noun, Beamforming is a signal processing technique used in sensor arrays for directional signal transmission or reception. This spatial selectivity is achieved by using adaptive or fixed receive/transmit beampatterns. The improvement compared with an omnidirectional reception/transmission is known as the receive/transmit gain."Beamforming" uses interference to shape directionality of the signal. When transmitting, a beamformer controls the phase and relative amplitude of the signal at each transmitter, in order to create a pattern of constructive and destructive interference in the wavefront. Information received at different sensors is combined in such a way that the expected pattern of radiation is interpreted optimally . This way signals from multiple transmitters can be timed to arrive simultaneously or at known times to return in a planned series, what we call spatial multiplexing. Instead of just spitting out a signal equally in every direction with equal power to be received at-will, MIMO shapes and manipulates it's contour to create a known quantity in reception.
SIMO is a multiple antenna technique. It stands for Single Input Multiple Output. The system uses a single antenna at the transmitter but multiple antennas at the receiver. The receiver can select whichever antenna receives the optimal signal or combine received signals to created the best result possible. It's called switch diversity. More here.
MISO is a system that uses multiple antennas at the transmitter and a single antenna at the receiver. Appropriately the acronym stands for Multiple Input Single Output. It uses Space Time Coding (STC) at the transmitter to transmit signals by two antennas at two different times consecutively. It's all still received at the single receiving antenna that can parse, combine or select between the two signals for lowest noise, and best signal.
SISO is conceptually easier to grasp. SISO stands for Single Input Single Output. It's what we do now. The transmitter and the receiver have one RF chain, as they call it. It's cheap and simple and it works... but it ain't perfect.