Discrete Implementation and Linearization of a New Polar Modulator-Based Mixerless Wireless Transmitter Suitable for High Reconfigurability

Abstract

A novel polar modulator-based transmitter architecture that uses an analog RF variable gain amplifier (VGA) and an analog phase shifter is proposed and implemented using discrete elements. This architecture translates the baseband phase signal to RF without using mixers and quadrature up-converter circuits. Accordingly, spurs and distortions that are typically associated with the mixers are absent over a wide frequency band. As such, no filtering is needed at the output of the proposed transmitter. The absence of RF band pass filter provides wider RF bandwidth which would make the transmitter design reconfigurable and more suitable for integration. The proposed transmitter architecture has unusual signal distortion effects due to amplitude and phase nonlinearity in the VGA and the phase shifter. Indeed, this distortion consists not only in amplitude-to-amplitude (AM/AM) and amplitude-to-phase (AM/PM) distortions, but also includes phase-to-amplitude (PM/AM) and phase-to-phase (PM/PM) distortion. Hence, a new behavioral model using augmented complex memory polynomial is proposed to mitigate these complex nonlinear effects. The performance of the linearized transmitter is tested using long-term evolution (LTE) signals and the signal quality is assessed in terms of error vector magnitude (EVM). The measured EVM of the LTE signal of 1.4 MHz bandwidth at the output of the transmitter improved from 9.82% to 0.43% using digital predistortion technique. The measured adjacent channel leakage-power ratio (ACLR) of the proposed transmitter is 59 dBc.