Adjacent Channel Power
Understanding Adjacent Channel Power (ACP/ACPR)
When you look at the spectrum of a perfect 5G or Wi-Fi signal, it should look like a sharp, rectangular block. All the RF power stays strictly inside the assigned 20 MHz frequency channel. However, power amplifiers are physically incapable of being perfectly linear. As the amplifier is driven near its maximum power output (compression), the severe non-linear distortion creates a phenomenon known as "Spectral Regrowth." The sharp edges of the signal begin to bleed outward, spilling RF noise into the neighboring frequency channels. Adjacent Channel Power Ratio (ACPR) is the measurement of this destructive leakage.
ACPR is heavily regulated by law. If a Verizon cell tower amplifier is driven too hard, its spectral regrowth will spill into AT&T's adjacent frequency block. The AT&T cell tower will experience this leakage as a massive wall of white noise, completely jamming their customers. The FCC enforces strict "Emission Masks" that dictate exactly how low the ACPR must be (often demanding the adjacent channel be -45 dB to -60 dB weaker than the main channel).
The Trade-off with Efficiency
ACPR is the ultimate nemesis of amplifier efficiency. To make the ACPR drop (cleaning up the signal), the engineer must "back-off" the transmit power, running the amplifier in its highly linear, low-power region. However, running a Class-AB amplifier backed-off destroys its power efficiency, turning most of the electricity into heat. To achieve both high efficiency and excellent ACPR, modern systems rely on heavy digital intervention, using Digital Predistortion (DPD) to mathematically correct the spectral regrowth before it happens.
ACPR (dBc) = 10 × log10 ( Padjacent_channel / Pmain_channel )
Because the non-linear distortion (Intermodulation) grows exactly 3 times faster than the main signal power, every 1 dB increase in your main transmit power will cause a massive 3 dB degradation in your ACPR.
Comparison
| Signal / Standard | Modulation Type | Typical ACPR Requirement | Linearity Demand |
|---|---|---|---|
| Old FM Radio | Constant Envelope | Not applicable (No regrowth) | Zero (Can use nonlinear Class-C) |
| WCDMA (3G) | QPSK | -45 dBc | Moderate |
| LTE / 5G NR | 256-QAM / OFDM | -50 dBc to -60 dBc | Extreme (Requires DPD) |
Frequently Asked Questions
How is ACPR measured on a Spectrum Analyzer?
You cannot simply look at the peak height of the signal. The analyzer must mathematically integrate the power across the entire bandwidth. It calculates the total Area Under the Curve for the main 20 MHz channel, and then calculates the Area Under the Curve for the adjacent 20 MHz channel. The ratio of those two total power integrals is the ACPR.
What is the difference between ACPR and ACLR?
They are physically the exact same measurement. ACPR (Adjacent Channel Power Ratio) is the traditional term used in general RF engineering and older standards. ACLR (Adjacent Channel Leakage Ratio) is the specific terminology adopted by the 3GPP governing body for cellular standards like WCDMA, LTE, and 5G. If you are building a cell tower, you call it ACLR.
Can a bandpass filter fix bad ACPR?
Usually, no. Spectral regrowth happens immediately adjacent to the main signal, literally touching the edge of your assigned frequency block. To physically filter out the adjacent channel without harming your main channel, you would need a bandpass filter with a 'brick-wall' roll-off. Mechanical cavity filters that sharp are incredibly massive, heavy, and introduce severe group-delay distortion. It is vastly cheaper and better to fix the ACPR at the amplifier level using Digital Predistortion (DPD).