Linear Equalizers for Quasi-Synchronous Block Spreading CDMA Systems

Recently, a block spreading code division multiple access (BS-CDMA) technique was presented whereby user-specific preceding along with orthogonal spreading codes are used to achieve multi-user interference (MUI) free when all users arrive at the base station simultaneously. In practice however, imperfect synchronization destroys the orthogonality among users and MUI occurs. This paper investigates the design of linear frequency domain equalizers to reduce the MUI in a quasi-synchronous BS-CDMA system. An optimal frequency domain linear minimum-mean squared error (LMMSE) equalizer is derived. Further simplification leads to a novel sub-optimal equalizer with reduced computational complexity. It is shown through simulation that the proposed equalizers effectively suppress the error floor due to quasi-synchronous reception when channel coding is applied.