Board Thread:Naruto Discussions/@comment-24347194-20150212224652/@comment-26010916-20150212225620

Sasuke received a rinnegan due to him being the transmigrate of Indra, leading the Uchiha to contact Hagoromo so that he could activate this power from within him.

I'm assuming that's why Sasuke received a Rinnegan which behaves and acts like a Sharingan, but with Rinnegan properties/abilities as-well. As you can see even though Sasuke has only one eye, it is as strong as Madara who had two normal rinnegan eyes. So it's safe to say it's a combination of both Rinnegan and Sharingan, due to Hagoromo's influence of course.