Efficient determination of RF exposure for MIMO transmitters

Abstract

Aspects of the present disclosure facilitate evaluating radio frequency (RF) exposure from a wireless device that supports multiple-input multiple-output (MIMO) transmission using multiple antennas. In certain aspects, a MIMO RF exposure distribution for one or more MIMO transmissions is determined and the MIMO RF exposure distribution is stored in a memory. To assess RF exposure of a MIMO transmission, a processor may retrieve the corresponding MIMO RF exposure distribution from the memory, linearly combine the MIMO RF exposure distributions to obtain a combined MIMO RF exposure distribution, and assess RF exposure compliance based on the combined MIMO RF exposure distribution. In one example, the MIMO RF exposure profile may include a MIMO specific absorption rate (SAR) profile.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This patent application claims priority to pending U.S. non-provisional application No. 17 / 952,169, filed on September 23, 2022, which is assigned to the assignee of the present application and is hereby expressly incorporated herein by reference as if fully set forth below and for all applicable purposes. Background Art Technical Field

[0003] Aspects of the present disclosure relate generally to wireless devices, and more particularly to assessing radio frequency (RF) exposure from wireless devices. Background Art

[0005] Modern wireless devices (e.g., cellular phones) are generally required to limit the user's exposure to RF radiation according to radio frequency (RF) exposure limits set by domestic and international regulatory agencies. In order to ensure that wireless devices comply with RF exposure limits, technologies have been developed that enable wireless devices to assess the RF exposure from the wireless device in real time and adjust the transmit power of the wireless device accordingly to comply with the RF exposure limits. Summary of the invention

[0006] The following content presents a simplified summary of one or more embodiments in order to provide a basic understanding of such embodiments. This summary is not an extensive overview of all contemplated embodiments, and is neither intended to identify the key or important elements of all embodiments, nor to describe the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a preface to the more detailed description presented subsequently.

[0007] A first aspect relates to a wireless device. The wireless device includes: a plurality of transmit circuits, wherein the plurality of transmit circuits are configured to transmit a plurality of radio frequency (RF) signals via a plurality of antennas; and a processor, the processor being coupled to the transmit circuits. The processor is configured to combine a multiple-input multiple-output (MIMO) radio frequency (RF) exposure distribution into a combined MIMO RF exposure distribution, wherein the combined MIMO RF exposure distribution comprises a linear combination of the MIMO RF exposure distributions. The processor is further configured to determine a maximum allowable power level based on the combined MIMO RF exposure distribution, wherein each of the maximum allowable power levels corresponds to a corresponding one of the transmit circuits. The processor is further configured to set a corresponding power level limit for each of the transmit circuits based on the corresponding one of the maximum allowable power levels.

[0008] A second aspect relates to a method for operating a wireless device including a plurality of transmit circuits, wherein the transmit circuits are configured to transmit a plurality of radio frequency (RF) signals via a plurality of antennas. The method includes combining a multiple-input multiple-output (MIMO) radio frequency (RF) exposure distribution into a combined MIMO RF exposure distribution, wherein the combined MIMO RF exposure distribution includes a linear combination of the MIMO RF exposure distributions. The method also includes determining a maximum allowable power level based on the combined MIMO RF exposure distribution, wherein each of the maximum allowable power levels corresponds to a corresponding one of the transmit circuits. The method also includes setting a corresponding power level limit for each of the transmit circuits based on the corresponding one of the maximum allowable power levels.

[0009] A third aspect relates to a computer-readable medium. The computer-readable memory includes instructions stored thereon for combining multiple-input multiple-output (MIMO) radio frequency (RF) exposure profiles into a combined MIMO RF exposure profile, wherein the combined MIMO RF exposure profile comprises a linear combination of the MIMO RF exposure profiles. The computer-readable memory also includes instructions for: determining a maximum allowable power level based on the combined MIMO RF exposure profile, wherein each of the maximum allowable power levels corresponds to a respective one of a plurality of transmit circuits; and setting a respective power level limit for each of the transmit circuits based on the respective one of the maximum allowable power levels.

[0010] To achieve the foregoing and related purposes, one or more embodiments include the features fully described below and specifically pointed out in the claims. The following description and the accompanying illustrations set forth in detail certain illustrative aspects of the one or more embodiments. However, these aspects are merely indicative of several of the various ways in which the principles of the various embodiments may be employed, and the described embodiments are intended to include all such aspects and their equivalents. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 An example of a wireless device is shown in which aspects of the present disclosure may be implemented.

[0012] Figure 2 Examples of wireless devices including transmit circuits, each of which includes a respective amplifier, are shown in accordance with certain aspects of the present disclosure.

[0013] Figure 3 Shown is a visual representation of a combined specific absorption rate (SAR) distribution in accordance with certain aspects of the present disclosure.

[0014] Figure 4 is a flow chart illustrating an example method for determining a maximum allowed power level for a transmit antenna in accordance with certain aspects of the present disclosure.

[0015] Figure 5 Shown is a visual representation of a time-averaged SAR distribution in accordance with certain aspects of the present disclosure.

[0016] Figure 6 is a flow chart illustrating an exemplary method for determining a maximum allowable power level for a transmit antenna that complies with time-averaged RF exposure limits in accordance with certain aspects of the present disclosure.

[0017] Figure 7 is a flow chart illustrating a method for operating a wireless device in accordance with certain aspects of the present disclosure. DETAILED DESCRIPTION

[0018] The specific embodiments described below in conjunction with the accompanying drawings are intended as descriptions of various configurations and are not intended to represent the only configurations with which the concepts described herein can be practiced. In order to provide a comprehensive understanding of the various concepts, the specific embodiments include specific details. However, it is apparent to those skilled in the art that these concepts can be practiced without these specific details. In some instances, in order to avoid obscuring such concepts, well-known structures and components are shown in block diagram form.

[0019] Figure 1 An example of a wireless device 100 is shown in which aspects of the present disclosure described herein may be implemented. The wireless device 100 may include a mobile wireless device (e.g., a cellular phone), a laptop computer, a wireless access point, an Internet of Things (IoT) device, consumer premises equipment (CPE), a base station, or another wireless device.

[0020] The wireless device 100 includes a processor 110 and a memory 115. The memory 115 is coupled to the processor 110 and may store instructions that, when executed by the processor 110, cause the processor 110 to perform one or more of the operations described herein. The memory 115 may include a random access memory (RAM), a read-only memory (ROM), a flash memory, registers, or any combination thereof. The processor 110 may be implemented with a general purpose processor, a digital signal processor (DSP), a baseband modem (also referred to as a baseband processor), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device (PLD), discrete gate logic components, discrete hardware components, or any combination thereof configured to perform one or more of the operations described herein.

[0021] The wireless device 100 further includes a plurality of transmission circuits 120-1 to 120-N and a plurality of antennas 130-1 to 130-N. Figure 1In the example shown, each of the transmit circuits 120-1 to 120-N has a respective input 122-1 to 122-N and a respective output 125-1 to 125-N. For each of the transmit circuits 120-1 to 120-N, the respective input 122-1 to 122-N may be coupled to a respective output 118-1 to 118-N of the processor 110, and the respective output 125-1 to 125-N may be coupled to a respective one of the antennas 130-1 to 130-N. As discussed further below, each of the transmit circuits 120-1 to 120-N may be configured to output a respective one of a plurality of RF signals to a respective antenna 130-1 to 130-N via a respective output 125-1 to 125-N. The antennas 130-1 to 130-N may be arranged in a one-dimensional array, in a two-dimensional array, in a three-dimensional array, placed in different locations on the wireless device 100, etc. Each of the antennas 130-1 to 130-N may be implemented with a patch antenna or another type of antenna. The antennas 130-1 to 130-N may also be referred to as antenna elements or another term. Each of the transmitting circuits 120-1 to 120-N may also be referred to as a transmitter or another term.

[0022] In certain aspects, the transmitting circuits 120-1 to 120-N are configured to transmit RF signals via the antennas 130-1 to 130-N using one or more wireless communication technologies, including but not limited to third generation (3G) technology (e.g., CDMA), fourth generation (4G) technology (also known as long term evolution (LTE)), fifth generation (5G) technology, one or more technologies based on one or more IEEE 802.11 or 802.15 or 802.16 protocols (e.g., IEEE 802.11ac, IEEE802.11n, IEEE 802.11ad, IEEE 802.11ax, IEEE 802.11ay, etc.), Bluetooth technology and / or one or more other technologies.

[0023] In certain aspects, the wireless device 100 uses multiple-input multiple-output (MIMO) transmission to transmit data and / or control information to another wireless device (not shown) (e.g., to increase data throughput between the wireless device 100 and the other wireless device). In these aspects, the transmit circuits 120-1 to 120-N may transmit multiple RF signals simultaneously via the antennas 130-1 to 130-N. For example, the transmit circuits 120-1 to 120-N may transmit multiple RF signals at the same transmit frequency (e.g., the multiple RF signals may have the same center frequency, the same subcarrier, etc.). As used herein, "transmit frequency" may refer to a center frequency (e.g., of a frequency band), a subcarrier, a group of subcarriers, a frequency band, etc. For MIMO transmission, the wireless device 100 may employ spatial multiplexing, diversity coding, precoding, beamforming, multi-user MIMO, etc. For an example of MIMO transmission, the transmit circuits 120-1 to 120-N may transmit multiple RF signals (e.g., coherent RF signals) having the same transmit frequency via the antennas 130-1 to 130-N (e.g., using the same radio access technology (RAT)), where each of the transmit circuits 120-1 to 120-N transmits a corresponding one of the RF signals via a corresponding one of the antennas 130-1 to 130-N. For example, MIMO may be used to provide spatial diversity and / or increased transmit power. It should be appreciated that in some use cases, a subset of the transmit circuits 120-1 to 120-N and a subset of the antennas 130-1 to 130-N may be used to transmit MIMO transmissions rather than all of the transmit circuits 120-1 to 120-N and all of the antennas 130-1 to 130-N.

[0024] In order to transmit data and / or control information, the processor 110 may process the data and / or control information into one or more signals (e.g., one or more baseband signals or intermediate frequency (IF) signals). The processing performed by the processor 110 may include decoding the data and / or control information, and modulating the decoded data and / or control information (e.g., using any of a variety of different modulation schemes, including BPSK, QPSK, QAM, etc.). For the example of MIMO, the processor 110 may also perform spatial multiplexing, diversity decoding, pre-coding, etc. The processor 110 outputs the one or more signals to one or more of the transmit circuits 120-1 to 120-N. The one or more transmit circuits 120-1 to 120-N are configured to convert the one or more signals from the processor 110 into one or more RF signals, and output the one or more RF signals to one or more of the antennas 130-1 to 130-N for transmission. For the example of MIMO, the processor 110 may output multiple signals (also referred to as streams) to the transmit circuits 120-1 to 120-N, where each of the transmit circuits 120-1 to 120-N receives a corresponding one of the signals. Each transmit circuit 120-1 to 120-N may process a corresponding one of the signals into a corresponding RF signal and output the corresponding RF signal to a corresponding one of the antennas 130-1 to 130-N for transmission. The processing performed by each of the transmit circuits 120-1 to 120-N may include filtering, frequency up-conversion, power amplification, and / or any combination thereof.

