in order to decrease the probility of occuring fading for transmitting signal, we have to send it in two seperate channel. transmission of the signal in two seperate time is named time division duplex, TDD transmission of the signal in two seperate frequency is named freq. division duplex FDD, and transmission of the signal in two different direction is named space division duplex, SDD
since the probility of frequency selective fading in two different freq. is independent, the prob. of fading decrease by using FDD