fsp is the cell array, and fsp{1,1} is the vector contained in the first element (first cell) of fsp.
You are taking the length of the vector fsp{1,1}, divided by 2, which is 8192, and using that to try to index into the cell array fsp, which apparently has 3 elements (three cells).
If you want sp{1,1} to contain the first half of the vector fsp{1,1}, do this:
sp{1,1} = fsp{1,1}(1:numel(fsp{1,1})/2);
where I've used numel instead of length for clarity, but that does not affect the behavior if fsp{1,1} is a vector.
Or more simply:
sp{1,1} = fsp{1,1}(1:end/2);
Also, in case the vector fsp{1,1} had an odd number of elements, the above would give you a warning since numel(fsp{1,1})/2 (or length(fsp{1,1})/2) would not be an integer, so to handle that case as well, you could do:
sp{1,1} = fsp{1,1}(1:floor(end/2)); % don't include the middle element in case there's an odd number of elements
or:
sp{1,1} = fsp{1,1}(1:ceil(end/2)) % do include the middle element in case there's an odd number of elements
depending on how you want it to work.
A concrete example:
fsp = {1:7};
fsp{1,1}(1:end/2)
fsp{1,1}(1:floor(end/2))
fsp{1,1}(1:ceil(end/2))
