说明：  mimo下行的线性预编码，分别基于ZF准则和MMSE准则,分别对基于zf准则的线性预编码和基于mmse准则的线性预编码进行比较
(mimo downlink linear precoding, respectively, based on the ZF criterion and the MMSE criterion, respectively zf criteria based linear precoding and the linear pre-coding based on mmse criterion compare)