我需要解释以下代码行。每行代码的目的是什么？该代码用于MIMO OFDM 2 * 2信道
% Simulate freq.-domain channels for 2x2 links
HA1 = randn(64,1) + randn(64,1) * i;
HA1 = HA1 ./ abs(HA1) * sqrt(Prx);
HB1 = randn(64,1) + randn(64,1) * i;
HB1 = HB1 ./ abs(HB1) * sqrt(Prx);
HA2 = randn(64,1) + randn(64,1) * i;
HA2 = HA2 ./ abs(HA2) * sqrt(Prx);
HB2 = randn(64,1) + randn(64,1) * i;