CDMA receiver structures for overlay systems employing transmitter filtering

In order to take advantage of the unused bandwidth available in sparsely populated narrowband systems, the concept of code-division multiple-access (CDMA) overlay has been examined (Milstein et al., 1992). It was shown in Rainbolt and Miller (1997) that even a CDMA system that is lightly loaded can cause significant degradation to the performance of the narrowband system, perhaps precluding the possibility of overlay. However, by notching the CDMA signal in those frequencies at which a narrowband user is operating, the effect of the CDMA interference on the performance of a narrowband user was shown to be greatly alleviated. This paper examines the effects of notch filtering on the performance of the CDMA system in an overlay scenario, and it is shown that the minimum mean-squared error (MMSE) receiver (Miller 1995; Rapajic and Vucetic 1994; Madhow and Honig 1994; Pateros and Saulnier 1996) can be used successfully, and offers a substantial improvement over the conventional matched-filter receiver. The results of this paper, along with those of Rainbolt and Miller, give strong support to both the feasibility and usefulness of CDMA overlay.