Measurement and Compensation of Tx/Rx Imperfections
| Imperfection |
Measurement Method |
Compensation Technique |
| Power amplifier non-linearity |
Spectrum analyzer (ACLR), EVM |
Digital predistortion, PA back-off |
| Phase noise |
Phase noise analyzer |
High-quality oscillator, PLL filtering |
| IQ imbalance (Tx) |
VSA, image rejection |
IQ correction algorithms |
| Frequency offset |
Pilot-based frequency error estimation |
CFO estimation and correction |
| Quantization noise |
SNR estimation, simulation |
Higher bit-depth, dithering |
| Clock jitter |
Jitter analyzer, oscilloscope TIE |
Low-jitter clocks, PLL cleanup |
| Signal clipping |
Digital scope, EVM analysis |
AGC, PAPR reduction |
| Digital predistortion errors |
Compare PA input/output models |
Adaptive DPD feedback |
| LNA non-linearity |
Two-tone test, IP3 measurement |
High-linearity LNA, operate linearly |
| Mixer leakage |
Spectrum analyzer (LO spur detection) |
Improve LO isolation and layout |
| IQ imbalance (Rx) |
Image rejection, VSA |
Digital baseband correction |
| Thermal noise |
Noise figure analyzer |
Low-noise components, shielding |
| ADC quantization |
SNR, ENOB measurement |
Higher-resolution ADCs, noise shaping |
| Timing recovery errors |
Eye diagram, timing error analysis |
Timing recovery loops (e.g., Gardner) |
| Channel estimation inaccuracies |
Pilot sequence error analysis |
Adaptive estimation and interpolation |
| Equalization artifacts |
EVM/BER analysis |
Robust equalizers (MMSE, DFE) |
Communication System Measurement Parameters and Acceptable Limits
| Parameter |
Definition |
Typical Max Acceptable Value |
| EVM (Error Vector Magnitude) |
Measures modulation accuracy; deviation of received symbols from ideal |
1.75% to 8% (depends on modulation & standard) |
| BER (Bit Error Rate) |
Fraction of bits received incorrectly |
1e-5 to 1e-3 (depends on FEC & system) |
| SNR (Signal-to-Noise Ratio) |
Ratio of signal power to noise power |
≥ 20 dB (higher for higher modulations) |
| SINR |
SNR including interference from other signals |
≥ 18 dB (LTE min requirement) |
| ACLR (Adjacent Channel Leakage Ratio) |
Signal leakage into adjacent frequency bands |
≤ -30 dBc (can vary with channel spacing) |
| SEM (Spectrum Emission Mask) |
Limits power emitted outside intended frequency band |
Must meet standard-specific spectral mask |
| TX Power / Output Power |
Power level of transmitted signal |
+23 to +30 dBm (varies by system) |
| OBW (Occupied Bandwidth) |
Bandwidth containing 99% of transmitted signal power |
99% of power in allocated channel |
| Frequency Error |
Offset between actual and expected carrier frequency |
±0.1 to ±0.5 ppm (standard-dependent) |
| Phase Noise |
Frequency stability of local oscillator |
< -100 dBc/Hz @ 1 MHz offset (typical) |
Error Vector Magnitude (EVM)
EVM quantifies the deviation of received symbols from ideal constellation points, reflecting overall modulation accuracy.
| Form |
Equation |
| Normalized |
( \text{EVM}_{\text{RMS}} = \sqrt{ \frac{ \sum |
| Percentage |
\( \text{EVM}(\%) = \text{EVM}_{\text{RMS}} \times 100 \) |
| dB |
\( \text{EVM}_{\text{dB}} = 20 \cdot \log_{10}(\text{EVM}_{\text{RMS}}) \) |
Where:
- \( S_i \): Ideal symbol
- \( R_i \): Received symbol
- \( N \): Number of symbols
Visual Concept
In I/Q constellation:
- Ideal point: expected symbol position
- Measured point: actual received position
- Error vector: \( \text{EVM}_i = |S_i - R_i| \)
Interpretation
| EVM Value |
Meaning |
| Low (e.g. <3%) |
High signal quality, minimal distortion |
| High |
Indicates noise, non-linearities, or RF impairments |
Why EVM Matters in Standards
1. Standard Compliance
| Standard |
Modulation |
EVM Limit |
| LTE |
64-QAM |
≤ 8% |
| 5G NR |
256-QAM |
≤ 3.5% |
| Wi-Fi 6 |
1024-QAM |
≤ 1.75% |
| Bluetooth |
GFSK |
≤ 20% |
- Devices must meet EVM limits to ensure certification and interoperability.
2. Captures Key Impairments
EVM reflects combined effects of:
- IQ imbalance
- Phase noise
- PA non-linearity
- Frequency offset
- ADC/DAC resolution
- Noise floor
3. Correlates with Other Metrics
| Metric |
Relationship with EVM |
| BER |
↑ EVM → ↑ Bit Error Rate |
| SNR |
↑ EVM → ↓ Signal-to-Noise Ratio |
4. Used in Testing and Manufacturing
- Fast diagnostic tool for verifying signal integrity
- Supports calibration and real-time tuning in production lines
EVM Measurement
| Method |
Description |
Tools/Environment |
Notes |
| Vector Signal Analyzer |
Measures EVM by demodulating the signal and comparing to ideal constellation |
Keysight, R\&S, NI VSA |
Industry standard; fast and accurate |
| Software/DSP Tools |
Use MATLAB, Python, or other DSP tools to calculate EVM from captured IQ data |
MATLAB, Python (NumPy), Simulink |
Requires access to ideal symbol mapping and synchronized samples |
| Built-in in SDR Tools |
SDR tools (e.g., GNU Radio, LabVIEW) may have EVM blocks or add-ons |
SDR test platforms |
Real-time or post-processing support |
| Simulation-Based |
Direct computation using known ideal and simulated received symbols |
MATLAB, SystemVue, Simulink |
Ideal for design/testbench validation |
Summary
EVM is a comprehensive, fast, and standard-compliant metric to evaluate modulation accuracy and system integrity in modern digital communication systems.
Measuring Signal-to-Noise Ratio (SNR)
| Method |
Description |
Tools Used |
Notes |
| Spectrum Analyzer |
Measure peak signal vs. noise floor near carrier |
Spectrum Analyzer |
Set correct resolution bandwidth (RBW); used in RF systems |
| Digital Sample Analysis |
Compute power of signal and noise using ADC samples |
ADC, DSP software, MATLAB/Python |
Must isolate signal and noise samples; accurate in simulations |
| VSA / Oscilloscope |
Use built-in SNR function based on demodulated or raw waveform |
Vector Signal Analyzer, Oscilloscope |
Fast and standardized; supports modulated signals |
| EVM-Based Estimation |
Approximate SNR from EVM when noise is dominant |
VSA or EVM measurement |
Valid if distortion is negligible: SNR ≈ -20·log10(EVM_RMS) |
| Baseband Simulation |
Extract signal and noise power from known model or testbench |
MATLAB, Simulink, etc. |
Most controlled; ideal for design phase |
_Last updated: June 06, 2025