The quest for 5G excellence isn’t just about speed; it’s about squeezing every watt from your network, keeping PAs linear, and slashing energy waste. Peak-to-average power ratio (PAPR) is the invisible mountain every signal must climb. But what if you could reshape that mountain? In Part 2 of our series, we shine a spotlight on the dynamic duo of crest factor reduction (CFR) and digital predistortion (DPD), which are making high-efficiency, high-performance base stations a reality.
The Two Pillars of Power: CFR and DPD
The traditional approach to avoiding PA distortion is power back-off, which is simply turning down the signal to stay well within the linear zone. But this strategy inevitably wastes power and limits coverage; a non-starter in the age of gigaspeed data and climate-conscious design.
Modern 5G systems demand a bolder play: run the PA hotter, but safely control peaks and linearization with precision.
Figure 1: The AM-AM Curve with the 1 dB Compression Point
The ideal response (straight line) diverges as the PA approaches saturation; the 1 dB compression point marks where linearity ends and distortion begins. The key is to eke out as much output as possible without crossing the red line.
Enter CFR and DPD, working in tandem to rewrite what’s possible:
- CFR squashes those rogue peaks, reducing the PAPR so amplifiers stay within their safe zone (and do so more often).
- DPD mathematically pre-corrects input waveforms so they come out linear; even when the PA it’s driving is straining near its limits.
CFR: Taming the Spikes
Most OFDM signals, even with their wild envelope, spend much of their time well within acceptable PA input levels. The issue? Occasional super-peaks that threaten system reliability if left unchecked.
With CFR, only those outlying spikes are brought into line, without turning down the whole signal. Peak reduction is targeted, preserving power and system fidelity.
Figure 2: Detected Signal Peaks Above the Threshold Level Are Reduced
Nonlinear peaks are clipped selectively above a set threshold, and the result is a smoother, safer waveform feeding the PA.
There’s a trade-off: overzealous clipping can introduce in-band distortion (hurting BER) and bleed noise into adjacent channels (hurting error vector magnitude (EVM) and adjacent channel leakage ratio (ACLR)). The ultimate CFR solution must strike a balance: enough suppression for safety, but not so much as to compromise performance.
Think of it like high-speed highway driving: you want to keep the speed up, but avoid nasty potholes. CFR fills in the biggest potholes so you stay safely in your lane.
DPD: The Art of Anticipation
Where CFR shapes the peaks, DPD counteracts the PA’s inherent distortion.
DPD works by creating an inverse of the PA’s AM-AM and AM-PM nonlinearities. It essentially pre-distorts the digital baseband signal, so that, after the PA’s own nonlinear response, the output emerges (almost magically) as a clean copy of the intended signal.
Figure 3: Generic Concept of DPD for Linearizing the PA Response
The predistorter (DPD) applies gain expansion in exact opposition to the PA’s compression. The result: the system output remains linear, even when the hardware itself isn’t.
There are two main flavors:
- Memoryless DPD: Works well for narrowband signals and PAs with fast, instantaneous behavior. Lookup tables (LUTs) translate each input value to its perfectly corrected output.
- DPD with Memory: Necessary for wideband operation, where thermal and circuit effects mean the PA’s present output depends on previous inputs. Here, sophisticated algorithms (including Volterra series and generalized memory polynomials) track and correct these subtler memory effects.
Figure 4: AM-AM Response of a PA with and without DPD on a 2× 100 MHz, 400 MHz Bandwidth Signal
It’s clear: with DPD active, output magnitude tracks input cleanly, while raw performance lags as the PA compresses.
Integration Delivers Real-World Results
While competitors piece together solutions using power-hungry, high-cost FPGAs, our advanced transceivers integrate CFR, DPD, and digital front-end processing right on-chip.
Our priority? System reliability, performance, and always-on efficiency, in a single, streamlined package.
Conclusion
The CFR/DPD combination offers a quantum leap in performance and efficiency for 5G transmitters. Instead of ham-fisted back-off or brute force, engineers have precision control, extracting max value from every watt and MHz. Analog Devices leads the way, giving you these tools in the most integrated and efficient Silicon solution on the market.
In the final part of our series, we’ll see how these features play out in real RF design, and why the ADRV9040 sets the standard for the future of wireless.
Read the full technical article Simplifying Your 5G Base Transceiver Station Transmitter Line-Up, Design, and Evaluation.
Read all the blogs in the 5G Transmitter Design series.