All-Digital Transmitter Architectures — Challenges and Opportunities
Multi-band multi-mode operation of 4G/5G broadband mobile communication creates several new design challenges to traditional RF transmitter architectures. For reconfigurable and flexible spectrum usage with compact and low-cost implementation, digital-intensive transmitter architectures have been recently proposed with high-efficiency power amplifiers in compound semiconductors. 4G/5G carrier aggregation, particularly non-contiguous multi-band transmission, demands staggering design requirements on bandwidth and linearity, which are difficult to manage with conventional digital modulation techniques such as pulse-width modulation (PWM) and delta-sigma modulation (DSM). In order to overcome these challenges of digital-intensive transmitters for non-contiguous multi-band transmission, we present advanced power encoding techniques, demonstrated with a proof-of-concept digital outphasing RF power amplifiers, real-time multi-band all-digital transmitter prototypes. For broadband operation, the quantization noise of digital modulation often becomes performance bottleneck, consequently requiring high-order RF output filters with high insertion loss. We present novel out-of-band noise cancellation techniques for digital-intensive transmitters. The trends, challenges and opportunities of all-digital transmitter architecture for future radio will be discussed.