Linearization technique achieves -50-dBc ACPR and 60 percent efficiency for LTE signal at 29-dBm

February 15, 2013 // By Paul Buckley
Nujira Ltd has released performance results for an LTE handset RF Power Amplifier which achieves an Adjacent Channel Power Ratio (ACPR) better than -50 dBc and an efficiency of 60% with an LTE waveform, without the use of Digital Pre-Distortion. 

The measurements were made using Nujira's patented ISOGAIN technique to linearize the amplifier in combination with Nujira's high performance Coolteq ET power supply modulator.  The results exceed the regulatory requirements for 4G LTE terminals by a factor of 100 (20 dB).     

“In addition to the well-publicised power saving benefits of ET, the ability to linearize the PA at no extra cost, power or complexity is a huge benefit for smartphone OEMs and chipset vendors”, said Tim Haynes, Nujira's CEO.  “Our chipset partners can reduce or eliminate the cost, power consumption and development complexity associated with Digital Pre-Distortion in handset applications, giving them significant competitive and time-to-market advantage”.    

The traditional approach to correct nonlinearities in RF PAs has been to apply computationally intensive Digital Pre Distortion to 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 requires digital logic, and complex software algorithms which adapt the correction coefficients as the PA's RF performance varies with power levels and temperature.     

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 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.     

“ET introduces a disruptive shift into the RF front end – the characteristics of the PA are now precisely controlled by a lookup table in the digital domain, rather than the analog domain” explained Jeremy Hendy, Nujira's VP Sales and Marketing.  “It's effectively a software-defined PA.  Although these headline linearity results we've achieved are