  This paper considers the implementation and application possibilities of a
compact full duplex multiple-input multiple-output (MIMO) architecture where
direct communication exists between users, e.g., device-to-device (D2D) and
cellular link coexisting on the same spectrum. For the architecture of the
compact full duplex radio, we combine an analog self-interference canceler
based dual-polarization with high cross-polarization discrimination (XPD) and
Long Term Evolution (LTE)-based per-subcarrier digital self-interference
canceler. While we consider the compactness and power efficiency of an analog
solution, we focus on the digital canceler design with robustness to a
frequency-selective channel and high compatibility with a conventional LTE
system. For an over-the-air wireless experiment of full duplex testbed with a
two-user-pair, we implement a full duplex MIMO physical layer (PHY), supporting
20 MHz bandwidth, on an FPGA-based software-defined radio platform. Further, we
propose a novel timing synchronization method to construct a more viable full
duplex MIMO link. By having the full duplex link prototype fully operating in
real-time, we present the first characterization of the proposed compact full
duplex MIMO performance depending on the transmit power of the full duplex
node. We also show the link quality between nodes. One of the crucial insights
of this work is that the full duplex operation of a user is capable of
acquiring the throughput gain if the user has self-interference capability with
guaranteed performance.
