  In this article, we address the challenges of transmitter-receiver isolation
in \emph{mobile full-duplex devices}, building on shared-antenna based
transceiver architecture. Firstly, self-adaptive analog RF cancellation
circuitry is required, since the capability to track time-varying
self-interference coupling characteristics is of utmost importance in mobile
devices. In addition, novel adaptive nonlinear DSP methods are also required
for final self-interference suppression at digital baseband, since mobile-scale
devices typically operate under highly nonlinear low-cost RF components.
  In addition to describing above kind of advanced circuit and signal
processing solutions, comprehensive RF measurement results from a complete
demonstrator implementation are also provided, evidencing beyond 40~dB of
active RF cancellation over an 80 MHz waveform bandwidth with a highly
nonlinear transmitter power amplifier. Measured examples also demonstrate the
good self-healing characteristics of the developed control loop against fast
changes in the coupling channel. Furthermore, when complemented with nonlinear
digital cancellation processing, the residual self-interference level is pushed
down to the noise floor of the demonstration system, despite the harsh
nonlinear nature of the self-interference. These findings indicate that
deploying the full-duplex principle can indeed be feasible also in mobile
devices, and thus be one potential technology in, e.g., 5G and beyond radio
systems.
