How can I optimize this code?

Good day to all! I have a snippet of code that is called very often. Since it is described through for loops, it is too slow. Are there any techniques to improve this part?

j = 1;
k = 1;
for l1 = 0:a
	for m1 = -l1:l1
        l2 = abs(m1);
            if(l2 <= b)
                for l2 = abs(m1):b
                    n = l2 * (l2 + 1) + m1 + 1;
                    d(j) = d(j) + G(k) * t(n);
                    k = k + 1;
                end
            else
                k = k + 1;
            end
            if (l2 <= c) 
                k = k + c - l2; 
            end
        j = j + 1;
	end
end