Multiple user maximum likelihood code-timing acquisition for uplink DS-CDMA systems

Future CDMA systems will sometimes be required to operate with low processing gain in order to accommodate high rate users. The resulting increase in channel dispersion can have a detrimental impact on code-timing acquisition. Two maximum likelihood code-timing acquisition algorithms are proposed for multipath channels: a multiuser estimator and a single-user estimator. Multipath diversity is exploited in the estimators via maximum ratio combining to increase the signal-to-interference ratio. In addition, correlations between signals from different paths are explicitly incorporated in the test statistics of both estimators. The Cramer-Rao bounds are found and the performances of the estimators are examined via simulations. The results indicate that the estimators are robust to near-far power ratio and channel dispersion. Moreover, the gain achieved by incorporating channel dispersion in estimator design is significant.