One frequency is used to send out a signal, a second one is used to receive. The first three generations use FDD because they have no way to isolate the interference in the sending out and receiving back in the cellular system. According to Lee, this innovation allows a signal to be sent on one time slot and return in another time slot on the same frequency. With this system, you don't need complicated technology. Lee says much of the research done to make wireless better has been focused on isolating interference.
Interference is the killer.
But now we have these special codes. This is simple, high-tech efficiency in using the spectrum, which is very expensive. Lee knows something about the cost of wireless. He remembers when the first mobile system was available commercially in the United States. It was rolled out just about the same time as the 1984 Los Angeles Olympics.