Sign language is just radio communication but on another frequency
Because you use light to communicate in sign language, and light is just an electromagnetic wave like radio but on another frequency. You just need to be in line of sight
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"Modulation is the process of converting data into radio waves by adding information to an electronic or optical carrier signal. A carrier signal is one with a steady waveform -- constant height, or amplitude, and frequency." Which stream of photons is the carrier signal?
Oh, so it's a bunch of different ones that contain different signals that can't be decoded into any additional information other than the presence of an object i.e. ordinary eyesight.
The information is actually encoded in the spatial position of the receivers(i.e. retinal cells)? That's not radio communication, anymore than a radio telescope detecting a pulsar is "communication" from a star. If there was some intentional encoding of information in a manner that was relatively indifferent to the position of the observers of the signal, I might agree. Such as, opening and closing ones hand in a Morse code. That would be much more akin to radio communication.
Exactly, I'd still place bursts of radio signals (like that of a pulsar or morse code transmitter) under the umbrella term "radio communication". Just because it uses the radiation that is at the radio frequency. No matter what the format/arrangement/method, which includes encoding and using carrier waves.
Then you also have electric waves that travel through conductors, although this is only a part the electromagnetic wave used in the air. But you can change the frequency of this electric wave perhaps to the radio frequency. So perhaps there is also radio communication through wires