| Code-division multiple-access (CDMA) is a promising technology for future wireless mobile communications. However, multiaccess interference (MAI) and time-varying multipath effects are limiting factors to the performance of CDMA systems. Several multiuser detection schemes that are resistant to MAI have been proposed recently; however, most of them require an accurate estimate of code timing and/or carrier offset, which are vital to reliable symbol detection.; Joint estimation of code timing and carrier frequency offset for (DS) direct sequence CDMA system is investigated. The goal is to provide an algebraic solution to the joint estimation problem, in a near-far time-varying frequency-selective fading channel. First, the multiuser estimation is decoupled into a series of single-user estimation problems; then, analytical tools of polynomial matrices are invoked for joint carrier and code timing estimation of the desired user. The performance of the proposed study is examined analytically, where expressions for the variance of propagation delay and frequency offset estimation errors are derived. Also, a lower bound on the performance of the estimator, the Cramér-Rao Bound (CRB), is derived. At high signal-to-noise ratios (SNR), the analytical and simulations results agree well, and both approach the CRB.; A subspace code-timing estimator for CDMA systems that utilize bandlimited chip waveforms is presented. The proposed estimator relies on converting the received signal to the frequency domain, then, the code-timing estimates are obtained by exploiting the unique structure of the spectra of the spreading waveform. The proposed method can be implemented both blindly and in a training-assisted fashion. Further, the proposed scheme is near-far resistant, and can deal with time and frequency selective channel fading. Numerical examples are presented to evaluate and compare the proposed and several other code-timing estimators for bandlimited CDMA systems. |
