# Aperture averaging

Aperture averaging is a technique in free-space optical communications in which a receiver collects the transmitted beam over a large aperture rather than a point, so that turbulence-induced irradiance fluctuations arriving at different points across the aperture partially cancel. It is quantified by the aperture averaging factor, defined as the ratio of the received-power scintillation index for a receiver with aperture diameter \( D \) to that for a point receiver under the same propagation conditions.<sup>[1](https://repository.library.noaa.gov/view/noaa/31243/noaa_31243_DS1.pdf)</sup> The technique addresses scintillation, the rapid fading of received power caused by atmospheric turbulence. Because the atmospheric cut-off frequency can be as low as 100 Hz, temporal filtering cannot remove this fading in high-speed communications, and increasing the receiver aperture is the simplest way to reduce it.<sup>[2](https://elib.dlr.de/7341/1/LASE2004-5338-29_Perlot_ApAv_measur_HANDOUT.pdf)</sup> Aperture averaging is described as the simplest form of spatial diversity, requiring no added hardware, software complexity, or power consumption beyond the larger optic itself.<sup>[3](https://ietresearch.onlinelibrary.wiley.com/doi/10.1049/iet-opt.2015.0042)</sup>

| Key fact | Value |
|---|---|
| Definition | Ratio of power fluctuations on an aperture of diameter \( D \) to those on a point receiver<sup>[1](https://repository.library.noaa.gov/view/noaa/31243/noaa_31243_DS1.pdf)</sup> |
| First quantification | D. L. Fried, Journal of the Optical Society of America, 1967<sup>[4](https://doi.org/10.1364/josa.57.000169)</sup> |
| Weak-turbulence scaling | Variance proportional to the −7/3 power of \( D \) over the Fresnel zone size for large apertures<sup>[1](https://repository.library.noaa.gov/view/noaa/31243/noaa_31243_DS1.pdf)</sup> |
| Onset threshold | Significant averaging begins when the receiver diameter exceeds roughly twice the Fresnel zone size<sup>[5](https://www.spiedigitallibrary.org/journalArticle/Download?urlId=10.1117%2F1.OE.63.12.120801)</sup> |
| Measured BER gain | Over a 1 km, 1550 nm link, BER improved from about \( 10^{-3} \) (2-inch aperture) to essentially error-free (8-inch aperture)<sup>[6](https://ntrs.nasa.gov/api/citations/20110024181/downloads/20110024181.pdf)</sup> |
| Satellite downlink gain | About 2 dB of noise-endurance improvement per aperture doubling at \( \mathrm{BER} = 10^{-8} \)<sup>[7](https://www.mdpi.com/2076-3417/8/12/2496)</sup> |
| Practical size limit | Some designs limit aperture size, typically to 20–25 cm, to control solar background noise, but the appropriate aperture is system-dependent and no general maximum applies<sup>[8](https://link.springer.com/article/10.1007/s44291-025-00141-3)</sup> |

## How it works

Turbulence splits the optical field into many eddies that imprint independent fluctuations on different patches of the receiver plane. The collecting aperture acts as a spatial filter of the irradiance in the receiver plane: by integrating the power over an area larger than the correlation scale of the fluctuations, it averages out variations that are uncorrelated across the aperture, reducing the received signal variance.<sup>[2](https://elib.dlr.de/7341/1/LASE2004-5338-29_Perlot_ApAv_measur_HANDOUT.pdf)</sup>

The controlling length scale differs by regime. In weak conditions the correlation scale of irradiance fluctuations is set by the [Fresnel zone](https://www.edgepedia.org/fresnel-zone) size \( (\lambda L)^{1/2} \), so significant aperture averaging takes place only when the receiver diameter exceeds roughly twice that scale.<sup>[5](https://www.spiedigitallibrary.org/journalArticle/Download?urlId=10.1117%2F1.OE.63.12.120801)</sup> For large apertures and weak path-integrated turbulence with small inner scale, the variance of signal fluctuations is proportional to the −7/3 power of the ratio of aperture diameter to the Fresnel zone size; with large inner scale, the variance instead scales as the −7/3 power of aperture diameter over the inner scale.<sup>[1](https://repository.library.noaa.gov/view/noaa/31243/noaa_31243_DS1.pdf)</sup> In strong path-integrated turbulence the behavior splits into two scales: the small-scale variance portion is proportional to the −2 power of aperture diameter over the phase coherence length, and the large-scale portion to the −7/3 power of aperture diameter over the scattering disk; these approximations fall within a factor of 2 of measurements.<sup>[1](https://repository.library.noaa.gov/view/noaa/31243/noaa_31243_DS1.pdf)</sup> A useful statistical consequence is that even in the saturation regime the irradiance [probability density function](https://www.edgepedia.org/probability-density-function) can be accepted as lognormal when the receiving aperture is bigger than the coherence radius.<sup>[9](https://doi.org/10.1109/sopo.2010.5504015)</sup>

