Downlink channel state estimation method for cell-free millimeter wave beam space system

The invention discloses a downlink channel state estimation method for a cell-free millimeter wave beam space system. The design is suitable for downlink channel state estimation of the cell-free millimeter wave beam space system. A special pilot sequence codebook is designed, and a scheme for directly estimating downlink channel state information is provided based on the codebook, so that the codebook storage amount and the overall channel estimation time can be reduced. Each wireless access point in the system needs to be equipped with an antenna or an antenna array capable of emitting millimeter wave beams, and needs to know information of millimeter wave beam radiation. The geographic locations of all the wireless access points are stored in a central processor. In addition, single/multiple base stations working in a centimeter wave frequency band are also needed. All the wireless access points and the base stations are connected to the central processor through optical fibers. A user is provided with two