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Multipath fading is inherent in wireless communication systems. Diversity is the techniques which takes the advantage of multipath and mitigate the effect of fading and increase signal strength. Space Time Block codes (STBC) are used in MIMO systems to improve the performance by maximizing transmit and/or receive diversity. Among different schemes based on STBC, Quasi orthogonal space time block code (QOSTBC) is able to achieve full rate transmission for more than two transmit antennas. Constellation rotation quasi orthogonal space time block code (CR-QOSTBC), a modified QOSTBC improves performance further and gets full diversity along with full rate, to overcome the limitation of QOSTBC, which is unable to maintain orthogonality amongst the codes transmitted by different antennas. Objective of this paper is to observe improvement in BER with reference to number of receive antennas, as higher diversity can be achieved by increasing uncorrelated paths between transmitter and receivers using higher number of receive antennas. Simulations were carried out under ideal as well as realistic environment using least square technique with four antennas at transmitter side and variable receive antennas. Results of simulations presented in this paper indicate performance improvement of CR-QOSTBC over QOSTBC in flat fading channel environment. Simulation results also shows performance degradation in BER when channel is estimated at the receiver.