MIMO (Multiple-input Multiple-output) systems are used in wireless communications to ensure high reliability, high data-rate & high capacity transmission. STBCs (Space-Time Block Coding) used in MIMO systems for increasing diversity gain. In MIMO system, high-data rates are obtained through Spatial multiplexing which also helps in combatting the effect of multipath fading. For SIMO (Single-input Multiple-output) schemes, MRC receiver is used to handle same information signal over two or more fading channel. In this paper, we investigate the Ergodic Capacity of different MIMO & SIMO systems & comparison of Bit Error Rate performance of un-coded coherent BPSK employing Alamouti STBC as transmit diversity & MRC (Maximal Ratio combining) as receiving diversity in Rayleigh fading channel.