Employing MIMO

The use of multiple antennas at the transmitter and receiver in wireless systems, popularly known as MIMO (multiple input multiple output) technology, has rapidly gained in popularity over the past decade due to its powerful performance enhancing capabilities. It offers a number of benefits that enables to meet the challenges posed by both the resource constraints and the impairments in wireless communication. In addition to the time and frequency dimension that are exploited in single-antenna system, multiple antennas exploit spatial dimension. Over the past few years, it has been shown that using multiple antennas can significantly increase the capacity and robustness of communication systems in fading environments. Capacity grows with the number of antennas used.
To achieve the capacities promised using MIMO scheme Bell Labs proposed an architecture known as Bell Labs Space Time (BLAST) architecture. To overcome the implementation complexities modified version known as Vertical Bell Labs Space Time or VBLAST was proposed. In this architecture a single data stream is demultiplexed into M sub streams, and each sub stream is then encoded into symbols and fed to its respective transmitter. It is assumed that the same constellation is used for each sub stream and that the transmissions are organized into bursts of L symbols. The rich scattering environment that is considered is rayleigh fading channel characterized by weak LoS and strong scatterers. At the receiver side each antenna receives signals from all the transmitters and the various versions of the signal are combined, then estimated and decoded to get the symbol.