This paper examines and compares the use of orthogonal frequency division multiple access (OFDMA)and generalized frequency division multiple access(GFDMA) multicarrier formats in next-generation passive optical networks (PONs) through experimentation and numerical simulations. It is demonstrated that GFDMA presents a high resilience toward multiple access interference (MAI) compared to OFDMA at the uplink channel of aPON. Moreover, two methods are proposed for achieving synchronization in the uplink channel of PONs, enabling high spectral efficiency. These proposed methods can lead to the elimination of inter-ONU guard bands, and facilitate overlapping inter-ONU subcarriers, while eliminating the effects of MAI. Additionally, the performance of these multi-carrier formats was analyzed in the downlink channel at a data rate of 15.6 Gb/s and a transmission span of 50 km where neglible performance penalty was observed