lz应多看看协议。36.213和36.212中讲得很明确：
36.213中明确tm8，PDCCH为DCI 2B时，为antenna port 7 and 8，或者antenna port7，或者antenna port 8
36.212中DCI 2B介绍中有一段话：
In case one of the transport blocks is disabled, the number of layers equals one; the transport block to codeword mapping is specified according to Table 5.3.3.1.5 2; and the antenna port for single-antenna port transmission is according to Table 5.3.3.1.5B-1.
提示一下，自己再研究研究。