Optical Upconversion of OFDM Signals for Next Generation Fronthauling Using Frequency Doubling and Quadrupling

Fronthaul data rates in case of millimeter wave (mmWave) wireless systems scale up to several Gbps in the currently deployed digitized radio over fiber solutions, resulting in large complexity remote radio heads (RRHs). We propose a promising solution involving optical frequency doubling and quadrupling, where all complex digital processing are moved to the centralized base band unit (BBU), thereby allowing placement of large number of low-complex RRHs at long distances. We experimentally demonstrate optical frequency doubling and quadrupling for the generation and transport of millimeter wave mmWave signals with OFDM containing QPSK, 16/64/256-QAM modulation formats over a distance of 2 km. These schemes enable the use of low-frequency RF signals at the BBU for the generation of mmWaves at the RRH. We evaluate the performance of the systems in terms of error vector magnitude (EVM) and out-of-band emissions of the upconverted data. Our results show that the proposed schemes achieve EVM values within the 3GPP 5G standard requirements. We also investigate the tunability and scalability of the proposed schemes across the n258 band (24.25 - 27.5 GHz) and discuss the feasibility of implementing dense wavelength division multiplexing (DWDM) for servicing multi-channel operation required for multisector antennas. The proposed schemes can support high data rate transmission for future wireless networks including 6G.