This project investigates novel self-encoded multiple access (SEMA) communications for the transmission of digital information in multiuser wireless channels. The approach is based on the unconventional self-encoded spread spectrum technique that has been developed at the University of Nebraska for the modulation and detection of spread spectrum signals. The application of self-encoded spread spectrum to multiple access communications entails a number of considerations in system and coding designs. They arise from the fact that self-encoding does not guarantee pair-wise code isolations between the spreading sequences. Multiuser channel coding can mitigate code collisions and improve the system performance. The research will characterize the system performance under various wireless channel disturbances such as noise, interference and fading. The results from this project demonstrate the feasibility of SEMA communications that will have a developmental impact on mobile communications technology.
The focus of this project is the analysis and applications of code-modulated SEMA communications. An analytical model of the system is developed that incorporates multiuser channel coding to control statistical code collisions between the self-encoding sequences. Cooperative channel coding among the users is exploited to determine the balanced trade-offs between signal despreading, capacity (code rate) and error correction capability. The bit-error-rate and capacity of the system are analyzed under various channel conditions. The research includes investigating the use of training sequences and preambles for synchronization acquisition and tracking of self-encoded signals. The development of SEMA multiuser detectors is of particular interest given the fact that the spreading sequences are random, time varying, and not known a priori. SEMA turbo receiver that employs iterative, joint multiuser detector and channel decoding is also studied. The goal of the research is to develop the theoretical foundation for self-encoded multiple access communications, with the potential applications spanning from terrestrial wireless to satellite and optical fiber communications systems.
