We establish and describe a physical transmission channel analysis model combining with the mode coupling and
differential mode group delay theory over the weakly guiding graded-index few-mode optical fiber with special design
base on the positive and negative few-mode fiber segment cascade structure. Numerical simulations illustrate the
relationship between the differential mode group delay and bit error rate under the condition of weakly mode-coupling.
The model can be a basic formalism for analyzing and controlling of mode coupling/dispersion induced by distortion, in
the given optical multiple-input multiple-output scheme over FMFs.