The unit step function can be represented as sum of shifted unit impulses.

The total response of the system is referred to as the CONVOLUTION SUM or superposition sum of the sequences x[n] and h[n]. The result is more concisely stated as y[n] = x[n] * h[n].

The convolution sum is realized as follows

1. Invert h[k] about k=0 to obtain h[-k].

2. The function h[n-k] is given by h[-k] shifted to the right by n (if n is positive) and to the left (if n is negative) (note the sign of the independent variable).

3. Multiply x[k] and h[n-k] for same coordinates on the k axis. The value obtained is the response at n i.e. Value of y[n] at a particular n the value chosen in step 2.