PAS-driven optimization in orbital angular momentum-multiplexed UWB systems

Ultra-wideband (UWB) technology enables high-speed and low-power consumption transmission for communication systems by utilizing an extremely wide bandwidth. However, its large frequency bandwidth poses significant challenges for integration with conventional wavelength-division multiplexing (WDM) techniques, thereby limiting further enhancement of communication capacity density. Although orbital angular momentum (OAM) multiplexing, operating independently of the wavelength dimension, offers a promising avenue to increase the capacity density of UWB communication systems, mode coupling in few-mode fibers (FMF) leads to severe inter-mode crosstalk, resulting in signal distortion and a substantial degradation in communication performance. To address this issue, we propose incorporating probabilistic amplitude shaping (PAS) into an OAM-multiplexed UWB system. This scheme combines a constant composition distribution matcher (CCDM) with a low-density parity-check (LDPC) coding module to achieve joint shaping and error correction. Concurrently, discrete Fourier transform (DFT) precoding is introduced to equalize the signal-to-noise ratio (SNR) across subcarriers and suppress high peak-to-average power ratio (PAPR). Experimental results demonstrate the successful transmission of a 4-channel OAM-multiplexed UWB signal at 1.89 × 4 Gbit/s over 5 km FMF. Compared to the original 64-QAM signal, the unshaped (uniformly distributed) 64-QAM signal achieved improvements in optical receiver sensitivity of 7.5 dB, 6.5 dB, and 6.0 dB at symbol rates of 3.0, 3.6, and 4.0 bits/QAM symbol, respectively. The application of probabilistic shaping, which reduces the occurrence probability of high-amplitude symbols, yielded an additional sensitivity gain of 0.5 dB. These results validate the effectiveness of the proposed scheme and its significant potential for enhancing the performance of multiplexed transmission systems.