Analog Pre-Distortion (APD) is a technology that improves the non-linearity characteristics of power amplifiers. APD is used to provide significantly
higher efficiency compared to classical power amplifier designs. This results in making total system smaller and less power consumption.

The input and output signals of the amplifier are monitored and used to create the inverse function of the non-linearity characteristics of the amplifier.
This inverse signal is then added back to the input of the amplifier to cancel out the non-linearity.

DPD (Digital Pre-Distortion) is a technology that is widely commercialized in the base station / repeater industry. Its function is maximizing the linear range of the power amplifier by
applying digital signal processing (DSP).

As mobile data traffic increases dramatically, next-generation base stations will be designed to achieve higher transmission bandwidths of up to 60 MHz or more. To linearize in the range
of 60MHz bandwidth, the pre-distortion feedback loop algorithm for the linearization must treat up to the 5th order intermodulation product, resulting in a required digital bandwidth of 300 MHz.
As time goes on and Moore’s Law continues, DPD will continue to become more competitive in price and efficiency than APD in multi-band, multi-path and high power systems – even with wider bandwidths.

Doherty technology has been commercialized in power amplifiers since early 2000 to improve the power consumption of mobile communication BTS, DAS, and repeater systems.

The signal is passed through two amplifier paths (Carrier Amplifier and Peaking Amplifier) with the same peak power through both paths and then the outputs are carefully matched together.

The bias of Carrier Amplifier is set to Class AB and the bias of Peaking Amplifier is set to Class B or C. For low input power, only the Carrier Amplifier operates and the Peaking
Amplifier does not operate. For maximum efficiency, the Peaking Amplifier starts to operate at the 6dB back-off point from the saturation power (Psat) of the power amplifier.

Asymmetrical Doherty technology does not use the same power Carrier Amplifier and Peaking Amplifier as Doherty, but applies different Power ratios.
The power at the Peaking Amplifier is applied about 2 ~ 3 times higher than Carrier Amplifier, and the efficiency increases at lower power as the power of Peaking Amplifier increases.

The Doherty has the maximum efficiency at the 6dB back-off point from the saturation power of the power amplifier, while the Asymmetrical Doherty has the maximum efficiency at the 7-12dB
back-off point according to the power of the peaking Amplifier. Considering the PAPR of the service signal, it can be designed to have the maximum efficiency at the rated output power
or lower output power of the power amplifier, but the drawback is that the cost is increased due to the high power Peaking Amplifier that is required.