Clipping and Filtering
Understanding Clipping and Filtering
OFDM signals have high PAPR because multiple subcarriers can constructively align, creating amplitude peaks 8 to 12 dB above the average. The PA must be backed off to accommodate these peaks, wasting DC power. Clipping and filtering attacks this directly: simply remove the peaks and clean up the spectral mess.
The clipping operation is: x̂(n) = x(n) × min(1, Aclip/|x(n)|). This preserves the signal phase while limiting the amplitude to Aclip. The clipping ratio CR = Aclip/σrms controls the aggressiveness. CR = 1.4 (3 dB above rms) removes most damaging peaks while keeping EVM under 3%. After clipping, a digital bandpass filter (typically 256-tap FIR) removes the out-of-band spectral regrowth to meet emission mask requirements.
x̂(n) = x(n) × min(1, Aclip/|x(n)|)
Clipping Ratio:
CR = Aclip / σrms (typically 1.2 to 2.0)
CR in dB = 20 log10(CR)
EVM from clipping (approximate):
EVM ≈ √(2/π) × e−CR² / CR (for Gaussian OFDM)
CR=1.0: EVM ≈ 12%. CR=1.4: EVM ≈ 3%. CR=2.0: EVM ≈ 0.5%.
CFR Technique Comparison
| Technique | PAPR Reduction | EVM Impact | Complexity | Standard Use |
|---|---|---|---|---|
| Clipping + Filtering | 3-6 dB | 0.5-3% EVM | Low (FIR filter) | All base stations |
| Peak Cancellation (PC-CFR) | 3-5 dB | <1% EVM | Medium | LTE/5G gNB |
| ACE | 1.5-3 dB | None (improved) | Medium | DVB-T2 |
| Tone Reservation | 2-4 dB | None | Medium | DVB-T2, 802.11ax |
Frequently Asked Questions
How does clipping and filtering reduce PAPR?
Limit amplitude to Aclip while preserving phase. This removes peaks but creates spectral regrowth (out-of-band) and in-band noise (EVM). Bandpass filtering removes out-of-band regrowth. In-band distortion remains as the EVM cost. Lower clipping ratio = more reduction but more EVM.
What is the EVM impact?
CR=1.0: ~12% EVM (too high for 64-QAM). CR=1.4: ~3% EVM (OK for 64-QAM). CR=2.0: ~0.5% (OK for 256-QAM). Clip just enough to drop PAPR 3-4 dB while staying within EVM budget for the highest MCS in use.
Why use iterative clipping?
Filtering can regenerate peaks via constructive interference. Iterating (clip-filter-clip-filter, 2-5 times) reduces residual peaks without significantly increasing EVM. Achieves 1-2 dB more reduction than single-pass. Most commercial CFR uses 2-3 iterations.