On the Construction of Polar Codes in Two-User Downlink NOMA with Power Allocation and Constellation-Rotation

We consider the joint construction of polar codes and power allocation based on rotated QPSK constellation in the two-user downlink non-orthogonal multiple access (NOMA) system. Our work is suitable for IoT-like networks where lightweight devices employ simple constellations with low data rate traffic, and the traditional coding performance analysis assuming Gaussian inputs and infinite blocklength may become less accurate. With the goal of achieving fairness and minimizing the maximum block-error-rate (BLER) among the NOMA users, the construction of polar codes, which takes into consideration the BLER estimated by the Gaussian-approximation-based density evolution (GA-DE) method, is jointly determined with the constellation-rotation angle and the power allocation over the NOMA users. Simulations verify that the proposed approach yields generally comparable BLER performance as the LDPC codes over the range of SNR considered while having shorter blocklength.