I. Introduction
DS-CDMA systems are interference-limited. In order to compete with wireless LANs and WiMax techniques in the long term, the capacity and data rates of DS-CDMA systems need to be substantially improved. Recently, successive interference cancellation (SIC), which is one of promising multi-user detection (MUD) techniques, draws more and more attentions for its low complexity and high performance [1]–[3]. Implementing SIC technique on the uplink of a DS-CDMA system can reduce multiple access interference and improve system capacity. The receiver with SIC technique detects (or decodes) the signals of individual users in sequence. Once a user is decoded, the received signal for this user can be estimated and reconstructed by appropriate channel estimation, and the reconstructed signal will be cancelled from the composite signal prior to decoding of subsequent users. The decoding order of SIC is of significance to the received signal qualities of individual users, as well as the overall system performance. Different decoding orders will inevitably result in different system performance. It is obvious that a system with active users has possible decoding orders. Examining the system feasibility of possible decoding orders may bring in prohibitively high computational complexity and could become a barrier for implementing admission control and/or scheduling mechanisms which need the system feasibility information.