[0025] In certain aspects, processor 110 may set the transmit power level of each of transmit circuits 120 - 1 through 120 -N using any of a variety of techniques. Figure 2 An example is illustrated in FIG. 1 , where each of the transmit circuits 120-1 to 120-N includes a respective amplifier 250-1 to 250-N. An input terminal 252-1 to 252-N of each amplifier 250-1 to 250-N is coupled to a respective input terminal 122-1 to 122-N, and an output terminal 254-1 to 254-N of each amplifier 250-1 to 250-N is coupled to a respective output terminal 125-1 to 125-N. Each amplifier 250-1 to 250-N may include a respective power amplifier, a variable gain amplifier, or another type of amplifier.

[0026] It should be understood that each of the transmitting circuits 120-1 to 120-N may include a Figure 2The one or more additional components in each of the transmit circuits 120-1 to 120-N may include a mixer (e.g., for frequency conversion), an additional amplifier, a filter, or any combination thereof. In some aspects, the one or more additional elements in each of the transmit circuits 120-1 to 120-N may be coupled with the respective amplifiers 250-1 to 250-N in a chain (e.g., an RF chain), but is not limited to this example.

[0027] Each amplifier 250-1 to 250-N may have a corresponding control input 256-1 to 256-N, such as Figure 2 As shown in the example of . In this example, each amplifier 250-1 to 250-N is configured to receive a corresponding signal (e.g., a corresponding RF signal) at a corresponding input terminal 252-1 to 252-N, amplify the corresponding signal at a corresponding gain, and output the corresponding amplified signal at a corresponding output terminal 254-1 to 254-N. Each amplifier 250-1 to 250-N is also configured to receive a corresponding gain control signal (labeled as "Gain Control 1" to "Gain Control N") at a corresponding control input terminal 256-1 to 256-N and set a corresponding gain based on the corresponding gain control signal.

[0028] exist Figure 2 In the example of FIG. 1 , the processor 110 is coupled to the control inputs 256-1 to 256-N of the amplifiers 250-1 to 250-N. In this example, the processor 110 can set the output transmit power level of each of the transmit circuits 120-1 to 120-N by correspondingly setting the gain of the corresponding amplifiers 250-1 to 250-N using the corresponding gain control signal. For example, the processor 110 can set the transmit power level of one of the transmit circuits 120-1 to 120-N to a higher transmit power level by increasing the gain of the corresponding amplifier 250-1 to 250-N.

[0029] In some aspects, the processor 110 may set the transmit power level of each of the antennas 130-1 to 130-N by setting the gain of the corresponding amplifiers 250-1 to 250-N accordingly. In one example, the processor 110 may individually set the transmit power levels of the antennas 130-1 to 130-N by individually setting the gain of the corresponding amplifiers 250-1 to 250-N using corresponding gain control signals. It should be understood that the present disclosure is not limited to this example, and the processor 110 may use other techniques to set the transmit power levels of the antennas 130-1 to 130-N. For example, in some specific implementations, the gains of the amplifiers 250-1 to 250-N may be controlled by the same gain control signal. In these specific implementations, the processor 110 may increase or decrease the gains of the amplifiers 250-1 to 250-N in unison.

[0030] It should be understood that the various aspects of the present disclosure are not limited to Figure 2 The examples shown are for reference only, and processor 110 may use other techniques to control the transmit power level of each of transmit circuits 120 - 1 through 120 -N.

[0031] In certain aspects, the processor 110 may use an open power control loop and / or a closed power control loop to set the transmit power levels of the antennas 130-1 to 130-N. For the example of the open power control loop, the wireless device 100 may receive a pilot signal from another wireless device (not shown) via a receiver (not shown). In this example, the processor 110 estimates the channel conditions between the wireless device 100 and the other wireless device based on the received pilot signal, and sets the transmit power levels of the antennas 130-1 to 130-N based on the estimated channel conditions. For the example of the closed power control loop, the wireless device 100 receives a feedback signal from the other wireless device via a receiver (not shown), wherein the feedback signal indicates the channel conditions between the wireless device 100 and the other wireless device. In this example, the processor 110 sets the transmit power levels of the antennas 130-1 to 130-N based on the indicated channel conditions.

[0032] Processor 110 may also set the transmit power levels of antennas 130-1 through 130-N based on the data rate. For example, processor 110 may increase (ie, boost) the transmit power levels to transmit short data bursts.

[0033] In addition, processor 110 may limit the transmit power levels that may be set for antennas 130-1 through 130-N to keep the RF exposure to which a user is exposed from wireless device 100 within RF exposure limits set by regulatory agencies, as discussed further below. In this case, the transmit power levels of antennas 130-1 through 130-N are constrained by the RF exposure limits (i.e., the transmit power levels cannot be set to levels that exceed the RF exposure limits).

[0034] For situations where the wireless device 100 (e.g., a cellular phone) is used in close proximity to a user, the wireless device 100 may need to limit the user's exposure to RF radiation in accordance with exposure limits set by domestic and international regulatory agencies. RF exposure may be expressed in terms of specific absorption rate (SAR), which measures energy absorption per unit mass of human tissue 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 Unit of.

[0035] For example, SAR can be determined at a certain location based on the electric field induced by the RF transmission of the wireless device as follows:

[0036]

[0037] Where E is the electric field (E field) induced by RF transmission at the location, σ is the conductivity, and ρ is the mass density of human tissue or human tissue model. In the case of MIMO transmission using multiple antennas (e.g., antennas 130-1 to 130-N), the electric field can be the sum of the electric fields induced by RF transmission from the multiple antennas.

[0038] To assess RF exposure of transmissions from wireless device 100, wireless device 100 may include a plurality of SAR distributions stored in memory 115. Each of the SAR distributions may correspond to a respective one of a plurality of transmission scenarios supported by wireless device 100. The transmission scenarios may correspond to various combinations of antennas 130-1 through 130-N, frequency bands, channels, and / or body locations, as discussed further below.

[0039] The SAR distribution for each transmission scenario may be generated based on measurements (e.g., electric field measurements) performed in a test laboratory using a human body model. After the SAR distribution is generated, the SAR distribution is stored in the memory 115 to allow the processor 110 to evaluate the RF exposure in real time, as discussed further below. Each SAR distribution may include a set of SAR values. For example, the SAR distribution may include two SAR values, 5 or more SAR values, dozens of SAR values, or even hundreds or thousands of SAR values. The SAR values ​​in the distribution may correspond to the SAR variation of the transmission scenario. In some examples, the SAR distribution represents a map, so that each SAR value in the distribution corresponds to a different spatial location (e.g., on a human body model) or part of the user's tissue. In some aspects, the SAR value of each location in the SAR distribution may be determined by an electric field measurement at the location (e.g., based on formula (1)). For examples where the SAR distribution includes a set of SAR values, the number of SAR values ​​in the SAR distribution may depend on, for example, a trade-off between the spatial resolution of the SAR distribution and the computational load. For example, a larger number of SAR values ​​may be used to increase the resolution of the SAR distribution, while a smaller number of SAR values ​​may be used to reduce the computational load required to process the SAR distribution and / or the amount of memory space required to store the SAR distribution. In some examples, the values ​​in the SAR distribution represent exposure at a given time.

[0040] The SAR value in each SAR distribution corresponds to a particular transmit power level (e.g., the transmit power level at which the electric field of the SAR value was measured in a test lab). Since SAR scales with transmit power level, processor 110 may scale the SAR distribution for any transmit power level by multiplying each SAR value in the SAR distribution by the following transmit power scaling factor:

[0041]

[0042] Among them, Tx c is the current transmit power level of the corresponding transmit scenario, and Tx SAR is the transmit power level corresponding to the SAR value in the stored SAR distribution (e.g., the transmit power level when the electric field of the SAR value is measured in a test laboratory). In the following discussion, Tx SAR is referred to as the reference transmit power level. Therefore, once the SAR distribution is determined at the reference transmit power level, the SAR distribution can be determined for another transmit power level by scaling the SAR distribution by the transmit power scaling factor in equation (2).

[0043] As discussed above, the wireless device 100 may support multiple transmission scenarios. In some aspects, the transmission scenario may be specified by a parameter set. The parameter set may include one or more of the following: antenna parameters indicating one or more antennas (i.e., active antennas) used for transmission, band parameters indicating one or more frequency bands (i.e., active bands) used for transmission, channel parameters indicating one or more channels (i.e., active channels) used for transmission, body position parameters indicating the position of the wireless device 100 relative to the user's body position (head, torso, away from the body, etc.), the distance of human tissue from the device, and / or other parameters. In the case where the wireless device 100 supports a large number of transmission scenarios, it may be very time-consuming and expensive to perform measurements for each transmission scenario in a test environment (e.g., a test laboratory). In order to reduce test time, measurements may be performed for a transmission scenario subset to generate a SAR distribution for the transmission scenario subset. In this example, the SAR distribution of each of the remaining transmission scenarios may be generated by combining two or more of the SAR distributions of the transmission scenario subset, as further discussed below.

[0044] For example, SAR measurement may be performed for each of the antennas 130-1 to 130-N to generate a SAR distribution for each of the antennas 130-1 to 130-N. In this example, for a transmission scenario in which two or more of the antennas 130-1 to 130-N are in an active state, the SAR distribution of the transmission scenario may be generated by combining the SAR distributions of the two or more active antennas.

[0045] In certain aspects, a SAR distribution for a transmission scenario may be generated by combining two or more SAR distributions. The SAR distributions may correspond to different antennas, different frequency bands, different transmitters, any combination thereof, etc. For the case where different transmit power levels are used for the SAR distributions, each of the SAR distributions may be scaled by a corresponding transmit power scaling factor before the SAR distributions are combined into a combined SAR distribution (also referred to as a total SAR distribution) for the transmission scenario. The combined SAR distribution may be given by the following formula:

[0046]

[0047] SAR combined is the combined SAR distribution of the transmission scene, k is the index of the SAR distribution, and SAR k is the kth SAR distribution, Tx k is the current transmit power level of the kth SAR distribution, Tx SARk is the reference transmit power level corresponding to the kth SAR distribution, and m is the number of SAR distributions. SARk The k-th SAR distribution is obtained by measuring the SAR distribution in the transmission scenario. Therefore, the combined SAR distribution of the transmission scenario is the sum of the individual SAR distributions, which is a linear operation. First, the corresponding transmission power scaling factor (i.e., the ratio Tx k / Tx SARk ) each of the SAR distributions is scaled by 100,000 and the SAR distributions are combined. It should be appreciated that the number of SAR distributions (i.e., m) may be less than, equal to, or greater than the number of antennas 130-1 to 130-N. For example, when a subset of the antennas 130-1 to 130-N are used (e.g., for non-MIMO transmissions such as single-input single-output (SISO) transmissions), the number of SAR distributions may be less than the number of antennas 130-1 to 130-N. In another example, when multiple RF signals are transmitted from the same antenna (e.g., in different frequency bands), the number of SAR distributions may be greater than the number of antennas 130-1 to 130-N.

[0048] In certain aspects, the SAR distribution may be normalized relative to the SAR limit by dividing each SAR value in the SAR distribution by the SAR limit. In this case, when the normalized SAR value is greater than 1, the normalized SAR value exceeds the SAR limit, and when the normalized SAR value is less than 1, the normalized SAR value is below the SAR limit. In these aspects, each of the SAR distributions stored in the memory 115 may be normalized relative to the SAR limit. In one example, the combined SAR distribution given in equation (3) may be rewritten as follows:

[0049]

[0050] SAR norm_combined is the normalized combined SAR distribution, and SAR norm_k is the kth normalized SAR distribution. In some aspects, the SAR limit may be a regulatory SAR limit set by a regulatory body (e.g., the Federal Communications Commission (FCC) or another regulatory body) to limit a user's RF exposure. In some aspects, the SAR limit may be set to a value lower than the regulatory SAR limit, which may be referred to as a SAR design target. For example, the SAR design target may be lower than the regulatory SAR limit to account for device uncertainties and / or to budget sufficient SAR margin to meet the total RF exposure in a simultaneous transmission scenario with other transmitters. As used herein, a regulatory exposure limit refers to an exposure limit set by a regulatory body (e.g., the FCC).

