"My query is what does the term stacking indicates..."
MATLAB does not use the term "stacking" to refer to these kinds of operations, so you would have to ask the author of that paper. Some possibly related terms in MATLAB: concatenation, reshape, permute, transpose...
"...and how does the dimension of "
As far as I can tell, the author is simply vertically concatenting some 1x2N vectors into an Lx2N matrix.
Here are three ways to concatenate some 1x2N vectors vertically into an Lx2N matrix:
[d1;d2;d3]
cat(1,d1,d2,d3)
vertcat(d1,d2,d3)
Instead of directly concatenating them vertically, the author seems to concatenate their transposes horizontally and then transposes the entire resulting matrix. You would have to ask its author why.
