  Incorporating full duplex operation in Multiple Input Multiple Output (MIMO)
systems provides the potential of boosting throughput performance. However, the
hardware complexity of the analog self-interference canceller in emerging full
duplex MIMO designs mostly scales with the number of transmit and receive
antennas, thus exploiting the benefits of analog cancellation becomes
impractical for full duplex MIMO transceivers, even for moderate number of
antennas. In this paper, we present two novel architectures for the analog
canceller comprising of reduced number of cancellation elements, compared to
the state of the art, and simple multiplexers for efficient signal routing
among the transmit and receive radio frequency chains. One architecture is
based on analog taps (tap refers to a line of fixed delay, variable phase
shifter, and attenuator) and the other on AUXiliary (AUX) Transmitters (TXs)
that locally generate the cancellation signal. In contrast to the available
analog cancellation architectures, the values for each tap or each AUX TX and
the configuration of the multiplexers are jointly designed with the digital
transmit and receive beamforming filters according to certain performance
objectives. Focusing on a narrowband flat fading channel model as an example,
we present a general optimization framework for the joint design of analog
self-interference cancellation and digital beamforming. We also detail the sum
rate optimization objective together with its derived solution for the latter
architectural components. Representative computer simulation results
demonstrate the superiority both in terms of hardware complexity and achievable
performance of the proposed low complexity full duplex MIMO schemes over the
lately available ones.
