 of the 4G system architecture as the wireless systems continue to strive for even higher data rates. Digital communication using multiple-input multiple-output (MIMO), also called a 'volume-to-volume' wireless link, has emerged as one of the most important breakthroughs in modern communications. The MIMO systems can be defined as a general wireless communication link where a link for the transmission and receiver is equipped with multiple antenna elements, as shown Figure 5.3. The MIMO systems are chosen ahead of the single antenna systems because of their ability to turn multipath propagation, a pitfall of wireless communication, into a benefit for the user. They take advantage of random fading and multipath delay spread for multiplying transfer rates. Principles of MIMO Systems The idea behind MIMO is that the signals on the transmit (TX) antennas on one end and the receiver (RX) antennas on the other end are combined in such a way that the quality (bit error rate, or BER) or the data rate (bits per second) of the communication for each MIMO user will be improved. The main idea is that of space-time signal processing, in which time is complemented with the spatial dimension inherent in the use of multiple spatially distributed antennas. As shown in Figure 5.2, a compressed digital source in the form of a binary data stream is fed to a simplified transmitting block encompassing the functions of error control coding and mapping to complex modulation symbols. The transmitter produces several separate symbol streams, which range from independent to partially redundant to fully redundant. Each is then mapped on to one of the multiple TX antennas. Mapping may include linear spatial weighting of the antenna elements or linear antenna space-time precoding. After upward frequency conversion, filtering and amplification, the signals are then launched into the wireless channels. At the receiver the signals are captured by possibly multiple antennas and demodulation and demapping operations are performed to recover the me