> Determine the autocorrelation $r_{xx}(l)$ of the discrete signal $$x(n) = (\sin{2\pi fn)}.$$
$n$ and $l$ are obviously integers. 

Using the definition I get
$$r_{xx}(l) = \sum_{n=-\infty}^{\infty}x(n)x(n-l) = \sum_{n=-\infty}^{\infty}\sin{(2\pi fn)\sin{(2\pi f(n-l))}}$$
$$= \sum_{n=-\infty}^{\infty}\sin{(2\pi fn)\sin{(2\pi fn - 2\pi fl))}}$$
but I can't seem to figure it out from here. I've tried using different trigonometric identities without result. I'm guessing it's something simple I'm missing.