This paper proposes a channel estimation method that adaptively estimates the upstream cross-polarization-induced signal fading channel. This method solves one of the most critical problems; minimizing the interference from the multi-user access in the orthogonal frequency-division multiple access (OFDMA) passive optical network (PON) system with polarization multiplexing (POLMUX) and direct detection (DD). The system reduces the receiver-end hardware compared to coherent detector, and while increasing spectral and data efficiency. In the upstream channel estimation process, the optical network unit (ONU) transmits the pilot signals and the optical line terminator (OLT) estimates the channel response of the subcarriers between pilot tones, which inherently interferes the data signals because of the polarization rotation of the fibers. In the proposed method, the pilot signals power increases gradually to mitigate the interference, and the OLT continuously employs the adaptive filter for the channel estimation to cope with the change of the channel response caused by the migration of the access network. This paper shows the configuration, operation principles, and performance of the channel estimation method. It also shows that the method can reduce the bit error rate and system performance that approximately approaches the level offered by an OFDM transmission system that has no signal fading.

passive optical network (PON), orthogonal frequency-division multiple access (OFDMA), polarization multiplexing, direct detection, channel estimation.