A CDMA overlay system using frequency-diversity spread spectrum

The frequency bands currently used by existing narrow-band users might be s hared with a code-division multiple-access (CDMA) spread-spectrum (SS) over lay system. Thus, the spectral efficiency is improved, providing more capac ity for future personal communications services. Since both narrow- and wid e-band signals interfere with each other, an SS modulation scheme with a go od resistance to narrow-band interference results in an increased number of acceptable overlay users. We study a frequency-diversity SS modulation sch eme for which optimal detection in the presence of narrow-band interference is easy to implement. The acceptable number of overlay users is evaluated and compared to that of conventional direct-sequence SS (DS-SS) modulation schemes,vith and without interference rejection filters. The proposed syste m also presents the following advantage: by suppressing transmission of rep licas where narrow-band signals (NBS's) are present, the mutual interferenc e can be avoided completely.