Digital Predistortion Linearization of Wideband Transmitter for High Data Rate Satellite Communications

Abstract

High speed satellite communication link are of considerable interest to both commercial and government customers. To support up to Gigabit per second (Gbps) data rates, wide modulation signal bandwidth over hundreds MHz and spectrum efficient high order modulation schemes need to be used. Consequently, transmitter linearity is becoming a very crucial issue. To correct distortion caused by nonlinearity of the high power amplifier as well as transmitter phase and amplitude variation over the wide signal band, this paper presents a linearization technique combining high speed digital time-domain transmitter equalization and high speed digital predistortion (DPD). A dynamic nonlinear model is developed based on the memory polynomial to enhance its accuracy for modelling wideband system. We implemented the high-speed digital linearizer in a FPGA-based testbed and demonstrate its performance by linearizing an in-house developed X-band satellite down link transmitter with a 30W GaN power amplifier, operating within the ITU allocated 375 MHz X-band spectrum and exploiting high-order APSK modulation based on DVB-S2x standard to support transmission data rates of multiple Gbps. Measured results for a 300Msps 256-APSK signal shows that with linearization spectra regrowth is reduced by some 8dB and messy signal constellation is cleaned up with an EVM of around 2.1%. This validates that the transmitter linearity is largely improved such that high data rate transmission can be supported.