Hi Tayyab Mehmood,
Here i am posting the link in which Damian Marek and Dhiman Kakati sir have discussed about error-vector-magnitude. As we also know Error vector magnitude (EVM) is a measure of modulation quality and error performance in complex wireless systems. Dhiman has discussed relationship between error-vector-magnitude and BER.
http://staging.optiwave.com/forums/topic/error-vector-magnitude/