Abstract
The envelope pulse-width modulation (EPWM) transmitter has been proposed to address the power efficiency issue in the linear amplification of multicarrier signals. However, the delta-sigma (Δ-Σ) modulator in the EPWM transmitter generates quantization noise that degrades the output signal quality. In this paper, noise and distortion characteristics of the EPWM transmitter in the amplification of the OFDM signal are presented. First, quantization noise and distortion due to amplitude clipping are analyzed. Theoretical noise power spectral density (PSD) and error vector magnitude (EVM) are obtained as functions of the Δ-Σ modulator and input signal parameters. Then, simulations to validate the noise and distortion characteristics are done using the IEEE 802.11a OFDM signal and first- and second-order Δ-Σ modulators. The effects of bandpass filtering on EVM and adjacent channel leakage power ratio (ACLR) are also obtained by simulation. Results showed good agreement with the analytical results despite the use of the linear-approximation gain plus noise model. The EPWM transmitter that employed the first-order Δ-Σ modulator with a 0.1% clipping probability, an oversampling ratio of 32 and a three-pole Butterworth bandpass filter yielded an EVM of 1.8% and an ACLR of −37.9dB, which are sufficiently lower than the OFDM transmitter specification.
