  In this paper an exact baseband equivalent model of a quadrature modulated
all-digital transmitter is derived. No restrictions on the number of levels of
a digital switched-mode power amplifier (SMPA) driving input, nor the pulse
encoding scheme employed, are made. This implies a high level of generality of
the proposed model. We show that all-digital transmitter (ADT) can be
represented as a series connection of the pulse encoder, discrete-time Volterra
series model of fixed degree and memory depth, and a linear time-varying system
with special properties. This result suggests a new analytically motivated
structure of a digital predistortion (DPD) of SMPA nonlinearities in ADT.
Numerical simulations in MATLAB are used to verify proposed baseband equivalent
model.
