR values and/or distributions for the subset of transmit scenarios. In this example, the SAR values and/or distributions for each of the remaining transmit scenarios may be generated by combining two or more of the SAR values and/or distributions for the subset of transmit scenarios, as discussed further below. 
     For example, SAR measurements may be performed for each one of the antennas to generate a SAR value or distribution for each one of the antennas. In this example, a SAR value or distribution for a transmit scenario in which two or more of the antennas are active may be generated by combining the SAR values or distributions for the two or more active antennas. 
     In another example, SAR measurements may be performed for each one of multiple frequency bands to generate a SAR value or distribution for each one of the multiple frequency bands. In this example, a SAR value or distribution for a transmit scenario in which two or more frequency bands are active may be generated by combining the SAR values or distributions for the two or more active frequency bands. 
     In certain aspects, a SAR distribution may be normalized with respect to a SAR limit by dividing each SAR value in the SAR distribution by the SAR limit. In this case, a normalized SAR value exceeds the SAR limit when the normalized SAR value is greater than one, and is below the SAR limit when the normalized SAR value is less than one. In these aspects, each of the SAR distributions stored in the memory may be normalized with respect to a SAR limit. Similarly, a single or individual SAR value may be normalized with respect to a SAR limit. 
     In certain aspects, the normalized SAR value or distribution for a transmit scenario may be generated by combining two or more normalized values or SAR distributions. For example, a normalized SAR value or distribution for a transmit scenario in which two or more antennas are active may be generated by combining the normalized SAR values or distributions for the two or more active antennas. For the case in which different transmission power levels are used for the active antennas, the normalized SAR value or distribution for each active antenna may be scaled by the respective transmission power level before combining the normalized SAR values or distributions for the active antennas. The normalized SAR value or distribution for simultaneous transmission from multiple active antennas may be given by the following: 
     
       
         
           
             
               
                 
                   
                     SAR 
                     
                       norm 
                       ⁢ 
                       _ 
                       ⁢ 
                       combined 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         i 
                         = 
                         1 
                       
                       
                         i 
                         = 
                         K 
                       
                     
                     
                       
                         
                           Tx 
                           i 
                         
                         
                           Tx 
                           SARi 
                         
                       
                       · 
                       
                         
                           SAR 
                           i 
                         
                         
                           SAR 
                           lim 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     where SAR lim  is a SAR limit, SAR norm_combined  is the combined normalized SAR value or distribution for simultaneous transmission from the active antennas, i is an index for the active antennas, SARI is the SAR value or distribution for the i th  active antenna, Tx i  is the transmission power level for the i th  active antenna, Tx SARi  is the transmission power level for the SAR distribution for the i th  active antenna, and K is the number of the active antennas. 
     Equation (2) may be rewritten as follows: 
     
       
         
           
             
               
                 
                   
                     SAR 
                     
                       norm 
                       ⁢ 
                       _ 
                       ⁢ 
                       combined 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         i 
                         = 
                         1 
                       
                       
                         i 
                         = 
                         K 
                       
                     
                     
                       
                         
                           Tx 
                           i 
                         
                         
                           Tx 
                           SARi 
                         
                       
                       · 
                       
                         SAR 
                         
                           norm 
                           ⁢ 
                           _ 
                           ⁢ 
                           i 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     3 
                     ⁢ 
                     a 
                   
                   ) 
                 
               
             
           
         
       
     
     where SAR norm_i  is the normalized SAR value or distribution for the i th  active antenna. In the case of simultaneous transmissions using multiple active antennas at the same transmitting frequency (e.g., multiple input, multiple output (MIMO)), the combined normalized SAR value or distribution may be obtained by summing the square root of the individual normalized SAR values or distributions and computing the square of the sum, as given by the following: 
     
       
         
           
             
               
                 
                   
                     SAR 
                     
                       norm 
                       ⁢ 
                       _ 
                       ⁢ 
                       combined 
                       ⁢ 
                       _ 
                       ⁢ 
                       MIMO 
                     
                   
                   = 
                   
                     
                       
                         [ 
                         
                           
                             ∑ 
                             
                               i 
                               = 
                               1 
                             
                             
                               i 
                               = 
                               K 
                             
                           
                           
                             
                               
                                 
                                   Tx 
                                   i 
                                 
                                 
                                   Tx 
                                   SARi 
                                 
                               
                               · 
                               
                                 SAR 
                                 
                                   norm 
                                   ⁢ 
                                   _ 
                                   ⁢ 
                                   i 
                                 
                               
                             
                           
                         
                         ] 
                       
                       2 
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   
                     3 
                     ⁢ 
                     b 
                   
                   ) 
                 
               
             
           
         
       
     
     In another example, normalized SAR values or distributions for different frequency bands may be stored in the memory. In this example, a normalized SAR distribution for a transmit scenario in which two or more frequency bands are active may be generated by combining the normalized SAR distributions for the two or more active frequency bands. For the case where the transmission power levels are different for the active frequency bands, the normalized SAR value or distribution for each of the active frequency bands may be scaled by the respective transmission power level before combining the normalized SAR values or distributions for the active frequency bands. In this example, the combined SAR value or distribution may also be computed using Equation (3a) in which i is an index for the active frequency bands, SAR norm_i  is the normalized SAR value or distribution for the i th  active frequency band, Tx i  is the transmission power level for the i th  active frequency band, and Tx SARi  is the transmission power level for the normalized SAR value or distribution for the i th  active frequency band. 
     To assess RF exposure from transmissions using the second technology (e.g., 5G in 24 to 60 GHz bands, IEEE 802.11ad, 802.11ay, etc.), the wireless communication device may include multiple PD values and/or distributions for the second technology stored in the memory (e.g., memory  282  of  FIG.  2    or memory  338  of  FIG.  3   ). Each of the PD values or distributions may correspond to a respective one of multiple transmit scenarios supported by the wireless communication device for the second technology. The transmit scenarios may correspond to various combinations of antennas (e.g., antennas  252   a  through  252   r  of  FIG.  2    or antenna  306  of  FIG.  3   ), frequency bands, channels, and/or body positions, as discussed further below. In some examples, the stored PD includes a single value (e.g., a peak value determined based on the description below, or a sum of peak values). 
     The PD values and/or distribution (also referred to as PD map) for each transmit scenario may be generated based on measurements (e.g., E-field measurements) performed in a test laboratory using a model of a human body. After generation, they are stored in the memory to enable the processor (e.g., processor  280  of  FIG.  2    or controller  336  of  FIG.  3   ) to assess RF exposure in real time, as discussed further below. Each PD distribution may include a set of PD values, where each PD value may correspond to a different location (e.g., on the model of the human body). 
     The PD values in each PD distribution correspond to a particular transmission power level (e.g., the transmission power level at which the PD values were measured in the test laboratory). Since PD scales with transmission power level, the processor may scale a PD value or distribution for any transmission power level by multiplying each PD value (e.g., in the PD distribution) by the following transmission power scaler: 
     
       
         
           
             
               
                 
                   
                     Tx 
                     c 
                   
                   
                     Tx 
                     PD 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     where Tx c  is a current transmission power level for the respective transmit scenario, and Tx PD  is the transmission power level corresponding to the PD values (e.g., the transmission power level at which the PD values were measured in the test laboratory). 
     As discussed above, the wireless communication device may support multiple transmit scenarios for the second technology. In certain aspects, the transmit scenarios may be specified by a set of parameters. The set of parameters may include one or more of the following: an antenna parameter indicating one or more antennas used for transmission (i.e., active antennas), a frequency band parameter indicating one or more frequency bands used for transmission (i.e., active frequency bands), a channel parameter indicating one or more channels used for transmission (i.e., active channels), a body position parameter (e.g., a DSI) indicating the location of the wireless communication device relative to the user&#39;s body location (head, trunk, away from the body, etc.), exposure category, and/or other parameters. In cases where the wireless communication device supports a large number of transmit scenarios, it may be very time-consuming and expensive to perform measurements for each transmit scenario in a test setting (e.g., test laboratory). To reduce test time, measurements may be performed for a subset of the transmit scenarios to generate PD values and/or distributions for the subset of transmit scenarios. In this example, the PD values and/or distributions for each of the remaining transmit scenarios may be generated by combining two or more of the PD values and/or distributions for the subset of transmit scenarios, as discussed further below. 
     For example, PD measurements may be performed for each one of the antennas to generate a PD value or distribution for each one of the antennas. In this example, a PD value or distribution for a transmit scenario in which two or more of the antennas are active may be generated by combining the PD values or distributions for the two or more active antennas. 
     In another example, PD measurements may be performed for each one of multiple frequency bands to generate a PD value or distribution for each one of the multiple frequency bands. In this example, a PD value or distribution for a transmit scenario in which two or more frequency bands are active may be generated by combining the PD values or distributions for the two or more active frequency bands. 
     In certain aspects, a PD distribution may be normalized with respect to a PD limit by dividing each PD value in the PD distribution by the PD limit. In this case, a normalized PD value exceeds the PD limit when the normalized PD value is greater than one, and is below the PD limit when the normalized PD value is less than one. In these aspects, each of the PD distributions stored in the memory may be normalized with respect to a PD limit. Similarly, a single or individual PD value may be normalized with respect to a PD limit. 
     In certain aspects, the normalized PD value or distribution for a transmit scenario may be generated by combining two or more normalized PD values or distributions. For example, a normalized PD value or distribution for a transmit scenario in which two or more antennas are active may be generated by combining the normalized PD values or distributions for the two or more active antennas. For the case in which different transmission power levels are used for the active antennas, the normalized PD value or distribution for each active antenna may be scaled by the respective transmission power level before combining the normalized PD values or distributions for the active antennas. The normalized PD value or distribution for simultaneous transmission from multiple active antennas may be given by the following: 
     
       
         
           
             
               
                 
                   
                     PD 
                     
                       norm 
                       ⁢ 
                       _ 
                       ⁢ 
                       combined 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         i 
                         = 
                         1 
                       
                       
                         i 
                         = 
                         L 
                       
                     
                     
                       
                         
                           Tx 
                           i 
                         
                         
                           Tx 
                           PDi 
                         
                       
                       · 
                       
                         
                           PD 
                           i 
                         
                         
                           PD 
                           lim 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     where PD lim  is a PD limit, PD norm_combined  is the combined normalized PD value or distribution for simultaneous transmission from the active antennas, i is an index for the active antennas, PD i  is the PD value or distribution for the i th  active antenna, Tx i  is the transmission power level for the i th  active antenna, Tx PDi  is the transmission power level for the PD distribution for the i th  active antenna, and L is the number of the active antennas. 
     Equation (5) may be rewritten as follows: 
     
       
         
           
             
               
                 
                   
                     PD 
                     
                       norm 
                       ⁢ 
                       _ 
                       ⁢ 
                       combined 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         i 
                         = 
                         1 
                       
                       
                         i 
                         = 
                         L 
                       
                     
                     
                       
                         
                           Tx 
                           i 
                         
                         
                           Tx 
                           PDi 
                         
                       
                       · 
                       
                         PD 
                         
                           norm 
                           ⁢ 
                           _ 
                           ⁢ 
                           i 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     6 
                     ⁢ 
                     a 
                   
                   ) 
                 
               
             
           
         
       
     
     where PD norm_i  is the normalized PD value or distribution for the i th  active antenna. In the case of simultaneous transmissions using multiple active antennas at the same transmitting frequency (e.g., MIMO), the combined normalized PD value or distribution may be obtained by summing the square root of the individual normalized PD values or distributions and computing the square of the sum, as given by the following: 
     
       
         
           
             
               
                 
                   
                     PD 
                     
                       norm 
                       ⁢ 
                       _ 
                       ⁢ 
                       combined 
                       ⁢ 
                       _ 
                       ⁢ 
                       MIMO 
                     
                   
                   = 
                   
                     
                       
                         [ 
                         
                           
                             ∑ 
                             
                               i 
                               = 
                               1 
                             
                             
                               i 
                               = 
                               L 
                             
                           
                           
                             
                               
                                 
                                   Tx 
                                   i 
                                 
                                 
                                   Tx 
                                   PDi 
                                 
                               
                               · 
                               
                                 PD 
                                 
                                   norm 
                                   ⁢ 
                                   _ 
                                   ⁢ 
                                   i 
                                 
                               
                             
                           
                         
                         ] 
                       
                       2 
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   
                     6 
                     ⁢ 
                     b 
                   
                   ) 
                 
               
             
           
         
       
     
     In another example, normalized PD values or distributions for different frequency bands may be stored in the memory. In this example, a normalized PD value or distribution for a transmit scenario in which two or more frequency bands are active may be generated by combining the normalized PD distributions for the two or more active frequency bands. For the case where the transmission power levels are different for the active frequency bands, the normalized PD value or distribution for each of the active frequency bands may be scaled by the respective transmission power level before combining the normalized PD values or distributions for the active frequency bands. In this example, the combined PD value or distribution may also be computed using Equation (6a) in which i is an index for the active frequency bands, PD norm_i  is the normalized PD value or distribution for the i th  active frequency band, Tx i  is the transmission power level for the i th  active frequency band, and Tx PDi  is the transmission power level for the normalized PD value or distribution for the i th  active frequency band. 
     Example RF Exposure Combinations 
     As discussed above, the UE  120  may simultaneously transmit signals using the first technology (e.g., 3G, 4G, IEEE 802.11ac, etc.) and the second technology (e.g., 5G, IEEE 802.11ad, etc.), in which RF exposure is measured using different metrics for the first technology and the second technology (e.g., SAR for the first technology and PD for the second technology). In this case, the processor  280  may determine a first maximum allowable power level for the first technology and a second maximum allowable power level for the second technology for transmissions in a future time slot that comply with RF exposure limits. During the future time slot, the transmission power levels for the first and second technologies are constrained (i.e., bounded) by the determined first and second maximum allowable power levels, respectively, to ensure compliance with RF exposure limits, as further below. In the present disclosure, the term “maximum allowable power level” refers to a “maximum allowable power level” imposed by an RF exposure limit unless stated otherwise. It is to be appreciated that the “maximum allowable power level” is not necessarily equal to the absolute maximum power level that complies with an RF exposure limit and may be less than the absolute maximum power level that complies with the RF exposure limit (e.g., to provide a safety margin). The “maximum allowable power level” may be used to set a power level limit on a transmission at a transmitter such that the power level of the transmission is not allowed to exceed the “maximum allowable power level” to ensure RF exposure compliance. Certain examples below (in this section and in other sections) are described with respect to SAR and/or PD distributions. It will be appreciated, however, that a distribution may not be required and that individual SAR or PD values may be utilized in most such examples. 
     The processor  280  may determine the first and second maximum allowable power levels as follows. The processor may determine a normalized SAR distribution for the first technology at a first transmission power level, determine a normalized PD distribution for the second technology at a second transmission power level, and combine the normalized SAR distribution and the normalized PD distribution to generate a combined normalized RF exposure distribution (referred to simply as a combined normalized distribution below). The value at each location in the combined normalized distribution may be determined by combining the normalized SAR value at the location with the normalized PD value at the location or another technique. 
     The processor  280  may then determine whether the first and second transmission power levels comply with RF exposure limits by comparing the peak value in the combined normalized distribution with one. If the peak value is equal to or less than one (i.e., satisfies the condition  1 ), then the processor  280  may determine that the first and second transmission power levels comply with RF exposure limits (e.g., SAR limit and PD limit) and use the first and second transmission power levels as the first and second maximum allowable power levels, respectively, during the future time slot. If the peak value is greater than one, then the processor  280  may determine that the first and second transmission power levels do not comply with RF exposure limits. The condition for RF exposure compliance for simultaneous transmissions using the first and second technologies may be given by: 
       SAR norm +PD norm ≤1  (7).
 
