Agilent Envelope Tracking and Digital Pre-Distortion PA Testing for LTE User Terminal Components - App Note free download

File name Envelope Tracking and Digital Pre-Distortion PA Testing for LTE User Terminal Components - App Note

Keysight Technologies Envelope Tracking and Digital Pre-Distortion Power Amplifier Testing for LTE User Terminal Components Application Note Introduction With the introduction of 4G LTE, mobile communications were able to achieve higher data throughput than ever before. However that achievement is accompanied by unprecedented battery requirements for mobile terminals. LTE employs the SC-FDMA modulation format in the uplink and it has a higher Peak-to-Average Ratio (PAR) than W-CDMA. One of the most power-hungry components of a mobile terminal is the power amplifier (PA). As shown in Figure 1, the power level of the LTE uplink signal stays relatively low most of the time and goes to peak power only occasionally, but the PA is designed to deliver the highest efficiency only at peak power. Since the high power supplied to the PA won't be used most of the time, it is mostly dissipated as heat and causes battery drain, impacting the thermal design power (TDP) of the mobile terminals. Envelope tracking (ET) has come to the forefront as a possible solution for this issue in mobile RF front end design. ET dynamically adjusts DC supply voltage based on the "envelope" of the PA input signal and delivers higher voltages only when needed, improving battery consumption and heat dissipation in the PA. Constant PA supply voltage Envelope tracking voltage Unused energy PA input signal Time Time Figure 1. Time domain view of the input signal and bias voltage. Another issue facing power amplifiers with high PAR signals, such as LTE, is non-linearity. When a high power signal is applied to the PA input, the PA can't linearly amplify the signal, resulting in a situation leading to gain saturation and distortion, ultimately leading to signal quality degradation, such as higher ACLR and EVM. Techniques such as crest factor reduction (CFR) and/or digital pre- distortion (DPD) are used to compensate for the non-linear behavior of the PA. DPD applies "inverse" distortion to the PA input signal to cancel the distortion generated by the PA. There are several types of DPD methods but a simple look-up-table (LUT)-based DPD that doesn't take memory effect into account is commonly used in mobile devices. This application note discusses measurement solutions for PA testing using ET and LUT-based DPD. 03 | Keysight | Envelope Tracking and Digital Pre-Distortion Power Amplifier Testing for LTE User Terminal Components - Application Note Envelope Tracking Overview When testing ET devices, you will need an IQ signal for PA input and the associated envelope signal for modulating the PA power supply. The envelope is generated from the IQ sample's absolute magnitude, typically with 3x t