Sony’s (NYSE: SNE) CEO Howard Stringer was quoted as saying in a German newspaper on today that its mobile-phone joint venture with Ericsson (NSDQ: ERIC) must do better, Reuters reported. “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. Asked whether Sony is planning to put an end to Sony Ericsson, Stringer said in German paper Die Welt: “It’s certainly been a difficult year but buying out a partner is never an easy thing.”

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Sony Ericsson, which is often being coined ‘the next Motorola’ (NYSE: MOT) for its troubles, reported last quarter that profit plunged 97 percent while its biggest market — Europe — is seeing phone sales fall, and mid-high end handset margins are coming down.

Stringer said one of the complexities of the business is being nimble while still checking in with two different companies. “You’re always engaged in discussion and negotiation. We and Ericsson are always discussing ways of making Sony Ericsson as successful as it possibly can be.”