Evaluating radio frequency (RF) exposure in real time

1. A wireless device, comprising:

a transmitter; and

a processor coupled to the transmitter, wherein the processor is configured to:

receive a power density (PD) limit for a time slot, the time slot comprising sub-time slots;

determine a PD allocation for the time slot based on the PD limit;

determine a maximum allowable power level for one of the sub-time slots based on a portion of the PD allocation used up by one or more of the sub-time slots preceding the one of the sub-time slots; and

set a transmission power limit for the transmitter based on the maximum allowable power level.

2. The wireless device of claim 1 , wherein, during the one of the sub-time slots, the transmitter is configured to transmit a signal at a transmission power level equal to or less than the transmission power limit.

3. The wireless device of claim 1 , wherein the determination by the processor of the PD allocation for the time slot comprises multiplying the PD limit by a number of the sub-time slots.

4. The wireless device of claim 1 , wherein the processor is configured to determine the PD allocation used up by the one or more of the sub-time slots preceding the one of the sub-time slots.

5. The wireless device of claim 4 , wherein the determination by the processor of the PD allocation used up by the one or more of the sub-time slots preceding the one of the sub-time slots comprises summing PDs of the one or more of the sub-time slots preceding the one of the sub-time slots.

6. The wireless device of claim 1 , wherein the processor is further configured to:

determine a portion of the PD allocation reserved for one or more of the sub-time slots succeeding the one of the sub-time slots; and

determine the maximum allowable power level for the one of the sub-time slots based also on the portion of the PD allocation reserved for the one or more of the sub-time slots succeeding the one of the sub-time slots.

7. The wireless device of claim 6 , wherein the determination by the processor of the portion of the PD allocation reserved for the one or more of the sub-time slots succeeding the one of the sub-time slots comprises multiplying a reserved PD by a number of the one or more of the sub-time slots succeeding the one of the sub-time slots.

8. The wireless device of claim 7 , wherein the processor is configured to determine the reserved PD based on the PD limit and a reserve ratio.

9. The wireless device of claim 8 , wherein the determination by the processor of the reserved PD comprises multiplying the PD limit by the reserve ratio.

10. The wireless device of claim 8 , wherein the processor is configured to set the reserve ratio based on at least one of a channel condition between the wireless device and another wireless device, or a scheduled data transmission for the wireless device.

11. The wireless device of claim 1 , wherein the determination by the processor of the maximum allowable power level for the one of the sub-time slots comprises:

determining an allowed PD for the one of the sub-time slots based on the portion of the PD allocation used up by the one or more of the sub-time slots preceding the one of the sub-time slots;

retrieving a transmission power level from a memory, the transmission power level corresponding to a reference PD; and

determining the maximum allowable power level for the one of the sub-time slots based on the retrieved transmission power level, the allowed PD, and the reference PD.

12. The wireless device of claim 11 , wherein the determination by the processor of the maximum allowable power level for the one of the sub-time slots based on the retrieved transmission power level, the allowed PD, and the reference PD comprises scaling the retrieved transmission power level by a ratio of the allowed PD to the reference PD.

13. The wireless device of claim 1 , wherein the processor is configured to:

determine a time-averaged radio frequency (RF) exposure distribution; and

determine the PD limit for the time slot based on the time-averaged RF exposure distribution.

14. The wireless device of claim 13 , wherein the determination of the time-averaged RF exposure distribution by the processor comprises:

determining a time-averaged specific absorption (SAR) distribution over a first time window;

determining a time-averaged power density (PD) distribution over a second time window; and

combining the time-averaged SAR distribution and the time-averaged PD distribution to generate the time-averaged RF exposure distribution.

15. The wireless device of claim 14 , wherein the first time window and the second time window have different lengths.

16. The wireless device of claim 13 , wherein the time-averaged RF exposure distribution comprises a time-averaged power density (PD) distribution.

17. A method implemented in a wireless device, comprising:

receiving a power density (PD) limit for a time slot, the time slot comprising sub-time slots;

determining a PD allocation for the time slot based on the PD limit;

determining a maximum allowable power level for one of the sub-time slots based on a portion of the PD allocation used up by one or more of the sub-time slots preceding the one of the sub-time slots; and

setting a transmission power limit for a transmitter based on the maximum allowable power level.

18. The method of claim 17 , further comprising:

determining a time-averaged radio frequency (RF) exposure distribution; and

determining the PD limit for the time slot based on the time-averaged RF exposure distribution.

19. The method of claim 18 , wherein determining the time-averaged RF exposure distribution comprises:

determining a time-averaged specific absorption (SAR) distribution over a first time window;

determining a time-averaged power density (PD) distribution over a second time window; and

combining the time-averaged SAR distribution and the time-averaged PD distribution to generate the time-averaged RF exposure distribution.

