I Introduction
The 5th generation (5G) and 6th generation (6G) of mobile communications will provide converged wireless and wired services in the optical access domain and require flexible, high capacity fronthaul and backhaul links, as well as high-rate transmission, massive amount of data processing and low latency communication. Orthogonal Frequency Division Modulation (OFDM) has been adopted in current 5G networks and has become one of the potential candidates for 6G systems. OFDM allocates various data capacities with flexible granularity in the frequency or wavelength domains, and presents many advantages, including high-spectrum efficiency. However, it requires a large power-consuming digital signal processing (DSP) to evaluate the discrete Fourier transform (DFT) for many subcarriers [1], and it cannot meet the future networking requirements related to end-to-end latency, reliability, and availability of extremely high data rates for the users. On the other hand, the optical implementation of DFT introduces a negligible processing time, thanks to fact that signals propagate inside an optical device at the speed of light. An arrayed waveguide grating (AWG) configuration has been proposed as an optical DFT device [2], and to reduce adjacent carriers crosstalk, a cascade configuration [3] and sparse subcarrier multiplexing [4] have been also proposed.