The systems of optical OFDM high-speed transmission over single-mode fiber and multimode access network by employing the long LDPC coding technique were investigated. The dispersion and frequency selective fading effects of optical fiber link were theoretically analyzed, and the transmission performance of LDPC coded OFDM signals with three different rates in the long-distance transmission and multi-mode fiber access networks through the numerical simulation were compared. Simulation results show that the coded modulation technique of long irregular LDPC coded OFDM with 0.75 coding rate is the most effective for long-distance single mode fiber transmission and multi-mode fiber access networks.