Adjacent Channel Leakage
Understanding ACLR
ACLR is the regulatory and practical measure of how well a transmitter confines its signal to its assigned channel. High ACLR (good performance) means very little power leaks into neighboring channels, minimizing interference to other users of the spectrum.
| Amplifier Type | Noise Figure | Gain | Output Power |
|---|---|---|---|
| LNA | 0.3-2 dB | 10-25 dB | -10 to +10 dBm |
| Driver | 3-8 dB | 10-20 dB | +15 to +25 dBm |
| Power Amp | N/A | 8-15 dB | +30 to +50 dBm |
| Distributed | 3-6 dB | 5-12 dB | +10 to +20 dBm |
ACLR Sources
- PA nonlinearity: 3rd and 5th-order intermodulation products spread into adjacent channels. This is the dominant source.
- Phase noise: Reciprocal mixing of phase noise sidebands into adjacent channels.
- Modulation quality: High EVM contributes to out-of-band emissions.
ACLR Requirements
- 5G NR base station: ACLR > 45 dBc.
- LTE base station: ACLR > 45 dBc at first adjacent, > 50 dBc at alternate.
- 5G NR handset: ACLR > 30 dBc.
Improving ACLR
- Reduce PA operating point (more backoff). Trade-off: lower efficiency.
- Apply DPD (Digital Predistortion). 15-25 dB ACLR improvement.
- Use higher-linearity PA technology (GaN Doherty).
Frequently Asked Questions
What is ACLR?
ACLR measures power leakage from a transmitter's channel into adjacent channels, mostly from PA nonlinearity. 5G NR requires > 45 dBc for base stations. Higher ACLR = less interference to neighboring users.
How does DPD improve ACLR?
DPD pre-distorts the baseband signal to cancel the PA's nonlinear distortion. It reduces adjacent channel emissions by 15-25 dB, enabling the PA to operate closer to compression (higher efficiency) while meeting ACLR requirements.
What is the relationship between ACLR and IP3?
ACLR is dominated by 3rd-order IM products near the channel edge. Higher IP3 means lower 3rd-order products and better ACLR. However, ACLR also depends on signal statistics (PAPR), modulation bandwidth, and memory effects, making it a more complete metric than IP3 alone.