Interference-Reducing Spreading Code Design for BS-CDMA with Quasi-Synchronous Reception

Using mutually shift orthogonal spreading codes, block spread code division multiple access (BS-CDMA) systems have been shown to achieve multiuser interference (MUI) free reception when signals of all the users arrive at the base station synchronously. In practice, imperfect synchronization destroys the orthogonality among users and causes MUI. We present in this paper the design of spreading and despreading codes to reduce the MUI due to quasi-synchronous reception. A quasi- synchronous BS-CDMA system using the proposed spreading and despreading codes is shown to achieve a performance close to that of a synchronous system, while maintaining a low receiver complexity. The cost is reduced bandwidth efficiency in transmission, which becomes less significant as the number of users increases.