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I was reading about multipath fading in optical wireless communication, and in many sources it is said that the multipath fading is negligible for two main reasons:

  1. In a system deploying direct-detection the path loss exponent is 4 instead of 2 due to the square-law nature of the PDs.
  2. The area of the PD is much larger than the wavelength of the optical signal, making for spatial diversity which mitigates multipath fading.

I understand 1) to some extent, but I don't get 2).

What does the wavelength of the transmitted signal (or its frequency) have to do with the surface area of the receiving photo-diode? What is the analogous case in RF?

Here is some text from sources I read:

... this type of links does not have diffuse multipath fading problems that are common in traditional RF wireless technologies. The reason is that the wavelength in VLC is only hundreds of nanometers, and the size of commonly used photodetectors is in an order of centimeters, which is sufficiently large to achieve effective space diversity for VLC signals, thereby mitigating the multipath fading.

and another:

Further, multipath fading can be neglected in optical wireless channel. In our channel model, the information carrier is a light wave whose frequency is about 10^14 Hz. Moreover, detector dimensions are in the order of thousands of wavelengths, leading to efficient spatial diversity, which prevents multipath fading. For the above reasons, multipath fading can be neglected

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What does the wavelength of the transmitted signal (or its frequency) have to do with the surface area of the receiving photo-diode?

Multipath fading happens when the signal from one path destructively interferes with the signal from another path when they intersect at the detector. It essentially means that the speckle pattern formed by the interference of the two (or more) beams has a dark spot at the detector.

However, the size of the dark and bright spots in the speckle pattern are sized roughly on the scale of the signal wavelength.

So if the detector is 100's of wavelengths in diameter, a single dark spot in the speckle pattern won't cover the whole detector. The low intensity received at a dark spot will be balanced by higher intensity received at a bright spot in the speckle pattern. The overall signal amplitude received will be averaged over the whole detector.

What is the analogous case in RF?

You could have an analogous case in RF if you built an antenna hundreds or thousands of wavelengths across. Or if you used multiple antennas spread over an area many wavelengths across and averaged the received signals appropriately (obviously this would only be compatible with crude modulation schemes like OOK, not with phase or frequency modulation schemes).

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