Multi-User Pilot-Domain NOMA Under Coherence Disparity and Channel State Feedback

Non-orthogonal transmission of data and pilots using product superposition is known to be highly efficient under unequal coherence conditions in a downlink channel, improving achievable rates and degrees of freedom (DoF). However, these techniques have not been used when transmit beamforming is present, as product superposition measures and utilizes composite (virtual) link gains, while beamforming requires knowledge of true (physical) link gains at the transmitter. This paper presents new techniques that enable the gains of pilot-domain non-orthogonal multiple access (NOMA) product superposition to be combined with transmit beamforming gain. The technical novelty of this paper lies in reconciling the requirements of transmit beamforming and product superposition under perfect or imperfect channel state feedback, and demonstrating its effectiveness under multi-user scenarios. The paper begins with a multi-user generalization of product superposition rate analysis in the absence of feedback. Then, a novel non-orthogonal scheme is proposed that harmoniously combines with either perfect or imperfect feedback under disparity in coherence time or coherence bandwidth among users. The proposed scheme includes efficient pilot placement strategies under multi-user scenarios with arbitrary coherence time and coherence bandwidth for different users. Numerical results illustrate the effectiveness of the proposed techniques.