Using MIMO with DS-CDMA and spread spectrum
Assuming one antenna transmits one symbol per time unit, then 16 symbols require 4 time units to be out. Then it is simply that

r_1 = H_1 * x_1(1:4)
r_2 = H_2 * x_1(5:8)
r_3 = H_3 * x_1(9:12)
r_4 = H_4 * x_1(13:16)


If channel H is fixed during these 4 time units,

[r_1 r_2 r_3 r_4] = H * [x_1(1:4) x_1(5:8) x_1(9:12) x_1(13:16)];


or

r = reshape(H*reshape(x_1,4,[]),[],1);


This can be generalized to any MIMO size and to any spreading size.