  The application of multiple-input multiple-output (MIMO) techniques to
non-orthogonal multiple access (NOMA) systems is important to enhance the
performance gains of NOMA. In this paper, a novel MIMO-NOMA framework for
downlink and uplink transmission is proposed by applying the concept of signal
alignment. By using stochastic geometry, closed-form analytical results are
developed to facilitate the performance evaluation of the proposed framework
for randomly deployed users and interferers. The impact of different power
allocation strategies, such as fixed power allocation and cognitive radio
inspired power allocation, on the performance of MIMO-NOMA is also
investigated. Computer simulation results are provided to demonstrate the
performance of the proposed framework and the accuracy of the developed
analytical results.