[0051] In some aspects, the reference transmit power level corresponding to the SAR design target (referred to as the maximum time-averaged power limit P limit ) can be used for each SAR distribution. For example, the maximum time-averaged power limit P for the kth SAR distribution is limitk It can be given by the following formula:

[0052] P limitk = Tx SARk * SAR design target / max(SAR k ) (5)

[0053] Where max(SAR k ) is the maximum value (i.e., the maximum SAR value) in the k-th SAR distribution. In this example, by limiting the time-averaged transmit power for the k-th SAR distribution to be less than or equal to P limitk , the time-averaged SAR can be limited to be less than or equal to the SAR design target. limitk It may vary with the technology, operating frequency band, transmitting antenna and / or the position of the device relative to the human body (referred to as the device state index). In these aspects, the above formula (4) can be modified to the following formula:

[0054]

[0055] SAR norm_combined is the normalized combined SAR distribution, P limitk is the P corresponding to the kth SAR distribution limit , and SAR norm_k It is in P limitk The transmission power level of SAR is measured at the kth SAR distribution corresponding to the SAR design target (i.e., the maximum value in the kth SAR distribution corresponding to the SAR design target) and normalized to the kth SAR distribution of the SAR design target.norm_k represents the k-th SAR distribution normalized with respect to its maximum value, which is obtained by dividing each value in the k-th SAR by the maximum value in the k-th SAR distribution, so SAR norm_k Each value in is less than or equal to 1. Therefore, in the present disclosure, the normalized SAR distribution may refer to a SAR distribution normalized with respect to a SAR limit (e.g., a regulatory SAR limit or a SAR design target) and / or a SAR distribution normalized with respect to a maximum value in the SAR distribution.

[0056] Figure 3 A visual representation of a plurality of SAR distributions 310-1 to 310-m that are combined to obtain a combined SAR distribution 320 is shown. Each of the SAR distributions 310-1 to 310-m may be normalized relative to a SAR limit (e.g., a regulatory SAR limit or a SAR design target). The SAR distributions 310-1 to 310-m may be spatially aligned, where SAR values ​​in the same location are combined to obtain a combined SAR distribution. Although each of the SAR distributions 310-1 to 310-m and 320 is spatially aligned, the SAR distributions 310-1 to 310-m and 320 may be spatially aligned. Figure 3 , but it should be understood that the present disclosure is not limited to this example.

[0057] In certain aspects, when the peak SAR value (i.e., the maximum SAR value) in the combined SAR distribution 320 is equal to or less than an RF exposure limit (e.g., a regulatory SAR limit, a SAR design target, etc.), the combined SAR distribution complies with the RF exposure limit. For examples where the SAR distributions 310-1 through 310-m are normalized (e.g., relative to a regulatory SAR limit, a SAR design target, etc.), the RF exposure limit may be a normalized RF exposure limit of 1. When temporal averaging within a time window is used to assess RF exposure compliance, the peak SAR value in the combined SAR distribution 320 may temporarily exceed the RF exposure limit as long as the temporal average of the peak SAR values ​​within the time window does not exceed the RF exposure limit.

[0058] In the case of MIMO where multiple antennas (e.g., all antennas 130-1 to 130-N or a subset of antennas 130-1 to 130-N) transmit simultaneously at the same transmission frequency (e.g., center frequency, subcarrier, etc.), the combined SAR distribution is obtained by summing the square roots of the individual SAR distributions and calculating the square of the sum. In this case, the combined SAR distribution may be given by the following formula:

[0059]

[0060] where the SAR of the kth SAR distribution may be plotted relative to a SAR limit (e.g., a regulatory SAR limit or a SAR design target) norm_kIn this example, each of the SAR distributions may correspond to a corresponding one of the plurality of antennas used for MIMO transmission (e.g., a corresponding one of the antennas 130-1 to 130-N) and be normalized by a corresponding transmit power scaling factor (i.e., ratio Tx k / P limitk ) to scale each of the SAR distributions. In formula (7), the square roots of the SAR distributions of the individual antennas used for MIMO transmission are calculated. This is done to sum the electric fields of the SAR distributions of the individual antennas used for MIMO transmission (note that the SAR value is proportional to the square of the electric field in formula (1)). The sum of the square roots is then squared to obtain the combined SAR distribution for MIMO transmission.

[0061] In some aspects, the transmit circuits 120-1 to 120-N may use both MIMO transmission and non-MIMO transmission (eg, SISO transmission) to simultaneously transmit signals. In these aspects, the combined SAR distribution may be given by the following formula:

[0062]

[0063] Where d is the total number of SAR distributions, SAR distributions k=1 to k=m correspond to MIMO transmissions, and SAR distributions k=m+1 to k=d correspond to non-MIMO transmissions. As shown in equation (8), the square root of the SAR distribution for MIMO transmission is calculated. As discussed above, this is done to sum the electric fields of the SAR distributions of the individual antennas that are part of the MIMO transmission (note that the SAR value is proportional to the square of the electric field in equation (1)). The sum of the square roots is then squared to obtain the combined SAR distribution for the MIMO transmission. In some aspects, a MIMO transmission may include the transmission of multiple RF signals (e.g., coherent RF signals) having the same transmission frequency (e.g., using the same RAT).

[0064] For non-MIMO transmission, the corresponding SAR distribution (ie, SAR distribution k=m+1 to k=d) is scaled by the corresponding transmit power scaling factor (ie, ratio Tx k / P limitk) are scaled and summed without the need for the square root of the SAR distributions because the antennas for these SAR distributions operate at different transmit frequencies. Non-MIMO transmissions may include transmitting two or more RF signals at different transmit frequencies (e.g., via different antennas 130-1 to 130-N). Non-MIMO transmissions may also include transmitting two or more RF signals having the same or similar transmit frequencies using different RATs (e.g., different RATs for each of the RF signals). The RATs may include, for example, IEEE 802.11 or 802.15 or 802.16 protocols (e.g., IEEE 802.11ac, IEEE 802.11n, IEEE802.11ad, IEEE 802.11ax, IEEE 802.11ay, etc.), Bluetooth technology, etc.

[0065] In certain aspects, multiple subsets of antennas (e.g., subsets of antennas 130-1 to 130-N) may be used for multiple MIMO transmissions. In these aspects, equation (8) may be generalized for multiple subsets of antennas used for multiple MIMO transmissions. For example, a first subset of antennas corresponding to SAR distributions k=1 to k=m1 may be used for WWAN MIMO transmissions, a second subset of antennas corresponding to SAR distributions k=m1+1 to k=m2 may be used for WLAN MIMO transmissions, and a third subset of antennas corresponding to SAR distributions k=m2+1 to d may be used for non-MIMO transmissions. In these aspects, equation (8) may be generalized as shown below:

[0066]

[0067] Where s subsets of antennas are used for MIMO transmission, SAR distributions k=1 to k=m1 correspond to the first subset of the s subsets, SAR distributions k=m1+1 to k=m2 correspond to the second subset of the s subsets, and so on. The remaining SAR distributions k=ms+1 to k=d correspond to non-MIMO transmission. Note that {P limitk ,SAR norm_k} is also related to frequency.

[0068] In formula (9), all d antennas can be divided into MIMO antennas or non-MIMO antennas, and each of these antennas can correspond to one transmission frequency. However, multiple frequencies can be transmitted simultaneously from one antenna, and one antenna can be used for both MIMO transmission and non-MIMO transmission at different frequencies at the same time. In this case, the SAR distribution of each of the multiple frequencies transmitted from a given antenna can be added to the above formula (9). For example, if k=1 to m1 antennas are used in MIMO transmission of WLAN 2.4GHz and the same k=1 to m1 antennas are also used in MIMO transmission of WLAN 5GHz, the k=1 to m1 antennas in formula (9) can be repeated twice, where {Tx k ,P limitk ,SAR norm_k} corresponds to the WLAN 2.4 GHz SAR distribution, and the {Tx k ,P limitk ,SAR norm_k The second set of} corresponds to the WLAN 5GHz SAR distribution. Similarly, an antenna can be used for MIMO transmission and non-MIMO transmission at different frequencies at the same time. In this case, the antenna should be part of the MIMO subset (i.e., part of the square root terms of other MIMO antennas involved in the subset) and part of the non-MIMO contribution (i.e., the last term in formula (9)). For the case of multiple MIMO transmissions without non-MIMO transmissions, the last term in formula (9) can be omitted.

[0069] Thus, equation (7) describes combining the SAR distributions of MIMO transmissions, equation (8) describes combining the SAR distributions of a MIMO transmission and multiple SISO transmissions, and equation (9) describes combining the SAR distributions of multiple MIMO transmissions and multiple SISO transmissions. For a given MIMO transmission (e.g., the MIMO term in equation (7), equation (8), or one of the s subsets in equation (9)), the operation for combining the SAR distributions of the MIMO transmissions is a nonlinear operation including calculating the square root of the scaled SAR distribution and calculating the square of the sum. Because the operation is nonlinear, the amount of computation required for processor 110 to perform the operation is large. In some cases, processor 110 may need to perform the above operation frequently to assess real-time RF exposure. In addition, for multiple MIMO transmissions, processor 110 may need to perform the above operation for each of the s subsets. In these cases, the above operation may consume a large portion of the computing resources of processor 110 and significantly increase the power consumption of processor 110. In some cases, the amount of computation required for the above operation may be too large for processor 110 to perform in real time.

[0070] To address the above issues, aspects of the present disclosure provide MIMO SAR distributions for MIMO transmissions that can be linearly combined in a linear operation, which significantly reduces the computational load on the processor 110 compared to the above nonlinear operations. The MIMO SAR distribution can be determined for each MIMO transmit antenna in one calculation, stored in the memory 115, and later retrieved from the memory 115 to evaluate the RF exposure of the MIMO transmission in real time, as further discussed below. Note that aspects of the present disclosure are described for one MIMO transmission. The MIMO transmission may correspond to one of the s subsets of MIMO antennas. For multiple MIMO transmissions, if the wireless device 100 supports multiple MIMO transmissions (e.g., WWAN MIMO transmissions with WLAN MIMO transmissions, or WLAN 2.4GHz MIMO transmissions with WLAN 5GHz MIMO transmissions), similar operations may be performed for each subset of MIMO antennas.

[0071] In certain aspects, the wireless device 100 supports a MIMO transmission scenario in which the wireless device 100 uses multiple transmit antennas to simultaneously transmit RF signals at the same frequency. In these aspects, for a MIMO transmission scenario of a given operating frequency band, the normalized MIMO SAR distribution of each of the m transmit antennas (i.e., active antennas) may be determined as follows:

[0072]

[0073] SAR norm_MIMO_k is the normalized MIMOSAR distribution of the kth antenna among the m transmitting antennas in the MIMO transmission scenario. The MIMO SAR distribution of the kth antenna shown in formula (10) includes a first component and a second component. The first component is the normalized SAR distribution of the kth antenna discussed above in formulas (7), (8) and (9), and the second component takes into account the correlation between the kth antenna and each of the other m antennas used for MIMO transmission. In this example, the second component includes the sum of multiple components, each of which includes the square root of the product of the SAR distribution of the kth antenna and the SAR distribution of a corresponding one of the other m antennas. It should be noted that in formula (10), the normalized SAR distributions of all antennas (i.e., SAR norm_k and SAR norm_i ) may have a maximum value of 1. However, the normalized MIMO SAR distribution (i.e., SAR) of the kth antenna norm_MIMO_k ) can have a maximum value greater than 1.

