Abstract
Since the first introduction of wireless cellular systems, wireless communication has attracted a lot of interests and become one of the most challenging research topics. In order to achieve the global wireless communications systems providing high-quality personal multimedia services, the third generation cellular systems, known as International Mobile Telecommunications - 2000 (IMT-2000), are extensively investigated. DS-CDMA communication, as the primary technology in IMT-2000, receives lots of researchers' interests due to its increasing capacity. To mitigate the performance degradation due to multiple access interference (MAI), multiuser detection was proposed. However, all the multiuser detectors assume the knowledge of received timings, carrier phases and perhaps amplitudes of all users. Multiuser synchronizers, which provide these necessary information, are therefore of paramount importance. Motivated by the introduction of optimal multiuser synchronization, two categories of multiuser synchronizers are proposed which are mathematically and numerically confirmed to be effective for an arbitrary number of users in the near-far environment with feasible complexity. The first category of multiuser synchronizers linearly combine the sufficient statistics as linear multiuser detectors do and have the following characteristics: * Suitable for general-purposes applications. * Complexity grows linearly to the number of users. * Near-far resistant for an arbitrary number of users. * Suboptimal performance. On the other hand, by maintaining frequency-domain orthogonality, another category of multiuser synchronizers aided by pilot signals are introduced. These multiuser synchronizers, however, own the properties: * Employing pilot signals and limited capacity. * Complexity similar to the one in point-to-point communications. * Near-far resistant. * Optimal performance. Both categories of estimators have their special advantages according to their characteristics and are applicable to practical DS/CDMA communications systems.