A SPREADING FUNCTION FOR BAND LIMITED DIRECT-SEQUENCE SPREAD SPECTRUM CODE DIVISION MULTIPLE ACCESS SYSTEMS

Abstract

In a band limited direct-sequence (DS) spread-spectrum (SS) code division multiple access (CDMA) system, the signal to co-user noise ratio (SNR) is determined by the frequency responses of the transmit and receive band limiting filters and the spectral shape of the spreading function. To optimize the SNR, it is important that the transmit and receive filters be phase matched and the in-band portion of the spectrum of the spreading function be whitened by the magnitude responses. In practice, the tolerances of the electronic components in the transmit and receive filters make it difficult if not impossible to phase match the filters, especially in the wide band systems where digital signal processing can not be used to implement band limiting filters. This thesis suggests a method of generating a spreading function that has a flat portion in the mainlobe of its spectrum and reasonable sidelobe rolloff. Compared with a conventional binary PN code spreading function, such a spectrum of the proposed spreading function results in a larger SNR, at the same time it is more immune to phase mismatch in the transmit and receive filters. The proposed spreading function can also be incorporated in a practical system as it does not require four quadrant multipliers.