Frequency overlay of GSM and cellular B-CDMA

The maximum capacity gain of a cellular broad-band direct-sequence code-division multiple-access (DS-CDMA) overlay on top of a cellular global system for mobile communications (GSM) system is estimated. Using geometrical arguments and a standard propagation model, all relevant contributions to the carrier-to-interference ratios (CIRs) of the GSM and CDMA systems are numerically evaluated. It is assumed that the base stations (BSs) 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. Adapting standard limits for the CIRs of 5-7 dB for CDMA and 9-12 dB for GSM, the total system capacity can be increased considerably over the GSM-only case: the capacity is increased by a factor of 1.8-2.9 if no macrodiversity is employed and by a factor of 1.9-3.3 if macrodiversity in the CDMA downlink is applied