| AWGN / Thermal Noise | Random noise added to the signal when signal power is insufficient compared to the noise floor. | Points spread randomly around ideal positions; larger circular clouds around each symbol. | Receiver noise floor, low signal power, poor LNA noise figure, or weak link budget. | Noise | SNR, EVM, BER, C/N |
| Phase Noise | Random phase variation caused mainly by oscillator instability. | Points smear in an angular or rotational direction, often forming arcs. | LO instability, PLL phase noise, or poor clock source. | Synchronization | EVM, phase-noise mask |
| Carrier Frequency Offset | TX and RX carrier frequencies are not perfectly aligned, or shift due to motion. | Entire constellation rotates over time; in OFDM, this leads to ICI. | Oscillator mismatch, Doppler shift, or poor frequency synchronization. | Synchronization | CFO, EVM, ICI |
| Timing Error | Receiver samples the symbol at the wrong instant or suffers from sampling clock mismatch. | Points spread or smear; eye diagram partially closes; constellation degrades over time. | Poor clock recovery, sampling clock offset, jitter, or PLL instability. | Synchronization | Timing error, eye opening, EVM, SCO, jitter |
| Residual Phase Error | Phase correction is incomplete after synchronization. | Constellation is rotated by a fixed angle or slowly drifting. | Poor carrier recovery or pilot tracking error. | Synchronization | EVM, phase error |
| IQ Imbalance | I and Q paths do not have equal gain or exact 90° phase separation. | Constellation becomes skewed, stretched, or elliptical. | Analog front-end mismatch, mixer imbalance, or calibration error. | Analog / RF | Image rejection, EVM |
| DC Offset / LO Leakage | Unwanted constant offset or LO energy appears in the signal path. | Constellation shifts away from the origin; energy appears near DC. | LO leakage, ADC offset, or direct-conversion receiver issues. | Analog / RF | DC offset, EVM |
| Gain Compression / AM/AM Distortion | Amplifier or converter response becomes nonlinear at high power levels. | Outer constellation points are pulled inward, compressed, or unevenly spaced. | PA driven near saturation, insufficient output back-off, or DAC/ADC saturation. | Analog / RF | EVM, ACLR, AM/AM, compression curve |
| AM/PM Distortion | Signal amplitude changes also cause unwanted phase shifts. | Outer points rotate depending on their amplitude. | PA nonlinear phase response. | Analog / RF | EVM, AM/PM |
| Clipping | Signal peaks are cut off due to hardware limits. | Outer points become flattened or scattered. | DAC, ADC, or PA clipping; high OFDM PAPR. | Analog / RF | EVM, crest factor, ACLR |
| AGC Error | Receiver gain is set incorrectly for the incoming signal level. | Constellation appears too large, too small, clipped, or noisy. | Poor gain control, saturation, or weak scaling. | Analog / RF | RSSI, EVM, ADC clipping |
| Nonlinear Memory Effects | Power amplifier distortion depends on previous signal history. | Constellation errors depend on the specific symbol sequence. | Wideband PA memory, thermal effects, or electrical effects. | Analog / RF | EVM, ACPR, ACLR |
| Image Interference | Mirror-frequency signal leaks into the desired signal band. | Distorted or skewed constellation. | IQ imbalance or poor image rejection. | Analog / RF | IRR, EVM |
| ISI — Inter-Symbol Interference | Adjacent symbols interfere with the current symbol due to channel or filtering limits. | Points smear in structured directions, often forming trails. | Insufficient filtering, multipath, timing error, or bandwidth limits. | Channel | Eye diagram, EVM, BER |
| Multipath / Fading | Signal arrives through multiple delayed paths, potentially causing frequency-selective distortion. | Distortion, spreading, or rotation; in OFDM, specific subcarriers may have worse EVM. | Reflections, indoor propagation, or channel nulls. | Channel | Channel response, EVM, BER |
| Flat Fading | Whole signal bandwidth is attenuated similarly. | Constellation shrinks toward the origin, possibly with noise spread. | Path loss, shadowing, or antenna orientation. | Channel | RSSI, SNR, EVM |
| Group Delay Distortion | Different frequency components arrive with different delays. | Symbol smearing and waveform distortion. | Analog filters, channel dispersion, or poor equalization. | Channel | EVM, group delay |
| Adjacent Channel Interference | Energy from nearby channels leaks into the desired channel. | Random or directional spreading, often asymmetric. | Poor filtering, nearby transmitter, or insufficient guard band. | Channel | ACLR, ACS, EVM |
| Co-channel Interference | Another signal overlaps the same frequency band. | Constellation becomes noisy or structured depending on the interferer. | Frequency reuse, external transmitter, or poor isolation. | Channel | SINR, EVM, BER |
| Doppler Spread | Channel changes over time due to motion. | Time-varying spreading and subcarrier degradation. | Fast fading, mobility, or multipath. | Channel | Coherence time, EVM |
| Quantization Noise | Limited ADC/DAC resolution adds error. | Fine random spreading around constellation points. | Low bit-depth ADC/DAC, poor scaling, or fixed-point truncation. | Noise | SQNR, EVM |
| Poor Equalization / Channel Estimation | Receiver fails to correctly estimate or undo channel distortion. | Points remain scattered, rotated, or smeared; in OFDM, correction is wrong per subcarrier. | Bad channel estimate, insufficient equalizer length, or pilot contamination. | DSP / Implementation | EVM, BER, channel-estimate error |
| Modulator Mapping / I/Q Swap | Logic errors occur in bit mapping or signal routing. | Symbols appear in wrong quadrants, wrong positions, mirrored, or swapped. | Wrong Gray coding, bit-order mismatch, or wiring/DSP sign bugs. | DSP / Implementation | BER, constellation sanity check |
| Filter / Rolloff Mismatch | TX and RX filters or symbol rates are not matched correctly. | ISI-like spreading, degraded eye diagram, or wrong occupied bandwidth. | Wrong RRC filter/rolloff, coefficient error, or sample-rate mismatch. | DSP / Implementation | EVM, eye diagram, occupied bandwidth |
| Fixed-Point Error / Scaling | DSP implementation loses precision or uses incorrect scaling. | Small systematic errors, or constellation has correct shape but wrong size. | Word-length limits, rounding, saturation, or gain mismatch. | DSP / Implementation | EVM, simulation mismatch, gain error |
| Common Phase Error — OFDM | All OFDM subcarriers share a common phase rotation. | Whole OFDM constellation rotates together. | Phase noise or residual CFO. | OFDM-specific | CPE, EVM |
| High PAPR — OFDM | OFDM waveform has large peaks compared with its average power. | Indirectly causes clipping and PA compression. | Many subcarriers adding constructively. | OFDM-specific | PAPR, crest factor, EVM |