## How it is done

A link designer works through three steps. First, choose the aperture diameter relative to the turbulence length scales. Aperture averaging is effective only for \( D \) greater than the spatial coherence radius; for a geostationary satellite-to-ground path the coherence radius is 0.928, 3.63, and 11.53 cm for \( C_{n}^{2} \) of \( 10^{-12} \), \( 10^{-13} \), and \( 10^{-14}\ \mathrm{m}^{-2/3} \) respectively, so apertures larger than a few centimeters already exhibit the effect.<sup>[3](https://ietresearch.onlinelibrary.wiley.com/doi/10.1049/iet-opt.2015.0042)</sup> Second, compute the residual scintillation with the aperture averaging factor. One analytic form is

\[ A = \left[ 1 + 1.062 \left( \frac{k D_{\mathrm{Rx}}^{2}}{4L} \right)^{7/6} \right]^{-1} \]

where \( k \) is the optical wavenumber, \( D_{\mathrm{Rx}} \) the receiver diameter, and \( L \) the path length.<sup>[5](https://www.spiedigitallibrary.org/journalArticle/Download?urlId=10.1117%2F1.OE.63.12.120801)</sup> Fried's formula for a circular clear aperture of diameter \( D \) is \( A = \left[ 1 + (D/d_{0})^{7/5} + (D/d_{0})^{7/3} \right]^{-1} \), with \( d_{0} \) a turbulence correlation distance.<sup>[10](https://ntrs.nasa.gov/api/citations/19750014049/downloads/19750014049.pdf)</sup> Third, set the link margin from the residual scintillation index and fade statistics. Measured results confirm the design logic: over a 1 km terrestrial link at 1550 nm, a 3-inch receive aperture was identified as probably the minimum diameter for good [BER](https://www.edgepedia.org/bit-error-rate) performance, and increasing the aperture from 1.0 inch to 1.25 inch improved BER from around \( 10^{-2} \)–\( 10^{-3} \) to often \( 10^{-6} \) to \( 10^{-12} \) or error-free.<sup>[6](https://ntrs.nasa.gov/api/citations/20110024181/downloads/20110024181.pdf)</sup>

## Origin

Aperture averaging of scintillation was first quantified by D. L. Fried in "Aperture Averaging of Scintillation," Journal of the Optical Society of America, 1967.<sup>[4](https://doi.org/10.1364/josa.57.000169)</sup> That paper derived the relationship between the statistics of log-amplitude fluctuations and irradiance fluctuations due to atmospheric turbulence, used it to evaluate the effect of a large aperture diameter in reducing the variance of a fluctuating signal, presented curves for the reduction factor, and worked out an application to space-to-ground communications system performance.<sup>[4](https://doi.org/10.1364/josa.57.000169)</sup> Fried, following a development by Tatarskii, concluded that the averaging factor is proportional to the inverse square of aperture diameter for large apertures in weak turbulence; this conclusion was based on incorrect numerical calculations, and the correct −7/3 power dependence was obtained later.<sup>[1](https://repository.library.noaa.gov/view/noaa/31243/noaa_31243_DS1.pdf)</sup> Early experiments comparing measured aperture-averaging factors with weak-turbulence theory found very poor agreement, probably because scintillation saturation was not understood and the measurements were not in the weak-turbulence regime.<sup>[1](https://repository.library.noaa.gov/view/noaa/31243/noaa_31243_DS1.pdf)</sup> Gracheva and Gurvich used measured covariance functions to obtain numerical estimates of the aperture-averaging factor in strong turbulence, concluding there was generally less aperture averaging than weak-turbulence theory predicts.<sup>[1](https://repository.library.noaa.gov/view/noaa/31243/noaa_31243_DS1.pdf)</sup>

