First digital sequential PA module for flexible efficiency tuning over wide power back-off range realized
The imminent launch of the 5G wireless communication standard represents a great challenge in the development of the future infrastructure. For one thing, next-generation wireless networks have to overcome present capacity limits with future data rates up to 10 Gbps and GHz modulation bandwidths. They also need to eliminate today's mobile communication constraints including network reliability, accessibility, energy efficiency and latency. To fulfill the future requirements, development trends involve a denser spatial distribution of base stations, simultaneous usage of several antennas (MIMO), frequency and service agility of the hardware components as well as integration of the RF power amplifiers (PA) into the antenna. FBH offers as a solution a highly energy-efficient microwave PA that can be tuned over a wide dynamic output power range (6 … 12 dB) while maintaining a small form factor and a large bandwidth.
Recently, FBH has presented the first GaN-based digital sequential PA (DSPA) module that allows to flexibly tune the efficiency over a wide output power range at the International Microwave Symposium 2018 in Philadelphia (USA). The DSPA concept applies two independently designed digital amplifiers combined by an isolating coupler at the output (see Fig. 1). This setup uses an H-bridge and a single-ended digital PA. Driving the H-bridge in digital Doherty operation generates an additional efficiency peak compared to common (analog) sequential amplifier solutions. The DSPA setup (see Fig. 2) is optimized for 900 MHz, but can easily be used for other frequencies due to its broadband design. One only has to change the output circuitry (coupler, filter).
Measurements were carried out using single-tone wavetable-based modulator input signals varying the power back-off from 0…. 12 dB. Moreover, the final-stage drain supply voltage of the H-bridge is varied from 20 … 40 V. The sequential PA reaches a maximum Pout of 5.9 W at 900 MHz The DSPA, however, achieves a final-stage drain efficiency of 67%, 74% and 71% at 5 dB, 8 dB and 10.5 dB power back-off, respectively (see. Fig. 3). The flexible efficiency peak tuning makes the DSPA suitable for operation with different modulated signals, varying in encoded power back-off and signal bandwidth. The presented setup is suitable for future MIMO base-stations, providing highest flexibility and compactness while supporting mobile communication standards of all generations, since only some hardware parts need to be adapted.
 A. Wentzel, W. Heinrich, "Digital Sequential PA for Flexible Efficiency Tuning Over Wide Power Back-Off Range", in IEEE MTT-S International Microwave Symposium Digest 2018, pp. 1226 – 1229, Philadelphia, USA.