Edwin M. Umali

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.

Power Spectral Analysis of the Envelope Pulse-Width Modulation (EPWM) Transmitter for High Efficiency Amplification of OFDM Signals

Multi-carrier systems such as Orthogonal Frequency Division Multiplexing (OFDM) require linear amplification and have low power efficiency because of high peak-to-average power ratio (PAPR). In an Envelope Pulse-Width Modulation (EPWM) transmitter, the envelope component of a linear modulation wave is converted to a PWM signal by a delta-sigma (Delta-Sigma) modulator while the phase component modulates the carrier signal. The distortion-free amplitude multiplexing of the PWM and PM signals enables the power amplifier to achieve maximum power efficiency. In this paper, EPWM transmitter characteristics such as noise power spectral density and SNR will be analyzed. The application of the EPWM transmitter to OFDM signals will also be explored. From the simulation results, the Delta-Sigma modulator with an oversampling ratio of 32 provides sufficient in-band SNR and adjacent channel power ratio required for the IEEE 802.11a OFDM system.

Power efficiency of OFDM signal amplification with Doherty and extended Doherty transmitters

OFDM (orthogonal frequency division multiplex) achieves high spectral efficiency in broadband wireless communications but has poor amplification efficiency due to its large peak-to-average power ratio. This paper presents achievable average power efficiency for the OFDM signal when the Doherty and extended Doherty transmitters are employed. By modelling the envelope of OFDM signal as a Rayleigh-distributed signal, average power efficiency for the OFDM signal is calculated as a function of clipping probability. Numerical calculation results assuming an ideal class-B amplifier and predistortion linearizer show that average drain efficiency of the two amplifiers for a clipping probability of 0.1% are 50.5 and 55.6%, respectively. It is concluded that the extended Doherty amplifier can improve power efficiency by several percents by optimizing the main amplifier contribution ratio alpha.

Effects of Quantization Noise and Distortion in EPWM Transmitters for OFDM Signal Amplification

The envelope pulse-width modulation (EPWM) transmitter has been studied to address the power efficiency issue on the linear amplification of OFDM signals. However, the delta- sigma (Delta-Sigma) modulator in the EPWM transmitter generates quantization noise that degrades signal quality. In this paper, OFDM in-band noise characteristics of EPWM transmitters are analyzed for first and second order Delta-Sigma modulators. Distortion noise power due to envelope clipping of OFDM signals is also analyzed. The combined effects of noise and distortion on the error vector magnitude (EVM) and adjacent channel leakage power ratio (ACLR) are examined. Simulations to validate the total performance were performed using the IEEE 802.11a OFDM signal. Results showed good agreement with analytical results considering a linear-approximation gain for the 1-bit quantizer in Delta-Sigma modulators. The first order EPWM transmitter provides slightly better EVM and ACLR values than second order EPWM transmitters, since the effective quantizer gain of second order Delta-Sigma modulators is smaller, that limits the noise- shaping capability of Delta-Sigma modulators. A 0.1% clipping probability for a first order EPWM transmitter with an oversampling ratio of 32 and a three-pole Butterworth bandpass filter gives an EVM of 1.8% and ACLR of-37.9 dB.