[0074] The normalized MIMO SAR distribution for the kth antenna in equation (10) corresponds to all m antennas transmitting in the MIMO transmission. However, it should be understood that fewer than m transmit antennas may be used for the MIMO transmission. For example, if only two antennas are active in the MIMO transmission, antenna k and antenna i, equation (10) may be recalculated for the dual-antenna MIMO transmission, where and Similarly, equation (10) can be calculated once for any given set of active antennas involved in a MIMO transmission.

[0075] The normalized MIMO SAR distribution for each of the m transmit antennas for the m-antenna MIMO transmission scenario may be calculated once based on formula (10), and the normalized MIMO SAR distribution may be stored in the memory 115 for later use. For example, a device (e.g., a computer) external to the wireless device 100 may calculate the normalized MIMO SAR distribution, and then the normalized MIMO SAR distribution may be loaded into the memory 115 (e.g., via a wired link or a wireless link) for later use. In another example, the processor 110 may calculate the normalized MIMO SAR distribution, and store the normalized MIMO SAR distribution in the memory 115 for later use.

[0076] After storing the normalized MIMO SAR distribution in the memory 115, the processor 110 may retrieve the normalized MIMO SAR distribution of the m transmit antennas from the memory 115 to evaluate the RF exposure in real time for the MIMO transmission scenario without having to recalculate the normalized MIMO SAR distribution in real time. The normalized MIMO SAR distribution may be calculated once by the processor 110 or by a device (e.g., a computer) external to the wireless device 100, and the normalized MIMO SAR distribution may be stored in the memory 115 for later use. Similarly, in the case where the device supports multiple MIMO transmissions, the normalized MIMO SAR distribution for each of the MIMO subset antennas and operating band combinations may be determined using formula (10). For example, if antennas k=1 to k=m1 are used for both WLAN 2.4 GHz MIMO transmissions and WLAN 5 GHz MIMO transmissions, the corresponding SAR at the 2.4 GHz band for the WLAN 2.4 GHz MIMO transmissions involving the k=1 to m1 antennas may be calculated using formula (10) norm_k distribution, and the corresponding SAR at the 5 GHz band for WLAN 5 GHz MIMO transmission involving k=1 to k=m1 antennas can also be calculated using formula (10): norm_k distributed.

[0077] The combined normalized SAR distribution for the MIMO transmission scenario can be calculated by scaling the individual normalized MIMO SAR distributions of the transmit antennas by the corresponding transmit power scaling factors and summing the scaled MIMO SAR distributions as follows:

[0078]

[0079] SAR norm_MIMO_combined is the combined normalized MIMO SAR distribution, k is the index of the normalized MIMO SAR distribution, SAR norm_MIMO_k is the normalized MIMOSAR distribution of the kth transmit antenna determined using equation (10) for a given operating band, Tx k is the current transmit power level of the kth transmit antenna, P limitk is the Plimit level for the kth transmit antenna for a given operating band, and m is the number of antennas used for a given MIMO transmission. The transmit power level of each of the transmit antennas (e.g., antennas 130-1 to 130-N or a subset of antennas 130-1 to 130-N) may correspond to the output transmit power level of a corresponding transmit circuit (e.g., a corresponding one of transmit circuits 120-1 to 120-N). As shown in equation (11), the combined normalized MIMO SAR distribution is a linear combination of the normalized MIMO SAR distributions of the individual antennas, where each of the normalized MIMO SAR distributions is scaled by a corresponding transmit power scaling factor (i.e., ratio Tx k / P limitk ) scaling. Therefore, equation (11) simplifies the nonlinear operation of any of equations (7), (8), and (9) by using a one-time calculation of equation (10). For the case where multiple antenna subsets are used for multiple MIMO transmissions, equation (11) can be modified as follows to calculate the combined normalized MIMO SAR distribution of multiple MIMO transmissions (i.e., all antenna subsets):

[0080]

[0081] There are s subsets of antennas used for MIMO transmission, MIMO SAR distributions k=1 to k=m1 correspond to the first subset of the s subsets, MIMO SAR distributions k=m1+1 to k=m2 correspond to the second subset of the s subsets, and so on.

[0082] For the case where there is also non-MIMO transmission (e.g., SISO transmission), the non-MIMO transmission term in equation (9) (i.e., the rightmost term in equation (9)) may be added to equation (11) or (12) to take into account the SAR contribution from the non-MIMO transmission. For example, for the case of simultaneous MIMO transmission and non-MIMO transmission, the non-MIMO transmission term may be added to equation (11) as follows:

[0083]

[0084] SAR norm_combined is a combined normalized SAR distribution, antennas k=1 to k=m are used for MIMO transmission, and antennas k=m+1 to k=d are used for non-MIMO transmission. The SAR distribution of non-MIMO transmission (e.g., SAR distribution of k=m+1 to k=d) may also be referred to as a non-MIMO SAR distribution. In this example, the combined normalized SAR distribution is a combination of the combined normalized MIMO SAR distribution and the combined normalized non-MIMO SAR distribution. For the case of simultaneous MIMO transmission and non-MIMO transmission, the non-MIMO term may also be added to formula (12) in a similar manner, where the wireless device 100 may use antennas k=ms+1 to k=d for non-MIMO transmission.

[0085] As discussed above, in some cases, the same antenna may be used for multiple frequency transmissions (e.g., multiple MIMO transmissions, multiple non-MIMO transmissions, or a combination of both). In one example, the same antennas k=1 to k=m may be used to transmit a first MIMO transmission at a first transmission frequency and a second MIMO transmission at a second transmission frequency (e.g., 2.4 GHz MIMO + 5 GHz MIMO). In this example, the combined normalized SAR distribution may be given by:

[0086]

[0087] SAR norm_MIMO_f1_k is the normalized MIMO SAR distribution of the first MIMO transmission at the first transmit frequency, and SAR norm_MIMO_f1_k is the normalized MIMO SAR distribution of the second MIMO transmission at the second transmission frequency. In this example, the wireless device 100 may use antennas k=1 to k=m for the two MIMO transmissions, and antennas k=m+1 to k=d for the non-MIMO transmissions. For the case where there is no simultaneous non-MIMO transmission, the non-MIMO transmission term in formula (14) may be omitted.

[0088] Although the normalized MIMO SAR distribution for each k-th antenna in formula (10) is nonlinear, for a given operating band and a given antenna subset, the normalized MIMO SAR distribution can be pre-calculated once based on formula (10) and stored in the memory 115. This allows the processor 110 to retrieve the normalized MIMO SAR distributions of all MIMO antennas from the memory 115 to calculate the combined normalized MIMO SAR without having to perform nonlinear operations to calculate the normalized MIMO SAR distribution in real time. In addition, the same set of normalized MIMO SAR distributions can be used to determine the combined normalized SAR distributions for various transmit power levels by scaling each normalized MIMO SAR distribution based on the corresponding transmit power level (i.e., the power level of the corresponding antenna) (e.g., formulas (11), (12), (13), and (14)). This is possible because for each antenna, the transmit power scaling factor of the antenna (i.e., Tx k / P limitk ) is outside the nonlinear expression of the corresponding normalized MIMO SAR distribution in formula (10). Therefore, the normalized MIMO SAR distribution of the primary antenna can be pre-calculated (e.g., formula (10)) and used for various transmit power levels of the antenna by scaling the normalized MIMO SAR distribution with the corresponding transmit power scaling factor (e.g., in formulas (11), (12), (13), and (14)). In contrast, in formulas (7), (8), and (9), the transmit power scaling factor for each MIMO antenna is within the corresponding square root term. Therefore, calculating the combined normalized SAR distribution for various transmit power levels based on formulas (7), (8), and (9) will require performing nonlinear operations for each transmit power level, which can consume a lot of computational resources.

[0089] The combined normalized SAR distribution allows processor 110 to evaluate RF exposure compliance of one or more MIMO transmissions. RF exposure compliance is met when the peak in the combined normalized SAR distribution is equal to or less than the normalized value of 1 (i.e., which may correspond to a SAR design target, which may be less than the RF exposure limit to account for device uncertainties).

[0090] Since the combined normalized SAR distribution is a linear combination of the normalized MIMO SAR distributions of the individual antennas, the combined normalized MIMO SAR can be calculated in a linear operation that is computationally similar to the linear combination operation for the non-MIMO case given in equation (6). Since the above operation is linear, the processor 110 can use significantly less computational resources and power to perform the above operation compared to the nonlinear operations given in equations (7), (8), and (9). This allows the processor 110 to efficiently calculate the combined normalized SAR distribution for MIMO transmissions in real time.

[0091] Although various aspects of the present disclosure are illustrated above using the example of a SAR distribution, it should be understood that various aspects of the present disclosure are not limited to the example of a SAR distribution. For example, various aspects of the present disclosure may be applied to situations where RF exposure is assessed using a power density (PD) distribution or a combination of a PD distribution and a SAR distribution, as discussed further below. In general, it should be understood that the present disclosure is not limited to a particular type of RF exposure distribution, and various aspects of the present disclosure are broadly applicable to other types of RF exposure distributions, as discussed further below.

[0092] Processor 110 may perform the above MIMO combining operations (e.g., equations (11), (12), (13), and (14)) to determine a set of maximum allowable power levels for transmit antennas (e.g., antennas 130-1 to 130-N) for future transmissions (e.g., in future time slots) that comply with RF exposure limits (e.g., SAR design targets that are lower than regulatory limits to account for device uncertainties and / or budget additional safety margins required for simultaneous transmission scenarios with other radio components). During future time slots, the transmit power levels of the transmit antennas are constrained (i.e., bounded) by the corresponding maximum allowable power levels to ensure RF exposure compliance, as further discussed below. In the present disclosure, unless otherwise noted, the term "maximum allowable power level" refers to the "maximum allowable power level" specified by the RF exposure limit (e.g., the time-averaged RF exposure limit). It should be understood that the "maximum allowable power level" is not necessarily equal to the absolute maximum power level that complies with RF exposure regulatory limits (e.g., regulatory agency SAR limits without safety margins), and may be less than the absolute maximum power level that complies with RF exposure limits (e.g., SAR design targets that are less than regulatory SAR limits to provide safety margins to account for device uncertainties and / or budgets for simultaneous transmission scenarios with other transmitters). As used herein, the term "future transmissions" refers to transmissions in the future relative to the time at which the processor 110 determines the maximum allowable power level for future transmissions. In other words, the maximum allowable power levels for future transmissions are calculated before the future transmissions require them. As used herein, the term "future time slots" refers to time slots in the future (i.e., time intervals or time durations) relative to the time at which the processor 110 determines the maximum allowable power level for the transmit antenna. Determining the maximum allowable power level for the future time slots in advance of the future time slots helps ensure that RF exposure compliance (e.g., time-averaged RF exposure compliance) is met during the future time slots.

[0093] In one example, the transmit circuits (e.g., transmit circuits 120-1 to 120-N) coupled to transmit antennas (e.g., antennas 130-1 to 130-N) have respective power level limits. In this example, the output transmit power level of each transmit circuit may be constrained (i.e., defined) by the respective power level limits. In this example, the processor 110 may constrain the transmit power level of the transmit antenna to the respective maximum allowable power level by setting the power level limit of the respective transmit circuit to the respective maximum allowable power level, as discussed further below.

