Partial interference subspace rejection in CDMA systems

Previously presented Interference Subspace Rejection (ISR) [3, 8, 41 proposed a family of new eficient multi-user detectors for CDMA. We reconsider in this paper, the modes of ISR using Decision Feedback (DF). DF modes share similarities with the PIC but attempt to cancel inteference by nulling rather than subtraction. However, like the PIC they are prone to wrong tentative decisions. We propose a modi,fication to DF modes that perfnrms partial ISR instead of complete interference cancellation. When tentative decisions are correct, interference is therefore not pefectly rejected anymore. This drawback is compensated by improved robustness to wrong tentative decisions. We show that in hard handoffsystems, partial ISR can only provide neg1ig’ble performance improvemen fs in high loaded systems outside the region of interest due to out-sector’ inteference. In situations where both in-sector and out-sector interferences are cancelled, as may occur in soft handofsituations, BER performance may improve by more than I dB.