       FIG.  4    is a diagram illustrating the normalized SAR distribution  410  and the normalized PD distribution  420 , in which the normalized SAR distribution  410  and the normalized PD distribution  420  are combined to generate a combined normalized distribution  430 .  FIG.  4    also shows the condition that the peak value in the combined normalized distribution  430  be equal to or less than one for RF exposure compliance. Although each of the distributions  410 ,  420 , and  430  is depicted as a two-dimensional distribution in  FIG.  4   , it is to be appreciated that the present disclosure is not limited to this example. 
     The normalized SAR distribution in Expression (7) may be generated by combining two or more normalized SAR distributions as discussed above (e.g., for a transmit scenario using multiple active antennas). Similarly, the normalized PD distribution in Expression (7) may be generated by combining two or more normalized PD distributions as discussed above (e.g., for a transmit scenario using multiple active antennas). In this case, the condition for RF exposure compliance in Expression (7) may be rewritten using Equations (3a) and (6a) as follows: 
     
       
         
           
             
               
                 
                   
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         
                           i 
                           = 
                           K 
                         
                       
                       
                         
                           
                             Tx 
                             i 
                           
                           
                             Tx 
                             SARi 
                           
                         
                         · 
                         
                           SAR 
                           
                             norm 
                             ⁢ 
                             _ 
                             ⁢ 
                             i 
                           
                         
                       
                     
                     + 
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         
                           i 
                           = 
                           L 
                         
                       
                       
                         
                           
                             Tx 
                             i 
                           
                           
                             Tx 
                             PDi 
                           
                         
                         · 
                         
                           PD 
                           
                             norm 
                             ⁢ 
                             _ 
                             ⁢ 
                             i 
                           
                         
                       
                     
                   
                   ≤ 
                   1. 
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     For the MIMO case, Equations (3b) and (6b) may be combined instead. As shown in Expression (8), the combined normalized distribution may be a function of transmission power levels for the first technology and transmission power levels for the second technology. All the points in the combined normalized distribution should meet the normalized limit of one in Expression (8). Additionally, when combining SAR and PD distributions, the SAR and PD distributions can be aligned spatially or aligned with their peak locations so that the combined distribution given by Expression (8) represents combined RF exposure for a given position of a human body. 
     Example RF Exposure Compliance 
     In certain cases, time averaging of RF exposure may be performed to be in compliance with the RF exposure limit within a specified running time window (T) (e.g., 2 seconds for 60 GHz bands, 100 or 360 seconds for bands less than or equal to 6 GHz, etc.) associated with the RF exposure limit.  FIG.  5 A  is a graph  500 A of a transmit power that varies over the time window (T) associated with a time-averaged RF exposure limit, in accordance with certain aspects of the present disclosure. As an example, the transmit power may exceed the maximum allowable time-averaged transmit power level P limit  in certain transmission occasions in the time window (T). In certain cases, a wireless device may transmit at P max , which is the maximum transmit power supported by the wireless device or the maximum instantaneous transmit power the wireless device is capable of outputting. In certain cases, the wireless device may transmit at a transmit power less than or equal to P limit  in certain transmission occasions. P limit  represents the time-averaged threshold for the RF exposure limit in terms of transmit power, and in certain cases, P limit  may be referred to as the average power level or limit. The graph  500 A also illustrates gaps between transmission bursts, where the gaps represent periods during which no transmission was sent from the device. 
     In certain cases, the transmit power may be maintained at the peak transmit power (e.g., P limit ) allowed for RF exposure compliance that enables continuous transmission during the time window. For example,  FIG.  5 B  is a graph  500 B of a transmit power over time illustrating an example where the transmit power is limited to P limit , in accordance with certain aspects of the present disclosure. As shown, the wireless device can transmit continuously at P limit  in the running time window (T) in compliance with the time-averaged RF exposure limit. P limit  represents the maximum transmit power that a wireless device can continuously use in the running time window associated with a time-averaged RF exposure limit. 
     Example RF Exposure Measurements 
     As discussed above, the UE  120  may simultaneously transmit signals using the first technology (e.g., 3G, 4G, IEEE 802.11ac, etc.) and the second technology (e.g., 5G, IEEE 802.11ad, etc.), in which RF exposure is measured using different metrics for the first technology and the second technology (e.g., SAR for the first technology and PD for the second technology). The RF exposure measurements may be performed differently for each transmit scenario and include, for example, electric field measurements using a model of a human body. RF exposure distributions (simulation and/or measurement) may then be generated per transmit antenna/configuration (beam) (as described above) on all evaluation surfaces/positions at all locations. 
       FIG.  6    is a diagram illustrating an example system  600  for measuring RF exposure values or distributions, in accordance with certain aspects of the present disclosure. As shown, the RF exposure measurement system  600  includes a processing system  602 , a robotic RF probe  604 , and a human body model  606 . The RF exposure measurement system  600  may take RF measurements at various transmit scenarios and/or exposure scenarios associated with the UE  120 . In some examples, these measurements may be used to assess suitable backoff factors for the transmit powers of the antenna(s)  252  in compliance with one or more RF exposure limits. In other words, the UE  120  may emit electromagnetic radiation via the antenna(s)  252  at various transmit powers, and the RF exposure measurement system  600  may take RF measurements via the robotic RF probe  604  (e.g., to determine the backoff factors for the antenna(s)  252 ). 
     The processing system  602  may include a processor  608  coupled to a memory  610  via a bus  612 . The processing system  602  may be a computational device such as a computer. The processor  608  may include a central processing unit (CPU), a graphics processing unit (GPU), a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device (PLD), discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. The processor  608  may be in communication with the robotic RF probe  604  via an interface  614  (such as a computer bus interface), such that the processor  608  may obtain RF measurements taken by the robotic RF probe  604  and control the position of the robotic RF probe  604  relative to the human body model  606 , for example. 
     The memory  610  may be configured to store instructions (e.g., computer-executable code) that when executed by the processor  608 , cause the processor  608  to perform various operations. For example, the memory  610  may store instructions for obtaining the RF exposure distributions associated with various RF exposure/transmit scenarios and/or adjusting the position of the robotic RF probe  604 . 
     The robotic RF probe  604  may include an RF probe  616  coupled to a robotic arm  618 . In aspects, the RF probe  616  may be a dosimetric probe capable of measuring RF exposures at various frequencies such as sub-6 GHz bands and/or mmWave bands. The RF probe  616  may be positioned by the robotic arm  618  in various locations (as indicated by the dotted arrows) to capture the electromagnetic radiation emitted by the antenna(s)  252  of the UE  120 . The robotic arm  618  may be a six-axis robot capable of performing precise movements to position the RF probe  616  to the location (on the human body model  606 ) of maximum electromagnetic field generated by the UE  120 . In other words, the robotic arm  618  may provide six degrees of freedom in positioning the RF probe  616  with respect to the antenna(s)  252  of the UE  120  and/or the human body model  606 . 
     The human body model  606  may be a specific anthropomorphic mannequin with simulated human tissue. For example, the human body model  606  may include one or more liquids that simulate the human tissue of the head, body, and/or extremities. The human body model  606  may simulate the human tissue for determining the maximum permissible transmission power of the antenna(s)  252  in compliance with various RF exposure limits. 
     While the example depicted in  FIG.  6    is described herein with respect to obtaining RF exposure values or distributions with a robotic RF probe to facilitate understanding, aspects of the present disclosure may also be applied to other suitable RF probe architectures, such as using multiple stationary RF probes positioned at various locations along the human body model  606 . 
     Example Transmit Antenna Grouping 
     Multi-mode/multi-band UEs have multiple transmit antennas, which can simultaneously transmit in sub-6 GHz bands and bands greater than 6 GHz bands, such as mmWave bands. As described herein, the RF exposure of sub-6 GHz bands may be evaluated in terms of SAR, and the RF exposure of bands greater than 6 GHz may be evaluated in terms of PD. Due to the regulations on simultaneous exposure, the wireless communication device may limit maximum transmit power for both sub-6 GHz bands and bands greater than 6 GHz. 
     In certain cases, although antennas may be positioned in different locations across a UE, a time-averaging algorithm for RF exposure compliance may assume all transmit antennas are collocated in a central location on the UE. Under such an assumption, the total transmit power of all transmit antennas may be limited regardless of the actual exposure scenario (e.g., head exposure, body exposure, or extremity exposure) of separate antennas. For example, suppose the user&#39;s hand covers the location of the collocated model, while specific antennas are not covered by the user&#39;s hand. That is, antennas may contribute to the RF exposure differently depending on the location of the exposure. Enforcing the collocated model may lead to limiting the transmit power of specific antennas not actually covered by the user&#39;s hand. That is, the assumption that the transmit antennas are collocated for RF exposure compliance may provide a needlessly low transmit power, which may affect uplink performance such as uplink data rates, uplink carrier aggregation, and/or an uplink connection at the edge of a cell. 
     Aspects of the present disclosure provide various techniques for grouping antennas, for example, to determine RF exposure compliance on a group basis. In aspects, the antenna groups may be defined and/or operated so as to be mutually exclusive of each other in terms of RF exposure. The RF exposure compliance and corresponding transmit power levels may be determined separately for each antenna group. The antenna grouping described herein may enable relatively higher transmit power for specific antenna groups. An antenna grouping may refer to a specific assignment (or grouping) of antennas into separate antenna groups. The higher transmit power may provide desirable uplink performance, such as desirable uplink data rates, uplink carrier aggregation, and/or an uplink connection at the edge of a cell. 
     In certain aspects, a plurality of antenna groups is defined. Each antenna group may include one or more antennas. For example, the antenna  252   a  may be categorized into a first antenna group, and the antennas  252   t  may be categorized into a second antenna group. In certain aspects, each antenna array (e.g., each phased array) is placed in a different group. The groups may be defined manually, for example by a designer or test operator, or in an automated fashion, for example by an algorithm operating prior to initialization of the device, at initialization, or during operation of the device. The groups may be established based on physical location (as described in greater detail below), operating frequency, form factor, associated method of calculating RF exposure, etc. 
       FIG.  7    is a flow diagram illustrating example operations  700  for grouping antennas for RF exposure compliance, in accordance with certain aspects of the present disclosure. The operations  700  may be performed, for example, by a processing system including a UE (e.g., the UE  120   a  in the wireless communication network  100 ), an RF exposure measurement system (e.g., the RF exposure measurement system  600 ), and/or a computational device, such as a computer. The operations  700  may be implemented as software components that are executed and run on one or more processors (e.g., controller/processor  280  of  FIG.  2    and/or the processor  608  of  FIG.  6   ). Further, the transmission and/or reception of signals by the UE or the RF exposure testing system in the operations  700  may be enabled, for example, by one or more antennas (e.g., antennas  252  of  FIG.  2    and/or the RF probe  616  of  FIG.  6   ). In certain aspects, the transmission and/or reception of signals by the UE may be implemented via a bus interface of one or more processors (e.g., controller/processor  280 ) obtaining and/or outputting signals. 
     The operations  700  may begin, at block  702 , where the processing system may determine (e.g., generate and/or receive) RF exposure distributions per transmit antenna configuration for a plurality of transmit antennas of a wireless communication device (such as the UE  120  as depicted in  FIG.  6   ). At block  704 , the processing system may assign the plurality of transmit antennas to a plurality of antenna groups based on the RF exposure distributions. Optionally, at block  706 , the UE and/or processing system may determine a backoff factor for at least one of the plurality of antenna groups, for example, associated with a specific exposure/transmit scenario. At block  708 , the UE may transmit, from at least one antenna in the at least one of the plurality of antenna groups, using a transmission power level based on the backoff factor. 
     In certain aspects, assigning the plurality of transmit antennas to the plurality of antenna groups at block  704  may involve the processing system determining backoff factors for each of the antenna groups, for example, as further described herein with respect to  FIG.  9   . As used herein, a backoff factor may be a specific number representing a fraction (or portion) of a maximum transmit power level supported by a UE, such as a number in the range of 0 to 1. For example, the processing system may generate normalized distributions of the RF exposure distributions, generate a normalized composite map of the normalized distributions for each of the antenna groups, and generate a total of the normalized composite maps for all of the antenna groups based on a backoff factor associated with each of the antenna groups. 
     In aspects, the normalized distributions may be generated by dividing the RF exposure distributions by a maximum RF exposure value for a corresponding transmit antenna configuration, for example, as described herein with respect to block  902 . In aspects, the normalized composite map may be generated by selecting a maximum of the normalized distributions as the normalized composite map for each of the antenna groups, for example, as described herein with respect to block  904 . 
     In certain aspects, generating the total of the normalized composite maps may be generated by multiplying the normalized composite map for each antenna group with the associated backoff factor to generate a weighted normalized composite map for each antenna group and summing the weighted normalized composite maps together, for example, as described herein with respect to block  908 . In certain aspects, at least one of the backoff factors may be adjusted and applied to calculating the total of the normalized composite maps until the total of the normalized composite maps is less than or equal to a first threshold (e.g.,  1 . 0 ). That is, the backoff factors associated with each antenna group may be updated and applied to the calculation of the normalized composite maps until the total of the normalized composite maps is less than or equal to the first threshold. 
     In certain cases, the processing system may assign each of the plurality of transmit antennas to one of the plurality of antenna groups based on the RF exposure distributions, such that no transmit antenna is in multiple antenna groups. In certain cases, the processing system may assign each of the plurality of transmit antennas to one of the plurality of antenna groups based on the RF exposure distributions, such that there is at least one transmit antenna in multiple antenna groups. 
     In aspects, the plurality of transmit antennas may be assigned to the plurality of antenna groups at block  704  based on the value of the determined backoff factors, for example, as further described herein with respect to  FIG.  10   . The transmit antennas may be redistributed or regrouped if one of the backoff factors is less than a second threshold (e.g., 0.5). For example, the processing system may determine the backoff factors for a first grouping of the antenna groups, for example, as described herein with respect to  FIG.  9   , and assign the transmit antennas to a second grouping of the antenna groups, if at least one of the backoff factors for the first grouping is less than a second threshold (e.g., 0.5). In certain cases, the first grouping may include a separate antenna group for each transmit antenna, and the second grouping may include at least one antenna group having multiple transmit antennas. That is, the first iteration of the antenna grouping procedure may involve determining backoff factors for each antenna and determining which transmit antennas to group together based on the backoff factors, and subsequent iterations may refine or adjust the assignment of antennas to specific antenna groups, for example, based on the determined backoff factors. 
     The processing system may repeat determining the backoff factors and assigning the transmit antennas to antenna groups until all of the backoff factors are greater than the second threshold. For example, the processing system may determine the backoff factors for the second grouping of the antenna groups, (e.g., repeating the operations described herein with respect to  FIG.  9   ) and assign the transmit antennas to a third grouping of the antenna groups, if at least one of the backoff factors for the second grouping is less than the threshold. In certain cases, the third grouping may include at least two antenna groups having multiple transmit antennas in each of the at least two antenna groups. That is, the assignment of the third grouping may further refine the antenna groups to include multiple antennas in more than two antenna groups. 
     In certain aspects, an antenna group may include mixed-mode antennas (e.g., sub-6 GHz and mmWave antennas). For example, at least one of the antenna groups may comprise a first antenna configured to transmit in a first mode and a second antenna configured to transmit in a second mode. The first mode may be a sub-6 GHz band transmission mode, and the second mode may be a mmWave band transmission mode. In other words, the first mode may be transmitting at one or more frequencies at or below 6 GHz (for example, 300 MHz to 6 GHz), and the second mode may be transmitting at one or more frequencies above 6 GHz (for example, 24 GHz to 53 GHz or beyond). That is, the first mode may include the first antenna being operable at one or more frequencies at or below 6 GHz, and the second mode may include the second antenna being operable at one or more frequencies above 6 GHz. 
     In aspects, a transmit antenna configuration may include a specific antenna or a transmit beam configuration of an antenna module having multiple antennas. In aspects, at least one of the transmit antennas is part of an antenna module having multiple antennas. As an example, at block  702 , RF exposure distributions may be generated (and/or indications thereof may be received) for each antenna in the plurality of antennas and/or for each transmit beam configuration supported by an antenna module among the plurality of antennas. In aspects, a transmit beam configuration may refer to a transmit radiation pattern from an antenna or antenna module in a certain azimuthal direction and/or elevation direction, which may be realized through beamforming. A transmit beam configuration may have a certain transmit power spread (e.g., a power angular spread associated with an angle of departure) in an azimuthal direction and/or elevation direction. 
     In certain cases, the antenna grouping may be used to determine RF exposure compliance and corresponding transmit power levels. For example, the UE may transmit a signal at a transmission power level based on enforcing the RF exposure compliance for at least one of the antenna groups. In certain aspects, enforcing the RF exposure compliance may include the UE transmitting the signal at the transmission power level that satisfies a certain RF exposure limit (e.g., a SAR limit of 1.6 watts per kilogram (1.6 W/kg) and/or a PD limit of 1.0 milliwatts per square centimeter (1.0 mW/cm 2 )). 
     In aspects, ensuring the RF exposure compliance may include evaluating the RF exposure compliance in terms of time-averaged RF exposure such as a time-averaged SAR or a time-averaged PD over a time window. In aspects, the time window may be in a range from 1 second to 360 seconds. For example, the time window may be 100 seconds or 360 seconds. The range from 1 second to 360 seconds is an example, and other suitable values for the time window may be used. In certain cases, the time window may be less than 1 second, such as 500 milliseconds. In certain cases, the time window may be greater than 360 seconds, such as 600 seconds. 
     In aspects, the UE may be communicating with a base station, such as the BS  110 . For example, at block  708 , the UE may be transmitting, to the base station, user data on a physical uplink shared channel (PUSCH) or various uplink feedback (e.g., uplink control information or hybrid automatic repeat request (HARD) feedback) on a physical uplink control channel (PUCCH). In certain cases, the UE may be communicating with another UE. For example, at block  708 , the UE may be transmitting, to the other UE, user data and/or various feedback on sidelink channels. 
       FIG.  8    is a block diagram illustrating an example grouping of multiple antennas of a wireless communication device  800 , in accordance with certain aspects of the present disclosure. In this example, the wireless communication device  800  (e.g., a UE  120 , such as a smartphone, or any of the wireless communication devices described herein) includes a first antenna  802   a , a second antenna  802   b , a third antenna  802   c , a fourth antenna  802   d , a fifth antenna  802   e , a sixth antenna  802   f , and a seventh antenna  802   g . In this example, the antennas  802   a - 802   g  are separated into three antenna groups  804 ,  806 ,  808 , which roughly correspond to a top of the device  800 , a bottom of the device  800 , and a side of the device  800 , when the device  800  is held in the upright position. Those of skill in the art will appreciate that more or less than seven antennas may be implemented, and/or more or less than three antenna groupings may be defined. Each of the illustrated antennas  802   a - 802   g  may represent a single antenna, an array (e.g., a phased array) of antennas, or a module including one or more antennas. The antenna groups  804 ,  806 ,  808  may each include one or more antennas that are configured to transmit in a certain frequency band (e.g., very high (e.g., mmWave bands), high (e.g., 6-7 GHz bands), medium (e.g., 3-6 GHz bands), or low (e.g., 400 MHz-3 GHz bands)), or the antenna groups may each include one or more antennas that are configured to transmit in multiple frequency bands. 
     In aspects, the antenna groupings described herein may be assigned into various antenna groupings (such as a mmWave grouping, a sub-6 GHz grouping, a low band grouping (e.g., 400 MHz-3 GHz bands), a mixed-mode grouping (e.g., mmWave and sub-6 GHz grouping)), for example, for differing transmit scenarios. As an example, under a mmWave grouping, each mmWave module (e.g., the first antenna  802   a , the third antenna  802   c , and the fifth antenna  802   e ) may be treated as a separate antenna group, where each mmWave module may have multiple antenna elements (e.g., 64 dual polarization antenna elements) arranged in one or more arrays. The mmWave module may be capable of transmitting various beams via predefined antenna configurations, where the beams may form a codebook. Under a sub-6 GHz grouping, sub-6 GHz antennas may be grouped into separate groups. For example, the second and fourth antennas  802   b ,  802   d  may be assigned to a group, and the sixth and seventh antennas  802   f ,  802   g  may be assigned to another group. In certain cases, the antennas  802   a - 802   g  may be assigned to a mixed-mode grouping, such as the three antenna groups  804 ,  806 ,  808 . 
     The groups may be defined and/or operated so as to be mutually exclusive in terms of RF exposure. In certain aspects, the transmit power of one or more of the groups (or of one or more of the antennas within one or more groups) may be reduced such that the (normalized) sum of the exposure of all antenna groups, or of the overlapped RF exposure distributions, are less than a particular value (e.g., 1.0). For example, backoff factors may be determined for one or more groups, or one or more antennas within one or more groups, and applied so as to limit transmission power for the antenna(s) and/or groups. 
     As an example, the backoff factor bf may be between [ 0 ,  1 ] for each antenna group, such that the maximum permissible transmit power for each antenna group equals the respective backoff factor times the transmit power limit of the antenna group (e.g., bf*Tx_power_limit), where bf=1 represents no backoff, where bf=0.3 signifies to operate the antenna group at 30% of the transmit power limit, and where the transmit power limit may be the maximum transmit power supported by that particular antenna and/or antenna group. 
       FIG.  9    is a flow diagram illustrating example operations  900  for determining backoff factors for antenna groups, in accordance with certain aspects of the present disclosure. The operations  900  may be performed, for example, by a UE (e.g., the UE  120   a  in the wireless communication network  100 ), an RF exposure measurement system (e.g., the RF exposure measurement system  600 ), and/or a processing system. In order to determine such backoff factors, at block  902 , RF exposure distributions (simulation and/or measurement) may be generated per transmit antenna/configuration (beam) (as described above) on all evaluation surfaces/positions at all locations, for example, using a processing system and/or the RF exposure measurement system  600 . In certain aspects, the RF exposure distributions may be generated via simulations, such as a simulation of the various exposure/transmit scenarios using a model of the human body being exposed to electromagnetic radiation from a wireless communication device. As previously described herein, an RF exposure distribution may include the RF exposure associated with various transmit scenarios that correspond to specific frequency bands and/or human body positions relative to the antenna. For example, the RF exposure distributions may be represented by the expression: RFexp(s,x,y,z,i), where s represents a particular surface or position, (x, y, z) represent a given location, and i represents a particular transmit configuration, such as a specific antenna or transmit beam. In certain cases, a transmit antenna may support multiple bands, so multiple RF exposure distributions for each band/channel (low/mid/high) may be available for a specific transmit antenna. In that case, the RF exposure distribution for a specific transmit antenna can represent the maximum exposure out of all technologies/bands/channels supported by the transmit antenna at each location/exposure surface. 
     Then, at block  904 , normalized distributions (maps) may be calculated by collecting exposures on all surfaces/positions per transmit antenna/beam and dividing by the corresponding maximum value. For example, the normalized distributions may be represented by the expression: normalized.map(s,x,y,z,i)={RFexp(1,x,y,z,i); RFexp(2,x,y,z,i); . . . ; RFexp(s,x,y,z,i)}/maxRFexp(i). 
     Thereafter, at block  906 , a normalized composite map per antenna group may be calculated, for example based on a maximum of the normalized distributions in the group. That is, generating the normalized composite map may include selecting the maximum normalized distribution among the normalized distributions in a particular antenna group. For example, a normalized composite map may be given by the expression: normalized.composite.map.AG k  (s,x,y,z)=maximum{normalized.map(s,x,y,z,i), ∀i=1 to n antennas/beams inside AG k }, where AG k  represents a specific antenna group (AG). 
     Further, at block  908 , a total normalized composite map may be calculated for all of the antenna groups, for example based on a sum of all of the normalized composite maps. As an example, the total normalized composite map may be given by the expression: 
     
