5G FR1 and FR2: exposure differences
Select RF exposure metrics from frequency and geometry, distinguish 5G range labels from exposure criteria, and interpret changing beams and measurement averages.
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ContinueA 5G label does not tell you the quantity to compare or the measurement to make. Start from the actual frequency, antenna aperture, accessible location and operating state. Then decide which exposure metric and averaging procedure the adopted framework requires.
Use field regions, power-density quantities and time averaging. This lesson applies those ideas to beamformed systems; it does not supply a new set of exposure limits.
Separate the radio designation from the exposure criterion
For terrestrial NR, ETSI TS 138 101-2 V17.20.0, Release 17, Table 5.1-1 defines FR1 as 410 MHz–7.125 GHz. FR2-1 spans 24.25–52.6 GHz and FR2-2 spans 52.6–71 GHz. Keep the specification version with those labels; a range is not a statement that every frequency within it is deployed.
The 6 GHz transition used by some exposure criteria lies inside FR1. Consequently, “FR1 means SAR; FR2 means power density” is an unreliable decision rule. Use frequency, exposure geometry and the exact criterion. Distinguish local dosimetry from whole-body assessment and basic restrictions from external reference levels.
Connect wavelength, aperture and absorption
With wave speed c, wavelength is λ = c/f. FR2 wavelengths are approximately 4–12 mm, allowing many elements in a compact array. Aperture size and phase control determine the beam pattern; a higher frequency alone does not determine the field at a person.
For an isolated aperture of largest active dimension D, r_FF ≈ 2D²/λ is a far-field screening estimate. Within it, distance alone cannot justify S = Eᵣₘₛ²/377. Beyond it, reflections and additional sources can still invalidate a single travelling-wave model. Use the actual active aperture rather than a device’s case dimensions.
At higher frequencies absorption tends to concentrate near the tissue surface. ICNIRP’s 2020 framework distinguishes local SAR and absorbed power density by frequency, with specified spatial and temporal averaging. Incident power density is an external quantity; it is not automatically absorbed power density. The above-6 GHz lesson develops that distinction.
Estimate an array’s field region
Worked example 1
Estimate the far-field boundary of a mmWave array
Problem
You want to know whether a point measurement can reasonably be treated as plane-wave-like.
Use f = 28 GHz and largest active aperture dimension D = 0.20 m for an isolated array in air.
Estimate the far-field boundary and assess whether measurements at 1–3 m can reasonably be treated as plane-wave-like.
Show full solution
Find the wavelength and aperture
Method
Convert the operating frequency to wavelength and identify the array aperture.
Reason
The Fraunhofer estimate depends on aperture size relative to wavelength.
Working
Answer
Assume:
- f = 28 GHz → λ ≈ (3 × 10⁸)/(28 × 10⁹) ≈ 0.0107 m
- array aperture D = 0.20 m
Estimate the far-field boundary
Method
Insert the aperture and wavelength into the Fraunhofer expression.
Reason
The boundary estimate screens whether the observation distance is sufficiently far from the array.
Working
Answer
Fraunhofer estimate:
r_FF ≈ 2D²/λ = 2(0.20)²/0.0107 ≈ 7.5 mInterpret the measurement region
Working
Answer
The estimated boundary is 7.5 m. Points at 1–3 m are inside it, so distance alone does not justify a plane-wave conversion. Use a method valid for the radiating near field. Beyond 7.5 m, this aperture estimate still does not rule out reflections or other sources that change the local E/H relationship.
Average one position without claiming every position
Beam steering and traffic scheduling can move a maximum. A time average at one point and an area average across several points answer different questions. Average the required power-like quantity over the specified time window; keep the location fixed while making that time record. A max-hold display stores a maximum rather than reconstructing its duration.
Guided practice 2
Interpret a changing beam at one location
Problem
At one fixed accessible location, a validated measurement reports incident power density of 2.00 W/m² for 120 s, 12.0 W/m² for 30.0 s and 1.00 W/m² for the remaining 210 s. These are complete, non-overlapping parts of an illustrative 360 s study window, with the same measurement geometry.
