Modulation encodes information onto a carrier wave by varying its amplitude, frequency, or phase. The choice of modulation scheme involves tradeoffs between spectral efficiency (bits/Hz), power efficiency (Eb/N0 required), and robustness to noise and interference.

Amplitude Modulation (AM)

The carrier amplitude varies with the message signal. Simple to demodulate but spectrally inefficient and susceptible to amplitude noise. Double-sideband suppressed carrier (DSB-SC) removes the carrier for power efficiency; single-sideband (SSB) halves the bandwidth.

Frequency Modulation (FM)

The carrier frequency varies with the message. Provides noise immunity above the FM threshold but requires wider bandwidth than AM. Used in broadcast radio (±75 kHz deviation) and VHF voice communications.

Phase-Shift Keying (PSK)

Digital data is encoded as discrete phase shifts. BPSK (2 phases, 1 bit/symbol) is robust; QPSK (4 phases, 2 bits/symbol) doubles spectral efficiency. Higher orders (8-PSK, 16-PSK) pack more bits per symbol but require higher SNR.

Quadrature Amplitude Modulation (QAM)

Combines amplitude and phase modulation. 16-QAM carries 4 bits/symbol; 64-QAM carries 6 bits/symbol; 256-QAM carries 8 bits/symbol. Used extensively in LTE, 5G, WiFi (802.11ax uses 1024-QAM), and cable TV. Higher QAM orders require much higher SNR and are sensitive to phase noise and nonlinearity.

OFDM

Orthogonal Frequency Division Multiplexing splits the channel into many narrow subcarriers, each modulated with QAM. The cyclic prefix guards against multipath inter-symbol interference. OFDM is the basis of WiFi (802.11a/g/n/ac/ax), LTE, 5G NR, and digital broadcast (DVB-T, DAB).