       
         
           
             
               
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     where bf k  represents the backoff factor for a specific antenna group. 
     In certain aspects, at block  910 , it may be determined whether the total normalized composite map is less than a threshold (for example, 1.0). If this condition is not satisfied, the expected or potential power for one or more antennas (or one or more antenna groups) may be reduced using an updated backoff factor. Antenna groups may contribute to the RF exposure at different levels, for example, due to the location of the antennas within a group, the supported bands of the antennas within the group, the maximum transmit power of the antennas within the group, etc. The contribution of an antenna group to the RF exposure (e.g., based on the total normalized composite map, where overlapping maps are at a peak, etc.) may be adjusted using the backoff factor for the antenna group. At block  912 , for example, backoff factor(s) may be adjusted (increased or decreased) for one or more of the antenna groups, and the total normalized composite map may be recalculated using the updated backoff factors at block  908 . The backoff factor for each antenna and/or group may be adjusted (or updated), and the total normalized composite map may be recalculated using the adjusted backoff factors, until the condition (e.g., the total normalized composite map being less than or equal to the threshold) at block  910  is satisfied. In some examples, the backoff factor for each transmitter (or antenna, or group of antennas or transmitters) may be determined based on a proportion of RF exposure attributable to each transmitter at a (e.g., peak) location and an amount of desired reduction in exposure. In some examples, the backoff factor may be determined based on a priority of a transmitter coupled to an antenna. In some examples, the backoff factor for an antenna that contributes most to the RF exposure at a (e.g., peak) is the largest backoff factor as compared to backoff factors for other antennas or groups. In some examples, the backoff factors are determined such that transmission power level for each one of several antennas or groups contribute approximately equally to RF exposure at a location. Backoff factors may be determined or applied uniformly to antennas in a group, or may vary across antennas in the group. 
     At block  914 , if the total normalized composite map is less than the threshold (for example, 1.0), the antenna groups are considered to be mutually exclusive in terms of RF exposure, and at block  916 , the final backoff factors for each antenna group may be obtained. The backoff factors may be used for determining transmission power levels for specific antenna groups, as further described herein, or for other purposes such as determining actual or potential interference. 
       FIG.  10    is a flow diagram illustrating example operations  1000  for assigning antennas to groups based on the backoff factors (for example as determined in the operations  900 ), in accordance with certain aspects of the present disclosure. The operations  1000  may be performed, for example, by a processing system including a UE (e.g., the UE  120   a  in the wireless communication network  100 ) and/or an RF exposure measurement system (e.g., the RF exposure measurement system  600 ). 
     At block  1002 , the backoff factor(s) for each antenna group may be obtained, for example, after completing the operations  900  with a certain antenna grouping. For example, the operations  900  may be first performed using a separate group for each of the antennas/beams to obtain the backoff factors for individual antennas at block  1002 . 
     At block  1004 , it may be determined whether each of the backoff factors is greater than or equal to a threshold (for example, 0.5). If this condition is not satisfied, then at block  1006 , the antennas may be reassigned or redistributed among the antenna groups. In certain cases, for antennas/antenna groups that have a low backoff factor (e.g., a backoff factor &lt;0.5), based on spatial distribution, some of the antennas can be grouped together into the same antenna group resulting in a reduction in the number of antenna groups. Suppose, for example, in the first iteration a separate group is used for each antenna, where antennas 1-7 are in antenna groups AG1 to AG7, respectively. The corresponding backoff factors are: bf1=bf2≈0.5, bf3≈1, bf4=bf5=bf6=bf7≈0.25. Then, updated antenna groups may be AG1={Ant4, Ant5, Ant6, Ant7}, AG2={Ant1, Ant2}, and AG3={Ant3}. In certain cases, specific antennas may be grouped together such that the sum of the backoff factors for the specific antennas is above the threshold at block  1004 . At block  1002 , the operations  900  or a portion (e.g., blocks  906 - 916 ) of the operations  900  may be repeated to determine the updated backoff factors for the reassigned antenna groups. The antenna grouping/backoff factor generation may be repeated until all of the backoff factors satisfy the conditions at both block  910  and block  1004 . If the conditions at these blocks are satisfied, the antenna group assignment may be considered complete. 
     The antenna grouping operations described herein may be determined and/or applied per device state index (DSI) and/or exposure category indicating a device&#39;s exposure scenario (e.g., head exposure, body exposure, or extremity exposure). For example, head exposure may have four exposure positions (right cheek, right tilt, left cheek, and left tilt), and these four positions can be collected together (e.g., at block  904 , into a normalized map; in certain cases, the value of s will range from [1, 4], to account for the four exposure positions, where s represents a particular surface or position). Body exposure may have two exposure positions (front surface and back surface), and these two exposure positions can be collected together (e.g., at block  904 ). Extremity exposure may have six exposure positions at 0 mm separation distance (front, back, left, right, top, and bottom surfaces of device), and these six positions can be collected together (e.g., at block  904 ). 
     In certain aspects, the antenna grouping operations described herein can be combined with existing approaches for some exposure configurations. For example, if the absolute sum of maximum RF exposure values for all antenna groups (e.g., total normalized composite map) is less than a regulatory limit, then the above procedure of adjusting the power/backoff factors may be skipped. 
     While the examples provided herein are described with respect to the UE performing various operations in determining the antenna grouping, aspects of the present disclosure may also apply to scenarios where the antenna grouping and backoff factor derivation operations are conducted in a laboratory setting (such as with the RF exposure measurement system  600 ), and certain calculations or simulations are performed external to the UE, for example, by a separate processing system (such as the processing system  602 ). That is, the various functions for antenna grouping and backoff factor derivation operations need not be done at the UE itself, but that the UE may be configured to store/access/utilize specific information derived from the antenna grouping operations, such as the backoff factors and antenna grouping assignments. For example, the antenna grouping assignments and corresponding backoff factors may be developed using (a prototype of) the wireless communication device in a laboratory setting (e.g., the RF exposure measurement system  600 ) to simulate various exposure/transmit scenarios during the RF exposure compliance certification process with a regulatory body, and the UE may be configured to store/access/utilize the backoff factors associated with the specific antenna groupings derived from the antenna grouping operations performed in the laboratory setting. 
     As an example, the UE may store and access various backoff factors associated with specific antenna groups and/or transmit beam configurations depending on the various RF exposure limit associated with the exposure/transmit scenarios (such as head exposure, body exposure, and/or extremity exposure at certain frequency bands). The backoff factors associated with the specific antenna groups and/or transit beam configurations may be developed according to the operations for assigning antenna groups as described herein, for example, using a prototype of the UE in a RF exposure testing laboratory. The backoff factors associated with the specific antenna groups may be arranged in a data structure, such as a table or database of backoff factors associated with specific antenna groupings at specific frequency bands and/or specific exposure/transmit scenarios. 
     While the examples provided herein are described with respect to the UE performing RF exposure compliance with the antenna grouping, aspects of the present disclosure are not limited to RF exposure use cases. For example, the stored values (e.g., the backoff factors and/or antenna grouping assignments) derived from the antenna grouping operations may be used for any number of applications. One application as further described below is to evaluate RF exposure compliance using the backoff factors and/or antenna groupings. Another application might be to determine self-interference among the antenna groupings based on transmission power levels. Other purposes are possible, as well. 
     In certain cases, an antenna may not meet exclusion criteria with another antenna group, and in such cases, that antenna can be incorporated into the other antenna group. In some cases, this may lead to all of the antennas being combined into a single antenna group, which implies that RF exposure from all antennas is collocated and does not take advantage of spatial diversity arising from antenna placement. One way to avoid this is to force the antenna to meet exclusion criteria by applying higher permanent backoff(s) to one or more antennas. 
     Aspects of the present disclosure relate to assigning an antenna to multiple antenna groups in a specific antenna grouping. For example, if an antenna does not meet the exclusion criteria with another antenna group, the antenna may be assigned to multiple antenna groups, which may avoid applying a permanent backoff to all of the antennas. The antenna grouping described herein may enable desirable transmit power for specific antenna groups and/or flexibility in complying with RF exposure limits per antenna group. 
     Aspects of the present disclosure relate to assigning one or more antennas to multiple sets of antenna groups (i.e., multiple antenna groupings), for example, for separate transmission scenarios. For example, the processing system may develop an antenna grouping for a specific country or region (which may be identified by public land mobile network (PLMN) code and/or a mobile country code (MCC), for example) due to separate RF exposure specifications for that country or region. In certain cases, the processing system may develop an antenna grouping for a specific exposure scenario, such as head exposure, body exposure, extremity exposure, and/or hotspot exposure (e.g., when the wireless communication device is not in close proximity to human tissue), and/or based on one or more operating conditions (whether MIMO is being utilized, for certain bands, when certain high priority applications or transmissions are likely to be active, etc.). Antenna groupings per transmission scenario (such as a specific region and/or exposure scenario) may provide flexibility for a wireless communication device to switch between antenna groupings depending on the transmission scenario encountered by the wireless communication device. 
     Returning to  FIG.  7   , the operations  700  may further involve the processing system (e.g., a UE, RF exposure measurement system, computer separate from the UE and/or any other device configured to perform the operations described herein) assigning at least one of the transmit antennas to two or more of the antenna groups at block  704 . For example, the processing system may assign an antenna to multiple antenna groups due to the antenna not meeting exclusion criteria with other antenna groups. At block  704 , the processing system may identify that at least one of the transmit antennas does not meet a mutually exclusive criterion with at least two of the antenna groups, and the processing system may assign the at least one of the transmit antennas to the at least two of the antenna groups in response to the identification. 
     In certain cases, an antenna may be assigned to multiple antenna groups based on the maximum time-averaged power limit (P limit ) associated with the antenna. The maximum time-averaged power limit may refer to the maximum constant transmit power an antenna can transmit continuously during the entire duration of a time window associated with an RF exposure limit in compliance with the RF exposure limit. For example, if a certain antenna has a relatively low P limit  compared to the other antennas, the processing system may not repeat assigning that particular antenna in multiple antenna groups to avoid consuming RF exposure margin in those antenna groups. As an example, if a specific antenna has a relatively high P limit , the processing system may assign that particular antenna to multiple antenna groups. Here, low or high P limit  for a specific antenna (and a specific technology/frequency band) can be quantified by comparing the P limit  against the maximum transmit power (P max ) supported by the hardware. In such scenarios, a peak-to-average-power ratio (PAPR) can be used as a metric to determine if P limit  is relatively low or high. The PAPR in dB may be given by P max −P limit , where P max  and P limit  may be in dBm. For example, if PAPR is positive (say, a few dB, for example 2 dB, 3 dB, or 6 dB), then P limit  may be considered low for that specific technology/band/antenna. Similarly, if PAPR is less than one of these example values or negative, then P limit  may be considered high. With respect to the operations  700 , the processing system may identify a maximum time-averaged power limit associated with each of the transmit antennas, and the processing system may assign at least one of the transmit antennas to at least two of the antenna groups based at least in part on the maximum time-averaged power limit associated with the at least one of the transmit antennas. 
     In certain aspects, the processing system may generate multiple antenna groupings. The antenna groupings may be developed for separate transmission scenarios, such as when a wireless communication device is located in a specific region and/or when the wireless communication device encounters a specific exposure scenario. With respect to the operations  700 , the processing system may assign the transmit antennas to a first grouping of the antenna groups for a first transmission scenario (e.g., when the UE is located in the United States) and assign the transmit antennas to a second grouping of the antenna groups for a second transmission scenario (e.g., when the UE is located in the European Union). 
     In certain aspects, the first grouping may have a different arrangement of transmit antennas in the plurality of antenna groups than the second grouping. At least one of the transmit antennas is in both the first grouping and the second grouping. For example, referring to  FIG.  8   , the antennas  802   a - 802   g  may be assigned to a first grouping where the first antenna  802   a , second antenna  802   b , third antenna  802   c , fourth antenna  802   d , and fifth antenna  802   e  are assigned to a first group; and the fifth antenna  802   e , sixth antenna  802   f , and seventh antenna  802   g  are assigned to a second group. The first group may be spatially separated from the second group to provide a mutually exclusive relationship in terms of RF exposure. In this first grouping, the fifth antenna  802   e  is assigned to two different antenna groups (namely, the first and second groups). Due to the fifth antenna  802   e  being positioned between the set of top and bottom antennas ( 802   a - d ,  802   f , and  802   g ), the fifth antenna  802   e  may be difficult to separate into a mutually exclusive exposure group. For example, the fifth antenna  802   e  may interact with the other antennas ( 802   a - d ,  802   f , and  802   g ), and to avoid applying a restrictive permanent backoff, the fifth antenna  802   e  may be assigned to the first group and second group. 
     The antennas  802   a - 802   g  may also be assigned to a second grouping, where the first antenna  802   a , second antenna  802   b , third antenna  802   c , and fourth antenna  802   d  are assigned to a third group; the sixth antenna  802   f  and seventh antenna  802   g  are assigned to a fourth group; and the fourth antenna  802   d , fifth antenna  802   e , and seventh antenna  802   g  are assigned to a fifth group. In this second grouping, the fourth antenna  802   d  is assigned to two different antenna groups (namely, the third and fifth groups) and the sixth antenna  802   g  is assigned to two different antenna groups (namely, the fourth and fifth groups). In this second grouping, the fifth antenna  802   e  may again be difficult to assign to a separate group, and the fifth antenna  802   e  may be grouped with antennas spatially arranged on the same side of the wireless communication device  800 , such as the fourth antenna  802   d  and seventh antenna  802   g.    
