Research into Memory Effects Compensation and Crest Factor Reduction
Inclusive Dates: 07/01/04 - 07/01/05
Background - Combining several carrier signals within a radio frequency (RF) transmitter's power amplifier creates large variations in amplifier output power, a condition described as high peak-to-average ratio (PAR). Under high average output power and high PAR, the amplifier temperature and direct current (DC) voltage variations, in turn, induce variations in the amplifier's gain characteristic called "memory effects." Memory effects compensation (MEC) and crest factor reduction (CFR) are promising areas of digital predistortion (DPD) research that can suppress memory effects and reduce RF signal PAR in ways that lead to improved microwave amplifier performance.
Approach - This project investigated alternative DPD methods for improving multicarrier RF amplifier output power, efficiency, and linear operation performance. The research developed and demonstrated highly effective CFR and MEC techniques for use with high-power, multicarrier RF amplifiers. Investigators used software simulations to refine and validate the techniques. The research converted CFR and MEC components within the simulation to their hardware equivalents. The combination of software and hardware was applied to RF amplifiers to demonstrate measurable performance improvements.
Accomplishments - The research team developed MEC and CFR techniques for improving RF amplifier performance. Computer simulations and hardware-in-the loop (HIL) measurements produced performance data and demonstrated the techniques applied to an amplifier. The research shows that CFR and MEC techniques each contribute to amplifier performance. Up to some maximum input power limit, either technique reduces amplifier output adjacent channel power ratio (ACPR), allowing the amplifier to operate at higher output powers without violating interference limits. Either technique can be applied to an amplifier with substantial amplifier performance improvement.
The greatest amplifier performance boost occurs when CFR and MEC are configured to work in unison. The CFR PAR reduction reduces extreme excursions by the input signal into the nonlinear region of the amplifier. The reduced PAR of the CFR-modified input signal improves MEC stability, reduces MEC computational load, and increases MEC ACPR improvements at higher amplifier output power levels. The combination of CFR and MEC techniques allow the amplifier to operate at even higher output powers than can be achieved using either technique alone.