What is the difference between coherent and non-coherent demodulation?

Non-coherent demodulation (envelope detection) does not need a phase reference to the carrier. A simple diode rectifier followed by a lowpass filter extracts the amplitude of an AM signal. It is simple but only works for amplitude-modulated signals and cannot recover phase information. Coherent demodulation multiplies the received signal with a locally generated replica of the carrier, requiring accurate knowledge of the carrier frequency and phase. This is essential for PSK and QAM, where information is encoded in the carrier phase. The local carrier is typically recovered from the received signal using a Costas loop or a pilot-based phase-locked loop. Coherent demodulation achieves 3 dB better sensitivity than non-coherent for the same modulation, but requires more complex circuitry.

How does I/Q demodulation work?

I/Q demodulation splits the received signal into two orthogonal components by mixing it with two copies of the local oscillator that are 90 degrees apart. The in-phase (I) mixer output represents the real component of the baseband signal; the quadrature (Q) mixer output represents the imaginary component. Together, they fully describe both the amplitude and phase of the received signal at every instant. After lowpass filtering and digitization by two ADCs, the digital I and Q streams are processed to recover symbols: for QPSK, each I/Q sample maps to one of four constellation points (2 bits). For 256QAM, each sample maps to one of 256 points (8 bits). The accuracy of the 90-degree phase split is critical; any I/Q imbalance causes image leakage and EVM degradation.

What impairments degrade demodulator performance?

Frequency offset between the transmitter and receiver oscillators causes the constellation to rotate continuously, requiring a frequency offset correction loop. Phase noise from the LO causes random constellation rotation (tangential smearing), contributing directly to EVM. I/Q gain imbalance makes the constellation elliptical instead of circular. I/Q phase imbalance (deviation from 90 degrees) skews the constellation. DC offset in the I or Q path shifts the entire constellation off-center. Timing errors cause inter-symbol interference as adjacent symbols bleed into each other. Modern digital demodulators correct all of these impairments in DSP using pilot symbols and training sequences embedded in the transmitted signal.

Signal Processing

Demodulation

A 5G base station receives a 3.5 GHz signal carrying a 256QAM OFDM waveform with 100 MHz bandwidth. The receiver must determine which of 256 possible constellation points the transmitter intended for each of 3,276 subcarriers, every 35.7 microseconds, while the signal is buried in noise, distorted by multipath, shifted in frequency by Doppler, and corrupted by the PA's nonlinearity. Demodulation is the process that accomplishes this: it mixes the signal down to baseband using an I/Q architecture, digitizes both quadrature components, corrects for every impairment (frequency offset, phase noise, timing error, I/Q imbalance), and maps each corrected sample to the nearest constellation point to recover the transmitted bits.

Category: Signal Processing

Key Architecture: I/Q (quadrature)

Quality Metric: EVM, BER

From RF to Bits

Demodulation Methods by Signal Type

Method	Carrier Reference	Complexity	Sensitivity	Applicable To
Envelope detection	Not needed	Very low (diode + LPF)	Poor (−3 dB vs coherent)	AM, ASK, OOK
Discriminator (FM)	Not needed	Low	Moderate	FM, FSK, GFSK
Differential (DPSK)	Previous symbol	Moderate	~1 dB loss vs coherent	DQPSK, Bluetooth
Coherent I/Q	Recovered (Costas/PLL)	High	Best (theoretical limit)	PSK, QAM, OFDM
Direct sampling (SDR)	Digital NCO	Highest (all in DSP)	Best (with correction)	Any modulation

I/Q demodulation:
I(t) = RF(t) × cos(2πf_LOt) → lowpass filter
Q(t) = RF(t) × sin(2πf_LOt) → lowpass filter

Recovered symbol:
Amplitude = √(I² + Q²)
Phase = arctan(Q/I)

Key impairments and budgets:
I/Q gain imbalance: <0.2 dB for 256QAM (<1% EVM)
I/Q phase error: <1° for 256QAM
Frequency offset: corrected by AFC/pilot tracking

Common Questions

Frequently Asked Questions

Coherent vs. non-coherent?

Non-coherent (envelope detection): no carrier phase needed, simple, 3 dB worse sensitivity, only for AM/ASK. Coherent (I/Q): needs recovered carrier phase (Costas loop or pilot PLL), works for PSK/QAM/OFDM, achieves theoretical sensitivity limit.

How does I/Q work?

Two mixers with LO signals 90° apart produce I (real) and Q (imaginary) baseband components. Together they fully describe amplitude and phase. After ADC: digital DSP maps each I/Q sample to constellation points. 90° accuracy is critical; I/Q imbalance degrades EVM.

What degrades demodulator performance?

Frequency offset (constellation rotation), phase noise (tangential smear), I/Q imbalance (elliptical constellation), DC offset (shifted center), timing error (ISI). Modern DSP corrects all using embedded pilot symbols and training sequences.

Related Terms

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Receiver Analysis

I/Q Impairment Simulator

Enter I/Q gain imbalance, phase error, DC offset, and LO phase noise. Visualize the effect on constellation quality and predict the resulting EVM and BER degradation.

Simulate Impairments