     In certain cases, the first transmission scenario may be associated with a first country or region (e.g., the United States), and the second transmission scenario may be associated with a second country or region (e.g., China or the European Union). That is, the first and second transmission scenarios may depend on a specific region where the UE is located to comply with specific RF exposure limits for that region. When the UE is located in that specific region (for example as determined based on a PLMN code and/or an MCC provided to the UE), the UE may use a certain antenna grouping associated with that region. 
     In certain cases, the first transmission scenario may be associated with a first exposure scenario (e.g., head exposure), and the second transmission scenario may be associated with a second exposure scenario (e.g., body exposure). That is, the first and second transmission scenarios may depend on a specific exposure scenario, such as head exposure, body exposure, extremity exposure, and/or hotspot exposure. When the UE encounters a specific exposure scenario, the UE may use a certain antenna grouping associated with that exposure scenario. 
     In certain cases, the transmission scenarios may be associated with when certain antennas are used for concurrent transmissions. For example, suppose the fourth antenna  802   d  and seventh antenna  802   g  will be commonly used for concurrent transmissions. The processing system may assign these antennas to different groups to facilitate efficient use of the RF exposure margin for these antennas. As an example, the processing system may develop the first grouping as described herein with respect to  FIG.  8    for when the fourth antenna  802   d  and seventh antenna  802   g  are used for concurrent transmissions to enable application of separate backoffs for these antennas. 
     With respect to the operations  700 , a UE may transmit from at least one transmit antenna in the first grouping, during the first transmission scenario, and the UE may transmit, from at least one transmit antenna in the second grouping, during the second transmission scenario. In other words, the UE may select which antenna grouping to use for a specific transmission scenario, and the UE may switch between antenna groupings when there is a change in the transmission scenario, such as when the UE moves from one region to another region, for example, as further described herein with respect to  FIG.  11   . 
     Example Time Averaged RF Exposure Per Transmit Antenna Group 
     Aspects of the present disclosure provide various techniques for determining time-averaged RF exposure compliance per transmit antenna group. As the antenna grouping described herein may provide mutually exclusive antenna groups in terms of RF exposure, the RF exposure compliance for each antenna group may be determined separately. In certain cases, the RF exposure compliance for the antenna groups may be conducted in parallel (e.g., concurrently together). The group-based RF exposure compliance described herein may enable desirable transmit power for specific antenna groups, for example, due to differing exposure scenarios encountered by each antenna group. The desirable transmit power may provide desirable uplink performance, such as desirable uplink data rates, uplink carrier aggregation, and/or an uplink connection at the edge of a cell. 
       FIG.  11    is a flow diagram illustrating example operations  1100  for wireless communication, in accordance with certain aspects of the present disclosure. The operations  1100  may be performed, for example, by a UE (e.g., the UE  120   a  in the wireless communication network  100 ). The operations  1100  may be implemented as software components that are executed and run on one or more processors (e.g., controller/processor  280  of  FIG.  2   ). Further, the transmission of signals by the UE in the operations  1100  may be enabled, for example, by one or more antennas (e.g., antennas  252  of  FIG.  2   ). In certain aspects, the transmission and/or reception of signals by the UE may be implemented via a bus interface of one or more processors (e.g., controller/processor  280 ) obtaining and/or outputting signals. 
     The operations  1100  may begin, at block  1102 , where the UE may access a stored backoff factor associated with an antenna group (e.g., the antenna group  804 ) among a plurality of antenna groups (e.g., the antenna groups  804 ,  806 ,  808 ). At block  1104 , the UE may transmit, from at least one transmit antenna (e.g., the antenna  802   a ) in the antenna group, a signal at a transmission power level based on the backoff factor in compliance with an RF exposure limit. 
     In certain cases, the grouping of the transmit antennas may not be an explicit indication of which antenna is in a specific group. In aspects, the grouping of the transmit antennas may be implicitly indicated by various backoff factors assigned to transmit antennas for specific exposure/transmit scenarios. That is, the antenna groupings and the antenna group assignments associated with the antenna grouping may be represented by backoff factors. For example, certain antennas may share the same backoff factor, such that these antennas are implicitly assigned to the same antenna group among a plurality of antenna groups. In aspects, the transmission power level may be based at least in part on at least one backoff factor of the backoff factors. 
     In certain aspects, the transmission power level may be determined based on a sum of the RF exposures being less than or equal to a threshold (e.g., 1.0). For example, the UE may transmit the signal at the transmission power level based on a sum of RF exposures for each of the antenna groups being less than or equal to a threshold. In some such scenarios, this is accomplished by applying the backoff factor(s) described above to transmission power levels. 
     In certain aspects, the transmission power level may be determined based on time-averaged RF exposure being less than the threshold. For example, the UE may transmit the signal at the transmission power level based on a time-averaged sum of RF exposures for each of the antenna groups being less than or equal to a threshold (e.g., 1.0). A backoff factor may be applied to the RF exposures for each of the antenna groups in the case of the sum of RF exposures or the time-averaged sum of RF exposures. 
     In aspects, the UE may determine time-averaged RF exposures for each of the antenna groups and use the group-based time-averaged RF exposures in determining RF exposure compliance. For example, the UE may transmit the signal at the transmission power level based on each of the time-averaged RF exposures being less than or equal to a threshold. In certain cases, because the antenna groups may be mutually exclusive from each other in terms of RF exposure, the UE may concurrently determine the time-averaged RF exposures for each of the antenna groups. In other words, the mutual exclusivity of the antenna groups may enable the UE to determine the time-averaged RF exposures for each of the antenna groups in parallel with (e.g., independent of) each other. Expressed another way, the UE may use parallel (or concurrent) processing to determine the time-averaged RF exposures for each or a portion of the antenna groups. For example, the UE may determine the time-averaged RF exposures associated with a first antenna group (e.g., the antenna group  804 ) while concurrently determining the time-averaged RF exposures associated with a second antenna group (e.g., the antenna group  806 ), and the UE may determine the transmit powers in compliance with RF exposure limits for each of the first and second antenna groups based on the respective time-averaged RF exposures and respective backoff factors. In certain cases, the UE may transmit the signal at the transmission power level based on enforcing RF exposure compliance for one of the plurality of antenna groups having a transmit power limit less than another one of the plurality of antenna groups. That is, the minimum of multiple transmit power limits may be enforced by the transmitter to ensure overall time-averaged RF exposure compliance. 
     In aspects, the antennas may have various antenna groupings, for example, as described herein with respect to the operations  700 . As an example, the UE may have backoff factors associated with antenna groups for mmWave bands, antenna groups for sub-6 GHz bands, and/or antenna groups for mixed-mode bands (sub-6 GHz bands and mmWave bands). In certain cases, the antenna grouping may be derived using the operations  700 ,  900 , or  1000 . For example, at least one of the antenna groups may include a first antenna configured to transmit in a first mode and a second antenna configured to transmit in a second mode. In certain cases, the first mode may be sub-6 GHz, and the second mode may be mmWave. That is, the first mode may be transmitting at a sub-6 GHz band, and the second mode may be transmitting at a mmWave band. In aspects, the first mode may include the first antenna being operable at a sub-6 GHz band, and the second mode may include the second antenna being operable at a mmWave band. 
     In certain aspects, the transmit antennas may include one or more first antennas configured to transmit in a first mode and one or more second antennas configured to transmit in a second mode. The first antenna(s) may be separately assigned to the antenna groups. That is, the first antennas may be divided into groups, such that some of the first antennas may be in the same group, but one of the first antennas may not be assigned to more than one group in a specific antenna grouping. The second antennas may be included in each or some of the antenna groups. In certain cases, each of the antenna groups may have all of the second antennas. In certain cases, the first mode may be transmitting at one or more frequencies below 6 GHz (e.g., at sub-6 GHz bands), and the second mode may be transmitting at one or more frequencies above 6 GHz (e.g., at mmWave bands). In other cases, the first mode may be transmitting at one or more frequencies above 6 GHz, and the second mode may be transmitting at one or more frequencies below 6 GHz. 
     In aspects, the transmit antennas are grouped such that each antenna group is mutually exclusive from all the other antenna groups in terms of RF exposure. The mutual exclusivity of the antenna groups may be accomplished using various techniques or criteria. For example, in a system of N antennas which are grouped into k antenna groups, first obtained normalized RF exposure distribution of each of i=1 to N antennas on all exposure surfaces of interest=normalized.map(s,x,y,z,i), represented in block  904 , such that the maximum value of RF exposure distributions out of all surfaces is max{normalized.map(s,x,y,z)}=1.0. Then, obtain composite map out of all n antennas inside antenna group k=normalized.composite.map.AG k (s,x,y,z)=max{normalized.map(s,x,y,z,i=1 to n)}, represented in block  906 , =normRFexposure(k,s,x,y,z). This normalized composite map is termed as normalized RF exposure for antenna group k. For example, the mutual exclusivity of the antenna groups may be provided if a sum of RF exposure of all antenna groups (k=1 to M)&lt;1.0 satisfies the following expression: 
       Σ k=1   M {normRFexposure( k,s,x,y,z )*predefined backoff( k )}&lt;1.0  (9)
 
     where the predefined backoff (k) is the backoff factor applied to all the antennas and/or antenna configuration of antenna group “k.” The backoff factor may be determined based on the operations  700 ,  900 , and/or  1000 , and/or the backoff factor may be stored by the UE (for example in the memory  282  or  338 ), and retrieved for use in performing the operations  1100 . In certain cases, existing regulatory approaches that meet predefined criteria like SAR peak location separation ratio (SPLSR) may be used to determine such mutual exclusivity (for example, as described in Section 4.3.2c of the FCC KDB 447498 D01 General RF Exposure Guidance v06). In certain cases, the mutual exclusivity of the antenna groups may be determined by the sum of overlapped RF exposure distributions at specific backoff factors being less than or equal to a threshold (for example, 1.0). The predefined backoff factors are between [0, 1] and are applied on all the antennas belonging to that antenna group. This can be accomplished by lowering the maximum time-averaged transmission power limit of each antenna belonging to antenna group k by predefined backoff (k). Alternatively, the total RF exposure for all the antennas in the antenna group k at all spatial locations (s,x,y,z) shouldn&#39;t exceed RFexposure limit *predefined backoff(k). 
     As the antenna groups are mutually exclusive in terms of RF exposure, (real-time) averaging of RF exposure can be performed per antenna group (e.g., irrespective of the other antenna groups) using the methods described above or using one or more other methods. For example, RF exposure of a given antenna at any time instant t may be directly proportional to the transmission power of the antenna at t. Therefore, RF exposure for antenna i belonging to antenna group k at a time instant t may be given by: 
     
       
         
           
             
               
                 
                   
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     Time-averaged RF exposure of all n antennas and/or antenna configurations in an antenna group k over time-window T may be given by: 
     
       
         
           
             
               
                 
                   
                     
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     The predefined backoff may be the backoff factor bf described herein. 
     When antennas and/or groups of antennas which use different mechanisms (e.g., SAR or PD) to calculate RF exposure are included in an antenna group, the exposures may be combined as described herein, or using one or more other methods or calculations. 
     Thus, transmission (power) using the antennas in the antenna groups may be controlled (e.g., by the processor  280 ) such that each group individually satisfies exposure limits, for example as defined by a regulator of a domestic or foreign jurisdiction. In some aspects, this may result in total power transmitted across all of the antenna groups being higher than if the antennas were not divided into mutually exclusive exposure groups. 
     In some cases, multiple sets of antenna groups (e.g., multiple antenna groupings) may be defined and used to determine (e.g., transmission power and/or backoff factors) settings for a plurality of transmitters and/or antennas. That is, the UE may be configured with multiple antenna groupings, where each antenna grouping has antenna groups that may be defined differently from the other antenna groupings. For example, referring to  FIG.  8   , the first antenna  802   a , third antenna  802   c , and fifth antenna  802   e  may be antenna modules having an antenna array configured to transmit at one or more mmWave bands (e.g., at approximately 24 GHz to 53 GHz or higher). The other antennas  802   b ,  802   d ,  802   f ,  802   g  may be configured to transmit at sub-6 GHz bands (e.g., 6 GHz or below). 
     A first antenna grouping (M1) may include three antenna groups, and a second antenna grouping (M2) may include two antenna groups. The antenna groups of the first antenna grouping (M1) may include a first antenna group (AG1) having all of the sub-6 GHz antennas  802   b ,  802   d ,  802   f ,  802   g  and the first antenna  802   a ; a second antenna group (AG2) having all of the sub-6 GHz antennas  802   b ,  802   d ,  802   f ,  802   g  and the third antenna  802   c ; and a third antenna group (AG3) having all of the sub-6 GHz antennas  802   b ,  802   d ,  802   f ,  802   g  and the fifth antenna  802   e . In aspects, the first antenna grouping (M1) may be expressed as follows:
         AG1: {all sub-6 GHz antennas, first mmWave module}   AG2: {all sub-6 GHz antennas, second mmWave module}   AG3: {all sub-6 GHz antennas, third mmWave module}       

     The antenna groups of the second antenna grouping (M2) may include a fourth antenna group (AG4) having the second antenna  802   b , fourth antenna  802   d , and all of the mmWave antennas  802   a ,  802   c , and  802   e , and a fifth antenna group (AG5) having the sixth antenna  802   f , seventh antenna  802   g , and all of the mmWave antennas  802   a ,  802   c , and  802   e . The second antenna grouping may be expressed as follows:
         AG4: {a first sub-group of sub-6 GHz antennas, all mmWave modules}   AG5: {a second sub-group of sub-6 GHz antennas, all mmWave modules}
 
where the first sub-group may include the sub-6 GHz antennas arranged at the top of the UE (such as the second antenna  802   b  and fourth antenna  802   d ), and the second sub-group of sub-6 GHz may include the sub-6 GHz antennas arranged at the bottom of the UE (such as the sixth antenna  802   f  and seventh antenna  802   g ).
       

