wireless communications, digital up-conversion, digital up-conversion architecture, high efficiency, wireless base stations, linear power amplifiers, PAs, switch mode power amplifiers, SMPAs, radio frequency, RF, circuits, SMPA drive signals, amplifier drive signals, analog, digital, digital signals, digital signal processing techniques, signal modulation, polar values, pulse widths, pulse positions, pulse position modulation, PPM, amplifiers, transmitters, Cartesian sigma delta architecture, polar quantisation plane, pulse modulation converters