  We consider baseband equivalent representation of transmission circuits, in
the form of a nonlinear dynamical system $\mathbf S$ in discrete time (DT)
defined by a series interconnection of a phase-amplitude modulator, a nonlinear
dynamical system $\mathbf F$ in continuous time (CT), and an ideal demodulator.
We show that when $\mathbf F$ is a CT Volterra series model, the resulting
$\mathbf S$ is a series interconnection of a DT Volterra series model of same
degree and memory depth, and an LTI system with special properties. The result
suggests a new, non-obvious, analytically motivated structure of digital
pre-compensation of analog nonlinear distortions such as those caused by power
amplifiers in digital communication systems. The baseband model and the
corresponding digital compensation structure readily extend to OFDM modulation.
MATLAB simulation is used to verify proposed baseband equivalent model and
demonstrate effectiveness of the new compensation scheme, as compared to the
standard Volterra series approach.
