Unequal Error Protection for Quasi-Synchronous BS-CDMA Systems

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 timing synchronization destroys the orthogonality among users and MUI occurs. In this paper, based on the fact that unequal interference power distribution occurs in an uplink BS-CDMA system where varying degrees of interference power occur among users and across the despread message, we apply unequal error protection (UEP) to mitigate the MUI due to quasi-synchronous reception. Two UEP methods are studied. It is shown through simulations that BS-CDMA systems using these methods achieve significant performance improvement in terms of packet-error-rate (PER) compared to conventional quasi-synchronous BS-CDMA systems where the same error correcting codes are applied to the entire information packet for all users.