Project Summary Signal processing is an area that already plays a significant role in the current GSM and IS-95 systems. This role will only increase in the projected third generation wireless communications, and in fact, the success in implementing these systems will strongly depend on the ability of the signal processing methods to resolve the new technical problems that will emerge with it. The underlying technology of the third generation systems will be based on the wideband CDMA (WCDMA) modulation scheme. In the core of this technology will be sequential signal processing algorithms with abilities to capture fast-changing characteristics of transmission channels very quickly and to exploit known system information optimally. The significant advances of latest CDMA signal processing methods notwithstanding, it is clear that the requirements for much lower bit error rates than in current systems will markedly increase the demands on signal processing capabilities of sequential algorithms for WCDMA signals. Additional challenge arises due to complexities that are a result from the need to handle high data rates and users with high mobility. Since the communication channels will be rapidly time varying, the signals will undergo quick attenuations and the new algorithms on channel estimation and tracking, channel equalization, interference rejection, and RAKE receiver adaptations must have extremely fast convergence rates. The objective of the proposed research is to develop algorithms that will meet the challenges of this new technology. The basic methodology for the proposed processing of WCDMA signals will be based on particle filters, which recently have gained much attention for their potential in handling nonlinear and non-Gaussian models. The underlying principle used in the design of such filters is the representation of the posterior distribution of state variables (the unknowns of the system) by a set of particles (samples). Each particle is given an importance weight so that the set of particles and their weights represent a random measure that approximates the desired posterior distribution. The particles may also represent means of density functions, usually Gaussians, in which case the particles have additional variables, the covariances of the Gaussians. As new information becomes available, these particles propagate recursively through the state space and their weights are modified using the principles of Bayesian theory. There are several ways of applying particle filters including sampling-importance-resampling, mixture Kalman filtering, and Monte Carlo and Metropolis-Hastings importance resampling. These approaches have their advantages and disad- vantages in performance, and impose different demands for real-time implementation. In the proposal, new schemes will be studied that naturally combine the best features of the existing schemes, and tailor them for processing of WCDMA signals. Not only will the new schemes be able to replicate or surpass the best possible performance of the known methods, but they will also be general enough to provide foundations for development of new task specific schemes. Four important topics will be investigated. The first is the examination of fundamental schemes for propagation of state particles. This issue is critical for two important reasons: (a) it aspects the performance of the algorithm and (b) it subsumes the implementation, which although parallelizible, is in some cases too computationally demanding and therefore not too practical. The second topic is task specific and is related to multiuser detection and channel estimation as well as exploitation of the physical characteristics of the channel and the base station/mobile asymmetry for development of improved algorithms. The third one is examination of the flexibility of the proposed methodology and the interaction of the various algorithms and tasks in order to improve their performances and robustness. Finally, the fourth topic will be related to investigation of computational requirements, and structures that would allow for real-time use.