## Variants

The theory is formulated separately for plane waves and spherical waves, and for beam waves. Using a Gaussian weighting function for the receiver aperture, a closed-form representation of the receiver-aperture averaging effect for beam waves was obtained.<sup>[11](https://opg.optica.org/josa/abstract.cfm?uri=josa-73-6-831)</sup> Unlike the plane-wave case, beam-wave power scintillations do not always decrease when the receiver aperture is increased, because on-axis intensity fluctuations of a coherent beam are smaller than off-axis ones and the averaging effect cannot appear when the whole beam lies within a coherent patch, that is, when the coherence length exceeds the beamwidth.<sup>[11](https://opg.optica.org/josa/abstract.cfm?uri=josa-73-6-831)</sup> In the saturation regime the aperture-averaging factor shows two-scale behavior: averaging is first determined by the spatial coherence scale \( \rho_{\mathrm{PW}} \), then a secondary roll-off related to the scattering disc \( L / ( k \cdot \rho_{\mathrm{pl}} ) \) appears.<sup>[5](https://www.spiedigitallibrary.org/journalArticle/Download?urlId=10.1117%2F1.OE.63.12.120801)</sup> Gamma-gamma and Málaga distributions are used in system studies; in one multi-aperture campaign the Málaga distribution fitted measured irradiance PDFs best, with fitting accuracy improved by 18.75% under weak and 13.16% under moderate turbulence versus other distributions.<sup>[12](https://www.mdpi.com/2304-6732/13/1/63)</sup>

## Applications

Aperture averaging is applied in terrestrial free-space optical links, where the [Kennedy Space Center](https://www.edgepedia.org/kennedy-space-center) 1 km, 1550 nm experiment showed BER improving from about \( 10^{-3} \) with a 2-inch aperture to essentially error-free with the full 8-inch aperture.<sup>[6](https://ntrs.nasa.gov/api/citations/20110024181/downloads/20110024181.pdf)</sup> It is used in satellite-to-ground links: for a coherent satellite downlink with log-normal turbulence and BPSK, aperture averaging reduces BER and outage probability and improves average capacity, with a larger benefit under stronger turbulence and larger zenith angles.<sup>[7](https://www.mdpi.com/2076-3417/8/12/2496)</sup> General agreement of experiment with theoretical predictions supports applying the theory to space-to-ground laser communications link analysis, an application already worked out in Fried's 1967 paper.<sup>[4](https://doi.org/10.1364/josa.57.000169)</sup><sup> • </sup><sup>[10](https://ntrs.nasa.gov/api/citations/19750014049/downloads/19750014049.pdf)</sup> Recent modeling finds that aperture averaging can substantially suppress scintillation in low Earth orbit (LEO) downlinks, but its effectiveness diminishes in medium Earth orbit ([MEO](https://www.edgepedia.org/medium-earth-orbit)) links due to the longer propagation path through free space.<sup>[13](https://link.springer.com/article/10.1007/s11082-025-08505-5)</sup>

## Limitations and alternatives

Returns diminish with size. With weak turbulence and small zenith angle, aperture-averaging improvement is limited once the diameter reaches 50 cm.<sup>[7](https://www.mdpi.com/2076-3417/8/12/2496)</sup> A primary drawback is that solar background noise increases with aperture size, so aperture size is typically kept at a maximum of 20–25 cm.<sup>[8](https://link.springer.com/article/10.1007/s44291-025-00141-3)</sup> For beam waves a larger aperture can even increase power scintillation when the coherence length exceeds the beamwidth.<sup>[11](https://opg.optica.org/josa/abstract.cfm?uri=josa-73-6-831)</sup> As the aperture increases, wavefront aberration can degrade coherent-link performance, which adaptive optics on the large ground receiver can compensate.<sup>[7](https://www.mdpi.com/2076-3417/8/12/2496)</sup>