[0094] The processor 110 may execute Figure 4 The illustrated exemplary method 400 determines the maximum allowable power level of a transmit antenna (eg, antennas 130-1 through 130-N) for future transmissions.

[0095] At block 410, the processor 110 initializes transmit power levels for transmit antennas. In one example, the transmit power levels for transmit antennas may be initialized based on one or more power control loops, one or more desired data rates, one or more desired beam directions or sectors, etc. In one example, the transmit power levels may be initialized to a set of default transmit power levels.

[0096] At block 420, processor 110 performs the following operations based on the transmit power level (e.g., Tx k ) to determine the combined normalized SAR distribution. To this end, the processor 110 may adjust the transmission power according to the corresponding transmission power scaling factor (e.g., Tx k / P limitk ) scales the normalized MIMO SAR distribution of each of the transmit antennas, and linearly combines the scaled normalized MIMO SAR distributions of the transmit antennas (e.g., based on any one of equations (11), (12), and (13)) to obtain a combined normalized SAR distribution. Processor 110 may retrieve the normalized MIMO SAR distributions of the individual antennas from memory 115, where the normalized MIMO SAR distributions are pre-calculated (e.g., based on equation (10)).

[0097] At block 430, processor 110 compares the peak value in the combined normalized SAR distribution to a normalized RF exposure limit (e.g., 1). For examples where the combined MIMO SAR distribution is not normalized, the RF exposure limit may be approximately equal to the RF exposure limit (e.g., a SAR design target, which may be less than the regulatory SAR limit to account for design uncertainties). If the peak value in the combined normalized SAR distribution is equal to or less than the normalized RF exposure limit (e.g., 1), processor 110 determines that the transmit power level complies with the RF exposure limit. In this case, method 400 ends at block 450, and processor 110 may use the transmit power level as the maximum allowable power level for the transmit antenna for future transmissions. In this case, the transmit power level during future transmissions is constrained by the maximum allowable power level.

[0098] If the peak in the combined normalized SAR distribution is greater than the normalized RF exposure limit (e.g., 1), the processor 110 adjusts the transmit power level at block 440. For example, the processor 110 may adjust the transmit power level by reducing one or more of the transmit power levels. In one example, the processor 110 may reduce the transmit power levels of the transmit antennas by the same scaling factor.

[0099] The processor 110 then repeats blocks 420 and 430 using the adjusted transmit power level (i.e., the adjusted transmit power level is used in block 420 to determine the combined normalized SAR distribution). The processor 110 may repeat blocks 440, 420, and 430 until the peak in the combined normalized SAR distribution is equal to or less than the normalized RF exposure limit (e.g., 1), at which point the transmit power level complies with the RF exposure limit (e.g., the SAR design target). The transmit power level that complies with the RF exposure limit may then be used as the maximum allowable power level for the transmit antenna for future transmissions.

[0100] After processor 110 determines the maximum allowable power level, processor 110 constrains (i.e., defines) the transmit power level of the transmit antenna during future transmissions to the respective maximum allowable power level. As discussed above, processor 110 may do this for each transmit antenna by setting the power level limit of the respective transmit circuit (i.e., the transmit circuit coupled to the antenna) to the respective maximum allowable power level. This prevents the output transmit power level of each of the transmit circuits from exceeding the respective maximum allowable power level.

[0101] In aspects where non-MIMO transmissions (e.g., SISO transmissions) are also present, the non-MIMO transmission term in equation (9) (i.e., the rightmost term in equation (9)) may be included in the combined normalized SAR distribution calculated in block 420 (e.g., equation (13)) to account for the SAR contribution from the non-MIMO transmissions. The non-MIMO transmission term combines the SAR distributions of the non-MIMO transmissions, where each of the SAR distributions may correspond to a respective one of the active antennas (e.g., k=m+1 to k=d) for MIMO transmissions. In this case, the combined normalized SAR distribution in block 420 is a combination of the combined normalized MIMO SAR distribution and the combined normalized non-MIMO SAR distribution (i.e., the SAR distribution of the non-MIMO transmissions).

[0102] It should be understood that the present disclosure is not limited to Figure 4The exemplary method 400 is illustrated, and other methods may be used to determine the maximum allowable power level of the transmit antenna that complies with the RF exposure limit. For example, the processor 110 may determine the maximum allowable power level so that the peak value in the combined normalized SAR distribution is equal to or less than 1 to perform a conservative approximation analysis to determine the maximum allowable power level with fewer calculations. For examples where the combined MIMO SAR distribution is not normalized, the peak value in the combined MIMO SAR distribution may be equal to or less than the RF exposure limit. The RF exposure limit may be equal to or less than the RF exposure limit set by the regulatory body (e.g., a SAR design target used to provide a safety margin to account for device uncertainties or to budget a SAR margin to comply with the total RF exposure in a simultaneous transmission scenario with other transmitters).

[0103] When the transmission scenario changes (i.e., changes in MIMO / SISO transmission and / or active antennas involved), the active transmission scenario can be re-evaluated. Figure 4 The illustrated exemplary method 400 is for determining a maximum allowable transmit power for a transmit antenna for a future transmission.

[0104] In some cases, RF exposure regulations require that the time average RF exposure within the averaging time window does not exceed the RF exposure limit. This allows the wireless device 100 to briefly exceed the RF exposure limit as long as the time average RF exposure within the averaging time window does not exceed the RF exposure limit.

[0105] In this regard, the processor 110 may evaluate the time-averaged RF exposure compliance as follows. The processor 110 may calculate the time-averaged combined normalized SAR distribution within an averaging time window (e.g., 6 minutes), and compare the peak value in the time-averaged combined normalized SAR distribution with 1 (where 1 corresponds to an RF exposure limit, such as a SAR design target) to evaluate the time-averaged RF exposure compliance. If the peak value is equal to or less than 1, the processor 110 may determine the time-averaged RF exposure compliance. If the time-averaged combined MIMO SAR distribution is not normalized, the peak value should be equal to or less than the RF exposure limit (e.g., SAR design target) to meet the requirements.

[0106] In this regard, Figure 5 An example is shown in which the processor 110 calculates a time-averaged combined normalized SAR distribution within an averaging time window 505 (e.g., 6 minutes). In this example, the averaging time window 505 is divided into a plurality of time slots (i.e., time intervals). For example, a 6-minute averaging time window may be divided into 5-second time slots. Figure 5In the example shown, there are p number of time slots 515(1) to 515(p) within the averaging time window 505, wherein each of the p time slots 515(1) to 515(p) has a corresponding combined normalized SAR distribution 510(1) to 510(p). Thus, in this example, the averaging time window 505 includes p time slots 515(1) to 515(p). Although each of the combined normalized SAR distributions 510(1) to 510(p) has a corresponding combined normalized SAR distribution 510(1) to 510(p), Figure 5 , but it should be understood that the present disclosure is not limited to this example. For example, the combined normalized SAR distribution 510(1) to 510(p) of each time slot 515(1) to 515(p) may be, for example, a combination of normalized SAR distributions (e.g., SAR distributions 310-1 to 310-m) of different antennas (e.g., antennas 130-1 to 130-N). In this example, each normalized SAR distribution 510(1) to 510(p) may be Figure 3 A single instance of combined SAR distribution 320 is illustrated.

[0107] The processor 110 may determine a combined normalized SAR distribution for each time slot (e.g., based on any one of equations (11), (12), (13), and (14)). The combined normalized SAR distribution for the time slot may be generated by combining a plurality of normalized MIMO SAR distributions, each of which corresponds to one of the transmit antennas (e.g., one of the m MIMO antennas among antennas 130-1 to 130-N). For a case where non-MIMO transmission is also present, the combined normalized SAR distribution for the time slot may also include the normalized SAR distribution of the non-MIMO transmission (e.g., the rightmost term in equation (9)). For a case where different transmit power levels are used for the transmit antennas, the processor 110 may scale the normalized MIMO SAR distribution of each transmit antenna by a corresponding transmit power scaling factor before combining. If the transmit power level of the transmit antenna varies within the time slot, the processor 110 may scale the corresponding normalized MIMO SAR distribution by a time average of the transmit power scaling factor of the antenna within the time slot.

[0108] The processor 110 may average the combined normalized SAR distributions 510(1) to 510(p) of the time slots 515(1) to 510(p) within the averaging time window 505 to generate a time-averaged combined normalized SAR distribution 520. For example, the processor 110 may calculate the time-averaged combined normalized SAR distribution 520 by combining the combined normalized SAR distributions 510(1) to 510(p) of the time slots 515(1) to 515(p) and dividing the resulting combined normalized SAR distribution by the number of time slots, as given by the following formula:

[0109]

[0110] SAR average is the time-averaged combined normalized SAR distribution within the averaging time window 505, j is the index of the time slots 515(1) to 515(p), and SAR norm_combined_j is the combined normalized SAR distribution of the j-th time slot 515(j). As discussed above, the combined normalized SAR distribution of the time slot is a combination of multiple normalized MIMO SAR distributions, each of which corresponds to a corresponding one of the transmit antennas. For the case where non-MIMO transmission is also present, the combined normalized SAR distribution of the time slot may also include the normalized SAR distribution of the non-MIMO transmission (e.g., the last term in equation (9)). The processor 110 may compare the peak value in the time-averaged combined normalized SAR distribution 520 with a normalized value of 1 (where 1 corresponds to an RF exposure limit (e.g., a SAR design target)) to evaluate the time-averaged RF exposure compliance. If the peak value is equal to or less than 1, the processor 110 may determine the time-averaged RF exposure compliance. For examples where the time-averaged combined MIMO SAR distribution is not normalized, the peak value in the SAR distribution 520 should be equal to or less than the RF exposure limit (e.g., a SAR design target).

[0111] In some aspects, processor 110 may determine a maximum allowable power level for future time slots to ensure time-averaged RF exposure compliance. In this regard, Figure 5 The time slots 515(1) to 515(p-1) in may correspond to previous transmissions of the wireless device 100, and the time slot 515(p) may correspond to a future time slot. In this regard, the time slot 515(p) is referred to below as a future time slot. In this case, formula (15) may be written as follows:

[0112]

[0113] SAR norm_combined_p is the combined normalized SAR distribution of the future time slot 515(p). Time slots 515(1) to 515(p-1) are previous time slots. In the present disclosure, the term "previous time slot" refers to a time slot before a future time slot.

[0114] In this example, it is assumed that the transmit power levels of the combined normalized SAR distributions 510(1) to 510(p-1) are known to the processor 110 because they correspond to previous transmissions of the wireless device 100. For example, the processor 110 may record the transmit power level of each of the time slots 515(1) to 515(p-1) in the memory 115 and determine the combined normalized SAR distributions 510(1) to 510(p-1) for the time slots using the recorded transmit power levels of the time slots 515(1) to 515(p-1). For the time slots 515(1) to 515(p-1), the combined normalized SAR distribution for the j-th time slot 515(j) may be determined, for example, based on any of equations (11), (12), (13), and (14) for all transmit antennas that were active during the j-th time slot 515(j).

