  We develop a low-complexity coding scheme to achieve covert communications
over binary-input discrete memoryless channels (BI-DMCs). We circumvent the
impossibility of covert communication with linear codes by introducing
non-linearity through the use of pulse position modulation (PPM) and multilevel
coding (MLC). We show that the MLC-PPM scheme exhibits many appealing
properties; in particular, the channel at a given index level remains
stationary as the number of level increases, which allows one to use families
of channel capacity- and channel resolvability-achieving codes to concretely
instantiate the covert communication scheme.
