Equal power allocation of IDMA systems: Feasibility, optimality, and throughput

Equal power allocation (EPA) has been truly successful when applied in IS-95 CDMA. It is therefore worthwhile to see whether EPA is also applicable in IDMA systems, which can be considered as a special case of the direct-sequence CDMA. However, the applicability of EPA in IDMA systems must be considered in light of two fundamental problems. First, EPA is not efficient in suppressing multi-user interference and hence often results in severe performance degradation (infeasibility). Second, EPA schemes often require higher received power than other schemes that allow unequal received powers (sub-optimality). In this paper, we derive the necessary and sufficient conditions for the feasibility and optimality of IDMA systems having equal received powers. The conditions show how feasibility and optimality are related to system parameters such as number of users, code rate and bit-error rate target. The conditions also suggest an upper limit on the throughput of IDMA systems with equal received powers. The results obtained in this paper may provide a theoretical foundation for designing future EPA schemes for IDMA.