A time-varying projection receiver for asynchronous DS/CDMA systems with narrowband interference over multipath fading channels

The authors present a projection receiver for an asynchronous DS/CDMA system, operating over a multipath fading channel and sharing a fraction of its bandwidth with a narrowband data-like interferer. The proposed receiver achieves narrowband interference rejection by projecting the received waveform onto the orthogonal complement of the subspace spanned by the interference: this operation is shown to entail in general a time-varying processing of the observables. In order to improve the system performance, the possibility of considering an observation interval larger than the bit signaling time T_b, is exploited as well as that of sampling the received waveform faster than the chip-rate. As to the performance assessment, we focus on the relevant case of Rayleigh distributed fading, and give closed-form formulas for the system bit error rate (BER) and near-far resistance. The results show that the system largely outperforms conventional detectors, in that it achieves a complete nullification of the interference, no matter its energy. Additionally, as expected, increasing the sampling rate and the processing window length yields a performance improvement