Sony's chief executive was quoted in a German newspaper on Wednesday as saying that the company's joint venture with cell phone maker Ericsson must do better, Reuters reports.

"It's certainly been a difficult year but buying out a partner is never an easy thing," CEO Howard Stringer said in Die Welt in response to a question on whether or not the companies could go their separate ways.

"We have to work together again as we did two years ago. Or the joint venture will have to find its own solution," Stringer said.

Sony Ericsson isn't doing well in the U.S. at all, although it fares much better overseas. Recently Sony finally added T-Mobile as a carrier partner, after years of staying exclusive with AT&T—an obvious move given how the company's GSM handsets work with virtually no modification on either network.