Analysis of Spectrum Band Sharing between CDMA and other Systems

Many mobile satellite systems have been proposed to employ Code Division Multiple Access (CDMA) technique at L/S band. These bands have been allocated to the mobile satellite services on a primary basis since 1992, in addition to the existing services. Therefore, the mobile satellite services will be subject to share spectrum with each other and with existing services. This paper discusses spectrum overlapping for two cases: i)- between a CDMA channel and narrowband signal channel; ii)- between CDMA channels sharing the same band. Analyses have been performed to increase the overall spectral efficiency in the frequency band, taking into account various factors such as the relative position of overlapping and the amount of overlapping. Positions of optimum overlapping for spectral efficiency are discussed. This spectrum sharing is referred to as a CDMA overlay, and has a potential to enhance both the spectral utilization and the flexibility of that utilization. From the results, the best position of the multiple narrow band signals, to allow sharing spectrum with CDMA signal without loosing significant capacity for both of them, is at the edge of the CDMA channel. Further improvement can be achieved for CDMA system, especially when narrow band signals have more power compared to the CDMA system, by implementing a notch filter at the specific location of the narrow-band signal. For CDMA/CDMA spectrum sharing, the overall spectral efficiency may be increased significantly by carefully choosing the overlapping position