In wireless communication systems, the balance between power amplifier (PA) linearity and efficiency is a critical design challenge. Engineers are continuously seeking advanced solutions that can deliver high linearity without compromising efficiency. One promising approach is Volterra-based adaptive digital predistortion (DPD), which offers flexibility and performance for wideband RF PAs. This article provides an in-depth review of various digital predistortion techniques and introduces a novel adaptive algorithm integrated into an innovative DPD linearization circuit.
Volterra-based DPD technology enables RF PAs to operate with improved linearity and efficiency, making it ideal for modern wireless applications. By extending the linear operating range of the PA and reducing the crest factor, this technique allows the amplifier to handle higher power levels more efficiently while maintaining signal integrity. It also helps meet stringent spectral efficiency and modulation accuracy requirements, ensuring compliance with industry standards.
A notable implementation of this technology is found in Texas Instruments’ GC5322 integrated transmitter solution. This system includes digital up-conversion (DUC), crest factor reduction (CFR), and digital pre-distortion (DPD) within a single application-specific standard product (ASSP). Fabricated using a 0.13-micron CMOS process, the GC5322 supports a 30 MHz signal bandwidth and is "modulation-agnostic," meaning it works across multiple standards such as CDMA2000, WCDMA, TD-SCDMA, WiMAX, and LTE. For 3G signals, it reduces peak-to-average power ratio (PAR) by up to 6 dB, while for OFDM signals, it improves adjacent channel power ratio (ACPR) by 4 dB without sacrificing error vector magnitude (EVM) performance.
The GC5322 can correct nonlinearities up to the 11th order and support PA storage effects of up to 200 ns. It enhances ACPR by over 20 dB and boosts power efficiency by more than four times. For base stations, it can reduce static power loss by as much as 60%, significantly lowering operational costs and environmental impact.
As wireless systems evolve, non-constant envelope modulation schemes—common in 3G and emerging standards—require higher spectral efficiency but result in higher peak-to-average ratios. This increases the demand on PAs, reducing their efficiency and increasing cooling and operating costs. With energy costs rising and green technologies becoming more important, improving PA linearity has become a key focus for next-generation base stations.
Traditional linearization methods like RF feedforward or predistortion have been used, but adaptive DPD has proven to be more efficient and cost-effective. The growing computational power of DSPs and ASSPs has made digital pre-distortion an increasingly attractive option. The GC5322 exemplifies this trend, combining DUC, CFR, and DPD in a highly integrated solution with real-time adaptive control.
For 3G CDMA and multi-carrier OFDM systems, handling high PAR signals is challenging due to strict EVM requirements. PAs must maintain linear amplitude and phase response, but their linear range is limited. Nonlinearities cause intermodulation distortion, leading to spectrum regrowth and reduced ACPR. A simple solution is to back off the input power, but this drastically reduces efficiency. Even advanced amplifier topologies like Doherty PAs exhibit nonlinearity at back-off levels, resulting in poor EVM and ACPR performance.
Conventional class AB amplifiers typically operate at 5–10% efficiency, but with crest factor reduction and DPD, efficiency can increase by 3–5 times. Newer technologies, such as GaN or GaAs transistors, combined with dynamic envelope tracking and DPD, can push efficiency close to 50%. These advancements highlight the importance of accurate modeling in predistortion schemes.
This article will continue to explore the need for highly accurate models in digital predistortion, focusing on hardware implementation and real-world performance. The first part introduced the GC5322 transmitter solution, and the following sections will delve deeper into the technical challenges and benefits of DPD in modern wireless systems.
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