  The challenge to in-band full-duplex wireless communication is managing
self-interference. Many designs have employed spatial isolation mechanisms,
such as shielding or multi-antenna beamforming, to isolate the
self-interference wave from the receiver. Such spatial isolation methods are
effective, but by confining the transmit and receive signals to a subset of the
available space, the full spatial resources of the channel be under-utilized,
expending a cost that may nullify the net benefit of operating in full-duplex
mode. In this paper we leverage an antenna-theory-based channel model to
analyze the spatial degrees of freedom available to a full-duplex capable base
station, and observe that whether or not spatial isolation out-performs
time-division (i.e. half-duplex) depends heavily on the geometric distribution
of scatterers. Unless the angular spread of the objects that scatter to the
intended users is overlapped by the spread of objects that backscatter to the
base station, then spatial isolation outperforms time division, otherwise time
division may be optimal.
