Utility of delayed CSI feedback in Mode Division Multiplexed Multimode Fiber Links

The use of multiple-input multiple-output (MIMO) communication using mode division multiplexing (MDM) in multimode fiber links increases the data rate of the fiber efficiently. However modal dispersion limits the data rate and increases processing requirements at the receiver and increasing complexity. The use of channel state feedback in MDM links can help, since transmissions can be precompensated to simplify receiver implementation, though the channel parameters may evolve in time by the time they are used at the transmitter. We develop a model that considers the temporal evolution of channel parameters, and study how using channel state feedback at the transmitter can help improve performance and how delay in feedback affects performance. We perform simulations to study how delayed feedback helps in links 10 Gb/s per mode systems that use few-mode and large core links, both in the weak and strong mode coupling regimes. We find that, as the length of the fiber ranges from 1 km to 100 km, for many practical lengths, the use of delayed feedback is effective for predistortion, but larger processing delays significantly diminish their utility.