A Hybrid Multiple Access Scheme Approaching Single User Performance

Recently, an alternative multiple access scheme was proposed by Li Ping. Its ability to separate the users accessing the common time and frequency resource is solely based on chip-level interleaving and it is therefore referred to as IDMA (interleavedivision multiple-access). Our contribution in this paper is to combine OCDMA (CDMA with orthogonal spreading sequences) with IDMA to obtain a new hybrid multiple access scheme referred to as OCDMA/IDMA. The new scheme is shown to outperform IDMA for a sufficiently large number of users and asymptotically approaches single user performance even for a bit load of as high as 3.75. The feasability of the soft decoder output values as reliability information is examined for IDMA and OCDMA/IDMA. It is shown that IDMA and OCDMA/IDMA are suitable to deliver correct reliability information. The application example of adaptive link transmission is considered in this context and an adaptive transmission scheme based on the reliability information at the decoder output is presented.