Broadband Linearity-Enhanced Doherty Power Amplifier Design Techniques for 5G Sub-6 GHz Applications

Abstract

The recently deployed fifth generation (5G) cellular networks represent a significant technological advancement over fourth generation (4G) networks. Specifically, new 5G frequency bands were allocated at sub-6 GHz and instantaneous signal bandwidths were increased to satisfy the rapidly growing needs for increased data rates. Furthermore, 5G uses more complex modulation schemes to improve spectrum efficiency. Finally, 5G introduced massive multiple input multiple output (MIMO), where multiple transceivers are used to direct the signal towards specific users, increasing channel capacity.

Conventional power amplifiers (PAs) are not suitable for 5G applications due to the increased signal and system complexity. For example, the Doherty power amplifier (DPA) technique is popular since DPAs can efficiently amplify signals with complex modulation schemes, but conventional DPAs have narrow bandwidth and poor linearity that preclude their use in 5G systems. This motivated research into DPA bandwidth and linearity improvements for use in 5G networks.

This work focuses on bandwidth and linearity enhancement for sub-6 GHz DPAs realized using discrete components on a printed circuit board (PCB). Bandwidth is improved using broadband architectures for the DPA output combiner network (OCN), the absorption of drain parasitics, and broadband input matching network (IMN) design. Linearity is enhanced by proper drain biasing network design, and careful selection of transistor source impedances. A 3.3–5.0 GHz DPA using these techniques is designed and fabricated. Under wideband modulated signal excitation, the DPA offers very good linearity with appropriate digital predistortion (DPD). A 2×2 array of DPAs is evaluated in fully digital MIMO setup using a 2×2 antenna array. The DPA array achieves excellent linearity characteristics under 100 MHz signals and use of dual-input single-output (DISO) DPD. The DPA remains the ideal choice in 5G MIMO systems when compared to the class AB PA since it can maintain a higher average drain efficiency and similar linearity.