Demodulation

Demodulation is the process of extracting the original information-bearing signal from a modulated carrier. It is the inverse operation of modulation and is performed at the receiver to recover the baseband or message signal. Demodulation must contend with channel impairments, noise, and distortion, and its design depends on the modulation scheme used to encode the information.

In amplitude modulation (AM), the information modulates the carrier amplitude: s(t) = [A + m(t)] cos(ωc t). Demodulation converts the received waveform back to m(t) using an envelope detector, a product detector, or coherent (synchronous) demodulation with a locally generated carrier. In frequency modulation (FM), s(t) = Ac cos(ωc t + β sin(ωm t)); demodulation recovers the instantaneous frequency via a discriminator, a phase-locked loop, or quadrature demodulation. Phase modulation (PM) encodes information in the instantaneous phase; demodulation typically employs a phase detector or PLL to recover m(t).

In digital communications, demodulation recovers symbolic or bit streams from modulated carriers. Coherent demodulation uses a locally synchronized carrier and matched filters to maximize information rate, followed by symbol decisions. Noncoherent methods rely on signal envelope or differential encoding. I/Q demodulation extracts in-phase and quadrature components for formats such as QPSK and QAM, enabling digital symbol mapping and error correction.

Key practical considerations include carrier recovery, timing and phase synchronization, filtering, and channel distortion. The chosen demodulation method affects sensitivity to noise, bandwidth efficiency, and implementation complexity. Advances in digital signal processing have expanded the range of practical demodulators, enabling robust operation in wireless, optical, and multimedia systems.