     In some aspects, sub-6 GHz (e.g., frequency range 1 (FR1)) RF exposure may be calculated via measurements, and mmWave (e.g., frequency range 2 (FR2)) RF exposure (for beams in the codebook) may be calculated via simulations (e.g., as described above). In such cases, sub-6 GHz antennas may be grouped into M2 groups (with all mmWave modules in each of the groups), and mmWave antennas are grouped into M1 groups (with all sub-6 GHz antennas in each of the groups), as described above. 
     Those of skill in the art will understand that the groupings M1 and M2 are merely examples for arranging the antennas into groups to facilitate understanding. Aspects of the present disclosure may also be applied to arranging the antennas into additional or alternative groups, such as the groupings described above with respect to assigning an antenna to multiple groups. For example, all of either the FR1 or FR2 radios could be assigned to all of the antenna groups, and the other of the FR1 or FR2 radios could be non-uniquely spread out among the antenna groups. In one such example, an antenna grouping (M3) may include the fourth antenna group AG4 and the fifth antenna group AG5, plus an additional antenna group (AG6), having the second antenna  802   b , the sixth antenna  802   f , and all of the mmWave antennas  802   a ,  802   c , and  802   e . In another such example, an antenna grouping (M4) may include the fourth antenna group AG4 and a seventh antenna group (AG7), having the second antenna  802   b , the sixth antenna  802   f , the seventh antenna  802   g , and all of the mmWave antennas  802   a ,  802   c , and  802   e.    
     In these examples, two or more determinations of time averaging may be performed (e.g., at least one for each set, for example according to one or more backoff values defined for the set). The processor  280  may determine to apply transmission settings to the antennas based on the results of the two or more determinations. In some aspects, the minimum of transmit power limits across the multiple antenna groupings (e.g., M1 versus M2, or M1 versus M3 and/or M4) may be selected and effected by the processor  280 , for example to ensure overall time-averaged RF exposure compliance. 
     In certain cases, the UE may access a stored backoff factor and transmit, from at least one antenna, the signal using a transmission power level based on the backoff factor in compliance with a radio frequency exposure limit, as described herein. The backoff factor may correspond to at least one antenna group of a plurality of antenna groups, where the at least one antenna is in the at least one antenna group. 
     Example Selection and Switching Between Sets of Antenna Groups 
     Aspects of the present disclosure relate to selecting a set of antenna groups (also referred to herein as a “grouping”) for operation by a wireless communications device (e.g., the UE  120 ). Operating with a particular set of antenna groups may be beneficial (e.g., by offering higher performance) for a certain radio transmission scenario. For example, when operating with the antenna groups (AG1, AG2, and AG3) of the first antenna grouping (M1), the mmW modules may get more combined total RF exposure margin, since each mmW module can get up to 100% RF exposure margin in this scenario (depending on how much margin the sub6 antennas consume). Therefore, when operating with LTE and Frequency Range 2 (FR2) in NR (e.g., an LTE+FR2 link), it may be beneficial to operate according to the M1 grouping. In contrast, when operating only with sub-6 GHz bands (e.g., LTE and Frequency Range 1 (FR1) in NR, such as in an LTE+FR1 link), it may be beneficial to operate according to the M2 grouping (with AG4 and AG5) or one of the M3 and M4 groupings, for example. 
     Aspects of the present disclosure also relate to switching between sets of antenna groups, such as when a wireless communications device (e.g., the UE  120 ) changes its operating parameters. When switching from one antenna grouping to another grouping (e.g., for performance benefits), RF exposure compliance should ideally be ensured, since the grouping assumptions may have changed. For example, when switching from the M1 grouping to the M2 grouping (or from the M1 grouping to one of the M3 and M4 groupings), if each mmW module had previously operated at 100% RF exposure margin, then upon switching to the M2 grouping (e.g., from an LTE+FR2 call to an LTE+FR1 call) or one of the M3 and M4 groupings, the time-history for all the antenna groups in the M1 grouping may now exceed the RF exposure compliance limit. Accordingly, aspects of the present disclosure provide one or more criteria for switching between different groupings (referred to herein as “switch criteria”), in an effort to maximize, or at least increase, the benefit of switching while simultaneously ensuring RF exposure compliance. 
     For example, assume there are a total of “B” mmW groups in the M1 grouping (with all sub-6 GHz groups added to each of the mmW groups) and “A” sub-6 GHz groups in the M2 grouping (with all mmW groups added to each of the sub6 groups). In this case, the total RF exposure margin available in the M1 grouping=100%−(sub6_1+sub6_2++sub6_A)−max{mmW_1, mmW_2, . . . , mmW_B}, and the total RF exposure margin available in the M2 grouping=100%−(mmW_1+mmW_2+ . . . +mmW_B)−max{sub6_1, sub6_2, . . . , sub6_A}. The switch criteria to change from the M1 grouping to the M2 grouping (e.g., LTE+FR2 call handover to LTE+FR1 call) may include the total available margin (TAM) in the M2 grouping being greater than the total available margin in the M1 grouping (TAM, M2&gt;TAM, M1). Similarly, the switch criteria to change from the M2 grouping to the M1 grouping (e.g., LTE+FR1 call handover to LTE+FR2 call) may include the total available margin in the M1 grouping being greater than the total available margin in the M2 grouping (TAM, M1&gt;TAM, M2). 
     The above example had only two different sets of antenna groups (the M1 and M2 groupings). However, this concept and the criteria for switching between antenna groups may be extended to more than two different groupings (e.g., to M3 and M4 also, or to using M3 and M4 in place of M2 in the example above). In general, the switch criteria to change from one radio configuration to another may include the total available margin in the new grouping being greater than the total available margin in the current (old) grouping. 
     Furthermore, the decision to switch between sets of antenna groups may be based on one or more criteria in addition to the total available margin. For example, in a combined transmission scenario of multiple sub-6 GHz radios and multiple mmW radios, the wireless communications device may use the priority of the radios to select operation with a specific set of antenna groups that provides the maximum total available margin to the highest priority radio. In other words, the wireless communications device may select the grouping that divides the antennas related to the highest priority radio into the maximum number of antenna groups. 
     Returning to  FIG.  11   , the operations  1100  may further involve the UE selecting a first grouping in the plurality of antenna groups based on one or more criteria. For certain aspects, the antenna group in block  1102  is in the first grouping, and the backoff factor in block  1102  is among a plurality of stored backoff factors for the first grouping and is associated with the antenna group in the first grouping. In this case, the one or more criteria may include total RF exposure margins available for different groupings in the plurality of antenna groups, including for the first grouping. For certain aspects, the one or more criteria may also include a priority of a radio type. 
     According to certain aspects, the operations  1100  may further involve the UE selecting a second grouping in the plurality of antenna groups based on the one or more criteria; accessing another stored backoff factor associated with an antenna group in the second grouping; and transmitting, from at least one transmit antenna in the antenna group in the second grouping, another signal at another transmission power level based on the other backoff factor in compliance with the RF exposure limit. In this case, the one or more criteria may include total RF exposure margins available for different groupings in the plurality of antenna groups, including for the first grouping and for the second grouping. In addition, the total RF exposure margin available for the second grouping may be greater than the total RF exposure margin available for the first grouping at a time of selecting the second grouping. 
     According to certain aspects, the accessing at block  1102  may include accessing a first stored backoff factor associated with a first antenna group in a first grouping in the plurality of antenna groups. In this case, the operations  1100  may further involve the UE accessing a second stored backoff factor associated with a second antenna group in a second grouping in the plurality of antenna groups, based on one or more switching criteria. 
     For certain aspects, a wireless communications device (e.g., a UE) may select a specific antenna grouping, such as M1, M2, or one of several antenna groupings where all FR2 antennas are assigned to every antenna group and FR1 antennas are distributed among the antenna groups such that at least one of the FR1 antennas is assigned to multiple antenna groups (e.g., select between M3 and M4). The wireless communications device may select the antenna grouping for a specific transmission scenario, for example, as described herein with respect to the operations  700  for grouping antennas for RF exposure compliance. As an example, the wireless communications device may select an antenna grouping for a specific region, when the wireless communications device is located in that region. In certain cases, the wireless communications device may select an antenna grouping for a specific exposure scenario, such as when the wireless communications device is in close proximity to a user&#39;s head, arm, or abdomen. In certain cases, the wireless communications device may select an antenna grouping for when certain antennas are being used for concurrent transmissions. Further, the wireless device may select an antenna grouping based on a combination of these factors, for example based on a specific region and total available margin in groupings that correspond to or can be used within that specific region. 
     RF Exposure Per Exposure Scenario or Category 
     As described herein, a wireless communication device may adjust certain RF exposure settings (e.g., backoff factor, antenna grouping, RF exposure limit, etc.) in response to a change in RF exposure scenario (e.g., head exposure, body exposure, extremity exposure, or hotspot exposure). A device state index (DSI) may be representative of or associated with a specific RF exposure scenario, such that a specific DSI may refer to a particular RF exposure scenario. During regulatory certification, the RF exposure may be characterized for different usage conditions or exposure scenarios (e.g., DSIs) including, for example, head, body-worn (trunk region), hotspot (trunk region), and extremity (grip sensor triggered—hands/feet region) scenarios. 
     Depending on use case, over time, the wireless communication device may expose different human tissue or different parts of human body to RF energy at different times.  FIG.  12    illustrates a diagram of example wireless device locations  1204   a - i  relative to a profile of a user&#39;s body  1202 . For example, in a first period of time, the wireless device may be held next to the head of a user for a voice call (e.g., at location  1204   a ,  1204   b ), where the RF exposure is to the head; and in a second period of time, the user may switch to using Bluetooth for the voice call and place the wireless device in a pocket (e.g., at location  1204   d ,  1204   g ,  1204   h ), where the RF exposure in the second time period is to both head (from a Bluetooth radio) and torso (from the wireless device). At other times, the user may position the wireless device in other locations, such as any of locations  1204   c - i.    
     Although nine different locations  1204   a - i  are shown in  FIG.  12   , the reader is to understand that there may be more or fewer than nine different locations being assessed for exposure. The number of different locations used for RF exposure tracking may depend, for example, on the sensing and/or memory capabilities of the wireless device, on the desired tissue exposure tracking resolution, etc. 
     The RF exposure history may be tracked as a function of time across different locations of the user&#39;s body (and as such, the RF exposure history may also be referred to as the “tissue exposure history”). The time-varying RF exposure may be recorded as a function of exposure f(exposure(i), t), where exposure(i) is the exposure recorded at time t for a particular tissue location (e.g., tissue). The tissue i  may represent a unique location (or region) across multiple locations or regions across the user&#39;s body. For example, the unique location may represent a particular tissue and/or portion of the human body, such as a right or left side of the user&#39;s head; a particular hand, wrist/, or arm (e.g., when the wireless device is positioned against the user&#39;s hand, wrist, or arm while exercising), fingers (e.g., when the wireless device is used for gaming), trunk (e.g., when the wireless device is in a pocket), etc. In certain cases (as described herein), for tracking and recording time varying exposure history, the exposed tissue may be grouped and classified into a certain number of exposure categories, and transmitting antennas may be grouped into different antenna groups. Each exposure category (described in more detail below) may be mutually exclusive of each other in terms of RF exposure over time, and each antenna group can transmit independently for a given time. For example, for a given time, the RF exposure from any antenna in one antenna group may have no contribution to the RF exposure of an antenna in other antenna groups. 
     In certain aspects, tissues may be classified per location on the user&#39;s body. The wireless device can track its location relative to the human body over time, for example, using sensor information. The RF exposure can be tracked as a function of time and tissue location (tissue) across the user&#39;s body. The approach can be generalized to any tissue location on a user&#39;s body, where tissue i  may represent a particular tissue location in multiple dimensions, e.g., x i , y i , z i . The RF exposure time averaging may be performed per tissue location (e.g., tissue). To demonstrate the compliance, the time-averaged RF exposure may be computed and assessed according to the following expression: 
     
       
         
           
             
               
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                 limit 
               
             
           
