Abstract: This paper is aimed to study the impact of mutual coupling, matching networks and polarization of antennas on Bit-Error-Ratio (BER) performance of MIMO systems employing spatial multiplexing based Bell lab LAyered Space-Time (BLAST) coding technique. An accurate signal analysis framework based on circuit network parameters is presented to describe the transmit/receive characteristics of the matched/unmatched antenna array, and Monte-Carlo numerical simulation are used to study the BER performance of spatial multiplexing MIMO systems. The studied arrays consist of matched/unmatched compact array and polarization diversity array. The simulation results demonstrate that the use of matching networks can reduce the BER of spatial multiplexing MIMO systems, and the BER performance deteoriation can also be compensated by use of polarization diversity antenna arrays.