38211的7.1.4.3节定义了PDCCH DMRS的时频资源。从这个定义可以看出,PDCCH的DMRS频域上映射在PDCCH所在每一个RB的子载波1,5,9,13…..,每4个子载波映射一个DMRS,位置固定;时域上映射在每一个PDCCH符号上(The quantity l is the OFDM symbol number within the slot.)
PUCCH DMRS
NR中的PUCCH又5种format,每种format的DMRS映射方式是不同的。
这里推荐一篇IEEE的论文<Physical Uplink Control Channel Design for 5G New Radio>,里面详细介绍了每种pucch format采用不同DMRS映射的仿真结果和分析,本篇文章也会引用其中的一些图片和结论。
PUCCH Format 0: format 0 是一种sequence,没有DMRS。论文当中提供了最初format 0的两种候选structure,一种是基于sequence的,一种是基于DMRS的。最后选择了使用基于sequence的format 0。
Both DMRS and sequence based structures were thoroughly studied and based on the BER performance comparison between these two structures [7], sequence based design was adopted for PF0 in 3GPP Release 15 specification of NR From Fig. 4, it is evident that under large delay spread scenario, the sequence based structure marginally outperforms DMRS based structure in terms of BER of missed ACK
PUCCH Format 1: Format 1采用的是TDM的方式映射DMRS。映射的符号协议有明确规定(l=0,2,4,…)。下面是一个长度为7个符号的的PUCCH Format 1的映射结果。
PUCCH Format 2: Format 2采用了FDM的方式映射DMRS。频域上映射的位置协议有规定(k = 3m + 1)。文章中对频域映射不同密度DMRS的方案进行了仿真和对比。最后的结论是1/3密度的方案最佳,且format2主要用于FR2。
Based on the link level simulation results, the DMRS with evenly distributed pattern and 1/3 overhead was adopted for PF2 in NR
PUCCH Format 3/4: Format 3/4采用TDM的方式映射DMRS。时域上映射的位置协议有规定,如下表所示。下面给出了一个长度为10个符号的 Format3/4的映射示例。
The position(s) of the DM-RS symbols is given by l and duration where
for PDSCH mapping type A, is the duration is between the first OFDM symbol of the slot and the last OFDM symbol of the scheduled PDSCH resources in the slot
for PDSCH mapping type B, is the duration is the number of OFDM symbols of the scheduled PDSCH resources
上面这个例子是一个4个DMRS port的例子,可以提供4个正交的DMRS序列,对应最大的PDSCH传输为4 layer。如果想提供更强大的MIMO能力,比如8 layer或者12 layer,那么就需要double symbol以及DMRS type 2的配置方式。下表是一个DMRS port数量与single/doubleDMRS以及configuration type1/2的关系总结。