Relationship between Carrier Frequency Offset and the Constellation Diagram
I think that you misunderstand the problem. The actual problem is that the frequency offset is unknown to the receiver. I won't give away the answer since this appears to be a homework project, but I'll try to help you by giving a hint. First, imagine what would happen if there was an unknown carrier phase offset at the receiver. How would this affect the constellation? If you know the answer to that question, then think of a carrier frequency offset as a continuing change in phase offset. So how would this affect the constellation?