Code-Timing Estimation for Long-Code CDMA Systems with Bandlimited Chip Waveforms

In this paper, we present a code-timing estimation scheme for asynchronous direct-sequence (DS) code-division multiple-access (CDMA) systems with aperiodic (long) spreading codes and bandlimited chip waveforms. The proposed scheme first converts the observed signal to the frequency domain by fast Fourier transform (FFT). Then, a nonlinear least-squares (NLS) criterion is invoked to fit the unknown parameters to the frequency-domain data. While the exact minimizer of the NLS criterion requires computationally prohibitive searches over a multi-dimensional parameter space, we propose an efficient approach that iteratively estimates one user/path at a time via simple linear processing and, furthermore, combines successive interference suppression (SIC) with parameter re-estimation for improved code acquisition performance. Simulation results show that the proposed scheme achieves a significantly larger user capacity and faster acquisition time than the SIC and a standard matched-filter based code acquisition techniques in time-varying fading channels.