CDMA is unfair: Transmit margin in an inhomogeneous user community

Abstract

In a CDMA system where multiple users share bandwidth in common, multiple access interference (MAI) acts as noise that adds to the receiver thermal noise. To overcome MAI, margin must be added to each user's transmit power. The amount of margin required to achieve this for the homogeneous case in which all users have the same parameters has been known for some time. The general solution for required transmit margin when the user parameters are arbitrary—the inhomogeneous case—is developed in this paper. The solution exhibits an “inverse Robin Hood” characteristic in which less demanding users “subsidize” the more demanding ones by providing a greater share of MAI margin. This behavior is an inherent attribute of CDMA that cannot be overcome except by the elimination of MAI through strictly orthogonal signals.