# Homework Help: Determine the unit-step response of the discrete-time LTI systems

1. Mar 24, 2013

### Mr.Tibbs

Determine the unit-step response of the discrete-time LTI
systems described by the following impulse responses:

h[n]=(0.9)$^{n}$e$^{j(\pi/2)n}$u[n]

So I am completely confused. . . I don't even know how to start. . . I want to say that I need to do a summation but the more examples and text I look up the more I'm in the dark. . . any help is appreciated.

The only thing I can think to start is you assume

x[n] = u[n]

2. Mar 24, 2013

### rude man

3. Mar 25, 2013

### Mr.Tibbs

My apologies for taking so long to reply. I was able to talk to my professor and this is what I have now.

$\sum$(0.9)$^{k}$e$^{j(\pi/2)k}$u[n-k] from k = -∞ to ∞.

this turns into

$\sum$(0.9)$^{k}$e$^{j(\pi/2)k}$ from k = 0 to n.

using the property of summation :

$\sum$ a$^{k}$ = $\frac{1-a^{n+1}}{1-a}$

my new snag is what do I define as a?