The nearest alternative is spatial diversity with multiple apertures. When the receiver is background noise limited, multiple apertures are largely preferred to a single large aperture under strong turbulence conditions, while a single aperture is likely preferred under moderate turbulence; when the receiver is thermal noise limited, multiple apertures are interesting only when working at a very low BER, even under strong turbulence.<sup>[14](https://opg.optica.org/jocn/abstract.cfm?uri=jocn-1-6-580)</sup> SIMO schemes increase system cost and power consumption with the number of detectors.<sup>[8](https://link.springer.com/article/10.1007/s44291-025-00141-3)</sup> In a 980 nm high-scintillation experiment, average SNR improved by 33.9% for aperture averaging over a single-input single-output link, versus 6.67% and 26.7% for 1×2 and 1×4 SIMO schemes, and average BER was reduced by 32.33% for aperture averaging versus 6.19% and 21.2% for the SIMO schemes.<sup>[8](https://link.springer.com/article/10.1007/s44291-025-00141-3)</sup> In a gamma-gamma turbulence model with Beer-Lambert weather attenuation, outage probability decreases as aperture diameter increases regardless of turbulence strength and weather conditions, but as the desired outage-probability performance level decreases, an array receiver becomes the preferred choice over a single receiver relying on aperture averaging.<sup>[15](https://www.degruyterbrill.com/document/doi/10.1515/joc-2014-0007/html)</sup> [Adaptive optics](https://www.edgepedia.org/adaptive-optics) with deformable mirrors and wavefront sensors is a complementary real-time turbulence mitigation method.<sup>[13](https://link.springer.com/article/10.1007/s11082-025-08505-5)</sup>

## References

1. [Aperture-Averaging Factor for Optical Propagation Through the Turbulent Atmosphere (NOAA repository)](https://repository.library.noaa.gov/view/noaa/31243/noaa_31243_DS1.pdf)
2. [Aperture-Averaging, Theory and Measurements (Perlot et al., DLR)](https://elib.dlr.de/7341/1/LASE2004-5338-29_Perlot_ApAv_measur_HANDOUT.pdf)
3. [Performance enhancement by aperture averaging in terrestrial and satellite free space optical links (IET Optoelectronics)](https://ietresearch.onlinelibrary.wiley.com/doi/10.1049/iet-opt.2015.0042)
4. [D. L. Fried (1967). Aperture Averaging of Scintillation. Journal of the Optical Society of America.](https://doi.org/10.1364/josa.57.000169)
5. [Communication performance in the focusing and saturation regimes inherent to the turbulent channel: a tutorial (Optical Engineering, SPIE)](https://www.spiedigitallibrary.org/journalArticle/Download?urlId=10.1117%2F1.OE.63.12.120801)
6. [Measurements of Aperture Averaging On Bit-Error-Rate (NASA NTRS)](https://ntrs.nasa.gov/api/citations/20110024181/downloads/20110024181.pdf)
7. [Performance Analysis of Satellite-to-Ground Coherent Optical Communication System with Aperture Averaging (Applied Sciences, MDPI)](https://www.mdpi.com/2076-3417/8/12/2496)
8. [Comparative analysis of aperture averaging & receiver diversity schemes in FSO channel performance under high scintillation regime (Springer, 2025)](https://link.springer.com/article/10.1007/s44291-025-00141-3)
9. [Laser Signal Intensity and Aperture Averaging Analysis in 16km Free-Space Optical Links](https://doi.org/10.1109/sopo.2010.5504015)
10. [Fried, David L., Optical Science Consultants, aperture averaging measurements and theory (NASA NTRS)](https://ntrs.nasa.gov/api/citations/19750014049/downloads/19750014049.pdf)
11. [Receiver-aperture averaging effects for the intensity fluctuation of a beam wave in the turbulent atmosphere (JOSA, 1983)](https://opg.optica.org/josa/abstract.cfm?uri=josa-73-6-831)
12. [Statistical Study of Free-Space Optical Transmission Using Multi-Aperture Receivers Under Real-Measured Atmospheric Turbulence (MDPI Photonics)](https://www.mdpi.com/2304-6732/13/1/63)
13. [Atmospheric modeling of free-space optical transmission: satellite downlinks and horizontal channels (Optical and Quantum Electronics, 2025)](https://link.springer.com/article/10.1007/s11082-025-08505-5)
14. [Fading Reduction by Aperture Averaging and Spatial Diversity in Optical Wireless Systems (JOCN, 2009)](https://opg.optica.org/jocn/abstract.cfm?uri=jocn-1-6-580)
15. [Comparison of Aperture Averaging and Receiver Diversity Techniques (Journal of Optical Communications)](https://www.degruyterbrill.com/document/doi/10.1515/joc-2014-0007/html)

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*Topic: Encyclopedia › Technology and the built world › Communications and everyday technology › Optical and fiber communication techniques*

*Initially written Sep 29, 2026 · Edited: Oct 15, 2026*

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