Abstract— The so-called receiver multiuser diversity aided multistage minimum mean-square error multiuser detector (RMD/MS-MMSE MUD), which was proposed previously by the author, is investigated in the context of the direct-sequence code-division multiple-access (DSCDMA) and space-division multiple-access (SDMA) systems that employ in- and quadrature-phase (I-Q) modulation schemes. A detection scheme is studied, which is operated in real domain in the principles of successive interference cancellation (SIC). The concept of noise recognition factor (NRF) is proposed for explaining the efficiency of SIC-type detectors and also for motivating to design other high-efficiency detectors. The achievable bit error rate (BER) performance of the RMD/MS-MMSE MUD is investigated for DS-CDMA and SDMA systems of either full-load or overload, when communicating over Rayleigh fading channels for the SDMA and over either additive white Gaussian noise (AWGN) or Rayleigh fading channels for the DS-CD...