Design Considerations for 5G mm-Wave Transmitters and Power Amplifiers
5G is turning into mm-waves for more bandwidth to secure yet higher speed and capacity demands. In this presentation, the transmitter design challenges are highlighted with a particular focus on the power amplifiers. System requirements in terms of analog-, hybrid-, and digital beamforming and antenna array size define the need for compact integration and required output power per power amplifier. Having few antennas with high output power is not desired from receiver point of view due to the limited output power in the uplink. As many antennas are required in the uplink, the output power from each antenna can be relatively low compared to classical macro base stations. The choice of process technology also affects how the system aspects can be fulfilled and what architectures are suitable. Silicon, CMOS (bulk, FD-SOI) and BiCMOS, gives the highest integration level, but also the lowest output power per power amplifier. GaAs and GaN deliver higher output power at the expense of higher complexity and more costly building practice. For the power amplifiers, the output power, efficiency at both peak and at back-off, and ACLR are obvious design targets. For mm-wave power amplifiers with large bandwidths, the digital predistortion (DPD) can be very power consuming and the order of the DPD must therefore be kept low. This prevents us from using very non-linear PA architectures to improve the efficiency. Besides that, due to the limited space in a mm-wave antenna array, there will only be a low-order antenna filter supressing the unwanted distortion from the PA to comply with the spectrum emission mask. The presented mm-wave design considerations account for beamforming, large array antennas, and how this relates to the building practice of the targeted system.