Question: Like ethernet cable, optical fiber. We have a square wave signal, do we just send that squre-wave signal directly down along the cable? If we do send it directly, why we can not send the square-wave signal directly into the air wireless channel? What is the reason behind? Is it because the cable or wire are more stable but wireless is very unstable?

Answer: 
tl; dr: RF needs a carrier, cable can use baseband. Both are band-limited, RF systems using carriers more so.

A perfect square wave has infinite harmonics. A good square wave has a large number of them. Because of this, square waves use a lot of bandwidth for the information they carry. They’re very inefficient. This limits their use to lower speed signaling, even on cables.

Look at serdes protocols like PCI Express or Gbit Ethernet. You will see that they don’t use square waves, but more optimized baseband coding to squeeze more throughput out of the cable.

Similarly, a square wave on an RF carrier is very inefficient use of spectrum (though it’s possible - primitive on-off keying does exactly that.) That said, it’s possible to send very-high bandwidth digital data over RF. The challenge is to do that efficiently, within the limits of the spectrum being used, and over a carrier that a receiver can work with and robustly recover the data.

And finally, wireless systems can’t represent a DC connection or low frequency directly. You can only get an approximate derivative of the changes in flux strength at the receiver side. For RF work need to use a coding system that would behave well as if it were AC coupled onto a wire. Using a carrier overcomes this problem.

That said, look into TEMPEST. This is a group of passive espionage techniques that extract information from emissions leaked digital and analog equipment - that is, from baseband waves (square and otherwise) that find their way into RF.
Along with all the bandwidth and other considerations mentioned, the main reason you don't transmit your data directly is that **everyone** would be transmitting at the same frequency.

Take [Ethernet.](https://en.wikipedia.org/wiki/Ethernet) It uses two (or more) pairs of twisted wires to carry two separate channels of information (your Ethernet card sends data on one pair and receives it on the other pair.)

If you just directly broadcast the signals from each pair, then you couldn't separate them.

The wires confine the signals do that they go pretty much only where they are intended to go.

If you just modulate each onto a carrier, you can separate them - but then you have know which carrier frequency is used for what.

That would work for a few channels, but would get difficult really quick as you add more channels (users.)

WiFi gets around that by having multiple channels and using some intelligence to decide who uses which carrier frequency for what and for how long. It also has WiFi routers which coordinate the end users and actively tries to avoid interference from other routers and users.