Wireless channels suffer from time-varying impairments such as multipath fading, interference, and noise. Diversity, such as time, frequency, space, polarization, or angle diversity, is typically used to mitigate these impairments. Diversity gain is achieved by receiving independent-fading replicas of the signal.
The multiple antenna system employs multiple antennas at either the transmitter or the receiver, and it can be either multiple-input single-output (MISO) for beamforming or transmit diversity at the transmitter, single-input multiple-output (SIMO) for diversity combining at the receiver, or MIMO, depending on the numbers of transmit and receive antennas. The MISO, SIMO, and MIMO channel models can be generated by using the angle-delay scattering function.
Multiple antenna systems are generally grouped as smart antenna systems and MIMO systems. A smart antenna system is a subsystem that contains multiple antennas; based on the spatial diversity and signal processing, it significantly increases the performance of wireless communication systems. Direction-finding and beamforming are the two most fundamental topics of smart antennas. Direction-finding is used to estimate the number of emitting sources and their DoAs, while beamforming is used to estimate the signal-of-interest (SOI) in the presence of interference.
A MIMO system consists of multiple antennas at both the transmitter and the receiver. They are typically used for transmit diversity and spatial multiplexing. Spatial multiplexing can maximize the system capacity by transmitting at each transmit antenna a different bitstream.