Frequency estimation and timing acquisition in the uplink of a DS-CDMA system

We consider the uplink of a direct-sequence code-division multiple-access system operating over a multipath fading channel, and we aim at estimating the propagation delay and the frequency offset of a new user entering the network. The joint estimation of these parameters is investigated using a weighted-least-squares approach. The exact solution turns out to be too complex for practical purposes, as it involves a numerical search over a bidimensional domain. As an alternative, we propose a suboptimal procedure, in which the propagation delay is first computed with a monodimensional search and then is exploited to derive the frequency offset in closed form. Both data-aided and nondata-aided estimation methods are considered. The proposed synchronizers have a reasonable complexity and are suited for third-generation cellular systems. Their performance is assessed by simulation in a scenario inspired by the specifications of the frequency-division duplexing component of the Universal Mobile Telecommunications Systems standard. It is found that they provide accurate estimates and are useful even in applications over rapidly varying channels.