An imbalances cancellation method with memory in a LINC transmitter for wideband systems

Abstract

Power amplifiers (PAs) are inherently nonlinear devices and they are present in all transmitters of a communication equipment. As a result of transmitter nonlinearities, the transmitted signal spectrum expands into adjacent channels. Several linearizing techniques for power amplifiers have been proposed, but the most existing architectures assume that the PA has a memoryless nonlinearity. For wideband signal transmission, such as WCDMA or OFDM, the PA memory effects can no longer be ignored. The LInear amplification using Nonlinear Components (LINC) technique is a well-known power amplifier linearization method to reduce out- of-band interferences in a nonconstant envelope modulation system, such as the OFDM modulation. In this work, authors propose and analyze an adaptive digital LINC transmitter structure including PA memory effects, applied to a OFDM signal transmission. The major drawback of a LINC structure is the inherent sensitivity to gain and phase imbalances between both amplifying branches. In this work a full-digital base band method is described which corrects any gain and phase imbalances in LINC transmitters mainly due to the un-matching of the two amplifier paths. The method uses adaptive signal processing techniques, includes memory effects and its main advantage is the ability to track the input signal variations and adapt to the changes of amplifier nonlinear characteristics.