  Design of energy-efficient access networks has emerged as an important area
of research, since access networks consume $80-90\%$ of the overall Internet
power consumption. TWDM-PON is envisaged to be one of the widely accepted
future access technologies. TWDM-PON offers an additional opportunity to save
energy at the OLT along with the existing energy-efficient ONU design. In this
paper, we focus on the energy-efficient OLT design in a TWDM-PON. While most of
the conventional methods employ a minimization of the number of wavelengths, we
propose a novel approach which aims at minimizing the number of voids created
due to scheduling. In the process, for the first time, we present a
low-complexity on-line scheduling algorithm for the upstream traffic
considering delay constraints. Our extensive simulations demonstrate a
significant improvement in energy efficiency of $\sim 25\%$ for high load at
the OLT receivers. Furthermore, we provide an analytical upper-bound on the
energy-efficiency of the OLT receivers and demonstrate that the proposed
protocol achieves an energy efficiency very close to the bound with a maximum
deviation $\sim 2\%$ for $64$ ONUs.
