Abstract: This paper investigates the effect of antenna diversity for a double transmit and multiple receive antenna supported wireless communication system that employs single user Alamouti’s space time block coding (STBC) and maximal ratio combining (MRC) scheme on secured text message transmission. The FEC encoded Alamouti-MRC transmission system under investigation implements RSA cryptographic algorithm and deploys various multi-level digital modulations (16- PSK, 16-DPSK and 16-QAM) techniques over an Additive White Gaussian Noise (AWGN) and Rayleigh Fading Channels. It has been observed from the study that in case of without receive antenna diversity the system shows comparatively worst performance in 16-DPSK scheme and satisfactory performance in 16-QAM. It is noticeable that the system performance is improved with increase in number of receive antenna. The performance analysis shows that with implemented Alamouti-MRC scheme (4 receive antenna) under 16-QAM digital modulation, the system provides excellent performance over a significant low signal to noise ratio(SNR) values.