What you are describing is basically CW which is how Morse code is transmitted over radio. It works well at the low data rates of Morse code. The higher the sym

What you are describing is basically CW which is how Morse code is transmitted over radio. It works well at the low data rates of Morse code. The higher the symbol rate, however, the higher resolution you need for measuring pulse length, which requires sharper edges from off to on. In practice it is usually easier to use different tones for each symbol (fsk) since the pulse length is less critical in that case. Quadrature modulation like bpsk is more efficient at the cost of additional complexity, and spread spectrum techniques allow for much better performance in noisy environments at the cost of using more bandwidth. So as usual in engineering, it’s a trade off that has to be decided on a case by case basis.