How do I calculate the peak to average power ratio of a multi-carrier OFDM signal?
OFDM PAPR Calculation
PAPR is the fundamental challenge of OFDM and multi-carrier systems. It forces the PA to operate at a large backoff from its maximum power point, wasting most of the PA's power capability as heat.
| Parameter | Class A | Class AB | Class F/Doherty |
|---|---|---|---|
| Max Efficiency | 50% | 50-78% | 70-90% |
| Linearity | Excellent | Good | Moderate (needs DPD) |
| P1dB Backoff | 0-3 dB | 3-6 dB | 6-10 dB |
| Complexity | Low | Low | High |
| Common Use | Test, small signal | General PA | Base station, broadcast |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Frequently Asked Questions
What PAPR do real standards specify?
WiFi 802.11a/g/n/ac (64-256 subcarriers): PAPR approximately 10-12 dB (at 10^-4 CCDF). LTE (300-1200 subcarriers): PAPR approximately 8-11 dB (with SC-FDMA uplink: approximately 6-8 dB, lower PAPR than OFDM). 5G NR (OFDM with CP-OFDM): PAPR approximately 10-13 dB (depending on bandwidth and modulation). DVB-T (1705 or 6817 subcarriers): PAPR approximately 12-15 dB.
Why does PAPR matter for battery life?
In mobile devices: the PA typically consumes 50-80% of the total transmit power. PAPR forces the PA to operate at backoff, reducing efficiency. For a Class-AB PA at 7 dB backoff: efficiency approximately 10-15% (vs. 40-50% at Psat). This means: the battery energy wasted as heat in the PA is 5-10× the useful RF output power. Reducing PAPR by 3 dB (using CFR or SC-FDMA) improves the PA efficiency by approximately 2× and extends the battery life proportionally. This is why LTE uses SC-FDMA (not OFDM) for the uplink.
What is EVM and how does it relate to PAPR?
EVM (Error Vector Magnitude) measures the modulation quality. When the PA clips the OFDM peaks: the modulation is distorted, increasing the EVM. The EVM must be below the standard's requirement: 802.11ac 256-QAM: EVM < -32 dB (2.5%). LTE 64-QAM: EVM < -8% (-22 dB). If the PAPR is not managed (insufficient backoff or no CFR): the EVM exceeds the requirement and the transmitted data has excessive bit errors. The PA backoff is set to maintain the required EVM under worst-case PAPR conditions.