Quantization Noise Suppression for Envelope Pulse-Width Modulation (EPWM) Transmitters

The envelope pulse-width modulation (EPWM) transmitter has been proposed to address the issue of low power efficiency in the linear amplification of multicarrier signals such as the OFDM. However, the delta-sigma (Σ-Δ) modulator in the EPWM transmitter generates quantization noise that degrades signal quality. In this paper, two new EPWM architectures called the envelope subtraction EPWM (ES-EPWM) and the amplitude compensated EPWM (AC-EPWM) are proposed to enhance quantization noise suppression. The architectures generate a narrowband noise-canceling signal that is either subtracted to the PWM envelope signal (ES-EPWM) or multiplied to the PM signal (AC-EPWM). Using the IEEE 802.11a OFDM signal, simulations were done with varying canceling signal bandwidth and oversampling ratio (OSR). Results showed that increasing the canceling signal bandwidth improved the performance of the ES-EPWM transmitter in terms of the measured error vector magnitude (EVM) and adjacent channel leakage power ratio (ACLR). A similar behavior was observed for the AC-EPWM transmitter, but only up to a certain canceling signal bandwidth. For an OSR of 32 and a canceling signal bandwidth of 40MHz, both ES-EPWM and AC-EPWM transmitters were able to improve the ACLR by 6 dB and reduce the EVM to 2/3.