IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 66, NO. 3, MARCH 2018 1475 Homodyne Digitally Assisted and Spurious-Free Mixerless Direct Carrier Modulator With High Carrier Leakage Suppression Weiwei Zhang, Graduate Student Member, IEEE , Abul Hasan, Graduate Student Member, IEEE , Fadhel M. Ghannouchi, Fellow, IEEE , Mohamed Helaoui, Member, IEEE, Yongle Wu, Senior Member, IEEE, Lingxiao Jiao, and Yuanan Liu, Member, IEEE Abstract—In this paper, a new method to design a digitally assisted and spurious-free direct carrier mixerless modulator based on the six-port correlator is proposed. The calibration of the modulator based on modified Cartesian memory poly- nomial (MCMP) is used to linearize and mitigate hardware impairment of the whole system. The modulation and the up conversion are performed by using the variable loads controlled by the differential in-phase and quadrature-phase baseband volt- ages together with common-mode voltages. The proposed MCMP is able to compensate for nonlinearity, frequency responses, residual carrier leakage, crosstalk between the in-phase and the quadrature-phase data. The proof-of-concept of digitally assisted mixerless modulator is developed and its performance is assessed at 2.6 GHz with modern communication signals. The error vector magnitudes between the input ideal baseband signals and the up-converted radio frequency signals are all between 2% and 4%. The residual carrier leakage, which remains present after imperfect suppression through hardware means, degrades the overall system performance and it can be suppressed completely by means of the proposed memory polynomial model. Index Terms— Carrier leakage, Cartesian memory polynomial, direct carrier modulator, memory effect, six-port correlator. Manuscript received April 26, 2017; revised June 22, 2017; accepted July 25, 2017. Date of publication August 15, 2017; date of current version March 5, 2018. This work was supported in part by the National Natural Science Foundations of China under Grant 61422103 and Grant 61671084, in part by the National Key Basic Research Program of China (973 Program) under Grant 2014CB339900, in part by BUPT Excellent Ph.D. Students Foundation under Grant CX2016303, in part by the China Scholarship Council, in part by the Alberta Innovates Technology Future, in part by the National Science and Engineering Research Council of Canada, and in part by the Canada Research Chairs Program. (Corresponding author: Weiwei Zhang.) W. Zhang is with the Beijing Key Laboratory of Work Safety Intelli- gent Monitoring, School of Electronic Engineering, Beijing University of Posts and Telecommunications, Beijing 100876, China, and also with the Intelligent RF Radio Laboratory, Department of Electrical and Computer Engineering, Schulich School of Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada (e-mail: clarence.zhang11@gmail.com). A. Hasan, F. M. Ghannouchi, and M. Helaoui are with the Intelli- gent RF Radio Laboratory, Department of Electrical and Computer Engi- neering, Schulich School of Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada (e-mail: ahasan@ucalgary.ca; fghannou@ucalgary.ca; mhelaoui@ucalgary.ca). Y. Wu, L. Jiao, and Y. Liu are with the Beijing Key Laboratory of Work Safety Intelligent Monitoring, School of Electronic Engineering, Beijing University of Posts and Telecommunications, Beijing 100876, China (e-mail: wuyongle138@gmail.com; jlx005@gmail.com; yuliu@bupt.edu.cn). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMTT.2017.2735408 I. I NTRODUCTION S INCE the six-port correlator was used for direct digital receiver for retrieving the phase shift keying (PSK) and quadrature amplitude modulation (QAM) baseband signals for the first time in [1], the homodyne structures based on the multiport techniques as radio frequency (RF) front-end, have attracted more attention. The RF front-ends using mul- tiport techniques can be classified into multiport demodula- tors [2]–[8] and multiport modulators [9]–[30]. Different from the multiport demodulator, the multiport modulator, which is shown in Fig. 1(a), can realize the function of frequency up-conversion from baseband signal to RF signal without a mixer and hence it can replace the traditional direct conver- sion modulator based on two mixers and one phase shifter, as shown in Fig. 1(b) in the modern wireless communication systems due to the easy fabrication, low-cost, low-power consumption, and wideband coverage. As shown in Fig. 1(a), the direct carrier mixerless modu- lator essentially consists of one six-port correlator and vari- able loads, which are controlled by the baseband signals to generate different reflection coefficients for RF modulation. Hence, the optimization and design of variable loads are very important as they directly affect the overall system performance. Many six-port modulators (SPMs) with different circuit structures [9]–[30] have been proposed since [9]. There are mainly three different methods to realize the variable loads, namely, RF switches [9]–[17], diodes [18]–[22], as well as transistors [23]–[30]. However, there are still many problems in the SPMs which deteriorate the system performance that need to be solved, and some of them are listed here. First, carrier leakage exists in almost all of the SPMs reported in the literature [12], [16], [18], [21], [26], and [28], which degrades the system performance and lowers the error vector magnitude (EVM). The carrier leakage is very prominent in [16] that uses only four ports for imple- mentation and measurement. The method based on phase shifters together with differential baseband signal is adopted in [18]–[21], [26], [28], and [29], however, the carrier leakage cannot be suppressed completely from the modulated RF signal if the diodes are not identical, the electrical length of the transmission line implementing 90° phase shift is not 0018-9480 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.