Why do mobile phone transmissions come in pulses?
Most digital cellphone systems use time division multiple access. They transmit their packets in short bursts so that other phones in the same cell coverage area can also use the frequency to transmit their own packets.

Also, transmitting in short bursts helps with managing the energy consumpution of the phone overall, for better battery life. The phone only has a certain amount of data to send per second, and by using the radio channel at its maximum rate for a short time saves power relative to sending at a slower rate continuously.