The maximum capacity gain on top of the GSM uplink due to a cellular broadband DS-CDMA overlay is estimated. It is assumed that the base stations of both systems are unilocated and that the power control in both systems is perfect. Furthermore, in the CDMA transmitters and receivers, ideal notch filtering is assumed around the occupied GSM frequencies of the same cell. Using geometrical arguments and a standard propagation model, all relevant contributions to the carrier-to-interference ratios (CIR) for the uplinks of the GSM and CDMA systems are numerically evaluated. Adapting standard limits for the CIRs, the capacity on the uplink can be increased by a factor of 4.8. However, the more critical down-link, whose derivation is more complicated and not presented in this paper, limits the overall capacity for the same assumptions to 3.3.
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