Minimum interference prolate based spreading codes design DS-CDMA overlaying narrow band systems

The overlaying of a CDMA system on an existing narrowband system has been proposed and investigated in several publications. This topic is critical in many environments where new broadband systems are required to coexist with legacy narrowband systems. This paper is focused on minimisation of the performance degradation of the narrow-band system(s) caused by the CDMA one. Common background in most of this topic deals with notch-filtering of CDMA signals in transmitter to minimise its interference over the narrowband signal. However, this solution results in effective spreading codes longer that bit time, causing ISI within the CDMA system. The solution that is proposed in this contribution is the design of spreading codes that notch the CDMA power spectrum density (PSD) on specified bandwidth intervals where narrow-band signals are transmitted. To achieve this, spreading codes are selected from the orthogonal set of polynomials arising from digital prolate functions (DPF) which have property of energy concentration in specified intervals. Provided solution guarantees null interference in CDMA system with orthogonal codes, strictly located within the bit period.