Performance and flexibility enhancement techniques for spread spectrum multiuser systems

Spread spectrum multiple-access systems allow many users to communicate simultaneously by intelligently scattering each user's information across the available bandwidth and then combining the information for each user at the receiver. In this way, each user's data appears as low level noise to the other users. Thus, instead of confining users to a limited portion of the available resources, this flexible system allows every user to access all available resources without colliding with the other users. The disadvantage is that the conventional receivers for these systems are "interference-limited" meaning that there exists a level of interference which prevents the desired user's transmission from being adequately decoded. For this reason, multiuser detection has been proposed which alleviates these problems by jointly decoding all users' information. However, multiuser detectors are more complex because they jointly decode everyone's information as opposed to just one user's data, and they lead to a less flexible system since joint detection requires that all users' parameters be known.; This thesis describes a technique called a Detector/Estimator (DE) which increases the flexibility of multiuser detection for direct sequence-code division multiple-access (DS/CDMA). These DEs allow a multiuser detector to update its knowledge of all users by determining which users have left the system as well as detecting a new user's presence and estimating that new user's parameters. Using these DEs, any multiuser detector becomes an adaptive detector which jointly decodes users' data without the use of a side channel or central controller.; Another important problem in spread spectrum multiple access is improving reception with multiuser detection for frequency hopping/code division multiple-access (FH/CDMA). Joint detection is more difficult in FH/CDMA than in direct sequence because the users' data is detected noncoherently. In this thesis, a reduced complexity multiuser detector called a multistage multiuser detector (MMD) is proposed which removes identifiable multiple-access interference from a FH/CDMA channel. The MMD is shown to significantly improve reception over conventional detection when users transmit synchronously. This detector has also been extended to the asynchronous environment. The asynchronous MMD yields a modest improvement over the conventional detector.