Prototype implementation of two efficient low-complexity digital predistortion algorithms

Abstract

Predistortion (PD) lineariser for microwave power amplifiers (PAs) is an important topicof research. With larger and larger bandwidth as it appears today in modern WiMaxstandards as well as in multichannel base stations for 3GPP standards, the relativelysimple nonlinear effect of a PA becomes a complex memory-including function, severelydistorting the output signal. In this contribution, two digital PD algorithms are investigated for the linearisation of microwave PAs in mobile communications. The first one is an efficient and low-complexityalgorithm based on a memoryless model, called the simplicial canonicalpiecewise linear (SCPWL) function that describes the static nonlinear characteristicof the PA. The second algorithm is more general, approximating the pre-inverse filterof a nonlinear PA iteratively using a Volterra model. The first simpler algorithm issuitable for compensation of amplitude compression and amplitude-to-phase conversion,for example, in mobile units with relatively small bandwidths. The second algorithmcan be used to linearise PAs operating with larger bandwidths, thus exhibiting memoryeffects, for example, in multichannel base stations. A measurement testbed which includes a transmitter-receiver chain with a microwave PA is built for testing and prototyping of the proposed PD algorithms. In the testing phase, the PD algorithms are implemented using MATLAB (floating-point representation) and tested in record-and-playback mode. The iterative PD algorithm is then implemented on a Field Programmable Gate Array (FPGA) using fixed-point representation.The FPGA implementation allows the pre-inverse filter to be tested in a real-timemode. Measurement results show excellent linearisation capabilities of both the proposedalgorithms in terms of adjacent channel power suppression. It is also shown thatthe fixed-point FPGA implementation of the iterative algorithm performs as well as thefloating-point implementation.