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Quantization and feedback of principal modes for high speed multimode fiber links

The larger core diameter of multimode fibers (MMFs) allows several propagating modes. The differential group delays of these modes cause modal dispersion, thus limiting the data rate. Recent results, however, have shown the existence of a set of principal modes (PMs) that allow dispersion free communication, to the first order. Moreover, the orthogonal nature of these PMs allows for convenient multiplexing in MMFs with low mode-dependent losses. To effectively utilize these PMs at the transmitter, it is essential to estimate them and feed them back to the transmitter. In this paper, we propose two quantization schemes that enable efficient encapsulation of the PMs for feedback. For shorter length MMF links, we generate a vector quantization codebook based on the Linde-Buzo-Gray algorithm, while for long links with strong mode coupling, we show analytically that the PMs are Haar distributed, and use a Grassmannian line packing based quantization codebook. Simulations reveal that the quantization effectively limits dispersion and the performance of the quantized PMs is within 2 dB of the ideal principal modes with only 6 bits used for quantization. In addition, the quantized PMs allow for efficient multiplexing with just 3% cross-talk.