An expert said that it is necessary to modulate low frequency signals to sent long distances.What are the different types of modulation

The type of communication system needed for a given signal depends on the band of frequencies in the signal. What are the different types of message signals used in a communication systems.

Modulation is a process employed to superimpose a carrier wave with a signal wave. A message signal of frequency 10 KHz and peak voltage of 10 volts is used to modulate a carrier wave of frequency 1 MHz and peak voltage of 20 volts.Determine the modulation index and frequencies of side bands.

In the broadcast of communication modulation is necessary:-What do you mean by modulation?

Various propagation modes are used in communication.If a signal of frequency omega_s used to modulate a is carrier wave of frequency omega_c which are the frequencies contained in the modulated signal other than omega_c ?