Idea Makers: Personal Perspectives on the Lives & Ideas of Some Notable People

by Stephen Wolfram

One of the single most prominent uses of shift register sequences is in cellphones, for what’s called CDMA (Code Division Multiple Access). Cellphones got their name because they operate in “cells”, with all phones in a given cell being connected to a particular tower. But how do different cellphones in a cell not interfere with each other? In the first systems, each phone just negotiated with the tower to use a slightly different frequency. Later, they used different time slices (TDMA, or Time Division Multiple Access). But CDMA uses maximum-length shift register sequences to provide a clever alternative. The idea is to have all phones essentially operate on the same frequency, but to have each phone encode its signal using (in the simplest case) a differently shifted version of a shift register sequence.