Improved Dynamic Range in RF Communication Over Optical Fiber

Radio-over-fiber (RoF) links are critical for millimeter-wave wireless communications infrastructure where antenna remoting is needed to provide cell coverage. However, nonlinearity is a key issue of electro-optical conversion and results in low dynamic range. Non linearity in the optical modulator also causes low dynamic spurious free dynamic range (SFDR) of fiber links, which presents an issue for 5G technologies that use antenna remoting.

Researchers at the University of California, Santa Barbara have developed an analog pre-distortion technique to linearize the nonlinear transfer function of an optical modulator such as a Mach-Zehnder modulator. The technique is implemented in Silicon photonics technology, and offers low-cost and large-scale integration compared to other technologies such as LiNb03, lnP and GaAs. Emerging 5G technologies that use antenna remoting to cover an environment (e.g., a room) with a wireless signal will distribute the signal to all antennas with electrical cables or fiber links. In contrast to electrical cables which experience high loss at higher frequencies, fiber links have negligible loss, but they suffer from low SFDR due to nonlinearity of the optical modulator. This technology circumvents this limitation through analog predistortion, achieving high SFDR radio-over-fiber links.