CDMA transmitter filtering for cellular overlay systems

The overlay of a CDMA system in a frequency band which is already servicing narrowband users has the potential for significant gains in user capacity. For the single-cell case, it was shown previously that interference caused to the narrowband system by CDMA users is significant, even when the CDMA system is very lightly-loaded. The use of notch filtering at the CDMA transmitter to avoid active narrowband users was shown to greatly alleviate the effects of the CDMA interference on the narrowband system. It was also demonstrated that the MMSE receiver can perform well in receiving these filtered CDMA signals. In this paper, overlay is applied to the cellular case, in which the narrowband users in each cell only utilize a fraction of the system bandwidth even when the system is fully loaded. As in the single-cell case, it is seen that overlay causes significant degradation to the narrowband system, which can again be alleviated greatly through the use of transmitter notch-filtering. It is demonstrated that overlay is very promising for cellular systems