What is the power spectral density mask for a given wireless standard and how does it constrain PA design?
PSD Mask and PA Design Constraints
The PSD mask is the primary regulatory and standards-based constraint on PA design. Every wireless standard (3GPP, IEEE 802.11, DVB) specifies a spectral mask that the transmitter's output spectrum must not exceed.
| Parameter | Free Space | Urban | Indoor |
|---|---|---|---|
| Path Loss Model | Friis (1/r²) | Okumura-Hata | IEEE 802.11 |
| Fading Margin | 0 dB | 10-30 dB | 5-15 dB |
| Multipath | None | Severe | Moderate-severe |
| Typical Range | Line of sight | 1-30 km | 10-100 m |
| Shadow Fading (σ) | 0 dB | 6-12 dB | 3-8 dB |
- 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
Frequently Asked Questions
What is the relationship between ACLR and OIP3?
For a memoryless nonlinear PA: ACLR ≈ 2 × (OIP3 - P_out) + 20log(BW_adj/BW_signal) [dB]. This shows that ACLR improves by 2 dB for every 1 dB increase in OIP3 relative to output power (i.e., more back-off = better ACLR). For a modulated signal with memory effects: this simple relationship breaks down, and the ACLR must be measured or simulated with the actual signal. In general: OIP3 provides a first-order estimate of ACLR, but the actual ACLR depends on the signal's PAPR, bandwidth, and the PA's memory effects.
How does DPD help meet the spectral mask?
DPD pre-distorts the digital signal to cancel the PA's AM-AM and AM-PM distortion. The result: the PA's output spectrum closely matches the ideal (undistorted) spectrum. ACLR improvement from DPD: typically 15-25 dB (reducing the spectral regrowth from -25 to -30 dBc to -45 to -55 dBc). This allows the PA to operate at 2-3 dB less back-off compared to no DPD, increasing efficiency from approximately 25% to approximately 40-50% for a typical Doherty PA. The DPD model must be adapted to the PA's actual nonlinearity (using a feedback observation receiver) and updated periodically to track temperature and aging effects.
What about emission limits in other bands?
In addition to the ACLR requirements for adjacent channels: the spurious emission mask specifies maximum emission levels in specific frequency ranges to protect other services. For example: a 3.5 GHz 5G BS must not exceed specific emission levels in the navigation satellite bands (1164-1300 MHz), the GPS band (1559-1610 MHz), or other protected bands. These requirements may necessitate: additional output filtering (cavity filters or duplexers with steep roll-off), PA harmonic suppression (the 2nd harmonic of 3.5 GHz falls at 7 GHz; the 3rd harmonic at 10.5 GHz), and receive-band filtering (in FDD systems: the TX filter must suppress emissions in the paired receive band by > 50 dB).