Transmit diversity is based on the principle that different fading paths are uncorrelated, and multiple antennas are uncorrelated as far as possible (the distance is greater than 10 times the wavelength), so as to offset the partial fading of the signal. The other method is based on polarization diversity, that is, the polarization directions of the two antennas are vertical. In this case, the distance between antennas can be relatively close. Beamforming is to point multiple antennas in the same direction, which is equivalent to improving the gain of antennas and strengthening the signal strength. In this sense, all antennas are beamforming now, just fixed beamforming, unlike smart antennas, the direction of the beam can be adjusted. For this reason, multiple antennas must be as coherent as possible (the distance is less than the wavelength). Transmit diversity is usually 2 antennas, and more antennas will become very complicated; Beamforming is usually 8 antennas. In addition, the current 8 antennas have adopted dual polarized 8 antennas, that is, a group of 4+4 and 4 antennas with the same polarization, which is actually a combination of transmit diversity and beamforming.