Channel-aided, decision-directed delay-locked loop for pilot symbol aided CDMA communications impaired by fast frequency-selective Rayleigh fading and user interference

Abstract

In this paper, a channel-aided, decision-directed delay-locked loop (CADD-DLL) is investigated for pilot-symbol-aided (PSA) code-division multiple-access (CDMA) communication. It is assumed that the initial pseudonoise (PN) code acquisition is accomplished with the aid of a conventional noncoherent PN code acquisition system, and that, upon acquiring the initial PN code epoch, the PN code tracking is performed using a channel-aided, decision-directed PN code tracking mechanism. The proposed tracking loop consists of the delayed and advanced PN correlators. The correlators are followed by data and phase correction as well as amplitude matching devices, the outputs of which are subtracted to form an error signal for code tracking purposes. The performance of the proposed tracking loop is assessed in terms of the variance of PN code tracking error and the mean-time-to-loss (MTTL) for a frequency-selective Rayleigh fading channel in the presence of user-induced interference. It is demonstrated that the impact of the decision-directed mechanism on the performance is to reduce the variance of timing error for moderate levels of signal-to-noise ratio (SNR) as compared to the conventional noncoherent delay-locked loop (NC-DLL) tracking mechanism. Also, the MTTL is shown to be highly dependent on the mobile's Doppler shift.