Fadel Shady Mohamed, Nosratinia Aria
Department of Electrical & Computer Engineering, The University of Texas at Dallas, Richardson, TX 75080, USA.
Entropy (Basel). 2020 Sep 1;22(9):976. doi: 10.3390/e22090976.
The broadcast channel may experience unequal link coherence times due to a number of factors including variation in node mobility or local scattering conditions. This means the block fading model for different links may have nonidentical block length, and the channel state information for the links may also not be identical. The faster the fading and the shorter the fading block length, the more often the link needs to be trained and estimated at the receiver, and the more likely that channel state information (CSI) is stale or unavailable at the transmitter. This paper investigates a MISO broadcast channel where some receivers experience longer coherence intervals and other receivers experience shorter coherence intervals and must estimate their receive-side CSI (CSIR) frequently. We consider a variety of transmit-side CSI (CSIT) conditions for the abovementioned model, including no CSIT, delayed CSIT, or hybrid CSIT. To investigate the degrees of freedom region, we employ interference alignment and beamforming along with a product superposition that allows simultaneous but noncontaminating transmission of pilots and data to different receivers. Outer bounds employ the extremal entropy inequality as well as a bounding of the performance of a discrete, memoryless, multiuser, broadcast channel. For several cases, inner and outer bounds are established that either partially meet, or the gap diminishes with increasing coherence times.
由于包括节点移动性变化或局部散射条件在内的多种因素，广播信道可能会经历不等的链路相干时间。这意味着不同链路的块衰落模型可能具有不同的块长度，并且链路的信道状态信息也可能不相同。衰落越快且衰落块长度越短，接收机就越需要频繁地对链路进行训练和估计，并且发射机处的信道状态信息（CSI）就越有可能过时或不可用。本文研究了一种多输入单输出（MISO）广播信道，其中一些接收机经历较长的相干间隔，而其他接收机经历较短的相干间隔，并且必须频繁估计其接收端CSI（CSIR）。对于上述模型，我们考虑了多种发射端CSI（CSIT）条件，包括无CSIT、延迟CSIT或混合CSIT。为了研究自由度区域，我们采用干扰对齐和波束成形以及乘积叠加，该叠加允许向不同接收机同时但无污染地传输导频和数据。外界采用极值熵不等式以及离散、无记忆、多用户广播信道性能的界定。对于几种情况，建立了内界和外界，它们要么部分重合，要么随着相干时间的增加差距减小。
