Decorrelating code-timing estimation for CDMA systems with long codes and bandlimited chip waveforms

This paper addresses the problem of multiuser code-timing estimation for asynchronous uplink CDMA systems with aperiodic spreading codes and bandlimited chip waveforms. Two decorrelating code-timing estimation schemes, namely the FLS and FWLS estimators, are developed, one based on least-squares (LS) while the other on weighted LS (WLS) processing in the frequency domain. The two proposed estimators offer different tradeoffs between complexity and estimation accuracy. Due to a unique signal structure in the frequency domain, both the FLS and FWLS estimators admit efficient recursive implementation, a subject also elaborated in this paper, which results in complexity reduction remarkably by orders of magnitude compared to direct implementation. Numerical results show that both of the proposed estimators can support overloaded systems (with more users than the processing gain) in multipath fading environments, and significantly outperform a conventional technique based on matched-filter processing.