From Wikipedia/MIMO: ”To correctly form a beam, the transmitter needs to understand the characteristics of the channel. This process is called channel sounding

From Wikipedia/MIMO: ”To correctly form a beam, the transmitter needs to understand the characteristics of the channel. This process is called channel sounding or channel estimation. A known signal is sent to the mobile device that enables it to build a picture of the channel environment. The mobile device sends back the channel characteristics to the transmitter. The transmitter can then apply the correct phase and amplitude adjustments to form a beam directed at the mobile device. This is called a closed-loop MIMO system.”