         
       
     
     where T is the time window associated with a time-averaged RF exposure limit (e.g., 360 seconds, 100 seconds, 60 seconds, 3 seconds, etc.), f(exposure(i), t) is the RF exposure encountered at tissue i  recorded at time t, ∀ tissue i  is the exposure recorded for all the tissue locations (tissue i ) in the exposure history at each instant in time t. In certain cases, the wireless communication device may expose multiple tissue locations (tissue) at the same instant in time. For example, the bottom half of wireless communication device may be held by hand, and the wireless communication device may be placed next to head (e.g., in a voice call situation). Here, multiple tissue locations (e.g., both hand tissue and head tissue) are exposed when the transmitting antenna is in the bottom half of wireless communication device. Similarly, in the same example, if the transmitting antenna is in the top half of the wireless communication device, then only head tissue may be exposed as hand tissue present at bottom half may be sufficiently far away. In certain cases, when multiple antennas are simultaneously transmitting, each transmitting antenna may expose different tissue locations (tissue) at the same instant in time. For example, the wireless communication device may be held by both hands (say, in landscape position for gaming) with transmitting antenna(s) in the top half of the wireless communication device exposing left hand tissues and transmitting antenna(s) in the bottom half of the wireless communication device exposing right hand tissues. In certain cases, the wireless communication device may evaluate the time-averaged RF exposure over a time window where the wireless communication device encountered different exposure scenarios during the time window. For example, during a first portion of the time window, the wireless communication device may be in a head exposure scenario (e.g., exp1), and during a second portion of the time window, the wireless communication device may be in a body-worn exposure scenario (e.g., exp2). The different exposures are correlated over the time window of the time-averaged RF exposure limit, such that the different exposures are evaluated in the same time-averaged function for determining the available transmit power margin (e.g., f(exp1, exp2, t)). Such a technique for determining the available transmit power margin may provide reduced wireless communication performance, for example, due to the application of the same RF exposure settings for the time-averaged function despite there being different RF exposure scenarios. 
     Aspects of the present disclosure provide techniques and apparatus for evaluating the time-averaged RF exposure per RF exposure scenario and/or RF exposure category. When exposure DSI changes from head to body-worn or vice versa, the exposure from an active radio may not expose the same tissue to RF energy, and therefore, the wireless communication device may take into account or consider the change in DSI when evaluating RF exposure compliance. As an example, the time-averaged RF exposure encountered for a head exposure scenario may be evaluated separately from the time-averaged RF exposure encountered for a body-worn exposure scenario, such that the RF exposure history for each RF exposure scenario may be tracked and evaluated separately, as further described herein. The RF exposure time averaging may be performed per exposure scenario or exposure category, which may include one or more exposure scenarios, as further described herein. DSIs may be grouped into exposure categories such that each exposure category is mutually exclusive from all other categories in terms of RF exposure. RF exposure during regulatory certification may be characterized for different usage conditions (DSIs): head, body-worn (trunk region), hotspot (trunk region) and extremity (grip sensor triggered—hands/feet region). As RF exposures for the DSIs may expose different parts of a human body, the RF exposure scenarios can be classified into different categories. 
     The techniques and apparatus for evaluating the time-averaged RF exposure per RF exposure scenario or category may enable desirable wireless communication performance (e.g., decreased latency, increased throughput, and/or increased signal quality), for example, due to increased transmit power margins being available for transmissions. Tracking RF exposure per tissue location and/or region may allow the wireless device to transmit at higher power levels in scenarios where different locations of the user&#39;s body are exposed to RF emissions at different time periods. For example, suppose the wireless device is positioned at location  1204   a  for a first period of time and positioned at location  1204   g  for a second period of time. As the wireless device is positioned in different locations over time, the wireless device may be allowed to effectively restart the time-averaged RF exposure compliance for each new location associated with the user&#39;s body. In other words, each tissue location may have a separate RF exposure history for tracking past transmissions. 
     Certain configurations are described herein for ensuring compliance with RF exposure limits across different tissues and/or locations on a user&#39;s body. As noted above, some examples of tracking and recording time varying exposure history include calculating or determining RF exposure based on tissues that have been grouped and/or classified into a certain number of exposure categories, and/or based on transmitting antennas that are grouped into different antenna groups. Other examples and configurations are also possible and are encompassed by this disclosure. 
     In some examples, a SAR or PD value or distribution is determined for certain tissue for each transmission. For example, SAR or PD measurements may be conducted in a laboratory for each potential tissue and/or location on a user&#39;s body (e.g., using a model or simulated tissue) during a transmission from each antenna and for each potential setting that may be used to transmit on that antenna (e.g., RAT, modulation, frequency, etc.). In some examples, a subset of these measurements is obtained, and the rest are determined using one or more techniques described above. The tissues and/or locations may correspond to discrete points or locations (e.g., such that a grid of points covers the body), or may be defined so as to be substantially continuous. Each such tissue and/or location may be tracked separately for the purpose of RF exposure. When a transmission is sent from an antenna, the exposure at a relevant tissue or location may be determined. In some examples, exposure for each such transmission is individually determined, without reference to an exposure category or antenna group. Rather, for each transmission, exposure for all affected tissue(s)/locations may be (individually) tracked. In some examples, however, discrete points (as referenced above) may correspond to or be commensurate with a DSI or other such location classification. In some examples, each transmission is associated with a distribution of exposure values, and tissue across an area or region of the body (or multiple areas or regions, for example a head and a hand) may experience RF exposure due to a transmission. As the transmitting device moves around, the distributions will also move and may vary due to contours or tissue type of the body (and the distributions may partially or wholly overlap), and thus the device (e.g., the exposure manager  122 ,  281 ) may track all locations or tissues which correspond to values in the distributions (and may add or accumulate exposure at a particular tissue or location for the purpose of time averaging). Similarly, simultaneous or serial transmissions may expose the same or additional tissue and the exposure from each of these may be tracked for each tissue and/or location. Thus, while settings and operation of the device (e.g., the UE  120 ) may be relevant to the determination of RF exposure, exposure may be tracked (e.g., by the device) in some examples from a tissue-centric perspective. 
     As described above, aspects of ensuring RF exposure compliance across different tissues and/or locations on a user&#39;s body may include tracking and recording time varying exposure history for the tissues and/or locations, where the exposed tissue may be grouped and classified into a certain number of exposure categories, and transmitting antennas may be grouped into different antenna groups. Some examples including exposure categories and/or antenna groups are described below. 
       FIG.  13    illustrates example RF exposure time windows, where the RF exposure is evaluated separately for each RF exposure scenario and/or category, in accordance with certain aspects of the present disclosure. In this example, the exposure categories may be divided into a head exposure category, which may only include a head exposure scenario (or multiple head exposure scenarios, for example a left cheek scenario, right cheek scenario, left tilt scenario, etc.), and a non-head exposure category, which may include body exposure, extremity exposure, and/or hotspot exposure scenarios. 
     In the first time series  1300 A, the wireless communication device encounters a head exposure scenario  1302  (exp1) in a first occasion, and then the wireless communication device switches to a non-head exposure scenario  1304  (exp2), such as a body exposure scenario, in a second occasion. For example, the wireless device may be used in a voice call next to a user&#39;s head (head exposure scenario) and switched from microphone to Bluetooth ON call with device placed on a belt buckle (non-head exposure scenario). Here, the head exposure scenario  1302  is evaluated in a first time window  1306  T 0 , and the non-head exposure scenario  1304  is evaluated in a second time window  1308  T 1  (which may overlap with the first time window T 0 ). T 0  and T 1  correspond to the regulatory time-averaging time windows for the RF transmissions in the exposure scenarios  1302 ,  1304  (exp1 and exp2), respectively. T 0  and T 1  may have the same or different time window lengths (durations) (for example, depending on whether the transmitting frequencies associated with the exposure scenarios  1302 ,  1304  (exp1 and exp2) are in the same or different bands for purposes of RF exposure limits). The exposure scenarios  1302 ,  1304  may be representative of sets of RF exposures, for example, where a first set of RF exposures may correspond to the head exposure scenario  1302 , and a second set of RF exposures may correspond to the non-head exposure scenarios  1304 . In some examples, a scenario corresponding to the wireless communication device being disposed at the location  1204   a  ( FIG.  12   ) and a scenario corresponding to the wireless communication device being disposed at the location  1204   b  may be included in a first set of RF exposures, and a scenario corresponding to the wireless communication device being disposed at the location  1204   g  and a scenario corresponding to the wireless communication device being disposed at the location  1204   h  may be included in a second set of RF exposures, as may scenarios in which the wireless communication device is disposed near other locations. In some scenarios, a set of RF exposures may include RF exposures corresponding to the same scenario at different times. 
     The wireless communication device may consider only the RF exposure attributable to the head exposure scenario  1302  (exp1) in determining the time-averaged RF exposure for the first time window  1306  T 0 . The time-averaged RF exposure in the first time widow  1306  T 0  may be calculated with the head exposure scenario  1302  (exp1), but without the non-head exposure scenario  1304  (exp2). The wireless communication device may consider only RF exposure attributable to the non-head exposure scenario  1304  (exp2) in determining the time-averaged RF exposure for the second time window  1308  T 1 . The time-averaged RF exposure in the second time widow  1308  T 1  may be calculated with the non-head exposure scenario  1304  (exp2), but without the head exposure scenario  1302  (exp1). The exposure for head and non-head scenarios  1302 ,  1304  may not be correlated to determine the time-averaged RF exposure for each scenario. Thus, exposure categories may be mutually exclusive of each other in terms of RF exposure. 
     In the second time series  1300 B, the wireless communication device may encounter a first head exposure scenario  1310  (exp1), and the wireless communication device may switch to a non-head exposure scenario  1312  (exp2) and then switch back to a second head exposure scenario  1314  (exp3). The exposure scenarios  1310 ,  1312 ,  1314  may be representative of sets of RF exposures, for example, where a first set of RF exposures may correspond to the head exposures scenarios  1310 ,  1314 , and a second set of RF exposures may correspond to the non-head exposure scenario  1312 . 
     As an example, the wireless device may be used for a voice call next to a user&#39;s head, the wireless device may be switched to a Bluetooth ON call with the device on a belt buckle, and then the wireless device may be moved next to the user&#39;s head with Bluetooth OFF. Here, the first head exposure scenario  1310  is evaluated in a first time window  1316  T 0 , the non-head exposure scenario  1312  is evaluated in a second time window  1318  T 1  (which may overlap with the first time window  1316  T 0 ), and the head exposure scenarios  1310 ,  1314  are evaluated in a third time window  1320  T 3 , for example, due to exposure continuity for the same tissue (exposure scenario or category). The wireless communication device may consider only the RF exposure attributable to the first head exposure scenario  1310  (exp1) in determining the time-averaged RF exposure for the first time window  1316  T 0 . The wireless communication device may consider the RF exposure attributable to the non-head exposure scenario  1312  (exp2) in a first evaluation for the time-averaged RF exposure for the second time window  1318  T 1  and separately/independently consider the RF exposure attributable to the head exposure scenarios  1310 ,  1314  (exp1 and exp3) in a second evaluation for the time-averaged RF exposure for the third time window T 3 . The wireless communication device may consider the head and non-head exposures in separate time-averaged evaluations for RF exposure compliance. 
       FIG.  14    illustrates example RF exposure settings for certain exposure scenarios and/or exposure categories, in accordance with certain aspects of the present disclosure, where P limit  represents the time-averaged transmit power level (P limit ) that corresponds to the RF exposure limit (e.g., after accounting for device uncertainty) for a given exposure scenario/RAT/frequency band/antenna. In this example, certain values for P limit  (e.g., p 0 , p 1 , p 2 , p 3 , etc.) may be assigned to various exposure scenarios and/or exposure categories per frequency band (e.g., B0, B1, B2, etc.) of a RAT (e.g., CDMA, LTE, NR, etc.) for an antenna (e.g., a radio or antenna module) and/or antenna group. As an example, for the antenna 1 (or antenna group 1), a P limit  of p 0  may be assigned to the head exposure scenario and/or head exposure category for the different RATs/bands, and the P limit  of p 1  may be assigned to the non-head exposure category and/or the non-head exposure scenarios. For an antenna group (e.g., antenna group 1), the values of P limit  may represent the smallest value of P limit  among the antennas in the antenna group.  FIG.  14    illustrates that a different P limit  (e.g., p 0 −p 23 ) may be assigned to each combination of exposure scenario, exposure category, RAT, band, antenna, and/or antenna group. In some examples, however, the P limit  for two or more combinations may be the same. 
     Those of skill in the art will understand that the parameters illustrated in  FIG.  14    are merely examples. Other parameters (e.g., backoff factors, or a power limit that corresponds to P limit  multiplied by a corresponding backoff factor for a respective combination of RAT, band, exposure scenario, exposure category, antenna, antenna group, etc.) or categories of parameters (e.g., antenna groups, antenna groupings, or exposure categorizations) may be used in addition to or instead of those illustrated. 
     In certain aspects, the wireless communication device may use a specific antenna grouping per exposure scenario and/or exposure category for evaluating time-averaged RF exposure, for example, as described herein with respect to  FIGS.  7 ,  8 , and  11   , such as the antenna groupings developed for separate transmission scenarios. For example, the various exposure scenarios may be grouped into certain categories such that each exposure category is mutually exclusive from all other categories in terms of RF exposure. As RF exposures for different DSIs are exposing different parts of the human body, the DSIs can be classified into different exposure categories for purposes of RF exposure compliance, for example, under the assumption that different DSIs may be exposing different human tissue. 
     In certain cases, exposure continuity is handled in two categories including head exposure versus non-head exposure. For example, a first exposure category may include the head region (head DSI), and a second exposure category may include a non-head region (body-worn DSI, hotspot DSI, or extremity DSI). The head exposure category may include exposure to the left cheek, left tilt, right cheek, and right tilt. The non-head exposure category may include the other exposure scenarios (e.g., body-worn, hotspot, extremity, etc.). For a given antenna and/or antenna group, all exposure in the head exposure category or non-head exposure category may be assumed to be collocated or assigned to that specific antenna group as described herein. 
     In certain cases, exposure continuity may be handled in three (or more) categories. As an example, a first exposure category may include a head region (head DSI), a second exposure category may include a trunk region (body-worn DSI, hotspot DSI), and a third exposure category may include hands/feet region (extremity DSI). All of the transmit antennas (e.g., radios or antenna modules) may be combined into one antenna group (e.g., all antennas are assumed collocated) or assigned to different antenna groups for each exposure category. In certain aspects, the exposure categories may provide a basis for grouping antennas into different antenna groups for certain DSIs, as further described herein. 
     The wireless communication device may perform time averaging per exposure category per antenna group. Once DSIs are classified into different exposure categories, the time-averaged RF exposure can be evaluated per exposure category by further classifying transmitting antennas into different antenna groups as described herein and performing time averaging per antenna group. In certain aspects, the RF exposure history may be tracked and stored separately per exposure category per antenna group, for example, as described herein with respect to  FIG.  13   , e.g., because each exposure category may be mutually exclusive of each other in terms of RF exposure, and each antenna group within each exposure category may be mutually exclusive of the other antenna groups within that exposure category in terms of RF exposure. The RF exposure for all of the exposure scenarios and/or categories may be tracked over time, for example, for evaluating the available transmit power margin for a future time interval within the time window of the time-averaged RF exposure limit. The time-averaged RF exposure for only active exposure scenarios (or categories) may be computed (for example, head/extremity DSIs, representing head exposure category and hand exposure category, may be active when a UE is held in the user&#39;s hand and against the user&#39;s head). Among the time-averaged RF exposures for the active exposure categories, the minimum allowed transmit power value determined per transmitting antenna may be selected to limit transmit power for the corresponding antenna and maintain compliance in the active exposure categories. For example, in the situation above in which the UE is held in the user&#39;s hand and against the user&#39;s head, a power limit for transmitting according to the head DSI may be determined and a power limit for transmitting according to the extremity DSI may be determined, and the lower of the two determined power limits may be selected. An active exposure category may include the one or more DSIs occurring within a time window for a time-averaged RF exposure limit. 
       FIG.  15    illustrates various antenna groupings for exposure categories, in accordance with certain aspects of the present disclosure. In a first example exposure categorization (or classification)  1500 A, there are four exposure categories  1502   a - d  (e.g., exposure category 1, exposure category 2, exposure category 3, and/or exposure category 4). Each exposure category  1502   a - d  may be associated with an antenna grouping having one or more antenna groups where all of the antennas for a wireless communication device may be assigned across the antenna groups for each exposure category. For example, in exposure category  1502   a , all of the antennas may be assigned across antenna group 1 and antenna group 2. In exposure category  1502   b , all of the antennas may be assigned across antenna groups 5-8, and so on for the other exposure categories. 
     In a second example exposure categorization  1500 B, there are also four exposure categories  1504   a - d , where a single antenna group is assigned to each of the categories. In this example, all of the antennas are assigned to the same antenna group. For example, it may be assumed that all of the antennas in the device overlap and that the exposure for any antenna will also contribute (partially or fully) to exposure for any other antenna. In another example, transmissions from individual antennas may be spatially averaged or evaluated with respect to how much exposure from one antenna overlaps with exposure from another antenna (e.g., the antennas are not assumed to be mutually exclusive, but the amount of contribution between multiple antennas is calculated based on where they are located with respect to each other and/or how their energy patterns overlap). 