Find the time-integrated incident energy density in J/m² and the time-averaged incident power density in W/m². Explain what the 12.0 W/m² maximum establishes, and what information is still missing for an area-averaged criterion or a maximum over operating states.
Make the decisions, then compare your working
Hints
Hint 1
Keep the location and time window fixed
Calculate ∑ SᵢΔ tᵢ. The quiet interval contributes energy too.
Hint 2
Separate a maximum from an average
Divide the total by 360 s. Neither the largest value nor an unweighted mean accounts for the unequal durations.
Show solution step by step
Accumulate incident energy
Method
Weight each power density by its duration.
Reason
Energy density has units (W/m²) s = J/m².
Working
Answer
Contributions are 2.00(120) = 240, 12.0(30.0) = 360 and 1.00(210) = 210 J/m². Their sum is 810 J/m².
Recover the time average
Method
Divide by the total observation duration.
Reason
All three states belong to the same point and complete window.
Working
Answer
S bar = 810/360 = 2.25 W/m². This lies between 1.00 and 12.0 W/m². The unweighted mean would treat three unequal durations as equal.
State the scope of the result
Method
Keep the measured statistic distinct from the required comparison.
Reason
A time record at one point neither maps an area nor bounds unobserved traffic and beam states.
Working
Answer
The observed maximum is 12.0 W/m², not the time average. The record alone does not establish an area average, a worst-case sliding window, brief-exposure criteria or a configured maximum. Compare those distinctions yourself; the checker verifies energy density and the study-window average only.
Reconsider the method when the band and geometry change
Check your understanding 1 A new band and a close measurement
A terrestrial FR1 device operates at 6.50 GHz with active aperture D = 0.100 m. A proposed close-to-body test is at 0.150 m. The assessor plans to infer SAR from a single electric-field reading because “it is FR1.”
Using c = 3.00 × 10⁸ m/s, assess the field-region argument and the metric choice. Identify the information required before choosing the comparison method.
Show answer
Here λ = 0.0462 m and 2D²/λ = 0.433 m. The proposed point is inside that screening boundary, so this estimate does not justify a plane-wave conversion. Nor does a field-in-air reading directly measure SAR.
The operating frequency exceeds 6 GHz despite its FR1 designation. Select the actual local or whole-body criterion from the adopted framework, including its incident/absorbed quantity, averaging and close-to-body method. The aperture estimate is a method screen, not a complete near-field dosimetry model. Compare your response for both failed assumptions and the missing comparison conditions.
Review the measurement decisions
Check your understanding 2
Why does a close-to-body assessment at a lower FR1 frequency commonly use SAR, while an FR2 assessment uses a surface metric?
Show answer
Lower-frequency energy can be deposited through a larger tissue volume, so SAR describes absorbed power per unit mass. FR2 absorption is more superficial, so a surface metric describes the local deposition. Select the exact metric from the adopted framework and frequency: FR1 itself extends above 6 GHz.
Check your understanding 3
Why can a “spot peak” be misleading for beamformed systems?
Show answer
Because beams move and scheduling changes the maximum over time and space. Many criteria are defined with time and/or spatial averaging, so a spot peak is not automatically the comparison quantity.
Check your understanding 4
An above-6 GHz criterion specifies an area average. Why can a single point reading not establish that average?
Show answer
The field can vary across the required area. A point reading samples one position; the criterion requires the metric averaged over the specified area, with any separate small-area focusing check applied too.
Keep frequency, geometry and averaging together
Record the active band, aperture, operating states, accessible region, measured quantity, detector response, averaging and uncertainty. A broadband total cannot identify 5G’s contribution without a justified attribution method; a quiet beam state cannot bound all beam states. Change the method when one of those conditions changes.
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Syllabus and review details
No official syllabus alignment is listed for this lesson.
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