Near-Optimal Detection of TAST Codes Over Multimode Optical Fiber Channels

In this work we propose a new low-complexity, essentially optimal detection algorithm for n × n TAST codes over n-mode optical fiber channels with Mode-Dependent Loss (MDL). Our algorithm is derived from the analysis of the Orthogonality Defect Factor (ODF) of the equivalent channel matrix where we derive its closed-form expression for general n as well as characterize its statistical properties. Using the ODF properties allows us to modify the detection algorithm such that it still achieves essentially optimal performance but at a much smaller cost than exhaustive or sphere decoding algorithms. The theoretical results are validated by simulations.