Why is channel DC gain sufficient to model optical wireless communications?

I came across the following question at ResearchGate, but I fail to comprehend the answer to it.

The question states:

Why do we only study the DC gain in VLC channel modeling?

In all the papers and books that I read, only the DC channel gain of the VLC system is studied without any further justification.

I looked at many relevant sources in the literature, and I agree that there is no explanation provided as to why the channel DC gain is sufficient to modeling the optical channel.

The answer might be so obvious that it does not require an explanation, however it is not immediately straightforward to me.

Literature closest to a satisfactory explanation were the following:

1)

The frequency responses of infrared channels are relatively flat near dc, so for most purposes, the single most important quantity characterizing a channel is the dc gain

2)

For intensity-in intensity-out channels, the zero-frequency (DC) value of their frequency responses can be expressed as [52]. The expression in equation (2) is commonly referred to as channel DC gain, which is the fraction of power emitted from a continuous-wave transmitter that is detected by the receiver

My understanding from the above is that, since optical wireless communication uses Intensity Modulation with Direct Detection (IM/DD), there is no frequency component that one should be concerned with, but on the other hand I do not understand what "the zero-frequency (DC) value of their (referring to the channels) frequency responses" means?

• When you say "channel" do you mean the path that conveys light from the transmitter to the receiver? (As opposed to, say, considering the receiver and decision circuit as parts of "the channel") May 25, 2022 at 16:53
• which answer are you not comprehending? May 25, 2022 at 17:13
• @ThePhoton That would be "the path that conveys light from the transmitter to the receiver". May 26, 2022 at 8:21

Say the LED has 10 nm linewidth, with a spectrum spanning 645 to 655 nm. Put in frequency terms ($$\f_o=c/\lambda\$$), that means the LED spectrum spans roughly 458 - 465 THz. It has a bandwidth of 7 THz.