  This paper aims to provide a comprehensive solution for the design, analysis,
and optimization of a multiple-antenna non-orthogonal multiple access (NOMA)
system for multiuser downlink communication with both time duplex division
(TDD) and frequency duplex division (FDD) modes. First, we design a new
framework for multiple-antenna NOMA, including user clustering, channel state
information (CSI) acquisition, superposition coding, transmit beamforming, and
successive interference cancellation (SIC). Then, we analyze the performance of
the considered system, and derive exact closed-form expressions for average
transmission rates in terms of transmit power, CSI accuracy, transmission mode,
and channel conditions. For further enhancing the system performance, we
optimize three key parameters, i.e., transmit power, feedback bits, and
transmission mode. Especially, we propose a low-complexity joint optimization
scheme, so as to fully exploit the potential of multiple-antenna techniques in
NOMA. Moreover, through asymptotic analysis, we reveal the impact of system
parameters on average transmission rates, and hence present some guidelines on
the design of multiple-antenna NOMA. Finally, simulation results validate our
theoretical analysis, and show that a substantial performance gain can be
obtained over traditional orthogonal multiple access (OMA) technology under
practical conditions.
