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Coherence Disparity in Broadcast and Multiple Access Channels

机译：广播和多路访问信道中的相干差异

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Individual links in a wireless network may experience unequal fading coherence times due to differences in mobility or scattering environment. This paper studies broadcast and multiple access channels whose nodes experience unequal fading block lengths. Channel state information (CSI) is not available at the transmitters, and the cost of acquiring CSI at the receivers is fully accounted for in the degrees of freedom. In the broadcast channel, the method of product superposition is employed to find the achievable degrees of freedom. When the number of symbols in any fading block is at least twice the number of antennas at any active node and the fading block lengths have integer ratios, achievable degrees of freedom meet the upper bound in four cases: when the transmitter has fewer antennas than the receivers, when all receivers have the same number of antennas, when the coherence time of one receiver is much shorter than all others, or when all receivers have identical block fading length. The degrees of freedom region of the broadcast under identical coherence times was also previously unknown and is settled by the results of this paper. The disparity of coherence times leads to gains that are distinct from those arising from other techniques, such as spatial multiplexing or multiuser diversity. This new class of gains is denoted coherence diversity. The inner bounds in the broadcast channel are further extended to fading block lengths of arbitrary ratio or alignment. In addition, in the multiple access channel with unequal coherence times, achievable and outer bounds on the degrees of freedom are obtained.
机译：由于移动性或散射环境的差异，无线网络中的各个链路可能会经历不平等的衰落相干时间。本文研究了广播和多路访问信道，它们的节点经历了不相等的衰落块长度。信道状态信息（CSI）在发送器上不可用，并且在接收器上获取CSI的成本完全由自由度解决。在广播频道中，采用产品叠加的方法来找到可实现的自由度。当任何衰落块中的符号数至少是任何活动节点上天线数的两倍，并且衰落块长度具有整数比时，在以下四种情况下可实现的自由度达到上限：当发射机的天线数少于天线数时。当所有接收器具有相同数量的天线时，或者当一个接收器的相干时间比所有其他接收器的相干时间短得多时，或者当所有接收器具有相同的块衰落长度时，这些接收器便成为接收器。相同相干时间下广播的自由度区域也是未知的，并由本文的结果解决。相干时间的差异导致了与其他技术（例如空间复用或多用户分集）所产生的收益截然不同的收益。这类新的增益称为相干分集。广播频道中的内部边界进一步扩展为任意比率或对齐方式的衰落块长度。另外，在相干时间不相等的多路访问信道中，获得了自由度上可实现的范围和外部范围。

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