MISO Load Modulated Power Amplifiers with Digital Predistortion

Fifth generation (5G) wireless communication systems operate at high data rates in a restricted spectrum. Higher order modulation schemes used like the 1024-QAM or the OFDM enhances the spectrum efficiency at the expense of the increase of the peak average power ratio. Linear power amplifiers are efficient at saturated power, but their efficiency significantly degrades at power back-off (PBO). Load Modulated amplifiers procures the advantage of maintaining high efficiency at high PBO. Among the load modulation techniques, the Doherty PA (DPA) is commonly employed in 5G base stations because of its lower overhead and relatively lower cost. Namely, the asymmetrical inverted Doherty amplifier is the main configuration used today in all wireless base stations as it maintains a high efficiency operation to up 8 dB PBO over relatively wide bandwidth. Recently, the load-modulated balanced amplifier (LMBA) architecture, has been proposed as an alternative to the Doherty PA as it enhances further the bandwidth while maintaining high efficiency at a much larger PBO than the DPA. The drawback of the conventional LMBA is its low efficiency of around 40% at PBO as the dc power consumption of the auxiliary stage significantly impacts the overall PA efficiency. To overcome the shortcoming of efficiency seen in the conventional LMBA, several variants of LMBA have been investigated. The pseudo-Doherty LMBA (PD-LMBA) is a promising architecture which seems to be a good substitute to the DPA. In this workshop, the bandwidth limitations as well as the linearizability of both the DPA and the PD-LMBA will be highlighted and discussed. The bandwidth enhancement with using multiple-input single output (MISO) configurations for both the DPA and the PD-LMBA will be explored. Some approaches of using Digital predistortion to linearize such MISO PAs will be also presented.