  The fundamental problem in the design of a full-duplex radio is the
cancellation of the self-interference (SI) signal generated by the
transmitter.Current techniques for suppressing SI rely on generating a copy of
the SI signal and subtracting it partly in the RF (radio frequency) and digital
domains. A critical step in replicating the self-interference is the estimation
of the multi-path channel through which the transmitted signal propagates to
the antenna. Since there is no prior model on the number of multipath
reflections, current techniques assume a tap delay line filter (in the RF and
digital domain) with a large number of taps, and estimate the taps in the
analog and the digital domain. Assuming such a model leads to a large
form-factor for the analog and RF circuits and increased complexity in the
digital domain.
  In this paper, using a linearization technique, we show that the
self-interference channel in an indoor environment can be effectively modelled
as $H(f)=C_0 + C_1f$ in the frequency domain. Thus, the effective
self-interference channel can be represented by two parameters $C_0$ and $C_1$,
irrespective of the multipath environment. We also provide experimental
evidence to verify the above channel model and propose novel low-complexity
designs for self-interference cancellation. Linearization not only aids in the
practicality of analog cancellation by reducing the form factor, but also
results in a simpler SI filter model in the digital domain due to
dimensionality reduction of the channel parameters. Therefore this method can
enable the widespread adoption of full-duplex techniques to portable devices in
addition to infrastructure base-stations.