[0115] In this example, the transmit power level of the combined normalized SAR distribution 510(p) corresponding to the future time slot 515(p) is a variable to be solved for by the processor 110. To determine the maximum allowable power level for the future time slot 515(p), the processor 110 may calculate a time-averaged combined normalized SAR distribution 520, where the transmit power level for the future time slot 515(p) is a variable in the time-averaged combined normalized SAR distribution 520 (i.e., the time-averaged combined normalized SAR distribution is a function of the transmit power level for the future time slot 515(p)). The processor 110 may then determine the transmit power level for the future time slot 515(p) such that the peak value in the time-averaged combined normalized SAR distribution is equal to or less than the normalized RF exposure limit (e.g., 1). The processor 110 may then use the determined transmit power level as the maximum allowable power level for the future time slot 515(p). The processor 110 may determine the maximum allowable power level for the future time slot 515(p) during the time slot 515(p-1) so that the maximum allowable power level for the future time slot 515(p) is ready at the start of the future time slot 515(p). For the case of MIMO, each of the maximum allowable power levels corresponds to a corresponding one of the transmit antennas in the future time slot 515(p). If the future time slot has both active MIMO and active non-MIMO transmissions, the rightmost term in equation (9) may be included to determine the combined normalized SAR distribution of 510(p), and the transmit power levels of all active MIMO and non-MIMO antennas may be solved as variables. During the future time slot 515(p), the processor 110 constrains the transmit power level of the transmit antenna to the corresponding maximum allowable power level. As discussed above, processor 110 may perform this operation for each transmit antenna (e.g., each of antennas 130-1 to 130-N) by setting the power level limit of the corresponding transmit circuit (e.g., a corresponding one of transmit circuits 120-1 to 120-N) to the corresponding maximum allowable power level.

[0116] The processor 110 may Figure 6 The illustrated exemplary method 600 determines a maximum allowable power level for a future time slot 515(p). At block 610, the processor 110 initializes a transmit power level for the future time slot 515(p). The transmit power level may be initialized based on a power control loop, a desired data rate, a desired beam direction or sector, etc. In one example, the transmit power level may be initialized to a set of default transmit power levels.

[0117] At block 620, the processor 110 determines a time-averaged combined normalized SAR distribution within the averaging time window 505 based on the transmit power level of the future time slot 515(p) and the transmit power levels of the previous time slots 515(1) through 515(p-1) at block 610. Note that the transmit power levels of the previous time slots 515(1) through 515(p-1) are known to the processor 110, as discussed above.

[0118] At block 630, the processor 110 compares the peak value in the time-averaged combined normalized SAR distribution to the normalized RF exposure limit 1 to evaluate the time-averaged RF exposure compliance. If the peak value is equal to or less than the normalized RF exposure limit, the method 600 ends at block 650. In this case, the processor 110 uses the transmit power level initialized at block 610 as the maximum allowable power level for the future time slot 515(p).

[0119] If the peak value is greater than 1, the processor 110 adjusts the transmit power level of the future time slot 515(p) at block 640. The processor 110 may adjust the transmit power level of the future time slot 515(p) by reducing one or more of the transmit power levels of the future time slot 515(p). In one example, the processor 110 may scale down the transmit power levels by the same scaling factor. For the example of MIMO, each of the transmit power levels corresponds to a respective one of the transmit antennas in the future time slot 515(p).

[0120] The processor 110 then repeats blocks 620 and 630 using the adjusted transmit power level for the future time slot 515(p). The processor 110 may repeat blocks 640, 620, and 630 until the peak in the time-averaged combined normalized SAR distribution is equal to or less than 1, at which point the transmit power level complies with the RF exposure limit. The processor 110 may then use the transmit power level that complies with the RF exposure limit as the maximum allowable power level for the future time slot 515(p).

[0121] After processor 110 determines the maximum allowable power level for the future time slot, processor 110 constrains (i.e., limits) the transmit power level of the transmit antenna during the future time slot 515(p) to the corresponding maximum allowable power level. As discussed above, processor 110 may do this for each transmit antenna by setting the power level limit of the corresponding transmit circuit (i.e., the transmit circuit coupled to the antenna) to the corresponding maximum allowable power level.

[0122] It should be understood that the present disclosure is not limited to Figure 6The exemplary method 600 illustrated, and other methods may be used to determine the transmit power level for the future time slot 515 (p) such that the time-averaged combined normalized SAR distribution complies with the normalized RF exposure limit of 1. For example, the processor 110 may determine the maximum allowable power level for the transmit antenna in the future time slot 515 (p) such that the peak value of the time-averaged combined normalized SAR distribution is equal to or less than 1 (i.e., the normalized RF exposure limit) to perform a conservative approximation analysis to determine the maximum allowable power level with fewer calculations. The RF exposure limit (e.g., SAR design target) discussed above may be equal to or less than the RF exposure limit set by the regulatory body (e.g., to provide a safety margin to account for device uncertainties and / or to budget additional margin to comply with the total RF exposure in a simultaneous transmission scenario with other transmitters).

[0123] The averaging time window 505 may be a moving time averaging window. In this case, each time the processor 110 determines the maximum allowable power level for a new future time slot, the processor 110 shifts the averaging time window 505 by one time slot. For example, in the above discussion, time slot 515(p) is given as a future time slot. In order to determine the maximum allowable power level for the next future time slot 515(p+1), the processor 110 shifts the averaging time window 505 by one time slot to cover time slots 515(2) to 515(p+1). Note that the first time slot 515(1) in the previously determined maximum allowable power level is discarded from the averaging time window 505, and the future time slot 515(p) in the previously determined maximum allowable power level becomes the last known exposure level (based on the most recent transmission) in the previous time slot in the averaging time window 505.

[0124] As discussed above, for the MIMO case, the linear MIMO combining operation according to various aspects of the present disclosure (e.g., based on any one of equations (11), (12), (13), and 914)) allows the processor 110 to efficiently calculate the combined normalized SAR distribution. This is particularly beneficial for the case where the processor 110 frequently calculates the combined normalized SAR distribution (e.g., at least once per time slot) to evaluate the time-averaged RF exposure compliance in the MIMO case (e.g., once per time slot). In this case, the computational complexity of the nonlinear MIMO combining operation given in equations (7), (8), and (9) may be too large for the processor 110 to perform at each time slot. Compared to the nonlinear MIMO combining operation given in equations (7), (8), and (9), the linear MIMO combining operation according to various aspects of the present disclosure (e.g., based on any one of equations (11), (12), (13), and (14)) significantly reduces the amount of computational resources and power required by the processor 110 to evaluate the time-averaged RF exposure in the MIMO transmission case.

[0125] As discussed above, the processor 110 may determine the maximum allowable power level of the transmit antenna during the future time slot, and set the power level limit of the corresponding transmit circuit based on the determined maximum allowable power level. In some aspects, setting the power level limit of the transmit circuit based on the determined maximum allowable power level prevents the output transmit power level of the transmit circuit from exceeding the maximum allowable power level at any time during the future time slot (i.e., the instantaneous transmit power level cannot exceed the maximum allowable power level during the future time slot).

[0126] In other aspects, setting the power level limit of the transmitting circuit based on the determined maximum allowable power level prevents the time average of the transmit power level of the transmitting circuit in the future time slot from exceeding the maximum allowable power level. This allows the instantaneous transmit power level of the transmitting circuit to temporarily exceed the maximum allowable power level in the future time slot, as long as the time average transmit power level of the transmitting circuit in the future time slot does not exceed the maximum allowable power level. For example, in a time division duplex (TDD) scheme with a fixed duty cycle (e.g., GSM) or in a time division duplex (TDD) scheme with a maximum duty cycle specified by the network in a radio resource control (RRC) message in the case of a dynamic duty cycle scheme (e.g., 5G NR), the instantaneous transmit power level may be equal to the maximum allowable power level / (fixed duty cycle or maximum RRC duty cycle), where 0≤duty cycle≤1. In these aspects, the transmitting circuit is allowed to transmit at a transmit power level exceeding the maximum allowable power level in a time interval shorter than the future time slot, as long as the time average transmit power level in the future time slot does not exceed the maximum allowable power level. In these aspects, the maximum allowed power level is the maximum allowed time-averaged power level in the future time slot.

[0127] During the future time slot, the transmit power level of the transmit circuits 120-1 to 120-N may be set by a power control operation performed by the processor 110. The power control operation may use a power control loop to set the transmit power level, as discussed above. The power control operation may also set the transmit power level based on the data rate and / or one or more other parameters. The power control operation may set the transmit power level of the transmit circuits 120-1 to 120-N, for example, by setting the gain of the amplifiers 250-1 to 250-N in the transmit circuits 120-1 to 120-N. In this example, setting the power level limits of the transmit circuits 120-1 to 120-N to the corresponding maximum allowable power levels constrains (i.e., defines) the transmit power level that can be set by the power control operation of the transmit circuits 120-1 to 120-N. In one example, the power control operation is prevented from setting the transmit power level to a level that exceeds the maximum allowable power level at any time during the future time slot. In another example, the power control operation is allowed to temporarily set the transmit power level to a level that exceeds the maximum allowed power level during a future time slot, as long as the time average of the transmit power level in the future time slot does not exceed the maximum allowed power level. In this example, the maximum allowed power level is the maximum allowed time average power level in the future time slot.

[0128] Although various aspects of the present disclosure are discussed above using the example of SAR distribution, it should be understood that various aspects of the present disclosure are not limited to SAR distribution and are applicable to other types of RF exposure distributions. For example, various aspects of the present disclosure may be applied to situations where RF exposure is assessed using a power density (PD) distribution or a combination of a PD distribution and a SAR distribution. For example, a MIMO PD distribution is calculated (e.g., based on any one or more of formulas (10) to (14)) by replacing each instance of a SAR distribution with a corresponding PD distribution and replacing the SAR limit with a PD limit (e.g., a PD limit set by a regulatory body or a PD limit that is lower than the PD limit set by a regulatory body to account for device uncertainty). In this example, the SAR distribution of an antenna (e.g., one of antennas 130-1 to 130-N) may be replaced by the PD distribution of the antenna, and the normalized PD distribution may be a PD distribution normalized relative to the PD limit. The PD distribution may include a set of PD values. For example, the PD distribution may include two PD values, 5 or more PD values, dozens of PD values, or even hundreds or thousands of PD values. The PD values ​​in the distribution may correspond to PD changes in the transmission scenario. In some examples, the PD distribution represents a map such that each PD value in the distribution corresponds to a different spatial location (e.g., on a human body model) or portion of a user's tissue. In some examples, the values ​​in the PD distribution represent exposure at a given time. In general, it should be understood that the present disclosure is broadly applicable to RF exposure distributions, of which SAR distributions and PD distributions are examples. An RF distribution may include a set of RF exposure values, wherein the RF exposure value is a measurement of RF exposure. In some examples, the values ​​in the RF distribution represent exposure at a given time.

[0129] Figure 7 7 is a flow chart illustrating a method 700 for operating a wireless device according to certain aspects of the present disclosure. A wireless device (e.g., wireless device 100) includes a plurality of transmit circuits (e.g., transmit circuits 120-1 to 120-N), wherein the plurality of transmit circuits are configured to transmit a plurality of signals via a plurality of antennas (e.g., antennas 130-1 to 130-N).

[0130] At block 710, a plurality of MIMO RF exposure distributions are combined into a combined MIMO RF exposure distribution, wherein the combined RF exposure distribution comprises a linear combination of a plurality of normalized MIMO RF exposure distributions. For example, each of the MIMO RF exposure distributions may comprise a corresponding MIMO specific absorption rate (SAR) distribution. In another example, each of the MIMO RF exposure distributions may comprise a corresponding MIMO power density (PD) distribution. For example, processor 110 may combine the MIMO RF exposure distributions in a linear operation (e.g., based on any one of equations (11), (12), (13), and (14) or any combination thereof).

