Structured multiuser channel estimation for block- synchronous DS/CDMA

Uplink channel estimation for a block-synchronous chip-asynchronous DS/CDMA system as proposed for the time-division duplex option of third-generation cellular systems is considered. Training midambles are employed for joint channel estimation of all users. The standard unstructured approach based o n modeling the effective user channels as unknown FIR filters is compared w ith two structured methods that exploit a priori knowledge about the user c hannels such as the maximum delay-spread, the transmit chip-shaping pulse a nd the path delays. Since these are usually unknown, a low-complexity estim ator for the path delays of all users is derived from a maximum-likelihood approach. For all channel estimators, optimal sets of training sequences ba sed on perfect root-of-unity sequences are found. For these optimal sets, i t is shown that the reduction in channel estimation mean-squared error of t he structured estimator versus the unstructured estimator is exactly the ra tio of the number of structured parameters to unstructured parameters. Simu lation results show that structured channel estimation provide advantages u p to 4 dB in terms of output signal-to-interference plus noise ratio with r espect to unstructured estimation, for linear MMSE detection. In contrast, for conventional single-user matched filtering, unstructured estimation pro ves to be sufficiently good.