  We propose a bit-allocation scheme for powerline orthogonal
frequency-division multiplexing (OFDM) that minimizes total transmit energy
subject to total-bit and delay constraints. Multiple delay requirements stem
from different sets of data that a transmitter must time-multiplex and transmit
to a receiver. The proposed bit allocation takes into account the channel
power-to-noise density ratio of subchannels as well as statistic of narrowband
interference and impulsive noise that is pervasive in powerline communication
(PLC) channels. The proposed scheme is optimal with 1 or 2 sets of data, and is
suboptimal with more than 2 sets of data. However, numerical examples show that
the proposed scheme performs close to the optimum. Also, it is less
computationally complex than the optimal scheme especially when minimizing
total energy over large number of data sets. We also compare the proposed
scheme with some existing schemes and find that our scheme requires less total
transmit energy when the number of delay constraints is large.
