ssage. The intelligence of a multiantenna system is located in the weight selection algorithm rather than in the coding side. A simple linear antenna array combination can offer a more reliable communications link in the presence of adverse propagation conditions such as multipath fading and interference. A key concept here is that of beam forming, by which the signal-to-noise ratio can be increased by focusing energy into desired directions, in either a transmitter or a receiver. If the response of each antenna element to a given desired signal, and possibly to interference signals, is estimated, the elements with weights selected as a function of each element response can be optimally combined. The average desired signal level can then be maximised or the level of other components, whether noise or co-channel interference, can be minimised. The prospect of many orders of improvement in wireless communication performance at no cost of extra spectra is largely responsible for the success of MIMO in high rate WLANs and 3G networks and Fourth Generation Mobile Networks Figure 5.4 Diversity mode is being planned in 4G networks. MIMO is basically the first wireless communications technology that can be used to multiply the link capacity. Multiplying the link capacity increases: Capacity Throughput Signal quality Low power consumption High data rates The MIMO systems multiply spectral efficiency and capacity by encoding, transmitting, receiving and decoding multipath signals. MIMO can be defined as an antenna technology for wireless communication in which multiple antennas are used both at the transmitter and the receiver ends. The antennas at both ends of transmission are combined to minimise errors and optimise data speed. MIMO operates basically in two modes: Diversity mode Spatial multiplexing Diversity Mode Diversity basically refers to the use of multiple antennas to increase the probability of a high quality signal path between the sender and the receiver and can be implemented at the receiver end or the transm