This paper studies MIMO relays with non-identical link coherence times, a frequently occurring condition when, e.g., the nodes in the relay channel do not all have the same mobility, or the scatterers around some nodes have different mobility compared with those around other nodes. Despite its practical relevance, this condition, known as coherence diversity, has not been studied in the relay channel. This paper studies the performance of MIMO relays and proposes efficient transmission strategies under coherence diversity. Since coherence times have a prominent impact on channel training, we do not assume channel state is available to the decoder for free; all channel training resources are accounted for in the calculations. A product superposition technique is employed at the source, which allows a more efficient usage of degrees of freedom when the relay and the destination have different training requirements. Varying configurations of coherence times are studied. The interesting case where the different link coherence intervals are not a multiple of each other, and therefore the coherence intervals do not align, is studied. Relay scheduling is combined with the product superposition to obtain further gains in degrees of freedom. The impact of coherence diversity is further studied in the presence of multiple parallel relays.

翻译：本文研究了具有非相同链路相干时间的MIMO中继系统，这种情况在实际中频繁出现，例如中继信道中各节点具有不同移动性，或某些节点周围散射体的移动性与其他节点不同。尽管具有实际意义，这种被称为相干性多样性的条件在中继信道中尚未被研究。本文分析了MIMO中继在相干性多样性下的性能，并提出了高效传输策略。由于相干时间对信道训练有显著影响，我们假设解码器不能免费获取信道状态信息；所有信道训练资源均纳入计算。在源端采用乘积叠加技术，当接收端与目的地具有不同训练需求时，该技术能更有效地利用自由度。研究了不同的相干时间配置，重点分析了不同链路相干区间互不为整数倍、导致相干区间无法对齐的特殊情况。通过将中继调度与乘积叠加相结合，进一步提升了自由度增益。最后，研究了多个并行中继存在时相干性多样性对系统性能的影响。