Multicarrier-Modulation-Based Digital Radio-over-Fiber System Achieving Unequal Bit Protection with Over 10 dB SNR Gain

In digital mobile fronthaul, multi-level modulation formats such as 4-level pulse amplitude modulation have been considered as viable solutions for bandwidth-efficient digital radio-over-fiber (DRoF). Unlike standard digital communication of 0/1 bit streams with equal bit importance, the quantized bits of wireless waveforms have varied importance, requiring unequal bit protection in DRoF transmission. In this paper, we propose and experimentally demonstrate a multicarrier-modulation-enabled unequal bit protection (MC-UBP) scheme for DRoF systems. Bit and power allocation are employed among digital subcarriers to ensure the superior performance of more important bits. Under the assumption of an additive white Gaussian noise (AWGN) channel, a theoretical analysis of the MC-UBP scheme is performed, including the theoretical derivation of the signal-to-noise ratio (SNR) of recovered wireless signals and the optimization results of the subcarrier power factors. In the experiment based on a coherent optical system, a SNR enhancement of 16.6 dB is achieved compared with the conventional single carrier scheme, which can support the transmission of up to 16384-QAM wireless signals. Different pre-emphasis filters have been used in the experiment to emulate the diverse transfer responses of the systems. The results indicate that the MC-UBP scheme is suitable for these systems with more than 10 dB SNR gain.