Energy saving in TWDM(A) PONs: Challenges and opportunities

Time- and Wavelength-Division Multiplexed (Multiple Access) TWDM(A) PONs are currently under standardization by ITU-T. Energy efficient schemes proposed for TDM(A) PON can still be applied to TWDM(A) PON for the subset of Optical Network Units (ONUs) transmitting and receiving at the same wavelength. However, because of the presence of multiple transmitters and receivers at the Optical Line Terminal (OLT), novel challenges and opportunities arise for improving the OLT energy efficiency. This paper surveys schemes, currently proposed in the literature, for improving TWDM(A) PON energy efficiency. Then it overviews the main challenges in providing energy efficiency at both the ONU and OLT side. Finally, it provides hints on how to tailor schemes to maximize energy efficiency while complying with the target cost and Quality of Service (QoS) requirements of PONs and user applications.