Multiuser receivers for randomly spread signals: Fundamental limits with and without decision-feedback

Synchronous code-division multiple-access (CDMA) communication systems,vith randomly chosen spreading sequences and capacity-achieving forward error c orrection coding are analyzed in terms of spectral efficiency. Emphasis is on the penalties paid by applying single-user coding in conjunction with su boptimal multiuser receivers as opposed to optimal joint decoding which inv olves complexity that is exponential in the number of users times the codew ord length. The conventional, the decorrelating, and the (re-encoded) decorrelating dec ision-feedback detectors are analyzed in the nonasymptotic case for spheric al random sequences. The re-en coded minimum mean-squared error (MMSE) deci sion feedback receiver achieving the same performance as joint multiuser de coding for equal power users is shown to be suboptimal in the case of equal rates.