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Rayleigh backscattering (RB) and Fresnel reflection (FR) effects can cause severe degradation on the performance of a WDM-PON system utilizing loop-back schemes that provide colorless upstream transmitters. We proposed cross-seeding schemes to overcome these problems and also to improve the utilization efficiency of the seeding lights. The operation principles of two types of cross-seeding schemes: mutual seeding and shared seeding are discussed in detail for different applications. A generalized formulation of the crosstalk from the RB and FR effects is derived for comparing the performance between a conventional WDM-PON and a cross-seeding WDM-PON. It is found that the cross seeding schemes can be most beneficial for the system with low loop-back gain or low reflection at the distribution fiber or drop fiber. This is especially advantageous for the shared seeding scheme in which the seeding light can be transmitted along a separate fiber with high power or by optical amplification such that the gain of the loop-back transmitter can be kept small. Application examples are used to demonstrate the merits and feasibility of the cross-seeding schemes.