Code-timing estimation for CDMA systems with bandlimited chip waveforms

In this paper, we present a novel code-timing estimation for asynchronous direct-sequence (DS) code-division multiple-access (CDMA) systems utilizing bandlimited chip waveforms. We start from a maximum likelihood (ML) approach that model the inter-symbol interference (ISI) and multiple-access interference (MAI) as a colored Gaussian process with unknown covariance matrix in the frequency domain. The exact ML estimator is highly nonlinear and requires iterative searches over multidimensional parameter space that is impractical to implement. To deal with this difficulty, we invoke asymptotic approximations of the ML criterion and reparameterization techniques, which lead to an asymptotic ML estimator that yields code-timing and channel estimates via efficient non-iterative quadratic optimizations. To benchmark the proposed estimator, we provide Cramer-Rao bound (CRB) analysis for the code-timing estimation problem. Numerical simulation results are presented to illustrate the performance of the proposed code-timing estimator for bandlimited CDMA in multipath fading channels.