Development of a Memory Effects Compensation Digital Predistortion Demonstration System, 10-R9573Printer Friendly Version
Inclusive Dates: 08/22/05 02/22/06
Background - This project and its predecessor, 10-R9496, investigated digital predistortion (DPD) methods for improving multicarrier radio frequency (RF) amplifier output power, efficiency, and linear operation performance. Investigators explored crest factor reduction (CFR) and memory effects compensation (MEC) techniques. CFR is a valuable technique for reducing the peak-to-average ratio (PAR) of the amplifier input signal. MEC is a sophisticated technique for improving the linearity of amplifiers exhibiting time-varying, nonlinear power gain and signal phase characteristics.
Approach - The investigators developed new CFR and MEC techniques within a framework of software and hardware-in-the-loop simulations. Software simulations identified likely algorithms and control mechanisms to be used within actual DPD systems. Investigators synthesized portions of the simulation software within field programmable gate array (FPGA), application specific standard part (ASSP), and test equipment hardware. A combination of the DPD hardware and simulation software was used to verify software simulation predictions. Simulations and hardware-in-the-loop experiments show that CFR and MEC substantially improve the linear operation of high-power, multi-carrier RF amplifiers.
Accomplishments - Individually, CFR and MEC each contribute to amplifier performance. Up to some maximum input power limit, MEC reduces amplifier output adjacent channel power ratio (ACPR), allowing the amplifier to operate at higher output powers without violating ACPR and spectral emissions mask (SEM) limits. CFR reduces the PAR of the amplifier input signal, allowing the amplifier to operate at higher output powers. Either type of DPD can be applied to an amplifier with substantial output power performance improvement (e.g., 2 to 4 dB).
The greatest 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 and improves amplifier output ACPR at higher output power levels. The combination of CFR and MEC digital predistortion techniques allows the amplifier to operate at even higher output powers than can be achieved using either technique alone. Using CFR and MEC together, it is possible to increase amplifier output power by as much as 6 dB; i.e., it is possible to quadruple the output power of the amplifier.