A NOMA Scheme for a Two-user MISO Downlink Channel with Unknown CSIT

Power-domain non-orthogonal multiple access (NOMA) for 5G essentially relies on the availability of the channel state information (CSI) at the transmitter. Feeding back the CSI from the users to the transmitter is clearly bandwidth consuming. In addition, when the CSI is unavailable at the transmitter (CSIT), NOMA techniques become inapplicable, and in such scenario, allocating power among users blindly, including equal power splitting, has been shown to yield poor probability of error performance. To this end, we develop a NOMA technique that does not require CSI knowledge at the transmitter, i.e., with unknown CSIT. The proposed technique allows the transmitter to communicate with multiple users simultaneously while keeping signals perfectly separable at their respective receivers. We apply the proposed technique to a two-user multiple-input single-output (MISO) two-user downlink channel with unknown CSI where the available degree of freedom (DoF) is one. We show that it is possible to allocate 1/2 DoF to each user, implying that the received signals can be perfectly separated at the receiver, i.e., without interference. We analyze the performance of the proposed scheme in terms of the achievable DoF per user and the symbol error rate (SER). We present numerical examples to validate the efficacy of the proposed scheme as a NOMA technique for 5G systems, and compare its performance in terms of the SER to that of existing schemes and demonstrate its superiority.