On the restriction of the coverage area of the interference TDMA co-channel slots in an overlaid CDMA and TDMA system

Theoretically the separation of CDMA and narrowband waveforms can be done through notch filters; however it has been demonstrated that the filtering techniques are not enough to eliminate the multi-cross interference problem. As a means of suppressing the interference that can not be eliminated by the use of filters this work presents a new proposal in which the reduction of the coverage area of the interference TDMA co-channel slots and the ordering in the use of this slots can significantly reduce the effect of the TDMA interference over the CDMA system. Thus, a TDMA sector is divided in two concentric zones, each one associated with a different set of slots. We divide the set of slots in two groups a general group C_G integrated by all non-co-channel slots and a restricted group C_R integrated by the co-channel slots. In the inner region the two groups of slots can be used but in the outer zone only the C_G group is used. Thus, with this strategy the effect of the co-channel interference is minimized. The performance evaluation of the overlaid system is derived analytically in terms of the Erlang capacity of the two systems for different scenarios, different concentric regions areas and for the reverse and forward links.