20. A wireless device, comprising:

a transmitter; and

a processor coupled to the transmitter, wherein the processor is configured to:

determine a time-averaged radio frequency (RF) exposure distribution over time slots;

determine an RF exposure limit for one of the time slots based on the time-averaged RF exposure distribution over the time slots, the one of the time slots comprising sub-time slots;

determine a time-averaged RF exposure distribution within the one of the time slots;

determine a maximum allowable power level for one of the sub-time slots based on the RF exposure limit and the time-averaged RF exposure distribution within the one of the time slots; and

set a transmission power limit for the transmitter based on the maximum allowable power level.

21. The wireless device of claim 20 , wherein the determination of the time-averaged RF exposure distribution over the time slots by the processor comprises:

determining a time-averaged specific absorption (SAR) distribution over a first time window;

determining a time-averaged power density (PD) distribution over a second time window; and

combining the time-averaged SAR distribution and the time-averaged PD distribution to generate the time-averaged RF exposure distribution over the time slots.

22. The wireless device of claim 21 , wherein the first time window and the second time window have different lengths.

23. The wireless device of claim 21 , wherein the time-averaged SAR distribution corresponds to a first technology and the time-averaged PD distribution corresponds to a second technology.

24. The wireless device of claim 23 , wherein the first technology comprises a sub-6 GHz technology and the second technology comprises a millimeter wave technology.

25. The wireless device of claim 23 , wherein a same time-slot duration is used for the first technology and the second technology.

26. The wireless device of claim 21 , wherein the time-averaged SAR distribution is normalized with respect to a SAR limit, and the time-averaged PD distribution is normalized with respect to a PD limit.

27. The wireless device of claim 21 , wherein the RF exposure limit comprises a PD limit.

28. The wireless device of claim 20 , wherein the time-averaged RF exposure distribution over the time slots comprises a time-averaged power density (PD) distribution, and the RF exposure limit comprises a PD limit.

29. The wireless device of claim 28 , wherein the sub-time slots are normalized with respect to the PD limit.

30. The wireless device of claim 20 , wherein the time-averaged RF exposure distribution over the time slots comprises a time-averaged specific absorption rate (SAR) distribution, and the RF exposure limit comprises a SAR limit.

31. The wireless device of claim 30 , wherein the sub-time slots are normalized with respect to the SAR limit.

32. The wireless device of claim 20 , wherein the one of the sub-time slots comprises second sub-time slots, and the processor is configured to:

determine a second RF exposure limit for the one of the sub-time slots;

determine a time-averaged RF exposure distribution within the one of the sub-time slots; and

determine a maximum allowable power level for one of the second sub-time slots based on the second RF exposure limit and the time-averaged RF exposure distribution within the one of the sub-time slots.

33. The wireless device of claim 20 , wherein the processor is configured to:

determine a second RF exposure limit for a second one of the time slots based on the time-averaged RF exposure distribution over the time slots, the second one of the time slots comprising second sub-time slots;

determine a time-averaged RF exposure distribution within the second one of the time slots; and

determine a maximum allowable power level for one of the second sub-time slots based on the second RF exposure limit and the time-averaged RF exposure distribution within the second one of the time slots.

34. The wireless device of claim 33 , wherein the first sub-time slots and the second sub-time slots have different time intervals.

35. The wireless device of claim 20 , wherein the RF exposure limit comprises a power density (PD) limit, and the processor is configured to:

determine a PD allocation based on the PD limit;

determine a portion of the PD allocation reserved for one or more of the sub-time slots succeeding the one of the sub-time slots; and

determine the maximum allowable power level for the one of the sub-time slots based also on the portion of the PD allocation reserved for the one or more of the sub-time slots succeeding the one of the sub-time slots.

36. The wireless device of claim 35 , wherein the time-averaged RF exposure distribution within the one of the time slots is averaged over one or more of the sub-time slots preceding the one of the time slots.

37. The wireless device of claim 35 , wherein the determination by the processor of the portion of the PD allocation reserved for the one or more of the sub-time slots succeeding the one of the sub-time slots comprises multiplying a reserved PD by a number of the one or more of the sub-time slots succeeding the one of the sub-time slots.

38. The wireless device of claim 37 , wherein the processor is configured to determine the reserved PD based on the PD limit and a reserve ratio.

39. The wireless device of claim 38 , wherein the determination by the processor of the reserved PD comprises multiplying the PD limit by the reserve ratio.

40. The wireless device of claim 38 , wherein the processor is configured to set the reserve ratio based on at least one of a channel condition between the wireless device and another wireless device, or a scheduled data transmission for the wireless device.