  Recent experimental results have shown that full-duplex communication is
possible for short-range communications. However, extending full-duplex to
long-range communication remains a challenge, primarily due to residual
self-interference even with a combination of passive suppression and active
cancellation methods. In this paper, we investigate the root cause of
performance bottlenecks in current full-duplex systems. We first classify all
known full-duplex architectures based on how they compute their cancelling
signal and where the cancelling signal is injected to cancel self-interference.
Based on the classification, we analytically explain several published
experimental results. The key bottleneck in current systems turns out to be the
phase noise in the local oscillators in the transmit and receive chain of the
full-duplex node. As a key by-product of our analysis, we propose signal models
for wideband and MIMO full-duplex systems, capturing all the salient design
parameters, and thus allowing future analytical development of advanced coding
and signal design for full-duplex systems.
