For a single FDR node, its performance is highly determined
by the capability of SI cancellation. Existing SI cancellation
techniques can be roughly grouped into either timedomain
or spatial-domain techniques. Time-domain techniques
are performed in digital/analog hardwares by subtracting the
known SI from the received signal [7] [8], while a residual
self interference (RSI) still remains after cancellation due to
hardware imperfections. On the other research track, multipleinput
multiple-output (MIMO) FDR is proposed to suppress SI
by exploiting different spatial-domain techniques at the relay.
Antenna placement and cross-polarization are investigated in
[9] and [10], respectively. A variety of beamforming-based techniques are proposed in [11]–[14]. By proper design of
the relay’s transmit and/or receive antenna beam patterns, the
intended signal is protected from the overwhelming SI in
orthogonal signal subspaces. It is worth noting that with the
help of massive arrays, such orthogonality can be achieved
with simple beamforming designs [15], despite that the limited
size of a cellular relay node might remain a problem. Recently,
the two parallel research tracks have started to merge as in [16]
[17], where spatial-domain and time-domain SI cancellation
are jointly considered in the system design. This design makes
better use of the spatial dimensions by allowing a low level
of SI received at the relay antennas and further mitigated by
subsequent time-domain SI cancellation modules, such that the
beamformers can be optimized more flexibly to yield a better end-to-end performance.