[0131] At block 720, a maximum allowable power level is determined based on the combined MIMO RF exposure distribution, wherein each of the maximum allowable power levels corresponds to a corresponding one of the plurality of transmit circuits. In one example, the maximum allowable power level may be determined by determining a transmit power level such that a peak value of the combined normalized RF exposure distribution is equal to or lower than an exposure limit (e.g., normalized exposure limit 1), wherein each of the MIMO RF exposure distributions in the combined RF exposure distribution is scaled based on the corresponding one of the transmit power levels. In another example, the combined normalized RF exposure (e.g., SAR) distribution corresponds to a future time slot (e.g., future time slot 515(p)). In this example, the maximum allowable power level may be determined by averaging the combined MIMO RF exposure distribution with the combined MIMO RF exposure distribution of previous time slots (e.g., previous time slots 515(1) to 515(p-1)) to obtain a time-averaged combined MIMO RF exposure distribution, and determining the transmit power level such that a peak value of the time-averaged combined MIMO RF exposure distribution is equal to or lower than the normalized exposure limit 1. The maximum allowable power level may be determined by processor 110 .

[0132] At block 730, a respective power level limit is set for each of the plurality of transmit circuits based on a respective one of the maximum allowable power levels. In this example, setting the power level limit of the transmit circuit to the respective maximum allowable power level constrains (i.e., defines) the transmit power level of the transmit circuit to the respective maximum allowable power level, or constrains (i.e., defines) the time average of the transmit power level of the transmit circuit in future time slots to the respective maximum allowable power level. The power level limit may be set by processor 110.

[0133] In certain aspects, each of the MIMO RF exposure distributions may include a first component including the RF exposure distribution of the corresponding one of the antennas and a second component that takes into account the correlation between the corresponding one of the antennas and each of the other of the antennas. In certain aspects, the second component of each of the MIMO RF exposure distributions includes a sum of multiple components, each of which includes the square root of the product of the RF exposure distribution of the corresponding one of the antennas and the RF exposure distribution of the corresponding one of the other of the antennas (e.g., based on formula (10)).

[0134] In some aspects, the memory 115 may include a computer-readable medium having stored thereon instructions that, when executed by the processor 110, cause the processor 110 to perform the methods and operations described herein. By way of example, the computer-readable medium may include 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 tangible, non-transitory storage medium, or any combination thereof.

[0135] Specific implementation examples are described in the following numbered clauses:

[0136] 1. A wireless device, comprising:

[0137] a plurality of transmit circuits, wherein the plurality of transmit circuits are configured to transmit a plurality of radio frequency (RF) signals via a plurality of antennas; and

[0138] a processor coupled to the transmitting circuit, wherein the processor is configured to:

[0139] combining multiple-input multiple-output (MIMO) radio frequency (RF) exposure profiles into a combined MIMO RF exposure profile, wherein the combined MIMO RF exposure profile comprises a linear combination of the MIMO RF exposure profiles;

[0140] determining maximum allowable power levels based on the combined MIMO RF exposure profile, wherein each of the maximum allowable power levels corresponds to a respective one of the transmit circuits; and

[0141] A respective power level limit is set for each of the transmit circuits based on the respective one of the maximum allowed power levels.

[0142] 2. The wireless device of clause 1, wherein each of the MIMO RF exposure profiles comprises a respective MIMO specific absorption rate (SAR) profile.

[0143] 3. The wireless device of clause 1, wherein each of the MIMO RF exposure profiles comprises a respective MIMO power density (PD) profile.

[0144] 4. A wireless device according to any of clauses 1 to 3, wherein the transmission circuit is configured to transmit the RF signals at the same transmission frequency.

[0145] 5. A wireless device according to any one of clauses 1 to 4, wherein each of the MIMO RF exposure distributions includes a first component and a second component, the first component including the RF exposure distribution of a corresponding one of the antennas, and the second component taking into account the correlation between the corresponding one of the antennas and each of the other antennas.

[0146] 6. A wireless device according to clause 5, wherein the second component of each of the MIMO RF exposure distributions comprises the sum of a plurality of components, each of the plurality of components comprising a square root of the product of the RF exposure distribution of the corresponding one of the antennas and the RF exposure distribution of the corresponding one of the other antennas.

[0147] 7. A wireless device according to clause 5 or 6, further comprising a memory storing the MIMO RF exposure profile, wherein the processor is configured to retrieve the MIMO RF exposure profile from the memory.

[0148] 8. A wireless device according to any of clauses 1 to 7, wherein the processor is configured to:

[0149] determining a transmit power level such that a peak value of the combined MIMO RF exposure distribution is equal to or less than an exposure limit; and

[0150] The maximum allowed power level is determined based on the determined transmit power level.

[0151] 9. The wireless device of clause 8, wherein each of the MIMO RF exposure profiles is scaled based on a respective one of the transmit power levels.

[0152] 10. A wireless device as recited in clause 8 or 9, wherein the MIMO RF exposure profile is normalized relative to the exposure limit.

[0153] 11. A wireless device as described in clause 10, wherein the exposure limit is equal to or less than a regulatory exposure limit.

[0154] 12. A wireless device as described in any of clauses 1 to 7, wherein the combined MIMO RF exposure profile corresponds to a future time slot, and the processor is configured to:

[0155] averaging the combined MIMO RF exposure distribution with a MIMO RF exposure distribution corresponding to a previous time slot to obtain a time average RF exposure distribution;

[0156] determining a transmit power level for the future time slot such that a peak value of the time-averaged RF exposure distribution is equal to or less than an exposure limit; and

[0157] The maximum allowed power level is determined based on the determined transmit power level for the future time slot.

[0158] 13. The wireless device of clause 12, wherein each of the MIMO RF exposure profiles in the combined MIMO RF exposure profile is scaled based on a respective one of the transmit power levels for the future time slot.

[0159] 14. The wireless device of clause 13, wherein the MIMO RF exposure profile is normalized relative to the exposure limit.

[0160] 15. The wireless device of clause 14, wherein the exposure limit is equal to or less than a regulatory exposure limit.

[0161] 16. A wireless device according to any of clauses 1 to 15, wherein the processor is further configured to:

[0162] combining the non-MIMO RF exposure distributions into a combined non-MIMO RF exposure distribution; and

[0163] The maximum allowable power level is determined based on a combination of the combined MIMO RF exposure profile and the combined non-MIMO RF exposure profile.

[0164] 17. The wireless device of clause 16, wherein each of the non-MIMO RF exposure profiles comprises a respective non-MIMO specific absorption rate (SAR) profile.

[0165] 18. The wireless device of claim 16, wherein each of the non-MIMO RF exposure profiles comprises a respective non-MIMO power density (PD) profile.

[0166] 19. A wireless device as recited in any of clauses 16 to 18, wherein the MIMO RF exposure profile corresponds to a first subset of the antennas and the non-MIMO RF exposure profile corresponds to a second subset of the antennas.

[0167] 20. The wireless device of any of clauses 16 to 19, wherein at least one of the MIMO RF exposure profiles and at least one of the non-MIMO RF exposure profiles correspond to a same one of the antennas.

[0168] 21. The wireless device of clause 20, wherein the at least one of the MIMO RF exposure profiles corresponds to a first transmit frequency and the at least one of the non-MIMO RF exposure profiles corresponds to a second transmit frequency.

[0169] 22. A wireless device as described in any of clauses 16 to 21, wherein the processor is configured to:

[0170] determining a transmit power level such that a peak value of said combination of said combined MIMO RF exposure distribution and said combined non-MIMO RF exposure distribution is equal to or less than an exposure limit; and

[0171] The maximum allowed power level is determined based on the determined transmit power level.

[0172] 23. A wireless device as recited in any of clauses 1 to 22, wherein a first one of the MIMO RF exposure profiles and a second one of the MIMO RF exposure profiles correspond to a same one of the antennas.

[0173] 24. The wireless device of clause 23, wherein the first of the MIMO RF exposure profiles corresponds to a first transmit frequency and the second of the MIMO RF exposure profiles corresponds to a second transmit frequency.

[0174] 25. A method for operating a wireless device comprising a plurality of transmit circuits, wherein the transmit circuits are configured to transmit a plurality of radio frequency (RF) signals via a plurality of antennas, the method comprising:

[0175] combining multiple-input multiple-output (MIMO) radio frequency (RF) exposure profiles into a combined MIMO RF exposure profile, wherein the combined MIMO RF exposure profile comprises a linear combination of the MIMO RF exposure profiles;

[0176] determining maximum allowable power levels based on the combined MIMO RF exposure profile, wherein each of the maximum allowable power levels corresponds to a respective one of the transmit circuits; and

[0177] A respective power level limit is set for each of the transmit circuits based on the respective one of the maximum allowed power levels.

[0178] 26. The method of clause 25, wherein each of the MIMO RF exposure profiles comprises a respective MIMO specific absorption rate (SAR) profile.

[0179] 27. The method of clause 25, wherein each of the MIMO RF exposure profiles comprises a respective MIMO power density (PD) profile.

[0180] 28. A method according to any of clauses 25 to 27, wherein the transmit circuit is configured to transmit the RF signals at the same transmit frequency.

[0181] 29. A method according to any one of clauses 25 to 28, wherein each of the MIMO RF exposure distributions comprises a first component and a second component, the first component comprising the RF exposure distribution of a corresponding one of the antennas, and the second component taking into account the correlation between the corresponding one of the antennas and each of the other antennas.

[0182] 30. A method according to clause 29, wherein the second component of each of the MIMO RF exposure distributions comprises the sum of a plurality of components, each of the plurality of components comprising a square root of the product of the RF exposure distribution of the corresponding one of the antennas and the RF exposure distribution of the corresponding one of the other antennas.

[0183] 31. The method of any of clauses 25 to 30, wherein determining the maximum allowed power level comprises:

[0184] determining a transmit power level such that a peak value of the combined MIMO RF exposure distribution is equal to or less than an exposure limit; and

[0185] The maximum allowed power level is determined based on the determined transmit power level.

[0186] 32. The method of clause 31, wherein each of the MIMO RF exposure profiles in the combined MIMO RF exposure profile is scaled based on a respective one of the transmit power levels.

[0187] 33. The method of clause 32, wherein the MIMO RF exposure distribution is normalized relative to the exposure limit.

[0188] 34. The method of clause 33, wherein the exposure limit is equal to or less than a regulatory exposure limit.

[0189] 35. A method as described in any of clauses 25 to 30, wherein the combined MIMO RF exposure profile corresponds to a future time slot, and determining the maximum allowed power level comprises:

[0190] averaging the combined MIMO RF exposure distribution with a MIMO RF exposure distribution corresponding to a previous time slot to obtain a time average RF exposure distribution;

[0191] determining a transmit power level for the future time slot such that a peak value of the time-averaged RF exposure distribution is equal to or less than an exposure limit; and

[0192] The maximum allowed power level is determined based on the determined transmit power level for the future time slot.

[0193] 36. The method of clause 35, wherein each of the MIMO RF exposure profiles in the combined MIMO RF exposure profile is scaled based on a respective one of the transmit power levels for the future time slot.

[0194] 37. The method of clause 36, wherein the MIMO RF exposure distribution is normalized relative to the exposure limit.

[0195] 38. The method of clause 37, wherein the exposure limit is equal to or less than a regulatory exposure limit.

