Partially blind adaptive MMSE interference rejection in asynchronous DS/CDMA networks over frequency-selective fading channels

In this work, the problem of joint suppression of multiple-access and narro w-band interference (NBI) for an asynchronous direct-sequence code-division multiple-access (CDMA) system operating on a frequency-selective fading ch annel is addressed. The receiver structure we consider can be deemed as a t wo-stage one: the first stage consists of a bank of minimum mean-square-err or (MMSE) filters, each keyed to a given replica of the useful signal, and aimed at suppressing the overall interference; the second stage, assuming k nowledge of the fading channel coefficients realizations, combines the MMSE filters outputs according to a maximal-ratio combining rule. Due to the pr esence of the NBI; the resulting structure is in general time-varying, and becomes periodically time-varying if the NBI bit-rate has a rational ratio to that of the CDMA system. Moreover, enlarging the observation window beyo nd the signaling interval and oversampling the signal space may yield a not iceable performance improvement. For the relevant case that the said ratio is rational, a new cyclic blind recursive least squares (RLS)-based algorit hm is introduced, capable of tracking the periodically tine-varying receive r structure, and allowing adaptive interference cancellation with a moderat e complexity increase. We also come up with a closed-form expression for the conditional bit-error rate (BER), which is useful both to evaluate semi-analytical methods to as sess the unconditional BER and to derive bounds on the system near-far resi stance. The results indicate that the receiver achieves very satisfactory p erformance in comparison to previously known structures. Computer simulatio ns also demonstrate that the cyclic blind RLS algorithm exhibits quite fast convergence dynamics.