Clock and Carrier Synchronized Multi-Band Wireless Communications Enabled by Frequency Comb Dissemination in Radio Access Networks

Next generation radio access network requires multi-band wireless communication between the distributed unit (DU) and the radio unit (RU) to maximize millimeter wave spectrum usage and thus improving data capacity. Besides, the increasing demand of high accuracy positioning and low latency applications such as self-driving car motivates the need for low timing jitter synchronous clock distribution in future radio access network. One of the key enabling technologies for such a system is using an optical frequency comb to distribute low phase noise and synchronous clock and RF carriers, which can 1) support simultaneous multi-band wireless communication and 2) enable clock synchronisation between DUs and RUs. We propose using optical frequency comb to achieve simultaneous synchronised clock and RF carrier distribution. In this proof-of-concept experiment, a frequency synchronised multi-band 16QAM wireless transmission at 12.5, 25 and 37.5GHz has been demonstrated, covering Ku, K and Ka bands at the same time. The optical frequency comb is distributed from the DU to the RU through a standard single mode fiber (SSMF) and a broadband photodetector generates the RF frequency comb through photomixing. We have demonstrated low phase noise RF carrier distribution over 22km SSMF transmission with 70fs root-mean-squared (rms) jitter. The low rms jitter enables high order modulation format and the ability of maintaining jitter performance after SSMF transmission validates the system scalability. Additionally, we analytically modeled the impact of the fiber chromatic dispersion on RF carrier power generated using the proposed scheme. Based on this model we have demonstrated a novel optical spectrum shaping technique to eliminate power fading of RF frequency comb after transmission. Finally, the impact of the fiber chromatic dispersion and the comb seed laser phase noise on the distributed RF carrier phase noise are experimentally investigated, providing benchmark for optical frequency comb design.