Integrating baseband-over-fiber and six-port direct modulation for high-speed high-frequency wireless communications

A complete scheme by integrating baseband-over-fiber link and six-port direct modulation is proposed and experimentally demonstrated for uplinks in future high-speed high-frequency wireless communications systems. To handle high frequencies, GaAs MESFET RF switches are used to interface the baseband in-phase (I) and quadrature (Q) signals to six-port modulator. The baseband I and Q signals are transported over parallel single-mode fiber (SMF) links with the help of commercial low-cost digital optics. The proposed scheme features low cost, low complexity and low power consumption in particular for high-speed high-frequency wireless communications. 120 Mbaud quadrature phase shift keying (QPSK) signals at 19 GHz are generated and evaluated after transmission over up to 10 km single-mode fiber. The experimental results show insignificant degradation by introducing baseband-over-fiber links, especially for larger local oscillator power. Due to baseband-over-fiber transmission, the performance holds as the fiber length goes to 10 km from 2 km.