  Recent research results have demonstrated the feasibility of full-duplex
wireless communication for short-range links. Although the focus of the
previous works has been active cancellation of the self-interference signal, a
majority of the overall self-interference suppression is often due to passive
suppression, i.e., isolation of the transmit and receive antennas. We present a
measurement-based study of the capabilities and limitations of three key
mechanisms for passive self-interference suppression: directional isolation,
absorptive shielding, and cross-polarization. The study demonstrates that more
than 70 dB of passive suppression can be achieved in certain environments, but
also establishes two results on the limitations of passive suppression: (1)
environmental reflections limit the amount of passive suppression that can be
achieved, and (2) passive suppression, in general, increases the frequency
selectivity of the residual self-interference signal. These results suggest two
design implications: (1) deployments of full-duplex infrastructure nodes should
minimize near-antenna reflectors, and (2) active cancellation in concatenation
with passive suppression should employ higher-order filters or per-subcarrier
cancellation.
