An optical-assisted microwave-mixing technique based on orthogonal and phase-coherent optical carriers is proposed and demonstrated. High-quality binary-phase-shift keying, amplitude-shift keying, and on–off keying microwave modulation at 20 GHz implemented in the optical domain have been tested. The ultra-high-bandwidth response of the proposed mixing device is also analyzed and compared. The phase stability of the photonic microwave signal in the radio-over-fiber system is improved by the proposed signal mixing technique. © 2014 Optical Society of America