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Interaction of Pilot Reuse and Channel State Feedback under Coherence Disparity

When the individual links in a downlink channel have different coherence intervals, previous studies have shown that pilot reuse can achieve not only rate gains, but also gains in degrees of freedom (DoF). Channel state feedback is another source of gains, but combining beamforming with pilot reuse presents new and interesting design questions. The performance of such a scheme has been an open problem under coherence disparity, the regime in which pilot reuse is most promising. We propose a new non-orthogonal transmission scheme for pilots and data that harmoniously combines with either perfect or imperfect channel state feedback. The proposed scheme employs both product superposition and zero-forcing beamforming within the same framework, and improves the resulting achievable rates. The developments include careful pilot placement and an efficient pilot reuse strategy under channel state feedback in a multi-user downlink channel. Numerical results are presented to corroborate our findings.