Multiple Access Noise Compensation in CO-OFDMA-PON Uplink Transmission Using Digital Phase Conjugated-Pilot Tones

In coherent orthogonal frequency division multiple accessing technique-based passive optical network (CO-OFDMA-PON) uplink transmission, two main multiple-access noises (MNs), which are multiple- state-of-polarization leakages (SOPLs) and phase noises (PNs) inevitably occur by various optical network units (ONUs) and interfere with the uplink signal in this system. These MNs are different by optical devices and paths of each ONU. In this regard, parallel MNs handling per ONU is necessary to provide uplink service. We propose digital phase conjugated pilot-tones (DPC-PTs) based MNs compensation technique. This can deal with MNs by using superposition of two adjacent pilot subcarriers. In the experiments conducted, we applied a pair of DPC-PTs per ONU to handle two MNs and performed parallel processing for each ONU. The transmission performance in terms of root-mean-square error, error-vector magnitude, bit-error-rate (BER), and the overhead of redundant PTs was improved by using the proposed technique in 20-km CO-OFDMA-PON uplink transmission. Both MNs were effectively compensated by comparing transmitted and received DPC-PTs. Minimum overhead of 0.39 % OFDMA symbol duration was verified to meet BER target in the given transmission condition. It guarantees more accurate tracking of SOPL than single-PT-based one due to the doubled number of estimator samples. Besides, more accurate tracking of PNs is demonstrated due to lower noise level between DPC-PTs. As a result, applicability to the phase and polarization diversities-based coherent multiple access is demonstrated through the proposed MNs tracking technique.