  This paper presents an adaptable steganography (information hiding) method
for digital radio communication. Many radio steganography methods exist, but
most are defined at higher levels of the protocol stack and are thus protocol
dependent. In contrast, this method is defined at the physical layer, which
makes it widely applicable regardless of the protocols used at higher layers.
This approach is also adaptive; the covertness of the hidden channel is simple
to control via a single continuous parameter either manually or automatically.
Several variations are introduced, each with performance evaluated by
simulation. Results show this to be a feasible method with a reasonable
trade-off between performance and covertness.
