关于degraded channel和non-degraded channel

广播信道下的问题
看了信息论(by Thomas M.Cover)上的定义是
p(y1,y2|x)=p(y1|x)p(y2|y1)
x为发射信号，y1,y2为接收信号；
但是在国内网站上搜索了一下看到的是
当发送端只有一个天线时，MIMO BC 信道是一个高斯退化信道，但发送端有多个天线时，信道将不再是退化的，
还有的论坛解释为MIMO 信道缺秩状态下就是退化的，
到底该怎么理解退化信道呢？

Nihar Jindal says: A broadcast channel is degraded if users can be ordered in terms of the quality of received signal. However, the multiple transmit antenna broadcast channel is in general (but no always) nondegraded. An intuitive explanation is that matrices can only be partially ordered, as opposed to the full ordering possible on scalars.