A data-modulation scheme defines how the data bits are mixed with the carrier signal, which is always a sine wave. There are three basic ways to modulate a carrier signal in a digital sense: amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK).

- In ASK the amplitude of the carrier signal is modified by the digital signal.
- In FSK the frequency of the carrier signal is modified by the digital signal.
- The PSK family is the most widely used modulation scheme in modern cellular systems.

**Digital Modulation - BPSK**

In binary phase shift keying (BPSK) modulation, each data bit is transformed into a separate data symbol. The mapping rule is 1 −> + 1 and 0 − > − 1. There are only two possible phase shifts in BPSK, 0 and π radians. NRZ means none return zero.

**Digital Modulation - QPSK**

The quadrature phase shift keying (QPSK) modulation has four phases: 0, π/2, π, and 3π/2 radians. Two data bits are transformed into one complex data symbol; A symbol is any change (keying) of the carrier.