OAM Mode Selection for High-Speed Optical Communications: A Bit Loading Approach

Occupying more channels increases transmission rate, however, crosstalk increases to an unacceptable level long before all 24 channels can be exploited. The crosstalk is not uniform between modes, hence occupying different subsets of channels leads to vastly different achievable transmission capacities. In addition to optimizing occupied subsets of channels, we adopt a bit loading approach. We examine several resource allocation strategies in a coherent detection system, starting with typical OAM mode group granularity (all channels in a group occupied) and ending with single channel granularity. By exhaustive search at mode group granularity, we find a bit load increasing the total capacity by
$\sim$
15% compared to a minimax solution for mode group allocation. Single channel granularity imposes great computational effort to optimize bit loading. We propose search algorithms that are computationally tractable and improve capacity by
$\sim$
30% vis-à-vis the minimax solution for mode groups. Finally, we examine the impact of signal-to-noise ratio (SNR) and receiver digital signal processing (DSP) complexity on the overall capacity. We include a discussion of DSP with limited or with no multi-input, multi-output (MIMO) processing.Our algorithms could be applied to any mode multiplexing fiber, as it only relies on knowledge of the crosstalk matrix across modes.