  This article addresses the modeling and cancellation of self-interference in
full-duplex direct-conversion radio transceivers, operating under practical
imperfect radio frequency (RF) components. Firstly, detailed self-interference
signal modeling is carried out, taking into account the most important RF
imperfections, namely transmitter power amplifier nonlinear distortion as well
as transmitter and receiver IQ mixer amplitude and phase imbalances. The
analysis shows that after realistic antenna isolation and RF cancellation, the
dominant self-interference waveform at receiver digital baseband can be modeled
through a widely-linear transformation of the original transmit data, opposed
to classical purely linear models. Such widely-linear self-interference
waveform is physically stemming from the transmitter and receiver IQ imaging,
and cannot be efficiently suppressed by classical linear digital cancellation.
Motivated by this, novel widely-linear digital self-interference cancellation
processing is then proposed and formulated, combined with efficient parameter
estimation methods. Extensive simulation results demonstrate that the proposed
widely-linear cancellation processing clearly outperforms the existing linear
solutions, hence enabling the use of practical low-cost RF front-ends utilizing
IQ mixing in full-duplex transceivers.
