Optimal Feedback Allocation for Zeroforcing Beamforming Transmission in Downlink NOMA

We consider non-orthogonal multiple access (NOMA) downlink channels with zeroforcing beamforming transmission. With limited feedback rate, channel direction information (CDI) needs to be quantized and fed back from mobile users to a base station. To increase spectral efficiency, two active users in some clusters share the same beamforming vector and thus, will interfere fully with each other. We derive the approximate achievable rates and outage probability for users in 1-user and 2-user clusters. With the approximate outage probability derived as a function of feedback rate, we find the optimal feedback allocation for both types of cluster that minimizes the maximum outage probability for all users. Numerical results show that significant gain over uniform feedback allocation is achieved in moderate-to-large SNR regimes.