WCDMA is wideband wireless access based on direct sequence code division multiple access aimed at third-generation mobile communications system. In this paper, I designed a baseband modulator and demodulator for the downlink transmission of WCDMA. The proposed design of modulator and demodulator system is based on the Technical Specification of 3GPP($3^{rd}$ Generation Partnership Project). The designs for each function are made by module type. Therefore these modules are assembled in total physical layer system of WCDMA downlink. The performance of this system is measured by the error bit count method. The performance measurement is performed in the 384kbps, and 64kbps downlink transmission.