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Modulations

To transmit the signal, we modulate it using various modulation techniques.

If we want to reduce the frequency bandwidth occupied by the signal, we use raised cosine or Gaussian pulses instead of square pulses.

We measure the efficiency (which we aim to maximize) of the modulation using the following formula:

p=RbBp = \frac{R_b}{B}

Caption:

  • p: efficiency
  • R_b: source bitrate
  • B: bandwidth

ASK

Amplitude-Shift Keying.

Modulation is done by varying the amplitude.

Operation
ASK Example

FSK

Frequency-Shift Keying (also called FSK - Frequency Shift Keying in French too).

Modulation is done by varying the frequency.

Operation
FSK

MSK

MSK (Minimum Shift Keying), used in GSM, is a variant of FSK where the signal is coherently demodulated.

GMSK

GMSK (Gaussian Minimum Shift Keying) is a variant of MSK that uses a Gaussian filter to reduce bandwidth.

PSK

Phase-Shift Keying.

Modulation is done by varying the phase.

There are several types of PSK modulations based on how many phases are used:

BPSK

Binary Phase-Shift Keying.
A PSK modulation with only two phases.

OperationConstellation Diagram
BPSKBPSK constellation

DPSK

DPSK is a phase modulation that uses the previous symbol’s phase to encode the current symbol.

QPSK

Quadrature Phase-Shift Keying (also called 4-PSK).

A PSK modulation with four different phases.

OperationConstellation Diagram
QPSKQPSK constellation

QAM

Quadrature Amplitude Modulation (in French: MAQ - modulation d’amplitude en quadrature).

QAM combines amplitude modulation and phase modulation to transmit more bits per symbol. It is widely used in modern communication systems like Wi-Fi, 4G/5G, and digital TV.