Effects of adaptive subcarrier modulation on MAC operation in OFDMA-PONs

We propose a MAC framework for OFDMA-PONs which allows exploiting both the subcarrier and time domains in a dynamic and flexible way. A dynamic subcarrier assignment algorithm that respects QoS by offering decreased average delay is proposed. Additionally, the proposed PON MAC framework for the first time leverages on a cross-layer optimization via the use of adaptive subcarrier modulation (ASM). ASM implies using a different modulation format per ONU depending on the transmission performance in individual links. The exact MAC procedures needed to support such a way of operation are discussed. Finally, delay performance of all schemes is evaluated using a hybrid PHY-MAC computer simulation tool. The simulation results on the one hand assist to determine the proper selection of MAC layer parameters (like the maximum number of subcarriers per ONU) and on the other demonstrate delay performance of the proposed scheduling scheme under realistic settings. Moreover, the employment of ASM is proven to provide significant delay reduction with acceptable physical layer performance degradation.