Software-defined beamforming enabled by spatial division multiplexing in the multicore fiber optical fronthaul

Abstract

This paper proposes and demonstrates experimentally, for the first time to our knowledge, a software-defined beamforming implementation based on the precise timing control of the samples generated by different digital-to-analog converters (DACs) modulating a single laser light and transmitted over a multicore fiber (MCF) optical fronthaul. Optical fronthaul systems based on standard single-mode fiber exhibit the problem of coherent signal distribution, as the signals for each antenna element are transmitted in wavelength division multiplexing (WDM), experiencing different delays from different chromatic dispersion (CD) values. Moreover, CD dynamics are different for each WDM signal due to temperature and vibration, making difficult to control the delay of the signals for the different antenna elements. This limitation can be solved using an optical fronthaul based on MCF, where the signals for the different antenna elements are transmitted through the different cores with spatial division multiplexing at the same wavelength from a single laser source. In this work, the software-defined beamforming functionality over a MCF optical fronthaul is proposed and demonstrated experimentally with the transmission of an LTE-like signal in the 700 MHz frequency band over 1 km of 7-core fiber. The LTE signal is steered ±45° by defining a delay of ±1 digital sample at 2 GS/s (±500 ps), ensuring error vector magnitude (EVM) compliant transmission after 1 km of MCF. This paper also demonstrates that, increasing the DAC sampling frequency to 5 GS/s, a higher density area can be covered.