Successive interference cancellation (SIC) is an effective technique to reduce multiple access interference and thereby increase capacity. While a conventional receiver attains best performance under equal received power for all users, a SIC-based receiver favors unequal powers. We consider ways to assign powers to the received user signals for the benefit of SIC-based multiuser detection. Two power assignment methods, both allocating user signal powers according to their sequence of detection in SIC, are investigated. Through numerical simulation, the methods are shown to greatly improve the bit error rate (BER) performance compared to equal-power assignment. The simulation results also show that, under these methods of power assignment, SIC yields superior performance to parallel interference cancellation (PIC) in high-data-rate transmission.