     When the wireless communication device encounters a specific exposure scenario, the wireless communication device may evaluate the time-averaged RF exposure using at least one of the antenna groups in the antenna group associated with corresponding RF exposure scenario of the exposure category. For example, suppose the first exposure category corresponds to head exposure. If the wireless communication device encounters head exposure, the wireless communication device may select one (or more in case of multi-transmission scenario) of the antenna groups in the first exposure category for evaluating time-averaged RF exposure. If multiple exposure categories are active, for example, when a device is held by hand (extremity exposure category) while placed next to the head (head exposure category), then depending on the transmitting antenna(s), the wireless communication device may select one or more active antenna groups for each of the active exposure categories for evaluating time-averaged RF exposure to determine allowed power limit for a future time interval to maintain time-averaged RF exposure compliance. In this regard, a final allowed power limit for each transmitting antenna may be determined to limit the transmit power of wireless communication device, for example, by taking the minimum out of all the computed allowed power limits obtained from evaluating all active antenna groups in all active exposure categories. 
       FIG.  16    is a flow diagram illustrating example operations  1600  for wireless communication, in accordance with certain aspects of the present disclosure. The operations  1600  may be performed, for example, by a wireless device (e.g., the UE  120   a  in the wireless communication network  100 ). The operations  1600  may be implemented as software components that are executed and run on one or more processors (e.g., controller/processor  280  of  FIG.  2   ). Further, the transmission of signals by the wireless device in the operations  1100  may be enabled, for example, by one or more antennas (e.g., antennas  252  of  FIG.  2   ). In certain aspects, the transmission and/or reception of signals by the wireless device may be implemented via a bus interface of one or more processors (e.g., controller/processor  280 ) obtaining and/or outputting signals. 
     The operations  1600  may optionally begin, at block  1602 , where the wireless device may track sets of RF exposures (e.g., corresponding to exposure scenarios and/or categories) separately across a plurality of locations (e.g., locations  1204 ) associated with a human body over time. In certain aspects, the wireless device may track RF exposures in terms of past transmit powers output by the wireless device over time. In certain cases, the past transmit powers may be normalized by P limit , for example. 
     At block  1604 , the wireless device may transmit a first signal at a first transmit power determined based on a time-averaged RF exposure limit and at least one of the tracked sets of RF exposures. For example, the wireless device may transmit a control and/or data signal at the first transmit power to another wireless communication device, such as a base station. The first transmit power may be less than or equal to a maximum allowed transmit power that satisfies the time-averaged RF exposure limit. 
     In certain aspects, the wireless device may perform the RF exposure tracking per exposure category and/or scenario, for example, as described herein with respect to  FIGS.  12 - 15   . The wireless device may track a first set of RF exposures (e.g., corresponding to a head exposure category) across a first set of the locations (e.g., head exposure locations) over time, and the wireless device may track a second set of RF exposures (e.g., corresponding to a non-head exposure category) across a second set of the locations (e.g., body-worn or extremity locations) over time. The first set of RF exposures may not contribute to the second set of RF exposures. The first set of RF exposures may be mutually exclusive of the second set of RF exposures, for example, due to the first set of RF exposures and the second set of RF exposures being associated with different exposure categories. The first set of the locations may be different from the second set of the locations. The first set of the locations may correspond to a first exposure category (e.g., a head exposure category), and the second set of the locations may correspond to a second exposure category (e.g., a non-head exposure category). 
     For certain aspects, the wireless device may evaluate the time-averaged RF exposure compliance based on the tracked RF exposure associated with each of the sets of RF exposures separately. The wireless device may determine the transmit power based on the first set of RF exposures if the wireless device will be transmitting in the locations associated with the first set of RF exposures. The wireless device may determine the first transmit power based at least in part on a maximum allowed transmit power that satisfies the time-averaged RF exposure limit in a time interval while considering the first set of RF exposures or the second set of RF exposures (whichever is relevant to the RF exposure in the time interval). Assuming that the wireless device will expose the second set of locations to RF signals in the time interval, the time-averaged transmit power across the time window satisfies the time-averaged RF exposure limit, where the time-averaged transmit power encompasses the transmit powers associated with the second set of RF exposures and the maximum allowed transmit power in the time interval. The time interval may be a future time interval in a time window associated with the time-averaged RF exposure limit. 
     In certain aspects, the wireless device may evaluate the time-averaged RF exposure per antenna group, for example, as described herein with respect to  FIGS.  8 ,  11 ,  14 , and  15   . The wireless device may track the sets of RF exposures per antenna group among a plurality of antenna groups (e.g., the antenna groups  804 ,  806 ,  808 ). The wireless device may track the first set of RF exposures per antenna group among a first set of one or more antenna groups (e.g., antenna group  804 ), and the wireless device may track the second set of RF exposures per antenna group among a second set of one or more antenna groups (e.g., antenna groups  806  and  808 ). The wireless device may determine the first transmit power based at least in part on the sets of RF exposures associated with the first set of one or more antenna groups or the second set of one or more antenna groups. 
     For certain aspects, the wireless device may treat the sets of RF exposures associated with different exposure scenarios or categories separately in terms of evaluating time-averaged RF exposure compliance. For example, when the wireless device switched from exposing human tissue in a first region or location of the human body (e.g., head region) to a second region or location (e.g., torso, hands, or arms), the wireless device may take into account the past exposure tracked for the second region or location to determine a maximum allowed transmit power that satisfies the time-averaged RF exposure limit. The wireless device may determine the first transmit power based at least in part on a first maximum allowed transmit power that satisfies the time-averaged RF exposure limit in a first time interval (e.g., a future time interval) while taking into account the first set of RF exposures (e.g., corresponding to head exposure). The wireless device may be repositioned such that the wireless device will expose RF to a different region or location of the human body in a second time interval (e.g., a subsequent future time interval). The wireless device may determine a second transmit power based at least in part on a second maximum allowed transmit power that satisfies the time-averaged RF exposure limit in a second time interval while taking into account the second set of RF exposures (e.g., corresponding to non-head exposure). The wireless device may transmit a second signal at the second transmit power, where the second transmit power may be less than or equal to the second maximum allowed transmit power. 
     In certain aspects, the wireless device may track the sets of RF exposures in RF exposure categories. As an example, an RF exposure category may correspond to a group or class of one or more exposure scenarios or locations associated with the human body. To track the sets of RF exposures, the wireless device may track the sets of RF exposures across a plurality of exposure categories. Each of the exposure categories may be representative of a different location or a different set of locations among the plurality of locations. Each of the exposure categories may be mutually exclusive of the other exposure categories in terms of tracking the RF exposures. For example, a head exposure category may be mutually exclusive of a non-head exposure category. In some cases, the exposure categories may include a first exposure category associated with head exposure and a second exposure category associated with non-head exposure. In certain aspects, the exposure categories may have only two categories: the first exposure category associated with head exposure and the second exposure category associated with non-head exposure. In certain cases, the exposure categories may include a first exposure category associated with head exposure, a second exposure category associated with trunk exposure, and a third exposure category associated with extremity exposure. 
     For certain aspects, the wireless device may track the sets of RF exposures across a running time window associated with the time-averaged RF exposure limit, for example, as described herein with respect to  FIG.  13   . In certain cases, different sets of RF exposures may apply different time windows associated with time-averaged RF exposure limits. For example, a first set of RF exposures may apply a first time window, and a second set of RF exposures may apply a second time window. The wireless device may determine the first transmit power based on the time-averaged RF exposure limit for a time window and the at least one of the tracked sets of RF exposures for the same time window. 
     In certain aspects, the wireless device may identify or predict where the wireless device is arranged in relation to the human body or human tissue. The wireless device may use one or more sensors to identify the RF exposure scenario (e.g., head, torso, extremity, or hotspot) exhibited by the wireless device. The transmitted first signal may be associated with an RF exposure distribution, and the wireless device may determine a subset of the plurality of locations which experience RF exposure based on the distribution and an output of one or more sensors of the wireless device. 
     It will be appreciated that RF exposure evaluated per exposure scenario or category may enable desirable uplink performance, such as desirable uplink data rates, uplink carrier aggregation, and/or an uplink connection at the edge of a cell. 
     Example Communications Device 
       FIG.  17    illustrates a communications device  1700  (e.g., the UE  120 ) that may include various components (e.g., corresponding to means-plus-function components) configured to perform operations for the techniques disclosed herein, such as the operations illustrated in  FIG.  7    and/or  FIG.  11   . The communications device  1700  includes a processing system  1702 , which may be coupled to a transceiver  1708  (e.g., a transmitter and/or a receiver). The transceiver  1708  is configured to transmit and receive signals for the communications device  1700  via an antenna  1710 , such as the various signals as described herein. The processing system  1702  may be configured to perform processing functions for the communications device  1700 , including processing signals received and/or to be transmitted by the communications device  1700 . 
     The processing system  1702  includes a processor  1704  coupled to a computer-readable medium/memory  1712  via a bus  1706 . In certain aspects, the computer-readable medium/memory  1712  is configured to store instructions (e.g., computer-executable code) that when executed by the processor  1704 , cause the processor  1704  to perform the operations illustrated in  FIG.  7    and/or  FIG.  11   , or other operations for performing the various techniques discussed herein for providing RF exposure compliance. In certain aspects, computer-readable medium/memory  1712  stores code for generating  1714 , code for assigning  1716 , code for accessing  1718 , code for transmitting  1720 , code for receiving (or obtaining)  1722 , and/or code for determining (e.g., code for generating and/or code for receiving)  1724 . In certain aspects, the processing system  1702  has circuitry  1726  configured to implement the code stored in the computer-readable medium/memory  1712 . In certain aspects, the circuitry  1726  is coupled to the processor  1704  and/or the computer-readable medium/memory  1712  via the bus  1706 . For example, the circuitry  1726  includes circuitry for generating  1728 , circuitry for assigning  1730 , circuitry for accessing  1732 , circuitry for transmitting  1734 , circuitry for receiving (or obtaining)  1736 , and/or circuitry for determining (e.g., circuitry for generating and/or circuitry for receiving)  1738 . 
       FIG.  18    illustrates a communications device  1800  (e.g., the UE  120 ) that may include various components (e.g., corresponding to means-plus-function components) configured to perform operations for the techniques disclosed herein, such as the operations illustrated in  FIG.  16   . The communications device  1800  includes a processing system  1802 , which may be coupled to a transceiver  1808  (e.g., a transmitter and/or a receiver). The transceiver  1808  is configured to transmit and receive signals for the communications device  1800  via an antenna  1810 , such as the various signals as described herein. The processing system  1802  may be configured to perform processing functions for the communications device  1800 , including processing signals received and/or to be transmitted by the communications device  1800 . 
     The processing system  1802  includes a processor  1804  coupled to a computer-readable medium/memory  1812  via a bus  1806 . In certain aspects, the computer-readable medium/memory  1812  is configured to store instructions (e.g., computer-executable code) that when executed by the processor  1804 , cause the processor  1804  to perform the operations illustrated in  FIG.  16   , or other operations for performing the various techniques discussed herein for providing RF exposure compliance. In certain aspects, computer-readable medium/memory  1812  stores code for tracking  1814 , code for determining  1816 , code for transmitting (or code for outputting for transmission)  1818 , or any combination thereof. In certain aspects, the processing system  1802  has circuitry  1820  configured to implement the code stored in the computer-readable medium/memory  1812 . In certain aspects, the circuitry  1820  is coupled to the processor  1804  and/or the computer-readable medium/memory  1812  via the bus  1806 . For example, the circuitry  1820  includes circuitry for tracking  1822 , circuitry for determining  1824 , circuitry for transmitting (or circuitry for outputting for transmission)  1826 , or any combination thereof. 
     Means for transmitting (or means for outputting for transmission) may include a transmitter and one or more antennas, such as the transceiver  254  and/or antenna(s)  252  of the UE  120   a  illustrated in  FIG.  2    and/or circuitry for transmitting  1826  of the communication device  1800  in  FIG.  18   . Means for tracking and/or means for determining may include a processing system, which may include one or more processors, such as the receive processor  258 , the transmit processor  264 , the TX MIMO processor  266 , and/or the controller/processor  280  of the UE  120   a  illustrated in  FIG.  2   , the processing system  1802  of the communication device  1800  in  FIG.  18   , and/or the circuitry for tracking  1822  or the circuitry for determining  1824 . 
     Example Aspects 
     In addition to the various aspects described above, specific combinations of aspects are within the scope of the disclosure, some of which are detailed below: 
     Aspect 1: A method of wireless communication by a wireless device, comprising: tracking sets of radio frequency (RF) exposures separately across a plurality of locations associated with a human body over time; and transmitting a first signal at a first transmit power determined based on a time-averaged RF exposure limit and at least one of the tracked sets of RF exposures. 
     Aspect 2: The method of Aspect 1, wherein tracking the sets of RF exposures comprises: tracking a first set of RF exposures across a first set of the locations over time; and tracking a second set of RF exposures across a second set of the locations over time, wherein the first set of RF exposures does not contribute to the second set of RF exposures, and wherein the first set of the locations is different from the second set of the locations. 
     Aspect 3: The method of Aspect 2, wherein: the first set of the locations corresponds to a first exposure category; and the second set of the locations corresponds to a second exposure category. 
     Aspect 4: The method of Aspect 2 or 3, further comprising determining the first transmit power based at least in part on a maximum allowed transmit power that satisfies the time-averaged RF exposure limit in a time interval while considering the first set of RF exposures or the second set of RF exposures. 
     Aspect 5: The method according to any of Aspects 2-4, wherein tracking the sets of RF exposures further comprises tracking the sets of RF exposures per antenna group among a plurality of antenna groups. 
     Aspect 6: The method of Aspect 5, wherein: tracking the sets of RF exposures further comprises: tracking the first set of RF exposures per antenna group among a first set of one or more antenna groups; and tracking the second set of RF exposures per antenna group among a second set of one or more antenna groups; and the method further comprises determining the first transmit power based at least in part on the sets of RF exposures associated with the first set of one or more antenna groups or the second set of one or more antenna groups. 
     Aspect 7: The method according to any of Aspects 2-6, further comprising: determining the first transmit power based at least in part on a first maximum allowed transmit power that satisfies the time-averaged RF exposure limit in a first time interval while taking into account the first set of RF exposures; determining a second transmit power based at least in part on a second maximum allowed transmit power that satisfies the time-averaged RF exposure limit in a second time interval while taking into account the second set of RF exposures; and transmitting a second signal at the second transmit power. 
     Aspect 8: The method according to any of Aspects 1-7, wherein: tracking the sets of RF exposures comprises tracking the sets of RF exposures across a plurality of exposure categories, each of the exposure categories is representative of a different location or a different set of locations among the plurality of locations, and each of the exposure categories is mutually exclusive of the other exposure categories in terms of tracking the RF exposures. 
     Aspect 9: The method of Aspect 8, wherein the exposure categories include a first exposure category associated with head exposure and a second exposure category associated with non-head exposure. 
     Aspect 10: The method of Aspect 8, wherein the exposure categories include: a first exposure category associated with head exposure; a second exposure category associated with trunk exposure; and a third exposure category associated with extremity exposure. 
     Aspect 11: The method according to any of Aspects 1-10, further comprising determining the first transmit power based on the time-averaged RF exposure limit for a time window and the at least one of the tracked sets of RF exposures for the same time window. 
     Aspect 12: The method according to any of Aspects 1-11, wherein the transmitted first signal is associated with an RF exposure distribution, and wherein the method further comprises determining a subset of the plurality of locations which experience RF exposure based on the distribution and an output of one or more sensors of the wireless device. 
     Aspect 13: An apparatus for wireless communication, comprising: a memory; and a processor coupled to the memory, the processor being configured to: track sets of radio frequency (RF) exposures separately across a plurality of locations associated with a human body over time, and output a first signal for transmission at a first transmit power determined based on a time-averaged RF exposure limit and at least one of the tracked sets of RF exposures. 
     Aspect 14: The apparatus of Aspect 13, wherein to track the sets of RF exposures, the processor is further configured to: track a first set of RF exposures across a first set of the locations over time; and track a second set of RF exposures across a second set of the locations over time, wherein the first set of RF exposures does not contribute to the second set of RF exposures, and wherein the first set of the locations is different from the second set of the locations. 
     Aspect 15: The apparatus of Aspect 14, wherein: the first set of the locations corresponds to a first exposure category; and the second set of the locations corresponds to a second exposure category. 
     Aspect 16: The apparatus of Aspect 14 or 15, wherein the processor is further configured to determine the first transmit power based at least in part on a maximum allowed transmit power that satisfies the time-averaged RF exposure limit in a time interval while considering the first set of RF exposures or the second set of RF exposures. 
     Aspect 17: The apparatus according to any of Aspects 14-16, wherein to track the sets of RF exposures, the processor is further configured to track the sets of RF exposures per antenna group among a plurality of antenna groups. 
