Digital Doherty Tx for Future SDR Mobile Infrastructures

Digital power amplifier (PA) concepts are highly attractive, especially when it comes to optimizing wireless infrastructure like mobile base stations. Their architecture is already fully digitized, except for the radio-frequency (RF) amplifier. However, processing all signals digitally up to the final stage would simplify the system set-up significantly, reducing space requirements and cost. In addition, digital PAs are more flexible in terms of signal frequency and are expected to reduce energy consumption when using modern mobile communication standards like 3G and 4G. This potential makes them particularly interesting for companies involved in the wireless market such as NEC in Japan. Given this background, joint experiments with NEC have been conducted. Recently, NEC and FBH proposed and verified a new digital Doherty transmitter architecture combining voltage-coupled class-D PAs with an efficient 1-bit EDSM scheme. The proposed Tx architecture was realized with a GaN H-bridge class-D PA module (FBH) and CMOS 1-bit modulator ICs (NEC, Japan).

As a result of the joint efforts, for the first time, the Doherty concept was proven for digital signals, successfully transferring it to the "digital world". So far, it has only been applied for analog input signals. The basic idea behind the Doherty approach is to operate two PAs in parallel – one of them is effectively only switched on at full-scale, which significantly enhances efficiency at power back-off. This is important because, according to modern communication standards, RF PAs in mobile base stations usually operate at 6…12 dB below full-scale at the input (power back-off). In the experiment, an FBH H-bridge PA was driven by an envelope delta-sigma modulator from NEC.

The fabricated digital Doherty Tx achieves final-stage drain efficiencies of 73.4% at 6 dB power back-off for 860 MHz CW and 56.5% for a CDMA signal with 1-MHz bandwidth and 6.5 dB peak-to-average power ratio, respectively. Compared to conventional balanced digital PAs the amplifier shows up to 20% higher efficiency when driven between 6 dB to 12 dB below full-scale. For broadband communication signals with modern modulation schemes like WCDMA the improvement in efficiency is around 10%. These results demonstrate the potential of the proposed digital Doherty Tx offering highly-efficient and flexible solutions for future software-defined radio mobile infrastructures.

Publication:

K. Motoi, A. Wentzel, M. Tanio, S. Hori, M. Hayakawa, W. Heinrich, K. Kunihiro, "Digital Doherty Transmitter with Envelope ΔΣ Modulated Class-D GaN Power Amplifier for 800 MHz band", IEEE International Microwave Symposium Digest 2014, TU4F-1, Tampa, USA.

FBH research: 26.08.2014