Nujira beats own world record for envelope tracking PA linearity

Cambridge, U.K. – May 23rd, 2013 –Nujira Ltd, the world’s leader in Envelope Tracking (ET) technology, will be presenting new performance results at the IEEE MTT International Microwave Symposium (IMS) this week demonstrating that the company has bettered its own world record linearity mark for an RF Power Amplifier (PA) operating under ET conditions. Using its patented ISOGAIN technique to linearize the amplifier Nujira achieved an adjacent channel leakage ratio (ACLR) of -53 dBc, without the use of modem assistance or digital predistortion (DPD). The result beats Nujira’s previous landmark of -50dBc, halving the signal distortion.
Nujira will be presenting the performance results at a workshop as part of IMS on 2nd June. The talk will also review fundamental issues and key techniques applied to envelope tracking systems.
Gerard Wimpenny, CTO of Nujira said: “These headline linearity results demonstrate yet again how much of a disruptive shift ET is for the RF front end. The fact that we have been able to significantly improve our own previous linearity record shows the power that our patented ISOGAIN technique puts in the hands of designers. Our ISOGAIN approach allows for precise control of the AM characteristics of the PA in a simple lookup table, meaning that designers can now quickly and simply choose the optimum trade off between linearity and efficiency for any PA device.”
The traditional approach to correct nonlinearities in RF PAs has been either to back off the output power, which increases cost and degrades energy efficiency, or to apply computationally intensive adaptive digital predistortion (DPD) techniques. DPD pre-distorts the I/Q signals which drive the RF input to the PA, in order to precisely ‘cancel out’ the nonlinear response of the PA. This can require complex software algorithms to adapt the correction coefficients as the PA’s RF performance varies with power levels, frequency bands, and temperature. DPD also represents a significant learning curve and software development effort for OEMs and chipset vendors.
Nujira’s ISOGAIN ET approach instead uses the power supply pin of the PA to directly linearize the PA, and 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 is stored in a lookup table in the modem 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 D/A converter controlling the ET power supply. Achieving these linearity results requires an Envelope Tracking modulator which can simultaneously deliver high bandwidth, a wide voltage swing range, low noise, and high efficiency.
The linearity results were obtained from a commercially available Multi Mode, Multi Band Gallium Arsenide (GaAs) handset PA operating in the 850 MHz band, with 0.9 to 4.6-V supply modulation provided by Nujira’s Coolteq.L Envelope Tracking IC for smartphones. Using the latest Nujira Coolteq.L IC together with ET optimised PAs it is possible to save approximately 400 mW at 27 dBm output power, representing a 30% reduction in energy consumption.
The IEEE MTT International Microwave Symposium is taking place in Seattle, USA, 2 – 7 June 2013.