Adaptive linearization using predistortion-experimental results

This paper describes an adaptive predistortion system to linearize class B and class C radio frequency high power amplifiers. The system, which can handle a 30 dB power control range, uses a digital signal processing microprocessor and memory look-up tables to predistort the baseband drive signals. Low data rate services such as are found in mobile satellite applications (<8.5 kbits/s) can be handled by this digital signal processing approach. Higher bit rates can be obtained with some software modifications and any further increase would require a more hardware intensive solution. The system gives a 30 dB improvement in IM₃ of a class B stage using a standard two-tone test. An improvement of 25 dB when using π/4-shift-QPSK modulation is obtained with a sample rate of B times the symbol rate. Experimental results show that AM to AM correction is much more important than AM to PM correction and in some cases this can lead to further simplifications