Highly Linear and Reconfigurable Three-Way Amplitude Modulation-Based Mixerless Wireless Transmitter

Abstract

A novel transmitter architecture that uses only envelope modulators is proposed. The complex baseband signal is decomposed into components using a three-coordinate decomposition technique. The individual components are translated to RF using three variable gain amplifiers (VGAs), which act as envelope modulators. The outputs of the VGAs are combined to generate the complex modulated RF signal. The proposed architecture does not have any phase modulator circuit and avoids phase noise and bandwidth expansion issues associated with polar topologies. This architecture avoids the use of any mixers or quadrature up-converters for frequency up-conversion. Accordingly, spurs that are associated with mixer circuits are avoided and, hence, no filtering is needed at the RF output of the transmitter. As RF filters are absent, this architecture offers wider RF bandwidth and would improve the design reconfigurability and integration capability. The signal at the output of the amplitude modulation-based transmitter has distortion due to gain and phase nonlinearity in the VGAs. A new memory polynomial-based modeling approach is used to linearize the transmitter. The performance of the implemented transmitter is evaluated using Long Term Evolution (LTE) signals and the measured error vector magnitude (EVM) and the adjacent channel leakage-power ratio (ACLR) are used to assess the signal quality. The measured EVM of the LTE signal of 1.4 MHz bandwidth, improved from 15% to 0.5% using digital predistortion technique, while the obtained ACLR is 54 dBc.