Nujira has used its Isogain technique to linearize an LTE handset RF power amplifier (PA) in combination with its Coolteq envelope tracking (ET) power supply modulator to achieve an adjacent channel power ratio (ACPR) better than –50 dBc and 60% efficiency with an LTE waveform, all without digital pre-distortion (DPD). These results, which the company calls a world record, exceed the regulatory requirements for 4G LTE terminals by a factor of 100 (20 dB).
Typically, computationally intensive DPD is applied to the I/Q signals that drive the RF input to the PA to precisely “cancel out” the PA’s nonlinear response and correct nonlinearities. But this requires significant digital logic and complex software algorithms that adapt the correction coefficients as the PA’s RF performance varies with power levels and temperature.
Instead, the Isogain ET approach uses the PA’s power supply pin to directly linearize the PA. It works by adapting the supply voltage at high speed to keep the gain constant as the RF amplitude is modulated. The mapping between instantaneous RF drive level and instantaneous supply voltage is static and stored in a lookup table in the modern chipset. The mapping is applied on a sample-by-sample basis to a magnitude signal calculated from I&Q and fed to a high-speed digital-to-analog converter controlling the ET power supply.
Alternatively, designers can trade off some of the linearity for further efficiency savings by loading a different set of numbers into the shaping table. Nujira obtained these linearity results from a gallium-arsenide (GaAs) PA operating in the 800-MHz band. Energy savings using ET were approximately 500 mW at 29-dBm output power, which the company calls a 30% reduction in energy consumption.