On Optimizing Transmit Power, Channel-Quantization Rate, and User Pairing for Power-Domain MIMO-NOMA Downlinks

We consider power-domain multiple-input multiple-output (MIMO) nonorthogonal multiple access (NOMA) down-links in which a multiple-antenna base station applies zeroforcing beamforming to transmit data to single-antenna users. Two users whose channel correlation coefficient exceeds a threshold are paired to share the same transmit beam. We derive an approximate of the expected signal-to-interference plus noise ratio (SINR) for each user, which depends on transmit power, the correlation threshold, and the number of quantization bits for channel direction information (CDI). A max-min SINR problem is formulated as a geometric program. We show that a near-optimal SINR can be achieved by optimizing only two CDI rates: one for unpaired users and the other for user pairs. A large SINR gain can be achieved by optimizing a pairing threshold when a total CDI rate is high.