Propagation delay estimation in asynchronous direct-sequence code-division multiple access systems

| In an asynchronous direct-sequence code-division multiple access (DS-CDMA) communication system the parameter estimation problem, i.e., estimating the propagation delay, attenuation and phase shift of each users' transmitted signal, may be complicated by the so-called near-far problem. The near-far problem occurs when the amplitudes of the users' received signals are very dissimilar, as the case might be in many important applications. In particular , the standard method for estimating the propagation delays will fail in a near-far situation. Several new estima-tors, the maximum likelihood, an approximative maximum likelihood and a subspace based estimator, are therefore proposed and are shown to be robust against the near-far problem. No knowledge of the transmitted bits is assumed and the proposed estimators can thus be used for both acquisition and tracking. In addition, the Cram er-Rao bound is derived for the parameter estimation problem.