Adaptive digital linearization of a DRP based edge transmitter for cellular handsets

Abstract

The digital RF processor (DRP) based EDGE small- signal polar transmitter utilizes a highly non-linear digitally controlled pre-power amplifier (DPA) for on-chip combination of the amplitude and phase modulation paths. This complex signal drives an off-chip power amplifier (PA), optimized for power added efficiency (PAE), causing further degradation in the linearity of transmission. We propose an effective adaptive predistortion scheme that takes advantage of the time division duplex (TDD) nature of the EDGE modulation and uses the on-chip receiver as a feedback path during transmission. The proposed linearization technique features automatic calibration of nominal AM-AM and AM-PM look-up tables (LUT). The temporal variations primarily caused by temperature, aging, impedance and voltage changes are compensated by an incremental predistortion function that is periodically adapted using a low complexity iterative algorithm. The proposed predistortion technique improves the EDGE TX error vector magnitude (EVM) contribution from -20 dB (or 10%) in absence of predistortion to -67 dB (0.04%) while the adjacent channel power ratio (ACPR) at 400 kHz offset improves from -46 dBc to -69 dBc.