What is modulation and why needed

Modulation is the process of varying a characteristic (amplitude, frequency or phase) of a high-frequency carrier signal in accordance with the instantaneous value of the low-frequency message (baseband) signal.

Why modulation is necessary

1. Practical antenna size: an antenna needs a length of about λ/4, with λ = c/f. For a 3 kHz audio signal λ = 100 km, so a 25 km antenna would be needed; for a 100 MHz carrier, λ/4 = 0.75 m.
2. Efficient radiation: low-frequency signals radiate poorly; high-frequency waves radiate and propagate well.
3. Multiplexing: many messages can share one channel by using different carrier frequencies (FDM). Without modulation, all would occupy the same band and interfere.
4. Reducing noise and interference: some types of modulation (such as FM) give better noise immunity.
5. Matching to the channel: it shifts the signal to a band that the medium can carry (e.g. digital data over a telephone line needs a modem).
6. Longer range and less attenuation at suitable frequencies, and easier equipment (amplifiers and filters) at higher frequencies.