Power Amplifier Design for Integrated Multi-beam Active Antenna Arrays

With the exponential growth in data traffic and connectivity needs, direct-to-cell satellite communication is anticipated to be the next milestone of future 6G networks, providing global connectivity. To enable a high throughput for a larger number of cell phone users, thousands of active antenna arrays are expected to be highly integrated into the direct-to-cell low Earth orbit (LEO) satellite, in which the traditional ferrite isolators placed between the antenna array and power amplifiers become unfavourable due to bulky and integration-incompatible. Due to mutual coupling among antenna elements, without isolators, there is a strong ‘load-pull’ interaction between the power amplifier (PA) and antenna array, and the PA loads become dynamic during beam scanning. Since the PA performance (i.e., output power and efficiency) is highly dependent on the PA loads, the traditional PA design approach, focusing on 50 Ohm, becomes impractical. This paper presents a new PA design approach for integrated active antenna arrays, where the PA will be matched to the average load (not 50-Ohm) within multi-beam implementation in the active antenna array. In this way, the PA performance in terms of output power and efficiency is improved compared with the PA matched to 50-Ohm. For verification, an 8×8 active antenna array consisting of 64 antenna elements and a Doherty power amplifier with 9 dB back-off connected to each antenna element is used, where a beamforming matrix is built to generate 8 beams, transmitting modulated signals to 8 users. The 64-quadrature amplitude modulation (64-QAM) modulated signals for each beam are generated based on the long-term evolution (LTE) orthogonal frequency-division multiplexing (OFDM) standard (12 subcarriers per beam). The simulation results show that when the Doherty power amplifier is matched to the average load (47-j*29 Ohm), the output power and efficiency have been improved by 0.47 dB to 0.9 dB and 2.44% to 7.85%, respectively, compared with 50-Ohm matching, and the specific improvement value is dependent on the input power level. This new PA approach, which has high efficiency and output power, will pave the way for thousands of integrated active antenna array implementations for direct-to-cell LEO satellites.