     Aspect 18: The apparatus of Aspect 17, wherein the processor is further configured to: track the first set of RF exposures per antenna group among a first set of one or more antenna groups; track the second set of RF exposures per antenna group among a second set of one or more antenna groups; and determine the first transmit power based at least in part on the sets of RF exposures associated with the first set of one or more antenna groups or the second set of one or more antenna groups. 
     Aspect 19: The apparatus according to any of Aspects 14-18, wherein the processor is further configured to: determine the first transmit power based at least in part on a first maximum allowed transmit power that satisfies the time-averaged RF exposure limit in a first time interval while taking into account the first set of RF exposures, determine a second transmit power based at least in part on a second maximum allowed transmit power that satisfies the time-averaged RF exposure limit in a second time interval while taking into account the second set of RF exposures, and output a second signal for transmission at the second transmit power. 
     Aspect 20: The apparatus according to any of Aspects 13-19, wherein: to track the sets of RF exposures, the processor is further configured to track the sets of RF exposures across a plurality of exposure categories, each of the exposure categories is representative of a different location or a different set of locations among the plurality of locations, and each of the exposure categories is mutually exclusive of the other exposure categories in terms of tracking the RF exposures. 
     Aspect 21: The apparatus of Aspect 20, wherein the exposure categories include a first exposure category associated with head exposure and a second exposure category associated with non-head exposure. 
     Aspect 22: The apparatus of Aspect 20, wherein the exposure categories include: a first exposure category associated with head exposure; a second exposure category associated with trunk exposure; and a third exposure category associated with extremity exposure. 
     Aspect 23: The apparatus according to any of Aspects 13-22, wherein the processor is further configured to determine the first transmit power based on the time-averaged RF exposure limit for a time window and the at least one of the tracked sets of RF exposures for the same time window. 
     Aspect 24: The apparatus according to any of Aspects 13-23, wherein the transmitted first signal is associated with an RF exposure distribution, and wherein the processor is further configured to determine a subset of the plurality of locations which experience RF exposure based on the distribution and an output of one or more sensors of the apparatus. 
     Aspect 25: The apparatus according to any of Aspects 13-24, further comprising a transmitter configured to transmit the first signal at the first transmit power. 
     Aspect 26: An apparatus for wireless communication, comprising: means for tracking sets of radio frequency (RF) exposures separately across a plurality of locations associated with a human body over time; and means for transmitting a first signal at a first transmit power determined based on a time-averaged RF exposure limit and at least one of the tracked sets of RF exposures. 
     Aspect 27: The apparatus of Aspect 26, wherein the means for tracking the sets of RF exposures is configured to: track a first set of RF exposures across a first set of the locations over time; and track a second set of RF exposures across a second set of the locations over time, wherein the first set of RF exposures does not contribute to the second set of RF exposures, and wherein the first set of the locations is different from the second set of the locations. 
     Aspect 28: The apparatus of Aspect 27, wherein: the first set of the locations corresponds to a first exposure category; and the second set of the locations corresponds to a second exposure category. 
     Aspect 29: The apparatus of Aspect 27 or 28, further comprising means for determining the first transmit power based at least in part on a maximum allowed transmit power that satisfies the time-averaged RF exposure limit in a time interval while considering the first set of RF exposures or the second set of RF exposures. 
     Aspect 30: The apparatus according to any of Aspects 27-29, wherein the means for tracking the sets of RF exposures is configured to track the sets of RF exposures per antenna group among a plurality of antenna groups. 
     Aspect 31: The apparatus of Aspect 30, wherein: the means for tracking the sets of RF exposures further is configured to: track the first set of RF exposures per antenna group among a first set of one or more antenna groups; and track the second set of RF exposures per antenna group among a second set of one or more antenna groups; and the apparatus further comprises means for determining the first transmit power based at least in part on the sets of RF exposures associated with the first set of one or more antenna groups or the second set of one or more antenna groups. 
     Aspect 32: The apparatus according to any of Aspects 27-31, further comprising: means for determining the first transmit power based at least in part on a first maximum allowed transmit power that satisfies the time-averaged RF exposure limit in a first time interval while taking into account the first set of RF exposures; means for determining a second transmit power based at least in part on a second maximum allowed transmit power that satisfies the time-averaged RF exposure limit in a second time interval while taking into account the second set of RF exposures; and means for transmitting a second signal at the second transmit power. 
     Aspect 33: The apparatus according to any of Aspects 26-32, wherein: means for tracking the sets of RF exposures is further configured to track the sets of RF exposures across a plurality of exposure categories, each of the exposure categories is representative of a different location or a different set of locations among the plurality of locations, and each of the exposure categories is mutually exclusive of the other exposure categories in terms of tracking the RF exposures. 
     Aspect 34: The apparatus of Aspect 33, wherein the exposure categories include a first exposure category associated with head exposure and a second exposure category associated with non-head exposure. 
     Aspect 35: The apparatus of Aspect 33, wherein the exposure categories include: a first exposure category associated with head exposure; a second exposure category associated with trunk exposure; and a third exposure category associated with extremity exposure. 
     Aspect 36: The apparatus according to any of Aspects 26-35, further comprising means for determining the first transmit power based on the time-averaged RF exposure limit for a time window and the at least one of the tracked sets of RF exposures for the same time window. 
     Aspect 37: The apparatus according to any of Aspects 26-36, wherein the transmitted first signal is associated with an RF exposure distribution, and wherein the apparatus further comprises means for determining a subset of the plurality of locations which experience RF exposure based on the distribution and an output of one or more sensors of the apparatus. 
     Aspect 38: An apparatus, comprising: a memory comprising computer-executable instructions; and one or more processors configured to execute the computer-executable instructions and cause the apparatus to perform a method in accordance with any of Aspects 1-12. 
     Aspect 39: An apparatus, comprising means for performing a method in accordance with any of Aspects 1-12. 
     Aspect 40: A non-transitory computer-readable medium comprising computer-executable instructions that, when executed by one or more processors of a processing system, cause the processing system to perform a method in accordance with any of Aspects 1-12. 
     Aspect 41: A computer program product embodied on a computer-readable storage medium comprising code for performing a method in accordance with any of Aspects 1-12. 
     The techniques described herein may be used for various wireless communication technologies, such as NR (e.g., 5G NR), 3GPP Long Term Evolution (LTE), LTE-Advanced (LTE-A), code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), orthogonal frequency division multiple access (OFDMA), single-carrier frequency division multiple access (SC-FDMA), time division synchronous code division multiple access (TD-SCDMA), and other networks. The terms “network” and “system” are often used interchangeably. A CDMA network may implement a radio technology such as Universal Terrestrial Radio Access (UTRA), cdma2000, etc. UTRA includes Wideband CDMA (WCDMA) and other variants of CDMA. cdma2000 covers IS-2000, IS-95 and IS-856 standards. A TDMA network may implement a radio technology such as Global System for Mobile Communications (GSM). An OFDMA network may implement a radio technology such as NR (e.g., 5G RA), Evolved UTRA (E-UTRA), Ultra Mobile Broadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, Flash-OFDMA, etc. UTRA and E-UTRA are part of Universal Mobile Telecommunication System (UMTS). LTE and LTE-A are releases of UMTS that use E-UTRA. UTRA, E-UTRA, UMTS, LTE, LTE-A and GSM are described in documents from an organization named “3rd Generation Partnership Project” (3GPP). cdma2000 and UMB are described in documents from an organization named “3rd Generation Partnership Project 2” (3GPP2). NR is an emerging wireless communications technology under development. 
     In 3GPP, the term “cell” can refer to a coverage area of a Node B (NB) and/or a NB subsystem serving this coverage area, depending on the context in which the term is used. In NR systems, the term “cell” and BS, next generation NodeB (gNB or gNodeB), access point (AP), distributed unit (DU), carrier, or transmission reception point (TRP) may be used interchangeably. A BS may provide communication coverage for a macro cell, a pico cell, a femto cell, and/or other types of cells. A macro cell may cover a relatively large geographic area (e.g., several kilometers in radius) and may allow unrestricted access by UEs with service subscription. A pico cell may cover a relatively small geographic area and may allow unrestricted access by UEs with service subscription. A femto cell may cover a relatively small geographic area (e.g., a home) and may allow restricted access by UEs having an association with the femto cell (e.g., UEs in a closed subscriber group (CSG), UEs for users in the home, etc.). A BS for a macro cell may be referred to as a macro BS. A BS for a pico cell may be referred to as a pico BS. A BS for a femto cell may be referred to as a femto BS or a home BS. 
     A UE as described herein may be configured as a mobile station, a terminal, an access terminal, a subscriber unit, a station, a customer premises equipment (CPE), a cellular phone, a smart phone, a personal digital assistant (PDA), a wireless modem, a wireless communication device, a handheld device, a laptop computer, a cordless phone, a wireless local loop (WLL) station, a tablet computer, a camera, a gaming device, a netbook, a smartbook, an ultrabook, an appliance, a medical device or medical equipment, a biometric sensor/device, a wearable device such as a smart watch, smart clothing, smart glasses, a smart wrist band, smart jewelry (e.g., a smart ring, a smart bracelet, etc.), an entertainment device (e.g., a music device, a video device, a satellite radio, etc.), a vehicular component or sensor, a smart meter/sensor, industrial manufacturing equipment, a global positioning system device, or any other suitable device that is configured to communicate via a wireless or wired medium. Some UEs may be considered machine-type communication (MTC) devices or evolved MTC (eMTC) devices. MTC and eMTC UEs include, for example, robots, drones, remote devices, sensors, meters, monitors, location tags, etc., that may communicate with a BS, another device (e.g., remote device), or some other entity. A wireless node may provide, for example, connectivity for or to a network (e.g., a wide area network such as Internet or a cellular network) via a wired or wireless communication link. Some UEs may be considered Internet-of-Things (IoT) devices, which may be narrowband IoT (NB-IoT) devices. 
     In some examples, access to the air interface may be scheduled. A scheduling entity (e.g., a BS) allocates resources for communication among some or all devices and equipment within its service area or cell. The scheduling entity may be responsible for scheduling, assigning, reconfiguring, and releasing resources for one or more subordinate entities. That is, for scheduled communication, subordinate entities utilize resources allocated by the scheduling entity. Base stations are not the only entities that may function as a scheduling entity. In some examples, a UE may function as a scheduling entity and may schedule resources for one or more subordinate entities (e.g., one or more other UEs), and the other UEs may utilize the resources scheduled by the UE for wireless communication. In some examples, a UE may function as a scheduling entity in a peer-to-peer (P2P) network, and/or in a mesh network. In a mesh network example, UEs may communicate directly with one another in addition to communicating with a scheduling entity. 
     The methods disclosed herein comprise one or more steps or actions for achieving the methods. The method steps and/or actions may be interchanged with one another without departing from the scope of the claims. In other words, unless a specific order of steps or actions is specified, the order and/or use of specific steps and/or actions may be modified without departing from the scope of the claims. 
     As used herein, a phrase referring to “at least one of” a list of items refers to any combination of those items, including single members. As an example, “at least one of: a, b, or c” is intended to cover a, b, c, a-b, a-c, b-c, and a-b-c, as well as any combination with multiples of the same element (e.g., a-a, a-a-a, a-a-b, a-a-c, a-b-b, a-c-c, b-b, b-b-b, b-b-c, c-c, and c-c-c or any other ordering of a, b, and c). 
     As used herein, the term “determining” encompasses a wide variety of actions. For example, “determining” may include calculating, computing, processing, deriving, generating, investigating, looking up (e.g., looking up in a table, a database or another data structure), ascertaining, and the like. Also, “determining” may include receiving (e.g., receiving information), accessing (e.g., accessing data in a memory), and the like. Also, “determining” may include resolving, selecting, choosing, establishing, and the like. 
     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 of the 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.” Unless specifically stated otherwise, the term “some” refers to one or more. 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. No claim element is to be construed under the provisions of 35 U.S.C. § 112(f) unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.” 
     The various operations of methods described above may be performed by any suitable means capable of performing the corresponding functions. The means may include various hardware and/or software component(s) and/or module(s), including, but not limited to a circuit, an application specific integrated circuit (ASIC), or processor. Generally, where there are operations illustrated in figures, those operations may have corresponding counterpart means-plus-function components with similar numbering. 
     The various illustrative logical blocks, modules and circuits described in connection with the present disclosure may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device (PLD), discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but in the alternative, the processor may be any commercially available processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     If implemented in hardware, an example hardware configuration may comprise a processing system in a wireless node. The processing system may be implemented with a bus architecture. The bus may include any number of interconnecting buses and bridges depending on the specific application of the processing system and the overall design constraints. The bus may link together various circuits including a processor, machine-readable media, and a bus interface. The bus interface may be used to connect a network adapter, among other things, to the processing system via the bus. The network adapter may be used to implement the signal processing functions of the physical (PHY) layer. In the case of a UE (see  FIG.  1   ), a user interface (e.g., keypad, display, mouse, joystick, etc.) may also be connected to the bus. The bus may also link various other circuits such as timing sources, peripherals, voltage regulators, power management circuits, and the like, which are well known in the art, and therefore, will not be described any further. The processor may be implemented with one or more general-purpose and/or special-purpose processors. Examples include microprocessors, microcontrollers, DSP processors, and other circuitry that can execute software. Those skilled in the art will recognize how best to implement the described functionality for the processing system depending on the particular application and the overall design constraints imposed on the overall system. 
     If implemented in software, the functions may be stored or transmitted over as one or more instructions or code on a computer-readable medium. Software shall be construed broadly to mean instructions, data, or any combination thereof, whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise. Computer-readable media include both computer storage media and communication media including any medium that facilitates transfer of a computer program from one place to another. The processor may be responsible for managing the bus and general processing, including the execution of software modules stored on the machine-readable storage media. A computer-readable storage medium may be coupled to a processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. By way of example, the machine-readable media may include a transmission line, a carrier wave modulated by data, and/or a computer-readable storage medium with instructions stored thereon separate from the wireless node, all of which may be accessed by the processor through the bus interface. Alternatively, or in addition, the machine-readable media, or any portion thereof, may be integrated into the processor, such as the case may be with cache and/or general register files. Examples of machine-readable storage media may include, by way of example, RAM (Random Access Memory), flash memory, ROM (Read-Only Memory), PROM (Programmable Read-Only Memory), EPROM (Erasable Programmable Read-Only Memory), EEPROM (Electrically Erasable Programmable Read-Only Memory), registers, magnetic disks, optical disks, hard drives, or any other suitable storage medium, or any combination thereof. The machine-readable media may be embodied in a computer program product. 
     A software module may comprise a single instruction, or many instructions, and may be distributed over several different code segments, among different programs, and across multiple storage media. The computer-readable media may comprise a number of software modules. The software modules include instructions that, when executed by an apparatus such as a processor, cause the processing system to perform various functions. The software modules may include a transmission module and a receiving module. Each software module may reside in a single storage device or be distributed across multiple storage devices. By way of example, a software module may be loaded into RAM from a hard drive when a triggering event occurs. During execution of the software module, the processor may load some of the instructions into cache to increase access speed. One or more cache lines may then be loaded into a general register file for execution by the processor. When referring to the functionality of a software module below, it will be understood that such functionality is implemented by the processor when executing instructions from that software module. 
     Also, any connection is properly termed a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared (IR), radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. Disk and disc, as used herein, include compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk, and Blu-ray® disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Thus, in some aspects computer-readable media may comprise non-transitory computer-readable media (e.g., tangible media). In addition, for other aspects computer-readable media may comprise transitory computer-readable media (e.g., a signal). Combinations of the above should also be included within the scope of computer-readable media. 
     Thus, certain aspects may comprise a computer program product for performing the operations presented herein. For example, such a computer program product may comprise a computer-readable medium having instructions stored (and/or encoded) thereon, the instructions being executable by one or more processors to perform the operations described herein, for example, instructions for performing the operations described herein and illustrated in  FIG.  16   . 
     Further, it should be appreciated that modules and/or other appropriate means for performing the methods and techniques described herein can be downloaded and/or otherwise obtained by a user terminal and/or base station as applicable. For example, such a device can be coupled to a server to facilitate the transfer of means for performing the methods described herein. Alternatively, various methods described herein can be provided via storage means (e.g., RAM, ROM, or a physical storage medium such as a compact disc (CD) or floppy disk, etc.), such that a user terminal and/or base station can obtain the various methods upon coupling or providing the storage means to the device. Moreover, any other suitable technique for providing the methods and techniques described herein to a device can be utilized. 
     It is to be understood that the claims are not limited to the precise configuration and components illustrated above. Various modifications, changes, and variations may be made in the arrangement, operation, and details of the methods and apparatus described above without departing from the scope of the claims.