On generalized downlink beamforming with NOMA

Recently, nonorthogonal multiple access (NOMA) has been studied to increase the spectral efficiency of downlink in a multiuser system by exploiting the notion of superposition coding with successive interference cancellation (SIC). NOMA can be employed with downlink beamforming for downlink transmissions from a base station (BS) equipped with an antenna array, which results in NOMA beamforming. In this paper, we formulate a multiuser NOMA beamforming problem as a semidefinite programming (SDP) problem and generalize it in order to include the conventional (multiuser) beamforming. A low-complexity approach to decide SIC sets for the generalized NOMA beamforming is studied using the correlation between channel vectors for better performance. From analysis and simulation results, we show that the (generalized) NOMA beamforming can outperform the conventional beamforming, especially under limited scattering environments.