Linearization Methods

Fraunhofer IIS develops application-specific linearization methods for RF power amplifiers that enable distortion-free and energy-efficient signal amplification. With our comprehensive know-how in the field of amplifier development for various radio communication standards, we are well-positioned to offer expert advice regarding the development, simulation and implementation of suitable linearization methods.

The following linearization methods have been successfully tested in real environments:

• Analog pre-distortion
• Adaptive digital pre-distortion
• Feed forward
• Cartesian and polar loop
• LiST (Linear amplification using sampling techniques)

During the development phase, simulations serve as the basis for a successful later implementation in different applications such as base stations, mobile devices or broadcast transmitters.

With the rise in the number of wireless communication connections, the demands placed on communication system RF amplifiers are increasing as well. The issue is that the amplifiers must achieve a high degree of energy efficiency while at the same time exhibiting extremely low adjacent channel interference. Existing linearization methods have reached their limits here.

Many digital modulation schemes such as QAM (quadrature amplitude modulation) and OFDM (orthogonal frequency division multiplex) lead temporarily to high signal peaks. These must be amplified with as little distortion as possible to avoid intermodulation products that increase the bit error rates and create adjacent channel interference. While high amplifier linearity is normally achieved with Class A operation, it results in very low efficiency. The increased power consumption thus leads to higher energy costs for stationary transmitters and basis stations, as well as short standby or talk times with battery-operated mobile devices.  

Distortion-free, energy-efficient signal amplification is being achieved through the use of high performance amplifiers. The transistors are operated in a more energy-efficient operating point (Class AB or B, or Class C through F if appropriate). Furthermore, technologies are being employed, which make sure that the power amplifier produces a high level of efficiency at back-off power levels, like Doherty or envelope tracking. In order to meet the linearization requirements in these cases, special linearization methods are used as well. Important decision criteria include bandwidth, out-of-band emission, implementation effort, power consumption, stability and adaptation to fluctuating operating conditions.