An efficient code-timing estimator for DS-CDMA signals

We present an efficient algorithm for estimating the code timing of a known training sequence in an asynchronous direct-sequence code division multiple access (DS-CDMA) system. The algorithm is a large sample maximum likelihood (LSML) estimator that is derived by modeling the known training sequence as the desired signal and all other signals including the interfering signals and thermal noise as unknown colored Gaussian noise that is uncorrelated with the desired signal. The LSML estimator is shown to be robust against the near-far problem and is also compared with several other code timing estimators via numerical examples. It is found that the LSML approach can offer noticeable performance improvement, especially when the loading of the system is heavy.