I've been working on document level sentiment analysis over the last year. Document level sentiment analysis provides the sentiment of the complete document. For example - The text "Nokia is good but vodafone sucks big time" would have a negative polarity associated with it as it would be agnostic to the entities Nokia and Vodafone.
How would it be possible to get entity level sentiment, like positive for Nokia but negative for Vodafone? Are there any research papers providing a solution to such problems ?