[0196] 39. The method according to any one of clauses 25 to 38, further comprising:

[0197] combining the non-MIMO RF exposure distributions into a combined non-MIMO RF exposure distribution;

[0198] Wherein determining the maximum allowable power level comprises determining the maximum allowable power level based on a combination of the combined MIMO RF exposure profile and the combined non-MIMO RF exposure profile.

[0199] 40. The method of clause 39, wherein each of the non-MIMO RF exposure profiles comprises a respective non-MIMO specific absorption rate (SAR) profile.

[0200] 41. The method of clause 39, wherein each of the non-MIMO RF exposure profiles comprises a respective non-MIMO power density (PD) profile.

[0201] 42. The method of any of clauses 39 to 41, wherein the MIMO RF exposure distribution corresponds to a first subset of the antennas and the non-MIMO RF exposure distribution corresponds to a second subset of the antennas.

[0202] 43. The method of clause 42, wherein at least one of the MIMO RF exposure profiles and at least one of the non-MIMO RF exposure profiles correspond to a same one of the antennas.

[0203] 44. The method of clause 43, wherein the at least one of the MIMO RF exposure profiles corresponds to a first transmit frequency and the at least one of the non-MIMO RF exposure profiles corresponds to a second transmit frequency.

[0204] 45. A method according to any one of clauses 39 to 44, wherein the method further comprises:

[0205] determining a transmit power level such that a peak value of said combination of said combined MIMO RF exposure distribution and said combined non-MIMO RF exposure distribution is equal to or less than an exposure limit;

[0206] Wherein determining the maximum allowable power level comprises determining the maximum allowable power level based on the determined transmit power level.

[0207] 46. ​​The method of any of clauses 25 to 45, wherein a first one of the MIMO RF exposure profiles and a second one of the MIMO RF exposure profiles correspond to a same one of the antennas.

[0208] 47. The method of clause 46, wherein the first of the MIMO RF exposure profiles corresponds to a first transmit frequency and the second of the MIMO RF exposure profiles corresponds to a second transmit frequency.

[0209] 48. A computer readable medium having stored thereon instructions for:

[0210] combining multiple-input multiple-output (MIMO) radio frequency (RF) exposure profiles into a combined MIMO RF exposure profile, wherein the combined MIMO RF exposure profile comprises a linear combination of the MIMO RF exposure profiles;

[0211] determining maximum allowable power levels based on the combined MIMO RF exposure profile, wherein each of the maximum allowable power levels corresponds to a respective one of a plurality of transmit circuits; and

[0212] A respective power level limit is set for each of the transmit circuits based on the respective one of the maximum allowed power levels.

[0213] It should be understood that the present disclosure is not limited to the exemplary terms used above to describe various aspects of the present disclosure, and the present disclosure covers equivalent terms. For example, it should be understood that an antenna may also be referred to as an antenna element or another term. In another example, it should be understood that a maximum allowable power level may also be referred to as a power level limit or another term.

[0214] As used herein, the term "about" with respect to a specified value or property is intended to indicate within 10% of the specified value or property.

[0215] Any reference to an element using designations such as "first," "second," etc. herein does not generally limit the number or order of those elements. Rather, these designations are used herein as a convenient method of distinguishing two or more elements or instances of elements. Thus, reference to a first element and a second element does not mean that only two elements can be used or that the first element must be located before the second element.

[0216] Within this disclosure, the word "exemplary" is used to mean "serving as an example, instance, or illustration." Any specific implementation or aspect described herein as "exemplary" is not necessarily to be construed as preferred or superior to other aspects of the disclosure. Likewise, the term "aspect" does not require that all aspects of the disclosure include the discussed feature, advantage, or mode of operation.

[0217] The previous description of the disclosure is provided to enable any person skilled in the art to make or use the disclosure. Various modifications to the disclosure will be apparent to those skilled in the art, and the general principles defined herein may be applied to other variations without departing from the spirit or scope of the disclosure. Thus, the disclosure is not intended to be limited to the examples described herein, but should be granted the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A wireless device, comprising: a plurality of transmit circuits, wherein the plurality of transmit circuits are configured to transmit a plurality of radio frequency (RF) signals via a plurality of antennas; and a processor coupled to the transmitting circuit, wherein the processor is configured to: combining multiple-input multiple-output (MIMO) radio frequency (RF) exposure profiles into a combined MIMO RF exposure profile, wherein the combined MIMO RF exposure profile comprises a linear combination of the MIMO RF exposure profiles; determining maximum allowable power levels based on the combined MIMO RF exposure profile, wherein each of the maximum allowable power levels corresponds to a respective one of the transmit circuits; as well as A respective power level limit is set for each of the transmit circuits based on the respective one of the maximum allowed power levels. 2 . The wireless device of claim 1 , wherein each of the MIMO RF exposure profiles comprises a respective MIMO specific absorption rate (SAR) profile. 3 . The wireless device of claim 1 , wherein each of the MIMO RF exposure profiles comprises a respective MIMO power density (PD) profile. The wireless device of claim 1 , wherein the transmission circuit is configured to transmit the RF signals at a same transmission frequency.

5. The wireless device of claim 1 , wherein each of the MIMO RF exposure distributions comprises a first component and a second component, the first component comprising the RF exposure distribution of a corresponding one of the antennas, and the second component taking into account correlation between the corresponding one of the antennas and each of the other antennas of the antennas.

6. A wireless device according to claim 5, wherein the second component of each of the MIMO RF exposure distributions comprises the sum of multiple components, each of the multiple components comprising the square root of the product of the RF exposure distribution of the corresponding one of the antennas and the RF exposure distribution of the corresponding one of the other antennas. 7 . The wireless device of claim 5 , further comprising a memory storing the MIMO RF exposure profile, wherein the processor is configured to retrieve the MIMO RF exposure profile from the memory.

8. The wireless device of claim 1 , wherein the processor is configured to: determining a transmit power level such that a peak value of the combined MIMO RF exposure distribution is equal to or less than an exposure limit; and The maximum allowed power level is determined based on the determined transmit power level.

9. The wireless device of claim 8, wherein each of the MIMO RF exposure profiles is scaled based on a corresponding one of the transmit power levels.

10. The wireless device of claim 1 , wherein the combined MIMO RF exposure profile corresponds to a future time slot, and the processor is configured to: averaging the combined MIMO RF exposure distribution with a MIMO RF exposure distribution corresponding to a previous time slot to obtain a time average RF exposure distribution; determining a transmit power level for the future time slot such that a peak value of the time-averaged RF exposure distribution is equal to or less than an exposure limit; and The maximum allowed power level is determined based on the determined transmit power level for the future time slot.

11. The wireless device of claim 10, wherein each of the MIMO RF exposure profiles in the combined MIMO RF exposure profile is scaled based on a corresponding one of the transmit power levels for the future time slot.

12. The wireless device of claim 1 , wherein the processor is further configured to: combining the non-MIMO RF exposure distributions into a combined non-MIMO RF exposure distribution; and The maximum allowable power level is determined based on a combination of the combined MIMO RF exposure profile and the combined non-MIMO RF exposure profile.

13. The wireless device of claim 12, wherein the MIMO RF exposure distribution corresponds to a first subset of the antennas and the non-MIMO RF exposure distribution corresponds to a second subset of the antennas.

14. The wireless device of claim 12, wherein at least one of the MIMO RF exposure profiles and at least one of the non-MIMO RF exposure profiles correspond to a same one of the antennas.

15. The wireless device of claim 14, wherein the at least one of the MIMO RF exposure profiles corresponds to a first transmit frequency and the at least one of the non-MIMO RF exposure profiles corresponds to a second transmit frequency.

16. A method for operating a wireless device comprising a plurality of transmit circuits, wherein the transmit circuits are configured to transmit a plurality of radio frequency (RF) signals via a plurality of antennas, the method comprising: combining multiple-input multiple-output (MIMO) radio frequency (RF) exposure profiles into a combined MIMO RF exposure profile, wherein the combined MIMO RF exposure profile comprises a linear combination of the MIMO RF exposure profiles; determining maximum allowable power levels based on the combined MIMO RF exposure profile, wherein each of the maximum allowable power levels corresponds to a respective one of the transmit circuits; as well as A respective power level limit is set for each of the transmit circuits based on the respective one of the maximum allowed power levels.

17. The method of claim 16, wherein each of the MIMO RF exposure profiles comprises a respective MIMO specific absorption rate (SAR) profile.

18. The method of claim 16, wherein each of the MIMO RF exposure profiles comprises a respective MIMO power density (PD) profile. The method of claim 16 , wherein the transmitting circuit is configured to transmit the RF signals at a same transmit frequency.

20. The method of claim 16, wherein each of the MIMO RF exposure distributions comprises a first component comprising the RF exposure distribution of a corresponding one of the antennas and a second component taking into account correlation between the corresponding one of the antennas and each of the other ones of the antennas.

21. The method of claim 20, wherein the second component of each of the MIMO RF exposure distributions comprises the sum of a plurality of components, each of the plurality of components comprising a square root of a product of the RF exposure distribution of the corresponding one of the antennas and the RF exposure distribution of the corresponding one of the other antennas.

22. The method of claim 16, wherein determining the maximum allowable power level comprises: determining a transmit power level such that a peak value of the combined MIMO RF exposure distribution is equal to or less than an exposure limit; as well as The maximum allowed power level is determined based on the determined transmit power level.

23. The method of claim 22, wherein each of the MIMO RF exposure profiles in the combined MIMO RF exposure profile is scaled based on a respective one of the transmit power levels.

24. The method of claim 16, wherein the combined MIMO RF exposure profile corresponds to a future time slot, and determining the maximum allowable power level comprises: averaging the combined MIMO RF exposure distribution with a MIMO RF exposure distribution corresponding to a previous time slot to obtain a time average RF exposure distribution; determining a transmit power level for the future time slot so that a peak value of the time-averaged RF exposure distribution is equal to or less than an exposure limit; as well as The maximum allowed power level is determined based on the determined transmit power level for the future time slot.

25. The method of claim 24, wherein each of the MIMO RF exposure profiles in the combined MIMO RF exposure profile is scaled based on a respective one of the transmit power levels for the future time slot.

26. The method according to claim 16, further comprising: combining the non-MIMO RF exposure distributions into a combined non-MIMO RF exposure distribution; Wherein determining the maximum allowable power level comprises determining the maximum allowable power level based on a combination of the combined MIMO RF exposure profile and the combined non-MIMO RF exposure profile.

27. The method of claim 26, wherein the MIMO RF exposure distribution corresponds to a first subset of the antennas and the non-MIMO RF exposure distribution corresponds to a second subset of the antennas.

28. The method of claim 26, wherein at least one of the MIMO RF exposure profiles and at least one of the non-MIMO RF exposure profiles correspond to a same one of the antennas.

29. The method of claim 28, wherein the at least one of the MIMO RF exposure profiles corresponds to a first transmit frequency and the at least one of the non-MIMO RF exposure profiles corresponds to a second transmit frequency.

30. A computer readable medium having stored thereon instructions for: combining multiple-input multiple-output (MIMO) radio frequency (RF) exposure profiles into a combined MIMO RF exposure profile, wherein the combined MIMO RF exposure profile comprises a linear combination of the MIMO RF exposure profiles; determining maximum allowable power levels based on the combined MIMO RF exposure profile, wherein each of the maximum allowable power levels corresponds to a respective one of a plurality of transmit circuits; and A respective power level limit is set for each of the transmit circuits based on the respective one of the maximum allowed power levels.

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